Normal Immune System Function
Karim Rafaat
Organs of the Immune System
Central (primary) lymphoid organs are the sites for generation and early maturation of lymphocytes
T cells mature in the thymus
Central lymphoid
organs
B for bursa of Fabricius (a lymphoid organ in birds)[After Hieronymus Fabricius (1537-1619), Italian anatomist]
(T for thymus)
B cells mature in the bone marrow
Lymphoid organs contain lymphocytes and non-lymphoid cells such as macrophages and dendritic cells (and epithelial cells)
Lymphoid organs are important for the generation and maturation of lymphocytes, the initiation of immune responses and the perpetuation of immune responses
*other stuff (not peripheral lymphoid organs)
Peripheral lymphoid organs
1. trap antigens
2. are the sites for initiation of most immune response
3. provide signals for recirculation of lymphocytes
4. Antigen delivery to regions of increased traffic
Flow of lymph
Gut associated lymphoid tissue (GALT)(tonsils, adenoids, Peyer’s patches, appendix)
• Immature B and T cells mature in the central lymphoid organs • Then, they circulate in the blood and through the peripheral
lymphoid organs. As long as they have not encountered the specific antigen that binds their antigen receptors (BCR or TCR), the circulating lymphocytes are mature naïve lymphocytes
When they encounter antigen (bind antigen in their antigen receptor), they
1. Stick in the lymph nodes (or other peripheral lymphoid organ)(they stop circulating, i.e., altered trafficking)
2. Proliferate (divide)
3. Differentiate
Innate and Specific immunity;
Cells of the Immune System
Cells of the Immune System
BasophilsNeutrophils
Eosinophils
Granulocytic
Langerhans &Macrophages
Kupffer cellsDendritic cells?
Monocytic
CytotoxicHelper
Suppressor
T-cells
Plasmacells
B-cells Dendriticcells?
lymphoid cellsMyeloid cells
Components of the Immune System
Humoral Cellular Humoral Cellular
SpecificNonspecific
complement, interferon, TNF etc.
macrophages, neutrophils
T cells; other effectors cellsantibodies
Innate Immunity Adaptive Immunity
Characteristics of Innate and Adaptive Immunity
No Immunologicmemory
Antigen independent
No time lag
Not antigen specific
Antigen dependent
A lag period
Antigen specific
Developmentof memory
Innate Immunity Adaptive Immunity
Components of Innate and Adaptive Immunity
skin, gut Villi, lung cilia,etc
many protein andnon-protein secretions
phagocytes, NK cell eosinophils, K cells
physical barriers
soluble factors
cells
none
Immunoglobulins(antibody)
T and B lymphocytes
Site Component Functions
Effector mechanisms in Innate Immunity
Peristalsis, low pHbile salts, fatty acids
columnar cellsGI tract
Skin squamous cellssweat
desquamationflushing, fatty acids
tracheal ciliaLung mucociliary elevatorsurfactants
Site Component Functions
Effector mechanisms in Innate Immunity
Nasopharynxand eye
mucus, saliva, tears flushing, lysozyme
PhagocytesBlood andLymphiodorgans
phagocytosis and intracellular killing
K, NK & LAK cells
direct and antibody dependent cytolysis
Site Component Functions
Effector mechanisms in Innate Immunity
Serum and other serous
fluids
lactoferrin, transferrin
iron deprivation
interferons, TNF-
antiviral proteins phagocyte activation
lysozyme peptidoglycan hydrolysis
Fibronectin & complement
opsonization, enhanced phagocytosis, inflammation
Phagocytes are the Most Important Cells
Influenced bythe work of
Eli Metchnikoff,
George Bernard Shaw wrote:
“There is at bottom only one genuine treatment for all diseases,…to stimulate the phagocytes. Drugs are a delusion. …(when) the phagocytes are stimulated; they devour the disease…”
Phagocytes:Macrophages
phagocytosis, intracellular and extra-cellular killing, tissue repair, antigen presentation for specific immune response
characteristic nucleus and CD14 membrane marker.
