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The majority of murine VH12‐expressing B cells are excluded from the peripheral repertoire in adults

We have previously demonstrated that at birth most productive (P) VH12 rearrangements in B10.H‐2aH‐4bp/Wts (2a4b) mice encode a ten‐amino acid CDR3, and that a significant fraction of the expected repertoire is absent. We have now examined the adult VH12 CDR3 repertoire involving all four JH gene segments in both peritoneum and spleen. Of the 74 P VH12 rearrangements from these tissues 67 encode a CDR3 of ten amino acids and include a Gly in the fourth position (designated 10/G4). Most of these rearrangements appear to derive from phosphatidylcholine (PtC)‐specific B cells, which also have a 10/G4 VHCDR3, since few 10/G4 P rearrangements were present in spleen cells depleted of PtC‐specific B cells. Thus, the VH12 B cell repertoire in adult mice is largely restricted to the use of a single CDR3 motif and to a single antigen specificity. This bias results from two selection events: (1) selective exclusion of most VH12 B cells from the peripheral repertoire, and (2) clonal expansion in the periphery of VH12 B cells that have a 10/G4 VHCDR3 and bind PtC. Analysis of VH12‐JH1 rearrangements in viable motheaten (mevJmev) mice, which have an abnormal B cell repertoire due to a defective phosphatase (Hcph) and have barely detectable numbers of PtC‐specific B cells, indicates that selective exclusion of VH12 B cells from the peripheral repertoire occurs normally, but that clonal expansion of 10/G4 VH12 B cells is minimal. This is evidence that the selective exclusion of VH12 B cells from the peripheral repertoire and the clonal expansion of VH12 B cells with a 10/G4 CDR3 are due to independent signaling events.