PHYSIOLOGY OF ANTIBODY SYNTHESIS

“It is not the strongest or the most intelligent who will survive but those who can best manage change.”

Charles Darwin

SpecificityDiversityMemory

Hematopoetic stem cell CD34+

Pro B cell

Heavy chain recombination starts

Pre B cell

Surface IgM+

Exposed to central tolerance

Transitional cell

Stage1

Bcr crosslinking leads to death

Receptor of BAFF & APRIL +ve

Transitional stage 2

Bcr crosslinking leads to proliferation

IgD+ve

Naïve b cell Marginal bcell

Mature bcell

Plasmacytes / memory bcells

B CELL ONTOGENY

IMMUNOGLOBULIN GENE REARRANGEMENT

This can occur when hairpin loop structures are created between the 2 strands of the DNA following cleavage at the RSS, & a subseq. cleavage of 1 strand creates an overhang which acts as a Template for the addn. of nucleotides, creating a Palindromic sequence

Nucleotides can be added in a Nontemplated fashion (N-region

diversity, indicated by the red nucleotides) by the enzyme terminal

deoxynucleotidyl transferase (TdT)

P-ELEMENTS AND N-REGION DIVERSITY

ALLELIC EXCLUSION

Splicing mechanism for the switch from the membrane to the secreted form of IgM.

• Alt. processing determines whether a secreted or membrane-bound form of the μheavy chain is produced. If transcription termination or cleavage occurs in the intron between Cμ4 & M secreted form is produced.

• If transcription continues through the membrane exons, then Cμ4 can be spliced to the M seq.The hydrophobic sequence encoded by the exons M, & M2 then anchors the receptor IgM to the membrane.

CD79A/ CD79B

This consists of two glycoprotein chains called Ig-a (CD79a) & Ig-ẞ

(CD79b) . Both Ig-a & Ig-ẞ have an extracellular immunoglobulin-type domain, but it is their C-terminal cytoplasmic domains which are obligatory for signaling & which become phosphorylated on cell

activation by antigen-induced cross-linking of the BCR, an event also

associated with rapid Ca2+ mobilization. Tyrosine-containing

structural motifs (immunoreceptortyrosine-based activation motifs,

ITAMs) are present in the cytoplasmic domains of the Ig-a/Ig-ẞ heterodimer

& it is these that undergo phosphorylation by tyrosine kinases

ITAM & ITIM (IMMUNORECEPTOR TYROSINE ACTIV./INH. MOTIF)

SELECTION IN BONE MARROW

CLOSE ENCOUNTER WITH THE OTHER KIND

BAFF

The cytokine milieu surrounding the B cell is diverse & spatially and temporarily

regulated. Although B cells are modulated by multiple cytokines, in recent years 2

members of the TNF family, BAFF & APRIL, have emerged as key survival factors, particularly at 2 regulatory points in

development and differentiation: the transition from an immature to a naïve B cell in the periphery & the survival of the newly

produced plasma cells. BAFF & APRIL are proteins produced by cells of innate response such as macrophages and dendritic cells, as

well as stromal cells, and are present as membrane-bound proteins or soluble

trimers. They have 3 known receptors (BAFF-R, TACI, and BCMA) that are present on the membrane of B cells from the T2 stage to their final differentiation to plasma cells

EVENTS INSIDE LYMPH NODE

AID dependent mutator complex

DNA replicationerror

ATG ... GGC TAT GCT CAC CGT ...

V CH1

T ...GGC, CCT...

Met ... Gly Tyr Ala His Arg ... ...Gly, Pro...

AID = Activation Induced Deaminase (-> deaminates Cytosine on Uracil

-> repair proteins then come in and this leads to error prone repair)

-> mutations are actively induced in the V-regions of the

antibody heavy and light chain genes

Val

SOMATIC MUTATION OF IG V REGION IN GC B CELL

ATG ... GGC TAT GTT CAC CGT ...

Met ... Gly Tyr Val His Arg ...

T

Val

...GGC, CCT...

...Gly, Pro...

V CH1

-> now encodes antibody molecule with slightly altered antigen

binding site

-> sometimes, by chance, this site will have an improved ability

to bind the inducing antigen (i.e. a higher affinity)

SOMATIC MUTATION OF IG V REGION IN GC

B CELL

An antigen eye view of immunoglobulin paratope

before & after SHM

THE CHEMOKINE TRAIL….

