A lethal mouse model for evaluating vaccine-associated enhanced respiratory disease during SARS-CoV-2 infection

A Mouse Model of Ovalbumin-Induced Airway Allergy Exhibits Altered Localization of SARS-CoV-2-Susceptible Cells in the Lungs, Which Reflects Omicron BA.5 Infection Dynamics, Viral Mutations, and Immunopathology

Asthma, an allergic disease of the airways, is a risk factor for severity of common respiratory viral infections; however, the relationship between asthma and severity in COVID-19 remains unclear. Here, we examined the effects of SARS-CoV-2 (Omicron BA.5 strain) infection in a mouse model of airway allergy. First, stimulation of allergic mice with OVA resulted in the appearance of ACE2-negative mucus-secreting goblet cells in the bronchiolar region, and an increase in the number of ACE2-expressing cells in the alveoli. As a result, ACE2-expressing cells, which are susceptible to SARS-CoV-2, were limited to the distal portion of the bronchioles while they increased in the alveolar area. After viral infection, the peak infectious viral load in the OVA group was 100-fold lower than that in the phosphate buffered saline (PBS) group; however, clearance of viral RNA from the upper/lower airways was delayed. There were notable differences in acquisition of nsp5 and nsp6 mutations by the Omicron BA.5 strain recovered from BALF samples obtained from the OVA and PBS groups. Immune responses associated with viral clearance were essentially the same, but expression of granulocyte-associated chemokines was higher, M2 macrophage responses were predominant, and the higher spike-specific IgG1/IgG2a ratio in the OVA group post-infection. Infection localized in the alveolar region earlier in the OVA group, resulting in more severe alveolar damage than in the PBS group. These data suggest a Th2-shifted immune background and altered localization of SARS-CoV-2 susceptible cells in mice with OVA-induced airway allergy, which reflect Omicron BA.5 infection dynamics, viral mutations, and immunopathology.

Mucosal Immunization with an Influenza Vector Carrying SARS-CoV-2 N Protein Protects Naïve Mice and Prevents Disease Enhancement in Seropositive Th2-Prone Mice

Background/Objectives: Intranasal vaccination enhances protection against respiratory viruses by providing stimuli to the immune system at the primary site of infection, promoting a balanced and effective response. Influenza vectors with truncated NS1 are a promising vaccine approach that ensures a pronounced local CD8+ T-cellular immune response. Here, we describe the protective and immunomodulating properties of an influenza vector FluVec-N carrying the C-terminal fragment of the SARS-CoV-2 nucleoprotein within a truncated NS1 open reading frame. Methods: We generated several FluVec-N recombinant vectors by reverse genetics and confirmed the vector's genetic stability, antigen expression in vitro, attenuation, and immunogenicity in a mouse model. We tested the protective potential of FluVec-N intranasal immunization in naïve mice and seropositive Th2-prone mice, primed with aluminium-adjuvanted inactivated SARS-CoV-2. Immune response in immunized and challenged mice was analyzed through serological methods and flow cytometry. Results: Double intranasal immunization of naïve mice with FluVec-N reduced weight loss and viral load in the lungs following infection with the SARS-CoV-2 beta variant. Mice primed with alum-adjuvanted inactivated coronavirus experienced substantial early weight loss and eosinophilia in the lungs during infection, demonstrating signs of enhanced disease. A single intranasal boost immunization with FluVec-N prevented the disease enhancement in primed mice by modulating the local immune response. Protection was associated with the formation of specific IgA and the early activation of virus-specific effector and resident CD8+ lymphocytes in mouse lungs. Conclusions: Our study supports the potential of immunization with influenza vector vaccines to prevent respiratory diseases and associated immunopathology.

Amino acid T25 in the substrate-binding domain of SARS-CoV-2 nsp5 is involved in viral replication in the mouse lung

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) non-structural protein 5 (nsp5) is a cysteine protease involved in viral replication and suppression of the host immune system. The substrate-binding domain of nsp5 is important for its protease activity. However, the relationship between nsp5 protease activity and viral replication remains unclear. We confirmed the importance of amino acid T25 in the nsp5 substrate-binding domain for viral replication using a split luciferase assay. By generating recombinant viruses using bacterial artificial chromosomes, we found that the proliferation of viruses with the T25I mutation in nsp5 was cell-dependent in culture. Furthermore, mice infected with the T25I mutant recombinant virus with a mouse acclimation backbone showed weight loss and increased lung viral load, similar to the wild-type (WT) infected group, up to 3 days after infection. However, after day 4, the lung viral load was significantly reduced in the T25I-infected group compared to that in the WT-infected group. This suggests that nsp5 T25 is involved in the pathogenesis of SARS-CoV-2.

An outbreak of SARS-CoV-2 omicron variant and deaths of three lions in a zoo

There have been reports of the transmission of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) from humans to various mammalian species. Some infected animals show clinical signs and may even die in rare cases. Outbreaks of SARS-CoV-2 have been reported in zoos where susceptible animals are bred in high population densities. However, there have been few reports of omicron variant outbreaks in zoo animals. From late 2022 to 2023, an outbreak of the SARS-CoV-2 omicron variant occurred in one Japanese zoo. A total of 24 lions were housed in the zoo; 13 of them showed respiratory symptoms, and the three oldest lions died. Molecular and histopathological analyses revealed that the deceased lions were infected with SARS-CoV-2 omicron BF.7.15. Virus-neutralization tests showed that all 21 lions were positive for antibodies against the omicron variant, but not against the delta variant. In addition, three tigers and one bear in the same or neighboring building as the lions possessed antibodies against the omicron variant. This is a very rare report on the outbreak of a SARS-CoV-2 omicron variant infection that resulted in the death of animals. This finding demonstrates the importance of continuous countermeasures to protect non-vaccinated animals from SARS-CoV-2 infection.

