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B‐1a, B‐1b and B‐2 B cells display unique VHDJH repertoires formed at different stages of ontogeny and under different selection pressures.

Analyses of VHDJH rearrangements isolated from murine peritoneal B‐1a cells (CD5+, IgMhi, B220lo), peritoneal B‐1b cells (CD5‐, IgMhi, B220lo), and conventional splenic B cells provide evidence that a unique repertoire of VH regions is displayed by each of these B‐cell subsets. The B‐1a subset is characterized by a low N‐region diversity, by a high frequency of sequence homologies in the VH‐D and D‐JH junctions, and by a limited exonuclease nibbling of the terminals of the joining gene segments. Through expansion in ageing mice, B‐1a clones with these properties are favoured. B‐1b cells are similar to conventional B‐2 cells with respect to N‐region diversity, but are unique in terms of D gene expression. Thus, while most murine pre‐B and B cells preferentially use DSP and DFL gene segments in a given reading frame (RF1), B‐1b cells frequently express D genes in another reading frame (RF2). Together, these findings provide structural evidence for a model where B‐1a, B‐1b and B‐2 cells are produced by separate progenitors that are active at different stages of ontogeny.