Further Evidence for Bats as the Evolutionary Source of Middle East Respiratory Syndrome Coronavirus

Positive selection, genetic recombination, and intra-host evolution in novel equine coronavirus genomes and other members of the Embecovirus subgenus

There are several examples of coronaviruses in the Betacoronavirus subgenus Embecovirus that have jumped from an animal to the human host. Studying how evolutionary factors shape coronaviruses in non-human hosts may provide insight into the coronavirus host-switching potential. Equids, such as horses and donkeys, are susceptible to equine coronaviruses (ECoVs). With increased testing prevalence, several ECoV genome sequences have become available for molecular evolutionary analyses, especially those from the United States of America (USA). To date, no analyses have been performed to characterize evolution within coding regions of the ECoV genome. Here, we obtain and describe four new ECoV genome sequences from infected equines from across the USA presenting clinical symptoms of ECoV, and infer ECoV-specific and Embecovirus-wide patterns of molecular evolution. Within two of the four data sets analyzed, we find evidence of intra-host evolution within the nucleocapsid (N) gene, suggestive of quasispecies development. We also identify 12 putative genetic recombination events within the ECoV genome, 11 of which fall in ORF1ab. Finally, we infer and compare sites subject to positive selection on the ancestral branch of each major Embecovirus member clade. Specifically, for the two currently identified human coronavirus (HCoV) embecoviruses that have spilled from animals to humans (HCoV-OC43 and HCoV-HKU1), we find that there are 42 and 2 such sites, respectively, perhaps reflective of the more complex ancestral evolutionary history of HCoV-OC43, which involves several different animal hosts.IMPORTANCEThe Betacoronavirus subgenus Embecovirus contains coronaviruses that not only pose a health threat to animals and humans, but also have jumped from animal to human host. Equids, such as horses and donkeys are susceptible to equine coronavirus (ECoV) infections. No studies have systematically examined evolutionary patterns within ECoV genomes. Our study addresses this gap and provides insight into intra-host ECoV evolution from infected horses. Further, we identify and report natural selection pattern differences between two embecoviruses that have jumped from animals to humans [human coronavirus OC43 and HKU1 (HCoV-OC43 and HCoV-HKU1, respectively)], and hypothesize that the differences observed may be due to the different animal host(s) that each virus circulated in prior to its jump into humans. Finally, we contribute four novel, high-quality ECoV genomes to the scientific community.

Cellular dynamics shape recombination frequency in coronaviruses

Coronavirus genomes have evolutionary histories shaped extensively by recombination. Yet, how often recombination occurs at a cellular level, or the factors that regulate recombination rates, are poorly understood. Utilizing experimental co-infections with pairs of genetically distinct coronaviruses, we found that recombination is both frequent and rare during coinfection. Recombination occurred in every instance of co-infection yet resulted in relatively few recombinant RNAs. By integrating a discrete-time Susceptible-Infected-Removed (SIR) model, we found that rates of recombination are determined primarily by rates of cellular co-infection, rather than other possible barriers such as RNA compartmentalization. By staggering the order and timing of infection with each virus we also found that rates of co-infection are themselves heavily influenced by genetic and ecological mechanisms, including superinfection exclusion and the relative fitness of competing viruses. Our study highlights recombination as a potent yet regulated force: frequent enough to ensure a steady influx of genetic variation but also infrequent enough to maintain genomic integrity. As recombination is thought to be an important driver of host-switching and disease emergence, our study provides new insights into the factors that regulate coronavirus recombination and evolution more broadly.

Molecular basis of convergent evolution of ACE2 receptor utilization among HKU5 coronaviruses

DPP4 was considered a canonical receptor for merbecoviruses until the recent discovery of African bat-borne MERS-related coronaviruses using ACE2. The extent and diversity with which merbecoviruses engage ACE2 and their receptor species tropism remain unknown. Here, we reveal that HKU5 enters host cells utilizing Pipistrellus abramus (P.abr) and several non-bat mammalian ACE2s through a binding mode distinct from that of any other known ACE2-using coronaviruses. These results show that several merbecovirus clades independently evolved ACE2 utilization, which appears to be a broadly shared property among these pathogens, through an extraordinary diversity of ACE2 recognition modes. We show that MERS-CoV and HKU5 have markedly distinct antigenicity, due to extensive genetic divergence, and identified several HKU5 inhibitors, including two clinical compounds. Our findings profoundly alter our understanding of coronavirus evolution and pave the way for developing countermeasures against viruses poised for human emergence.

ACE2 from Pipistrellus abramus bats is a receptor for HKU5 coronaviruses

The merbecovirus subgenus of coronaviruses includes Middle East Respiratory Syndrome Coronavirus (MERS-CoV), a zoonotic pathogen transmitted from dromedary camels to humans that causes severe respiratory disease. Viral discovery efforts have uncovered hundreds of merbecoviruses in different species across multiple continents, but few have been studied under laboratory conditions, leaving basic questions regarding their human threat potential unresolved. Viral entry into host cells is a critical step for transmission between hosts. Here, a scalable approach that assesses novel merbecovirus cell entry was developed and used to evaluate receptor use across the entire merbecovirus subgenus. Merbecoviruses are sorted into clades based on the receptor-binding domain of the spike glycoprotein. Receptor tropism is clade-specific, with the clade including MERS-CoV using DPP4 and multiple clades using ACE2, including HKU5 bat coronaviruses. Mutational analysis identified possible structural limitations to HKU5 adaptability and a cryo-EM structure of the HKU5-20s spike trimer revealed only 'down' RBDs.

