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Respiratory infections are a major health burden worldwide. Respiratory syncytial virus (RSV) is among the leading causes of hospitalization in both young children and older adults. The onset of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) pandemic and the public health response had a profound impact on the normal seasonal outbreaks of other respiratory viruses. However, little is known about how a prior respiratory virus infection impacts SARS-CoV-2 disease outcomes. In this study, we examine the impact of a previous RSV infection on the disease severity of a subsequent SARS-CoV-2 challenge in BALB/c mice. Mice infected with RSV, followed by a SARS-CoV-2 challenge, 30 days later, exhibited decreased weight loss and increased survival as compared to control groups. Our results suggest a prior RSV infection can provide protection against a subsequent SARS-CoV-2 infection., Importance: Severe acute respiratory syndrome coronavirus 2 and respiratory syncytial virus are respiratory viruses that are a major health burden worldwide. Severe acute respiratory syndrome coronavirus 2 and respiratory syncytial virus frequently have peak seasonal outbreaks during the winter months, and are capable of causing severe respiratory disease, often leading to hospitalization. The 2019 pandemic brought attention to the importance of understanding how co-circulating viruses can impact the disease severity of other respiratory viruses. It is known that many hospitalized patients are undergoing multiple viral infections at once, yet not much has been studied to understand the impact this has on other respiratory viruses or patients. How co-circulating viruses impact one another can provide critical knowledge for future interventions of hospitalized patients and potential vaccination strategies., Competing Interests: The authors declare no conflict of interest.

Coronavirus disease 2019 (COVID-19), caused by severe acute respiratory syndrome-coronavirus-2 (SARS-CoV-2) has resulted in substantial morbidity and mortality. The basis of severe disease in humans is difficult to determine without the use of experimental animal models. Mice are resistant to infection with ancestral strains of SARS-CoV-2, although many variants that arose later in the pandemic were able to directly infect mice. In almost all cases, viruses that naturally infected mice or were engineered to enable mouse infection required mouse passage to become virulent. In most cases, changes in structural and nonstructural changes occurred during mouse adaptation. However, the mechanism of increased virulence in mice is not understood. Here, using a recently described strain of mouse-adapted SARS-CoV-2 (rSARS2-MA30 N501Y ), we engineered a series of recombinant viruses that expressed a subset of the mutations present in rSARS2-MA30 N501Y . Mutations were detected in the spike protein and in three nonstructural proteins (nsp4, nsp8, and nsp9). We found that infection of mice with recombinant SARS-CoV-2 expressing only the S protein mutations caused very mild infection. Addition of the mutations in nsp4 and nsp8 was required for complete virulence. Of note, all these recombinant viruses replicated equivalently in cultured cells. The innate immune response was delayed after infection with virulent compared to attenuated viruses. Further, using a lineage tracking system, we found that attenuated virus was highly inhibited in the ability to infect the parenchyma, but not the airway after infection. Together, these results indicate that mutations in both the S protein and nonstructural proteins are required for maximal virulence during mouse adaptation.IMPORTANCEUnderstanding the pathogenesis of coronavirus disease 2019 (COVID-19) requires the study of experimental animals after infection with severe acute respiratory syndrome-coronavirus-2 (SARS-CoV-2). For this purpose, several mouse-adapted SARS-CoV-2 strains have been developed. Here, using a strain of mouse-adapted virus that causes a range of diseases ranging from mild to severe, we show that mutations in both a structural protein [spike (S) protein] and nonstructural proteins are required for maximal virulence. Thus, changes in the S protein, the most widely studied viral protein, while required for mouse adaptation, are not sufficient to result in a virulent virus., Competing Interests: The authors declare no conflict of interest.

