Selection at Multiple Checkpoints Focuses V_H12 B Cell Differentiation toward a Single B-1 Cell Specificity

Phosphatidyl choline (PtC)-specific B cells segregate to the B-1 subset, where they comprise up to 10% of the B-1 repertoire. About half express V_H12 and Vκ4/5H and are restricted in V_HCDR3. We have previously reported that anti-PtC V_HCDR3 is enriched among V_H12-expressing cells by selective elimination of pre-B cells. We report here a bias for Vκ4/5H expression among V_H12-expressing B cells, even among those that do not bind PtC and are not B-1. This is due in part to an inability of V_H12 to associate with many light (L) chains but must also be due to a selective advantage in survival or clonal expansion in the periphery for Vκ4/5H-expressing cells. Thus, the bias for Vκ4/5H expression is independent of PtC binding, and, as segregation to B-1 occurs after Ig gene expression, it precedes segregation to the B-1 subset. In 6-1 mice, splenic B-1 cells reside in follicles but segregate to follicles distinct from those that contain B-2 cells. These data indicate that selection at multiple developmental checkpoints ensures the co-expression of an anti-PtC V_HCDR3 and L chain in a high frequency of V_H12 B cells. This focus toward specificity for PtC facilitates the development of a large anti-PtC B-1 repertoire.