CLASS SWITCH RECOMBINATION

Class switch recombination is

achieved by a recombination process

which utilizes the specialized switch

sequences ( ) and leads to a loss of the intervening DNA loop

(μ, δ and γ3).

1ARY & 2NDARY ANTIBODY RESPONSE

THE CELL THAT DOES’NT FORGET

Fanum in 1847 described a measles epidemic on the Faroe Islands in the

previous year in which almost the entire population suffered from

infection except for a few old people who had been infected 65

years earlier!!

While this evidence favors the long half-life hypothesis, memory function of B-cells transferred to an irradiated syngeneic recipient is lost within a month unless antigen is given or the donor is transgenic for the bcl-2 gene (remember that signals in the germinal center which prevent apoptosis of centrocytic B-cells also upregulate bcl-2 expression). It is envisaged that B-cell memory is a dynamic state in which survival of the memory cells is maintained by recurrent signals from follicular dendritic cells in the germinal centers, the only long-term repository of antigen.

memory B cells ingest antigen and express Peptide MHC class II fragments. After antigen presentation of peptide to helper T cells, memory B cells undergo expansion and may differentiate to plasma cells.

Memory cells respond to antigen much faster, require lower amounts of antigen, and can even be induced in its absence by soluble mediators such as IL-2 or IL-15, in part because the BCR is already localized to lipid rafts.

in humans they are distinguished by the presence of the marker CD27

The CD40-CD40L interaction contributes to directing GC B cells to mature into long-lived memory B cells. The exact life span of memory B cells is unknown. It has been postulated that these B cells either persist throughout the lifetime of the host or are renewed constantly through either nonspecific or antigen specific stimulation.

MEMORY B CELL

MEMORY B CELL

IGA AND IT’S ROLE IN MUCOSAL IMMUNITY

The mechanism of IgA secretion at which drives the transport of IgA dimers to the

mucosal surface .

TRANSCYTOSIS OF IgG

BACTERIAL INFECTION

EVASION OF HUMORAL IMMUNITY BY VIRUSES

Changing antigens

Influenza (antigenic drift & shift), rhinovirus (protected site)

Mutation can produce antagonistic T-cell epitopes

HBV(Mutations which modify residues critical for recognition by MHC or TCR may generate partial agonists that can induce a profound and long-lasting state of T-cell anergy)

Antigen processing

EBV(EBNA- 1 inhibits Proteasome mediated processing of the virus), HSV(Peptide binding to TAP is prevented by ICP47), CMV(US6 prevents peptide transport through the TAP pore)

Sabotaging of humoral immune response

Pox viruses (via VCP) & HSV 1 (via surface glycoprotein), HHV6& measles(use cd55 as receptor), echo & coxsackie ( use cd46 as receptor), (HSV) types 1 and 2, coronavirus (bind immunoglobulin by Fc receptors)

ANTIBODY DEPENDENT ENHANCEMENT

DENGUEHIV

INFLUENZAENTEROVIRUS/ DIABETES

INTESTINAL PARASITES- A DAILY WAR

The parasite is 1st damaged by IgG antibody passing into the gut lumen, perhaps as a

consequence of IgE-mediated inflammation & possibly aided by accessory ADCC cells. Cytokines

released by antigen-specific triggering of Tcells stimulate

proliferation of goblet cells and secretion of mucous materials,

which coat the damaged worm & facilitate its expulsion from the

body by increased gut motility induced by mast cell mediators, such as LT-D4, &

diarrhea resulting from inhibition of gluc dependent Na+ absorption

by mast cell-derived histamine &PGE.

CONCEPT OF CONJUGATED VACCINE

CONTD.

IVIG

TOLERANCE OF ADAPTIVE IMMUNITY

AUTOIMMUNITY & BCELL

CHRONIC INFLAMMATION

leads to the release of soluble mediators such as the chemokines CCL21&

CXCL12, which recruit lymphocytes

These cells, once activated, secrete cytokines such as lymphotoxin that

act in a paracrine manner & contribute to the organization of a

GC-like structure that includes a dark and light zone with local

induction of AID

In contrast to the GC of the secondary lymphoid organs,

these structures are not encapsulated

The B cells in these structures, therefore, are continuously exposed both to the local antigens that might be absent from the lymphoid organs & to the inflammatory

μenvironment that may facilitate bypass of the regula-tory points in B cell differentiation, hence contributing to a potential autoimmune bias in these sites

TERTIARY FOLLICLES & AUTOIMMUNITY

THE IDIOTYPIC NETWORK THEORY