VH-replacement shapes the antibody repertoire by targeting non-pairing heavy-chains

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 V, D and J gene-segments. Many are non- functional and are negatively selected. Critically, this phase of development is impacted by ageing and inflammation. To understand this process in normal mice we have undertaken the first in-depth analysis of heavy-chain selection at this ‘pre-B cell transition’. We find independent selection acting on three regions of the CDR3 antigen binding site, with particularly heavy counter-selection of certain productive VH/JH combinations. This led us to hypothesise that VH-replacement is targeting productive VDJ rearrangements that cannot pair with the surrogate light-chain (SLC). We detect VH-replacement recombination products that closely follow the pattern of counter- selection of productive VDJ. This reveals a physiological role for VH-replacement: the developmental release of B-cells that are stalled, with a productive but not SLC-pairing VDJ, leading to re-modelling of the early VDJ repertoire toward locus-distal VH that we show are more tolerant of CDR3 composition.