Development of tumor cell resistance to syngeneic cell-mediated cytotoxicity during growth of ascitic mastocytoma P815Y.

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Abstract
The immune reactivity to tumor cells within a progressively growing tumor mass in the syngeneic host was analyzed by studying the cell-mediated cytolytic response of DBA/2 mice to the ascitic mastocytoma P815Y. Peritoneal cells from P815Y tumor-bearing hosts were fractionated by velocity sedimentation at unit gravity. Cell-mediated cytotoxicity of fractionated and unfractionated cells was measured by 51Cr-release from tumor target cells. The cell separation procedure revealed significant levels of specific cell-mediated cytotoxicity to P815Y within peritoneal cell populations at 8-16 days after tumor cell inoculation. Tumor cells purified from the peritoneal cell populations of mice injected with 103 tumor cells 10 days previously were as susceptible to syngeneic and allogeneic cell-mediated cytotoxicity as P815Y grown in vitro. However tumor cells obtained from mice 16 days after tumor inoculation were resistant to cytolysis by syngeneic, but not allogeneic, effector cells. Day 16 tumor cells also did not inhibit syngeneic cell-mediated cytotoxicity against P815Y grown in vitro. Immunoglobulin was not detected on day 16 tumor cells and no circulating antibody to P815Y was found in the ascitic fluid of day 16 tumor-bearing mice. Tumor cells may apparently escape immune attack by loss of expression of cell surface tumor-associated antigens in the absence of circulating antibody against tumor.