Submission summary : Hepatitis E virus (HEV) is an emerging zoonotic pathogen and the leading cause of enterically transmitted hepatitis worldwide. 4 genotypes of HEV (HEV-1 to HEV-4) are able to infect humans. Interestingly, genotypes HEV-3 and HEV-4 are zoonotic while the 2 others infect only humans (HEV-1 and HEV-2), raising the question of cross-species barrier determinants. Swine are the main reservoir of zoonotic HEV and transmission of the virus occurs via direct contact with infected pigs and consumption of contaminated meat. As HEV is largely enzootic in pig farms, it represents a major problem for public health and is part of the One Health target. Upon viral infection, interferon (IFN) is secreted and activates cellular pathways leading to the expression of hundreds of IFN-stimulated genes (ISGs). ISGs can restrict the replication of a specific virus and protect a species against a given pathogen, thus playing a critical role in preventing cross-species transmission. In this project, we aim to identify and characterise ISG-encoded proteins with antiviral activity against HEV that are involved in the inter-species transmission of HEV. In a first task, we will identify the spectrum of ISGs expressed in function of the genotype involved (HEV-1 vs HEV-3) and the species infected (human vs swine). To achieve this, customised PCR arrays will be used to detect the expression of hundreds of human and swine ISGs in human hepatocytes infected in vitro with HEV-1 and HEV-3 and in the liver of pigs experimentally infected with HEV-3. In a second task, we will assess the antiviral activity of the ISG-encoded proteins identified in the previous task as differentially expressed in a genotype- and species-dependent manner. This will be achieved by measuring HEV replication in human hepatic cells overexpressing the ISGs of interest. In a parallel task, we will also use an alternative medium-throughput approach to screen hundreds of human ISGs to identify ISG-encoded proteins with antiviral activities against HEV infection. Results from these 3 tasks will allow us to identify ISGs able to restrict HEV infection that might play a role in the cross-species transmission of HEV. In a final task, we will characterise the mechanisms of action of these ISGs of interest and determine whether these mechanisms are conserved between host species and are effective against both HEV genotypes. This will provide a better understanding of host and viral factors contributing to HEV adaptation and species barrier crossing.