14.doc

Allergy and Allergic Diseases

The adaptive immune response is a critical component of host defense againstinfection and is essential for normal health. Adaptive immune responses aresometimes elicited by antigens not associated with infectious agents, and thiscan cause disease. One circumstance in which this occurs is when harmfulimmunologically mediated hypersensitivity reactions known generally asallergic reactions are made in response to inherently harmless ‘environmen-tal’ antigens such as pollen, food, and drugs.

Hypersensitivity reactions due to immunological responses were classifiedinto four broad types by Coombs and Gell (Fig. 14.1). Type I hypersensitivityreactions in this classification are immediate-type allergic reactions mediatedby IgE antibodies, but many of the allergic diseases that are initiated by IgEantibodies, such as allergic asthma, have chronic features characteristic ofother types of immune response, particularly of TH2 cell-mediated type IVhypersensitivity (see Fig. 14.1). In most allergies, such as those to food, pollen,and house dust, reactions occur because the individual has become sensitizedto an innocuous antigen—the allergen—by producing IgE antibodies againstit. Subsequent exposure to the allergen triggers the activation of IgE-bindingcells, chiefly mast cells and basophils, in the exposed tissue, leading to aseries of responses that are characteristic of this type of allergic reaction. Inhay fever (allergic rhinoconjunctivitis), for example, symptoms occur whenallergenic proteins leached out of grass pollen grains come into contact withthe mucous membrane of the nose and eyes. Other allergic diseases, such asserum sickness, allergic contact dermatitis, and celiac disease, do not involveIgE and are due to type II, III, or TH1- or CD8 T-cell type IV hypersensitivity

2 Chapter 14: Allergy and Allergic Diseases

reactions (see Fig. 14.1).

Although everybody is exposed to common environmental allergens, most ofthe population does not develop allergic reactions to them. In surveys, up to40% of the population shows an exaggerated tendency to become sensitizedto a wide variety of common environmental allergens. A predisposition tobecome IgE-sensitized to environmental allergens is called atopy, and laterin the chapter we discuss the various factors—both genetic and environ-mental—that may contribute to predisposition. The importance of geneticfactors in predisposing to IgE-mediated allergic disease is shown by the factthat if both parents are atopic, a child has a 40-60% chance of developing anIgE-mediated allergy, whereas the risk for a child neither of whose parents isatopic is much lower, of the order of 10%, although this percentage is increas-ing, as we discuss later in the chapter.

The main biological role of IgE is thought to be in adaptive immunity toinfection with parasitic worms (see Chapter 10), which are prevalent in lessdeveloped countries. In the industrialized countries, IgE-mediated allergicresponses to innocuous antigens predominate and are an important cause ofdisease (Fig. 14.2). Almost half the population of North America and Europe


Type I Type II Type Ill Type IV

Immunereactant

lgE lgG lgG Th1 cells Th2 cells CTL

Antigen Solubleantigen

Cell- or matrix-associatedantigen

Cell-surface

receptor

Solubleantigen

Solubleantigen

Solubleantigen

Cell-associated

antigen

Effectormechanism

Mast-cellactivation

Complement,FcR+ cells(phagocytes,NK cells)

Antibodyalters

signaling

Complement,phagocytes

Macrophage

activation

lgE production,eosinophilactivation

Cytotoxicity

/n*m\

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plateletsP $+

complement0

rOU

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bloodvessel Ut^ v/J

+complement

0

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es,cytokines

, cytotoxin

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IL-5 0 eotaxin

cytotoxins,inflammatory mediators

©CTL

0

Example ofhypersensitivityreaction

Allergic rhinitis,allergic asthma,atopic eczema,systemic

Some drugallergies(e.g. penicillin)

Chronic urticaria

(antibody against

FceRI alpha

chain)

Serum sickness,Arthus reaction

Allergic contact

dermatitis,

tuberculin

reaction

Chronic asthma,chronic allergicrhinitis

Graft rejection,allergic contactdermatitis topoison ivy



Fig. 14.1 Immunological hypersensitivity reactions, orallergic reactions, are mediated by immune reactions thatcause tissue damage. Four types of allergic reaction aregenerally recognized. Types I-III are antibody-mediated andare distinguished by the different types of antigen recognizedand the different classes of antibody involved. Type I responsesare mediated by lgE, which induces mast-cell activation,whereas types II and Ill are mediated by lgG, which can engagecomplement-mediated and phagocytic effector mechanisms tovarying degrees, depending on the subclass of lgG and the natureof the antigen involved. Type II responses are directed against cell-surface or matrix antigens, whereas type Ill responses are directed

against soluble antigens, and the tissue damage involved iscaused by responses triggered by immune complexes. A specialcategory of type II responses involves lgG antibodies against cell-surface receptors that disrupt the normal functions of the receptor,either by causing uncontrollable activation or by blocking receptorfunction. Type IV hypersensitivity reactions are mediated by T cellsand can be subdivided into three groups. In the first group, tissuedamage is caused by the activation of macrophages by TH 1 cells,which results in an inflammatory response. In the second, damageis caused by the activation by TH2 cells of inflammatory responsesin which eosinophils predominate; in the third, damage is causeddirectly by cytotoxic T cells (CTLs).



is sensitized to one or more common environmental antigens and, althoughrarely life-threatening, allergic diseases initiated by contact with a specificallergen can cause much distress and lost time from school and work. The bur-den of allergic diseases in theWestern world is considerable, with a more thandoubling in prevalence in the past 15 years or so, and so most clinical and sci-entific attention has been paid to the role oflgE in allergic disease rather thanin its protective capacity. Until a few years ago, developing countries in Africaand the Middle East reported a relatively low prevalence of allergy (althoughthis situation is rapidly changing as a result ofWestern-style modernization).

In this chapter we first consider the mechanisms that favor the sensitization ofan individual to an allergen through the production of IgE. We then describethe IgE-mediated allergic reaction itself—the pathological consequences ofthe interaction between allergen and the IgE bound to the high-affinity Fc£receptor on mast cells and basophils. Finally, we consider the causes andconsequences of other types of immunological hypersensitivity reaction.

lgE and lgE-mediated allergic diseases

lgE and IgE-mediated allergic diseases.

Type I hypersensitivity reactions are those allergic reactions due to theproduction of IgE against innocuous antigens. IgE is produced both byplasma cells in lymph nodes draining the site of antigen entry and by plasmacells at the site of the allergic reaction—typically a mucosal tissue or the skin.In mucosal tissues, germinal centers develop within the inflamed tissue. IgEdiffers from other antibody isotypes in being predominantly localized in thetissues, where it is tightly bound to the surface of mast cells, and some othercell types, through the high-affinity IgE receptor FceRI (see Section 10-24).Binding of antigen to IgE cross-links these receptors, causing the release ofchemical mediators from the mast cells that can lead to allergic disease (Fig.14.3) . How an initial antibody response to environmental antigens comes tobe dominated by IgE production in atopic individuals is still being workedout. In this part of the chapter we describe the current understanding of the

lgE-mediated allergic reactions

Reaction or disease Common allergens Route of entry Response

Systemicanaphylaxis

DrugsVenoms

Food, e.g. peanutsSerum

Intravenous (eitherdirectly or followingoral absorptioninto the blood afteroral intake)

EdemaIncreased vascular

permeabilityLaryngeal edema

Circulatory collapseDeath

Acute urticaria(wheal-and-flare)

Animal hairInsect bites

Allergy testing

Through skinSystemic

Local increase inblood flow and

vascular permeabilityEdema

Seasonalrhinoconjunctivitis(hay fever)

Pollens (ragweed,trees, grasses)Dust-mite feces

Contact withconjunctiva of eye and

nasal mucosa

Edema of conjunctivaand nasal mucosa

Sneezing

Asthma Danders (cat)Pollens

Dust-mite feces

Inhalation leading tocontact with mucosallining of lower airways

Bronchial constrictionIncreased mucus

productionAirway inflammation

Food allergy PeanutsTree nutsShellfishFishMilkEggsSoyWheat

Oral VomitingDiarrhea

Pruritis (itching)Urticaria (hives)

Anaphylaxis (rarely)

Fig. 14.2 IgE-mediated reactions toextrinsic antigens. All IgE-mediatedresponses involve mast-celldegranulation, but the symptomsexperienced by the patient can be verydifferent depending, for example, onwhether the allergen is injected directlyinto the bloodstream, is eaten, or comesinto contact with the mucosa of therespiratory tract.


factors that contribute to this process.

13- 1 Sensitization involves class switching to lgE production on firstcontact with an allergen.

To produce an allergic reaction against a given antigen, an individual has firstto be exposed to the antigen and become sensitized to it by producing IgEantibodies. Atopic individuals often develop multiple types of allergic diseaseto multiple allergens—for example, atopic eczema developing in childhood inresponse to sensitization to food antigens is followed in a sizable proportionof these individuals by the development of allergic rhinitis and/or asthma

lgE and IgE-mediated allergic diseases 10

The enzyme Der p 1 cleaves

occludin in tight junctions and

Dendritic cell primes cellm lymph node

Plasma cell travels back tomucosa and produces Der p 1-

Der p 1-specitic lgE binds tomast cell; Der p 1 triggers

Airwaytic

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Der p 1 is taken up by dendriticcells for antigen presentation and

Th2 priming

Th2 cell induces B-cell switchto lgE production

lgE binds to FceRI receptoron mast cell

Mast-cell granule contentscause allergic symptoms



Fig. 14.3 Sensitization to an inhaledallergen. A common respiratory allergenis the protein Der p 1, found in fecalpellets of the house dust mite. On afirst encounter with Der p 1 in an atopicindividual, TH2 cells specific for Der p 1may be produced (first and secondpanels). Interaction of these T cells withDer p 1-specific B cells leads to theproduction of class-switched plasmacells producing Der p 1-specific lgE inthe mucosal tissues (third panel), andthis lgE becomes bound to Fc receptorson resident submucosal mast cells. On asubsequent encounter with Der p 1, theallergen binds to the mast-cell-boundlgE, triggering mast-cell activation andthe release of mast-cell granule contents,which cause the symptoms of the allergicreaction (last panel). Der p 1 is a proteasethat cleaves occludin, a protein thathelps to maintain the tight junctions; theenzymatic activity of Der p 1 is thought tohelp it to pass through the epithelium.caused by airborne allergens. Allergic reactions in non-atopic people, in con-trast, are predominantly due to sensitization to one specific allergen, such asbee venom or a drug such as penicillin, and can develop at any time of life.It is important to remember, however, that not all encounters with a poten-tial allergen will lead to sensitization, and not all sensitizations will lead to asymptomatic allergic response, even in atopic individuals.

The immune response leading to IgE production in response to antigen isdriven by two main groups of signals. The first consists of signals that favorthe differentiation of naive T cells to aTH2 phenotype. The second comprisesthe action of cytokines and co-stimulatory signals from Th2 cells thatstimulate B cells to switch to the production of IgE. The fate of a naive CD4 Tcell responding to a peptide presented by a dendritic cell is determined by thecytokines it is exposed to before and during this response, and by the intrinsicproperties of the antigen, the

antigen dose, and the route of presentation.Exposure to IL-4, IL-5, IL-9, and IL-13 favors the development of Th2 cells,whereas IFN-y and IL-12 (and its relatives IL-23 and IL-27) favor T^-celldevelopment (see Section 9-18).

Immune defenses against multicellular parasites are found mainly at thesites of parasite entry, namely under the skin and in the mucosal tissues ofthe airways and the gut; cells of the innate and adaptive immune systemsat these sites are specialized to secrete cytokines that promote a ^2-cellresponse to parasite infection. In the presence of infection, dendritic cellstaking up antigens in these tissues migrate to regional lymph nodes, wherethey tend to drive antigen-specific naive CD4 T cells to become effector Th2cells. Th2 cells themselves secrete IL-4, IL-5, IL-9, and IL-13, thus maintainingan environment in which further differentiation of Th2 cells is favored. Thecytokine IL-33, which can be produced by activated mast cells, also seemsto be important in amplifying the Th2 response. Allergic responses againstcommon environmental antigens are normally avoided by the propensity ofmucosal dendritic cells in the absence of infection to induce the productionof antigen-specific regulatoryT cells (Treg cells) from naive CD4 T cells. The Treg

cells suppress T-cell responses and produce a state of tolerance to the antigen(see Section 12-8).

The cytokines and chemokines produced by TH2 cells both amplify the TH2response and stimulate the class switching of B cells to IgE production. Aswe saw in Chapter 10, IL-4 or IL-13 provides the first signal that switchesB cells to IgE production. Cytokines IL-4 and IL-13 activate the Janus-family tyrosine kinases Jak1 and Jak3 (see Section 7-20), which ultimately


leads to phosphorylation of the transcriptional regulator STAT6 in T and Blymphocytes. Mice lacking functional IL-4, IL-13, or STAT6 have impaired TH2 CD40 Ligand Deficiency

responses and impaired IgE switching, demonstrating the key importance t

of these cytokines and their signaling pathways. The second signal is a «co-stimulatory interaction between CD40 ligand on the T-cell surface andCD40 on the B-cell surface. This interaction is essential for all antibody classswitching: patients with a genetic deficiency of CD40 ligand produce no IgG,lgA, or IgE, and display a hyper IgM syndrome phenotype (see Section 13-15).

Mast cells and basophils can also drive IgE production by B cells (Fig. 14.4).Mast cells and basophils express FceRI, and when they are activated by anti-gen cross-linking their FceRI-bound IgE, they express cell-surface CD40 lig-and and secrete IL-4. Like TH2 cells, therefore, they can drive class switchingand IgE production by B cells. The interaction between mast cells or basophilsand B cells can occur at the site of the allergic reaction, because B cells areobserved to form germinal centers at inflammatory foci. One goal of therapyfor allergies is to block this amplification process and thus prevent allergicreactions from becoming self-sustaining.

In humans, the IgE response, once initiated, can also be amplified by thecapture of IgE by Fee receptors on dendritic cells. Some populations ofhuman immature dendritic cells—for example, the Langerhans cells ofthe skin—express surface FceRI in an inflammatory setting, and onceanti-allergen IgE antibodies have been produced they can bind to thesereceptors. The bound IgE forms a highly effective trap for allergen, whichis then efficiently processed by the dendritic cell for presentation to naiveT cells, thus maintaining and reinforcing the TH2 response to the allergen.Eosinophils have also been reported to express IgE receptors, but this is stillcontroversial. However, eosinophils may act as antigen-presenting cells toT cells in a standard fashion after upregulation of eosinophil MHC class IImolecules and co-stimulatory molecules, although this probably occurs insites where activated T cells have migrated rather than in lymph nodes wherenaive T cells are primed by dendritic cells.



