Differentiation-Induced Post-Transcriptional Control of B7-H1 in Human Trophoblast Cells

Abstract 

Trophoblast expression of immunomodulatory proteins in the human placenta is among the mechanisms that are critical for ensuring lymphocyte tolerance to the semi-allogeneic fetus. High levels of B7-H1 on trophoblast cells together with the known role of this protein in establishment of peripheral tolerance suggest that B7-H1 mediates immunological protection of the placenta during gestation. In this study, we investigated the molecular mechanisms of regulation of B7-H1 in trophoblast cells by epidermal growth factor (EGF), a key regulator of trophoblast cell differentiation. EGF increased B7-H1 protein levels within 24h and mRNA levels within 4h of the initiation of treatment; by 24h B7-H1 mRNA levels were similar between control and EGF-treated cells. Analysis of two different potential promoter regions revealed strong promoter activity in response to IFN-γ. In contrast, no promoter activity could be induced by EGF, suggesting that this cytokine regulates B7-H1 expression post-transcriptionally in trophoblast cells. EGF-induced B7-H1 protein expression was completely blocked in the presence of inhibitors of the PI3Kinase/Akt/mTOR pathway, a pathway known to regulate gene expression at the translational level. Finally, analysis of monosomal and polysomal mRNA fractions of untreated and EGF-treated term trophoblast cells revealed that EGF induces a shift towards the translatable fractions and away from the untranslated fractions. These results highlight a novel mechanism for regulation of B7 family proteins in the placenta.

Keywords: Placenta, Trophoblast, Syncytiotrophoblast, Epidermal growth factor, Interferon-gamma, mTOR, Polysomal RNA, Phosphatidylinositol 3-kinase, B7-H1, PD-L1, PD-1