After stimulation, B cells can undergo two types of genetic alteration in their immunoglobulin (Ig) genes: somatic hypermutation and class switch recombination (CSR), both of which are initiated by activation-induced cytidine deaminase (AID).

Although class switching requires cell proliferation, the mechanism is partly unknown. In Study I, analysis of the cell cycle distribution of newly switched cells showed that majority of the IgG1+ cells were in the S/G2/M phases, suggesting that switching ended in the late G1 or early S phase. Subsequent experiments with roscovitine treatment showed that Ig switching reduced dramatically upon the inhibition of CDK activity, suggesting the involvement of CDK during CSR. Interestingly the association of AID to the S region was compromised, while the expression levels of aicda, ung and germline transcripts were unchanged upon inhibition. This is probably due to the reduced accumulation of AID in the nucleus. In  study II, we identified a mouse strain with a spontaneous point mutation in AID, leading to an amino acid substitution of arginine 112 by histidine. In this work, we aimed at establishing a mouse model for type II hyper IgM syndrome. We found that both CSR and somatic hypermutation was completely abolished in the mutant B cells, indicating that R112 is essential for AID function. The mutant mice were characterized by big germinal centers even before immunization, and they had an elevated total B cell population with a relatively lower percentage of plasma cells, indicating that B cell differentiation is halted in these mice. In Study III, we analyzed how BCL6 was influenced by type I interferons (IFNs). We found that IFN-α down-regulated BCL6 mRNA in a JAK/STAT-dependent way and promoted BCL6 protein degradation in the germinal center-derived cell lines. Similar result was found also in primary germinal center B cells. This suggests a mechanism for the impact of the innate immune response on the adaptive immune response.