Bar Mitzvah for B-1 Cells: How Will They Grow Up?

The discovery of CD5⁺ B cells by Hayakawa et al. (1) initiated a 13 year-long discussion about the origin and functional properties of these cells. The CD5⁺ B cells (B-1a) and their phenotypic CD5⁻ “twins” (B-1b cells) differ from conventional peripheral B cells (B-2) by anatomical location, developmental origin, surface marker expression, antibody repertoire and growth properties (2–4). B-1 cells form a dominant population of B lineage cells in the peritoneal cavity, but are rare in the spleen and lymph nodes of adult mice (3–6). The progenitors of B-1a cells are abundant in the fetal omentum and liver but in contrast to the progenitors of conventional B-2 cells, are missing in the bone marrow of adult mice (3, 7–9). The progenitors of B-1b cells are present in the fetal omentum and...