Dissertation

Effectiveness of the Type I Interferon Response Correlates to Host Resilience During Bluetongue Virus Infection

Hardy, Alexandra Caroline Elodie

2019

Overview

Bluetongue is a major disease of ruminants and is caused by bluetongue virus (BTV), an arbovirus transmitted by Culicoides biting midges. BTV infects essentially all domestic and wild ruminants, however, the clinical outcome of infection differs substantially between host species. Clinical disease induced by BTV, including haemorrhagic fever in severe cases, is normally evident only in sheep, which are therefore susceptible hosts. Conversely, cattle are more resilient to BTV infection, as they rarely develop clinical signs despite showing high levels of viraemia and being considered potential reservoirs of the virus. The variability in the clinical outcome of BTV infection is determined by both virus and host factors. However, the exact mechanisms involved in bluetongue pathogenesis, in particular the differences between host species is not yet fully understood. In this study, we aimed to investigate whether susceptibility or resilience to BTV could correlate with features of viral replication and cellular antiviral responses that could be assessed in vitro. We concentrated on studying virus-host cell interactions using primary cells derived from sheep and cattle, the two animal species exhibiting a different clinical outcome to BTV infection. We established that BTV reaches higher titres in ovine cells compared to cells derived from cattle. This phenotype was consistent using two types of primary cells (endothelial cells and fibroblasts) and two different BTV serotypes. An interferon (IFN) production assay validated that antiviral cytokines were produced in all cell types. Importantly, the species-specific differences in the replication kinetics of BTV were abolished by inhibiting the JAK/STAT signalling pathway. In addition, pre-treatment with type-I IFN severely hampered BTV replication in bovine, but not in ovine primary cells. These data suggest that bovine IFN-stimulated genes (ISGs), unlike their ovine orthologues, are effective in controlling BTV replication and could be important host restriction factors. Using a high-throughput flow cytometry approach, we subsequently screened a library of over 1200 genes, including 300 bovine ISGs, to identify proteins with antiviral properties against BTV. We successfully identified a subset of bovine ISGs such as IFIT1, RSAD2 and OAS2 which negatively impacted BTV replication. The restricting potential of the sheep orthologues to the bovine ISGs of interest was also investigated using similar over-expression assays. This approach has identified species-specific differences, with the ovine genes showing either similar or decreased levels of restriction compared to their bovine counterparts. Finally, RNA sequencing was performed in ovine and bovine primary cells to fully characterise and compare the host anti-viral responses upon BTV infection. We specifically observed that genes responding to IFN were more highly up- (or down)-regulated in bovine compared to ovine cells. We also demonstrated the expression of inflammatory mediators and highlighted the up-regulation of genes associated with an activation of the endothelium. Our study offers novel insights on how bovine cells might control BTV replication and provides an intellectual framework to understand how virus-host interactions may influence host resilience and susceptibility to virus infections.

• Publisher: ProQuest Dissertations & Theses
• Subject: Cattle / Clinical outcomes / Infections / Sheep