Characterization of an immunosuppressive factor secreted by a human trophoblast-derived choriocarcinoma cell line

Factors and cells of placental origin have been considered to be important in mediating local active immunosuppression that regulates maternal immune reactivity to aid fetal survival. In this context, we investigated the immunosuppressive capabilities of supernatants from human trophoblast-derived choriocarcinoma cell lines (HCS). We have previously reported the inhibitory effects of HCS on proliferative responses of T-lymphocytes both in vitro and in vivo (L. Krishnan, E. Menu, G. Chaouat, G. P. Talwar, and R. Raghupathy, Cell. Immunol. 138, 113, 1991; L. Krishnan, R. Kinsky, G. Chaouat, G.P. Talwar, and R. Raghupathy, Cell. Immunol. 150, 376, 1993). We now show that HCS also suppresses LPS-induced proliferation of murine lymphocytes but does not inhibit the constitutive proliferation of lymphoma cell lines and B cell hybridomas indicating that HCS has no inhibitory effects on terminally differentiated or transformed cells. Furthermore, we have succeeded in isolating and partially characterizing the HCS-derived suppressor factor (HCSf) from culture supernatants of a human choriocarcinoma cell line JEG-3. Purification of the factor has been accomplished by sequential fractionation on anion-exchange, gel filtration, and reverse-phase HPLC columns. The suppressor factor is a low-molecular-weight compound in the range of 5-6 kDa, composed predominantly of hydrophilic amino acids. Protease digestion of the factor revealed that the peptide moiety in HCSf is important for its inhibitory activity. HCSf mediates a dose-dependent suppression of proliferative responses of both human and murine lymphocytes.