The discovery of an emerging viral disease, severe fever with thrombocytopenia syndrome (SFTS), caused by SFTS virus (SFTSV), has prompted the need to understand pathogenesis of SFTSV. We are unique in establishing an infectious model of SFTS in C57/BL6 mice, resulting in hallmark symptoms of thrombocytopenia and leukocytopenia. Viral RNA and histopathological changes were identified in the spleen, liver, and kidney. However, viral replication was only found in the spleen, which suggested the spleen to be the principle target organ of SFTSV. Moreover, the number of macrophages and platelets were largely increased in the spleen, and SFTSV colocalized with platelets in cytoplasm of macrophages in the red pulp of the spleen. In vitro cellular assays further revealed that SFTSV adhered to mouse platelets and facilitated the phagocytosis of platelets by mouse primary macrophages, which in combination with in vivo findings, suggests that SFTSV-induced thrombocytopenia is caused by clearance of circulating virus-bound platelets by splenic macrophages. Thus, this study has elucidated the pathogenic mechanisms of thrombocytopenia in a mouse model resembling human SFTS disease.animal model | phlebovirus | pathology | reduction of platelets | macrophage infection S evere fever with thrombocytopenia syndrome (SFTS) is a recently identified emerging viral infectious disease in China that is caused by a novel phlebovirus in the family Bunyaviridae, SFTS virus (SFTSV) (1). Clinical features of SFTS patients include abrupt high fever, thrombocytopenia, leukocytopenia, and gastrointestinal symptoms. Laboratory tests commonly show elevated serum levels of alanine aminotransferase (ALT), aspartate aminotransferase (AST), blood urea nitrogen (BUN), lactate dehydrogenase, creatine kinase, creatine kinase MB fraction, as well as elongated activated partial-thromboplastin time (1). These abnormally changed laboratory parameters are indicative of the pathological lesions that occur in multiple organs and altered homeostasis of the coagulation systems of SFTS patients. Pathological studies of SFTS are absent because patient tissues are rarely donated after death in rural areas of China. Therefore, the causes of illness and death, as well as pathological changes within organs, remain unclear. To systematically investigate the pathogenic mechanisms of SFTS and understand key symptoms, such as thrombocytopenia, infectious animal models for SFTSV are urgently needed. ResultsEstablishment of a SFTSV Pathogenic Mouse Model. In our initial study, to identify an infectious animal model that could mimic most clinical features during SFTSV infection, the susceptibilities of three commonly used laboratory rodent strains for phlebovirus (2-4), C57/BL6 mice, BalB/C mice, and Syrian hamsters, were examined. The SFTSV strain HB29 was inoculated at 10 5 TCID 50 (50% tissue culture infective dose) per mouse or 5 × 10 5 TCID 50 per hamster through four different routes of infection, including intravenous, intramuscular, intraperitoneal, and intracerebral...

The papain-like protease (PL pro ) of SARS-CoV-2 is a validated antiviral drug target. Through a fluorescence resonance energy transfer-based high-throughput screening and subsequent lead optimization, we identified several PL pro inhibitors including Jun9-72-2 and Jun9-75-4 with improved enzymatic inhibition and antiviral activity compared to GRL0617 , which was reported as a SARS-CoV PL pro inhibitor. Significantly, we developed a cell-based FlipGFP assay that can be applied to predict the cellular antiviral activity of PL pro inhibitors in the BSL-2 setting. X-ray crystal structure of PL pro in complex with GRL0617 showed that binding of GRL0617 to SARS-CoV-2 induced a conformational change in the BL2 loop to a more closed conformation. Molecular dynamics simulations showed that Jun9-72-2 and Jun9-75-4 engaged in more extensive interactions than GRL0617 . Overall, the PL pro inhibitors identified in this study represent promising candidates for further development as SARS-CoV-2 antivirals, and the FlipGFP-PL pro assay is a suitable surrogate for screening PL pro inhibitors in the BSL-2 setting.

Severe fever with thrombocytopenia syndrome bunyavirus is a newly discovered bunyavirus with high pathogenicity to human. The transmission model has been largely uncharacterized. Investigation on a cluster of severe fever with thrombocytopenia syndrome cases provided evidence of person-to-person transmission through blood contact to the index patient with high serum virus load.

The invitro antiviral activity of lactoferrin against common human coronaviruses and SARS-CoV-2 is mediated by targeting the heparan sulfate co-receptor, Emerging Microbes & Infections,