Emerging viral infections are difficult to control because heterogeneous members periodically cycle in and out of humans and zoonotic hosts, complicating the development of specific antiviral therapies and vaccines. Coronaviruses (CoVs) have a proclivity to spread rapidly into new host species causing severe disease. Severe acute respiratory syndrome CoV (SARS-CoV) and Middle East respiratory syndrome CoV (MERS-CoV) successively emerged, causing severe epidemic respiratory disease in immunologically naïve human populations throughout the globe. Broad-spectrum therapies capable of inhibiting CoV infections would address an immediate unmet medical need and could be invaluable in the treatment of emerging and endemic CoV infections. We show that a nucleotide prodrug, GS-5734, currently in clinical development for treatment of Ebola virus disease, can inhibit SARS-CoV and MERS-CoV replication in multiple in vitro systems, including primary human airway epithelial cell cultures with submicromolar IC₅₀ values. GS-5734 was also effective against bat CoVs, prepandemic bat CoVs, and circulating contemporary human CoV in primary human lung cells, thus demonstrating broad-spectrum anti-CoV activity. In a mouse model of SARS-CoV pathogenesis, prophylactic and early therapeutic administration of GS-5734 significantly reduced lung viral load and improved clinical signs of disease as well as respiratory function. These data provide substantive evidence that GS-5734 may prove effective against endemic MERS-CoV in the Middle East, circulating human CoV, and, possibly most importantly, emerging CoV of the future.

The emergence of a novel coronavirus (2019-nCoV) has awakened the echoes of SARS-CoV from nearly two decades ago. Yet, with technological advances and important lessons gained from previous outbreaks, perhaps the world is better equipped to deal with the most recent emergent group 2B coronavirus.

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2)—a new coronavirus that has led to a worldwide pandemic¹—has a furin cleavage site (PRRAR) in its spike protein that is absent in other group-2B coronaviruses². To explore whether the furin cleavage site contributes to infection and pathogenesis in this virus, we generated a mutant SARS-CoV-2 that lacks the furin cleavage site (ΔPRRA). Here we report that replicates of ΔPRRA SARS-CoV-2 had faster kinetics, improved fitness in Vero E6 cells and reduced spike protein processing, as compared to parental SARS-CoV-2. However, the ΔPRRA mutant had reduced replication in a human respiratory cell line and was attenuated in both hamster and K18-hACE2 transgenic mouse models of SARS-CoV-2 pathogenesis. Despite reduced disease, the ΔPRRA mutant conferred protection against rechallenge with the parental SARS-CoV-2. Importantly, the neutralization values of sera from patients with coronavirus disease 2019 (COVID-19) and monoclonal antibodies against the receptor-binding domain of SARS-CoV-2 were lower against the ΔPRRA mutant than against parental SARS-CoV-2, probably owing to an increased ratio of particles to plaque-forming units in infections with the former. Together, our results demonstrate a critical role for the furin cleavage site in infection with SARS-CoV-2 and highlight the importance of this site for evaluating the neutralization activities of antibodies.

The ongoing pandemic of COVID-19, caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), underscores the urgency to develop experimental systems for studying this virus and identifying countermeasures. We report a reverse genetic system for SARS-CoV-2. Seven complimentary DNA (cDNA) fragments spanning the SARS-CoV-2 genome were assembled into a full-genome cDNA. RNA transcribed from the full-genome cDNA was highly infectious after electroporation into cells, producing 2.9 × 106 plaque-forming unit (PFU)/mL of virus. Compared with a clinical isolate, the infectious-clone-derived SARS-CoV-2 (icSARS-CoV-2) exhibited similar plaque morphology, viral RNA profile, and replication kinetics. Additionally, icSARS-CoV-2 retained engineered molecular markers and did not acquire other mutations. We generated a stable mNeonGreen SARS-CoV-2 (icSARS-CoV-2-mNG) by introducing this reporter gene into ORF7 of the viral genome. icSARS-CoV-2-mNG was successfully used to evaluate the antiviral activities of interferon (IFN). Collectively, the reverse genetic system and reporter virus provide key reagents to study SARS-CoV-2 and develop countermeasures.

Acute respiratory distress syndrome (ARDS) is immune-driven pathologies that are observed in severe cases of severe acute respiratory syndrome coronavirus (SARS-CoV) infection. SARS-CoV emerged in 2002 to 2003 and led to a global outbreak of SARS. As with the outcome of human infection, intranasal infection of C57BL/6J mice with mouse-adapted SARS-CoV results in high-titer virus replication within the lung, induction of inflammatory cytokines and chemokines, and immune cell infiltration within the lung. Using this model, we investigated the role of the complement system during SARS-CoV infection. We observed activation of the complement cascade in the lung as early as day 1 following SARS-CoV infection. To test whether this activation contributed to protective or pathologic outcomes, we utilized mice deficient in C3 (C3^–/–), the central component of the complement system. Relative to C57BL/6J control mice, SARS-CoV-infected C3^–/– mice exhibited significantly less weight loss and less respiratory dysfunction despite equivalent viral loads in the lung. Significantly fewer neutrophils and inflammatory monocytes were present in the lungs of C3^–/– mice than in C56BL/6J controls, and subsequent studies revealed reduced lung pathology and lower cytokine and chemokine levels in both the lungs and the sera of C3^–/– mice than in controls. These studies identify the complement system as an important host mediator of SARS-CoV-induced disease and suggest that complement activation regulates a systemic proinflammatory response to SARS-CoV infection. Furthermore, these data suggest that SARS-CoV-mediated disease is largely immune driven and that inhibiting complement signaling after SARS-CoV infection might function as an effective immune therapeutic. IMPORTANCE The complement system is a critical part of host defense to many bacterial, viral, and fungal infections. It works alongside pattern recognition receptors to stimulate host defense systems in advance of activation of the adaptive immune response. In this study, we directly test the role of complement in SARS-CoV pathogenesis using a mouse model and show that respiratory disease is significantly reduced in the absence of complement even though viral load is unchanged. Complement-deficient mice have reduced neutrophilia in their lungs and reduced systemic inflammation, consistent with the observation that SARS-CoV pathogenesis is an immune-driven disease. These data suggest that inhibition of complement signaling might be an effective treatment option following coronavirus infection.