Presence of α₂-Macroglobulin in Normal but Not in Malignant Human Syncytiotrophoblasts¹

α₂-Macroglobulin (α₂M) was demonstrated in normal syncytiotrophoblasts of both early and full-term human placentas using immunocytological staining. α₂M was also detected in hydatidiform moles, the benign tumors of proliferating syncytiotrophoblasts. In contrast, no α₂M was detected in invasive moles or choriocarcinomas. In culture conditions, both normal syncytiotrophoblasts and choriocarcinoma cells, identified by production of human chorionic gonadotropin, were negative when stained for α₂M or when studied using metabolic labeling and immunoprecipitation or radioimmunoassay. However, α₂M was taken up from added human serum by the cultured syncytiotrophoblasts, whereas choriocarcinoma cells remained negative also under these conditions. The possible role of α₂M in the regulation of proteolysis in cell invasion is considered.