Abstract
The diversity of antibodies underpins robust immune responses. During the formation of the antibody repertoire in early bone marrow B-cells, random antibody heavy-chain proteins are generated from recombined VH, DH, and JH gene segments. Many are non-functional and are negatively selected. To understand this process in normal mice, we have undertaken an in-depth analysis of heavy-chain selection at this pre-B cell transition. We find independent selection acting on three regions of the complementarity-determining region 3 (CDR3) antigen-binding site, with particularly heavy counter-selection against certain productive VH/JH combinations. This led us to hypothesise that VH-replacement, where the VH gene segment in an existing VDJ combination is replaced, targets productive VDJ rearrangements that code for non-functional heavy chains. We detect VH-replacement recombination products that closely follow the pattern of selection of functional and non-functional VDJ rearrangements. This reveals a physiological role for VH-replacement in the developmental release of B-cells that are stalled by non-functional heavy-chains. This leads to re-modelling of the restricted early VDJ repertoire toward the use of other VH gene segments throughout the IgH locus.