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Adapting the Roberts and Hayry sponge allograft model, we have demonstrated the presence of an enriched, specifically cytotoxic population of cells which infiltrate rejecting sponge allografts. The number of cells infiltrating a rejecting sponge allograft peaks on day 14 after transplantation. Utilizing a short-term 51chromium cytotoxicity assay, peak antiallogeneic killing was demonstrable on day 14 also. Only T cell killing was apparent for the first 15 days after transplantation. After day 20, specific cytolysis was present which was not sensitive to anti-θ serum and complement. The infiltrating cytotoxic cells are large, specifically cytotoxic, do not proliferate in culture, do not respond to mitogen, and do not respond in mixed lymphocyte culture even to the same alloantigen to which the animal had been sensitized. In contrast, spleens from sponge-bearing animals kill poorly, respond to mitogen, and respond vigorously in mixed lymphocyte culture to specific and nonspecific alloantigens.The following hypotheses are set forth with regard to the cytotoxic lymphocytes. (1) Such cells may be end stage and cannot proliferate. (2) The cytotoxic cells may kill the stimulator cells more rapidly than they can be stimulated to proliferate. (3) The sponge cell population may be enriched for nonspecific supressor cells. (4) The sponge cells may be devoid of helper T cells.