Comparison of transforming growth factor beta and a human tumour-derived suppressor factor

Serum-free supernatants from the human melanoma cell line G361 contain a factor that can potently suppress the generation of tumouricidal lymphokine-activated killer (LAK) cells in response to interleukin-2. To characterise the suppressive factor of tumour origin we performed a number of physicochemical and functional comparisons with another immunosuppressive protein, transforming growth factor beta (TGF beta). The bioactivity of tumour-derived suppressor factor (TDSF), assayed by suppression of LAK cell generation, was unaffected by a reducing agent but lost when denatured with a chaotropic agent. In contrast, TGF beta was inactivated by reduction but not denaturation. TDSF lost bioactivity in conditions of pH less than 4, whereas TGF beta showed no loss of activity. The TDSF moiety has an estimated pI of 4.3 and a molecular mass of 69-87 kDa. This differs from published values of pI 9.5, and 25 kDa molecular mass for TGF beta. Anti-TGF beta antiserum reversed the effects of TGF beta but did not affect the suppression of LAK cell generation caused by TDSF. These findings provide compelling evidence that the TDSF moiety is not TGF beta, and may be a novel immunoregulatory cytokine.