Activation-induced cytidine deaminase: a dual role in class-switch recombination and somatic hypermutation.

Maturation of the antibody repertoire is mediated by two different mechanisms: class-switch recombination (CSR) and somatic hypermutation (SHM). These two processes are T cell dependent and occur in the germinal centers of secondary lymphoid organs. CSR leads to the production of antibodies of different isotypes whereas SHM leads to the selection of B cells expressing a BCR with high affinity for antigen. The activation-induced cytidine deaminase (AID) was recently shown to play a key role in these two mechanisms, demonstrating for the first time that these maturation processes share a common mechanism. There is evidence that AID is involved in the somatic DNA alterations required for CSR and SHM. The mechanism of action of AID is unclear. As AID and APOBEC-1 display a sequence similarity, AID may act as an RNA-editing enzyme. However, the immunological abnormalities observed in uracil-N glycosylase deficiency in a recent study indirectly suggest that AID may edit DNA directly.