U3L4: Mature B cell responses II

Do TD antigens need to be able to cluster the BCR?

YES → this provides signal 1

Do TD antigens provide components that can provide signal 2

no

What provides signals 2 and 3 for B cells responding to TD antigens

2: CD40 signaling from CD4+ T cells 

3: cytokines

what is the germinal center critical for

selection of high-affinity B cells, somatic hypermutation, isotype switching, and differentiation of B cells into long-lived plasma cells or memory cells

Where do activated CD4 T cells and B cells migrate

toward one another to interact at the border of the T cell zone and B cell follicle

what does the formation of the extrafollicular focus lead to?

B cell proliferation, class-switching, and differentiation into short lived plasma cells → provides a quick burst of antibody production

How do CD4 T cells leads to the formation of the germinal center

some CD4 T cells in the extrafollicular focus develop into Tfh cells and migrate into B cell follicles to orchestrate GC reaction

what leads to the formation of the extrafollicular focus

activated CD4 T cells upregulate CXCR5 and downregulate CCR7 and some activated B cells upregulate CCR7 —> causes them to migrate to the T:B cell border

How to CD4 cells provide help to activated B cells

• CD40L on CD4 provides co-stimulation through CD40 on B cell (proliferation and differentiation)

• CD4 secrete cytokines like IL-21 → stimulate B cell responses

Tfh1 transcription factor

T-bet

Tfh2 transcription factor

GATA3

Tfh3 transcription factor

RORyt

initiatl Tfh transcription factor

Stat3

Tfh1 cause B cells to release

IgG

Tfh2 cells cause B cells to release

IgE

Tfh3 cells cause B cells to release 

IgA

What cytokines cause T cells to differentiate into pre-Tfh cells

IL-6 and IL-21

what happens to pre-Tfh cells after they receive IL-6 and IL-21 interaction

upregulate CSCR5 and interact with B cells that the T:B border through interaction with the ICOS receptor on T cells → Tfh diffferentiation

What happens after pre-Tfh cells differentiate into Tfh cells

they migrate to the light zone of the GC reaction and form stable conjugates with GCB cells in a SAP dependent manner → GC Tfh cells provide help signals through CD40L, IL04 and IL-21

What are the key processes that occur in the germinal center

• proliferation

• somatic hypermutation

• isotype switching

  • terminal differentiation

somatic hypermutation

mutational and selection process designed to improve affinity of antibodies for antigen (requires continued signals 2 and 3 from Tfh cells)

What does isotype switching in the GC require

continued signals 2 and 3 from Tfh cells

what is terminal differentiaton in the GC

formation of long-lived antibody-secreting cells, or plasma cells; generation of memory B cells

structure of a GC (outside in)

1. mantle zone

2. light zone

3. dark zone

*light zone and dark zone make up the actual germinal center

What occurs in the light zone of a GC

B cells undergo affinity maturation (receive help from Tfh cells and undergo class switching)`

What occurs in the dark zone of the germinal center

B cell proliferation and somatic hyper mutation

somatic hypermutation

V segment generated by VDJ recombination is fixed but can become mutated by somatic hypermutation (occurs for both the heavy and the light chains); point mutations that can alter the affinity of antibodies for their target

AID

crucial enzyme for both somatic hypermuttion and class switch recombination in activated B cells

what exactly does AID do in activated B cells

converts C → U!

downstream effects of AID conversion of C→U

• mismatch repair and BAse excision repair mecanisms that can results in random insertions or mutations at A:T base pairs

• APE1 can generate some nicks in the nontemplate strand as well as some nicks on the template strand → ds breaks are important for class switch recombinases

what is isotype switching initiated by?

selection and opening of “switch” regions in the antibody heavy chain gene → these regions are made more accessible by CD40L interactions (CD4-provided signal 2) and the specific regions selected are dependent on the cytokines present (signal 3)

how are follicular dendritic cells involved in B cell selection and affinity maturation?

FDCs express complement receptors as well as FC receptors and display antigen to B cells → only B cells with high affinity receptors recognize and capture the antigen from FDCs and then in turn process and present peptide to Tfh cell

where does affinity maturation occur?

in the light zone of the GC

Where does somatic hyper mutation occur

in the dark zone of the GC

what are long-lived plasma cells specialized to produce

antibody molecules

what are long-lived plasma cells derived from?

B cells that have undergone isotype switching and affinity maturation and produce high affinity Abs

what is a significant feature of long-lived plasma cells

down regulate membrane bound BCR and MHC II → these cells are thought to no longer interact directly with antigen or present peptide to CD4 T cells

where are long-lived plasma cells initially found, and where do they migrate to?

initially found in lymphoid organs proximal to the germinal center → eventually migrate to the bone marrow where they survive for a long time and consistently secrete antibody

where do memory B cells localize

circulate and distribute throughout the secondary lymphoid organs

can some memory B cells launch rapid plasma cell differentiation following secondary response?

YES! IgG memory cells preferentially differentiate into plasma cells, and IgM memory B cells preferentially seed the GC and interact with Tfh cells