TNF-α exacerbates SARS-CoV-2 infection by stimulating CXCL1 production from macrophages

Since most genetically modified mice are C57BL/6 background, a mouse-adapted SARS-CoV-2 that causes lethal infection in young C57BL/6 mice is useful for studying innate immune protection against SARS-CoV-2 infection. Here, we established two mouse-adapted SARS-CoV-2, ancestral and Delta variants, by serial passaging 80 times in C57BL/6 mice. Although young C57BL/6 mice were resistant to infection with the mouse-adapted ancestral SARS-CoV-2, the mouse-adapted SARS-CoV-2 Delta variant caused lethal infection in young C57BL/6 mice. In contrast, MyD88 and IFNAR1 KO mice exhibited resistance to lethal infection with the mouse-adapted SARS-CoV-2 Delta variant. Treatment with recombinant IFN-α/β at the time of infection protected mice from lethal infection with the mouse-adapted SARS-CoV-2 Delta variant, but intranasal administration of recombinant IFN-α/β at 2 days post infection exacerbated the disease severity following the mouse-adapted ancestral SARS-CoV-2 infection. Moreover, we showed that TNF-α amplified by type I IFN signals exacerbated the SARS-CoV-2 infection by stimulating CXCL1 production from macrophages and neutrophil recruitment into the lung tissue. Finally, we showed that intravenous administration to mice or hamsters with TNF protease inhibitor 2 alleviated the severity of SARS-CoV-2 and influenza virus infection. Our results uncover an unexpected mechanism by which type I interferon-mediated TNF-α signaling exacerbates the disease severity and will aid in the development of novel therapeutic strategies to treat respiratory virus infection and associated diseases such as influenza and COVID-19.

Necroptosis in alveolar epithelial cells drives lung inflammation and injury caused by SARS-CoV-2 infection

COVID-19, caused by SARS-CoV-2 infection, results in irreversible or fatal lung injury. We assumed that necroptosis of virus-infected alveolar epithelial cells (AEC) could promote local inflammation and further lung injury in COVID-19. Since CD8+ lymphocytes induced AEC cell death via cytotoxic molecules such as FAS ligands, we examined the involvement of FAS-mediated cell death in COVID-19 patients and murine COVID-19 model. We identified the occurrence of necroptosis and subsequent release of HMGB1 in the admitted patients with COVID-19. In the mouse model of COVID-19, lung inflammation and injury were attenuated in Fas-deficient mice compared to Fas-intact mice. The infection enhanced Type I interferon-inducible genes in both groups, while inflammasome-associated genes were specifically upregulated in Fas-intact mice. The treatment with necroptosis inhibitor, Nec1s, improved survival rate, lung injury, and systemic inflammation. SARS-CoV-2 induced necroptosis causes cytokine induction and lung damage, and its inhibition could be a novel therapeutic strategy for COVID-19.

Recombinant RSV G protein vaccine induces enhanced respiratory disease via IL-13 and mucin overproduction

The G protein expressed on the surface of respiratory syncytial virus (RSV) is important for adhesion to host cells and as a vaccine target antigen. The corresponding vaccines can effectively eliminate RSV. However, they exacerbate pulmonary immunopathology including eosinophilic infiltration in the lungs after an RSV challenge in animal models, raising concerns about enhanced respiratory disease (ERD); thus, approaches that mitigate these effects are urgently needed. Herein, we aimed to examine the mechanisms of G protein vaccine-induced ERD in mice, using recombinant G protein as a vaccine antigen. After the RSV challenge, G protein-vaccinated mice exhibited lung weight gain, lung tissue damage, and increased infiltration of eosinophils, neutrophils, and CD4+ T cells into the lungs. We set lung weight gain as the endpoint for ERD and examined the impact of each infiltrating cell on lung weight gain. We observed that CD4+ T cells, but not eosinophils or neutrophils, that infiltrate the lungs are responsible for lung weight gain. In addition, T helper 2 cell-mediated IL-13 induced mucin hypersecretion and lung weight gain. Mucin hypersecretion may contribute to weight gain in the lungs. In conclusion, our results indicate a novel mechanism of G protein vaccine-induced ERD via IL-13 and mucin hypersecretion, which could lead to the development of safe G protein vaccines and the elucidation of the causes of ERD associated with other vaccines.

Anti-SARS-CoV-2 gapmer antisense oligonucleotides targeting the main protease region of viral RNA

Given the worldwide risk for the outbreak of emerging/re-emerging respiratory viruses, establishment of new antiviral strategies is greatly demanded. In this study, we present a scheme to identify gapmer antisense oligonucleotides (ASOs) targeting severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) RNA that efficiently inhibit viral replication. We synthesized approximately 300 gapmer ASOs designed to target various SARS-CoV-2 RNA regions and evaluated their activity in cell-based assays. Through a multistep screening in cell culture systems, we identified that ASO#41, targeting the coding region for viral main protease, reduced SARS-CoV-2 RNA levels in infected cells and inhibited virus-induced cytopathic effects. Antiviral effect of ASO#41 was also observed in iPS cell-derived human lung organoids. ASO#41 depleted intracellular viral RNAs during genome replication in an endogenous RNaseH-dependent manner. ASO#41 showed a wide range of antiviral activity against SARS-CoV-2 variants of concern including Alpha, Delta, and Omicron. Intranasal administration to mice exhibited intracellular accumulation of ASO#41 in the lung and significantly reduced the viral infectious titer, with milder body weight loss due to SARS-CoV-2 infection. Further chemical modification with phosphoryl guanidine-containing backbone linkages provided an elevation of anti-SARS-CoV-2 activity, with 23.4 nM of 50% antiviral inhibitory concentration, one of the strongest anti-SARS-CoV-2 ASOs reported so far. Our study presents an approach to identify active ASOs against SARS-CoV-2, which is potentially useful for establishing an antiviral strategy by targeting genome RNA of respiratory viruses.

Generating prophylactic immunity against arboviruses in vertebrates and invertebrates

The World Health Organization recently declared a global initiative to control arboviral diseases. These are mainly caused by pathogenic flaviviruses (such as dengue, yellow fever and Zika viruses) and alphaviruses (such as chikungunya and Venezuelan equine encephalitis viruses). Vaccines represent key interventions for these viruses, with licensed human and/or veterinary vaccines being available for several members of both genera. However, a hurdle for the licensing of new vaccines is the epidemic nature of many arboviruses, which presents logistical challenges for phase III efficacy trials. Furthermore, our ability to predict or measure the post-vaccination immune responses that are sufficient for subclinical outcomes post-infection is limited. Given that arboviruses are also subject to control by the immune system of their insect vectors, several approaches are now emerging that aim to augment antiviral immunity in mosquitoes, including Wolbachia infection, transgenic mosquitoes, insect-specific viruses and paratransgenesis. In this Review, we discuss recent advances, current challenges and future prospects in exploiting both vertebrate and invertebrate immune systems for the control of flaviviral and alphaviral diseases.

