ONGOING DIVERSIFICATION OF THE REARRANGED IMMUNOGLOBULIN LIGHT-CHAIN GENE IN A BURSAL LYMPHOMA CELL-LINE

The chicken immunoglobulin light-chain gene (Ig(L)) encodes only a single variable gene segment capable of recombination. To generate an immune repertoire, chickens diversify this unique rearranged V(L) gene segment during B-cell development in the bursa of Fabricius. Sequence analysis of Ig(L) cDNAs suggests that both gene conversion events derived from V(L) segment pseudogene templates (ΨV(L)) and non-template-derived single-base-pair substitutions contibute to this diversity. To facilitate the study of postrecombinational mechanisms of immunoglobulin gene diversification, avian B-cell lines were examined for the ability to diversify their rearranged Ig(L) gene during in vitro passage. One line that retains this ability, the avian leukosis virus-induced bursal lymphoma cell line DT40, has been identified. After passage for 1 year in culture, 39 of 51 randomly sequenced rearranged V-J segments from a DT40 population defined novel subclones of the parental tumor. All cloned V-J segments displayed the same V-J joint, confirming that the observed diversity arose after V-J rearrangement. Most sequence variations that we observed (203 of 220 base pairs) appeared to result from ΨV(L)-derived gene conversion events; 16 of the 17 novel single nucleotide substitutions were transitions. Based on these data, it appears that immunoglobulin diversification during in vitro passage of DT40 cells is representative of the diversification that occurs during normal B-cell development in the bursa of Fabricius.