Murine malignant cells synthesize a 19,000-dalton protein that is physicochemically and antigenically related to the immunosuppressive retroviral protein, P15E.

Abstract

Murine malignant and nonmalignant cell lines, as well as primary tumors, were examined for retroviral P15E or a related protein. Tumor lines examined included the Hep 129 hepatocarcinoma, BP8 fibrosarcoma, RL1 lymphoma, and 3 variants of the B16 melanoma. Tumor lines were virus negative by EM. Nonmalignant cells examined included ST0, 3T3/BALB, and 3T3/L1 fibroblasts and murine splenocytes. All tumor lines synthesized a .apprx. 19,000-dalton protein that co-migrated with retroviral P15E on SDS-PAGE [sodium dodecyl sulfate-polyacrylamide gel electrophoresis]. None of the nonmalignant cells synthesized this protein. Two-dimensional gel electrophoresis of the proteins from B16 melanoma showed them to the physico-chemically similar to P15E. Competition ELISA [enzyme-linked immunosorption assay] assay confirmed the results obtained by metabolite labeling and demonstrated P15E-related antigens in the tumor cell lines and in the ascites fluid of mice injected with Hep 129 cells. P15E antigens were expressed in both a spontaneous mammary adenocarcinoma and in a primary methylcholanthrene-induced fibrosarcoma. Nonmalignant tissues from tumor-bearing animals contained no detectable P15E antigen. Intrathigh injections of primary fibrosarcomas'' extracts, inhibited the intraperitoneal accumulation of inflammatory macrophages. The inhibitory activity was specifically removed by absorption with monoclonal antibody to P15E. Synthesis of the immunosuppressive retroviral protein P15E, or a very similar protein, routinely occurred during the growth of spontaneous murine primary tumors as well as in all murine tumor cell lines tested.