PROJECT SUMMARY/ABSTRACT Understanding novel mechanisms by which animal viruses enter cells, evade immunity, and cause disease has scientific and public health importance. Arteriviruses are an understudied family of RNA viruses (related to coronaviruses) that infect a wide variety of mammals. The host and viral factors that determine arterivirus disease, persistence, and cross-species transmission remain unknown and unpredictable. This lack of understanding has implications for predicting/thwarting arterivirus emergence in humans, as some arteriviruses have been shown to infect human cells. Macrophages are exclusively infected by most arteriviruses, but the subpopulation(s) of macrophages that support arterivirus replication remain poorly defined. The process by which arteriviruses enter cells is highly novel and also poorly understood. Each virion displays an unusually large set of surface glycoproteins (7-11 depending on the virus) that are unlike any known viral fusion machinery. The macrophage-specific molecule CD163 is a required arterivirus receptor, yet CD163 by itself is insufficient to mediate arterivirus entry. We recently identified the neonatal Fc receptor (FcRn) as an important entry factor that arteriviruses use together with CD163 to gain entry into cells. Aim 1 of this project builds upon this discovery to define the molecular details of the arterivirus:FcRn interaction. In Aim 1a, we will map the site(s) on FcRn that are critical for arterivirus engagement by creating chimeric FcRn molecules that incorporate features of arterivirus-permissive and -resistant FcRn orthologs, with the goal of defining the domains, motifs, and residues involved in arterivirus/FcRn interactions. In Aim 1b, we will generate chimeric arteriviruses that contain combinations of glycoproteins from different arteriviruses, seeking to define the viral glycoproteins, domains, motifs, and residues involved in FcRn engagement. In Aim 1c, we will continue to develop our understandi