The Coronavirus Disease 2019 (COVID-19) pandemic continues to cause extraordinary loss of life and economic damage. Animal models of severe acute respiratory syndrome-coronavirus-2 (SARS-CoV-2) infection are needed to better understand disease pathogenesis and evaluate preventive measures and therapies. While mice are widely used to model human disease, mouse angiotensin converting enzyme 2 (ACE2) does not bind the ancestral SARS-CoV-2 spike protein to mediate viral entry. To overcome this limitation, we "humanized" mouse Ace2 using CRISPR gene editing to introduce a single amino acid substitution, H353K, predicted to facilitate S protein binding. While H353K knockin Ace2 (mACE2 H353K ) mice supported SARS-CoV-2 infection and replication, they exhibited minimal disease manifestations. Following 30 serial passages of ancestral SARS-CoV-2 in mACE2 H353K mice, we generated and cloned a more virulent virus. A single isolate (SARS2 MA-H353K ) was prepared for detailed studies. In 7-11-month-old mACE2 H353K mice, a 10 4 PFU inocula resulted in diffuse alveolar disease manifested as edema, hyaline membrane formation, and interstitial cellular infiltration/thickening. Unexpectedly, the mouse-adapted virus also infected standard BALB/c and C57BL/6 mice and caused severe disease. The mouse-adapted virus acquired five new missense mutations including two in spike (K417E, Q493K), one each in nsp4, nsp9, and M and a single nucleotide change in the 5' untranslated region. The Q493K spike mutation arose early in serial passage and is predicted to provide affinity-enhancing molecular interactions with mACE2 and further increase the stability and affinity to the receptor. This new model and mouse-adapted virus will be useful to evaluate COVID-19 disease and prophylactic and therapeutic interventions.IMPORTANCEWe developed a new mouse model with a humanized angiotensin converting enzyme 2 (ACE2) locus that preserves native regulatory elements. A single point mutation in mouse ACE2 (H353K) was sufficient to confer in vivo infection with ancestral severe acute respiratory syndrome-coronavirus-2 virus. Through in vivo serial passage, a virulent mouse-adapted strain was obtained. In aged mACE2H353K mice, the mouse-adapted strain caused diffuse alveolar disease. The mouse-adapted virus also infected standard BALB/c and C57BL/6 mice, causing severe disease. The mouse-adapted virus acquired five new missense mutations including two in spike (K417E, Q493K), one each in nsp4, nsp9, and M and a single nucleotide change in the 5' untranslated region. The Q493K spike mutation arose early in serial passage and is predicted to provide affinity-enhancing molecular interactions with mACE2 and further increase the stability and affinity to the receptor. This new model and mouse-adapted virus will be useful to evaluate COVID-19 disease and prophylactic and therapeutic interventions., Competing Interests: The authors declare no conflict of interest.

Importance: The function of betacoronavirus internal protein has been relatively understudied. The earliest report on the internal protein of mouse hepatitis virus suggested that the internal protein is a structural protein without significant functions in virus replication and virulence. However, the internal proteins of s evere a cute r espiratory s yndrome c oronavirus (SARS-CoV), Middle-East respiratory syndrome coronavirus, and SARS-CoV-2 have been shown to evade immune responses. Despite the reported functions of the internal protein in these highly pathogenic human coronaviruses, its role in mediating pathogenesis in experimentally infected animals has not been characterized. Our data indicated that despite the similar genomic location and expression strategy of these internal proteins, their effects on virulence are vastly different and virus specific, highlighting the complexity between host-virus interaction and disease outcome., Competing Interests: N.C.W. consults for HeliXon. The authors have no other competing interests, financial or non-financial.

Safe, passive immunization methods are required against severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) and its variants. Immunization of chickens with antigen is known to induce specific IgY antibodies concentrated in the egg yolk and has a good safety profile, high yield of IgY per egg, can be topically applied, not requiring parenteral delivery. Our data provide the first evidence of the prophylactic efficacy of Immunoglobulin Y antibodies against SARS-CoV-2 in mice. Lohmann hens were injected with recombinant SARS-CoV-2 RBD protein; IgY-Abs were extracted from the eggs and characterized using SDS-PAGE. Antiviral activity was evaluated using plaque reduction neutralization tests. In additional experiments, IgY-RBD efficacy was examined in mice sensitized to SARS-CoV-2 infection by transduction with Ad5-hACE2 (mild disease) or by using mouse-adapted virus (severe disease). In both cases, prophylactic intranasal administration of IgY-Abs reduced SARS-CoV-2 replication, and reduced morbidity, inflammatory cell infiltration, hemorrhage, and edema in the lungs and increased survival compared to control groups that received non-specific IgY-Abs. These results indicate that further evaluation of IgY-RBD antibodies in humans is warranted., Competing Interests: The authors have declared that no competing interests exist.

