Stimulation of tumor cell growth in humans by a mononuclear cell-derived factor

Serum-free supernatants of peripheral blood mononuclear cell cultures significantly stimulated [3H]thymidine incorporation of human hematopoietic and nonhematopoietic tumor cell lines. For assay we used a low number of tumor cells per well and medium enriched with dithiothreitol-treated fetal calf serum. The growth-stimulatory activity was detected in the supernatant of peripheral blood mononuclear cell culture within the first 24 h and decreased thereafter. Treatment of mononuclear cells with OKT3 monoclonal antibodies and rabbit complement decreased only moderately the factor production while treatment with anti-Leu-M2 and rabbit complement decreased it significantly. Supernatants of concanavalin A-stimulated peripheral blood mononuclear cell cultures enhanced nonsignificantly [3H]thymidine incorporation by tumor cell cultures unless antibodies against tumor necrosis factor alpha and gamma-interferon were added to the supernatants. Growth-stimulatory activity was heat inactivated partially at 60 degrees C and totally at 80 degrees C. It was abolished at pH 2.5 within 2 h as well as by treatment with dithiothreitol and partially lost by dialysis.