Immunoregulatory factors derived from human tumors. I. Immunologic and biochemical characterization of factors that suppress lymphocyte proliferative and cytotoxic responses in vitro

immunoregulatory factors (IRF) that suppress normal human peripheral blood mononuclear cell (PBM) function can be extracted from a variety of fresh human tumors. Three molar potassium chloride extracts of 18 fresh human tumor specimens, including sarcomas, melanomas, and lung carcinomas, markedly inhibited ³H-leucine and ³H-thymidine incorporation by PBM in response to mitogens (phytohemagglutinin, concanavalin A, Pokeweed mitogen) and pooled mitomycin C-treated allogeneic PBM. Induction of human allospecific cell-mediated cytotoxicity was also inhibited. Extracts of human normal and fetal tissues were not inhibitory. IRF could be extracted from either cultured tumor cells or from spent culture media from a melanoma cell line grown in chemically defined, serum-free media, demonstrating IRF is a tumor product. Although IRF were extracted from three histologically distinct tumors, the extracts had several common characteristics. Preincubation of PBM with IRF followed by washing prevented inhibition of lectin-stimulated proliferation. Inhibition was greatest when IRF were added immediately to cultures. IRF were heat-stable and resisted inactivation by papain, chymotrypsin, protease, and lipase; they could be partially inactivated by trypsin treatment. Inhibitory activity was removed from extracts by lentil lectin affinity chromatography and recovered by elution with 3% α-methyl-D-mannoside indicating the active moiety contained a carbohydrate. Inhibitory activity was also abolished by extreme acid or base conditions and treatment with 2-mercaptoethanol. These results demonstrate that factors produced by human tumors suppress a variety of cell-mediated immune responses. Factors from histologically different tumors possess similar biochemical and immunobiologic characteristics.