Coronavirus disease 2019 (COVID-19) is a worldwide pandemic caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) that has affected millions of lives. Individuals who survive severe COVID-19 can experience sustained respiratory symptoms that persist for months after initial infection. In other airway diseases, abnormal airway mucus contributes to sustained airway symptoms. However, the impact of SARS-CoV-2 on airway mucus has received limited attention. In the current review, we assess literature describing the impact of SARS-CoV-2 on airway pathophysiology with specific emphasis on mucus production. Accumulating evidence suggests that the 2 major secreted airway mucin glycoproteins, MUC5AC and MUC5B, are abnormal in some patients with COVID-19. Aberrations in MUC5AC or MUC5B in response to SARS-CoV-2 infection are likely due to inflammation, though the responsible mechanisms have yet to be determined. Thus, we also provide a proposed model highlighting mechanisms that can contribute to acute and sustained mucus abnormalities in SARS-CoV-2, with an emphasis on inflammatory cells and mediators, including mast cells and histamine. Last, we bring to light the challenges of studying abnormal mucus production in SARS-CoV-2 infections and discuss the strengths and limitations of model systems commonly used to study COVID-19. The evidence to date suggests that ferrets, nonhuman primates, and cats may have advantages over other models to investigate mucus in COVID-19.

Coronavirus disease 2019 (COVID-19) is especially severe in aged populations 1 . Vaccines against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) are highly effective, but vaccine efficacy is partly compromised by the emergence of SARS-CoV-2 variants with enhanced transmissibility 2 . The emergence of these variants emphasizes the need for further development of anti-SARS-CoV-2 therapies, especially for aged populations. Here we describe the isolation of highly virulent mouse-adapted viruses and use them to test a new therapeutic drug in infected aged animals. Many of the alterations observed in SARS-CoV-2 during mouse adaptation (positions 417, 484, 493, 498 and 501 of the spike protein) also arise in humans in variants of concern 2 . Their appearance during mouse adaptation indicates that immune pressure is not required for selection. For murine SARS, for which severity is also age dependent, elevated levels of an eicosanoid (prostaglandin D 2 (PGD 2 )) and a phospholipase (phospholipase A2 group 2D (PLA 2 G2D)) contributed to poor outcomes in aged mice 3,4 . mRNA expression of PLA 2 G2D and prostaglandin D 2 receptor (PTGDR), and production of PGD 2 also increase with ageing and after SARS-CoV-2 infection in dendritic cells derived from human peripheral blood mononuclear cells. Using our mouse-adapted SARS-CoV-2, we show that middle-aged mice lacking expression of PTGDR or PLA 2 G2D are protected from severe disease. Furthermore, treatment with a PTGDR antagonist, asapiprant, protected aged mice from lethal infection. PTGDR antagonism is one of the first interventions in SARS-CoV-2-infected animals that specifically protects aged animals, suggesting that the PLA 2 G2D-PGD 2 /PTGDR pathway is a useful target for therapeutic interventions., (© 2022. The Author(s), under exclusive licence to Springer Nature Limited.)

