Selection at multiple checkpoints focuses V(H)12B cell differentiation toward a single B-1 cell specificity

Phosphatidyl choline (PtC)-specific B cells segregate to the B-1 subset, wh ere they comprise up to 10% of the B-1 repertoire. About half express V(H)1 2 and V kappa 4/5H and are restricted in V(H)CDR3. We have previously repor ted that anti-PtC V(H)CDR3 is enriched among V(H)12-expressing cells by sel ective elimination of pre-B cells. We report here a bias for V kappa 4/5H e xpression among V(H)12-expressing B cells, even among those that do not bin d PtC and are not B-1. This is due in part to an inability of V(H)12 to ass ociate with many light (L) chains but must also be due to a selective advan tage in survival or clonal expansion in the periphery for V kappa 4/5H-expr essing cells. Thus, the bias for V kappa 4/5H 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 res ide in follicles but segregate to follicles distinct from those that contai n B-2 cells. These data indicate that selection at multiple developmental c heckpoints ensures the co-expression of an anti-PtC V(H)CDR3 and L chain in a high frequency of V(H)12 B cells. This focus toward specificity for PtC facilitates the development of a large anti-PtC B-1 repertoire.