Phagocyte Response to Infection
The SOS Signals–N-formyl methionine–Clotting system
peptides–Complement products
Phagocyte response–Vascular adherence–Diapedesis–Chemotaxis–Activation–Phagocytosis and killing
Initiation of Phagocytosis
Attachment via
IgG FcR
ScavengerR
CRToll-like R
Respiratory BurstOxygen Dependent Myeloperoxidase
Independent Reactions
2O2- + H2O2
.OH + OH- + 1O2
Glucose +NADP+
G-6-P-dehydrogenasePentose-P + NADPH
NADPH + O2
Cytochrome b558
NADP++ O2-
2O2- + 2H+
Superoxide dismutase
H2O2 + 1O2
Effector Molecule Function
Mediators of Oxygen Independent Killing in the Phago-lysosome
Cationic proteins (cathepsin) Damage to microbial membranes
Lysozyme Hydrolyses mucopeptides in the cell wall
Lactoferrin Deprives pathogens of iron
Hydrolytic enzymes (proteases) Digests killed organisms
Non-specific Killer Cells
NK and LAK cells
ADCC (K) cell
Activated
macrophages
Eosinophils
They all kill foreign and altered self targets
Natural Killer (NK) cells
also known as large granular lymphocytes (LGL)
kill infected and malignant cells
are identified by the presence of CD56 & CD16 and absence of CD3
activated by IL2 and IFN-γ to become LAK cells
Lymphokine Activated Killer (LAK) cell
IL2
IFNIFN
IL2
kills malignant
cells
kills transformed
and malignant cells
Innate Immunity
• The complement system
Complement:history
Discovered in 1894 by Bordet
It represents lytic activity of fresh serum
Complement functions
Host benefit:opsonization to enhance phagocytosisphagocyte attraction and activationlysis of bacteria and infected cellsregulation of antibody responsesclearance of immune complexesclearance of apoptic cells
Host detriment:Inflammation, anaphylaxis
Proteins of the complementsystem (nomenclature)
C1(qrs), C2, C3, C4, C5, C6, C7, C8, C9
factors B, D, H and I, properdin (P)
mannose binding lectin (MBL), MBL associated serine proteases (MASP-1 MASP-2)
C1 inhibitor (C1-INH, serpin), C4-binding protein (C4-BP), decay accelerating factor (DAF),
C1 receptor (CR1), protein-S (vitronectin)
Pathways of complement
activationCLASSICALPATHWAY
ALTERNATIVEPATHWAY
activationof C5
LYTIC ATTACKPATHWAY
antibodydependent
LECTINPATHWAY
antibodyindependent
Activation of C3 andgeneration of C5 convertase
Components of the Classical Pathway
C4C2 C3
C1 complex
Ca++
C1r C1s
C1q
Ca++
C1r C1s
C1q
C4
C4a
b
Classical Pathway Generation of C3-
convertase
Classical Pathway Generation of C3-
convertase
C4b
Mg++
C4a
Ca++
C1r C1s
C1q
C2
C2ba
C2a
C4b2a is C3 convertase
Classical Pathway Generation of C5-
convertase
C4b
Mg++
C4a
Ca++
C1r C1s
C1q
C2b
C2a
C3
C3a
b
C4b2a3b is C5 convertase; it leads into the Membrane
Attack Pathway
Components of mannose-binding lectin pathway
C4
MBL C2 MASP1
MASP2
Mannose-binding lectin pathway
C4
MBL
C4b
C4a
C4b
C2
C2b
C2a
C2a
C4b2a is C3 convertase; it will lead to the generation of
C5 convertaseMASP1
MASP2
Components of thealternative pathway
C3 B
D
P
Spontaneous C3 activation
C3
H2O
i B
D
Generation of C3 convertase
C3iBb complex has a very short half life
b C3
C3a
b
B
D
bC3b
If spontaneously-generated C3b is not degraded
C3-activationthe amplification loop
C3C3a b
C3a
B
D
BbC3b
C3 b
C3-activationthe amplification
loop
C3b
C3a
b
C3a
C3a BbC3b
C3bC3 BbB
D
bb
C3a
C3-activationthe amplification
loop
C3b
C3a
C3a BbC3b
BbBbC3b
C3a
C3-activationthe amplification
loop
C3bC3b
C3a
C3a BbC3b
BbBb
C3a
C3-activationthe amplification
loop
C3bC3b
Control of spontaneousC3 activation via DAF
C3bDAF prevents
the binding of