The coronavirus disease 2019 (COVID-19) pandemic has caused significant morbidity and mortality on a global scale. The etiologic agent, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), initiates host cell entry when its spike protein (S) binds to its receptor, angiotensin-converting enzyme 2 (ACE2). In airway epithelia, the spike protein is cleaved by the cell surface protease TMPRSS2, facilitating membrane fusion and entry at the cell surface. This dependence on TMPRSS2 and related proteases suggests that protease inhibitors might limit SARS-CoV-2 infection in the respiratory tract. Here, we tested two serine protease inhibitors, camostat mesylate and nafamostat mesylate, for their ability to inhibit entry of SARS-CoV-2 and that of a second pathogenic coronavirus, Middle East respiratory syndrome coronavirus (MERS-CoV). Both camostat and nafamostat reduced infection in primary human airway epithelia and in the Calu-3 2B4 cell line, with nafamostat exhibiting greater potency. We then assessed whether nafamostat was protective against SARS-CoV-2 in vivo using two mouse models. In mice sensitized to SARS-CoV-2 infection by transduction with human ACE2 , intranasal nafamostat treatment prior to or shortly after SARS-CoV-2 infection significantly reduced weight loss and lung tissue titers. Similarly, prophylactic intranasal treatment with nafamostat reduced weight loss, viral burden, and mortality in K18- hACE2 transgenic mice. These findings establish nafamostat as a candidate for the prevention or treatment of SARS-CoV-2 infection and disease pathogenesis. IMPORTANCE The causative agent of COVID-19, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), requires host cell surface proteases for membrane fusion and entry into airway epithelia. We tested the hypothesis that inhibitors of these proteases, the serine protease inhibitors camostat and nafamostat, block infection by SARS-CoV-2. We found that both camostat and nafamostat reduce infection in human airway epithelia, with nafamostat showing greater potency. We then asked whether nafamostat protects mice against SARS-CoV-2 infection and subsequent COVID-19 lung disease. We performed infections in mice made susceptible to SARS-CoV-2 infection by introducing the human version of ACE2, the SARS-CoV-2 receptor, into their airway epithelia. We observed that pretreating these mice with nafamostat prior to SARS-CoV-2 infection resulted in better outcomes, in the form of less virus-induced weight loss, viral replication, and mortality than that observed in the untreated control mice. These results provide preclinical evidence for the efficacy of nafamostat in treating and/or preventing COVID-19.

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection continues to be a serious global public health threat. The 3C-like protease (3CLpro) is a virus protease encoded by SARS-CoV-2, which is essential for virus replication. We have previously reported a series of small-molecule 3CLpro inhibitors effective for inhibiting replication of human coronaviruses including SARS-CoV-2 in cell culture and in animal models. Here we generated a series of deuterated variants of a 3CLpro inhibitor, GC376, and evaluated the antiviral effect against SARS-CoV-2. The deuterated GC376 displayed potent inhibitory activity against SARS-CoV-2 in the enzyme- and the cell-based assays. The K18-hACE2 mice develop mild to lethal infection commensurate with SARS-CoV-2 challenge doses and were proposed as a model for efficacy testing of antiviral agents. We treated lethally infected mice with a deuterated derivative of GC376. Treatment of K18-hACE2 mice at 24 h postinfection with a derivative (compound 2) resulted in increased survival of mice compared to vehicle-treated mice. Lung virus titers were decreased, and histopathological changes were ameliorated in compound 2-treated mice compared to vehicle-treated mice. Structural investigation using high-resolution crystallography illuminated binding interactions of 3CLpro of SARS-CoV-2 and SARS-CoV with deuterated variants of GC376. Taken together, deuterated GC376 variants have excellent potential as antiviral agents against SARS-CoV-2., Competing Interests: Competing interest statement: Kansas State University, Wichita State University, and The University of Iowa have jointly filed a provisional patent application covering compound 1-11 as well as related compounds, with K.-O.C., Y.K., S.P., and W.C.G. as inventors. Title: Improvement of efficacy and pharmacokinetics of protease inhibitors via deuteration (application 63036542). No author is involved in consulting regarding this study., (Copyright © 2021 the Author(s). Published by PNAS.)

Worse outcomes occur in aged compared with young populations after infections with respiratory viruses, including pathogenic coronaviruses (SARS-CoV, MERS-CoV, and SARS-CoV-2), and are associated with a suboptimal lung milieu ("inflammaging"). We previously showed that a single inducible phospholipase, PLA2G2D, is associated with a proresolving/antiinflammatory response in the lungs, and increases with age. Survival was increased in naive Pla2g2d-/- mice infected with SARS-CoV resulting from augmented respiratory dendritic cell (rDC) activation and enhanced priming of virus-specific T cells. Here, in contrast, we show that intranasal immunization provided no additional protection in middle-aged Pla2g2d-/- mice infected with any of the 3 pathogenic human coronaviruses because virtually no virus-specific antibodies or follicular helper CD4+ T (Tfh) cells were produced. Using MERS-CoV-infected mice, we found that these effects did not result from T or B cell intrinsic factors. Rather, they resulted from enhanced, and ultimately, pathogenic rDC activation, as manifested most prominently by enhanced IL-1β expression. Wild-type rDC transfer to Pla2g2d-/- mice in conjunction with partial IL-1β blockade reversed this defect and resulted in increased virus-specific antibody and Tfh responses. Together, these results indicate that PLA2G2D has an unexpected role in the lungs, serving as an important modulator of rDC activation, with protective and pathogenic effects in respiratory coronavirus infections and immunization, respectively.

