Reversal of Tumor-Mediated Suppression of Immune Reactivity by in Vitro Incubation of Spleen Cells

Abstract

Abstract The cellular immune response of hamsters to PARA-7 tumor isografts was investigated with the 51Cr-release assay. It was found that spleen cells obtained from tumor-bearing animals were not cytotoxic for PARA-7 target cells on direct testing. However, after overnight incubation in vitro the cells became specifically reactive. Activation was temperature dependent occurring at 37°C but not at 4°C. Spleen cells from SV40-immunized animals behaved differently. They produced significant cytotoxicity without preincubation. if the virus-immunized hamsters were given a tumor graft then incubation was required. Conversely, if the graft was excised from tumor-bearing animals 3 days before spleen cell harvest, an immune response was obtained on direct testing. Spleen cells pretreated with sera from PARA-7 tumor bearers were significantly less cytotoxic than cells exposed to normal or heterologous tumor bearer sera. These results indicate that the tumor, or more likely its by-products, can inhibit the activity of sensitized spleen cells, and that this phenomenon can be reversed by incubation of the cells in a tumor-free environment.