B cell Activation and Germinal Center Responses - 4

8.3 and 8,4 - Immunology

8.3

B cell activation

Cytoplasmic tails of the BCR complex are

phosphorylated by src family kinases

Phosphorylated ITAMS recruit

kinase Syk, which phosphorylates BLNK

B cell activation requires 3 critical transcription factors

NFAT, AP-1, and NF-KB by BLNK signalosome

• increases transcription

TI antigens induce

earlies B cell response without requiring T cell help

Thymus independent (TI) antigens

• typically carbs (not proteins)

• Do NOT prime T cell responses

• Detected during first phases of infection

DCs secrete IL-6 in SLOs to

induce differentiation of T follicular helper cells (Tfh)

• Tfh cells migrate to lymphoid follicles to provide T cell help to naive B cells

Thymus-Dependent antigens

induce endocytosis and are processed for presentation to Tfh cells

TD antigens induce

enhanced B cell activation via T cell help

• Naive B cell and Tfh cell exchange signals that initiate B cell activation in a process called T cell help

CD40 provides key

transcriptional activation in naive B cells recognizing TD antigens

T cell help also involves secretion 

of key Tfh–derived cytokines

• Tfh release the signature cytokine IL21

• STAT3 induces genes promoting proliferation + differentiation

• Other Tfh derived cytokines selectively induce class-switch recombination

Tfh-derived cytokines determine

class switching in activated B cells

Response Type 1 - IFN-gamma targets

targets IgG isotype

Response Type 2 - IL-4 targets

targets IgE isotype

Response Mucosal - with IL-21 and TGF-beta

targets IgA isotype

Summary 8.3

B cell activation occurs when BCRs are crosslinked

• Src/Syk family kinases recruit downstream signals including transcription factors

• Source of NFkB is variable depending on timing and type of antigen

Different types of antigens recruit different B cell responses

• TI antigens are likely carbohydrates and do not recruit T cell help

• TD antigens enhance B cell activation using additional signals

Summary 8.3 pt. 2

Tfh cells provide critical help for the B cell to develop enhanced responses

• Tfh cells influence the survival and proliferation of B cells

• Tfh cells secrete cytokines to polarize the type of immunoglobulin

8.4

Germinal Center Responses

Specialized chemokine gradients guide naïve lymphocytes to

appropriate SLO compartments

• primary follicle - naive B cell (CXCR5 with CXCL13)

• paracortex - naive T cell (CCR7 with CCL19/21)

Activated B and Tfh cells find each other

at the T/B boundary

Upon activation, B cells

downregulate CXCR5, and upregulate CCR7

T cell activation results in opposite

downregulation of CCR7 and upregulation of CXCR5

B cell interactions with Tfh cells result in two major outcomes

Leave follicle or remain in follicle

if B cell leaves follicle

no sustained Tfh interations

• Primary focus

  • B cells become plasmablasts

  • produce larger quantities of IgM

  • Do not undergo CSR

  • Short lived (around 10 days)

if B cell remain in follicle

sustained Tfh interactions

• Germinal center (GC)

  • B cells become GC B cells

  • Can undergo CSR

  • can undergo somatic hypermutation

  • can differentiate into memory cells

The germinal center is comprised of different zones

where antibody responses are fine tuned

Mantle zone

Peripheral edges of follicle

• Contains resting B cells (not activated, not proliferating)

Dark Zone

Contains rapidly proliferating B cells (centroblasts) that express CXCR4 → attracted to CXCL12 produced in dark zone

• Densely packed region of proliferating cells (appears dark)

• Region where immunoglobulin genes are mutated

Light Zone

Contains TFH cells

• Contains slower proliferating B cells (centrocytes) that lose CXCR4 and express CXCR5 → attracted to CXCL13 in light zone

• Region where B cells that have undergone immunoglobulin mutation interact with TFH to test reactivity of new Ig

• Cells can return to dark zone for additional rounds of mutation

B cells in dark zones activate

AID

• result in Ig V regions = somatic hypermutation

Activation-induced deaminase (AID) initiates somatic hypermutation (SHM) by converting cytosine to uracil in the DNA of immunoglobulin (Ig) variable (V) regions, leading to high-rate nucleotide substitutions. This process, occurring in germinal center B cells, increases antibody affinity for antigens, enabling affinity maturation.

purpose of somatic hypermutation

to increase antibody affinity

somatic hypermutation and increase antibody affinity

Initially the BCR of a B cell may be specific enough for an antigen to bind successfully, but weakly

• the Antibodies from this B cell may not be very effective

Somatic hypermutation

results in new clones whose Ig binds the antigen more tightly, resulting in antibodies more likely to promote effector functions

Mutated B cells are “tested” in light cells

using Tfh cells

• only high affinity clones survive (affinity maturation)

Low antigen affinity

Germinal center B cell with low affinity mutated surface Ig

• BCR is not cross-linked and the B cell cannot present Ag to T cells

High antigen affinity

Germinal center B cell with high affinity mutated surface Ig

• T cell help and BCR cross linking sustains B cell proliferation and maturation

Germinal center responses yield

higher quality antibodies than those expressed during early infection

TI antigen-responsive B cells

TD antigen-responsive plasmablasts

Rapid production of low affinity IgM

• Important initial protection but not completely effective

TD antigen-responsive GC B cells

Slower production of higher affinity antibodies

• Class switch recombination unlocks new effector functions

• Differentiation into memory cells can provide durable protection

Summary 8.4

Tfh and B cells must physically interact to provide help

• Both B and T cells move toward the boundaries of follicles to find one another

• Availability of this interaction is dependent on chemokine signaling

B cell responses are dynamically influenced by T cell help

• Without T cell help, B cells form primary foci that secrete low affinity IgM

• Tfh signals help B cells form high affinity antibodies of specific isotype

Summary 8.4 pt 2

Germinal center reactions are a combined effort to produce high affinity antibodies

• Dark zone centroblasts are undergoing somatic hypermutation in V regions

• Light zone centrocytes interact with Tfh to receive survival signals