Ally, adversary, or arbitrator? The context-dependent role of eosinophils in vaccination for respiratory viruses and subsequent breakthrough infections

Eosinophils are a critical type of immune cell and central players in type 2 immunity. Existing literature suggests that eosinophils also can play a role in host antiviral responses, typically type 1 immune events, against multiple respiratory viruses, both directly through release of antiviral mediators and indirectly through activation of other effector cell types. One way to prime host immune responses toward effective antiviral responses is through vaccination, where typically a type 1-skewed immunity is desirable in the context of intracellular pathogens like respiratory viruses. In the realm of breakthrough respiratory viral infection in vaccinated hosts, an event in which virus can still establish productive infection despite preexisting immunity, eosinophils are most prominently known for their link to vaccine-associated enhanced respiratory disease upon natural respiratory syncytial virus infection. This was observed in a pediatric cohort during the 1960s following vaccination with formalin-inactivated respiratory syncytial virus. More recent research has unveiled additional roles of the eosinophil in respiratory viral infection and breakthrough infection. The specific contribution of eosinophils to the quality of vaccine responses, vaccine efficacy, and antiviral responses to infection in vaccinated hosts remains largely unexplored, especially regarding their potential roles in protection. On the basis of current findings, we will speculate upon the suggested function of eosinophils and consider the many potential ways by which eosinophils may exert protective and pathological effects in breakthrough infections. We will also discuss how to balance vaccine efficacy with eosinophil-related risks, as well as the use of eosinophils and their products as potential biomarkers of vaccine efficacy or adverse events.

A measles-vectored vaccine candidate expressing prefusion-stabilized SARS-CoV-2 spike protein brought to phase I/II clinical trials: candidate selection in a preclinical murine model

In the early COVID-19 pandemic with urgent need for countermeasures, we aimed at developing a replicating viral vaccine using the highly efficacious measles vaccine as vector, a promising technology with prior clinical proof of concept. Building on our successful pre-clinical development of a measles virus (MV)-based vaccine candidate against the related SARS-CoV, we evaluated several recombinant MV expressing codon-optimized SARS-CoV-2 spike glycoprotein. Candidate V591 expressing a prefusion-stabilized spike through introduction of two proline residues in HR1 hinge loop, together with deleted S1/S2 furin cleavage site and additional inactivation of the endoplasmic reticulum retrieval signal, was the most potent in eliciting neutralizing antibodies in mice. After single immunization, V591 induced similar neutralization titers as observed in sera of convalescent patients. The cellular immune response was confirmed to be Th1 skewed. V591 conferred long-lasting protection against SARS-CoV-2 challenge in a murine model with marked decrease in viral RNA load, absence of detectable infectious virus loads, and reduced lesions in the lungs. V591 was furthermore efficacious in an established non-human primate model of disease (see companion article [S. Nambulli, N. Escriou, L. J. Rennick, M. J. Demers, N. L. Tilston-Lunel et al., J Virol 98:e01762-23, 2024, https://doi.org/10.1128/jvi.01762-23]). Thus, V591 was taken forward into phase I/II clinical trials in August 2020. Unexpected low immunogenicity in humans (O. Launay, C. Artaud, M. Lachâtre, M. Ait-Ahmed, J. Klein et al., eBioMedicine 75:103810, 2022, https://doi.org/10.1016/j.ebiom.2021.103810) revealed that the underlying mechanisms for resistance or sensitivity to pre-existing anti-measles immunity are not yet understood. Different hypotheses are discussed here, which will be important to investigate for further development of the measles-vectored vaccine platform.IMPORTANCESARS-CoV-2 emerged at the end of 2019 and rapidly spread worldwide causing the COVID-19 pandemic that urgently called for vaccines. We developed a vaccine candidate using the highly efficacious measles vaccine as vector, a technology which has proved highly promising in clinical trials for other pathogens. We report here and in the companion article by Nambulli et al. (J Virol 98:e01762-23, 2024, https://doi.org/10.1128/jvi.01762-23) the design, selection, and preclinical efficacy of the V591 vaccine candidate that was moved into clinical development in August 2020, 7 months after the identification of SARS-CoV-2 in Wuhan. These unique in-human trials of a measles vector-based COVID-19 vaccine revealed insufficient immunogenicity, which may be the consequence of previous exposure to the pediatric measles vaccine. The three studies together in mice, primates, and humans provide a unique insight into the measles-vectored vaccine platform, raising potential limitations of surrogate preclinical models and calling for further refinement of the platform.

Adjuvant-dependent impact of inactivated SARS-CoV-2 vaccines during heterologous infection by a SARS-related coronavirus

Whole virus-based inactivated SARS-CoV-2 vaccines adjuvanted with aluminum hydroxide have been critical to the COVID-19 pandemic response. Although these vaccines are protective against homologous coronavirus infection, the emergence of novel variants and the presence of large zoonotic reservoirs harboring novel heterologous coronaviruses provide significant opportunities for vaccine breakthrough, which raises the risk of adverse outcomes like vaccine-associated enhanced respiratory disease. Here, we use a female mouse model of coronavirus disease to evaluate inactivated vaccine performance against either homologous challenge with SARS-CoV-2 or heterologous challenge with a bat-derived coronavirus that represents a potential emerging disease threat. We show that inactivated SARS-CoV-2 vaccines adjuvanted with aluminum hydroxide can cause enhanced respiratory disease during heterologous infection, while use of an alternative adjuvant does not drive disease and promotes heterologous viral clearance. In this work, we highlight the impact of adjuvant selection on inactivated vaccine safety and efficacy against heterologous coronavirus infection.

