PROJECT SUMMARY Hantaviruses are a family of zoonotic RNA viruses found in insectivore and rodent hosts worldwide. The Old World hantaviruses, Hantaan virus (HTNV) and Seoul virus (SEOV), are the etiologic agents of hemorrhagic fever with renal syndrome (HFRS), the most common hemorrhagic fever disease in Asia. Hantaviruses primarily target endothelial cells for infection and drive vascular leakage and dysregulation, through poorly defined means. In their respective reservoir hosts, hantaviruses establish asymptomatic, persistent infections. The mechanisms underlying these divergent infection outcomes remain unknown and no therapeutic exists to treat HFRS. Our previously published data demonstrates that type I interferon (IFN) effectively limits HTNV replication and that the cytoplasmic RNA recognition receptors RIG-I and MDA5 are required for initiating the type I IFN response in human endothelial cells during hantavirus infection. Here we show that, in contrast, SEOV infection in its natural reservoir host does not result in innate immune transcriptional activation in vitro and, more interestingly, IFNβ treatment does not limit SEOV replication in reservoir cells. Further, global transcriptomic analysis of human and rat endothelial cells infected with SEOV reveal striking, host-specific patterns of differential gene expression for the IFN and leukocyte extravasation pathways. Predicted network analysis identified differential regulation of the vascular endothelial growth factor (VEGF) receptor signaling pathway, with increased gene expression related to activation and migration in human endothelial cells compared to reservoir endothelial cells. These data, along with substantial literature in the field demonstrating a significant effect of type I IFN on vascular function, lead us to hypothesize that type I IFN signaling following SEOV infection in humans drives endothelial cell activation, vascular dysregulation, and recruitment of proinflammatory neutrophils that are the hallmarks of HFRS. Further, we hypothesize that the reservoir host for SEOV, the common rat, escapes severe disease through viral-induced inhibition of type I IFN activation and reduced inflammation. We will test this hypothesis by: 1) dissecting the virus-host interactions that initiate and antagonize reservoir and non-reservoir innate immunity, 2) investigating the contribution of type I IFN signaling to vascular dysfunction in SEOV-infected endothelial cells, and 3) defining the role of type I IFN signaling in neutrophil activation and extravasation across SEOV-infected endothelial cells from reservoir and human hosts. While similar comparative immunology approaches to understanding pathogenesis for other zoonotic RNA viruses have been successful, the limited tractability of most reservoir rodent hosts has significantly limited progress in the hantavirus field. Thus, the intentional selection to study SEOV and its natural reservoir, the common laboratory rat, provides us with the genomic and immunologic resources to complete a novel, in depth investigation into the immunopathogenic mechanisms underlying hantavirus disease in humans.

PROJECT NARRATIVE Hantavirus disease is a form of zoonotic viral hemorrhagic fever (VHF) that results when humans inhale excreta from hantavirus-infected rodents, with case-fatality rates exceeding 40% for some hantaviruses. In its natural reservoir host, the brown rat, Seoul virus causes asymptomatic, lifelong infection, posing a persistent threat to humans worldwide due to the global rat distribution. The work proposed here leverages the genetic, immunological, and commercially available tools of the laboratory rat reservoir model to investigate the innate immune mechanisms driving differential disease outcomes in reservoir and non-reservoir hosts for hantavirus.