13- 2 Allergens are usually delivered transmucosally at low dose, a routethat favors lgE production.

Most airborne allergens are relatively small, highly soluble proteins that arecarried on dry particles such as pollen grains or mite feces (Fig. 14.5). On con-tact with the mucus-covered epithelia of the eyes, nose, or airways, the solu-ble allergen is eluted from the particle and diffuses into the mucosa, where itcan be picked up by dendritic cells and provoke sensitization (see Fig. 14.3).Allergens are typically presented to the immune system at low concentra-tions. It has been estimated that the maximum exposure of a person to thecommon pollen allergens in ragweed (Ambrosia species) does not exceed 1 j.lg

Fig. 14.4 Antigen binding to lgE onmast cells or basophils leads toamplification of lgE production. Leftpanel: lgE secreted by plasma cells bindsto the high-affinity lgE receptor on mastcells (illustrated here) and basophils.Right panel: when the surface-boundlgE is cross-linked by antigen, thesecells express CD40 ligand (CD40L) andsecrete IL-4, which in turn binds to IL-4receptors (IL-4R) on the activated B cell,stimulating class switching by B cellsand the production of more lgE. Theseinteractions can occur /n v/vo at thesite of allergen-triggered inflammation,for example in bronchus-associatedlymphoid tissue.

lgE secreted by plasma cells binds to ahigh-affinity Fc receptor FceRI on mast cells

Activated mast cells provide contact and secretedsignals to B cells to stimulate lgE production

igE

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per year. Yet these minute doses of allergen can provoke irritating and evenlife-threateningTH2-driven IgE antibody responses in atopic individuals.

It seems likely that presenting an antigen across a mucosal epithelium andat very low doses is a particularly efficient way of inducing TH2-driven IgEresponses. In mice, IgE antibody production requires help from Th2 cells thatproduce interleukin-4 (IL-4) and IL-13, and it can be inhibited byT^ cellsthat produce interferon-y (IFN-y) (see Fig. 9.28). Low doses of antigen canfavor the activation of^2 cells over Th1 cells, and many common allergensare delivered in low doses to the respiratory mucosa. In the mucosa theseallergens encounter dendritic cells that take up and process protein antigensvery efficiently. In some circumstances, mast cells, basophils, and eosinophilscan also present allergen-derived antigen to activated T cells that have alreadybeen primed by dendritic cells, further promoting the responses of^2 cells.

Many parasitic worms invade their hosts by secreting proteolytic enzymesthat break down connective tissue and allow the parasite access to internaltissues, and it has been proposed that these enzymes are particularlyactive at promoting Th2 responses. One ubiquitous protease allergen isthe cysteine protease Der p 1 present in the feces of the house dust mite{Dermatophagoid.es pteronyssimus), which provokes allergic reactions inabout 20% of the North American population. This enzyme has been foundto cleave occludin, a protein component of intercellular tight junctions.This reveals one possible reason for the allergenicity of certain enzymes. Bydestroying the integrity of the tight junctions between epithelial cells, Der p 1may gain abnormal access to subepithelial antigen-presenting cells (see Fig.13.3) . The tendency of proteases to induce IgE production is highlighted byindividuals with Netherton’s syndrome (Fig. 14.6), which is characterized byhigh levels oflgE and multiple allergies. The defect in this disease is the lack ofa protease inhibitor called SPINKS, which is thought to inhibit the proteasesreleased by bacteria such as Staphylococcus aureus, thus raising the possibilitythat protease inhibitors might be novel therapeutic targets in some allergicdisorders. The cysteine protease papain, derived from the papaya fruit, is usedas a meat tenderizer and causes allergic reactions in workers preparing theenzyme; such allergies are called occupational allergies. Not all allergens areenzymes, however; for example, two allergens identified from filarial wormsare enzyme inhibitors, and, in general, allergenic pollen-derived proteins donot seem to possess enzymatic activity. Thus, there seems to be no systematicassociation between enzymatic activity and allergenicity.

Knowledge of the identity of allergenic proteins can be important to publichealth and can have economic significance, as illustrated by the followingcautionary tale. Some years ago, the gene for a protein from brazil nuts that isrich in methionine and cysteine was transferred by genetic engineering intosoy beans intended for animal feed. This was done to improve the nutritionalvalue of soy beans, which are intrinsically poor in these sulfur-containingamino acids. This experiment led to the discovery that the protein, 2S albu-min, was the major brazil nut allergen. Injection of extracts of the genetically

Features of airborne allergens that maypromote the priming ofTtt2 cells

that drive lgE responses

Protein, oftenwith carbohydrateside chains

Only proteins induceT-cell responses

Low dose Favors activation of I L-4-producing CD4 T cells

Low molecularweight

Allergen can diffuse out ofparticles into the mucosa

Highly soluble Allergen can be readilyeluted from particle

Stable Allergen can survive indesiccated particle

Contains peptidesthat bind hostMHC class II

Required forT-cell priming

Fig. 14.5 Properties of inhaledallergens. The typical characteristicsof inhaled allergens are described inthis table.


modified soy beans into the epidermis triggered an allergic skin response in


udermis

Dermisfit..


Fig. 14.6 Netherton’s syndrome illustrates the association of proteases with thedevelopment of high levels of lgE and allergy. This 26-year-old man with Netherton’ssyndrome, caused by a deficiency in the protease inhibitor SPINK5, had persistenterythroderma, recurrent infections of the skin and elsewhere, and multiple food allergiesassociated with high serum lgE levels. In the top photograph, large erythematousplaques covered with scales and erosions are visible over the upper trunk. The lowerpanel shows a section through the skin of the same patient. Note the psoriasis-likehyperplasia of the epidermis. Neutrophils are also present in the epidermis. In the dermis,a perivascular infiltrate containing both mononuclear cells and neutrophils is evident.Source: Sprecher, E., eta/.: Clin. Exp. Dermatol. 2004, 29:513-517.


people allergic to brazil nuts. As there could be no guarantee that the modi-fied soy beans could be kept out of the human food chain if they were pro-duced on a large scale, development of this genetically modified food wasabandoned.



13- 3 Genetic factors contribute to the development of lgE-mediatedallergic disease.

The risk of developing allergic disease has both genetic and environmentalcomponents. In studies performed in Western industrialized countries, upto 40% of the test population shows an exaggerated tendency to mount IgEresponses to a wide variety of common environmental allergens. This is thestate called atopy. It has a strong familial basis and is known to be influencedby multiple genetic loci. Atopic individuals have higher total levels of IgEin the circulation and higher levels of eosinophils than their non-atopiccounterparts and are more susceptible to developing allergic diseases such asallergic rhinoconjunctivitis, allergic asthma, or atopic eczema.

Genome-wide linkage scans have uncovered several distinct susceptibilitygenes for the allergic skin condition atopic eczema (also known as atopicdermatitis) and for allergic asthma, although there is little overlap between thetwo sets of genes, suggesting that the genetic predisposition differs somewhat(Fig. 14.7). In addition, there are many ethnic differences in the susceptibilitygenes for a given allergic disease. Several of the chromosome regionsassociated with allergy or asthma are also associated with the

inflammatorydisease psoriasis and with autoimmune diseases, suggesting the presence ofgenes that are involved in exacerbating inflammation (see Fig. 14.7).

One candidate susceptibility gene for both allergic asthma and atopiceczema, at chromosome llq12-13, encodes the p subunit of the high-affinityIgE receptor FceRI. Another region of the genome associated with allergicdisease, 5q31-33, contains at least four types of candidate gene that mightFig. 14.7 Susceptibility loci identifiedby genome screens for asthma,atopic dermatitis, and other immunedisorders. Only loci with significantlinkages are indicated. Clustering ofdisease-susceptibility genes is found forthe MHC on chromosome 6p21, and alsoin several other genomic regions. There isin fact little overlap between susceptibilitygenes for asthma and atopic dermatitis,suggesting that specific genetic factorsare involved in both. There is also someoverlap between susceptibility genesfor asthma and those for autoimmunediseases, and between those for theinflammatory skin disease psoriasis andatopic dermatitis. Adapted from Cookson,W.: Naf. Rev. lmmunol. 2004, 4:978-988.




be responsible for increased susceptibility. First, there is a cluster of tightlylinked genes for cytokines that enhance IgE class switching, eosinophil sur-vival, and mast-cell proliferation, all of which help to produce and maintainan IgE-mediated allergic response. This cluster includes the genes for IL-3,IL-4, IL-5, IL-9, IL-13, and granulocyte-macrophage colony-stimulating fac-tor (GM-CSF). In particular, genetic variation in the promoter region of thegene encoding IL-4 has been associated with raised IgE levels in atopic indi-viduals. The variant promoter directs increased expression of a reporter genein experimental systems and thus might produce increased IL-4 in vivo. Atopyhas also been associated with a gain-of-function mutation of the a subunit ofthe IL-4 receptor that causes increased signaling after ligation of the receptor.

A second set of genes in this region of chromosome 5 is the TIM family(for T cell, immunoglobulin domain, and mucin domain), which encodeT-cell-surface proteins. In mice, Tim-3 protein is specifically expressed onTH1 cells and negatively regulates TH1 responses, whereas Tim-2 (and to alesser extent Tim-1) is preferentially expressed in, and negatively regulates,TH2 cells. Mouse strains that carry different variants of the Tim genes differboth in their susceptibility to allergic inflammation of the airways and in theproduction of IL-4 and IL-13 by their T cells. Inherited variation in the TIMgenes in humans has been correlated with levels of airway hyperreactivityor hyperresponsiveness. In this condition, contact not only with allergen butalso with nonspecific irritants causes airway narrowing with wheezy breath-lessness similar to that seen in asthma. The third candidate susceptibilitygene in this part of the genome encodes p40, one of the two subunits ofIL-12. This cytokine promotes TH1 responses, and genetic variation in p40expression that could cause reduced production of IL-12 was found to beassociated with more severe asthma. A fourth candidate susceptibility gene,that encoding the P-adrenergic receptor, is also encoded in this region.Variation in this receptor might be associated with alteration in smooth-muscle responsiveness to endogenous and pharmacological ligands.


This complexity illustrates a common challenge in identifying the genetic basisof complex disease traits. Relatively small regions of the genome, identified ascontaining genes for altered disease susceptibility, may contain many goodcandidates, judging by their known physiological activities. Identifying thecorrect gene, or genes, may require studies of several very large populationsof patients and controls. For chromosome 5q31-33, for example, it is still tooearly to know how important each of the different polymorphisms is in thecomplex genetics of atopy.

A second type of inherited variation in IgE responses is linked to the HLA classII region (the human MHC class II region) and affects responses to specificallergens, rather than a general susceptibility to atopy. IgE production inresponse to particular allergens is associated with certain HLA class II alleles,implying that particular peptide:MHC combinations might favor a strongTH2 response; for example, IgE responses to several ragweed pollen allergensare associated with haplotypes containing the HLA class II allele DRB1*1501.Many people are therefore generally predisposed to make TH2 responses andare specifically predisposed to respond to some allergens more than others.However, allergic responses to drugs such as penicillin show no associationwith HLA class II or with the presence or absence of atopy.There are also likely to be genes that affect only particular aspects of allergicdisease. In asthma, for example, there is evidence that different genes affectat least three aspects of the disease—IgE production, the inflammatoryresponse, and clinical responses to particular treatments. Polymorphismof the gene on chromosome 20 encoding ADAM33, a metalloproteinaseexpressed by bronchial smooth muscle cells and lung fibroblasts, has beenassociated with asthma and bronchial hyperreactivity. This is likely to be anexample of genetic variation in the pulmonary inflammatory response and


Fig. 14.8 Candidate susceptibilitygenes for asthma.

in the pathological anatomical changes that occur in the airways (airwayremodeling).

Some of the many genes that have been associated with asthma are shown inFig. 14.8, where they are grouped into several categories of immune responsein which they participate.

14- 4 Environmental factors may interact with genetic susceptibility tocause allergic disease.

Studies of susceptibility suggest that environmental factors and genetic vari-ation each account for about 50% of the risk of developing a disease such asallergic asthma. The prevalence of atopic allergic diseases, and of asthma inparticular, is increasing in economically advanced regions of the world, andthis is likely to be due to changing environmental factors.

The main candidate environmental factors for the increase in allergy arechanges in exposure to infectious diseases in early childhood; the changefrom ‘traditional' rural societies that has meant less early exposure to animalmicroorganisms and microorganisms in the soil, for example; and changes inthe intestinal microbiota, which performs an important immunomodulatoryfunction (discussed in Chapter 12). Changes in exposure to ubiquitous micro-organisms as a possible cause of the increase in allergy has received muchattention since the idea was first mooted in 1989, and this is known as thehygiene hypothesis (Fig. 14.9). The original proposition was that less hygienicenvironments, specifically environments that predispose to infections earlyin childhood, help to protect against the development of atopy and allergicasthma. It was originally proposed that the protective effect might be due tomechanisms that skewed immune responses away from the production ofTh2 cells and their associated cytokines, which dispose toward IgE produc-tion, and toward the production ofTH1 cells, whose cytokines do not induceclass switching to IgE.


