IMMUNOGLOBULIN HEAVY-CHAIN CLASS SWITCHING, MU TO GAMMA, IN A HUMAN LYMPHOMA CELL-LINE FL-318 CARRYING A T(14-18)(Q32-Q21) CHROMOSOMAL TRANSLOCATION

Previously, we reported the establishment of a human lymphoma cell lin e, FL-318, carrying a t(14;18)(q32;q21) chromosomal translocation. FL- 318 cells had mu-heavy chain on the cell surface, while they expressed 'sterile' germ-line gamma transcripts, suggesting that the chromatin structure of the immunoglobulin heavy chain (IGH) locus was 'accessibl e' to class switch recombination. After several months of in vitro cel l culture, we found a small population of FL-318 cells expressing the gamma-chain. Using a limiting dilution method, a mu-producing cell clo ne FL-318M, and gamma-producing FL-318G were isolated. Hybridization s tudies with various DNA probes from the IGH locus as well as the BCL2 gene demonstrated that the mu-constant gene was deleted on the functio nal IGH allele of FL-318G cells, and that the cells produced abundant productive gamma-chain messages. These studies indicated that FL-318 c ells underwent spontaneous class switching during in vitro cell cultur e, unrelated to T cell interaction or antigenic stimulation.