Studies of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2)-infected patients and experimentally infected animals indicate a critical role for augmented expression of proinflammatory chemokines and cytokines in severe disease. Here, we demonstrate that SARS-CoV-2 infection of human monocyte-derived macrophages (MDMs) and monocyte-derived dendritic cells was abortive, but induced the production of multiple antiviral and proinflammatory cytokines (interferon-α, interferon-β, tumor necrosis factor, and interleukins 1β, 6, and 10) and a chemokine (CXCL10). Despite the lack of efficient replication in MDMs, SARS-CoV-2 induced profound interferon-mediated cell death of host cells. Macrophage activation and death were 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 coronavirus disease 2019 pathogenesis, even in the absence of productive infection., (© The Author(s) 2020. Published by Oxford University Press for the Infectious Diseases Society of America. All rights reserved. For permissions, e-mail: journals.permissions@oup.com.)

The coronavirus disease 2019 (COVID-19) pandemic continues to cause morbidity and mortality. Since SARS coronavirus 2 (SARS-CoV-2) was identified as the cause for COVID-19, some have questioned whether exposure to seasonal common cold coronaviruses (CCCs) could provide tangible protection against SARS-CoV-2 infection or disease. In this issue of the JCI, Sagar et al. examined SARS-CoV-2 infections and outcomes of patients who had previously tested positive or negative for CCC infection (CCC+ or CCC-) by a comprehensive respiratory panel using PCR. No differences were seen between groups in terms of susceptibility to SARS-CoV-2 infection. However, hospitalized patients with a documented history of CCC infection had lower rates of intensive care unit (ICU) admissions and higher rates of survival than hospitalized CCC- patients. While these findings are associative and not causative, they highlight evidence suggesting that previous CCC infection may influence the disease course of SARS-CoV-2 infection.

The ongoing coronavirus disease 2019 (COVID-19) pandemic is associated with substantial morbidity and mortality. Although much has been learned in the first few 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 anosmia show no or only minor respiratory symptoms 1 . Studies in animals infected experimentally with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the cause of COVID-19, provide opportunities to study aspects of the disease that are not easily investigated in human patients. Although the severity of COVID-19 ranges from asymptomatic to lethal 2 , most experimental infections provide insights into mild disease 3 . Here, using K18-hACE2 transgenic mice that were 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. Furthermore, we show that infusion of convalescent plasma from a recovered patient with COVID-19 protected against lethal disease. Mice developed anosmia at early time points after infection. Notably, although pre-treatment with convalescent plasma prevented most signs of clinical disease, it did not prevent anosmia. Thus, K18-hACE2 mice provide a useful model for studying the pathological basis of both mild and lethal COVID-19 and for assessing therapeutic interventions.

Although Omicron RBD of SARS-CoV-2 accumulates many mutations, the backbone region (truncated RBD) of spike protein is highly conserved. Here, we designed several subunit vaccines by keeping the conserved spike backbone region of SARS-CoV-2 Omicron BA.1 subvariant (S-6P-no-RBD), or inserting the RBD of Delta variant (S-6P-Delta-RBD), Omicron (BA.5) variant (S-6P-BA5-RBD), or ancestral SARS-CoV-2 (S-6P-WT-RBD) to the above backbone construct, and evaluated their ability to induce immune responses and cross-protective efficacy against various SARS-CoV-2 variants and SARS-CoV. Among the four subunit vaccines, S-6P-Delta-RBD protein elicited broad and potent neutralizing antibodies against all SARS-CoV-2 variants tested, including Alpha, Beta, Gamma, and Delta variants, the BA.1, BA.2, BA.2.75, BA.4.6, and BA.5 Omicron subvariants, and the ancestral strain of SARS-CoV-2. This vaccine prevented infection and replication of SARS-CoV-2 Omicron, and completely protected immunized mice against lethal challenge with the SARS-CoV-2 Delta variant and SARS-CoV. Sera from S-6P-Delta-RBD-immunized mice protected naive mice against challenge with the Delta variant, with significantly reduced viral titers and without pathological effects. Protection correlated positively with the serum neutralizing antibody titer. Overall, the designed vaccine has potential for development as a universal COVID-19 vaccine and/or a pan-sarbecovirus subunit vaccine that will prevent current and future outbreaks caused by SARS-CoV-2 variants and SARS-related CoVs. [ABSTRACT FROM AUTHOR]

