Immunoglobulin gene conversion: insights from bursal B cells and the DT40 cell line.

Chicken B cells diversify their immunoglobulin (Ig) light and heavy chain genes by pseudogene templated gene conversion within the bursa of Fabricius. Although Ig gene conversion was initially believed to occur only in birds, it is now clear that most farm animals also use this elegant mechanism to develop an immunoglobulin gene repertoire. The best model to study Ig gene conversion remains the chicken Ig light chain locus due to its compact size and the fact that all the pseudogene donors are sequenced. Furthermore, gene conversion continues in the bursa-derived DT40 cell line whose genome can be easily modified by targeted integration of transfected constructs. Disruption of the AID gene, which had been shown to control somatic hypermutation and switch recombination in mammals leads to a complete block of gene conversion in DT40 indicating that all B-cell specific repertoire formation is controlled by the same gene. Here, we review the genetics and the molecular mechanism of Ig gene conversion based on sequence analysis of bursal B cells and gene disruption studies in the DT40 cell line.