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 line, 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 'accessible' to class switch recombination. After several months of in vitro cell culture, we found a small population of FL-318 cells expressing the gamma-chain. Using a limiting dilution method, a mu-producing cell clone FL-318M, and gamma-producing FL-318G were isolated. Hybridization studies with various DNA probes from the IGH locus as well as the BCL2 gene demonstrated that the mu-constant gene was deleted on the functional IGH allele of FL-318G cells, and that the cells produced abundant productive gamma-chain messages. These studies indicated that FL-318 cells underwent spontaneous class switching during in vitro cell culture, unrelated to T cell interaction or antigenic stimulation.