Diversification of rabbit VH genes by gene-conversion-like and hypermutation mechanisms

Summary: Where, when and how does Vu diversification occur in the rabbit? Early diversification by gene-conversion and somatic hypermutation in rabbit appendix and chicken bursa of Fabricius are similar processes; the chicken bursa and the rabbit appendix have homologous functions. However, diversification in bursa starts during embryonic development whereas it starts in rabbit appendix about 2 weeks after birth in the presence of antigens and superantigens that ma) contribute to positive and negative selection, affect B-cell expansion and mold the repertoire. The biochemical steps leading to diversification by gene conversion are unknown. However elevated levels of RAD51 mRNA in both chicken bursa and young rabbit appendix suggest that repair of double stand breaks may be involved. The base changes found in expressed rabbit V_H sequences derived from rearrangement of known germline V_H genes followed by one or more gene conversions occur with frequencies similar to those found in analyses of somatic hypermutation. The Ser codons in CDR1 and CDR2 of rabbit V_Hl I genes are all AGY rather than TCN, suggesting that they may represent intrinsic hotspots for hypermutation comparable to those described in human and mouse V_Hr. Somatic hypermutation may further refine antibody affinities in rabbit germinal centers.