The immunobiology of SARS-CoV-2 infection and vaccine responses: potential influences of cross-reactive memory responses and aging on efficacy and off-target effects

Immune responses to both SARS-CoV-2 infection and its associated vaccines have been highly variable within the general population. The increasing evidence of long-lasting symptoms after resolution of infection, called post-acute sequelae of COVID-19 (PASC) or "Long COVID," suggests that immune-mediated mechanisms are at play. Closely related endemic common human coronaviruses (hCoV) can induce pre-existing and potentially cross-reactive immunity, which can then affect primary SARS-CoV-2 infection, as well as vaccination responses. The influence of pre-existing immunity from these hCoVs, as well as responses generated from original CoV2 strains or vaccines on the development of new high-affinity responses to CoV2 antigenic viral variants, needs to be better understood given the need for continuous vaccine adaptation and application in the population. Due in part to thymic involution, normal aging is associated with reduced naïve T cell compartments and impaired primary antigen responsiveness, resulting in a reliance on the pre-existing cross-reactive memory cell pool which may be of lower affinity, restricted in diversity, or of shorter duration. These effects can also be mediated by the presence of down-regulatory anti-idiotype responses which also increase in aging. Given the tremendous heterogeneity of clinical data, utilization of preclinical models offers the greatest ability to assess immune responses under a controlled setting. These models should now involve prior antigen/viral exposure combined with incorporation of modifying factors such as age on immune responses and effects. This will also allow for mechanistic dissection and understanding of the different immune pathways involved in both SARS-CoV-2 pathogen and potential vaccine responses over time and how pre-existing memory responses, including potential anti-idiotype responses, can affect efficacy as well as potential off-target effects in different tissues as well as modeling PASC.

Defining Parameters That Modulate Susceptibility and Protection to Respiratory Murine Coronavirus MHV1 Infection

Patients infected with SARS-CoV-2 experience variable disease susceptibility, and patients with comorbidities such as sepsis are often hospitalized for COVID-19 complications. However, the extent to which initial infectious inoculum dose determines disease outcomes and whether this can be used for immunological priming in a genetically susceptible host has not been completely defined. We used an established SARS-like murine model in which responses to primary and/or secondary challenges with murine hepatitis virus type 1 (MHV-1) were analyzed. We compared the response to infection in genetically susceptible C3H/HeJ mice, genetically resistant C57BL/6J mice, and genetically diverse, variably susceptible outbred Swiss Webster mice. Although defined as genetically susceptible to MHV-1, C3H/HeJ mice displayed decreasing dose-dependent pathological changes in disease severity and lung infiltrate/edema, as well as lymphopenia. Importantly, an asymptomatic dose (500 PFU) was identified that yielded no measurable morbidity/mortality postinfection in C3H/HeJ mice. Polymicrobial sepsis induced via cecal ligation and puncture converted asymptomatic infections in C3H/HeJ and C57BL/6J mice to more pronounced disease, modeling the impact of sepsis as a comorbidity to β-coronavirus infection. We then used low-dose infection as an immunological priming event in C3H/HeJ mice, which provided neutralizing Ab-dependent, but not circulating CD4/CD8 T cell-dependent, protection against a high-dose MHV-1 early rechallenge. Together, these data define how infection dose, immunological status, and comorbidities modulate outcomes of primary and secondary β-coronavirus infections in hosts with variable susceptibility.

Viral uptake and pathophysiology of the lung endothelial cells in age-associated severe SARS-CoV-2 infection models

Thrombosis is the major cause of death in severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection, and the pathology of vascular endothelial cells (ECs) has received much attention. Although there is evidence of the infection of ECs in human autopsy tissues, their detailed pathophysiology remains unclear due to the lack of animal model to study it. We used a mouse-adapted SARS-CoV-2 virus strain in young and mid-aged mice. Only mid-aged mice developed fatal pneumonia with thrombosis. Pulmonary ECs were isolated from these infected mice and RNA-Seq was performed. The pulmonary EC transcriptome revealed that significantly higher levels of viral genes were detected in ECs from mid-aged mice with upregulation of viral response genes such as DDX58 and IRF7. In addition, the thrombogenesis-related genes encoding PLAT, PF4, F3 PAI-1, and P-selectin were upregulated. In addition, the inflammation-related molecules such as CXCL2 and CXCL10 were upregulated in the mid-aged ECs upon viral infection. Our mouse model demonstrated that SARS-CoV-2 virus entry into aged vascular ECs upregulated thrombogenesis and inflammation-related genes and led to fatal pneumonia with thrombosis. Current results of EC transcriptome showed that EC uptake virus and become thrombogenic by activating neutrophils and platelets in the aged mice, suggesting age-associated EC response as a novel finding in human severe COVID-19.

Th2 and Th17-associated immunopathology following SARS-CoV-2 breakthrough infection in Spike-vaccinated ACE2-humanized mice

Vaccines have demonstrated remarkable effectiveness in protecting against COVID-19; however, concerns regarding vaccine-associated enhanced respiratory diseases (VAERD) following breakthrough infections have emerged. Spike protein subunit vaccines for SARS-CoV-2 induce VAERD in hamsters, where aluminum adjuvants promote a Th2-biased immune response, leading to increased type 2 pulmonary inflammation in animals with breakthrough infections. To gain a deeper understanding of the potential risks and the underlying mechanisms of VAERD, we immunized ACE2-humanized mice with SARS-CoV-2 Spike protein adjuvanted with aluminum and CpG-ODN. Subsequently, we exposed them to increasing doses of SARS-CoV-2 to establish a breakthrough infection. The vaccine elicited robust neutralizing antibody responses, reduced viral titers, and enhanced host survival. However, following a breakthrough infection, vaccinated animals exhibited severe pulmonary immunopathology, characterized by a significant perivascular infiltration of eosinophils and CD4+ T cells, along with increased expression of Th2/Th17 cytokines. Intracellular flow cytometric analysis revealed a systemic Th17 inflammatory response, particularly pronounced in the lungs. Our data demonstrate that aluminum/CpG adjuvants induce strong antibody and Th1-associated immunity against COVID-19 but also prime a robust Th2/Th17 inflammatory response, which may contribute to the rapid onset of T cell-mediated pulmonary immunopathology following a breakthrough infection. These findings underscore the necessity for further research to unravel the complexities of VAERD in COVID-19 and to enhance vaccine formulations for broad protection and maximum safety.

Resolution of Eosinophilic Pneumonia after Coronavirus Disease 2019 without Systemic Corticosteroids

Pulmonary and extrapulmonary complications after coronavirus disease 2019 (COVID-19) have been major public health concerns during the COVID-19 pandemic. Although post-COVID-19 pulmonary manifestations cover a wide spectrum, eosinophilic pneumonia (EP) has rarely been reported. To date, only four cases of EP potentially triggered by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection have been reported, all of which required systemic corticosteroid therapy. We herein report the first case of post-COVID-19 EP resolution without systemic corticosteroid therapy. We also review the literature regarding EP associated with SARS-CoV-2 infection and vaccination.

