Studies of allograft immunity in mice. I. Induction, development and in vitro assay of cellular immunity.

Cellular immunity induced by tumour allografts in inbred mice was studied with the help of an assay system measuring the cytotoxic effect of sensitized lymphocytes on Cr-labelled target cells. It is shown that lymphoid cells from spleen, lymph nodes and blood of the allograft recipients reach a peak of cytotoxic activity on days 10–11 after immunization. Incubated with labelled target cells at a ratio of 100:1, the sensitized lymphocytes caused the specific release of up to 70 per cent of the radioactivity within 1 hour. A second population of target cells added to the same cell suspension was destroyed at a slightly accelerated rate, suggesting stimulation of the effector cells by the first interaction. The cytotoxic activity of circulating lymphocytes was found to reach a plateau between days 20 and 60 after immunization, while the activity of spleen cells dropped to low levels in the same time period. The specificity of target cell destruction by sensitized lymphocytes is demonstrated by the lack of lytic activity for syngeneic target cells, and by the selective destruction of target cells carrying a tumour specific antigen. Tumour cells, lymphocytes and embryonic fibroblasts of the donor strain are shown to differ considerably in their sensitivity to lysis by immune lymphocytes.