Antigenic drift of SARS-CoV-2 is typically defined by mutations in the N-terminal domain and receptor binding domain of spike protein. In contrast, whether antigenic drift occurs in the S2 domain remains largely elusive. Here, we perform a deep mutational scanning experiment to identify S2 mutations that affect binding of SARS-CoV-2 spike to three S2 apex public antibodies. Our results indicate that spatially diverse mutations, including D950N and Q954H, which are observed in Delta and Omicron variants, respectively, weaken the binding of spike to these antibodies. Although S2 apex antibodies are known to be nonneutralizing, we show that they confer protection in vivo through Fc-mediated effector functions. Overall, this study indicates that the S2 domain of SARS-CoV-2 spike can undergo antigenic drift, which represents a potential challenge for the development of more universal coronavirus vaccines. [ABSTRACT FROM AUTHOR]

SARS-CoV-2, the causative agent of COVID-19 encodes at least 16 nonstructural proteins of variably understood function. Nsp3, the largest nonstructural protein contains several domains, including a SARS-unique domain (SUD), which occurs only in Sarbecovirus. The SUD has a role in preferentially enhancing viral translation. During isolation of mouse-adapted SARS-CoV-2, we isolated an attenuated virus that contained a single mutation in a linker region of nsp3 (nsp3-S676T). The S676T mutation decreased virus replication in cultured cells and primary human cells and in mice. Nsp3-S676T alleviated the SUD translational enhancing ability by decreasing the interaction between two translation factors, Paip1 and PABP1. We also identified a compensatory mutation in the nucleocapsid (N) protein (N-S194L) that restored the virulent phenotype, without directly binding to SUD. Together, these results reveal an aspect of nsp3-N interactions, which impact both SARS-CoV-2 replication and, consequently, pathogenesis. [ABSTRACT FROM AUTHOR]

The article focuses on investigating the long-term sequelae of SARS-CoV-2 infection, known as postacute sequelae of COVID-19 (PASC), with an emphasis on understanding the potential role of persistently infected cells in contributing to PASC. The topics include the use of a lineage-tracing approach in mice to track previously infected cells, the assessment of SARS-CoV-2 infection in various organs over time.

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection continues to be a serious global public health threat. The 3C-like protease (3CLpro) is a virus protease encoded by SARS-CoV-2, which is essential for virus replication. We have previously reported a series of small-molecule 3CLpro inhibitors effective for inhibiting replication of human coronaviruses including SARS-CoV-2 in cell culture and in animal models. Here we generated a series of deuterated variants of a 3CLpro inhibitor, GC376, and evaluated the antiviral effect against SARS-CoV-2. The deuterated GC376 displayed potent inhibitory activity against SARS-CoV-2 in the enzyme and the cell-based assays. The K18-hACE2 mice develop mild to lethal infection commensurate with SARS-CoV-2 challenge doses and were proposed as a model for efficacy testing of antiviral agents. We treated lethally infected mice with a deuterated derivative of GC376. Treatment of K18-hACE2 mice at 24 h postinfection with a derivative (compound 2) resulted in increased survival of mice compared to vehicle-treated mice. Lung virus titers were decreased, and histopathological changes were ameliorated in compound 2-treated mice compared to vehicle-treated mice. Structural investigation using high-resolution crystallography illuminated binding interactions of 3CLpro of SARS-CoV-2 and SARS-CoV with deuterated variants of GC376. Taken together, deuterated GC376 variants have excellent potential as antiviral agents against SARS-CoV-2. [ABSTRACT FROM AUTHOR]