Th2 and Th17-Associated Immunopathology Following SARS-CoV-2 Breakthrough Infection in Spike-Vaccinated ACE2-humanized Mice

Vaccines have demonstrated remarkable effectiveness in protecting against COVID-19; however, concerns regarding vaccine-associated enhanced respiratory diseases (VAERD) following breakthrough infections have emerged. Spike protein subunit vaccines for SARS-CoV-2 induce VAERD in hamsters, where aluminum adjuvants promote a Th2-biased immune response, leading to increased type 2 pulmonary inflammation in animals with breakthrough infections. To gain a deeper understanding of the potential risks and the underlying mechanisms of VAERD, we immunized ACE2-humanized mice with SARS-CoV-2 Spike protein adjuvanted with aluminum and CpG-ODN. Subsequently, we exposed them to increasing doses of SARS-CoV-2 to establish a breakthrough infection. The vaccine elicited robust neutralizing antibody responses, reduced viral titers, and enhanced host survival. However, following a breakthrough infection, vaccinated animals exhibited severe pulmonary immunopathology, characterized by a significant perivascular infiltration of eosinophils and CD4+ T cells, along with increased expression of Th2/Th17 cytokines. Intracellular flow cytometric analysis revealed a systemic Th17 inflammatory response, particularly pronounced in the lungs. Our data demonstrate that aluminum/CpG adjuvants induce strong antibody and Th1-associated immunity against COVID-19 but also prime a robust Th2/Th17 inflammatory response, which may contribute to the rapid onset of T cell-mediated pulmonary immunopathology following a breakthrough infection. These findings underscore the necessity for further research to unravel the complexities of VAERD in COVID-19 and to enhance vaccine formulations for broad protection and maximum safety.

Preclinical evaluation of PHH-1V vaccine candidate against SARS-CoV-2 in non-human primates

SARS-CoV-2 emerged in December 2019 and quickly spread worldwide, continuously striking with an unpredictable evolution. Despite the success in vaccine production and mass vaccination programs, the situation is not still completely controlled, and therefore accessible second-generation vaccines are required to mitigate the pandemic. We previously developed an adjuvanted vaccine candidate coded PHH-1V, based on a heterodimer fusion protein comprising the RBD domain of two SARS-CoV-2 variants. Here, we report data on the efficacy, safety, and immunogenicity of PHH-1V in cynomolgus macaques. PHH-1V prime-boost vaccination induces high levels of RBD-specific IgG binding and neutralizing antibodies against several SARS-CoV-2 variants, as well as a balanced Th1/Th2 cellular immune response. Remarkably, PHH-1V vaccination prevents SARS-CoV-2 replication in the lower respiratory tract and significantly reduces viral load in the upper respiratory tract after an experimental infection. These results highlight the potential use of the PHH-1V vaccine in humans, currently undergoing Phase III clinical trials.

Adenosine deaminase augments SARS-CoV-2 specific cellular and humoral responses in aged mouse models of immunization and challenge

Despite numerous clinically available vaccines and therapeutics, aged patients remain at increased risk for COVID-19 morbidity. Furthermore, various patient populations, including the aged can have suboptimal responses to SARS-CoV-2 vaccine antigens. Here, we characterized vaccine-induced responses to SARS-CoV-2 synthetic DNA vaccine antigens in aged mice. Aged mice exhibited altered cellular responses, including decreased IFNγ secretion and increased TNFα and IL-4 secretion suggestive of TH2-skewed responses. Aged mice exhibited decreased total binding and neutralizing antibodies in their serum but significantly increased TH2-type antigen-specific IgG1 antibody compared to their young counterparts. Strategies to enhance vaccine-induced immune responses are important, especially in aged patient populations. We observed that co-immunization with plasmid-encoded adenosine deaminase (pADA)enhanced immune responses in young animals. Ageing is associated with decreases in ADA function and expression. Here, we report that co-immunization with pADA enhanced IFNγ secretion while decreasing TNFα and IL-4 secretion. pADA expanded the breadth and affinity SARS-CoV-2 spike-specific antibodies while supporting TH1-type humoral responses in aged mice. scRNAseq analysis of aged lymph nodes revealed that pADA co-immunization supported a TH1 gene profile and decreased FoxP3 gene expression. Upon challenge, pADA co-immunization decreased viral loads in aged mice. These data support the use of mice as a model for age-associated decreased vaccine immunogenicity and infection-mediated morbidity and mortality in the context of SARS-CoV-2 vaccines and provide support for the use of adenosine deaminase as a molecular adjuvant in immune-challenged populations.

Results from a preclinical study in rodents and a Phase 1/2, randomized, double-blind, placebo-controlled, parallel-group study of COVID-19 vaccine S-268019-a in Japanese adults

BACKGROUND

In early 2020, developing vaccines was an urgent need for preventing COVID-19 from a contingency perspective.

METHODS

S-268019-a is a recombinant protein-based vaccine against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), comprising a modified recombinant spike protein antigen adjuvanted with agatolimod sodium, a Toll-like receptor-9 agonist. In the preclinical phase, it was administered intramuscularly twice at a 2-week interval in 7-week-old mice. Immunogenicity was assessed, and the mice were challenged intranasally with mouse-adapted SARS-CoV-2 at 2 and 8 weeks, respectively, after the second immunization. After confirming the preclinical effect, a Phase 1/2, randomized, parallel-group clinical study was conducted in healthy adults (aged 20-64 years). All participants received 2 intramuscular injections at various combinations of the antigen and the adjuvant (S-910823/agatolimod sodium, in μg: 12.5/250, 25/250, 50/250, 25/500, 50/500, 100/500, 10/500, 100/100, 200/1000) or placebo (saline) in an equivalent volume at a 3-week interval and were followed up until Day 50 in this interim analysis.

