STABLE NONTUMORIGENIC PHENOTYPE OF SOMATIC-CELL HYBRIDS BETWEEN MALIGNANT BURKITTS-LYMPHOMA CELLS AND AUTOLOGOUS EBV-IMMORTALIZED B-CELLS DESPITE INDUCTION OF CHROMOSOMAL BREAKAGE AND LOSS

Fusion of the highly tumorigenic Burkitt's lymphoma (BL) cell line BL6 0-P7 with the nontumorigenic autologous EBV-immortalized lymphoblastoi d cell line (LCL) IARC 277 results in suppression of the tumorigenic p henotype of the parental cell line BL60-P7 after s.c. inoculation into T cell-deficient nude mice, We analyzed whether, after long-term cult ivation of these lymphoma hybrid cells, expression of tumorigenicity c ould be observed and correlated to the loss of particular chromosomes or chromosomal fragments, akin to numerous nonlymphoid hybrid cell mod els described previously. Two years after fusion, in vitro proliferati on of some BL x LCL hybrid cells accelerated, and they partially lost LCL-typical aggregation, However, no major changes in the expression p attern of B cell-associated surface antigens and the EBV latent membra ne protein LMP 1 were observed, Cytogenetic evaluation of these cells revealed spontaneous loss of chromosomes. Karyotyping of long-term cul tivated hybrid cells demonstrated the occurrence of disomy for each ch romosome in at least one metaphase analyzed, Therefore, if suppression of tumorigenicity in these hybrid cells would have been the result of the presence of a single LCL-derived chromosome, there should have be en a high probability of its loss, leading to tumorigenic segregants, Surprisingly, the tumorigenic phenotype remained suppressed in nude mi ce. To induce chromosomal breakage and maldistribution, in addition to spontaneous chromosomal loss, the hybrid cell lines were irradiated a t various doses. Again, none of the hybrid cell clones treated in this manner became tumorigenic in nude mice, Immunohistological analysis o f the regressing hybrid cell tumors revealed that the hybrid cells had retained their LCL-like differentiation phenotype in vivo, In additio n, infiltration with mononuclear cells of murine origin was observed i n these regressing hybrid grafts. We conclude that suppression of the tumorigenic Burkitt's Lymphoma phenotype in these hybrid cells cannot be attributed to a function encoded by a distinct chromosome or chromo somal fragment. Rather, the unexpected stable nontumorigenic phenotype reflects a LCL-specific activated B-cell phenotype of these hybrids, most probably induced by the expression of numerous copies of episomal latent EBV proteins.