Insight into the mechanism(s) through which TNF promotes the generation of T cell-mediated antitumor cytotoxicity by tumor bearer splenic cells.

We have shown previously that addition of TNF to stimulation cultures of MOPC-315 tumor bearer splenic cell suspensions containing metastatic tumor cells capable of secreting TGF-beta greatly enhances the generation of anti-MOPC-315 lytic activity by their CD8+ T cells. The current studies were undertaken to gain some insight into the mechanism(s) through which TNF potentiates the in vitro generation of anti-MOPC-315 cytotoxicity by such tumor bearer splenic cells. Here we show that TNF is capable of 1) preventing completely the inhibitory activity of TGF-beta for CTL generation when both cytokines are added at the time of initiation of a 5-day stimulation culture and 2) reversing at least part of the inhibitory activity of TGF-beta when TNF is added as late as 3 days after TGF-beta addition. The costimulatory molecule B7-2 is shown here to be important for the realization of the potentiating activity of TNF for CTL generation by tumor bearer splenic cells. However, despite the importance of the B7-2 molecule, TNF does not mediate its immunopotentiating activity for CTL generation through up-regulation in IL-2 production. In addition, we show here that GM-CSF, but not IL-12, is important for the potentiating effect of TNF for CTL generation by tumor bearer splenic cells. Taken together, these studies identify several factors that are important for the realization of the potentiating effect of TNF for the in vitro generation of antitumor cytotoxicity by tumor-infiltrated splenic cells. It is not known at present, however, whether these factors utilize distinct and/or overlapping mechanisms in realizing the TNF effect.