During the first trimester of pregnancy endogenous expression of tumour necrosis factor (TNF)-α has been detected in villous, as well as in proliferating and invading extravillous, trophoblasts suggesting that the protein could be involved in trophoblast differentiation. To gain insights into the putative role of the TNF-α signalling pathway, we investigated expression of its receptors, TNFR I and II, in first trimester placentae and early trophoblasts, and studied the influence of the cytokine on cell proliferation and apoptosis. ELISA and RT-PCR revealed secretion/ expression of TNFRI protein/ mRNA in immortalized ED27 cells and purified first trimester cytotrophoblasts, while soluble TNFRII was undetectable in cell culture supernatants. In agreement, immunohistochemical analyses of first trimester placentae showed that TNFRI is localized to the villous cyto- and syncytiotrophoblast, to the proliferating cytotrophoblasts of the cell islands and cell columns, as well as to extravillous cells invading decidual tissue. TNFRII, however, was absent in early trophoblast populations. Interleukin (IL)-1 and phorbol 12-myristate 13-acetate (PMA) induced shedding of TNFRI from ED27 and primary cells suggesting that under inflammatory conditions the soluble receptor protein may protect from cytotoxic effects of TNF-α. Upon incubation with increasing amounts of TNF-α no significant changes in DNA-content or cell numbers were found, suggesting that the cytokine does not augment proliferation of primary cytotrophoblasts. High doses of TNF-α, however, provoked growth arrest in ED27 cells as evaluated by cell counting, but did not induce necrosis/ apoptosis as was assessed by TUNEL assay. In first trimester cells addition of elevated amounts of TNF-α resulted in the appearance of TUNEL-positive cells and an increase in caspase-3 enzyme activity suggesting that the TNF-α-dependent apoptotic cascade is executed in a portion of the early cytotrophoblasts. (C) 2000 Harcourt Publishers Ltd.