factor B to C3b
B
Autologous cell membrane
DA
F
CR1
Autologous cell membrane
C3b C3b Bb
H
I
iC3b
Control of spontaneousC3 activation via CR1
B b
I
iC3b DA
F
CR1DA
F
CR1
C3b stabilization andC5 activation
C3b
C3b finds an activator (protector) membrane
C3
C3a
bB
D
b
P This is stable C5 convertase of the alternative pathway
C3b regulation on self and activator surfaces
C3b
C5-convertase of the two pathways
C3b Bb C3b
C5-convertase of the Alternative Pathway
C4b C2a C3b
C5-convertase of the Classical and lectin
Pathways
Generation of C5 convertase leads to the activation of the
Lytic pathway
Lytic pathway
Components of the lytic pathway
C6
C9
C8
C7
C5
Lytic pathwayC5-activation
C3b C2 aC4b
C5 b
C5a
Lytic pathwayassembly of the lytic
complex
C5 b
C6
C7
Lytic pathway:insertion of lytic complex into cell
membrane
C5 b
C6
C7C8
C9C
9C9 C
9C9
C9 C
9 C9
C9
Biological effects of C5a
Opsonization and phagocytosis
Product Biological Effects Regulation
Biological properties of C-activation products
C2b (prokinin) edema C1-INH
C3a (anaphylatoxin)
mast cell degranulation; enhanced vascular permeability; anaphylaxis
carboxy-peptidase- B (C3-INA)
Product Biological Effects Regulation
Biological properties of C-activation products
as C3, but less potent
(C3-INA)C4a (anaphylatoxin)
opsonization; phagocytosis
C4b (opsonin)
C4-BP, factor I
C3b (opsonin)
opsonization; phagocyte activation
factors H & I
Product Biological Effects Regulation
Biological properties of C-activation products
anaphylactic as C3, but much more potent;attracts & activates PMN causes neutrophil aggregation, stimulation of oxidative metabolism and leukotriene release
C5a (chemotactic factor)
carboxy-peptidase-C (C3-INA)
C5b67 protein-Schemotaxis, attaches to other membranes
Antigens
Factors Influencing ImmunogenicityContribution of the Immunogen• Foreignness• Size
Conformational determinants
Sequence determinants
• Chemical Composition– Primary Structure– Secondary Structure– Tertiary Structure– Quarternary Structure
Factors Influencing ImmunogenicityContribution of the Immunogen• Foreigness• Size• Chemical Composition• Physical Form• Degradability
– Ag processing by Ag Presenting Cells (APC)
Factors Influencing ImmunogenicityContribution of the Biological System• Genetics
– Species– Individual
• Responders vs Non-responders• Age
Factors Influencing ImmunogenicityMethod of Administration• Dose• Route
– Subcutaneous > Intravenous > Intragastric
• Adjuvant– Substances that enhance an immune
response to an Ag
Chemical Nature of Immunogens• Proteins• Polysaccharides• Nucleic Acids• Lipids
– Some glycolipids and phosopholipids can be immunogenic for T cells and illicit a cell mediated immune response
Types of AntigensT-independent• Polysaccharides• Properties
– Polymeric structure
– Polyclonal B cell activation
– Resistance to degradation
• Examples– Pneumococcal polysaccharide,
lipopolysaccharide– Flagella
Types of AntigensT-dependent• Proteins• Structure• Examples
– Microbial proteins
– Non-self or Altered-self proteins
Antigenic Determinants Recognized by B cells and Ab• Composition
– Proteins, polysaccharides, nucleic acids
• Size– 4-8 residues
Antigenic DeterminantsRecognized by T cells• Composition
– Proteins (some lipids)– Sequence determinants
• Processed• MHC presentation (lipid presentation by
MHC-like CD1)• Size
– 8 -15 residues• Number
– Limited to those that can bind to MHC
Foreign bit receptorBiological
Consequence of Interaction
Microbial cell wall components
