BCR-dependent lineage plasticity in mature B cells.

B2 cells engage in classical antibody responses, whereas B1 cells are considered carriers of innate immunity, biased toward recognizing epitopes present on the surfaces of common pathogens and self antigens. To explore the role of B cell antigen receptor (BCR) specificity in driving B1 cell differentiation, we developed a transgenic system allowing us to change BCR specificity in B cells in an inducible and programmed manner. Mature B2 cells differentiated into bona fide B1 cells upon acquisition of a B1 cell-typical self-reactive BCR through a phase of proliferative expansion. Thus, B2 cells have B1 cell differentiation potential in addition to their classical capacity to differentiate into memory and plasma cells, and B1 differentiation can be instructed by BCR-mediated self-reactivity, in the absence of B1-lineage precommitment.