Immortalization of EBV-infected B cells is not influenced by exogenous signals acting on B cell proliferation. Effects of mutant EL-4 thymoma cells and transforming growth factor-beta.

We have recently developed a culture system in which 90% of B cells from human peripheral blood or spleen are induced to strongly proliferate and generate short-term clones of a mean of about 400 antibody-secreting cells. B cells are stimulated by mutant EL-4 thymoma cells in conjunction with T cell supernatant. In the present study, we first investigated whether the frequency of B cell immortalization by EBV would be higher in this system than in a conventional system by using PBMC as fillers. The results showed that the EBV-dependent cloning frequency (0.7%) was not increased compared with the system with the use of PBMC (2.1%). However, the short term proliferation of EBV-infected B cells was 20 times increased in the EL-4 system and EBV nuclear Ag-positive cells participated in this response. Recent reports showed that transforming growth factor-beta (TGF-beta) inhibited the growth of normal B cells, whereas the growth of EBV-immortalized (lymphoblastoid) cells was not inhibited. We have studied the effects of this cytokine in the EL-4 system. TGF-beta (200 pg/ml) was found to inhibit the proliferative response of normal B cells by greater than 95% and the short term response of EBV-infected B cells by 78%. In contrast, the EBV-dependent long term cloning frequency, as well as the proportion of clones producing IgM, IgG, or IgA, was not influenced at all by TGF-beta. In conclusion, potent modulation of the early proliferative response of an EBV-infected B cell population by either stimulatory or inhibitory exogenous signals did not influence the number of cells that subsequently became immortalized.