Asthma susceptibility genes

Genes triggering the immune responseor directing CD4 TH cell differentiation

Pattern recognition receptors: CD14, TLR2, TLR4,TLR6, TLR10, NODI, NOD2lmmunoregulatory cytokines: IL-10, TGFf31

Transcription factors: STAT3

Antigen presentation: HLA-DR, HLA-D0, HLA-DP alleles

Prostaglandin receptor: PDGER2

Genes regulating TH2 cell differentiationand effector function

GATA3, TBX21, IL-4, IL-13, IL4RA, FCER1B,IL-5, IL5RA, IL12B

Genes expressed in epithelial cells Chemokines: CCL5, CCL11, CCL24, CCL26Antimicrobial peptides: DEFB1

CC16

Epithelial cell barrier: SPINK5, FLG

Genes identified by positional cloning ADAM33, DPP10, PHF11, GPRA, HLA-G, IRAKM,COL29A1


Fig. 14.9 Genes, the environment,and atopic allergic diseases. Bothinherited and environmental factors areimportant determinants of the likelihoodof developing atopic allergic disease.Some genes known to influence thedevelopment of asthma are shown inFig. 14.8. The postulate of the ‘hygienehypothesis’ or ‘counter-regulationhypothesis’ is that exposure to someinfections and to common environmentalmicroorganisms in infancy and childhooddrives the immune system toward ageneral state of non-atopy. In contrast,children with genetic susceptibility toatopy and who live in an environmentwith low exposure to infectious diseaseand environmental microorganismsare thought not to develop efficientimmunoregulatory mechanisms and to bemost susceptible to the development ofatopic allergic disease.The biggest drawback to this interpretation, however, was the strong negativecorrelation between infection by helminths (such as hookworm and schisto-somes) and the development of allergic disease. A study in Venezuela showedthat children treated for a prolonged period with antihelminthic agents hada higher prevalence of atopy than did untreated and heavily parasitized chil-dren. As helminths provoke a strongTH2-mediated IgE response, this seemedto run counter to the hygiene hypothesis.

These observations led to a modification of the hypothesis known as thecounter-regulation hypothesis. This proposes that all types of

infectionmight protect against the development of atopy by driving the productionof cytokines such as IL-10 and transforming growth factor (TGF)-p, whichdownregulate both TH1 and TH2 responses (see Sections 9-18 and 9-19). Alarge proportion of allergic reactions are initiated by antigens that enterthough mucosal surfaces such as the respiratory or intestinal epithelium. Asdescribed in Chapter 12, the human mucosal immune system has evolvedmechanisms of regulating responses to commensal flora and environmentalantigens (such as food antigens) that involve the generation ofIL-10/TGF-p-producing regulatory T cells. The idea underlying the current version of thehygiene hypothesis is that decreased early exposure to common microbialpathogens and commensals in some way makes the body less efficient atproducing these regulatory T cells, thus increasing the risk of making anallergic response to a common environmental antigen.

In support of the counter-regulation hypothesis is evidence that exposure tocertain types of childhood infection, with the important exception of somerespiratory infections that we consider below, helps to protect against thedevelopment of allergic disease. Younger children from families with three ormore older siblings, and children aged less than 6 months who are exposed toother children in daycare facilities—situations linked to a greater exposure toinfections—are somewhat protected against atopy and asthma. Furthermore,early colonization of the gut by commensal bacteria such as lactobacilli andbifidobacteria, or infection by gut pathogens such as Toxoplasma gondii orHelicobacter pylori, is associated with a reduced prevalence of allergic disease.

A history of infection with measles or hepatitis A virus, or a positive tuber-culin skin test (suggesting previous exposure and an immune response toMycobacterium tuberculosis), also seems to have a negative association withatopy. The human counterpart of the murineTim-1 protein (see Section 14-3)is the cellular receptor for hepatitis A virus. The infection ofT cells by hepatitisA virus could thus directly influence their differentiation and cytokineproduction, limiting the development of an IgE-generating response.

In contrast to these negative associations between childhood infection and

Environment

Early exposure toubiquitousmicroorganismsHelminth infectionHepatitis A virusComposition of gutcommensalmicrobiota

Atopic

Non-atopic


the development of atopy and asthma is evidence that children who havehad attacks of bronchiolitis associated with respiratory syncytial virus (RSV)infection are more prone to developing asthma later on. Children hospital-ized with RSV infection have a skewed ratio of cytokine production away fromIFN-y toward IL-4, the cytokine that induces^2 responses. This effect of RSVmay depend on age at first infection. Infection of neonatal mice with RSVwas followed by a decreased IFN-y response compared with mice challengedat 4 or 8 weeks of age. '^hen the mice were rechallenged at 12 weeks of agewith RSV infection, animals that had been primarily infected as neonates hadmore severe lung inflammation than animals infected at 4 or 8 weeks of age.

Other environmental factors that might explain the increase in allergy arechanges in diet, allergen exposure, atmospheric pollution, and tobaccosmoke. Pollution has been blamed for an increase in the prevalence of non-allergic cardiopulmonary diseases such as chronic bronchitis, but an associa-tion with allergic disease has been less easy to demonstrate. There is, however,increasing evidence for an interaction between allergens and pollution,


particularly in genetically susceptible individuals. Diesel exhaust particlesare the best-studied pollutant in this context; they increase IgE production20-50-fold when combined with allergen, with an accompanying shift to TH2cytokine production. Reactive oxidant chemicals such as ozone are gener-ated as a result of such pollution, and individuals less able to deal with thisonslaught may be at increased risk of allergic disease.

Genes that might be governing this aspect of susceptibility are GSTP1 andGSTMl, members of the glutathione-S-transferase superfamily that areimportant in preventing oxidant stress. Individuals who were allergic to rag-weed pollen and who carried particular variant alleles of these genes showedan increased airway hyperreactivity when challenged with the allergen plusdiesel exhaust particles, compared with the allergen alone. A study in MexicoCity on the effects of atmospheric ozone levels on atopic children with aller-gic asthma also found that the children carrying the null allele of GSTMlwere more susceptible to airway hyperreactivity than were noncarriers whenexposed to given levels of ozone. Indeed, genetic factors such as these, and thecomplexity of genetic and environmental interactions, may explain why theepidemiological evidence for an association between pollution and allergyremains moderate at best.

14- 5 RegulatoryT cells can control allergic responses.

Peripheral blood mononuclear cells (PBMCs) from atopic individuals havea tendency to secrete Th2 cytokines after nonspecific stimulation via theT-cell receptor, whereas those from non-atopic individuals do not. This hasled to the suggestion that regulatory mechanisms have an important role inpreventing IgE responses to allergens. Regulatory T cells, in particular, arereceiving considerable attention with regard to all types of immunologicallymediated disease. The different types of regulatory T cells (see Section 9-19)may all have a role in modulating allergy. Circulating CD4 CD25 Treg

cellsfrom atopic individuals are defective in suppressingTH2 cytokine productioncompared with those from non-atopic individuals, and this defect is evenmore pronounced during the pollen season. More evidence comes from micedeficient in the transcription factor FoxP3, the master switch for


producingboth natural (thymus-derived) and some induced Treg cells. These micedevelop manifestations of allergic disease including eosinophilia, hyper IgE,and allergic airway inflammation, suggesting that these symptoms resultfrom the absence of regulatoryT cells. This syndrome could be partly reversedby a concomitant deficiency in STAT6, which independently prevents thedevelopment of a Th2 response (see Section 14-1).

Regulatory T cells can also be induced by the actions of the anti-inflammatoryenzyme indoleamine 2,3-dioxygenase (IDO), which is expressed in a varietyof cell types in response to stimulation by certain cytokines, such as IFN-y,or by unmethylated CpG DNA acting via the receptor TLR-9. IDO activity inresident dendritic cells in the lung stimulated in this way has been shown toameliorate experimental asthma in mice. IDO breaks down tryptophan tometabolites called kynurenines, which are thought to be the active agents inthe various effects reported for IDO on immune system functions.

Summary.

Type I allergic reactions are the result of the production of specific IgE anti-body against common, innocuous antigens. Allergens are small antigens thatcommonly provoke an IgE antibody response. Such antigens normally enterthe body at very low doses by diffusion across mucosal surfaces and thus trig-ger a TH2 response. The differentiation of naive allergen-specific T cells intoTH2 cells is also favored by cytokines such as IL-4 and IL-13. Allergen-specific


Th2 cells producing IL-4 and IL-13 drive allergen-specific B cells to produceIgE. The specific IgE produced in response to the allergen binds to the high-affinity receptor for IgE on mast cells and basophils. IgE production can beamplifiedby these cells because, upon activation, they produce IL-4 and CD40ligand. The tendency to IgE overproduction is influenced by both genetic andenvironmental factors. Once IgE has been produced in response to an allergen,reexposure to the allergen triggers an allergic response. Immunoregulationis critical in the control of allergic disease through a variety of mechanisms,including regulatory T cells. We describe the mechanism and pathology of theallergic responses themselves in the next part of the chapter.

Effector mechanisms in lgE-mediated

allergic

reactions.

Allergic reactions are triggered when allergens cross-link preformed IgEbound to the high-affinity receptor FCERI on mast cells. Mast cells line exter-nal mucosal surfaces and serve to alert the immune system to local infection.Once activated, they induce inflammatory reactions by secreting pharma-cological mediators such as histamine stored in preformed granules and bysynthesizing prostaglandins, leukotrienes, and platelet-activating factor fromthe plasma membrane. They also release various cytokines and chemokinesafter activation. In the case of an allergic reaction, they provoke unpleasantreactions to innocuous antigens that are not associated with invading patho-gens that need to be expelled. The consequences of IgE-mediated mast-cellactivation depend on the dose of antigen and its route of entry; symptomsrange from the swollen eyes and rhinitis associated with contact of pollenwith the conjunctiva of the eye and the nasal epithelium, to the life-threaten-ing circulatory collapse that occurs in anaphylaxis (Fig. 14.10). The immediatereaction caused by mast-cell degranulation is followed, to a greater or lesser

Fig. 14.10 Mast-cell activation has


different effects on different tissues.

Gastrointestinal tract

Increased fluid secretion,increased peristalsis

Mast-cell activationand granule release

Eyes, nasal passages,

and airwaysDecreased diameter,increased mucus secretion

Blood vessels

Increased blood flow,increased permeability


Expulsion of gastrointestinaltract contents

(diarrhea, vomiting)


Congestion and blockage of

airways (wheezing, coughing,phlegm)

Swelling and mucus secretionin nasal passagesIncreased fluid in tissuescausing increased flow oflymph to lymph nodes,increased cells and proteinin tissues, increased effectorresponse in tissuesHypotension potentiallyleading to anaphylactic shock

Effector mechanisms in IgE-mediated allergic reactions

extent depending on the disease, by a more sustained inflammation, which isdue to the recruitment of other effector leukocytes, notablyTH2 lymphocytes,eosinophils, and basophils.

14- 6 Most lgE is cell-bound and engages effector mechanisms ofthe immune system by different pathways from those of otherantibody isotypes.

Antibodies engage effector cells such as mast cells by binding to receptors spe-cific for the Fc constant regions. Most antibodies engage Fc receptors only afterthe antibody variable region has bound specific antigen, forming an immunecomplex of antigen and antibody. IgE is an exception, because it is capturedby the high-affinity FC£ receptor (FceRI) in the absence of bound antigen. Thismeans that, unlike other antibodies, which are found mainly in body fluids,IgE is mostly found fixed on cells that carry this receptor—mast cells in tissuesand basophils in the circulation. The ligation of the cell-bound IgE antibodyby specific antigen triggers the activation of these cells at the sites of antigenentry into the tissues. The release of inflammatory lipid mediators, cytokines,and chemokines at sites of IgE-triggered reactions recruits eosinophils andbasophils to augment the type I hypersensitivity response. It also recruits TH2cells, which can then mount a local TH2 type IV hypersensitivity response.

There are two types of IgE-binding Fc receptor. The first, Fc£RI, is a high-affinity receptor of the immunoglobulin superfamily that binds IgE on mastcells and basophils (see Section 10-24). '^hen the cell-bound IgE is cross-linked by specific antigen, Fc£RI transduces an activating signal. High levelsof IgE, such as those that exist in people with allergic diseases or parasiteinfections, can result in a marked increase in FceRI on the surface of mastcells, an enhanced sensitivity of such cells to activation by low concentrationsof specific antigen, and a markedly increased IgE-dependent release ofchemical mediators and cytokines.

The second IgE receptor, FceRII, usually known as CD23, is a C-type lectinand is structurally unrelated to FceRI; it binds IgE with low affinity. CD23 is


present on many cell types, including B cells, activated T cells, monocytes,eosinophils, platelets, follicular dendritic cells, and some thymic epithelialcells. This receptor was thought to be crucial for the regulation of IgE levels,but mouse strains in which the gene for CD23 has been inactivated still dev-elop relatively normal polyclonal IgE responses. Nevertheless, CD23 doesseem to be involved in enhancing IgE antibody levels in some situations.Responses against a specific antigen are known to be increased in thepresence of the antigen complexed with IgE, but such enhancement failsto occur in mice that lack the gene for CD23. This has been interpreted toindicate that CD23 on antigen-presenting cells has a role in the capture ofantigen complexed with IgG.

14- 7 Mast cells reside in tissues and orchestrate allergic reactions.

Mast cells were described by Ehrlich in the mesentery of rabbits and namedMastzellen (‘fattened cells'). Like basophils, mast cells contain granules richin acidic proteoglycans that take up basic dyes. Mast cells are derived fromhematopoietic stem cells but mature locally, often residing near surfacesexposed to pathogens and allergens, such as mucosal tissues and theconnective tissues surrounding blood vessels. Mucosal mast cells differ insome of their properties from submucosal or connective tissue mast cells, butboth can be involved in allergic reactions.

The major factors for mast-cell growth and development include stem-cellfactor (the ligand for the receptor tyrosine kinase Kit), IL-3, andTH2-associated


cytokines such as IL-4 and IL-9. Mice with defective Kit lack differentiatedmast cells, and although they produce IgE they cannot make IgE-mediatedinflammatory responses . This shows that such responses depend almostexclusively on mast cells . Mast-cell activation depends on the activation ofphosphatidylinositol 3-kinase (PI 3-kinase) in mast cells by Kit, and pharma-cological inactivation of the pllOS isoform of PI 3-kinase has been shown toprotect mice against allergic responses .

Mast cells express FCERI constitutively on their surface and are activatedwhen antigens cross-link IgE bound to these receptors (see Fig. 10.37). Arelatively low level of allergen is sufficient to trigger degranulation. Thereare many mast-cell precursors in tissues, which can rapidly differentiate tomature mast cells in conditions of allergic inflammation, thus aiding thecontinuation of the allergic response. Mast-cell degranulation begins withinseconds of antigen binding, releasing an array of preformed and newlygenerated inflammatory mediators (Fig. 14.11). Granule contents include theshort-lived vasoactive amine hist^ine, serine esterases, and proteases suchas chymase and tryptase .

