B Cell Activation

Question 1

T Cell-Dependent Antigen v. T Cell-Independent Antigen

Answer 1

• T-dependent antigens - proteins
  
  • Usually show more isotope switching, affinity maturation, long-lived plasma cells and memory cells
  • Usually follicular B cells
• T-independent antigens- polysaccharides, lipids and other non-proteins
  
  • Mainly IgM (maybe some IgG)
  • Low affinity antibodies and short-lived plasma cells
  • Usually marginal zone B cells (lipids and sugars in blood from splenic white pulp) or B-1 cells (non-protein antigens in mucosal tissues)
  • antigens have repeating epitopes so easier to form crosslinks –> greater activation (less need for T cell help)
• These antigens also bind TLRs and activate complement –> more B cell activation

Question 2

How does B cell antigen recognition lead to activation? (5 steps)

Answer 2

• 1- Must bind multiple antigens to diff Ig on membrane - then crosslink the Ig’s (cross linking is induced by antigen binding)
• 2- ITAM motif on Ig-alpha and Ig-beta are phosphorylated by other kinases of the BCR complex
• 3- Phosphotyrosines now recruit the Syk tyrosine kinase —> phosphorylates tyrosine residues on adaptor proteins
• 4- GTP-Ras/Rac cascade —> ERK/J K and PLC-gamma cascade —> inc Ca++ and DAG activate protein kinase C
• 5- Leads to activation of transcription factors (Myc, NFAT, AP-1, NF-kB)

Question 3

2 Examples of B Cell Second Signals

Answer 3

• C3d of complement system may bind CR2 or CD21 of B cell —> enhances activation process
• PAMPs if microbe can bind TLRs on B cell at same time as antigen —> activating signals that work in concert w/ activation signals in B cell

Question 4

5 Steps of T-B Cell Interaction

Answer 4

• 1- Naive T cell exposed to antigen via APC in T cell zone —> activated and makes cytokines
• 2- Naive B cell exposed to same antigen —> activated in follicles
• 3- Activated T and B cells migrate toward ea other
  
  • T cells inc expression of CXCR5 and dec expression of CCR7 (recognizes chemokine in T zone)
  • B cells are opposite; inc expression of CCR7 and dec expression of CXCR5
• 4- interact @ parafollicular areas (outside follicle)
  
  • B cells endocytose peptide antigen and present it to helper T cells on MHC class II
  • B cell can bind 1 epitope of microbe BUT present different epitope to T cells
• 5- Some cells stay outside follicle and produce plasma cells (usually short-lived) OR some migrate back to follicle to form germinal center where antibody response fully develops (more long-lived plasma cells and memory cells)

**T cell CD40 L binds B cell CD40 (must be in physical contact) AND cytokines prod by T cell bind to B cell receptors–> lead to B cell proliferation, differentiation and antibody production

Question 5

Thf Cells

Answer 5

• T cells w/ high CXCR5 migrate back to follicle (called Thf cells - follicular helper T cells); have ICOS which binds ICOS-L on B cells
• Thf secrete IFN-gamma, IL-4 and IL-17

Question 6

Heavy Chain Isotype Switching

Answer 6

• Requires CD40 binding CD40L
• Delete DNA in between C-mu constant region and downstream other isotype constant region; then bring rearranged VDJ exon upstream of a new heavy chain constant region so that it is now transcribed
• AID= activation induced cytidine deaminase= DNA is deleted by converting C —> U then another enzyme removes U’s —> nicks in both strands —> eventual dbl strand breaks

Question 7

X-linked Hyper-IgM Syndrome

Answer 7

mutations in CD40L gene —> almost all IgM in circulation

no isotype switching

Question 8

Affinity Maturation

Answer 8

• Also requires helper T cells
• Point mutation in hypervariable regions encoding antibodies (somatic hypermutation + selection of those B cells w/ highest affinity)
• AID has a role b/c produces U’s then DNA repair tries to change them back to T’s during replication —> mutations
• Only mutated B cells that can bind free antigen or antigen on follicular dendritic cells survive; as more antigen is bound the remaining B cells have less and less antigen concentration to work w/, thus only those w/ highest affinity can still bind antigen

Question 9

Antibody Feedback

Answer 9

• IgG antibody binds antigen still in blood; forms immune complex
• B cells bind the complex by binding to the antigen w/ their Ig membrane receptors
• Fc receptor on B cells (FcγRIIB) also binds Fc tail of the IgG antibody —> inhibitory signals that shut down further antibody production

Question 10

Haptens

Answer 10

small molecules that can bind to larger substances; haptens then react w/ BCR or TCR —> elicit immune response

• Ex) penicillin is a hapten —> allergies to antibiotics

Question 11

Which cytokine induces switching to IgA?

Answer 11

TGF-beta

Question 12

Which cytokine induces switching to IgE?

Answer 12

IL-4