Purified plasminogen activating factor produced by malignant lymphoid cells abrogates lymphocyte cytotoxicity.

Immunosuppression is a generally observed phenomenon in patients with malignancies. Here we report that plasminogen activating factor (PAF) produced by human (P3HR-1) and simian (B95-8) lymphoid cells of malignant origin abrogates lymphocyte cytotoxicity. PAF has been purified from Epstein-Barr (EB) virus genome carrying lymphocyte cytotoxicity. PAF has been purified from Epstein-Barr (EB) virus genome carrying lymphoid lines by affinity chromatography using lysine-Sepharose columns. Purified PAF consistently inhibited Killer cell activity against the following targets: K-562, EB virus superinfected Raji cells and in vitro EB virus transformed autologous B lymphocytes. Furthermore PAF also inhibited the antibody-dependent cellular cytotoxicity. The results presented also indicate that PAF affects the effector lymphocytes and not the target cells. Taken together, these observations emphasize the importance of factors such as PAF, released by malignant cells, as inhibitors/modulators of immune mechanisms effective against tumour cells.