Human mast cells are classified on the basis of their protease content. Mastcells of one class (MCT) predominantly express tryptase only, and thesepredominate in mucosal epithelia, whereas those of another type (MCCT)express tryptase, chymase, carboxypeptidase A, and cathepsin G and pre-dominate in the submucosa and other connective tissues. Histamine actsvia Hj receptors on local blood vessels to cause an immediate increase inlocal blood flow and vessel permeability. It also has immunomodulatoryand inflammatory activity. Acting through the Hj receptor on dendritic cells,histamine can increase antigen-presenting capacity and TH1 priming; actingthrough Hj on T cells, it can enhance TH1 proliferation and IFN-y production .



Fig. 14.11 Molecules released by mastcells on activation. Mast cells producea wide variety of biologically activeproteins and other chemical mediators.The enzymes and toxic mediators listedin the first two rows are released fromthe preformed granules. The cytokines,chemokines, and lipid mediators aresynthesized after activation.

Class of product

Examples Biological effects

Enzyme Tryptase, chymase,cathepsin G,carboxypeptidase

Remodel connective tissue matrix

Toxic mediator Histamine, heparin Toxic to parasitesIncrease vascular permeabilityCause smooth muscle contractionAnticoagulation

Cytokine IL-4, IL-13 Stimulate and amplify TH2-cell response

IL-3, IL-5, GM-CSF Promote eosinophil production and activation

TNF-a (some storedpreformed in granules)

Promotes inflammation, stimulates cytokineproduction by many cell types,activates endothelium

Chemokine CCL3 Attracts monocytes, macrophages,and neutrophils

Lipid mediator Prostaglandins D2, E2Leukotrienes C4, D4, E4

Smooth muscle contractionChemotaxis of eosinophils, basophils, and TH2 cellsIncrease vascular permeabilityStimulate mucus secretionBronchoconstriction

Platelet-activating factor Attracts leukocytesAmplifies production of lipid mediatorsActivates neutrophils, eosinophils, and platelets


The proteases released by the mast cells activate matrix metalloproteinases,which break down extracellular matrix proteins, causing tissue disintegrationand damage. Large amounts of the cytokine tumor necrosis factor (TNF)-aare also released by mast cells after activation. Some comes from stores inthe granules; some is newly synthesized by the activated mast cells. TNF-aactivates endothelial cells, resulting in increased expression of adhesionmolecules, which in turn promotes the influx of pro-inflammatory leukocytesand lymphocytes into the affected tissue (see Chapter 3).

On activation, mast cells also synthesize and release chemokines, cytokines,and lipid mediators—prostaglandins, leukotrienes, thromboxanes (collect- >? Aliergic Asthma

ively called eicosanoids), and platelet-activating factor. Mucosal andsubmucosal mast cells, for example, produce the cytokine IL-4, which helps Aperpetuate theTH2 response. These secreted products contribute to both acuteand chronic inflammatory responses. The lipid mediators, in particular, actrapidly to cause smooth muscle contraction, increased vascular permeability,and the secretion of mucus, and also induce the influx and activation ofleukocytes, which contribute to the allergic inflammation.

Eicosanoids derive mainly from the fatty acid arachidonic acid. This iscleaved from membrane phospholipids by phospholipase A2, which is act-ivated at the plasma membrane as a result of cell activation. Arachidonicacid can be modified by either of two pathways to give rise to lipid mediators.Modification via the cyclooxygenase pathway produces the prostaglandinsand thromboxanes, whereas leukotrienes are produced via the lipoxygenasepathway. Prostaglandin D2 is the major prostaglandin produced by mast cellsand recruits Th2 cells, eosinophils, and basophils, all of which express itsreceptor (PTGDR). Prostaglandin D2 is critical to the development of allergicdiseases such as asthma, and polymorphisms in the PTGDR gene have beenlinked to an increased risk of developing asthma. The leukotrienes, especiallyC4, D4, and E4, are also important in sustaining inflammatory responses intissues. Nonsteroidal anti-inflammatory drugs such as aspirin and ibuprofenexert their effects by preventing prostaglandin production. They inhibitthe cyclooxygenases that act on arachidonic acid to form the ring structurepresent in prostaglandins.

IgE-mediated activation of mast cells thus orchestrates an important inflam-matory cascade that is amplified by the recruitment of several cell typesincluding eosinophils, basophils, TH2 lymphocytes, and B cells. The physio-


logical importance of this reaction is as a defense against parasite infection(see Section 10-25). In an allergic reaction, however, the acute and chronicinflammatory reactions triggered by mast-cell activation have importantpathophysiological consequences, as seen in the diseases associated withallergic responses to environmental antigens. The role of mast cells is not lim-ited to IgE-driven pro-inflammatory responses, however. Increasingly, mastcells are also considered to have a role in immunoregulation. They can secretethe immunosuppressive cytokine IL-10, and interaction with regulatory Tcells may prevent degranulation.

14- 8 Eosinophils and basophils cause inflammation and tissue damagein allergic reactions.

Eosinophils are granulocytic leukocytes that originate in bone marrow. Theyare so called because their granules, which contain arginine-rich basic pro-teins, are colored bright orange by the acidic stain eosin. Only very smallnumbers of these cells are normally present in the circulation; most eosi-nophils are found in tissues, especially in the connective tissue immediatelyunderneath respiratory, gut, and urogenital epithelium, implying a likely rolefor these cells in defense against invading organisms. They possess numerous

Class of product

Examples Biological effects

Enzyme Eosinophil peroxidase Toxic to targets by catalyzing halogenationTriggers histamine release from mast cells

Eosinophil collagenase Remodels connective tissue matrix

Matrix metalloproteinase-9 Matrix protein degradation

Toxic protein Major basic protein Toxic to parasites and mammalian cellsTriggers histamine release from mast cells

Eosinophil cationic protein Toxic to parasitesNeurotoxin

Eosinophil-derived neurotoxin Neurotoxin

Cytokine IL-3, IL-5, GM-CSF Amplify eosinophil production by bone marrowEosinophil activation

TGF-a, TGF-13 Epithelial proliferation,myofibroblast formation

Chemokine CXCL8 (IL-8) Promotes influx of leukocytes

Lipid mediator Leukotrienes C4, D4, E4 Smooth muscle contractionIncrease vascular permeabilityIncrease mucus secretionBronchoconstriction

Platelet-activating factor Attracts leukocytesAmplifies production of lipid mediatorsActivates neutrophils, eosinophils, and platelets

Fig. 14.12 Eosinophils secrete a rangeof highly toxic granule proteins andother inflammatory mediators.


cell-surface receptors, including receptors for cytokines (such as IL-5), Feyand Fca receptors, and the complement receptor C3, through which they canbe activated and stimulated to degranulate. For example, parasites coatedwith IgG, C3b, or IgA can cause eosinophil degranulation. In allergic tissuereactions, the large concentrations ofIL-5/IL-3 and GM-CSF that are typicallypresent are likely inducers of degranulation.

Eosinophils have two kinds of effector function. First, on activation theyrelease highly toxic granule proteins and free radicals, which can kill micro-organisms and parasites but also cause significant tissue damage in allergicreactions (Fig. 14.12). Second, activation induces the synthesis of chemicalmediators such as prostaglandins, leukotrienes, and cytokines. These amplifythe inflammatory response by activating epithelial cells and by recruiting andactivating more eosinophils and leukocytes. Eosinophils also secrete proteinsinvolved in airway tissue remodeling.

What were later to be defined as eosinophils were observed in the 19th cent-ury in the first pathological description of fatal status asthmaticus, but theprecise role of these cells in allergic disease generally is still unclear. In aller-gic tissue reactions, for example, those that lead to chronic asthma, mast-celldegranulation and Th2 activation cause eosinophils to accumulate in largenumbers and to become activated. Among other things, eosinophils secrete


TH2-type cytokines and in vitro can promote the apoptosis of ThI cells bytheir expression of IDO and consequent production of kynurenine, whichacts on the ThI cells. Their apparent promotion of TH2-cell expansion maythus be partly due to a relative reduction in T^-cell numbers. The continued


presence of eosinophils is characteristic of chronic allergic inflammation, andeosinophils are thought to be major contributors to tissue damage.

The activation and degranulation of eosinophils is strictly regulated, becausetheir inappropriate activation would be harmful to the host. The first levelof control acts on the production of eosinophils by the bone marrow. Feweosinophils are produced in the absence of infection or other immunestimulation. But when TH2 cells are activated, cytokines such as IL-5 arereleased that increase the production of eosinophils in the bone marrow andtheir release into the circulation. However, transgenic animals overexpressingIL-5 have increased numbers of eosinophils (eosinophilia) in the circulationbut not in their tissues, indicating that the migration of eosinophils from thecirculation into tissues is regulated separately, by a second set of controls.The key molecules in this case are CC chemokines. Most of these causechemotaxis of several types of leukocyte, but three are particularly importantin attracting and activating eosinophils, and have been named the eotaxins:CCLII (eotaxin 1), CCL24 (eotaxin 2), and CCL26 (eotaxin 3).

The eotaxin receptor on eosinophils, CCR3, is quite promiscuous and bindsother CC chemokines, including CCL7, CCL13, and CCL5, which also induceeosinophil chemotaxis and activation. Identical or similar chemokinesstimulate mast cells and basophils. For example, eotaxins attract basophilsand cause their degranulation, and CCL2, which binds to CCR2, similarlyactivates mast cells in both the presence and the absence of antigen. CCL2can also promote the differentiation of naive T cells to^2 cells; TH2 cells alsocarry the receptor CCR3 and migrate toward eotaxins. It is striking that theseinteractions between different chemokines and their receptors show a highdegree of overlap and redundancy; we do not understand the significance ofthis complexity. However, these findings show that families of chemokines, aswell as cytokines, can coordinate certain kinds of immune response.

Basophils are also present at the site of an inflammatory reaction, andgrowth factors for basophils are very similar to those for eosinophils; theyinclude IL-3, IL-5, and GM-CSF. There is evidence for reciprocal


control ofthe maturation of the stem-cell population into basophils or eosinophils.For example, TGF-P in the presence of IL-3 suppresses eosinophil different-iation and enhances that of basophils. Basophils are normally present invery low numbers in the circulation and seem to have a similar role to that ofeosinophils in defense against pathogens. Like eosinophils, they are recruitedto the sites oflgE-mediated allergic reactions. Basophils express high-affinityFceRI on the cell surface and so have IgE bound. On activation by antigenbinding to IgE or by cytokines, they release histamine from the basophilicgranules after which they are named; they also produce IL-4 and IL-13.

Eosinophils, mast cells, and basophils can interact with each other. Eosinophildegranulation releases major basic protein (see Fig. 14.12), which in turncauses the degranulation of mast cells and basophils. This effect is augmentedby any of the cytokines that affect eosinophil and basophil growth, different-iation, and activation, such as IL-3, IL-5, and GM-CSF.

14- 9 IgE-mediated allergic reactions have a rapid onset but can also leadto chronic responses.

Under laboratory conditions, the clinical response of a sensitized individualto challenge by intradermal allergen or inhalation of allergen can be dividedinto an ‘immediate reaction' and a ‘late-phase reaction' (Fig. 14.13). Theimmediate reaction is due to IgE-mediated mast-cell activation and startswithin seconds of allergen exposure. It is the result of the actions of hista-mine, prostaglandins, and other preformed or rapidly synthesized mediatorsreleased by mast cells. These cause a rapid increase in vascular permeability,


12 Time

Immediate

Late phase

=2-jaAntigenchallenge

10

30minute

s

60 6hours

PEFR

(liters mirr1) 400

200 -

100 -

300

Effector mechanisms in IgE-mediated allergic reactionsFig. 14.13 Allergic reactions in responseto test antigens can be divided into animmediate response and a late-phaseresponse. Left panel: the response toan inhaled antigen can be divided intoearly and late responses. An asthmaticresponse in the lungs with narrowing ofthe airways caused by the constrictionof bronchial smooth muscle anddevelopment of edema can be measuredas a fall in the peak expiratory flow rate(PEFR). The immediate response peakswithin minutes after antigen inhalation andthen subsides. Six to eight hours afterantigen challenge, there is a late-phaseresponse that also results in a fall in thePEFR. The immediate response is causedby the direct effects on blood vessels andsmooth muscle of rapidly metabolizedmediators such as histamine and lipidmediators released by mast cells. Thelate-phase response is caused by thecontinued production of these mediatorsand by the production of vasoactivecompounds that dilate blood vessels andproduce edema. Right panel: a wheal-and-flare allergic reaction develops withina minute or two of intradermal injectionof antigen and lasts for up to 30 minutes.The more widespread edematousresponse characteristic of the latephase develops approximately 6 hourslater and can persist for some hours.The photograph shows an intradermalskin challenge with allergen showing a15-minute wheal-and-flare (early-phase)reaction (left) and a 6-hour late-phasereaction (right). The allergen was grasspollen extract. Photograph courtesy ofS.R. Durham.resulting in visible edema and reddening of the skin (in a skin response) andairway narrowing as result of edema and the constriction of smooth muscle(in an airway response). In the skin, histamine acting on H1 receptors on localblood vessels causes an immediate increase in vascular permeability, whichleads to extravasation of fluid and edema. Histamine also acts on H1 receptorson local nerve endings, leading to vasodilation of cutaneous blood vesselsand local reddening of the skin. The resulting skin lesion is called a wheal-and-flare reaction.

^Whether a late-phase reaction occurs is dependent on allergen dose; at

doses that are deemed safe in tests of asthmatic reactions, for example, a latereaction occurs in about 50% of individuals who show an immediate response(see Fig. 14.13, left panel). The late reaction peaks between 3 and 9 hours afterantigen challenge and in skin tests becomes obvious as a much increasedarea and degree of edema (see Fig. 14.13, right panel). The late-phase reactionis caused by the continued synthesis and release of inflammatory mediatorsby mast cells, especially vasoactive mediators such as calcitonin gene relatedpeptide (CGRP) and vascular endothelial growth factor (VEGF), which causevasodilation and vascular leakage resulting in edema. In experimentalresponses to inhaled allergens, the late-phase reactions are associated with asecond phase of airway narrowing with sustained edema.

Allergists take advantage of the immediate response as an aid to the assess-ment and confirmation of sensitization, in the light of a clinical history ofallergic disease, and to determine which allergens are responsible. Minuteamounts of potential allergens are introduced into the skin by skin prick—one site for each allergen—and if the individual is sensitive to any of theallergens tested, a wheal-and-flare reaction will occur at the site within a fewminutes (see Fig. 14.13). Although the reaction after the administration ofsuch small amounts of allergen is usually very localized, there is a still a smallrisk of inducing anaphylaxis. Another standard test for allergy is to measurecirculating concentrations oflgE antibody specific for a particular allergen ina sandwich ELISA (see Appendix I, Section A-6).

