CD25 B-1a Cells Express .

B-1a cells are innate-like B-lymphocytes producing natural antibodies. Activation-induced cytidine deaminase (AID), a product of the gene, plays a central role in class-switch recombination and somatic hypermutation in B cells. Although a role for in B-1a cells has been suggested on the basis of experiments with knock out (KO) mice, whether B-1a cells express , and if so, which B-1a cell subpopulation expresses , remains unknown. Here, we demonstrate that B-1 cells express , but at a level below that expressed by germinal center (GC) B cells. We previously reported that B-1a cells can be subdivided based on CD25 expression. We show here that B-1a cell expression is concentrated in the CD25 B-1a cell subpopulation. These results suggest the possibility that previous studies of memory B cells identified on the basis of expression may have inadvertently included an unknown number of CD25 B-1a cells. Although B-1a cells develop normally in the absence of , a competitive reconstitution assay reveals enhanced vigor for AID KO B-1a cell bone marrow (BM) progenitors, as compared with wild-type BM B-1 cell progenitors. These results suggest that AID inhibits the development of B-1a cells from BM B-1 cell progenitors in a competitive environment.