Exploration of immunological responses underpinning severe fever with thrombocytopenia syndrome virus infection reveals IL-6 as a therapeutic target in an immunocompromised mouse model

Two amino acid pairs in the Gc glycoprotein of severe fever with thrombocytopenia syndrome virus responsible for the enhanced virulence

Severe fever with thrombocytopenia syndrome (SFTS) is a significant public health concern, with a high fatality rate in humans and cats. In this study, we explored the genetic determinants that contribute to the different virulence of SFTS virus (SFTSV) based on Tk-F123 and Ng-F264 strains isolated from cats. Tk-F123 was 100% lethal in type I interferon receptor-knockout mice, whereas Ng-F264 exhibited no fatality. We identified a pair of amino acid residues in the Gc protein, glycine and serine, at residues 581 and 934, respectively, derived from Tk-F123, leading to a fatal infection. Those in Ng-F264 were arginine and asparagine. These results suggest that this pair of residues affects the Gc protein function and regulates SFTSV virulence. Our findings provide useful clues for the elucidation of viral pathogenicity and the development of effective live-attenuated vaccines and antiviral strategies.

N-glycosylation of viral glycoprotein is a novel determinant for the tropism and virulence of highly pathogenic tick-borne bunyaviruses

Severe fever with thrombocytopenia syndrome (SFTS) virus, a tick-borne bunyavirus, causes a severe/fatal disease termed SFTS; however, the viral virulence is not fully understood. The viral non-structural protein, NSs, is the sole known virulence factor. NSs disturbs host innate immune responses and an NSs-mutant SFTS virus causes no disease in an SFTS animal model. The present study reports a novel determinant of viral tropism as well as virulence in animal models, within the glycoprotein (GP) of SFTS virus and an SFTS-related tick-borne bunyavirus. Infection with mutant SFTS viruses lacking the N-linked glycosylation of GP resulted in negligible usage of calcium-dependent lectins in cells, less efficient infection, high susceptibility to a neutralizing antibody, low cytokine production in macrophage-like cells, and reduced virulence in Ifnar-/- mice, when compared with wildtype virus. Three SFTS virus-related bunyaviruses had N-glycosylation motifs at similar positions within their GP and a glycan-deficient mutant of Heartland virus showed in vitro and in vivo phenotypes like those of the SFTS virus. Thus, N-linked glycosylation of viral GP is a novel determinant for the tropism and virulence of SFTS virus and of a related virus. These findings will help us understand the process of severe/fatal diseases caused by tick-borne bunyaviruses.

The Adaptive Immune Response against Bunyavirales

The Bunyavirales order includes at least fourteen families with diverse but related viruses, which are transmitted to vertebrate hosts by arthropod or rodent vectors. These viruses are responsible for an increasing number of outbreaks worldwide and represent a threat to public health. Infection in humans can be asymptomatic, or it may present with a range of conditions from a mild, febrile illness to severe hemorrhagic syndromes and/or neurological complications. There is a need to develop safe and effective vaccines, a process requiring better understanding of the adaptive immune responses involved during infection. This review highlights the most recent findings regarding T cell and antibody responses to the five Bunyavirales families with known human pathogens (Peribunyaviridae, Phenuiviridae, Hantaviridae, Nairoviridae, and Arenaviridae). Future studies that define and characterize mechanistic correlates of protection against Bunyavirales infections or disease will help inform the development of effective vaccines.

Construction and Characterization of Severe Fever with Thrombocytopenia Syndrome Virus with a Fluorescent Reporter for Antiviral Drug Screening

