A humanized ACE2 mouse model recapitulating age- and sex-dependent immunopathogenesis of COVID-19.

In the ongoing battle against coronavirus disease 2019 (COVID-19), understanding its pathogenesis and developing effective treatments remain critical challenges. The creation of animal models that closely replicate human infection stands as a critical step forward in this research. Here, we present a genetically engineered mouse model with specifically-humanized knock-in ACE2 (hiACE2) receptors. This model, featuring nine specific amino acid substitutions for enhanced interaction with the viral spike protein, enables efficient severe acute respiratory syndrome coronavirus 2 replication in respiratory organs without detectable infection in the central nervous system. Moreover, it mirrors the age- and sex-specific patterns of morbidity and mortality, as well as the immunopathological features observed in human COVID-19 cases. Our findings further demonstrate that the depletion of eosinophils significantly reduces morbidity and mortality, depending on the infecting viral dose and the sex of the host. This reduction is potentially achieved by decreasing the pathogenic contribution of eosinophil-mediated inflammation, which is strongly correlated with neutrophil activity in human patients. This underscores the model's utility in studying the immunopathological aspects of COVID-19 and represents a significant advancement in COVID-19 modeling. It offers a valuable tool for testing vaccines and therapeutics, enhancing our understanding of the disease mechanisms and potentially guiding more targeted and effective treatments.

A natural variation in the RNA polymerase of severe fever with thrombocytopenia syndrome virus enhances viral replication and in vivo virulence.

Severe fever with thrombocytopenia syndrome (SFTS) is an emerging tick-borne disease with high mortality in Eastern Asia. The disease is caused by the SFTS virus (SFTSV), also known as Dabie bandavirus, which has a segmented RNA genome consisting of L, M, and S segments. Previous studies have suggested differential viral virulence depending on the genotypes of SFTSV; however, the critical viral factor involved in the differential viral virulence is unknown. Here, we found a significant difference in viral replication in vitro and virulence in vivo between two Korean isolates belonging to the F and B genotypes, respectively. By generating viral reassortants using the two viral strains, we demonstrated that the L segment, which encodes viral RNA-dependent RNA polymerase (RdRp), is responsible for the enhanced viral replication and virulence. Comparison of amino acid sequences and viral replication rates revealed a point variation, E251K, on the surface of RdRp to be the most significant determinant for the enhanced viral replication rate and in vivo virulence. The effect of the variation was further confirmed using recombinant SFTSV generated by reverse genetic engineering. Therefore, our results indicate that natural variations affecting the viral replicase activity could significantly contribute to the viral virulence of SFTSV.

A bivalent form of a RBD-specific synthetic antibody effectively neutralizes SARS-CoV-2 variants.

Coronavirus Disease 2019 (COVID-19) pandemic is severely impacting the world, and tremendous efforts have been made to deal with it. Despite many advances in vaccines and therapeutics, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) variants remains an intractable challenge. We present a bivalent Receptor Binding Domain (RBD)-specific synthetic antibody, specific for the RBD of wild-type (lineage A), developed from a non-antibody protein scaffold composed of LRR (Leucine-rich repeat) modules through phage display. We further reinforced the unique feature of the synthetic antibody by constructing a tandem dimeric form. The resulting bivalent form showed a broader neutralizing activity against the variants. The in vivo neutralizing efficacy of the bivalent synthetic antibody was confirmed using a human ACE2-expressing mouse model that significantly alleviated viral titer and lung infection. The present approach can be used to develop a synthetic antibody showing a broader neutralizing activity against a multitude of SARS-CoV-2 variants.