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Effects of Tumor-like Assay Conditions, Ionizing Radiation, and Hyperthermia on Immune Lysis of Tumor Cells by Cytotoxic T-Lymphocytes¹

Laboratory of Radiobiology, University of California, San Francisco, California 94143

Cytotoxic T-lymphocytes (CTL's) harvested from mixed splenic lymphocyte cultures (DBA/2 + C57BL) were tested for their ability to lyse allogeneic P815 mastocytoma cells under various tumor-like assay conditions, with or without previous exposure to ionizing radiation or hyperthermia (43°). There was little or no decrease of immune cytolysis when CTL's were assayed by ⁵¹Cr release under tumor-like conditions (plateau-phase target cells, low pH, or anoxia) or after irradiation, but cytolytic activity was greatly reduced when CTL's were exposed to heat; 45 min of hyperthermic treatment decreased activity by 99% while reducing the apparent cell viability (as indicated by trypan blue exclusion) by only 30%. When the P815 target cells rather than the CTL's were exposed to heat, their susceptibility to immune lysis was not affected even after treatment times that were lethal to the tumor cells. Despite the dissimilar heat sensitivities of CTL and P815 cells, the dose-response curves for inhibition of protein synthesis by heat, as indicated by [³H]leucine incorporation, were similar for both cell types; neither the depression of protein synthesis in heated CTL's nor the decreased cytolytic ability of these cells was reversed within 3 hr. When irradiated or heated P815 cells were incubated with CTL's, the resulting survival curves were always additive, indicating that neither irradiation nor heat treatment affected the susceptibility of the tumor cells to immune attack. The extreme heat sensitivity of cytotoxic T-lymphocytes raises important questions about the possible effects of hyperthermic treatment on the immune competence of cancer patients.

¹ Work performed under the auspices of the United States Energy Research and Development Administration. Work was begun while the author was on sabbatical leave in the Department of Immunology, Swiss Institute for Experimental Cancer Research, Lausanne, Switzerland.

Received 2/ 4/76. Accepted 4/15/76.