B cell activation and antibody production

Question 1

Activation results from

Answer 1

antigen binding to antigen specific membrane IgM and IgD on mature, naïve B cells

Question 2

A single B cell can generate

Answer 2

5000 plasma cells that produce > 1012 Ab

molecules per day (1,000,000,000,000)

Question 3

The type and amount of Ab produced depends on:

Answer 3

Antigen driving response
T cell involvement
Prior antigen exposure
Anatomic site of infection

Question 4

Antibody responses to peptide antigen require

Answer 4

internalization, processing

and presentation to CD4+ helper T lymphocytes and TH can then costimulate and further activate B cells

Question 5

what Ab responses do not require T cell costimulation

Answer 5

Ab response to polysaccharides, some lipids and NA with repeating, multivalent, non-protein determinants do not require T cell costimulation for activation

• May activate many BCR simultaneously
• involves B1 cells or marginal zoneB cells and short lived plasma cells

Question 6

typical in helper T –

dependent humoral responses

Answer 6

Isotype switching and affinity maturation

Question 7

Most mature naïve B cells are

Answer 7

follicular B cells located in lymph nodes
–>Enter follicles, or B cell zones, of lymph nodes
Guided by cytokines

Question 8

Antigen Delivery

many forms and routes

Answer 8

1. Most antigens from tissues arrive in lymph nodes through lymphatic system. (small, soluble antigens). they enter B cell zone via conduits
2. Subcapsular macrophages in lymph nodes capture large microbes and antigen-antibody complexes and deliver to follicles
3. Many large antigens enter the lymph node from lymphatics and are not captured by subcapsular macrophages and are too large to enter conduits so they are Captured by resident dendritic cells and transported to follicles for B cell activation
4. Antigens in immune complexes may bind to complement receptors on marginal zone B cells
   (These cells transfer antigen to follicular B cells)
   5.Blood-borne microbes captured
   by plasmacytoid dendritic cells
   are transported to spleen for
   potential delivery to marginal zone B cells
5. Polysaccharide antigens can be
   captured by macrophages in
   marginal zone (spleen) and
   transferred to Marginal zone B cells

Question 9

Antigen binding to BCR results in two main processes

Answer 9

1. Signal transduction for activation

2. Receptor internalizations of the bound antigen =Antigen processing and presentation to T helper cells

Question 10

Antigen binding can initiate activation but Usually requires additional stimuli including to allow proliferation and differentiation of naive B cells

Answer 10

Complement proteins
pattern recognition receptors
and in the case of protein antigens, helper T cells

Question 11

complement activation of B cells. the plasma protein is activated by 1 of 2 ways

Answer 11

-usually seen with microbes which activate this system with direct binding in the absence of antibodies by the alternative and lectin pathways
or
-in the presence of antibodies (Antigen-Ab complex) by the classical pathway

• Protein antigens can be bound by pre-
  existing Abs, or Abs produced early in
  the response

Question 12

complement activation results in complement fragments being generated that bind to the microbes

Answer 12

Complement activation cleaves complement proteins
C3 (key component) cleavage = C3b and C3a
C3b binds to microbe or antigen-Ab complex.
C3b cleaved to C3d which remains on microbial surface
-the C3d fragment on antigen is recognized by the type 2 complement receptor on the B cells called CR2(aka CD21), which functions as a coreceptor for B cells as it enhances the strength of B cell signalling

Question 13

C3d-antigen or C3d-antigen-Ab complex

bind to B cells through

Answer 13

Ig AND CR2

Enhances activation signals

Question 14

Myeloid cells activated through PRR

help B cell activation indirectly in two ways:

Answer 14

Dendritic cells contribute to T
cell activation
and
Secrete cytokines to induce T-
independent activation

Question 15

Antigen-mediated crosslinking of BCR by different types of antigens induces distinct cellular events

Answer 15

Multivalent antigens initiate B cell proliferation and differentiation
vs
Protein antigens prepare B cells for T interaction

Question 16

Some activated B cells differentiate into short-lived plasma cells and the Survival of B cells is promoted by

Answer 16

the production of anti-apoptotic proteins (Bcl-2)

Question 17

B cell activation by antigen binding results in:

Answer 17

Increases MHC II expression
and
B7 costimulator expression
-Activated B cells express more receptors for T cell derived cytokines and may move from follicles to T cell location
*therefore antigen stimulated B cells are more efficient activators of helper T cells compared to naive B cells

Question 18

protein antigens require processing by B cells and present them to helper T cells via MHC 11
-both TCR and BCR recognize protein antigen in peripheral lymphoid organs and then come together to initiate humoral immune response:

Answer 18

Antigen taken up by DC and presented to naïve T cells, Helper T cell activation results and then Activated helper T migrate toward follicle (B cell zone) and express CD40L (T cells also secrete cytokines that promote isotope switching). B cells in follicle are activated by antigen (soluble or displayed). B cells process antigen and migrate toward T cell zone and then
activated Helper T cells and B cells interact and B cells become activated. B cells undergo low level isotype switching, somatic mutation and generate short-lived plasma cells. Some activated T cells differentiate into T follicule cells. Activated B cells and T cells migrate back into the follicle and form germinal centres where the more specialized antibody responses are induced and include:

Extensive B cell proliferation
Isotype switching 
Somatic mutation
Affinity maturation
Memory B cell generation
Induction of long-lived plasma cells that migrate to bone marrow

Question 19

A protein antigen that elicits a T-dependent B cell response uses two epitopes to fully activate B cells

Answer 19

1. Surface epitope on native protein
   recognized by BCR
   2.Linear internal epitope recognized by TCR

Question 20

Germinal Centre in lymphoid follicles is site of:

Answer 20

Affinity maturation
Isotype switching
Generation of memory B cells
Long-lived plasma cell differentiation

Question 21

describe the more slowly developing but more effective germinal center response

Answer 21

activation mediated by B-T cell interaction

-TFH cells begin to develop 4-7 days after antigen exposure (Triggered by B cells)

Question 22

Some progeny of IgD and IgM expression B cells undergo isotype switching following CD40L and cytokine activation

Answer 22

IFN-γ induces switching to IgG
and
IL-4 induces switching to IgE
(cytokines from activated helper T cells)

Question 23

Isotype switching involves

Answer 23

the process called switch recombination in which the Ig heavy chain in B cells is cut and recombined such that a previously formed VDJ exon that encodes the V domain is placed adjacent to a downstream C region, and the intervening DNA is deleted (old heavy
chain constant region is replaced but the variable region remains the same)

Question 24

Affinity Maturation

Answer 24

result of somatic mutations of Ig genes followed by
selective survival of B cells with the highest affinities for the antigen

(Antigen recognition stimulates Bcl-2 expression)
(leads to increased affinity of antibodies for a particular antigen)->-As the process progresses the affinities get higher and higher
-Generates Abs that more efficiently neutralize and eliminate microbes
Only generated in T-dependent B cell activation

Question 25

Somatic hypermutation

Answer 25

=Proliferating germinal center B cells undergo point mutations in Ig V genes at 1/1000 V gene bp/cell division

• 1000x higher than spontaneous rate
• Can result in differences of up to 10 amino acids
• the presence of mutations is correlated with increasing affinities of the antibodies for the antigen that induced the response

Question 26

Two main types of plasma cells`

Answer 26

1. Short-lived:
   - T cell-independent activation or early T-cell dependent
   - Found in secondary lymphoid organs and peripheral non-lymphoid tissues
2. Long-lived:
   - T cell-dependent activation
   - Respond to protein antigens
   - Plasmablasts in circulation travel to bone marrow for terminal differentiation
   - Some survive for life of the host

Question 27

Generation of Memory B cells

Answer 27

• Generated during germinal centre reaction
• Capable of making rapid responses to subsequent infection
• Generated in T-dependent immune responses
• Express high levels of Bcl-2
• Some in lymphoid organ that generated them and some will circulate
• Effective anti-microbial vaccines will only be generated if they are capable of stimulating helper T cells

Question 28

Conjugate vaccine

Answer 28

Can conjugate a polysaccharide bacterial coat component with a foreign protein to get strong Ab response to polysaccharide
-activates helper T cells

Question 29

T Cell-independent Ab Reponses

Answer 29

Response to polysaccharide, lipid, NA…… non-protein
Generally generate low-affinity Ab
Mostly IgM
Limited isotype switching

Question 30

Marginal zone vs B-1 B cell subsets

Answer 30

1. Marginal zone B cells respond mainly to polysaccharides
   Differentiate into short-lived IgM producing plasma cells
2. B-1 cells mainly respond in peritoneum and mucosal sites
   * TI Ab responses can be initiated in spleen, bone marrow, peritoneal cavity and mucosal sites

Question 31

*Best TI antigens are

Answer 31

polysaccharides, glycolipids and NA
->Not processed and presented on MHC II
->Multivalent (Repeated identical epitopes Induce maximal cross-linking of BCR)
-Many TI antigens also active complement system by
alternative pathway (Lots of C3d)
-Also sometimes activate TLR pathway

Question 32

Antibody Feedback

Answer 32

regulation of humoral immune responses by Fc receptors
-secreted antibodies inhibit continuing B cell activation by forming antigen-antibody complexes that simultaneously bind to antigen receptors and inhibitory Fcy receptors on antigen-specific B cells