Worse outcomes occur in aged compared with young populations after infections with respiratory viruses, including pathogenic coronaviruses (SARS-CoV, MERS-CoV, and SARS-CoV-2), and are associated with a suboptimal lung milieu ("inflammaging"). We previously showed that a single inducible phospholipase, PLA2G2D, is associated with a proresolving/antiinflammatory response in the lungs, and increases with age. Survival was increased in naive Pla2g2d-/- mice infected with SARS-CoV resulting from augmented respiratory dendritic cell (rDC) activation and enhanced priming of virus-specific T cells. Here, in contrast, we show that intranasal immunization provided no additional protection in middle-aged Pla2g2d-/- mice infected with any of the 3 pathogenic human coronaviruses because virtually no virus-specific antibodies or follicular helper CD4+ T (Tfh) cells were produced. Using MERS-CoV-infected mice, we found that these effects did not result from T or B cell intrinsic factors. Rather, they resulted from enhanced, and ultimately, pathogenic rDC activation, as manifested most prominently by enhanced IL-1β expression. Wild-type rDC transfer to Pla2g2d-/- mice in conjunction with partial IL-1β blockade reversed this defect and resulted in increased virus-specific antibody and Tfh responses. Together, these results indicate that PLA2G2D has an unexpected role in the lungs, serving as an important modulator of rDC activation, with protective and pathogenic effects in respiratory coronavirus infections and immunization, respectively. [ABSTRACT FROM AUTHOR]

However, monkeys do not develop clinical disease from SARS-CoV-2 and pulmonary lesions are often focal and mild, and they rapidly resolve with virologic control. The cause was identified as a novel CoV, SARS-CoV, which may have originated from wild animals in a Chinese city food market. However, the NHP model that develops mild asymptomatic COVID-19 pneumonia complements the Syrian hamster model that develops clinical disease with severe pneumonia, for mechanistic studies into the disparate clinicopathological manifestations of SARS-CoV-2. We believe that this special issue with emphasis on SARS-CoV-2 pathology will complement and clarify our current perspective of COVID-19 from experimental as well as natural diseases in animals. [Extracted from the article]

Coronavirus disease 2019 (COVID-19), caused by severe acute respiratory syndrome-coronavirus-2 (SARS-CoV-2) has resulted in substantial morbidity and mortality. The basis of severe disease in humans is difficult to determine without the use of experimental animal models. Mice are resistant to infection with ancestral strains of SARS-CoV-2, although many variants that arose later in the pandemic were able to directly infect mice. In almost all cases, viruses that naturally infected mice or were engineered to enable mouse infection required mouse passage to become virulent. In most cases, changes in structural and nonstructural changes occurred during mouse adaptation. However, the mechanism of increased virulence in mice is not understood. Here, using a recently described strain of mouse-adapted SARS-CoV-2 (rSARS2-MA30N501Y), we engineered a series of recombinant viruses that expressed a subset of the mutations present in rSARS2-MA30N501Y. Mutations were detected in the spike protein and in three nonstructural proteins (nsp4, nsp8, and nsp9). We found that infection of mice with recombinant SARS-CoV-2 expressing only the S protein mutations caused very mild infection. Addition of the mutations in nsp4 and nsp8 was required for complete virulence. Of note, all these recombinant viruses replicated equivalently in cultured cells. The innate immune response was delayed after infection with virulent compared to attenuated viruses. Further, using a lineage tracking system, we found that attenuated virus was highly inhibited in the ability to infect the parenchyma, but not the airway after infection. Together, these results indicate that mutations in both the S protein and nonstructural proteins are required for maximal virulence during mouse adaptation. IMPORTANCE Understanding the pathogenesis of coronavirus disease 2019 (COVID-19) requires the study of experimental animals after infection with severe acute respiratory syndrome-coronavirus-2 (SARS-CoV-2). For this purpose, several mouse-adapted SARS-CoV-2 strains have been developed. Here, using a strain of mouse-adapted virus that causes a range of diseases ranging from mild to severe, we show that mutations in both a structural protein [spike (S) protein] and nonstructural proteins are required for maximal virulence. Thus, changes in the S protein, the most widely studied viral protein, while required for mouse adaptation, are not sufficient to result in a virulent virus. [ABSTRACT FROM AUTHOR]

