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) V_H12 rearrangements in B10.H-2^aH-4^bp/Wts (2^a4^b) mice encode a ten-amino acid CDR3, and that a significant fraction of the expected repertoire is absent. We have now examined the adult V_H12 CDR3 repertoire involving all four J_H gene segments in both peritoneum and spleen. Of the 74 P V_H12 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 V_HCDR3, since few 10/G4 P rearrangements were present in spleen cells depleted of PtC-specific B cells. Thus, the V_H12 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 V_H12 B cells from the peripheral repertoire, and (2) clonal expansion in the periphery of V_H12 B cells that have a 10/G4 V_HCDR3 and bind PtC. Analysis of V_H12-J_H1 rearrangements in viable motheaten (me^vJme^v) 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 V_H12 B cells from the peripheral repertoire occurs normally, but that clonal expansion of 10/G4 V_H12 B cells is minimal. This is evidence that the selective exclusion of V_H12 B cells from the peripheral repertoire and the clonal expansion of V_H12 B cells with a 10/G4 CDR3 are due to independent signaling events.