Expression and subcellular localization of TLR-4 in term and first trimester human placenta

Toll-like receptor 4 (TLR-4) mediates Gram-negative bacterial-induced inflammatory responses, including production of pro-inflammatory cytokines. Maternal infection and inflammation play an important role in preterm birth and neonatal brain damage. The localization of placental TLR-4 as well as changes during normal gestation are critical issues in understanding the role of toll-like receptors in defending the placento-fetal unit from maternal infection. We therefore investigated, by immunohistochemistry (IHC) and Western blot, the subcellular localization of TLR-4 in first trimester and term human placenta. In both term placenta (n=4) and first trimester placenta villous samples (n=5), immunoreactivity for TLR-4 was found in the cytoplasm of the syncytiotrophoblast, with darker staining in some areas of the maternal facing plasma membrane (MVM). In addition, TLR-4 was found to be expressed in the first trimester cytotrophoblast cells. Using Western blot analysis, TLR-4 was identified in both placental homogenates and isolated MVM and the fetal facing basal membrane (BM). TLR-4 expression in MVM was significantly higher in term (n=9) as compared to first trimester (n=2) samples. We have shown for the first time that the subcellular localization of TLR-4 in term placenta is preferentially in the MVM compared to BM. The MVM is continuously bathed in maternal blood, suggesting that from this vantage point TLR-4 can initiate a rapid response to maternal bacterial infection.