Complement Opsonization; Complement activation
Mannose-containing carbohydrates
Mannose-binding protein
Opsonization; Complement activation
Polyanions Scavenger receptors PhagocytosisLipoproteins of Gram + bacteriaYeast cell wall components
TLR-2 (Toll-like receptor 2)
Macrophage activation; Secretion of inflammatory cytokines
Foreign bit receptorBiological
Consequence of Interaction
Double stranded RNA
TLR-3 Production of interferon (antiviral)
LPS (lipopolysaccharide of Gram – bacteria
TLR-4 Macrophage activation; Secretion of inflammatory cytokines
Flagellin (bacterial flagella)
TLR-5 Macrophage activation; Secretion of inflammatory cytokines
Immunoglobulins:Structure and Function
Immunoglobulins:Structure and Function• Definition: Glycoprotein molecules that
are produced by plasma cells in response to an immunogen and which function as antibodies
General Functions of Immunoglobulins
• Effector functions – Fixation of
complement– Binding to various
cells
(Usually require Ag binding)
• Ag binding– Can result in
protection
Immunoglobulin Structure• Heavy & Light
Chains• Disulfide
bonds– Inter-chain– Intra-chain CH1
VL
CL
VH
CH2 CH3
Hinge Region
Carbohydrate
Disulfide bond
Immunoglobulin Structure
• Variable & Constant Regions– VL & CL
– VH & CH
• Hinge RegionCH1
VL
CL
VH
CH2 CH3
Hinge Region
Carbohydrate
Disulfide bond
Immunoglobulin Fragments: Structure/Function RelationshipsAg Binding
Complement Binding Site
Placental Transfer
Binding to Fc Receptors
IgG• Structure
– Monomer (7S)
IgG1, IgG2 and IgG4 IgG3
IgG• Properties
– Major serum Ig– Major Ig in extravascular spaces– Placental transfer – Does not require Ag
binding – Fixes complement Binds to Fc receptors
Phagocytes - opsonization• K cells - ADCC
IgM
• Structure– Pentamer (19S)– Extra domain
(CH4)– J chain C4
J Chain
IgM• Properties
– 3rd highest serum Ig
– First Ig made by fetus and B cells
– Fixes complement
Fixation of C1 by IgG and IgM Abs
C1r C1s
C1q C1r C1s
C1q
No activation Activation
IgM• Properties
– 3rd highest serum Ig
– First Ig made by fetus and B cells
– Fixes complement
Tail Piece
– Agglutinating Ig– Binds to Fc
receptors– B cell surface Ig
B Cell Antigen Receptor (BcR)
Ig-Ig- Ig-Ig-
IgA• Structure
– Serum - monomer– Secretions (sIgA)
• Dimer (11S)• J chain• Secretory
component
J ChainSecretory Piece
IgA• Properties
– 2nd highest serum Ig– Major secretory Ig (Mucosal or Local
Immunity)• Tears, saliva, gastric and pulmonary
secretions – Does not fix complement (unless
aggregated)– Binds to Fc receptors on some cells
IgD
• Structure– Monomer– Tail piece
Tail Piece
IgD• Properties
– 4th highest serum Ig– B cell surface Ig– Does not bind complement
IgE• Structure
– Monomer– Extra domain
(CH4)
C4
IgE• Properties
– Least common serum Ig• Binds to basophils and mast cells (Does not
require Ag binding)– Allergic reactions– Parasitic infections (Helminths)
• Binds to Fc receptor on eosinophils– Does not fix complement
Nature of Ag/Ab Reactions
• Lock and Key Concept• Non-covalent Bonds
– Hydrogen bonds– Electrostatic bonds– Van der Waal forces– Hydrophobic bonds
• Reversible
• Multiple Bonds
Source: Li, Y., Li, H., Smith-Gill, S. J., Mariuzza, R. A., Biochemistry 39, 6296, 2000
http://www.med.sc.edu:85/chime2/lyso-abfr.htm
Ab formation
Hallmarks of the Immune Response
• Self/Non-self Discrimination• Memory• Specificity
Kinetics of the Ab ResponseT-dependent Ag; 1o Response
• Lag phase• Log phase• Plateau phase• Decline phase Ag