Severe fever with thrombocytopenia syndrome (SFTS) caused by a novel bunyavirus (SFTSV) is an emerging infectious disease with up to 30% case fatality. Currently, there are no specific antiviral drugs or vaccines for SFTS. Here, we constructed a reporter SFTSV in which the virulent factor nonstructural protein (NSs) was replaced by eGFP for drug screening. First, we developed a reverse genetics system based on the SFTSV HBMC5 strain. Then, the reporter virus SFTSV-delNSs-eGFP was constructed, rescued, and characterized in vitro. SFTSV-delNSs-eGFP showed similar growth kinetics with the wild-type virus in Vero cells. We further detected the antiviral efficacy of favipiravir and chloroquine against wild-type and recombinant SFTSV by the quantification of viral RNA, and compared the results with that of fluorescent assay using high-content screening. The results showed that SFTSV-delNSs-eGFP could be used as a reporter virus for antiviral drug screening in vitro. In addition, we analyzed the pathogenesis of SFTSV-delNSs-eGFP in interferon receptor-deficient (IFNAR^-/-) C57BL/6J mice and found that unlike the fatal infection of the wild-type virus, no obvious pathological change or viral replication were observed in SFTSV-delNSs-eGFP-infected mice. Taken together, the green fluorescence and attenuated pathogenicity make SFTSV-delNSs-eGFP a potent tool for the future high-throughput screening of antiviral drugs.

Virus-host interactions during tick-borne bunyavirus infection

The Bunyavirales order is the largest grouping of RNA viruses, comprising emerging and re-emerging human, plant and animal pathogens. Bunyaviruses have a global distribution and many members of the order are transmitted by arthropods. They have evolved a plethora of mechanisms to manipulate the regulatory processes of the infected cell to facilitate their own replicative cycle, in hosts of disparate phylogenies. Interest in virus-vector interactions is growing rapidly. However, current understanding of tick-borne bunyavirus cellular interaction is heavily biased to studies conducted in mammalian systems. In this short review, we summarise current understandings of how tick-borne bunyaviruses utilise major cellular pathways (innate immunity, apoptosis and RNAi responses) in mammalian or tick cells to facilitate virus replication.

Coinfections in hospitalized patients with severe fever with thrombocytopenia syndrome: A retrospective study

Severe fever with thrombocytopenia syndrome (SFTS) is an emerging tick-borne disease with a high case fatality rate. Few studies have been performed on bacterial or fungal coinfections or the effect of antibiotic therapy. A retrospective, observational study was performed to assess the prevalence of bacterial and fungal coinfections in patients hospitalized for SFTSV infection. The most commonly involved microorganisms and the effect of antimicrobial therapy were determined by the site and source of infection. A total of 1201 patients hospitalized with SFTSV infection were included; 359 (29.9%) had microbiologically confirmed infections, comprised of 292 with community-acquired infections (CAIs) and 67 with healthcare-associated infections (HAIs). Death was independently associated with HAIs, with a more significant effect than that observed for CAIs. For bacterial infections, only those acquired in hospitals were associated with fatal outcomes, while fungal infection, whether acquired in hospital or community, was related to an increased risk of fatal outcomes. The infections in the respiratory tract and bloodstream were associated with a higher risk of death than that in the urinary tract. Both antibiotic and antifungal treatments were associated with improved survival for CAIs, while for HAIs, only antibiotic therapy was related to improved survival, and no effect from antifungal therapy was observed. Early administration of glucocorticoids was associated with an increased risk of HAIs. The study provided novel clinical and epidemiological data and revealed risk factors, such as bacterial coinfections, fungal coinfections, infection sources, and treatment strategies associated with SFTS deaths/survival. This report might be helpful in curing SFTS and reducing fatal SFTS.

Tocilizumab therapy for IL-6 increment in a patient with non-fatal severe fever with thrombocytopenia syndrome

We report the administration of an anti-interleukin (IL)-6 antibody in a case of severe fever with thrombocytopenia syndrome (SFTS) with an increase in IL-6. On the day of admission, SFTS viral load and IL-6 concentration were 93 831 copies/ml and 5.4 pg/ml, respectively, and tocilizumab was administered. SFTS viral load decreased to 17 821.1 copies/ml on the 3^rd day of admission, while IL-6 levels increased to 104.9 pg/ml; SFTS viral load and IL-6 levels had decreased to 2876.4 copies/ml and 48.2 pg/ml on 7^th day of admission, respectively. The patient fully recovered no tocilizumab adverse events.