Helper factor(s) generated by tumor-immune rats for lymphoproliferative responses to syngeneic tumor cells.

New Zealand Black ( NBR ) rats that are innately immune to challenge with a syngeneic 3-methylcholanthrene [(MCA) CAS: 56-49-5]-induced fibrosarcoma have spleen cells that produce helper effects for in vitro lymphoproliferative responses in the presence of individual MCA-induced fibrosarcoma cells. Spleen cells from MCA-induced fibrosarcoma progressor rats (which lack innate tumor immunity) do not produce demonstrable helper activity. Supernatants from 48-hour cocultures of spleen cells from tumor-immune (TI) rats and syngeneic MCA-induced fibrosarcoma cells replaced the spleen cell helper activity. Comparable spleen cell supernatants from tumor progressor rats or unchallenged rats (controls) as well as supernatants from MCA-induced fibrosarcoma cells cultured alone did not produce any helper factor activity. Supernatants from TI rat spleen cells following inoculation with MCA-induced fibrosarcoma cells did not affect lymphoproliferative responses of NBR spleen cells induced by concanavalin A or alloantigens. The supernatants also did not contain detectable interleukin 2 activity as determined with the use of the thymocyte costimulator assay. These data indicate that the production of soluble helper factors by TI rat spleen cells may be involved in the augmentation of a protective host antitumor response.