D a y s A f t e r I m m u n i z a t i o n
A b
T i
t e r
LAG LOG DECLINEPLATEAU
Kinetics of the Ab ResponseT-dependent Ag; 2o Response
* Specificity
• Lag phase• Log phase• Plateau phase• Decline phase
1o Ag 2o Ag
D a y s A f t e r I m m u n i z a t i o n
A b
T i
t e r
Qualitative Ab Changes during 1o and 2o Responses
• Class variation– 1o - IgM– 2o - IgG, IgA or IgE 1o Ag 2o Ag
Total Ab
IgM Ab
IgG Ab
D a y s A f t e r I m m u n i z a t i o n
A b
T i
t e r
Cellular Events in 1o Response to T-dependent Ags
• Lag– Clonal selection
• Log– IgM– Class switching
• Stationary• Decline• Memory Cell
Pool
IgM
Memory Cells
IgG
1o Ag
Cellular Events in 2o Response to T-dependent Ags• Lag phase– Virgin cells– Memory cells
• Log phase– Pool size– IgG, IgA or IgE
• Stationary• Decline
– Sustained production
IgM
Memory Cells
IgG
IgG
Memory Cells
Memory Pool
Virgin B cell
Cell-Cell Interactions, T-Independent Antigens, CD5 B Cells, Cytokines
T Cell-B Cell Interactions(hapten-carrier effect)• Th cells recognize carrier, B cells
recognize hapten• Th and B cells cooperate by
interacting• Interactions are class II self-MHC
restricted
Uniqueness of B Cells• Express both immunoglobulin (Ig)
and class II MHC on cell surface• Capable of producing antibody of
same specificity as that of its surface Ig
AND• Capable of functioning as an
antigen presenting cell
Mechanism of Hapten-Carrier• Hapten recognized by Ig
receptor on B cell• Hapten-carrier endocytosed• Carrier processed and
presented on class II MHC to Th cell
• Activated Th cell produces cytokines
• Cytokines enable B cell to be activated to produce anti-hapten antibodies
CD40
Immunoglobulinreceptor
MHC II
B7 CD28
TCRT helpercell
Antigen
1. Antigen presentation toTh cell
2. B7 expressed 3. Th cell is activated
and expresses CD40 ligand,
Cytokines secreted
CD40 ligand
Cytokine
Cytokine receptor
Bcell
Bcell
Bcell
T helpercell
4. Cytokine binds to cytokine receptor,
CD40 ligand binds to CD40
Bcell
Bcell
Bcell
5. B cell activated
6. B cells proliferate, differentiate, secrete Ig
Class II MHCAPCTh cell B
cellTh cell
Bcell
Bcell
B cell takes up and presents antigen
Th cell Th cellB
cell
Th cells are primed by antigen-presenting cell
B-T cell cooperationB cells receive signals from T cells
B cells divide
Bcell
Bcell
Bcell
Bcell
Antibody formingcell
Antibody formingcell
Antibody formingcell
Bmemory
cell
B Cells In Secondary Responses
• Memory cells created during primary response
• Have high-affinity Ig receptors• Can therefore take up antigens at
much lower concentrations than other antigen presenting cells that lack Ig antigen receptors
Cytokines Non-antibody proteins acting as
mediators between cells, termed:• Monokines – mononuclear
phagocytes• Lymphokines – activated T cells,
especially helper T cells• Interleukins – abbreviated IL with
a number
Properties of Cytokines1. Produced by cells involved in both
natural and specific immunity2. Mediate and regulate immune
responses3. Secretion brief and limited
- not stored pre-formed - synthesis initiated by gene transcription - mRNA short-lived - cytokines produced as needed
Properties of Cytokines(continued)
4.Can be produced by many cell types and act on many cell types (pleiotropic)
5.Can have similar actions (redundant)
Properties of Cytokines (continued)6. Can influence synthesis of other
cytokines - produce cascades
- enhance or suppress production of
other cytokines
- exert positive or negative regulatory mechanisms for immune responses
