Evidence for V_H Gene Replacement in Human Fetal B Cells

Background: In contrast to evidences of Ig H chain receptor editing in transformed cell lines and transgenic mouse models, there has been no direct evidence that this phenomenon occurs in human developing B cells. Methods: $V_HDJ_H$ rearrangements were obtained from genomic DNA of individual $IgM^-$ B cells from liver and $IgM^+B$ cells from bone marrow of 18 wk of gestation human fetus by PCR amplification and direct sequencing. Results: We found three examples of H chain receptor editing from $IgM^+$ and $IgM^-human$ fetal B cells. Two types of $V_H$ replacements were identified. The first involved $V_H$ hybrid formation, in which part of a $V_H$ gene from the initial VDJ rearrangement is replaced by part of an upstream $V_H$ gene at the site of cryptic RSS. The second involved a gene conversion like replacement of CDR2, in which another $V_H$ gene donated a portion of its CDR2 sequence to the initial VDJ rearrangement. Conclusion: These data provide evidence of receptor editing at the H chain loci in developing human B cells, and also the first evidence of a gene conversion event in human Ig genes.