COMPREHENSIVE IMMUNE PROFILING OF BLUETONGUE VIRUS INFECTION IN SHEEP REVEALS ACUTE IMMUNOSUPPRESSIVE EFFECTS ON T CELL RESPONSES
Bluetongue virus (BTV), the prototypical Orbivirus that belongs to the Sedoreoviridae family, is transmitted by the bite of infected Culicoides midges and affects domestic and wild ruminants producing great economic losses. The infection induces an IFN response, followed by an adaptive immune response that plays a critical role in disease clearance. BTV can nonetheless impair IFN and humoral responses. The main goal of this study was to gain a more detailed understanding of BTV pathogenesis and its effects on immune cell populations. To this end, we combined flow cytometry and transcriptomic analyses of several immune cells at different times post-infection (pi). Four sheep were infected with BTV serotype 8 and blood samples collected at days 0, 3, 7 and 15pi to isolate peripheral blood mononuclear cells. Transcriptomic analysis of B-cell marker+, CD4+, CD8+, and CD14+ sorted cells showed that the maximum number of differentially expressed genes occurred at day 7pi, which coincided with the peak of infection. KEGG pathway enrichment analysis in B-cell marker+, CD4+, and CD14+ cells indicated that genes belonging to virus sensing and immune response initiation pathways were enriched and day 3 and 7 pi. Transcriptomic analysis also showed that T cell exhaustion pathway was enriched in CD4+ cells at day 7pi, while CD8+ cells showed downregulated immune response initiation pathways at this timepoint. When T cell functionality was assessed, ELISpot assays, intracellular cytokine staining, and proliferation assays demonstrated that BTV produced an acute inhibition of CD4+ and CD8+ T cell activation at the peak of replication. Furthermore, this coincided with PD-L1 upregulation on the surface of CD4+ and CD8+ T cells as well as monocytes. Taken together, these data indicate that BTV exploits the PD1/PD-L1 immune checkpoint to impair T cell responses. These findings identify several mechanisms in the interaction between host and BTV, which could help develop better tools to combat the disease. Overall design: Four sheep were infected with BTV serotype 8 and blood samples collected at days 0, 3, 7 and 15pi to isolate peripheral blood mononuclear cells. Then, RNA was extracted and sequenced and transcriptomic analysis of B-cell marker+, CD4+, CD8+, and CD14+ sorted cells were carried out.