7. Influence action of other cytokines - can be antagonistic, additive, synergistic
Properties of Cytokines (continued)8. Bind to receptors with high affinity9. Cells responding to cytokine can be: - same cell (autocrine) - nearby cell (paracrine) - distant cell by circulation
(endocrine)10.Cellular responses to cytokines are
slow, require new mRNA and protein synthesis
• Tumor Necrosis Factor-alpha (TNF-α)
• Interleukin-1 (IL-1) • Chemokines (Chemotactic
cytokines)• Type I Interferons (IFN-α and IFN-
β)• Interleukin-12 (IL-12)• Interleukin-10 (IL-10)
Mediators and Regulators of Natural Immunity
Tumor Necrosis Factor (TNF-α)• Produced by activated macrophages• Most important mediator of acute
inflammation in response to microbes, especially Gram-negative bacteria (LPS)
• Mediates recruitment of neutrophils and macrophages to site of inflammation
• Acts on hypothalamus to produce fever
• Promotes production of acute phase proteins
Interleukin-1 (IL-1)• Produced by activated
macrophages• Effects similar to those of TNF-α
Chemokines• Produced by many different
leukocytes and tissue cells• Large family of >50 substances• Recruit leukocytes to sites of
infection• Play a role in lymphocyte
trafficking
Type I Interferons (IFN-α and β)
• IFN-α a family of many proteins produced by macrophages, IFN-β a single protein produced by many cells
• Both IFNs inhibit viral replication• Both increase expression of class
I MHC• Both activate NK cells
Interleukin-12 (IL-12)• Produced by activated
macrophages and dendritic cells
• Stimulates production of IFN-γ• Induces differentiation of Th
cells to become Th1 cells• Enhances cytolytic functions of
cytotoxic T cells and NK cells
Interleukin-10 (IL-10)• Produced by activated
macrophages, Th2 cells• An inhibitory cytokine• Inhibits cytokine production by
activated macrophages• Inhibits expression of class II
MHC and costimulatory molecules on macrophages
Mediators and Regulators of Specific Immunity• Interleukin-2 (IL-2)• Interleukin-4 (IL-4)• Interleukin-5 (IL-5)• Interleukin-10 (IL-10)• Interferon-gamma (IFN-γ)
Interleukin-2 (IL-2)• Produced by Th>>Tc• Main growth factor for T cells
IL-2secretion
T cell
NKIncrease in NK
Cell activity
B cell Stimulationof division
T cellStimulation
of division and IFN gamma release (and other
mediators)
Monocyte
Activation
Interleukin-4 (IL-4)• Produced by Th2 cells• Stimulates Ig class switching to
IgE isotype• Stimulates development of Th2
cells from naïve Th cells• Promotes growth of
differentiated Th2 cells
Interleukin-5 (IL-5)• Produced by Th2 cells• Promotes growth and
differentiation of eosinophils• Activates mature eosinophils• IL-4 and IL-5 can work together
Helminths opsonized with IgE can be killed by activated eosinophils
Interleukin-10 (IL-10)• Produced by activated
macrophages, Th2 cells• Inhibits production of IFN-γ by
Th1 cells needed to activate macrophages
Interferon-gamma (IFN-γ)• Produced by Th cells >> Tc and NK
cells• Numerous functions in both natural
and specific immunity
Bcell
MacrophageNK
T cell
Th1 cell > Tc cell
NK
Many cell types Many cell types
Induction of class I and class II MHC
Increase in NK
cell activity
Differentiation,Stops cell divisionT cell activation
Weak anti-viral activity,Stops cell division,
Stops hematopoiesis
Activation
Granulocyte
ActivationEndothelial cell
ActivationIFN gammasecretion
Major Histocompatibility Complex and T Cell Receptor
Class-I expressed on all nucleated cells in man, and also on erythrocytes in mice.
