Antibody Genetics II Flashcards
What happens if B cells do not encounter antigens in the secondary lymph organs?
They die within a few weeks
How are B cells activated?
Rearrangement of genes and expression of IgM in BM
Enter circulation and may encounter Ag in secondary lymph organs
What are T-independent Ag?
Do not need T cells
Particularly resistant to degradation
What are the 2 groups of TI Ag?
TI-1 and TI-2
What are TI-1?
Mainly bacterial cell wall components eg. LPS
In high conc they can polyclonally activate B cells
What are TI-2?
Predominantly large polysaccharides with repeating antigenic determinants
Thought to x-link B cells, causing clustering
Require help from cytokines
What do TI Ag generate?
IgM and do not induce memory
What do TI Ag activate?
CD5+ B cells
What are some examples of T-independent antigens?
lipopolysaccharide (LPS)
Ficoll
Dextran
Levan
Poly-D amino acids
Polymeric bacterial flagellin
What is the T dependent response?
T and B cells recognise diff parts of the antigen
B cells see epitope, T cells see processed peptide fragment
T cells need peptide presenting on APC
T helper cells involved
What is the role of T helper cells?
Interact with B cells and aid in activation and division
How do Th cells interact with B cells?
Surface IgM binds Ag and internalises it
Peptide presents on MHCII
TCR and MHC polarise on cell surfaces
Co-stimulatory molecules bind
T cell produces cytokines allowing proliferation and differentiation of B cells into memory cells or AFC (antibody forming cells)
What do T follicular cells do?
Localise to germinal centres and produce IL-21 and direct Ig class switching
What does IL-4 do?
Induces activation and differentiation in B cells
What does IL-5 do?
Similar effect to IL-4 but additional effects on eosinophils
What does IL-6 do?
Induces B cells to become AFC
What does IL-10 do?
Growth and differentiation of B cells, blocks Th1
What does IL-13 do?
Directs response to IgE
What are the 2 outcomes of B cell activation?
Prod of antibody forming cells - secrete antibodies to clear antigens, mostly die within 2 weeks
Prod of memory B cells - long lived and responsible for long term Ab production
Where does B cell activation occur?
In the germinal centres of secondary lymphoid tissue, spleen (PALS - periarteriolar lymphatic sheath) and lymph nodes
What is the sequence of events in B cell activation?
- Ag taken up by DCs (TLR etc)
- DCs activate Th in Ln (DC only cell to activate naïve T cells)
- B cells in the lymphoid tissue activated by soluble Ag
- B cells present to T cells and get some Ig production-extrafollicular activation, low
level somatic mutation. - Some T cells develop into TFH and these cells move to follicles
- B cells move to follicles
- T and B cells cooperate to form germinal centres where extensive somatic hypermutation, affinity maturation and Ig class switching takes place
When does Ig class switching occur?
In the secondary response
Where does Ig class switching occur?
The germinal centre
What does class switching do?
Adds plasticity to the response
Determines the functionality of the antibody
Determined by cytokines
What do cytokines do in the immune response?
Direct the way in which the class switch goes
Give examples of how cytokines direct class switching
- Th1 cells are activated by viruses and bacteria to
produce interferon gamma. This causes switch to IgG - main complement fixing isotype. - Helminths produce IL-4 from Th2, IL-4 directs
IgE production which target eosinophils to
helminths
– Mucosal tissue results in the switch to IgA under
the influence of TGFb
What does CD40 L on T cells do?
Ligates to CD40 on B cell
What does the CD40L-CD40 interaction do?
Induces expression of activation-induced deaminase (AID)
Involved in class switching and somatic hypermutation
What happens in switch recombination?
- Rearranged VDJ exon encoding the heavy chain V domain recombines with new C region
- Intervening DNA is deleted
- Switch regions are in the introns between J and each CH region
- Upstream of the switch (S) region there is an initiation (I) sequence
- Transcription occurs along the strand but strand breaks occur at the switch regions
- S mu strand break recombines with selected downstream switch region determined by cytokines
- Translation of the protein composed of original VDJ and new C
What does AID do?
Converts C to U in single strand DNA switch region
Enzyme removes U, causing abasic DNA
Endonucleases cleave DNA at this site, causing nicks
Nicks on both strands = double stranded breaks
Complementary DSBs in switch regions combine leading to new heavy chain isotype produced by B cell
What do follicular dendritic cells do?
Express C receptors and FcR
They hold Ag bound to these structures and present them to B cells
What doe Tfh cells do?
Produce IL-21, driving B cells into apoptosis unless they are rescued by Ag recognition
What happens as maturation progresses?
Ag conc drops so select for higher affinity B cells
What contributes the long life span of memory B cells?
Bcl-2
How are so many B cell specificities generated from 35000 genes?
2 chains per receptor
Multiple VDJ segments
Junctional diversity
Somatic hypermutation
C region switching
