Mutation of highly conserved residues in loop 2 of the coronavirus macrodomain demonstrates that enhanced ADP-ribose binding is detrimental to infection (preprint)

All coronaviruses (CoVs) encode for a conserved macrodomain (Mac1) located in nonstructural protein 3 (nsp3). Mac1 is an ADP-ribosylhydrolase that binds and hydrolyzes mono-ADP-ribose from target proteins. Previous work has shown that Mac1 is important for virus replication and pathogenesis. Within Mac1, there are several regions that are highly conserved across CoVs, including the GIF (glycine-isoleucine-phenylalanine) motif. To determine how the biochemical activities of these residues impact CoV replication, the isoleucine and the phenylalanine residues were mutated to alanine (I-A/F-A) in both recombinant Mac1 proteins and recombinant CoVs, including murine hepatitis virus (MHV), Middle East respiratory syndrome coronavirus (MERS-CoV), and severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). The F-A mutant proteins had ADP-ribose binding and/or hydrolysis defects that led to attenuated replication and pathogenesis in cell culture and mice. In contrast, the I-A mutations had normal enzyme activity and enhanced ADP-ribose binding. Despite increased ADP-ribose binding, I-A mutant MERS-CoV and SARS-CoV-2 were highly attenuated in both cell culture and mice, indicating that this isoleucine residue acts as a gate that controls ADP-ribose binding for efficient virus replication. These results highlight the function of this highly conserved residue and provide unique insight into how macrodomains control ADP-ribose binding and hydrolysis to promote viral replication and pathogenesis.

Klebsiella pneumoniae Co-infection Leads to Fatal Pneumonia in SARS-CoV-2-infected Mice (preprint)

SARS-CoV-2 patients have been reported to have high rates of secondary Klebsiella pneumoniae infections. Klebsiella pneumoniae is a commensal that is typically found in the respiratory and gastrointestinal tracts. However, it can cause severe disease when a person's immune system is compromised. Despite a high number of K. pneumoniae cases reported in SARS-CoV-2 patients, a co-infection animal model evaluating the pathogenesis is not available. We describe a mouse model to study disease pathogenesis of SARS-CoV-2 and K. pneumoniae co-infection. BALB/cJ mice were inoculated with mouse-adapted SARS-CoV-2 followed by a challenge with K. pneumoniae. Mice were monitored for body weight change, clinical signs, and survival during infection. The bacterial load, viral titers, immune cell accumulation and phenotype, and histopathology were evaluated in the lungs. The co-infected mice showed severe clinical disease and a higher mortality rate within 48 h of K. pneumoniae infection. The co-infected mice had significantly elevated bacterial load in the lungs, however, viral loads were similar between co-infected and single-infected mice. Histopathology of co-infected mice showed severe bronchointerstitial pneumonia with copious intralesional bacteria. Flow cytometry analysis showed significantly higher numbers of neutrophils and macrophages in the lungs. Collectively, our results demonstrated that co-infection of SARS-CoV-2 with K. pneumoniae causes severe disease with increased mortality in mice.

Toll-like receptor 7 (TLR7)-mediated antiviral response protects mice from lethal SARS-CoV-2 infection (preprint)

SARS-CoV-2-induced impaired antiviral and excessive inflammatory responses cause fatal pneumonia. However, the key pattern recognition receptors that elicit effective antiviral and lethal inflammatory responses in-vivo are not well defined. CoVs possess single-stranded RNA (ssRNA) that is abundantly produced during infection and stimulates both antiviral interferon (IFN) and inflammatory cytokine/ chemokine responses. Therefore, in this study, using wild-type control and TLR7 deficient BALB/c mice infected with a mouse-adapted SARS-COV-2 (MA-CoV-2), we evaluated the role of TLR7 signaling in MA-CoV-2-induced antiviral and inflammatory responses and disease outcome. We show that TLR7-deficient mice are more susceptible to MA-CoV-2 infection as compared to infected control mice. Further evaluation of MA-CoV-2 infected lungs showed significantly reduced mRNA levels of antiviral type I and type III IFNs, IFN stimulated genes (ISGs, ISG15 and CXCL10), and several pro-inflammatory cytokines/chemokines in TLR7 deficient compared to control mice. Reduced lung IFN/ISG levels and increased morbidity/mortality in TLR7 deficient mice correlated with high lung viral titer. Detailed examination of total cells from MA-CoV-2 infected lungs showed high neutrophil count in TLR7 deficient mice compared to control mice. Additionally, blocking TLR7 activity post-MA-CoV-2 infection using a specific inhibitor also enhanced disease severity. In summary, our results conclusively establish that TLR7 signaling is protective during SARS-CoV-2 infection, and despite robust inflammatory response, TLR7-mediated IFN/ISG responses likely protect the host from lethal disease. Given similar outcomes in control and TLR7 deficient humans and mice, these results show that MA-CoV-2 infected mice serve as excellent model to study COVID-19.

