Humoral Immunity Flashcards
What are the receptors you would expect to see on a mature, naive B cell?
- BCR: IgM, IgD, Igα, Igβ
- Co-BCR: CD19, CD81, CR2 (CD21)
- HLA II
- CD40
- CD20
What are the 3 types of B cells?
- follicular B-2 cells: the majority, re-circulating B cells
- marginal B-2 cells: reside in the spleen, blood-borne polysaccharide Ags
- B-1 cells: mucosa, limited Ag specificity
How do B cells migrate through lymph and lymph nodes?
- enter lymph nodes through high endothelium venules (HEV) in the cortex
- slow down: L-selectin, CXCR5 (follilcular chemokine receptor); stable arrest: LFA-1
- migrate to primary follicles to sample Ags and receive survival signals form follicular DC’s (FDC’s)
- if there are too many B cells and not enough FDC’s, naive B cells will die within a week in absence of Ag
- if no antigen is encountered, the B cell leaves and travels to next node
- particulate Ag filters through the lymph draining through the sinuses of lymphoid tissues
- these cells retain Ags in their follicles
- not hematopoietic and do not process antigen or express MHC/HLA II, NOT AN APC
- express receptors for C3b (CR1) and IgG (FcγR)
- concentrate unprocessed opsonized Ag for naive B cells to sample for activation and activated B cells selecting of highest affinity Abs
- secrete cytokines for B cell recruitment, survival, and differentiation
follicular dendritic cells (FDC’s)
What is the first signal of B cell activation?
recognition of antigen by BCR specific for that antigen epitope
What are the two types of second signals that occur for B cell activation?
- T cell dependent: Th responses for second signal for protein antigens
- T cell independent: second signal from overwhelming BCR linkage, mainly long repeating epitopes like lipids and/or polysaccharides
What are the 3 different pathways that occur to initiate the first signal in B cell activation?
- crosslinking of several BCR’s with signaling through Igα and Igβ ITAMs
- crosslinking of BCR with co-BCR with signaling through Igα and Igβ ITAMs and CR2 and CD19 signaling motifs
- crosslinking of BCR with TLRs with signaling through Igα and Igβ ITAMs and TLR signaling motifs
(all 3 can happen simultaneously)
What is the general process of first signal crosslinking of several BCRs with signaling through Igα and Igβ ITAMs?
- antigen binds to mIg’s
- must crosslink 2 or more BCR for signaling to occur
- signaling occurs through ITAMs on Igα and Igβ cytoplasmic tails
- Syk: B cell phosphorylation
- intracellular steps are identical to T cell’s, only difference is in kinase involved in initial signaling steps
What is the general process of first signal crosslinking of BCR with co-BCR with signaling through Igα and Igβ ITAMs and CR2 and CD19 signaling motifs during B cell activation?
- co-BCR: CR2, CD19, CD81
- crosslinking of BCR by antigen generates a signal this necessary but not sufficient to activate B cells
- antigen with bound C3d recognized by mIg’s and CR2 (opsonized)
- CR2 provides cross linkage for signaling
- signaling occurs through Igα and Igβ, CR2, and CD19 cytoplasmic tails
- if C3d is attached to protein antigen, antigen is ~1000 fold more immunogenic
What is the general process of crosslinking of BCR with TLR’s with signaling through Igα and Igβ ITAMs and TLR signaling motifs during B cell activation?
- crosslinking of BCR by antigen generates a signal that is necessary but not sufficient in activating B cells
- TLR signaling: binding of antigen to both BCR and TLR
- binding of PAMP leads to TLR signaling through cytoplasmic domains
- amplifies signaling response of Igα and Igβ
How does the B cell that received a first signal “prepare” for the second signal? (5)
- express proteins that promote survival and cell cycling (increased survival, proliferation)
- endocytosis of BCR/antigen complex, processing/presentation of antigen through MHC II, increased B7 expression (interaction w/ T helper cells)
- increased expression of cytokine receptors (responsiveness to cytokines)
- increased expression of CCR7 (migration from follicle to T cell areas)
- secretion of IgM
How do B and T cells “meet in the middle” so to say during activation within the lympd nodes?