RESULTS

In the preclinical studies, S-268019-a was safe and elicited robust immunoglobulin G (IgG) and neutralizing antibody responses in mice. When challenged with SARS-CoV-2, all S-268019-a-treated mice survived and maintained weight until 10 days, whereas all placebo- or adjuvant-treated (without antigen) mice died within 6 days. In the Phase 1/2 trial, although S-268019-a was well tolerated in adult participants, was safe up to Day 50, and elicited robust anti-spike protein IgG antibodies, it did not elicit sufficient neutralizing antibody levels.

CONCLUSIONS

The S-268019-a vaccine was not sufficiently immunogenic in Japanese adults despite robust immunogenicity and efficacy in mice. Our results exemplify the innate challenges in translating preclinical data in animals to clinical trials, and highlight the need for continued research to overcome such barriers. (jRCT2051200092).

Impact of Reinfection with SARS-CoV-2 Omicron Variants in Previously Infected Hamsters

The diversity of SARS-CoV-2 mutations raises the possibility of reinfection of individuals previously infected with earlier variants, and this risk is further increased by the emergence of the B.1.1.529 Omicron variant. In this study, we used an in vivo, hamster infection model to assess the potential for individuals previously infected with SARS-CoV-2 to be reinfected with Omicron variant and we also investigated the pathology associated with such infections. Initially, Syrian hamsters were inoculated with a lineage A, B.1.1.7, B.1.351, B.1.617.2 or a subvariant of Omicron, BA.1 strain and then reinfected with the BA.1 strain 5 weeks later. Subsequently, the impact of reinfection with Omicron subvariants (BA.1 and BA.2) in individuals previously infected with the BA.1 strain was examined. Although viral infection and replication were suppressed in both the upper and lower airways, following reinfection, virus-associated RNA was detected in the airways of most hamsters. Viral replication was more strongly suppressed in the lower respiratory tract than in the upper respiratory tract. Consistent amino acid substitutions were observed in the upper respiratory tract of infected hamsters after primary infection with variant BA.1, whereas diverse mutations appeared in hamsters reinfected with the same variant. Histopathology showed no acute pneumonia or disease enhancement in any of the reinfection groups and, in addition, the expression of inflammatory cytokines and chemokines in the airways of reinfected animals was only mildly elevated. These findings are important for understanding the risk of reinfection with new variants of SARS-CoV-2. IMPORTANCE The emergence of SARS-CoV-2 variants and the widespread use of COVID-19 vaccines has resulted in individual differences in immune status against SARS-CoV-2. A decay in immunity over time and the emergence of variants that partially evade the immune response can also lead to reinfection. In this study, we demonstrated that, in hamsters, immunity acquired following primary infection with previous SARS-CoV-2 variants was effective in preventing the onset of pneumonia after reinfection with the Omicron variant. However, viral infection and multiplication in the upper respiratory tract were still observed after reinfection. We also showed that more diverse nonsynonymous mutations appeared in the upper respiratory tract of reinfected hamsters that had acquired immunity from primary infection. This hamster model reveals the within-host evolution of SARS-CoV-2 and its pathology after reinfection, and provides important information for countermeasures against diversifying SARS-CoV-2 variants.

Phenothiazines inhibit SARS-CoV-2 cell entry via a blockade of spike protein binding to neuropilin-1

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) enters cells using angiotensin-converting enzyme 2 (ACE2) and neuropilin-1 (NRP-1) as the primary receptor and entry co-factor, respectively. Cell entry is the first and major step in initiation of the viral life cycle, representing an ideal target for antiviral interventions. In this study, we used a recombinant replication-deficient vesicular stomatitis virus-based pseudovirus bearing the spike protein of SARS-CoV-2 (SARS2-S) to screen a US Food and Drug Administration-approved drug library and identify inhibitors of SARS-CoV-2 cell entry. The screen identified 24 compounds as primary hits, and the largest therapeutic target group formed by these primary hits was composed of seven dopamine receptor D2 (DRD2) antagonists. Cell-based and biochemical assays revealed that the DRD2 antagonists inhibited both fusion activity and the binding of SARS2-S to NRP-1, but not its binding to ACE2. On the basis of structural similarity to the seven identified DRD2 antagonists, which included six phenothiazines, we examined the anti-SARS-CoV-2 activity of an additional 15 phenothiazines and found that all the tested phenothiazines shared an ability to inhibit SARS2-S-mediated cell entry. One of the phenothiazines, alimemazine, which had the lowest 50% effective concentration of the tested phenothiazines, exhibited a clear inhibitory effect on SARS2-S-NRP-1 binding and SARS-CoV-2 multiplication in cultured cells but not in a mouse infection model. Our findings provide a basis for the development of novel anti-SARS-CoV-2 therapeutics that interfere with SARS2-S binding to NRP-1.

Toward Establishing an Ideal Adjuvant for Non-Inflammatory Immune Enhancement

The vertebrate immune system functions to eliminate invading foreign nucleic acids and foreign proteins from infectious diseases and malignant tumors. Because pathogens and cancer cells have unique amino acid sequences and motifs (e.g., microbe-associated molecular patterns, MAMPs) that are recognized as "non-self" to the host, immune enhancement is one strategy to eliminate invading cells. MAMPs contain nucleic acids specific or characteristic of the microbe and are potential candidates for immunostimulants or adjuvants. Adjuvants are included in many vaccines and are a way to boost immunity by deliberately administering them along with antigens. Although adjuvants are an important component of vaccines, it is difficult to evaluate their efficacy ex vivo and in vivo on their own (without antigens). In addition, inflammation induced by currently candidate adjuvants may cause adverse events, which is a hurdle to their approval as drugs. In addition, the lack of guidelines for evaluating the safety and efficacy of adjuvants in drug discovery research also makes regulatory approval difficult. Viral double-stranded (ds) RNA mimics have been reported as potent adjuvants, but the safety barrier remains unresolved. Here we present ARNAX, a noninflammatory nucleic acid adjuvant that selectively targets Toll-like receptor 3 (TLR3) in antigen-presenting dendritic cells (APCs) to safely induce antigen cross-presentation and subsequently induce an acquired immune response independent of inflammation. This review discusses the challenges faced in the clinical development of novel adjuvants.

