Studies of inflammatory responses in hantavirus infection

Sammanfattning: Throughout the world, orthohantaviruses cause severe, acute infections in humans. Orthohantaviruses, commonly referred to as hantaviruses, are zoonotic viruses with a single stranded RNA genome of negative sense. Hantavirus strains endemic to Europe and Asia cause a systemic infection with renal involvement referred to as hemorrhagic fever with renal syndrome (HFRS). In the Americas, hantaviruses cause hantavirus pulmonary syndrome (HPS) - a severe and highly fatal infection characterized by severe pulmonary compromise. Individuals infected with hantaviruses typically display increased levels of cytokines, decreased platelet counts, and vascular leakage. As specific treatments are lacking, the longterm goal of the studies within this thesis was to provide leads that will aid in the development of such. Specifically, this thesis aimed to characterize inflammatory responses and MAIT cell responses during hantavirus infection, as a step to increase the understanding of protective versus detrimental immune responses. Moreover, this thesis aimed to investigate the role of the cytokine interleukin-6 (IL-6) in the pathogenesis of hantavirus infection. In both HFRS patients and HPS patients, we observed increased systemic levels of many proinflammatory cytokines and other inflammatory markers. In HPS patients, serum levels of IL-6 were found to be associated with increased odds of developing severe disease. On the contrary, serum levels of complement factor (C) 5/C5a and B cell activating factor were associated with decreased odds of developing severe disease. Intestinal fatty acid-binding protein (I-FABP), a systemic marker of intestinal damage, was increased during HPS and associated with increased odds of a fatal outcome. Next, we demonstrated that IL-6 trans-signaling in hantavirus-infected endothelial cells led to increased pro-inflammatory responses and increased monolayer permeability. In HFRS patients, we observed an altered balance of soluble IL-6 receptors in plasma, which may increase the likelihood of IL-6 trans-signaling in patients. The imbalance in these markers was associated with an increased need for supplemental oxygen treatment. When investigating the phenotype of peripheral blood MAIT cells in HFRS patients, we observed a strong decline in MAIT cell numbers during the acute disease. MAIT cells remaining in the circulation were highly activated and exhibited decreased expression of mucosal tissue homing markers. In vitro, we were able to recapitulate these findings, and show that MAIT cell activation mediated by hantavirus was dependent on type I interferons (IFNs) produced by antigen-presenting cells or endothelial cells. In conclusion, this thesis adds to the view that HFRS and HPS are diseases characterized by strong inflammatory responses. The identification of IL-6 and I-FABP as markers of disease severity and fatality, respectively, may help in the understanding of hantavirus pathogenesis and the development of treatment options. The demonstration of the effects of IL-6 on hantavirus-infected endothelial cells suggest a potential mechanism behind IL-6-driven pathogenesis. Finally, this thesis provides further evidence on the involvement of MAIT cells during acute viral infection, and highlights type I IFNs as important mediators in MAIT cell activation.