- B cells change their chemokine receptor expression after activation and migrate to the edge of the follicular zone for interaction w/ T helper cells for second signal
- T cells also change their chemokine receptors and move towards the edge of the follicular zone
- B cells: downregulate CXCR5 and upregulate CCR7, migrate towards paracortex, increase expresion of MHC II and B7 (CD80)
- T cells: downregulate CCR7 and upregulate CXCR5, migrate towards follicles, increase expression of CD40L and cytokine secretion
B CELLS ARE GOING OUT, T CELLS ARE COMING IN
What is the second signal in B cell activiation dependent on?
antigen composition
T-dependent: proteins (small)
T-independent: all others, long repeating epitopes that are able to crosslink several hundred BCRs to provide strong enough signal to by pass second signal from T cells
What is the general process of T dependent second signal during B cell activation?
- activated T helper cell recognizes antigen displayed by B cell within MHC II
- B7 on B cell binds to CD28 on T cell
- CD40 on B cell binds to CD40L on T cell
- cytokines binds to cytokine receptors on B cell
- induced expression of activation-induced deaminase (AID) enzyme
- proliferation and clonal expansion
What is the purpose of having a 2 signal approach during B cell activation?
- first signal is important for recognition of antigenic epitope by BCR
- second signal maintains the specificity of response to specific epitope
results in large number of antigen-specific plasma cells and antibodies from rare antigen-specific naive B cells
What happens to B cells that recognize an antigen but BCR does not crosslink, no binding of co-stimulatory ligands, or no cytokine support?
- the B cell will become anergic or tolerant
- IgM surface receptors internalized or capped, cell matures to express IgD
What happens after the B cell is fully activated after receiving the second signal from T cells?
- B cells change their chemokine receptor expression (decrease CCR7, increase CXCR5) and migrate to follicular area and establish germinal centers in follicles
- B cells begin cytokine mediated isotype switching and affinity maturation (somatic hypermutation) of BCR genes
- successful rearrangements are selected by T follicular helper cells and FDC’s
- type of T helper cell
- expresses low levels of CD25
- secretes IL-21: facilitates B cell survival, expansion, and differentiation
- secretes T helper cytokines to influence isotype switching
- continues CD40L binding for B cell induction of AID
T follicular helper cells
What are the 2 general functions of cytokines released by Th and Tfh cells?
- induce heavy chain class switching, do this by opening switch regions in heavy chain gene for somatic recombination
- augment B cell differentation and proliferation into plasma cells
Why is isotype switching in B cells only possible in T-dependent antigens?
isotype switching requires CD40/CD40L interaction between B and T cells
How does CD40:CD40L ligation trigger isotype switching and affinity maturation?
- opening of cytokine defined switch regions upstream of heavy chain constant regions
- expression of activation induced cytidine deaminase (AID) enzyme
- VDJ gene segment recombines with a downstream C region gene and the intervening DNA is deleted
- uses normal DNA repair enzymes
- can only occur in B cells
- introduction of point mutations in switch regions of variable areas of heavy and light Ig genes result in generation of high-affinity antigen specific antibodies
- somatic hypermutation: 10^3 to 10^6 times more higher than normal spontaneous mutation rates
- key enzyme: AID, converts C’s to U’s
- mutations can be useful, sometimes not
- isoptype switching and this process often occur at the same time
affinity maturation
How do FDC’s select for high affinity in B cell antibodies?
- selective survival of B cells prod highest affinity Abs occurs in germinal centers
- FDC and Tfh interactions with high affinity B cells are necessary for survival of B cell
- FDC’s provide intact antigen to the new BCR for specificity sampling
- if the BCR binds w/ higher affinity, the B clone is selected for further differentiation to plasma/memory cell
