Serum inhibition of autochthonous lymphocyte cytotoxicity for tumour cells has been studied in 112 cases of colonic carcinoma. Addition of patient's serum to the lymphocyte tumour cell reaction mixture resulted in decreased cytotoxic reactivity of lymphocytes from 8 of 39 cytotoxic positive cases. It was also shown that sera could inhibit if separately preincubated with the lymphocytes (4 cases) or the target cells (2 cases). A tumour antigen preparation inhibited only when incubated with the lymphocytes. Inhibition by serum or antigen appeared to be specific for colon carcinoma. Four cases were specially studied to determine the mode of lymphocyte killing of tumour cells: in 3 it was mediated largely if not entirely by T lymphocytes, and in the fourth by both T and non-T cells. The findings support the view that T lymphocytes lose their anti-tumour reactivity in vivo in the presence of circulating antigen or antigen-antibody complexes such as would occur with progressive tumour growth
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