It has previously been suggested that some tumours may elaborate substances which can suppress the functional activity of lymphokine activated killer (LAK) cells. We have investigated the effects of secreted products from the colorectal cancer cell line LoVo on the pattern of expression of T lymphocyte subsets and natural killer cell phenotype and activation antigens during in vitro treatment of mixed populations of human peripheral blood mononuclear cells with recombinant human IL-2 (rhIL-2). Tumour cell line products inhibited the proliferation of cells expressing the NK cell antigen CD56 and of cells expressing the CD8 T cell subset antigen. Increased expression of the IL-2 receptor alpha chain component CD25, and the activation associated antigens CD71 and HLA-DR induced by rhIL-2 was inhibited by the presence of tumour cell products. Enhanced cytotoxic activity against standard target. cell lines Daudi and K562 induced by treatment with rhIL-2 was diminished by tumour cell supernatants. These mechanisms may contribute to the resistance of some tumours to immune response modulation treatment regimens which depend upon induction of lymphokine activation responses by rhIL-2.

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