Inhibitors of lymphocyte activation secreted by human melanoma cell lines

The concentrated supernatants of nine human melanoma cell line cultures were analyzed for the presence of factors that inhibit in vitro immunological reactions. All cell lines secreted a factor that inhibited LPS-induced proliferation of murine B cells; eight cell lines released a factor that inhibited PHA-induced proliferation of murine T cells; all of the three cell lines investigated secreted a factor that inhibited the allogeneic stimulation of BALB/c spleen cells by mitomycin-C-treated C57BL spleen cells. Further analysis of the M21 cell supernatant indicated that its continuous presence was required for the inhibition of the PHA but not of the LPS response suggesting a different mechanism of action. High levels of PHA, but not of LPS, could overcome the inhibitory effect of M21 supernatants. Fractionation of M21 supernatants by sucrose gradient centrifugation, DEAE chromatography and gel filtration suggested that the anti-PHA and the anti-LPS activities were due to different factors. These factors differed from a serine proteinase inhibitor that is also released by M21 cells. Since these inhibitory factors may have a role in protecting a melanoma tumor from attack by the immune system of the host, their consideration may be helpful in designing protocols for therapy which include methods to boost the antitumor responses of the host.