The Coronavirus Disease 2019 (COVID-19) pandemic continues to cause extraordinary loss of life and economic damage. Animal models of severe acute respiratory syndrome-coronavirus-2 (SARS-CoV-2) infection are needed to better understand disease pathogenesis and evaluate preventive measures and therapies. While mice are widely used to model human disease, mouse angiotensin converting enzyme 2 (ACE2) does not bind the ancestral SARS-CoV-2 spike protein to mediate viral entry. To overcome this limitation, we "humanized" mouse Ace2 using CRISPR gene editing to introduce a single amino acid substitution, H353K, predicted to facilitate S protein binding. While H353K knockin Ace2 (mACE2H353K) mice supported SARS-CoV-2 infection and replication, they exhibited minimal disease manifestations. Following 30 serial passages of ancestral SARS-CoV-2 in mACE2H353K mice, we generated and cloned a more virulent virus. A single isolate (SARS2MA-H353K) was prepared for detailed studies. In 7-11-month-old mACE2H353K mice, a 104 PFU inocula resulted in diffuse alveolar disease manifested as edema, hyaline membrane formation, and interstitial cellular infiltration/thickening. Unexpectedly, the mouse-adapted virus also infected standard BALB/c and C57BL/6 mice and caused severe disease. The mouse-adapted virus acquired five new missense mutations including two in spike (K417E, Q493K), one each in nsp4, nsp9, and M and a single nucleotide change in the 5' untranslated region. The Q493K spike mutation arose early in serial passage and is predicted to provide affinity-enhancing molecular interactions with mACE2 and further increase the stability and affinity to the receptor. This new model and mouse-adapted virus will be useful to evaluate COVID-19 disease and prophylactic and therapeutic interventions. IMPORTANCE We developed a new mouse model with a humanized angiotensin converting enzyme 2 (ACE2) locus that preserves native regulatory elements. A single point mutation in mouse ACE2 (H353K) was sufficient to confer in vivo infection with ancestral severe acute respiratory syndrome-coronavirus-2 virus. Through in vivo serial passage, a virulent mouse-adapted strain was obtained. In aged mACE2H353K mice, the mouse-adapted strain caused diffuse alveolar disease. The mouse-adapted virus also infected standard BALB/c and C57BL/6 mice, causing severe disease. The mouse-adapted virus acquired five new missense mutations including two in spike (K417E, Q493K), one each in nsp4, nsp9, and M and a single nucleotide change in the 5' untranslated region. The Q493K spike mutation arose early in serial passage and is predicted to provide affinity-enhancing molecular interactions with mACE2 and further increase the stability and affinity to the receptor. This new model and mouse-adapted virus will be useful to evaluate COVID-19 disease and prophylactic and therapeutic interventions. [ABSTRACT FROM AUTHOR]

Transmission-blocking vaccines are urgently needed to reduce transmission of SARS-CoV 2, the cause of the COVID-19 pandemic. The upper respiratory tract is an initial site of SARS-CoV-2 infection and, for many individuals, remains the primary site of virus replication. An ideal COVID-19 vaccine should reduce upper respiratory tract virus replication and block transmission as well as protect against severe disease. Here, we optimized a vaccine candidate, parainfluenza virus 5 (PIV5) expressing the SARS-CoV-2 S protein (CVXGA1), and then demonstrated that a single-dose intranasal immunization with CVXGA1 protects against lethal infection of K18-hACE2 mice, a severe disease model. CVXGA1 immunization also prevented virus infection of ferrets and blocked contact transmission. This mucosal vaccine strategy inhibited SARS-CoV-2 replication in the upper respiratory tract, thus preventing disease progression to the lower respiratory tract. A PIV5-based mucosal vaccine provides a strategy to induce protective innate and cellular immune responses and reduce SARS-CoV-2 infection and transmission in populations. [ABSTRACT FROM AUTHOR]