Characteristics and development of the murine B-1b (Ly-1 B sister) cell population

In this paper we have outlined the evidence for two distinct branches of the B-1 cell lineage. The data show that phenotypically B-1a and B-1b cells are essentially identical, distinguished only by the presence or absence of the CD5 antigen. Functionally no differences between the two populations have yet been identified. Both produce anti-PtC antibodies, a specificity not observed in conventional B cells. Both produced high levels of IgM as measured in adoptive transfer experiments. Developmentally, B-1a and B-1b cells are indistinguishable with respect to generation from progenitors present in fetal liver and omentum, feedback regulation of new B-1a and B-1b cells from bone marrow, self-replenishment from Ig+ cells following adoptive transfer, and the generation of clonal populations. The major difference in the two populations is seen in the development of B-1a and B-1b cells from B220- progenitors in the adult bone marrow. Although B220- B-1a progenitors are rare in adult (greater than 6 weeks) bone marrow, the progenitors for B-1b cells persist well into adulthood. Our understanding of B-1b cell ontogeny is at a stage similar to that of B-1a cells five years ago. We have evidence from transfer experiments that strongly suggests the existence of two distinct progenitors for B-1a and B-1b, but we have yet to physically separate these progenitors as Solvansen et al. have done for B-1 and conventional B cells. Furthermore we must determine whether the B-1b cells that develop from fetal liver and bone marrow are functionally and developmentally equivalent to those that develop from adult bone marrow. As with B-1a cells, the role of B-1b cells in the immune system is unclear. Although we have not yet discerned functional differences between B-1a and B-1b, given the recent identification of CD72 (Lyb-2) as the ligand for CD5, it is tempting to speculate that B-1a cells are more involved in B-B cell interactions such as idiotype-anti-idiotype regulation of the early B-cell repertoire and that B-1b cells are more involved in B-T cell interactions. Whatever their function, it is clear that in trying to understand the role of the B-1 lineage it is important to consider both the B-1a and B-1b lineages.