SARS-CoV-2 Mac1 is required for IFN antagonism and efficient virus replication in mice (preprint)

Several coronavirus (CoV) encoded proteins are being evaluated as targets for antiviral therapies for COVID-19. Included in this set of proteins is the conserved macrodomain, or Mac1, an ADP-ribosylhydrolase and ADP-ribose binding protein. Utilizing point mutant recombinant viruses, Mac1 was shown to be critical for both murine hepatitis virus (MHV) and severe acute respiratory syndrome (SARS)-CoV virulence. However, as a potential drug target, it is imperative to understand how a complete Mac1 deletion impacts the replication and pathogenesis of different CoVs. To this end, we created recombinant bacterial artificial chromosomes (BACs) containing complete Mac1 deletions ({Delta}Mac1) in MHV, MERS-CoV, and SARS-CoV-2. While we were unable to recover infectious virus from MHV or MERS-CoV {Delta}Mac1 BACs, SARS-CoV-2 {Delta}Mac1 was readily recovered from BAC transfection, indicating a stark difference in the requirement for Mac1 between different CoVs. Furthermore, SARS-CoV-2 {Delta}Mac1 replicated at or near wild-type levels in multiple cell lines susceptible to infection. However, in a mouse model of severe infection, {Delta}Mac1 was quickly cleared causing minimal pathology without any morbidity. {Delta}Mac1 SARS-CoV-2 induced increased levels of interferon (IFN) and interferon-stimulated gene (ISG) expression in cell culture and mice, indicating that Mac1 blocks IFN responses which may contribute to its attenuation. {Delta}Mac1 infection also led to a stark reduction in inflammatory monocytes and neutrophils. These results demonstrate that Mac1 only minimally impacts SARS-CoV-2 replication, unlike MHV and MERS-CoV, but is required for SARS-CoV-2 pathogenesis and is a unique antiviral drug target.

Time-Dependent Increase in Susceptibility and Severity of Secondary Bacterial Infection during SARS-CoV-2 Infection (preprint)

Secondary bacterial infections can exacerbate SARS-CoV-2 infection, but their prevalence and impact remain poorly understood. Here, we established that a mild to moderate SARS-CoV-2 infection increased the risk of pneumococcal coinfection in a time-dependent, but sex-independent, manner in the transgenic K18-hACE mouse model of COVID-19. Bacterial coinfection was not established at 3 d post-virus, but increased lethality was observed when the bacteria was initiated at 5 or 7 d post-virus infection (pvi). Bacterial outgrowth was accompanied by neutrophilia in the groups coinfected at 7 d pvi and reductions in B cells, T cells, IL-6, IL-15, IL-18, and LIF were present in groups coinfected at 5 d pvi. However, viral burden, lung pathology, cytokines, chemokines, and immune cell activation were largely unchanged after bacterial coinfection. Examining surviving animals more than a week after infection resolution suggested that immune cell activation remained high and was exacerbated in the lungs of coinfected animals compared with SARS-CoV-2 infection alone. These data suggest that SARS-CoV-2 increases susceptibility and pathogenicity to bacterial coinfection, and further studies are needed to understand and combat disease associated with bacterial pneumonia in COVID-19 patients.

Effective interferon (IFN)-lambda treatment regimen to control lethal MERS-CoV infection in mice (preprint)

Effective broad-spectrum antivirals are critical to prevent and control emerging human coronavirus (hCoV) infections. Despite considerable progress made towards identifying and evaluating several synthetic broad-spectrum antivirals against hCoV infections, a narrow therapeutic window has limited their success. Enhancing the endogenous interferon (IFN) and interferon-stimulated gene (ISG) response is another antiviral strategy known for decades. However, the side effects of pegylated type-I IFNs (IFN-Is) and the pro-inflammatory response detected after delayed IFN-I therapy have discouraged their clinical use. In contrast to IFN-Is, IFN-λ, a dominant IFN at the epithelial surface, is shown to be less pro-inflammatory. Consequently, we evaluated the prophylactic and therapeutic efficacy of IFN-λ in hCoV infected airway epithelial cells and mice. Human primary airway epithelial cells treated with a single dose of IFN-I (IFN-a) and IFN-λ showed similar ISG expression, whereas cells treated with two doses of IFN-λ expressed elevated levels of ISG compared to IFN-a treated cells. Similarly, mice treated with two dose IFN-λ were better protected compared to mice receiving a single dose, and a combination of prophylactic and delayed therapeutic regimens completely protected mice from lethal MERS-CoV- infection. A two dose IFN-λ regimen significantly reduced lung viral RNA and inflammatory cytokine levels with marked improvement in lung inflammation. Collectively, we identify an ideal regimen for IFN-λ use and demonstrate the protective efficacy of IFN-λ in MERS-CoV infected mice.

Higher mortality in men from COVID19 infection-understanding the factors that drive the differences between the biological sexes. (preprint)

The emergent global pandemic caused by the rapid spread of Severe Acute Respiratory syndrome- Coronavirus-2 (SARS-CoV-2) has led to increased mortality and negatively impacted day to day activities of humankind within a short period of time. As the data is rapidly emerging from earlier outbreak locations around the world, there are efforts to assimilate this with the knowledge from prior epidemics and find rapid solutions for this. One of the observations and a recurring theme is the disproportionate differences in the incidence of infection and the consequent mortality between males and females. We, therefore, analyzed retrospective datasets from the previous epidemics and the ongoing pandemic in order to address these differences in clinical outcomes. The data shows that even though the infection rates are similar, the odds ratio of male mortality remains high, indicating a divergence in the crosstalk between the three pathogenic human Coronavirus (hCoVs)- the SARS-CoV, MERS-CoV and the SARS-CoV-2 and immune effectors in the two sexes. One proximate cause is the sex-specific modulation of the X-linked genes that can influence susceptibility to infection. Future studies are needed to confirm these findings, which can form the basis for developing rational strategies for ending the current and preventing future pandemics.