Secondary in vitro generation of CTL specific for MM antigen by stimulation with somatic tumor hybrid, but not with parental tumor cells

Somatic tumor hybrid cells (L-FM3A#2), obtained by hybridization of MM tumor cells (FM3A/R) with HGPRT-less L cells, could induce CTL directed against MM antigen, a tumor-associated transplantation antigen that is expressed on some ascitic mammary tumor cell lines of C3H/He mice; parental tumor cells (FM3A/R) could not produce such CTL in syngeneic mice. In this study, the mechanisms of the generation of CTL by stimulation with L-FM3A#2 hybrid cells were investigated. In the secondary in vitro stimulation system, L cell component(s) that were introduced into hybrid cells by cell fusion play a role in the induction of CTL, as shown by the fact that stimulation with a mixture of L and FM3A/R cells could induce MM antigen-specific CTL, and that the killer helper effect of L cells could be replaced by cell free culture supernatants obtained from co-cultures of unprimed C3H spleen and L cells. IFN activity, but no IL 2 activity, was detected in the culture supernatants. Both IFN and killer helper activity were lost with pH 2 treatment; furthermore, CTL were generated by stimulation of primed spleen cells with FM3A/R cells in the presence of mouse beta-IFN, but not in its absence. These results suggest that IFN liberated by the helper cells that recognize L cell component(s) on the surfaces of tumor hybrid cells plays an essential role in the generation of CTL specific for MM antigen.