The late-phase reaction described above occurs under controlled experi-mental conditions to a single relatively high dose of allergen and so does notreflect all the effects of long-term natural exposure. In IgE-mediated allergicdiseases, a long-term sequel of allergen exposure is chronic allergic inflamm-ation, which is in essence a TH2 type IV hypersensitivity reaction (see Fig.14.1) . These chronic reactions contribute to much serious long-term illness,such as chronic asthma. Inflammatory mediators released by mast cells and


basophils recruit other leukocytes, mainly TH2 cells and eosinophils, to thesite of inflammation. In chronic asthma, for example, the cytokines releasedby Th2 cells and the effector molecules released by eosinophils (see Fig. 14.12)result in persistent edema, which narrows the airways, and in airway tissueremodeling, a change in the bronchial tissue due to smooth muscle hyper-trophy (an increase in size due to cell growth) and hyperplasia (an increasein the number of cells). The chronic phase of asthma is characterized by thepresence of both TH1 cytokines (such as IFN-y) and TH2 cytokines, althoughthe latter seem to predominate.

In the natural situation, the clinical symptoms produced by an IgE-mediatedallergic reaction depend critically on several variables: the amount of aller-gen-specific IgE present, the route by which the allergen is introduced, thedose of allergen, and most probably some underlying defect in barrier func-tion in the particular tissue or organ affected. The outcomes produced bydifferent combinations of allergen dose and route of entry are summarizedin Fig. 14.14. When exposure to allergen in a sensitized individual triggers anallergic reaction, both the immediate and the chronic effects are focused onthe site at which mast-cell degranulation occurs.

14- 10 Allergen introduced into the bloodstream can cause anaphylaxis.

If allergen is introduced directly into the bloodstream, for example by a beeor wasp sting, or is rapidly absorbed into the bloodstream from the gut in Acute Systemic

a sensitized individual, connective-tissue mast cells associated with blood ; Anaphylaxis

vessels throughout the body can become immediately activated, resulting ina widespread release of histamine and other mediators that causes a reactioncalled anaphylaxis. The symptoms of anaphylaxis can range in severity froma mild urticaria (hives) to potentially fatal anaphylactic shock (see Fig.14.14, first and last panels). Acute urticaria is a response to ingested allergensentering the bloodstream and reaching the skin. Histamine released by mastcells activated by allergen in the skin causes large, itchy, red swellings all overthe body, a disseminated version of the wheal-and-flare reaction. Althoughacute urticaria is commonly caused by an IgE-mediated reaction against anallergen, the causes of chronic urticaria, in which the urticarial rash recursover long periods, are mostly unknown. It seems likely that from one-third

Effector mechanisms in IgE-mediated allergic reactionsto one-half of the cases of chronic urticaria are caused by autoantibodiesagainst either the a chain of FceRI or against IgE itself, and are thus due toautoimmunity. Interaction of the autoantibody with the receptor triggersmast-cell degranulation, with resulting urticaria. This is an example of a typeII hypersensitivity reaction (see Fig. 14.1).

In anaphylactic shock, a widespread increase in vascular permeability result-ing from a massive release of histamine leads to a catastrophic loss of bloodpressure, resulting in shock; airways constrict, causing difficulty in breath-ing; and swelling of the epiglottis can cause suffocation. The major causes ofanaphylaxis are wasp and bee stings, or allergic responses to foods in sensi-tized individuals—anaphylaxis in response to peanuts is relatively common.Severe anaphylactic shock can be rapidly fatal if untreated, but can usuallybe controlled by the immediate injection of epinephrine, which reverses theaction of histamine at the Hi receptor, thus relaxing the bronchial smoothmuscle, and inhibits the immediately life-threatening cardiovascular effects.Sensitization to, and anaphylactic reactions to, insect venom, drugs, and foodsuch as peanuts are not particularly associated with atopy, even when they areIgE-mediated. This may be because these antigens are delivered at high dosescompared with the very low doses of airborne antigens such as pollen.

A frequent IgE-mediated allergic reaction to drugs occurs to penicillin andits relatives. In people with IgE antibodies against penicillin, injection of thedrug can cause anaphylaxis and even death. Penicillin acts as a hapten (see


Appendix I, Section A-1); it is a small molecule with a highly reactive |3-lactamring that is crucial for its antibacterial activity. This ring reacts with aminogroups on host proteins to form covalent conjugates. When penicillin isingested or injected, it forms conjugates with self proteins, and the penicillin-modified self peptides provoke a TH2 response in some individuals. These TH2cells then activate penicillin-binding B cells to produce IgE antibody againstthe penicillin hapten. Thus, penicillin acts both as the B-cell antigen and,

Mast-cell activation



General release of histamine, whichacts on blood vessels to increasepermeability, leading to systemic

anaphylaxic shockLocal release of histamine

causes wheal-and-flare reaction.Airborne allergens penetrating

skin can also be a cause ofatopic eczema

Allergic rhinitis (upper airway),caused by increased mucus

production and nasal irritation.Asthma (lower airway) due tocontraction of bronchial smooth muscleand increased mucus secretion

^X/7 =

Contraction of intestinal smooth muscleinduces vomiting. Outflow of fluid intogut causes diarrhea. Antigen diffuses

into blood vessels and is widelydisseminated, causing urticaria (hives),

anaphylaxis, or atopic eczema

W® ® W* m ©/

Xs^

f°( )°]^0/

respiratory tractO I o II O' II O | O II o I o



Fig. 14.14 The dose and route of allergen administrationdetermine the type of lgE-mediated allergic reaction thatresults. There are two main anatomical distributions of mast cells:those associated with vascularized connective tissues, calledconnective tissue mast cells, and those found in submucosallayers of the gut and respiratory tract, called mucosal mast cells.In an allergic individual, all of these mast cells are loaded with lgEdirected against specific allergens. The response to an allergenthen depends on which mast cells are activated. Allergen inthe bloodstream (intravenous) activates connective tissue mastcells throughout the body, resulting in the systemic release ofhistamine and other mediators. Entry of allergen through the skinactivates local connective tissue mast cells, leading to a localinflammatory reaction. In an experimental skin prick with allergenor after some types of insect bite, this manifests as a wheal-and-flare reaction. In atopic individuals, airborne allergens penetratingthe skin may lead to atopic eczema. Inhaled allergen, penetratingrespiratory mucosal epithelia, activates mainly mucosal mastcells, causing increased secretion of mucus by the mucosalepithelium and irritation in the nasal mucosa, leading to allergicrhinitis, or to the additional constriction of smooth muscle in thelower airways, leading to asthma. Ingested allergen penetratesthe gut epithelium, causing vomiting due to intestinal smoothmuscle contraction and diarrhea due to outflow of fluid acrossthe gut epithelium. Food allergens can also be disseminated inthe bloodstream, causing a widespread urticaria (hives) when theallergen reaches the skin, or they may cause eczema.


by modifying self peptides, as the T-cell antigen. When penicillin is injectedintravenously into an allergic individual, the penicillin-modified proteins cancross-link IgE molecules on tissue mast cells and circulating basophils andthus cause anaphylaxis.

Another drug that is known to provoke anaphylaxis is the anesthetic hexa-methonium. Great care should be taken to avoid giving a drug to patients witha past history of allergy to that drug or a close structural relative.

14- 11 Allergen inhalation is associated with the development of rhinitisand asthma.



The respiratory tract is an important route of allergen entry (see Fig.14.14, third panels). Many atopic people react to airborne allergens withan IgE-mediated allergic reaction known as allergic rhinitis. This resultsfrom the activation of mucosal mast cells beneath the nasal epithelium byallergens such as pollens that release their soluble protein contents, whichthen diffuse across the mucous membranes of the nasal passages. Allergicrhinitis is characterized by intense itching and sneezing, local edemaleading to blocked nasal passages, a nasal discharge, which is typically richin eosinophils, and irritation of the nasal mucosa as a result of histaminerelease. A similar reaction to airborne allergens deposited on the conjunctivaof the eye is called allergic conjunctivitis. Allergic rhinitis and conjunctivitisare commonly caused by environmental allergens that are present onlyduring certain seasons of the year. For example, hay fever (known clinicallyas seasonal allergic rhinoconjunctivitis) is caused by a variety of allergens,including certain grass and tree pollens. Summer and autumnal symptomsmay be caused by weed pollen, such as that of ragweed, or the spores of fungisuch as Alternaria. Ubiquitous allergens such as cat dander and house dustmites can be a cause of year-round allergic rhinoconjunctivitis.

A more serious IgE-mediated respiratory disease is allergic asthma, whichis triggered by allergen-induced activation of submucosal mast cells in thelower airways (Fig. 14.15). This leads within seconds to bronchial constrictionand an increased secretion of fluid and mucus, making breathing more

difficult by trapping inhaled air in the lungs. Patients with allergic asthmausually need treatment, and asthmatic attacks can be life threatening. Thesame allergens that cause allergic rhinitis and conjunctivitis commonly causeasthma attacks. For example, respiratory arrest caused by severe attacks ofasthma in the summer or autumn has been associated with the inhalation ofAlternaria spores.

Allergic Asthma

Fig. 14.15 The acute response inallergic asthma leads to TH2-mediatedchronic inflammation of the airways.In sensitized individuals, cross-linkingof specific lgE on the surface of mastcells by an inhaled allergen triggersthem to secrete inflammatory mediators,causing increased vascular permeability,contraction of bronchial smooth muscle,and increased secretion of mucus. Thereis an influx of inflammatory cells, includingeosinophils and TH2 cells, from theblood. Activated mast cells and TH2 cellssecrete cytokines that augment eosinophilactivation and degranulation, whichcauses further tissue injury and the entryof more inflammatory cells. The result ischronic inflammation, which can causeirreversible damage to the airways.


Chronic response caused by cytokinesand eosinophil products

.. eosinophil granulecytokines pr

Hotein

Ms

e . . .


Chronic response


• V - * - * W * *

>■ .-. • • -- v. :*

/ • ’• * — ■ "• w —>*

"I

i? -

Fig. 14.16 Morphological evidence ofchronic inflammation in the airwaysof an asthmatic patient. Panel ashows a section through a bronchus ofa patient who died of asthma; there isalmost total occlusion of the airway bya mucus plug. In panel b, a close-upview of the bronchial wall shows injuryto the epithelium lining the bronchus,accompanied by a dense inflammatoryinfiltrate that includes eosinophils,neutrophils, and lymphocytes.Photographs courtesy ofT. Krausz.An important feature of asthma is chronic inflammation of the airways,which is characterized by the continued presence of increased numbers ofTh2 lymphocytes, eosinophils, neutrophils, and other leukocytes (Fig. 14.16).The concerted actions of these cells cause airway remodeling—a thickeningof the airway walls due to hyperplasia and hypertrophy of the smooth musclelayer, with the eventual development of fibrosis. This remodeling leads toa permanent narrowing of the airways, and is responsible for many of theclinical manifestations of chronic allergic asthma. In chronic asthmatics, ageneral hyperreactivity of the airways to nonimmunological

stimuli also oftendevelops. Bronchial epithelial cells can produce at least two of the chemo-kine ligands—CCL5 and CCL11—of the receptor CCR3 expressed on Th2 cells,macrophages, eosinophils, and basophils. These chemokines enhance theTh2 response by attracting more Th2 cells and eosinophils to the damagedlungs. The direct effects ofT^ cytokines and chemokines on airway smoothmuscle and fibroblasts lead to the apoptosis of epithelial cells and airwayremodeling, induced in part by the production ofTGF-p, which has numerouseffects on the epithelium, ranging from inducing apoptosis to stimulatingcell proliferation. The direct action ofT^ cytokines such as IL-9 and IL-13on airway epithelial cells may also have a dominant role in another majorfeature of chronic allergic asthma, the induction of goblet-cell metaplasia,which is the increased differentiation of epithelial cells as goblet cells, anda consequent increase in mucus secretion. CD1d-restricted invariant NKTcells (iNKTs, a type of innate-like lymphocyte; see Sections 3-24 and 8-9), alsoseem to have an important role in the development of airway hyperreactivity,whether allergen-induced or nonspecific. Animal models of asthma haveshown that airway hyperreactivity requires the presence of iNKT cells.

Mice do not naturally develop asthma, but a disease resembling humanasthma develops in mice that lack the transcription factor T-bet, whichis required for TH1 differentiation (see Section 9-18), and in which T-cellresponses are thought to be skewed to TH2. These mice have increased levelsof the TH2 cytokines IL-4, IL-5, and IL-13, and develop airway inflammationinvolving lymphocytes and eosinophils (Fig. 14.17). They also develop non-specific airway hyperreactivity to nonimmunological stimuli, similar to thatseen in human asthma. These changes occur in the absence of any exogenousinflammatory stimulus and show that, in extreme circumstances, a geneticimbalance toward TH2 responses can cause allergic disease. The involvementof eosinophils in asthma seems somewhat different in humans and in mice.In human asthma patients, the number of eosinophils is directly associatedwith the severity of asthma. In mice deficient in eosinophils, however, theonly consistent finding relevant to asthma pathophysiology is a

Effector mechanisms in IgE-mediated allergic reactionsreduction inairway remodeling without a reduction in airway hyperreactivity.

Although allergic asthma is initially driven by a response to a specific aller-gen, the subsequent chronic inflammation seems to be perpetuatedeven in the apparent absence of exposure to allergen. The airways becomecharacteristically hyperreactive, and factors other than antigen can triggerasthma attacks. Asthmatics characteristically show hyperresponsiveness toenvironmental chemical irritants such as cigarette smoke and sulfur dioxide.Viral or, to a smaller extent, bacterial respiratory tract infections can alsoexacerbate the disease by inducing aT^-dominated local response.

14- 12 A genetically determined defect in the skin’s barrier functionincreases the risk of atopic eczema.