Non-Neutralizing Epitopes Shade Neutralizing Epitopes against Omicron in a Multiple Epitope-Based Vaccine

The ongoing coronavirus disease 2019 pandemic has raised concerns about the risk of re-infection. Non-neutralizing epitopes are one of the major reasons for antibody-dependent enhancement. Past studies on the ancestral severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) have revealed an infectivity-enhancing site on the ancestral SARS-CoV-2 spike protein. However, infection enhancement associated with the SARS-CoV-2 Omicron strain remains elusive. In this study, we examined the antibodies induced by a multiple epitope-based vaccine, which showed infection enhancement for the Omicron strain but not for the ancestral SARS-CoV-2 or Delta strain. By examining the antibodies induced by single epitope-based vaccines, we identified a conserved epitope, IDf (450-469), with neutralizing activity against ancestral SARS-CoV-2, Delta, and Omicron. Although neutralizing epitopes are present in the multiple epitope-based vaccine, other immunodominant non-neutralizing epitopes such as IDg (480-499) can shade their neutralizing activity, leading to infection enhancement of Omicron. Our study provides up-to-date epitope information on SARS-CoV-2 variants to help design better vaccines or antibody-based therapeutics against future variants.

Immune response and protective efficacy of the SARS-CoV-2 recombinant spike protein vaccine S-268019-b in mice

Vaccines that efficiently target severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the etiological agent for coronavirus disease (COVID-19), are the best means for controlling viral spread. This study evaluated the efficacy of the COVID-19 vaccine S-268019-b, which comprises the recombinant full-length SARS-CoV-2 spike protein S-910823 (antigen) and A-910823 (adjuvant). In addition to eliciting both Th1-type and Th2-type cellular immune responses, two doses of S-910823 plus A-910823 induced anti-spike protein IgG antibodies and neutralizing antibodies against SARS-CoV-2. In a SARS-CoV-2 challenge test, S-910823 plus A-910823 mitigated SARS-CoV-2 infection-induced weight loss and death and inhibited viral replication in mouse lungs. S-910823 plus A-910823 promoted cytokine and chemokine at the injection site and immune cell accumulation in the draining lymph nodes. This led to the formation of germinal centers and the induction of memory B cells, antibody-secreting cells, and memory T cells. These findings provide fundamental property of S-268019-b, especially importance of A-910823 to elicit humoral and cellular immune responses.

Essential role of TMPRSS2 in SARS-CoV-2 infection in murine airways

In cultured cells, SARS-CoV-2 infects cells via multiple pathways using different host proteases. Recent studies have shown that the furin and TMPRSS2 (furin/TMPRSS2)-dependent pathway plays a minor role in infection of the Omicron variant. Here, we confirm that Omicron uses the furin/TMPRSS2-dependent pathway inefficiently and enters cells mainly using the cathepsin-dependent endocytosis pathway in TMPRSS2-expressing VeroE6/TMPRSS2 and Calu-3 cells. This is the case despite efficient cleavage of the spike protein of Omicron. However, in the airways of TMPRSS2-knockout mice, Omicron infection is significantly reduced. We furthermore show that propagation of the mouse-adapted SARS-CoV-2 QHmusX strain and human clinical isolates of Beta and Gamma is reduced in TMPRSS2-knockout mice. Therefore, the Omicron variant isn't an exception in using TMPRSS2 in vivo, and analysis with TMPRSS2-knockout mice is important when evaluating SARS-CoV-2 variants. In conclusion, this study shows that TMPRSS2 is critically important for SARS-CoV-2 infection of murine airways, including the Omicron variant.

Structural bases for the higher adherence to ACE2 conferred by the SARS-CoV-2 spike Q498Y substitution

A remarkable number of SARS-CoV-2 variants and other as yet unmonitored lineages harbor amino-acid substitutions with the potential to modulate the interface between the spike receptor-binding domain (RBD) and its receptor ACE2. The naturally occurring Q498Y substitution, which is present in currently circulating SARS-CoV-2 variants, has drawn the attention of several investigations. While computational predictions and in vitro binding studies suggest that Q498Y increases the binding affinity of the spike protein for ACE2, experimental in vivo models of infection have shown that a triple mutant carrying the Q498Y replacement is fatal in mice. To accurately characterize the binding kinetics of the RBD Q498Y-ACE2 interaction, biolayer interferometry analyses were performed. A significant enhancement of the RBD-ACE2 binding affinity relative to a reference SARS-CoV-2 variant of concern carrying three simultaneous replacements was observed. In addition, the RBD Q498Y mutant bound to ACE2 was crystallized. Compared with the structure of its wild-type counterpart, the RBD Q498Y-ACE2 complex reveals the conservation of major hydrogen-bond interactions and a more populated, nonpolar set of contacts mediated by the bulky side chain of Tyr498 that collectively lead to this increase in binding affinity. In summary, these studies contribute to a deeper understanding of the impact of a relevant mutation present in currently circulating SARS-CoV-2 variants which might lead to stronger host-pathogen interactions.

Vaccine-associated enhanced disease in humans and animal models: Lessons and challenges for vaccine development

The fight against infectious diseases calls for the development of safe and effective vaccines that generate long-lasting protective immunity. In a few situations, vaccine-mediated immune responses may have led to exacerbated pathology upon subsequent infection with the pathogen targeted by the vaccine. Such vaccine-associated enhanced disease (VAED) has been reported, or at least suspected, in animal models, and in a few instances in humans, for vaccine candidates against the respiratory syncytial virus (RSV), measles virus (MV), dengue virus (DENV), HIV-1, simian immunodeficiency virus (SIV), feline immunodeficiency virus (FIV), severe acute respiratory syndrome coronavirus 1 (SARS-CoV-1), and the Middle East respiratory syndrome coronavirus (MERS-CoV). Although alleviated by clinical and epidemiological evidence, a number of concerns were also initially raised concerning the short- and long-term safety of vaccines against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), which is causing the ongoing COVID-19 pandemic. Although the mechanisms leading to this phenomenon are not yet completely understood, the individual and/or collective role of antibody-dependent enhancement (ADE), complement-dependent enhancement, and cell-dependent enhancement have been highlighted. Here, we review mechanisms that may be associated with the risk of VAED, which are important to take into consideration, both in the assessment of vaccine safety and in finding ways to define models and immunization strategies that can alleviate such concerns.