Class-II expressed primarily on antigen presenting cells (dendritic cells, macrophages and B cells, etc.)
Differential expression of MHC antigens
Structure of Class I MHC
NH2
Alloantigenicsites
CHO
NH2
COOH
COOH
P
α1
α2
α3
β2
OH
Plasma membrane
Disulfide bridge
Papain cleavage
Cytoplasm
NH2
Structure of Class II MHC
Plasma membrane
Cytoplasm
CHO
CHO
CHO
NH2 NH2
COOH COOH
α1
α2 β2
β1
Aspects of MHC1.MHC molecules are membrane-
bound. Recognition by T cells requires cell-cell contact.
2.Peptide from cytosol associates with class I MHC and is recognized by Tc cells. Peptide from vesicles associates with class II MHC and is recognized by Th cells.
Aspects of MHC (continued)3.Although there is a high degree of polymorphism for a species, an individual has maximum of six different class I MHC products and only slightly more class II MHC products.
A peptide must associate with a given MHC of that individual, otherwise no immune response can occur. That is one level of control.
Aspects of MHC (continued)4.Mature T cells must have a T
cell receptor that recognizes the peptide associated with MHC. This is the second level of control.
5.Each MHC molecule has only one binding site. The different peptides a given MHC molecule can bind all bind to the same site, but only one at a time.
Structure of T Cell Receptor
CHO CHO
CHOCHO
Variable region “V”
Constant region “C”
Hinge “H”
Alphachain
Betachain
Disulfide bridge
Transmembrane region
Cytoplasmic tail
++ +
Structure of T Cell Receptor(continued)• Hypervariable regions in V
contribute to diversity of TCR• TCR recognizes portions of MHC
molecule and peptide bound in the groove
• Small population of T cells has a TCR comprised of γ and δ chains – γδ TCR specificity differs from αβ TCR
Antigen Processing and Presentation
What Does The B Cell Immunoglobulin (Ig) Receptor Recognize?
1.Proteins (conformational determinants, denatured or proteolyzed determinants)
2.Nucleic acids3.Polysaccharides4.Some lipids5.Small chemicals (haptens)
What Does the αβ T Cell Receptor (TCR) Recognize?1.Only fragments of proteins
(peptides) associated with MHC molecules on surface of cells
• Helper T cells (Th) recognize peptide associated with MHC class II molecules
• Cytotoxic T cells (Tc) recognize peptide associated with MHC class I molecules
Antigen Processing and Presentation• Fragmentation of protein into
peptides• Association of peptide with an
MHC molecule• Transport to cell surface for
expression• Different cellular pathways for
association of peptide with MHC class I and class II molecules
Class I MHC Pathway Viral protein is made
on cytoplasmicribosomes
Plasma membrane
Proteasome degrades protein topeptides
Peptide transporterprotein moves peptide into ER
MHC class I alpha and beta proteinsare made on the rER
Peptide associateswith MHC-I complex
Peptide with MHCgoes to Golgi body
Peptide passeswith MHC from Golgi
body to surface
Peptide is presentedby MHC-I to CD8cytotoxic T cell
Golgi body
rER
Globular viralprotein - intact
Class II MHC PathwayGlobular protein
Endocytosis
Protein is processed topeptides in endosome
or lysosome
Endosome
Lysosome
Fusion of endosomeand exocytic vesicle
Endoplasmic reticulum
Class II MHCSynthesis
3 chains: α,β and Ii
Golgibody
Exocytic vesicle fuseswith endosome
releasing Ii from αβ dimer
αβ Ii
Immunodominantpeptide binds
to class II MHC
Endosome fuses withplasma membrane
Peptide MHC-II complex is presentedto CD4 helper T cell
CD4 helper T cell
Points Concerning Antigen Processing and Presentation1. Location of pathogen • viruses in cytosol, MHC class I
pathway, Tc response
• extracellular bacteria, MHC class II pathway, Th2 response, Ab formation
• intracellular bacteria, MHC class II pathway, Th1 response
Points Concerning Antigen Processing and Presentation2.Peptides derived from both self
and non-self proteins can associate with MHC class I and class II molecules.
