The Role of Activation-Induced Cytidine Deaminase in Antibody Diversification and B Cell Malignancies
Activation-induced cytidine deaminase (AID) is indispensable for somatic hypermutation (SHM) and class switch recombination (CSR) in B cells. SHM alters the affinity of an antibody for its corresponding antigen by introducing point mutations within the variable region of the immunoglobulin (Ig) gene heavy chain locus. CSR replaces the Ig μ constant region with one of the downstream constant region, thereby changing the physical properties of the antibody. To study the molecular mechanism of AID in these two antibody diversification events, I cloned, expressed and purified the recombinant AID protein from E. coli and showed that it can deaminate cytidines within singlestranded DNA molecules in vitro. In addition, I showed that AID does not have a preference for mutational hotspot sequences within a short DNA oligo in vitro. By studying truncated recombinant AID protein and AID point mutants I confirmed that AID activity is due to its typical cytidine deaminase active site. Hence, the role of AID in SHM and CSR is beneficial to an organism as it gives rise to antibodies of high affinity and avidity crucial in clearance of the antigen by humoral immune response. Aberrant AID activity, however, can lead to c-myc/IgH chromosomal translocations and ultimately to B cell lymphomas and plasmocytomas in mice. Such translocations can also be detected in B cells stimulated to switch in vitro. I showed that switching B cells also upregulate Rae-1, a ligand for the NKG2D activating receptor found on natural killer (NK) cells, which renders them sensitive to NK attack. Furthermore, I demonstrated that IgH/c-myc translocations can be detected in vivo in germinal center (GC) B cells of immunized mice, and that these cells also express Rae-1. However, these calls are found in same frequencies in NK + and NK- mice, implying that NK cells may be actively excluded from the GC. In addition, my data suggest a role for NK cells in clearance of B cells bearing IgH/c-myc translocations outside the protective environment of the germinal center.
AID, SHM, CSR, cytidine deamination, IgH/c-myc translocation, NK cells, Life Sciences
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