5 - Adaptive Immune System Part 2

Somatic hypermutation in immunoglobulin genes

• After being stimulated, B cells can use this mechanism to further diversity the BCR and increase its affinity for the antigen

• SHM introduces point mutations into variable regions of rearranged heavy and light chain genes

• Normally occurs in germinal centres (form in secondary lymphoid organs after exposure to antigen)

• Affect all four bases, more likely in hotspots where AID acts most frequently, recognises WRCY

• W = A or T

• R = A or G

• C = cytidine

• Y = C or T

Role of activation induced cytidine

• Enzyme only expressed in B cells, required for SHM and CSR

• Expression triggers SMH

• Those with hyper IgM syndrome harbour mutations in gene coding AID, leads to low affinity antibodies, cannot class switch, patient susceptible to infections

Mechanism of ID mediated HSM

• Activation induced cytidine deaminase converts cytidine residues to uridine in hot spots by loss of amino group, creates G-U mismatch (instead of G-C)

• Replication based mechanism - causes mutations (transitions) from G-A and C-T (2 cycles = two normal, 1 AU and 1 AT)

• UNG dependant base excision repair - uracil N-glycosylase removes uracil base, polymerase may then add any nucleotide during replication, one normal and one synthesis error strand

• Error prone Msh2 / Msh6 and exonuclease 1 processing - the proteins recognise mismatches and recruit polymerase N to add random base pairs outside of hotspots 

Purpose of class switch recombination

• To change the class of antibody that a B cell makes without changing its antigen specificity

• IgH loci (heavy chain) contain a number of constant region (C) and switch region (S) segments

• After VDJ recombination B lymphocytes only produce IgM (sometimes IgD)

• VDJ must me just before the constant region of interest, the exon can be moved downstream by CSR to express other Ig subclasses

• Recombination occurs between donor and acceptor switch regions - tandem repeats of short G rich sequences containing target sites for AID

• AID will cause breaks in DNA and different S regions will be joined by repair pathways

Expression of membrane bound IgM and IgD

• Start off with IgM (Cμ) but also express IgD (Cδ) receptors as they mature

• Both regions encoded by the same transcript and decision of which to translate is made at the level of RNA splicing

• The switch from IgM to any other class operates at the level of DNA recombination, DNA in between is spliced out, permanent

• The entire heavy chain gene is transcribed into pre-mRNA

• If the first polyadenylation site is read, soluble IgM is produced by splicing

• Second is membrane bound IgM, and fourth is membrane bound IgD

Signals for class switch recombination

• Cytokines - stimulate transcription from different promoters located upstream of each class switch region e.g. IL-4 can influence switch from IgM to IgG or IgE (Cy/Ce)

• IL-5 for IgA

• Signalling from interaction between CD40 on B cell and C40L (ligand)

• X-linked hyper IgM syndrome - T helper cells fail to express CD40L so patients predominantly express IgM