The inflammatory skin rash known as eczema is common in the generalpopulation, especially on the hands, and may be due either to an allergicreaction or to a non-allergic cause (such as contact with irritant chemicals).Allergic eczema is likely to represent a constellation of clinically similar

T-bet+/+

Airway inflammation with lymphocytesand eosinophils

Normal lung biopsy


Fig. 14.17 Mice lacking thetranscription factor T-bet developasthma and T-cell responses polarizedtoward TH2. T-bet binds to the promoterof the gene encoding IL-2 and is presentin TH 1 but not TH2 cells. Mice with agene-targeted deletion of T-bet (T-bet+)developed a spontaneous asthma-likephenotype in the lungs. Left-hand panels:lung and airways in normal mice. Right-hand panels: T-bet-deficient mice showedlung inflammation, with lymphocytes andeosinophils around the airway and bloodvessels (top) and airway remodeling withincreased collagen around the airway(bottom). Photographs courtesy ofL. Glimcher.

Normal airway


conditions with various underlying allergic mechanisms, not all of whichpredominantly involve IgE. Although allergy is often considered solely inthe context of a TH2 phenotype, both THl and TH2 cytokines can contributeto the immunopathogenesis. About one-third of patients with eczemashow minimal, if any, rise in

circulating IgE, and TH1-cell development ispreferentially observed in the lesions of people with a history of persistentdisease. However, early-onset and/ or chronic eczema occurring in youngchildren, often in response to food antigens such as those in cow's milk, isoften the first indication of atopy, and in such cases sensitization to an allergenand IgE-mediated allergic reactions are usually eventually demonstrated.The terms atopic eczema or atopic dermatitis are now recommended foruse only in such cases. Atopic eczema is the result of a chronic inflammatoryresponse with features of tissue remodeling and fibrosis similar to those seenin the bronchial walls of patients with asthma. In both atopic and non-atopic Atopic Dermatitisallergic eczema, the apoptosis of keratinocytes induced by T-cell-producedIFN-y and TNF-a contributes to the pathophysiology.

A recently discovered association between a deficiency in the epidermalprotein filaggrin and atopic eczema is throwing new light on the condition.Filaggrin binds to keratin fibers in epidermal cells, contributing to the phys-ical barrier at the skin surface that keeps skin waterproof and prevents theentry of airborne allergens. Many patients with atopic eczema have mutationsin the filaggrin gene that lead to the nonproduction of filaggrin, and havea ‘leaky skin,' which is more liable to become dry owing to increased waterloss. A recent survey in Denmark showed a positive association between thefilaggrin-null mutation and the occurrence of eczema in atopic

Airway remodeling with increasedcollagen deposited around airway


patients witha history of the disease, whereas there was no significant association betweenthe occurrence of hand eczema and filaggrin deficiency in non-atopicpeople. It is proposed that the filaggrin defect facilitates the sensitizationof atopic people to airborne allergens by allowing the allergens to penetrate

Fig. 14.18 Risk factors for thedevelopment of food allergy.


Risk factors for the developmentof food allergy

Immature mucosal immune system

Early introduction of solid food

Hereditary increase in mucosal permeability

IgA deficiency or delayed lgA production

Inadequate challenge of the intestinal immunesystem by commensal flora

Genetically determined bias toward aTH2 environment

Polymorphisms of TH2 cytokine orlgE receptor genes

Impaired enteric nervous system

Immune alterations (e.g., low levels of TGF-[3)

Gastrointestinal infectionsthe skin more easily. The filaggrin-null mutation is therefore a good exampleof a polymorphism that contributes to the risk of sensitization and allergicdisease by reducing the barrier function of a tissue.

Th2 responses in atopic eczema may lead indirectly to exacerbation of thecondition by making the individual more susceptible to certain infections.For example, at the time when children were routinely vaccinated againstsmallpox with a live vaccinia virus vaccine, it was observed that this oftenled to a disseminated vaccinia infection in the skin of children with atopiceczema; atopic individuals are now excluded from voluntary smallpoxvaccination. The antimicrobial peptide cathelicidin (see Chapter 2), which is

produced by keratinocytes in reponse to infection, can inhibit vaccinia virus.

The spread of the virus in atopic skin seems to be due to the actions of theTh2 cytokines IL-4 and IL-13, which are overexpressed in atopic eczema andinhibit the production of cathelicidin. Thus, one can envisage a vicious circleof infection triggering atopic eczema resulting in increased susceptibility tofurther infection. Activation ofTLRs and other innate receptors on epithelialcells can also occur either by microbial antigens or by allergens and canexacerbate atopic eczema and other allergic reactions.

14- 13 Allergy to particular foods causes systemic reactions as well assymptoms limited to the gut.

Adverse reactions to particular foods are common, but only some are due toan immune reaction. 'Food allergy' can be classified into IgE-mediated aller-gic reactions, non-IgE-mediated food allergy (celiac disease, discussed inSection 14-18), idiosyncrasies, food intolerance, and food fads. Idiosyncrasiesare abnormal responses to particular foods whose cause is unknown butwhich can provoke symptoms resembling those of an allergic response. Foodintolerances are nonimmune adverse reactions due mainly to metabolic defi-cits, such as intolerance of cows milk due to the inability to digest lactose.

IgE-mediated food allergies affect about 1-4% of American and Europeanadults, with allergies being slightly more frequent in children (around 5%).About 25% of this is to peanuts, and peanut allergy is increasing in incidence.Figure 14.18 illustrates the risk factors for the development of IgE-mediatedfood allergy. IgE-mediated food allergy can manifest itself in a variety ofways, ranging from a swelling of the lips and oral tissue on contact with theallergen, to gastrointestinal symptoms, urticaria, asthma, and in the mostsevere cases a severe anaphylactic reaction leading to cardiovascular collapse(see Section 14-10). Local gastrointestinal symptoms are due to activation ofmucosal mast cells, leading to transepithelial fluid loss and smooth musclecontraction, causing diarrhea and vomiting. Food allergens that subsequentlyreach the bloodstream can lead to urticaria or to a severe anaphylacticreaction. Certain foods, most importantly peanuts, tree nuts, and shellfish,are particularly associated with severe anaphylaxis. Around 150


deaths occureach year in the United States as a result of a severe allergic reaction to food,with peanut and tree nut allergies accounting for most of the deaths. Peanutallergy is a significant public-health problem, especially in school: childrenmay be unwittingly exposed to peanuts, which are present in many foods.

One of the characteristic features of food allergens is a high degree of resistanceto digestion by pepsin in the stomach. This allows them to reach the mucosalsurface of the small intestine as intact allergens. Cases of IgE-mediated foodallergies arising in previously unaffected adults who were taking antacidsor proton-pump inhibitors for ulcers or acid reflux have been proposed tobe due to impaired digestion of potential allergens in the less-acid stomachconditions produced by these medications.


14- 14 lgE-mediated allergic disease can be treated by inhibiting the effectorpathways that lead to symptoms or by desensitization techniques thataim at restoring tolerance to the allergen.

Most of the current drug treatments for allergic disease either treat only thesymptoms—examples of such drugs are antihistamines and p-antagonists—or are general anti-inflammatory drugs such as the corticosteroids (Fig.14.19). Treatment is largely palliative, rather than curative, and often needs tobe taken throughout life. Anaphylactic reactions are treated with epinephrine,which stimulates the re-formation of endothelial tight junctions, promotesthe relaxation of constricted bronchial smooth muscle, and stimulates theheart. Antihistamines, which block the Hi receptor, reduce the symptoms thatfollow the release of histamine from mast cells in allergic rhinoconjunctivitisand IgE-triggered urticaria. In urticaria, for example, the relevant Hi receptorsinclude those on blood vessels and unmyelinated nerve fibers in the skin.



Fig. 14.19 Approaches to the treatmentof allergic disease. Examples oftreatments in current clinical use forallergic reactions are listed in the tophalf of the table, with approaches underinvestigation listed below.

Treatments for allergic disease

Target step Mechanism of treatment Specific approach

In clinical use

Mediator action Inhibit effects of mediatorson specific receptors

Inhibit synthesis ofspecific mediators

Antihistamines, 13-blockers

Lipoxygenase inhibitors

Chronic inflammatoryreactions

General anti-inflammatoryeffects

Corticosteroids

TH2 response Induction of regulatoryT cells

Desensitization therapy byinjections of specific antigen

lgE binding to mast cell Bind to lgE Fc region andprevent lgE binding to Fereceptors on mast cells

Anti-lgE antibodies(omalizumab)

Proposed or under investigation

TH2 activation Induction of regulatoryT cells

Injection of specific antigenpeptidesAdministration of cytokines,e.g., IFN-'/, IL-10, IL-12, TGF-13Use of adjuvants such as CpGoligodeoxynucleotides tostimulate T H1 response

Activation of B cell toproduce lgE

Block co-stimulationInhibit TH2 cytokines

Inhibit CD40LInhibit IL-4 or IL-13

Mast-cell activation Inhibit effects of lgEbinding to mast cell

Blockade of lgE receptor

Eosinophil-dependentinflammation

Block cytokine andchemokine receptors thatmediate eosinophilrecruitment and activation

Inhibit IL-5Block CCR3


Antileukotriene drugs act as antagonists of leukotriene receptors on smoothmuscle, endothelial cells, and mucous-gland cells, and are also used torelieve the symptoms of allergic rhinoconjunctivitis and in asthma. Inhaledbronchodilators that act on p-adrenergic receptors to relax constrictedmuscle relieve acute asthma attacks. In chronic allergic disease it is extremelyimportant to treat and prevent the chronic inflammatory injury to tissues,and regular use of inhaled corticosteroids is now recommended in persistentasthma to help suppress inflammation. Topical corticosteroids are used tosuppress the chronic inflammatory changes seen in eczema.

The treatments noted above have been in use for many years. Ways are stillbeing sought to more precisely and effectively suppress the T-cell responseto a given allergen through immunological approaches. Two types ofimmunotherapy are already used in the clinic. One relies on active immuno-therapy—known as desensitization immunotherapy or allergen-specificimmunotherapy—that manipulates the immune response itself. The otherrelies on passive immunotherapy, such as the blockade of IgE by anti-IgEantibodies. Several other approaches are still in the experimental or clinicaltrial stage.

In desensitization immunotherapy the aim is to restore tolerance to theallergen by reducing its tendency to induce IgE production. Patients aredesensitized by injection with escalating doses of allergen, starting with tinyamounts, an injection schedule that gradually decreases the IgE-dominatedresponse. The mechanisms underlying desensitization therapy are complex,but the key to success seems to be the induction of regulatory T cells secretingIL-10 and/or TGF-p, which skew the response away from IgE production(see Section 14-3). For example, beekeepers exposed to repeated stings areoften naturally protected from severe allergic reactions such as anaphylaxisthrough a mechanism that involves IL-10-secreting T cells. Similarly, specificallergen immunotherapy for sensitivity to insect venom and airborneallergens induces the increased production of IL-10 and in some casesTGF-p, as well as the induction of IgG isotypes, particularly IgG4, an isotypeselectively promoted by IL-10. Recent evidence shows that


desensitization isalso associated with a reduction in the numbers of inflammatory cells at thesite of the allergic reaction. A potential complication of the desensitizationapproach is the risk of inducing an IgE-mediated allergic response, and it iscontraindicated in patients with severe asthma. When a patient is allergicto a drug that is essential for their treatment (such as an antibiotic, insulin,or a chemotherapeutic agent), a high-risk procedure called acute or rapiddesensitization is sometimes used to induce temporary tolerance. In this casethe drug is introduced in repeated increasing subthreshold doses over a shortperiod of time, from hours to days, until a total cumulative therapeutic doseis reached. The tolerance lasts only for as long as the medication continues.

An alternative, and still experimental, approach to desensitization is vaccina-tion with peptides derived from common allergens. This procedure inducesTreg cells, with the accompanying production ofIL-10. An IgE-mediated aller-gic reaction is not induced by the peptide because it is too short to cross-linkIgE. A potential difficulty with this approach is that an individual’s response topeptide antigens is restricted by their MHC class II alleles (see Section 6-14);patients with different MHC class II molecules therefore respond to differentallergen-derived peptides. One possible solution is the use of peptides thatcontain short sequences with multiple overlapping MHC-binding motifs thatwould provide coverage for most of the population. A vaccination strategythat shows promise in experimental animal models of allergy is the use ofoligodeoxynucleotides rich in unmethylated CpG as adjuvants for desens-itization regimes. These oligonucleotides mimic the CpG motifs in bacterial


DNA and strongly promote TH1 responses, probably through the stimulationofTLR-9 in dendritic cells and the suppression of^2 responses. The mechan-ism of action of adjuvants is discussed in Appendix I, Section A-4.

An alternative approach to manipulating the T-cell response is to administercytokines that promote T^-type responses. IFN-y, IFN-a, and IL-12 haveeach been shown to reduce IL-4-stimulated IgE synthesis in vitro, and IFN-yand IFN-a have been shown to reduce IgE synthesis in vivo. Administration ofIL-12 to patients with mild allergic asthma caused a decrease in the number ofeosinophils in blood and sputum but had no effect on immediate or late-phaseresponses to inhaled allergen. The treatment with IL-12 was accompanied byquite severe flu-like symptoms in most patients, which is likely to limit itspossible therapeutic value. Cytokines that enhance the switching of B cells toIgE production are also potential targets for therapy with cytokine inhibitors.Inhibitors ofIL-4, IL-5, and IL-13 would be predicted to reduce the productionof IgE, but redundancy in the activities of these cytokines might make thisapproach difficult to implement in practice.

One successful approach to passive immunotherapy has been the develop-ment of anti-IgE antibodies that bind the Fc region of free IgE and prevent itfrom binding to FceRL The humanized mouse monoclonal anti-IgE antibodyomalizumab is mainly used in cases of chronic allergic asthma in which othertreatments have failed to control the disease. In clinical trials, omalizumabtreatment reduced circulating IgE levels by more than 95%, which was accom-panied by downregulation of the numbers of high-affinity IgE receptors onbasophils and mast cells. Omalizumab also seems to exert its therapeuticeffect in chronic allergic asthma by reducing IgE-mediated antigen trappingand presentation by dendritic cells, thus preventing the activation of newallergen-specificTH2 cells.

A further approach to the treatment of allergic disease would be to block therecruitment of eosinophils to sites of allergic inflammation. The eotaxin recep-tor CCR3 is a potential target in this context. The production of eosinophilsin bone marrow and their exit into the circulation might also be


reduced bya blockade of IL-5 action. Anti-IL-5 antibody (mepolizumab) is of benefit intreating hypereosinophilic syndrome, in which chronic overproduction ofeosinophils causes severe organ damage. Clinical trials of anti-IL-5 treatmentin asthma, however, show that any beneficial effect is in practice likely to belimited to a small subset of asthma patients with prednisone-dependent eosi-nophilic asthma, in which IL-5 seems to reduce the number of exacerbationsof asthma when the corticosteroid dose is reduced.