Vaccine-associated enhanced respiratory pathology in COVID-19 hamsters after TH2-biased immunization

Vaccine-associated enhanced respiratory disease (VAERD) is a severe complication for some respiratory infections. To investigate the potential for VAERD induction in coronavirus disease 2019 (COVID-19), we evaluate two vaccine leads utilizing a severe hamster infection model: a T helper type 1 (T_H1)-biased measles vaccine-derived candidate and a T_H2-biased alum-adjuvanted, non-stabilized spike protein. The measles virus (MeV)-derived vaccine protects the animals, but the protein lead induces VAERD, which can be alleviated by dexamethasone treatment. Bulk transcriptomic analysis reveals that our protein vaccine prepares enhanced host gene dysregulation in the lung, exclusively up-regulating mRNAs encoding the eosinophil attractant CCL-11, T_H2-driving interleukin (IL)-19, or T_H2 cytokines IL-4, IL-5, and IL-13. Single-cell RNA sequencing (scRNA-seq) identifies lung macrophages or lymphoid cells as sources, respectively. Our findings imply that VAERD is caused by the concerted action of hyperstimulated macrophages and T_H2 cytokine-secreting lymphoid cells and potentially links VAERD to antibody-dependent enhancement (ADE). In summary, we identify the cytokine drivers and cellular contributors that mediate VAERD after T_H2-biased vaccination.

Intranasal vaccination induced cross-protective secretory IgA antibodies against SARS-CoV-2 variants with reducing the potential risk of lung eosinophilic immunopathology

To control the coronavirus disease 2019 (COVID-19) pandemic, there is a need to develop vaccines to prevent infection with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) variants. One candidate is a nasal vaccine capable of inducing secretory IgA antibodies in the mucosa of the upper respiratory tract, the initial site of infection. However, regarding the development of COVID-19 vaccines, there is concern about the potential risk of inducing lung eosinophilic immunopathology as a vaccine-associated enhanced respiratory disease as a result of the T helper 2 (Th2)-dominant adaptive immune response. In this study, we investigated the protective effect against virus infection induced by intranasal vaccination of recombinant trimeric spike protein derived from SARS-CoV-2 adjuvanted with CpG oligonucleotides, ODN2006, in mouse model. The intranasal vaccine combined with ODN2006 successfully induced not only systemic spike-specific IgG antibodies, but also secretory IgA antibodies in the nasal mucosa. Secretory IgA antibodies showed high protective ability against SARS-CoV-2 variants (Alpha, Beta and Gamma variants) compared to IgG antibodies in the serum. The nasal vaccine of this formulation induced a high number of IFN-γ-secreting cells in the draining cervical lymph nodes and a lower spike-specific IgG1/IgG2a ratio compared to that of subcutaneous vaccination with alum as a typical Th2 adjuvant. These features are consistent with the induction of the Th1 adaptive immune response. In addition, mice intranasally vaccinated with ODN2006 showed less lung eosinophilic immunopathology after viral challenge than mice subcutaneously vaccinated with alum adjuvant. Our findings indicate that intranasal vaccine adjuvanted with ODN2006 could be a candidate that can prevent the infection of antigenically different variant viruses, reducing the risk of vaccine-associated enhanced respiratory disease.

Immunogenicity and protective efficacy of SARS-CoV-2 recombinant S-protein vaccine S-268019-b in cynomolgus monkeys

The vaccine S-268019-b is a severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) spike (S)-protein vaccine consisting of full-length recombinant SARS-CoV-2 S-protein (S-910823) as antigen, mixed with the squalene-based adjuvant A-910823. The current study evaluated the immunogenicity of S-268019-b using various doses of S-910823 and its vaccine efficacy against SARS-CoV-2 challenge in cynomolgus monkeys. The different doses of S-910823 combined with A-910823 were intramuscularly administered twice at a 3-week interval. Two weeks after the second dosing, dose-dependent humoral immune responses were observed with neutralizing antibody titers being comparable to that of human convalescent plasma. Pseudoviruses harboring S proteins from Beta and Gamma SARS-CoV-2 variants displayed approximately 3- to 4-fold reduced sensitivity to neutralizing antibodies induced after two vaccine doses compared with that against ancestral viruses, whereas neutralizing antibody titers were reduced >14-fold against the Omicron variant. Cellular immunity was also induced with a relative Th1 polarized response. No adverse clinical signs or weight loss associated with the vaccine were observed, suggesting safety of the vaccine in cynomolgus monkeys. Immunization with 10 µg of S-910823 with A-910823 demonstrated protective efficacy against SARS-CoV-2 challenge according to genomic and subgenomic viral RNA transcript levels in nasopharyngeal, throat, and rectal swab specimens. Pathological analysis revealed no detectable vaccine-dependent enhancement of disease in the lungs of challenged vaccinated monkeys. The current findings provide fundamental information regarding vaccine doses for human trials and support the development of S-268019-b as a safe and effective vaccine for controlling the current pandemic, as well as general protection against SARS-CoV-2 moving forward.

Isolation of human monoclonal antibodies with neutralizing activity to a broad spectrum of SARS-CoV-2 viruses including the Omicron variants

Monoclonal antibody therapy is a promising option for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection, and a cocktail of antibodies (REGN-COV) has been administered to infected patients with a favorable outcome. However, it is necessary to continue generating novel sets of monoclonal antibodies with neutralizing activity because viral variants can emerge that show resistance to the currently utilized antibodies. Here, we isolated a new cocktail of antibodies, EV053273 and EV053286, from peripheral blood mononuclear cells derived from convalescent patients infected with wild-type SARS-CoV-2. EV053273 exerted potent antiviral activity against the Wuhan wild-type virus as well as the Alpha and Delta variants in vitro, whereas the antiviral activity of EV053286 was moderate, but it had a wide-range of suppressive activity on the wild-type virus as well as the Alpha, Beta, Delta, Kappa, Omicron BA.1, and BA.2 variants. With the combined use of EV053273 and EV053286, we observed similar inhibitory effects on viral replication as with REGN-COV in vitro. We further assessed their activity in vivo by using a mouse model infected with a recently established viral strain with adopted infectious activity in mice. Independent experiments revealed that the combined use of EV053273 and EV053286 or the single use of each monoclonal antibody efficiently blocked infection in vivo. Together with data showing that these two monoclonal antibodies could neutralize REGN-COV escape variants and the Omicron variant, our findings suggest that the EV053273 and EV053286 monoclonal antibody cocktail is a novel clinically applicable therapeutic candidate for SARS-CoV-2 infection.