Highlights d Mice are sensitized for SARS-CoV-2 infection by Ad5-hACE2 transduction d Genetically deficient strains can be directly assessed without additional breeding d Mice useful for determining host factors necessary for optimal virus clearance d Useful for assessing efficacy of vaccines and therapies such as convalescent plasma

The ongoing COVID-19 pandemic is associated with substantial morbidity and mortality. While much has been learned in the first months of the pandemic, many features of COVID-19 pathogenesis remain to be determined. For example, anosmia is a common presentation and many patients with this finding show no or only minor respiratory signs 1 . Studies in animals experimentally infected with SARS-CoV-2, the cause of COVID-19, provide opportunities to study aspects of the disease not easily investigated in human patients. While COVID-19 severity ranges from asymptomatic to lethal 2 , most experimental infections provide insights into mild disease 3 . Here, using K18-hACE2 mice that we originally developed for SARS studies 4 , we show that infection with SARS-CoV-2 causes severe disease in the lung, and in some mice, the brain. Evidence of thrombosis and vasculitis was detected in mice with severe pneumonia. Further, we show that infusion of convalescent plasma (CP) from a recovered COVID-19 patient protected against lethal disease. Mice developed anosmia at early times after infection. Notably, while pretreatment with CP prevented significant clinical disease, it did not prevent anosmia. Thus K18-hACE2 mice provide a useful model for studying the pathological underpinnings of both mild and lethal COVID-19 and for assessing therapeutic interventions.

Pathogenic coronaviruses are a major threat to global public health, as exemplified by Severe Acute Respiratory Syndrome coronavirus (SARS-CoV), Middle East respiratory syndrome coronavirus (MERS-CoV) and the newly emerged SARS-CoV-2, the causative agent of coronavirus disease 2019 (COVID-19). We describe herein the structure-guided optimization of a series of inhibitors of the coronavirus 3C-like protease (3CLpro), an enzyme essential for viral replication. The optimized compounds were effective against several human coronaviruses including MERS-CoV, SARS-CoV and SARS-CoV-2 in an enzyme assay and in cell-based assays using Huh-7 and Vero E6 cell lines. Two selected compounds showed antiviral effects against SARS-CoV-2 in cultured primary human airway epithelial cells. In a mouse model of MERS-CoV infection, administration of a lead compound one day after virus infection increased survival from 0 to 100% and reduced lung viral titers and lung histopathology. These results suggest that this series of compounds has the potential to be developed further as antiviral drugs against human coronaviruses.

Studies of SARS-CoV-2-infected patients and experimentally infected animals indicate a critical role for augmented expression of pro-inflammatory chemokines and cytokines in severe disease. Here, we demonstrate that SARS-CoV-2 infection of human monocyte-derived macrophages (MDMs) and dendritic cells (MDDCs) was abortive, but induced the production of multiple antiviral and pro-inflammatory cytokines (IFN-α, IFN-β, TNF, IL-1β, IL-6 and IL-10) and a chemokine (CXCL10). Despite the lack of efficient replication in MDMs, SARS-CoV-2 induced profound IFN-mediated cell death of host cells. Macrophage activation and death was not enhanced by exposure to low levels of convalescent plasma, suggesting that antibody-dependent enhancement of infection does not contribute to cell death. Together, these results indicate that infection of macrophages and dendritic cells potentially plays a major role in COVID-19 pathogenesis, even in the absence of productive infection.

Our findings suggest a potentially novel therapeutic approach to seasonal, zoonotic avian, and pandemic influenza-the use of phosphoantigens to activate gammadelta T cells against influenza virus infections.