Normal immunosuppressive protein: inhibitory effect on immune response against tumour cells.

Normal immunosuppressive protein isolated from human plasma was found to inhibit the generation of primary cytotoxic effector lymphocytes against allogeneic tumour cells in vitro. Total inhibition was observed when NIP was present during the early stages of the sensitization process. In contrast, the generation of secondary cytotoxic lymphocytes in vitro was only slightly inhibited even though large amounts of NIP were used. The inhibition of target cell lysis by sensitized lymphocytes required long preincubation of a relatively small number of effector cells with large amounts of NIP and was most significant when tested at low effector: target cell ratios. Under the same conditions NIP showed no inhibitory effect on the cytotoxic activity of immune macrophages. The present in vitro experiments suggest that NIP exerts its effect through inhibition of DNA synthesis and cellular proliferation and to a limited extent only, by inducing specific suppressor cells.