Summary.

The allergic response to innocuous antigens reflects the pathophysiologicalaspects of a defensive immune response whose physiological role is to protectagainst helminth parasites. It is triggered by the binding of antigen to IgEantibodies bound to the high-affinity IgE receptor FceRI on mast cells. Mastcells are strategically distributed beneath the mucosal surfaces of the bodyand in connective tissue. Antigen cross-linking the IgE on their surface causesthem to release large amounts of inflammatory mediators. The resultinginflammation can be divided into early events, characterized by short-livedmediators such as histamine, and later events that involve leukotrienes,cytokines, and chemokines, which recruit and activate eosinophils andbasophils. This response can evolve into chronic inflammation, characterizedby the presence of effector T cells and eosinophils, which is most clearly seenin chronic allergic asthma.


Non-lgE-mediated allergic diseases.

In this part of the chapter we focus on immunological hypersensitivityresponses involving IgG antibodies (type II and type III responses in Fig. 14.1)and type IV responses involving antigen-specific Th1 cells or CD8 T cells (thetype IV Th2 responses that are characteristic of chronic IgE-initiated allergicdisease were discussed in the previous part of the chapter). These effectorarms of the immune response occasionally react with noninfectious antigensto produce acute or chronic allergic reactions. Although the mechanismsinitiating the various forms of hypersensitivity are different, much of thepathology is due to the same immunological effector mechanisms.

14- 15 Innocuous antigens can cause type II hypersensitivity reactions insusceptible individuals by binding to the surfaces of circulatingblood cells.

Antibody-mediated destruction of red blood cells (hemolytic anemia) orplatelets (thrombocytopenia) can be caused by some drugs, including theantibiotics penicillin and cephalosporin. These are examples of type II hyper-sensitivity reactions in which the drug binds to the cell surface and is a targetfor anti-drug IgG antibodies that cause destruction of the cell (see Fig. 14.1).The anti-drug antibodies are made in only a minority of people, and it is notclear why these individuals make them. The cell-bound antibody triggers theclearance of the cell from the circulation, predominantly by tissue macro-phages in the spleen, which bear Fey receptors.

14- 16 Systemic disease caused by immune-complex formation can followthe administration of large quantities of poorly catabolized antigens.

Type III hypersensitivity reactions can arise with soluble antigens (seeFig. 14.1). The pathology is caused by the deposition of antigen:antibodyaggregates, or immune complexes, in particular tissues and sites. Immunecomplexes are generated in all antibody responses, but their pathogenicpotential is determined, in part, by their size and by the amount, affinity,


and isotype of the responding antibody. Larger aggregates fix complementand are readily cleared from the circulation by the mononuclear phagocytesystem. However, the small complexes that form when antigen is in excesstend to be deposited in blood vessel walls. There they can ligate Fe receptorson leukocytes, leading to leukocyte activation and tissue injury.

A local type III hypersensitivity reaction called an Arthus reaction (Fig.14.20)can be triggered in the skin of sensitized individuals who possessIgG antibodies against the sensitizing antigen. When antigen is injectedinto the skin, circulating IgG antibody that has diffused into the skin formsimmune complexes locally. The immune complexes bind Fe receptors suchas FcyRIII on mast cells and other leukocytes, generating a local inflammatoryresponse and increased vascular permeability. Fluid and cells, especiallypolymorphonuclear leukocytes, then enter the site of inflammation from localblood vessels. The immune complexes also activate complement, leading tothe production of the complement fragment C5a. This is a key participantin the inflammatory reaction because it interacts with C5a receptors onleukocytes to activate these cells and attract them to the site of inflammation(see Section 2-5). Both C5a and FcyRIII have been shown to be required for theexperimental induction of an Arthus reaction in the lung by macrophages inthe walls of the alveoli, and they are probably required for the same reactioninduced by mast cells in the skin and the linings of joints (synovia).

Non-lgE-mediated allergic diseases



A systemic type III hypersensitivity reaction, known as serum sickness, canresult from the injection of large quantities of a poorly catabolized foreignantigen. This illness was so named because it frequently followed theadministration of therapeutic horse antiserum. In the pre-antibiotic era,antiserum made by immunizing horses was often used to treat pneumococcalpneumonia; the specific anti-pneumococcal antibodies in the horse serumwould help the patient to clear the infection. In much the same way, antivenin(serum from horses immunized with snake venoms) is still used today asa source of neutralizing antibodies to treat people suffering from the bitesof poisonous snakes. The increasing use of monoclonal antibodies in thetreatment of disease (for example, anti-TNF-a in rheumatoid arthritis) hasled to the development of serum sickness in a small minority of patients.

Serum sickness occurs 7-10 days after the injection of horse serum, an intervalthat corresponds to the time required to mount an IgG-switched primaryimmune response against the foreign antigens. The clinical features of serumsickness are chills, fever, rash, arthritis, and sometimes glomerulonephritis(inflammation of the glomeruli of the kidneys). Urticaria is a prominentfeature of the rash, implying a role for histamine derived from mast-celldegranulation. In this case, the mast-cell degranulation is triggered by theligation of cell-surface FcyRIII by IgG-containing immune complexes.

The course of serum sickness is illustrated in Fig. 14.21. The onset of diseasecoincides with the development of antibodies against the abundant solubleproteins in the foreign serum; these antibodies form immune complexeswith their antigens throughout the body. These immune complexes fixcomplement and can bind to and activate leukocytes bearing Fc andcomplement receptors; these in turn cause widespread tissue damage. Theformation of immune complexes causes clearance of the foreign antigen,so serum sickness is usually a self-limiting disease. Serum sickness after asecond dose of antigen follows the kinetics of a secondary antibody response(see Section 10-14), with symptoms typically appearing within a day or two.

Pathological immune-complex deposition is seen in other situations in whichantigen persists. One is when an adaptive antibody response fails to

clear theinfecting pathogen, as occurs in subacute bacterial endocarditis or chronicviral hepatitis. In these situations, the replicating pathogen is continuouslygenerating new antigen in the presence of a persistent antibody response,with the consequent formation of abundant immune complexes. These areFig. 14.20 The deposition of immunecomplexes in tissues causes a localinflammatory response known as anArthus reaction (type Ill hypersensitivityreaction). In individuals who havealready made lgG antibody against anantigen, the same antigen injected intothe skin forms immune complexes withlgG antibody that has diffused out of thecapillaries. Because the dose of antigenis low, the immune complexes are onlyformed close to the site of injection,where they activate mast cells bearingFey receptors (FcyRIII). The complementcomponent C5a seems to be importantin sensitizing the mast cell to respond toimmune complexes. As a result of mast-cell activation, inflammatory cells invadethe site, and blood vessel permeabilityand blood flow are increased. Plateletsalso accumulate inside the vessel atthe site, ultimately leading to vesselocclusion.

Drug-InducedSerum Sickness

injection arthritis, nephritis

Fig. 14.21 Serum sickness is a classicexample of a transient immunecomplex-mediated syndrome. Aninjection of a foreign protein or proteinsleads to an antibody response. Theseantibodies form immune complexeswith the circulating foreign proteins.The complexes are deposited in smallvessels and activate complement andphagocytes, inducing fever and thesymptoms of vasculitis, nephritis, andarthritis. All these effects are transientand resolve when the foreign proteinis cleared.

Type IV hypersensitivity reactions are mediated by antigen-specific effector T cells

Syndrome Antigen Consequence

Delayed-typehypersensitivity

Proteins:Insect venom

Mycobacterial proteins(tuberculin, lepromin)

Local skin swelling:ErythemaInduration

Cellular infiltrateDermatitis

Contacthypersensitivity

Haptens:Pentadecacatechol (poison ivy)

DNFBSmall metal ions:

NickelChromate

Local epidermal reaction:Erythema

Cellular infiltrateVesicles

lntraepidermal abscesses

Gluten-sensitive enteropathy(celiac disease)

Gliadin Villous atrophy in small bowelMalabsorption

Fig. 14.22 Type IV hypersensitivityresponses. These reactions are mediatedby T cells and all take some time todevelop. They can be grouped into threesyndromes, according to the route bywhich antigen passes into the body. Indelayed-type hypersensitivity the antigenis injected into the skin; in contacthypersensitivity it is absorbed into theskin; and in gluten-sensitive enteropathyit is absorbed by the gut. DNFB,dinitrofluorobenzene.


deposited within small blood vessels, with consequent injury in many tissuesand organs, including the skin, kidneys, and nerves.

Immune-complex disease also occurs when inhaled allergens provoke IgGrather than IgE antibody responses, perhaps because they are present atrelatively high levels in the air. "When a person is reexposed to high doses ofsuch allergens, immune complexes form in the walls of alveoli in the lung.This leads to the accumulation of fluid, protein, and cells in the alveolar wall,slowing blood-gas interchange and compromising lung function. This type ofreaction is more likely to occur in occupations such as farming, in which thereis repeated exposure to hay dust or mold spores, and the resulting disease isknown as farmer’s lung. If exposure to antigen is sustained, the lining of thelungs can be permanently damaged.

14- 17 Hypersensitivity reactions can be mediated byTH1 cellsand CDS cytotoxicT cells.

Unlike the immediate hypersensitivity reactions (type I and type II), whichare mediated by antibodies, type IV hypersensitivity reactions or delayed-type hypersensitivity are mediated by antigen-specific effector T cells. Wehave already seen the involvement of TH2 effector cells and the cytokinesthey produce in the chronic response in IgE-initiated allergic reactions.Here we consider the allergic diseases caused by type IV hypersensitivityreactions mediated by Th1 and CD8 cytotoxic T cells (Fig. 14.22). These cellsfunction in essentially the same way as they do in response to a pathogen,as described in Chapter 9, and the responses can be transferred betweenexperimental animals by purified T cells or cloned T-cell lines. Much of thechronic inflammation seen in some of the allergic diseases described earlieris due to type IV hypersensitivity reactions mediated by antigen-specific Th1cells as well as by Th2 cells.

The prototypic delayed-type hypersensitivity reaction is the tuberculin test(see Appendix I, Section A-38). This is a T^-type IV hypersensitivity reaction


(see Fig. 14.1) that is used to determine whether an individual has previouslybeen infected with M. tuberculosis. In the Mantoux test, small amountsof tuberculin—a complex mixture of peptides and carbohydrates derivedfrom M. tuberculosis—are injected intradermally. In people who have beenexposed to the bacterium, either by infection or by immunization with theBCG vaccine (an attenuated form of M. tuberculosis), a local T cell-mediated


inflammatory reaction evolves over 24-72 hours. The response is caused byTHl cells, which enter the site of antigen injection, recognize complexes ofpeptide:MHC class II molecules on antigen-presenting cells, and releaseinflammatory cytokines such as IFN-y and TNF-p. These stimulate theexpression of adhesion molecules on endothelium and increase local bloodvessel permeability, allowing plasma and accessory cells to enter the site, thuscausing a visible swelling (Fig. 14.23). Each of these phases takes several hoursand so the fully developed response only appears 24-48 hours after challenge.The cytokines produced by the activated THl cells and their actions are shownin Fig. 14.24.

Very similar reactions are observed in allergic contact dermatitis, which is animmune-mediated local inflammatory reaction in the skin caused by directskin contact with certain antigens—or sometimes by oral uptake of the anti-gen, when it is known as systemic allergic contact dermatitis. It is importantto note that not all contact dermatitis is immune-mediated and allergic innature; it can also be caused by direct damage to the skin by irritant or toxicchemicals.

Allergic contact dermatitis can be caused by the activation of CD4 or CD8 Tcells, depending on the pathway by which the antigen is processed. Typicalantigens that cause allergic contact dermatitis are highly reactive smallmolecules that can easily penetrate intact skin, especially if they causeitching that leads to scratching. These chemicals then react with self proteins,creating hapten:protein complexes that can be processed to hapten:peptidecomplexes capable of being presented by MHC molecules and recognizedby T cells as foreign antigens. As with other allergic responses, there are twophases to a cutaneous allergic response: sensitization and elicitation. Duringthe sensitization phase, Langerhans cells (dendritic cells) in the skin take upand process antigen, and migrate to regional lymph nodes, where they acti-vate T cells (see Fig. 9.13) with the consequent production of memory T cells,which end up in the dermis. In the elicitation phase, a subsequent exposure


to the sensitizing chemical leads to antigen presentation to memoryT cells inthe dermis, with the release ofT-cell cytokines such as IFN-y and IL-17. Thisstimulates the keratinocytes of the epidermis to release IL-l, IL-6, TNF-a,GM-CSF, the chemokine CXCL8, and the interferon-inducible chemokinesCXCLll (IP-9), CXCLlO (IP-10), and CXCL9 (Mig; monokine induced byIFN-y).These cytokines and chemokines enhance the inflammatory responseby inducing the migration of monocytes into the lesion and their maturationinto macrophages, and by attracting more T cells (Fig. 14.25).



Fig. 14.23 The stages of a delayed-type hypersensitivity reaction. Thefirst phase involves uptake, processing,and presentation of the antigen by localantigen-presenting cells. In the secondphase, TH 1 cells that have been primedby a previous exposure to the antigenmigrate into the site of injection andbecome activated. Because these specificcells are rare, and because there is littleinflammation to attract cells into the site,it can take several hours for a T cell ofthe correct specificity to arrive. Thesecells release mediators that activatelocal endothelial cells, recruiting aninflammatory cell infiltrate dominatedby macrophages and causing theaccumulation of fluid and protein. At thispoint the lesion becomes apparent.

Antigen is processed by tissue macrophagesand stimulates^ cells

Chemokines IFN"'Y

TNF-a and LT IL-3/GM-CSF


Fig. 14.24 The delayed-type (type IV)hypersensitivity response is directed bychemokines and cytokines released byantigen-stimulated TH 1 cells. Antigen inthe local tissues is processed by antigen-presenting cells and presented on MHCclass II molecules. Antigen-specific TH 1cells that recognize the antigen locally atthe site of injection release chemokinesand cytokines that recruit macrophagesto the site of antigen deposition.Antigen presentation by the newlyrecruited macrophages then amplifiesthe response. T cells can also affectlocal blood vessels through the releaseof TNF-a and lymphotoxin (LT), andstimulate the production of macrophagesthrough the release of IL-3 and GM-CSF.TH 1 cells activate macrophages throughthe release of IFN-yand TNF-a, and killmacrophages and other sensitive cellsthrough the cell-surface expression of theFas ligand.


