Efficient Induction of Ig Gene Hypermutation in Ex Vivo-Activated Primary B Cells

Activation-induced cytidine deaminase (AID) initiates both somatic hypermutation (SHM) and class switch recombination (CSR) of Ig genes. How AID is targeted to the Ig V gene and switch region to trigger SHM and CSR remains elusive. Primary B cells stimulated with CD40L plus IL-4 or LPS plus IL-4 undergo efficient CSR, but it has been difficult to induce SHM in these cells. In the current study, we used B cells from B1-8^hi mice carrying a prerecombined V_H186.2DFL16.1J_H2 Ab gene to investigate the induction of SHM under in vitro culture conditions. B1-8^hi splenic B cells stimulated with CD40L plus IL-4 or LPS plus IL-4 underwent robust CSR to IgG₁, but failed to generate SHM in the V_H186.2 gene. Remarkably, ectopic expression of AID in AID-deficient, but not wild-type, B1-8^hi B cells induced efficient SHM at a rate close to that observed in germinal center B cells. We further established an AID-deficient CH12 B lymphoma line and found that ectopic expression of AID in the mutant line, but not in AID-sufficient CH12 cells, induced efficient point mutations and deletions in the V gene. These results demonstrate that the endogenous AID in ex vivo-activated primary B and B lymphoma cells not only cannot induce SHM but also inhibit the induction of SHM by the exogenous AID. Our results further suggest that the spatiotemporal distribution and/or posttranslational modification of AID strongly affects the induction of SHM in ex vivo-activated primary B cells.