Differential expression of RLRs in the human placenta across gestation

Abstract

After the discovery that Zika virus (ZIKV) infection in pregnant women may result in severe adverse outcomes such as fetal microcephaly, ZIKV must be added to an ever-expanding list of teratogenic viruses. As only a small minority of newborns will display congenital abnormalities after maternal primary infection, innate immune mechanisms must exist in the placenta to prevent viral transmission to the fetus. Understanding the innate antiviral defenses of the placenta is critical to improving diagnosis, treatment, and prevention of adverse pregnancy outcomes stemming from viral infections. We hypothesized that RLRs are expressed in either one or both of the outer cell layers of the chorionic villi, either the syncytiotrophoblast (STB, outermost layer) or in the villous cytotrophoblast (CTB, inner layer), and that expression of these receptors will increase with advancing gestational age. In order to determine the expression of RLRs in placental tissue (6-32 weeks and full-term), we used immunohistochemistry to stain for the RIG-I-like receptors (RLRs) RIG-I, DHX58/LGP2, and MDA5, as well as the endosomal Toll-like receptor TLR7, that serve as antiviral innate immune receptors involved in detecting microbial ligands and cytoplasmic viral nucleic acids. Hofbauer (HB) cells stained positive for all receptors and served as a positive internal control. TLR7 was not present in either the STB or CTB throughout gestation. MDA5 was localized to the STB cytoplasm up to 13 weeks. After 13 weeks, MDA5 was localized to the CTB cytoplasm. DHX58/LGP2 was localized to the STB cytoplasm at 6 weeks of gestation, the STB apical and basal membranes in addition to cytoplasm at 7 weeks of gestation, and also CTB cytoplasm after 7 weeks of gestation. Lastly, RIG-I was localized to the CTB cytoplasm throughout gestation. The differential expression of these RLRs suggest an innate immune defense system unique to the placenta that is responsible for protecting the conceptus from viral attack. These findings will complement ongoing work in characterizing replication of teratogenic viruses in the placenta.