D-D recombination diversifies the CDR 3 region of chicken immunoglobulin heavy chains

The occurrence of D-D recombination during the embryonic differentiation of chicken B cells was studied. Ig heavy (H) chains were amplified by polymerase chain reaction from day-12 bursal cDNA, and 30 random V-D-J regions were analysed by DNA sequencing. No gene conversion events were observed in any of the V regions, indicating that diversification of the H chains by gene conversion is not yet activated at this stage of embryonic B-cell development. In contrast, the V-D-J joint regions were extremely heterogeneous. Most of the sequenced V-D-J joints were formed by direct joining of the single-germline V mu 1 gene, one of the multiple-germline D elements, and the single J gene. However, three V-D-J regions were clearly longer in size, and their D-region structure indicated recombination between two or three different germline D elements. Thus, the present data suggest that D-D recombination may have a role in diversification of the Ig H-chain repertoire.