The skewed heavy-chain repertoire in peritoneal B-1 cells is predetermined by the selection via pre-B cell receptor during B cell ontogeny in the fetal liver

As many as 5-15% of B-1 cells in the peritoneal cavity of adult mice produce antibodies reactive to phosphatidylcholine (PtC) and the vast majority of them express B cell receptors (BCRs) composed of V_H11-μH chains utilizing the J_H1 segment and Vκ9-L chains. This extremely skewed repertoire of PtC-reactive B-1 cells is traditionally attributed to the expansion of particular clones in response to self or exogenous antigens. Here, we show that the strong bias toward the J_H1 usage among V_H11-μH chains is already established prior to the BCR assembly, namely at the transition from the large to the small pre-B cell stage during B cell ontogeny in the fetal liver. Among V_H11-μH clones isolated from large pre-B cells where the J_H1 skewing was not established yet, the J_H1 users showed the highest ability to form pre-B cell receptor (pre-BCR) and to induce cellular proliferation and differentiation when expressed in fetal liver pro-B cells. Thus, the J_H1 users were positively selected and amplified at the pre-BCR checkpoint. When co-expressed with Vκ9-L chains to form BCR, the J_H1 users almost exclusively conferred the PtC reactivity on BCR even though other J_H users could also form BCR on the cell surface. Therefore, the pre-BCR-mediated positive selection of the J_H1 users among V_H11-μH chains appears to be beneficial to the efficient generation of 'innate-type' PtC-reactive B cells during the fetal B cell development, even before the self-renewal or the antigen-driven clonal expansion of B-1 cells takes place in the peritoneal cavity.