Project Summary: Hepatitis E virus (HEV) infects >20 million people worldwide annually leading to 3.3 million clinical cases of hepatitis and >44,000 deaths due to hepatobiliary diseases. As a non-enveloped virus, HEV is surprisingly present as quasi-enveloped exosome-like virions in circulating blood that resist neutralization. Genotype 1 HEV (HEV-1) infection is associated with fulminant hepatitis with high mortality (>25%) in pregnant women. HEV-1 replicates in placental tissues, and HEV-1 vertical transmission is associated with a high neonatal mortality. Due to the lack of an efficient cell culture or animal model for HEV-1, the mechanism of HEV-1-associated severe diseases during pregnancy is unknown. Significantly higher levels of TNF-α were found in HEV-infected pregnant women with fulminant hepatitis, and HEV-infected pigs with detectable HEV RNA in CNS tissues had significantly higher levels of proinflammatory cytokines (TNF-α and IL-18) than in pigs without detectable HEV RNA in CNS tissues. The long-term goal is to delineate the mechanisms contributing to HEV-associated high mortality during pregnancy. Unfortunately, we currently do not have a suitable system, in vitro or in vivo, to study HEV-1 infection at the maternal-fetal interface. In aim 1, we will establish an in vitro placental barrier in Transwell insert to study HEV-1 infection in the maternal-fetal interface. We hypothesize that HEV-1 in circulating blood during peak viremia crosses the placental barrier leading to fetal infection. We will develop a placental barrier culture mimicking the critical maternal blood- and fetal blood-facing layers that constitute the human placental barrier in vivo, by co-culturing BeWo placental trophoblastic cells and human umbilical vein endothelial cells on basolateral and apical sides of an extracellular matrix-coated Transwell insert. The integrity of the barrier will be confirmed by measuring TEER and barrier permeability to small molecules. We will determine whether HEV-1 can cross the barrier by infecting barrier cultures in the maternal chamber with HEV-1 and HEV-3, respectively, and measuring the amount of HEV in the fetal chamber of the barrier. In aim 2, we will determine the mechanisms of HEV-1 infection in the maternal-fetal interface leading to fetal infection. We hypothesize that quasi-enveloped exosome-like HEVs in circulating maternal blood during peak viremia more easily cross the placental barrier when the barrier is inflamed by pro-inflammatory cytokines such as TNF-α and IL- 18 that are consistently produced during HEV infection, and that HEV-1 infection in the barrier produces type III IFNs to limit viral infection. We will inflame the barrier cultures with TNF-α and IL-18, separately or in combination, and then infect them with non-enveloped HEV-1, HEV-3, quasi-enveloped HEV-1, HEV-3, respectively, to determine the amounts of HEV that have crossed the barrier. We will also determine the expression levels of IFN-α, IFN-β...