Systemic and in-situ natural killer activity in tumour-bearing rats

Single-cell suspensions prepared by enzymatic disaggregation of an immunogenic 3-methylcholanthrene-induced sarcoma (Mc40A) contain a significant proportion of infiltrating leucocytes (approximately 42%), comprising T lymphocytes, macrophages and non-phagocytic FcR+ lymphoid-like cells. Tumour-infiltrating lymphocytes (TIL) were isolated and purified by successive passage over Sephadex G-10 columns and their cytotoxic activity in vitro compared with that of lymphoid cells from normal rats and from tumour-bearers at different times after implantation. For this purpose, surviving target cells were quantified by incorporation of the gamma-emitting analogue of methionine, 75Sel-methionine, in a 48-h assay which detected both cytotoxic and cytostatic effects. The reactivity of TIL, which was consistently demonstrable from 11 days after tumour transplantation, was essentially similar to that of normal splenic lymphocytes in magnitude and specificity. Reciprocal cytotoxicity tests using TIL and cultured targets from an antigenically unrelated tumour of similar aetiology (Mc57) showed that the manifestation of TIL cytotoxicity was determined, not by the tumour of origin, but by the susceptibility of the target cells. Evidence that the effector function of TIL was mediated in part by natural killer (NK) cells was derived from concurrent experiments using human myeloid cells (K562) as targets in an 18h 51Cr-release assay. In this system the level of NK activity was critically dependent on the numbers of tumour cells in the TIL population; contamination in excess of 2% gave rise to dose-dependent inhibition of NK function. The results show that within a progressively growing tumour known to possess rejection antigens, NK reactivity was detected in the absence of a demonstrable tumour-specific cytotoxic component.