3.Chemical nature of MHC groove determines which peptides it will bind.
Self MHC Restriction• T cells recognize foreign antigen
associated with self MHC • No value for individual to have T
cells that recognize foreign antigen associated with foreign MHC
• Self MHC restriction occurs in thymus
Process of Self MHC Restriction in Thymus• T cells with TCR recognizing self MHC
molecules are retained – “positive selection”
• Retained T cells with TCR recognizing self peptide associated with self MHC are eliminated – “negative selection”
• Self MHC-restricted T cells are released
Functions of Th Cells, Th1 and Th2 Cells, Macrophages and Tc Cells. Immunoregulation
Critical Role of Th Cells in Specific Immunity
• Select effector mechanisms• Induce proliferation in appropriate effectors• Enhance functional activities of effectors
APC
Thcell
B cell
Tccell NK
AgAg
Ag
NK cell
Cytokines
Granulocyte Macrophage
Cytokines
Antigen-presenting cell
Functions of Th1 and Th2 Cells
Th1cell
Th2cell
Macrophage B cell
IFNγActivates
IL-4 IL-5
IL-10
Activates
Inhibits productionInhibits proliferation
Mast cell Eosinophil
Antibodies (including IgE)
Macrophage Macrophage
Thcell
Macrophage
Cytokines Lymphokines
CytokinesAnti-microbial functions
Anti-tumor functions
Activa
te
Invading agent Antigen presentation Activated macrophage
Central Role of Macrophages in Natural and Specific Immunity• Involved in initial defense and
antigen presentation and have effector functions
Detailed Functions of MacrophagesInflammation – Fever, Production
of: IL-6, TNF-alpha, IL-1 – act as pyrogen
ImmunitySelection of lymphocytes to be activated:IL-12 results in Th1 activationIL-4 results in Th2 activationActivation of lymphocytes:Production of IL-1Processing and presentation of antigen
Reorganization of tissues,Secretion of a variety of factors:Degradative enzymes (elastase, hyaluronidase, collagenase)Fibroblast stimulation factorsStimulation of angiogenesis
Damage to tissuesHydrolases, Hydrogen peroxide productionComplement C3aTNF alpha production
Antimicrobial actionO2–dependent production of: hydrogen peroxide, superoxide, hydroxyl radical, hypochlorous acidO2-independent production of: acid hydrolases, cationic proteins, lysozyme
Anti-tumor activity produced by:Toxic factorsHydrogen peroxideComplement C3aProteases, ArginaseNitric oxideTNF alpha
Cytolytic T (Tc) Cells• Tc exiting the thymus are pre-Tc
cells, i.e. have TCR that can recognize antigen, but are not mature and cannot kill until “armed”
• To become armed requires two signals:
1. Recognition by TCR of specific antigen associated with class I MHC, and
2. Exposure to cytokines (IL-2 and IFN-γ)
Mechanism of Arming Tc Cells
Pre-Tc cell
Tc cell
T helper cell
Class IMHC
Class II MHCAPC
1. Cell expressing class I MHC presents antigen ( )
to a pre-Tc cell
IFNIL-2
2. Antigen-presenting cell presents antigen in
association with class II MHC to Th cell3. Th cell
makes cytokines
4. Pre-Tc celldifferentiates to
functional Tc cell
5. Tc recognizes antigen onclass I MHC-expressing target cell
6. Target cellis killed
Features of Tc Killing• Antigen-specific• Requires cell-cell contact• Each Tc capable of killing many
target cells
Steps in Tc KillingTc cell
1. Tc recognizes antigen ontarget cell Target cell
Tc cell2. A lethal hit is delivered by the Tc using agents such as perforin or granzyme B
Target cell
Tc cell
3. The Tc detaches from the target cellTarget cell
4. Target cell dies by apoptosis
Target cell
Regulation of Immune Responses• Magnitude determined by balance
between the extent of lymphocyte activation and tolerance induced by an antigen
• Nature determined by specificities and functional classes of lymphocytes activated
• Regulatory mechanisms may act at the recognition, activation, or effector phases of an immune response