Recruitmacrophages tosite of antigen

deposition

Stimulate monocyteproduction by bonemarrow stem cells


Induces expression ofvascular adhesionmolecules.Activates macrophages,increasing release ofinflammatory

mediatorsCause local tissue

destruction.Increase expression

ofadhesion molecules

onlocal blood vesssels



The rash produced by contact with the poison ivy plant (Fig. 14.26) is aContact Sensitivity common example of allergic contact dermatitis in the United States and is

to Poison Ivy caused by a CD8 T-cell response to urushiol oil (a mixture of pentadecacate-

® chols) in the plant. These chemicals are lipid-soluble and so can cross thecell membrane and attach to intracellular proteins. The modified proteinsgenerate modified peptides within the cytosol, which are translocated into



Contact-sensitizing agentpenetrates the skin and binds toself proteins, which are taken up

by Langerhans cells

Langerhans cells present selfpeptides haptenated with thecontact-sensitizing agent toTHlcells, which secrete IFN"'Y andother cytokines

Activated keratinocytes secretecytokines such as IL-1 and

TNF-a and chemokines suchas CXCL8, CXCL11, and

CXCL9

The products of keratinocytesandTHl cells activatemacrophages to secretemediators of inflammation



TNF-a CXCL8 IL-1 CXCL11 CXCL9



Fig. 14.25 Elicitation of a delayed-type hypersensitivityresponse to a contact-sensitizing agent. A contact-sensitizingagent is a small highly reactive molecule that can easilypenetrate intact skin. It binds covalently as a hapten to a varietyof endogenous proteins, which are taken up and processed byLangerhans cells, the major antigen-presenting cells of skin.These present haptenated peptides to effector TH 1 cells (whichmust have been previously primed in lymph nodes and then havetraveled back to the skin). These then secrete cytokines such asIFN-ythat stimulate keratinocytes to secrete further cytokines andchemokines. These in turn attract monocytes and induce theirmaturation into activated tissue macrophages, which contribute tothe inflammatory lesions depicted in Fig. 14.26. NO, nitric oxide.

Fig. 14.26 Blistering skin lesions on thehand of a patient with allergic contactdermatitis caused by poison ivy.Photograph courtesy of R. Geha.


the endoplasmic reticulum and delivered to the cell surface bound to MHCclass I molecules. CD8 T cells recognizing the peptides cause damage eitherby killing the eliciting cell or by secreting cytokines such as IFN-y. The well-studied chemical picryl chloride produces a CD4 T-cell allergic contact der-matitis. Picryl chloride modifies extracellular self proteins, which are thenprocessed into modified self peptides that bind to self-MHC class II mole-cules and are recognized by TH 1 cells. When sensitized TH 1 cells recognizethese complexes, they produce extensive inflammation by activating macro-phages (see Fig. 14.25).

Some insect proteins also elicit a delayed-type hypersensitivity response.One example of this in the skin is a severe reaction to mosquito bites. Insteadof a small itchy bump, people allergic to proteins in mosquito saliva candevelop an immediate reaction such as urticaria and swelling or, much morerarely, anaphylactic shock (see Section 14-10). Some allergic individualssubsequently develop a delayed reaction to a bite in which the whole affectedlimb swells up.

Important delayed-type hypersensitivity responses to divalent cations suchas nickel have also been observed. These divalent cations can alter theconformation or the peptide binding of MHC class II molecules, and thusprovoke a T-cell response. In humans, nickel can also bind to the receptorTLR-4 and produce a pro-inflammatory signal. Sensitization to nickel iswidespread as a result of prolonged contact with nickel-containing itemssuch as jewelry, buttons, and clothing fasteners, but some countries now havestandards that specify that such products must have non-nickel coatings orpotentially release very low amounts of the metal, and this is reducing theprevalence of nickel allergy in those countries.

Finally, although this section has focused on the role ofTH 1 and cytotoxic Tcells in inducing delayed-type hypersensitivity reactions, there is evidencethat antibody and complement might also have a role. Mice deficient in Bcells, antibody, or complement show impaired contact hypersensitivityreactions. In particular, IgM antibodies (produced in part by B1 cells), whichactivate the complement cascade, facilitate the initiation of these reactions.

Celiac DiseaseI


14- 18 Celiac disease has features of both an allergic responseand autoimmunity.

Celiac disease is a chronic condition of the upper small intestine caused by animmune response directed at gluten, a complex of proteins present in wheat,oats, and barley. Elimination of gluten from the diet restores normal gut func-tion, but must be continued throughout life. The pathology of celiac disease ischaracterized by the loss of the slender, finger-like villi formed by the intesti-nal epithelium (a condition termed villous atrophy), together with an increasein the size of the sites in which epithelial cells are renewed (crypt hyperplasia)(Fig. 14.27). These pathological changes result in the loss of the mature epithe-lial cells that cover the villi and which normally absorb and digest food, andis accompanied by severe inflammation of the intestinal wall, with increasednumbers of T cells, macrophages, and plasma cells in the lamina propria,as well as increased numbers of lymphocytes in the epithelial layer. Glutenseems to be the only food protein that provokes intestinal inflammation inthis way, a property that reflects gluten's ability to stimulate both innate andspecific immune responses in genetically susceptible individuals.

Celiac disease shows an extremely strong genetic predisposition, with morethan 95% of patients expressing the HLA-DQ2 class II MHC allele, and thereis an 80% concordance in monozygotic twins (that is, if one twin develops it,there is an 80% probability that the other will) but only a 10% concordance indizygotic twins. Nevertheless, most individuals expressing HLA-DQ2 do not


Fig. 14.27 The pathological featuresof celiac disease. Left: the surface ofthe normal small intestine is folded intofinger-like villi, which provide an extensivesurface for nutrient absorption. Right: thelocal immune response against the foodprotein a-gliadin provokes destruction ofthe villi. In parallel, there is lengtheningand increased mitotic activity in theunderlying crypts where new epithelialcells are produced. There is also a markedinflammatory infiltrate in the intestinalmucosa, with increased numbers oflymphocytes in the epithelial layer andaccumulation of CD4 T cells, plasmacells, and macrophages in the deeperlayer, the lamina propria. Because the villicontain all the mature epithelial cells thatdigest and absorb foodstuffs, their lossresults in life-threatening malabsorptionand diarrhea. Photographs courtesy ofAllan Mowat.



develop celiac disease despite the almost universal presence of gluten in theWestern diet. Thus, other genetic factors must make important contributionsto susceptibility.

Most evidence indicates that celiac disease requires the aberrant priming ofIFN-y-producing CD4 T cells by antigenic peptides present in a-gliadin, oneof the major proteins in gluten. It is generally accepted that only a limitednumber of peptides can provoke an immune response leading to celiacdisease. This is likely to be due to the unusual structure of the peptide-binding groove of the HLA-DQ2 molecule. The key step in the immunerecognition of a-gliadin is the deamidation of its peptides by the enzymetissue transglutaminase (tTG), which converts selected glutamine residuesto negatively charged glutamic acid. Only peptides containing negativelycharged residues in certain positions bind strongly to HLA-DQ2, and thusthe transamination reaction promotes the formation of peptide:HLA-DQ2complexes, which can activate antigen-specific CD4 T cells (Fig. 14.28).Multiple peptide epitopes can be generated from gliadin. Activated gliadin-specific CD4 T cells accumulate in the lamina propria, producing IFN-y, acytokine that leads to intestinal inflammation.

Celiac disease is entirely dependent on the presence of a foreign antigen(gluten) and is not associated with a specific immune response againstantigens in the tissue—the intestinal epithelium—that is damaged duringthe immune response. But it does have some features of autoimmunity.Autoantibodies against tissue transglutaminase are found in all patients withceliac disease; indeed, the presence of serum IgA antibodies against thisenzyme is used as a sensitive and specific test for the disease. Interestingly, notTG-specific T cells have been found, and it has been proposed that gluten-reactive T cells provide help to B cells reactive to tissue transglutaminase. Insupport of this hypothesis, gluten can complex with the enzyme and thereforecould be taken up by tTG-reactive B cells (Fig. 14.29). There is no evidencethat these autoantibodies contribute to tissue damage, however.

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The activated T cells can killmucosal epithelial cells by

bindingFas. They also secrete

IFN"')',which activates the epithelial

cell

The bound peptide activatesgluten-specific CD4 T cells


Peptides naturally produced from

gluten do not bind to MHC

class II molecules

An enzyme, tissue transglutaminase

(tTG) modifies the peptides

sothey now can bind to theMHC class II molecules



Chronic T-cell responses against food proteins are normally prevented bythe development of oral tolerance (see Section 12-13). '^hy this breaks downin patients with celiac disease is unknown. The properties of the HLA-DQ2molecule provide a partial explanation, but there must be additional factorsbecause most HLA-DQ2-positive individuals do not develop celiac disease,and the high concordance rates in monozygotic twins indicate a role foradditional genetic factors. Polymorphisms in the gene for CTLA-4 or in otherimmunoregulatory genes may be associated with susceptibility. There couldalso be differences in how individuals digest gliadin in the intestine, so thatdiffering amounts survive for deamidation and presentation to T cells.

The gluten protein also seems to have several properties that contribute topathogenesis. As well as its relative resistance to digestion, there is mountingevidence that some gliadin-derived peptides stimulate the innate immunesystem by inducing the release of IL-15 by intestinal epithelial cells. Thisprocess is antigen-nonspecific and involves peptides that cannot be bound byHLA-DQ2 molecules or recognized by CD4 T cells. IL-15 release leads to theactivation of dendritic cells in the lamina propria, as well as the upregulationof MIC-A expression by epithelial cells. CD8 T cells in the mucosal epitheliumcan be activated via their NKG2D receptors, which recognize MIC-A, and theycan kill MIC-A-expressing epithelial cells via these same NKG2D receptors(Fig. 14.30). Triggering of these innate immune responses by a-gliadin maycreate some intestinal damage on its own and also induce some of theco-stimulatory events necessary for initiating an antigen-specific CD4 T-cellresponse to other parts of the a-gliadin molecule. The ability of gluten tostimulate both innate and adaptive immune responses may thus explain itsunique ability to induce celiac disease.

Summary.

Non-IgE-mediated immunological hypersensitivity also reflects normalimmune mechanisms that are inappropriately directed against innocuousantigens or inflammatory stimuli. It comprises both immediate-type anddelayed-type reactions. Immediate-type reactions are due to the binding of

Fig. 14.28 Molecular basis of immunerecognition of gluten in celiac disease.After the digestion of gluten by gutdigestive enzymes, deamidation ofepitopes by tissue transglutaminase leadsto their binding to HLA-DQ molecules andpriming of the immune system.

Fig. 14.29 A hypothesis to explainantibody production against tissuetransglutaminase (tTG) in the absenceof T cells specific for tTG in celiacpatients. tTG-reactive B cells endocytosegluten-tTG complexes and present glutenpeptides to the gluten-specific T cells.The stimulated T cells can now providehelp to these B cells, which produceautoantibodies against tTG.

JANEWAY'S

lntraepitheliallymphocytes (IELs)express NKG2D,which binds to MICmolecules andactivates the IELs tokill the epithelial cell

Gluten peptidesactivate mucosalepithelial cells to

express MICmolecules

of%to|IAA/W\AAAAAJ|

%AAA/1

Fig. 14.30 The activation of cytotoxicT cells by the innate immune systemin celiac disease. Gluten peptides caninduce the expression of the MHC classlb molecules MIC-A and MIC-B on gutepithelial cells. lntraepithelial lymphocytes(IELs), many of which are CD8 cytotoxicT cells, recognize these proteins via thereceptor NKG2D, which activates the IELsto kill the MIG-bearing cells, leading todestruction of the gut epithelium.

specific IgG antibodies to allergen-modified cell surfaces, as in drug-inducedhemolytic anemia (a type II reaction), or to the formation of immune comp-lexes of antibodies bound to poorly catabolized antigens, as occurs in serumsickness (a type III reaction). Type IV hypersensitivity reactions mediatedby THl cells and cytotoxic T cells develop more slowly. The THl-mediatedhypersensitivity reaction in the skin provoked by mycobacterial tuberculinis used to diagnose previous exposure to Mycobacterium tuberculosis. Theallergic reaction to poison ivy is due to the recognition and destruction bycytotoxic T cells of skin cells modified by a plant molecule, and to cytotoxic Tcell cytokines. These T cell-mediated responses require the induced synthesisof effector molecules and develop more slowly.

Summary to Chapter 14.

In susceptible individuals, immune responses to otherwise innocuousantigens can produce allergic reactions upon reexposure to the same antigen.Most allergic reactions involve the production of IgE antibody againstcommon environmental allergens. Some people are intrinsically prone tomaking IgE antibodies against many allergens, and such people are said tobe atopic. IgE production is driven by antigen-specific Th2 cells; the responseis polarized toward Th2 by an array of chemokines and cytokines that engagespecific signaling pathways. The IgE produced binds to the high-affinity IgEreceptor FceRI on mast cells and basophils. Specific effector T cells, mast cells,and eosinophils, in combination with Th 1 and Th2 cytokines and chemokines,orchestrate chronic allergic inflammation, which is the major cause of thechronic morbidity of asthma. Failure to regulate these responses can occurat many levels of the immune system, including defects in regulatory T cells.Antibodies of other isotypes and antigen-specific effectorT cells contribute toallergic hypersensitivity to other antigens.

Questions.

JANEWAY'S

14.1 List three allergic reactions that involve IgE and three that involve othermechanisms.

14.2 Describe how a person becomes sensitized to an allergen. Discuss thefactors predisposing to the production of lgE.

14.3 What are the key features that differentiate acute and chronic allergicreactions?

14.4 How can the innate immune system contribute to allergy? How do infectiousagents modulate allergy?

14.5 Which types of white blood cell participate in allergic responses, and whatdo they do?

14.6 Describe how an ingested food allergen can give rise to the allergic skinreaction urticaria.

14.7 How does desensitization therapy work?

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