Test 4 Immuno - B Cells and Antibodies

Professional antigen presenting cells

They are dendritic cells, B cells and macrophages

They express high levels of MHC I and II molecules

They express co-stimulatory molecules (e.g. B7)

They are differently distributed in lymphoid tissues

(LT).

Dendritic cells are particularly important for

initiation of T cell activation in lymphoid tissues.

Only these cells express B7

Different features of the three types of APC

Macrophages are good at presenting extracellular bacteria

DCs present bacterial and viral antigens

B cells can efficiently present soluble antigens

Types of B cells

B1 B cell

– Produced in fetus

– 5% of total B cell

population

– 95% of B cell in

peritoneum

– No memory

• No recall

– Broad specificity of

antibody response

– Low affinity

B2 B cell

– Produced in bone

marrow

– Generate memory

– Highly specific

– Require T cell help

– Classic B cell that we

have been talking about

in class

– Recall response

thymus dependent antigen

T-independent Ag

• Mostly IgM

• Lower affinity

• Do NOT require T cell help

• Can cross-link BCR

– Repeating epitopes

• LPS

• Can generate two signals

– BCR and TLR

native antigen is recognized by the BCR and C3b (compliment) (also CD21/19)

this gives two signals → key for internalization

the antigen goes through the endosomal pathway with MHC 2 and is then presented to the T Cell

helper t cell ?

thymus independent antigen

-dependent Ag

• Different isotypes

• SHM high affinity

• Require “help” co-

stimulation from T cells

• T cell help linked to BCR

response

uses a polyvalent antigen so that the BCR can cluster

mostly IGM , low affinity

Two types of Thymus-Independent Ags

TI-1 and TI-2

TI-1 - three different ways

LPS is an TI-1 antigen that activates LPS specific b cells, creates IgM antibodies for LPS

LPS also activates b cells specific for other bacterial surface antigens in a T-independent manner, creates IgM antibodies for other components of the bacterial cell surface

bacterial DNA is a TI-1 antigen that activates b cells specific for bacterial surface antigen, creates IgM antibodies for bacterial surface antigen

TI-2

bacterium binds to TI-2 antigen and b cell co receptor that activates cell

Most B cell Ags are…

T cell dependent

What is meant by ‘B cell activation’?

• Go -> G1 of cell cycle (increase in size) → becomes a blast

• upregulate

– MHC class II bc need to communicate with t cell

– costimulatory molecules (B7-2) to tell t cell to survive

– adhesion molecules (ICAM1) to make sticky for t cell

– cytokine receptors (IL-2R) to tell b cell what to do

• migrate to outer T zone

– altered response to chemokines

• become receptive to T cell help

– survival

lymph node structure

primary lymphoid follicle → mostly b cells

paracortical area → mostly t cells

medullary cords → macro and plasma cells

have follicles and germinary centers

DCs are kinda everywhere

B cells detect Ags in native (soluble?) form via BCR

• There is no need for antigen presentation

• Can bind intact bacteria, viruses etc…

• Can bind pieces of bacteria, viruses etc…

How do B cells get activated (T dependent Ag)

naive CD4 T cells are activated by antigens presented by DCs in lymph node

naive b cells activated by antigen are trapped in the t cell zone of the lymph node

antigen activated b cells present antigen to helper t cells forming conjugate interactions and conjugate pairs

B cell signaling requires …

crosslinking, T cell help or costimulation

clustering of antigen receptors allows receptor-associated kinases to phosphorylate the ITAMs

syk binds to doubly phosphorated ITAMs and is activated on binding → creates changes in gene expression in nucleus

the bcr and co receptor can cooperate in b cell activation → creates changes in gene expression in nucleus

specific t cells are activated by antigens that may reside within the viral particle, b cell that recognizes a surface epitope of a virus can process and present other antigen epitopes

viral specific Tfh cells can provide help to b cells that recognized a linked epitope → provides a robust antibody response

How and where do B cells interact with

T cells

then…

before activation resting b cells express CXCR5 and reside in follicles and T cells express CCR7 and reside in t cell zones

activated b cells induce CCR7 and EB12 and t cells induce CXCR5 and both cells migrate to follicular and inter follicular regions

interactions with t cells sustain EB12 expression on b cells which move to outer follicular and interfollicular regions

some b cells migrate to form a primary focus and differentiate into plasmablasts while some t cells become Tfh cells

Help from T cells drives B cells to

proliferate and secrete IgM

Clonal expansion

antigen recognition by Tfh cells induces signals that activate b cells

b cell proliferation generates plasma blasts that form the primary focus

further differentiation can proceed to the germinal center, resting memory cells, or antibody secreting plasma cells

Some of the B cell clones form a …

germinal center within the lymph node and continue to proliferate

dc, b cell, Tfh → CD40 interaction this promotes b cell differentiation and drives germinal center rxn

middle is dark zone, t cells surround outside

germinal b cells come in and replicate (clonal expansion)

in dark zone centroblasts replicate and go through somatic hypermutation

then go to light zone and are called centrocytes, interact withfollicular DC and Tfh

Role of CD40 in B cell activation

• TCR triggering up-regulates

CD40L on T cell

• CD40 signaling promotes B

cell activation

• CD40L-deficiency = 'hyper-

IgM syndrome' (no isotype

switching, no Germinal

Centers)

• CD40 signaling also

important in DC, MØ

function

Germinal Center Organization

Germinal Centers

Function: to generate B cells that produce antibodies with increased

affinity for the inducing antigen

=> affinity maturation

Germinal Center Reaction:

Activated B cells give rise to Centroblasts

- localize in follicle, undergo rapid cell division and turn on machinery that

causes somatic mutation in V-regions

Centroblasts give rise to Centrocytes

- migrate to the FDC-rich region of the Germinal Center

- survival is dependent on interaction with FDC-bound Ag and presentation of

Ag to T cells

- centrocytes that successfully compete to bind antigen (e.g. by having higher

affinity BCR) and to receive T cell help are selected and may differentiate

into long-lived plasma cells or memory B cells

somatic hypermutation is an iterative process….

cells

that survive return to the dark zone

to continue to replicate and class

switch their antibodies

FDCs have low ability to…

internalize

antigens

SHM occurs in the GC: make the best…

antibody possible

Balance of allo-reactivity with auto-reactivity…

Makes sure that high affinity interactions result in

cell death

Cytokines dictate the fate of effector B cells

antigen selected centrocytes mature under the influence of an IL-10 secreting helper t cell, then centrocytes differentiate into plasma cells and make antibodies that fight the current infection

antigen selected centrocytes mature under the influence of an IL-4 secreting helper t cell, then centrocytes differentiate into memory b cells

tfh cells help b cells become plasma cells

activated follicular b cells express CCR7 and migrate to T/B boundary

activated t cells express CXCR5 and reduce CCR7 expression and migrate towards b cells

b cells receive t cell help and differentiate into memory b cells or plasmablasts

IL-21 is critical for generation of plasma and memory cells, FHC make it

Isotype function and location IgA, IgG 1/3, IgM

IgA, IgG1/3 → fetal protection

IgM → compliment

IgGs → good for neutralizing (blocking stuff from entering the cell

somatic hypermutation

AID mutates BCR in variable region to alter specificity of bcr

can be good, bad, or neutral effect

after goes through a selection process via centrocytes and migrate to light zone?

survival is dependent on FDC, b cells interact with FDC and F helper cell if binds to both = strongest signal and does not die

can go back to dark zone to undergo more mutation so it can become better and divide

AID can also mutate constant region and do class switch recombination