ABSTRACT: Immunoregulatory factors of trophoblast origin may partially abrogate maternal immune responses to the fetus during pregnancy. We have previously shown that soluble factors extracted from hydatidiform mole trophoblast suppress interleukin-2 (IL-2)-dependent proliferation of a cloned murine cytotoxic T cell line (CTLL-2). To characterize human T cell responses to this trophoblast extract, we measured the effects of molar tissue extracts (HME) on IL-2-stimulated proliferation of human T-lymphocytes and mitogen (PHA) transformed T-cell blasts (PHA-blasts). HME significantly (P<0.05) suppressed T-lymphocyte proliferation in response to 5 and 10 units/ml of IL-2 at 500 and 250 μg/ml, while no effect was observed at the 100 μg/ml concentration. Suppression by HME of IL-2-stimulated T-cell proliferation was partially overcome by the addition of excess IL-2. HME also suppressed (P<0.05) IL-2-stimulated proliferation of PHA-blasts at 500 and 250 μg/well at both 5 and 10 units/ml of IL-2. As observed with resting T-cell responses, no suppression of PHA-blast proliferation was observed using 100 μg/ml of HME. In contrast to the response of the resting T-cells to excess IL-2, HME suppression of IL-2-stimulated blast proliferation was not affected by increasing the concentration of IL-2. These results indicate that extracts from hydatidiform mole trophoblast contain immunosuppressive factors that block human T-cell clonal expansion by inhibiting the utilization and/or production of IL-2. Furthermore, the effects of HME are not reversed by excess IL-2 when PHA-blasts are reacted compared to resting T-cell responses, which are partially reversed in the presence of excess IL-2. This suggests that human trophoblast-derived immunoregulatory factors that are present in hydatidiform mole inhibit human lymphocyte utilization of IL-2 in a fashion similar to the murine (CTLL) model, and exhibit irreversible effects on lymphocytes containing high affinity IL-2 receptors (PHA-blasts) compared to lymphocytes with only low affinity IL-2 receptors (resting T-cell). This property of the trophoblast may partially explain how hydatidiform mole, and possibly pregnancy trophoblast in general, escapes lethal immunological challenge by the maternal immune system.