Genetic loci regulate Sarbecovirus pathogenesis: A comparison across mice and humans

Coronavirus (CoV) cause considerable morbidity and mortality in humans and other mammals, as evidenced by the emergence of Severe Acute Respiratory CoV (SARS-CoV) in 2003, Middle East Respiratory CoV (MERS-CoV) in 2012, and SARS-CoV-2 in 2019. Although poorly characterized, natural genetic variation in human and other mammals modulate virus pathogenesis, as reflected by the spectrum of clinical outcomes ranging from asymptomatic infections to lethal disease. Using multiple human epidemic and zoonotic Sarbecoviruses, coupled with murine Collaborative Cross genetic reference populations, we identify several dozen quantitative trait loci that regulate SARS-like group-2B CoV pathogenesis and replication. Under a Chr4 QTL, we deleted a candidate interferon stimulated gene, Trim14 which resulted in enhanced SARS-CoV titers and clinical disease, suggesting an antiviral role during infection. Importantly, about 60 % of the murine QTL encode susceptibility genes identified as priority candidates from human genome-wide association studies (GWAS) studies after SARS-CoV-2 infection, suggesting that similar selective forces have targeted analogous genes and pathways to regulate Sarbecovirus disease across diverse mammalian hosts. These studies provide an experimental platform in rodents to investigate the molecular-genetic mechanisms by which potential cross mammalian susceptibility loci and genes regulate type-specific and cross-SARS-like group 2B CoV replication, immunity, and pathogenesis in rodent models. Our study also provides a paradigm for identifying susceptibility loci for other highly heterogeneous and virulent viruses that sporadically emerge from zoonotic reservoirs to plague human and animal populations.

Genomic Diversity and Recombination Analysis of the Spike Protein Gene from Selected Human Coronaviruses

Human coronaviruses (HCoVs) are seriously associated with respiratory diseases in humans and animals. The first human pathogenic SARS-CoV emerged in 2002-2003. The second was MERS-CoV, reported from Jeddah, the Kingdom of Saudi Arabia, in 2012, and the third one was SARS-CoV-2, identified from Wuhan City, China, in late December 2019. The HCoV-Spike (S) gene has the highest mutation/insertion/deletion rate and has been the most utilized target for vaccine/antiviral development. In this manuscript, we discuss the genetic diversity, phylogenetic relationships, and recombination patterns of selected HCoVs with emphasis on the S protein gene of MERS-CoV and SARS-CoV-2 to elucidate the possible emergence of new variants/strains of coronavirus in the near future. The findings showed that MERS-CoV and SARS-CoV-2 have significant sequence identity with the selected HCoVs. The phylogenetic tree analysis formed a separate cluster for each HCoV. The recombination pattern analysis showed that the HCoV-NL63-Japan was a probable recombinant. The HCoV-NL63-USA was identified as a major parent while the HCoV-NL63-Netherland was identified as a minor parent. The recombination breakpoints start in the viral genome at the 142 nucleotide position and end at the 1082 nucleotide position with a 99% CI and Bonferroni-corrected p-value of 0.05. The findings of this study provide insightful information about HCoV-S gene diversity, recombination, and evolutionary patterns. Based on these data, it can be concluded that the possible emergence of new strains/variants of HCoV is imminent.

Recombination analysis on the receptor switching event of MERS-CoV and its close relatives: implications for the emergence of MERS-CoV

BACKGROUND

PlMERS-CoV is a coronavirus known to cause severe disease in humans, taxonomically classified under the subgenus Merbecovirus. Recent findings showed that the close relatives of MERS-CoV infecting vespertillionid bats (family Vespertillionidae), named NeoCoV and PDF-2180, use their hosts' ACE2 as their entry receptor, unlike the DPP4 receptor usage of MERS-CoV. Previous research suggests that this difference in receptor usage between these related viruses is a result of recombination. However, the precise location of the recombination breakpoints and the details of the recombination event leading to the change of receptor usage remain unclear.

METHODS

We used maximum likelihood-based phylogenetics and genetic similarity comparisons to characterise the evolutionary history of all complete Merbecovirus genome sequences. Recombination events were detected by multiple computational methods implemented in the recombination detection program. To verify the influence of recombination, we inferred the phylogenetic relation of the merbecovirus genomes excluding recombinant segments and that of the viruses' receptor binding domains and examined the level of congruency between the phylogenies. Finally, the geographic distribution of the genomes was inspected to identify the possible location where the recombination event occurred.

RESULTS

Similarity plot analysis and the recombination-partitioned phylogenetic inference showed that MERS-CoV is highly similar to NeoCoV (and PDF-2180) across its whole genome except for the spike-encoding region. This is confirmed to be due to recombination by confidently detecting a recombination event between the proximal ancestor of MERS-CoV and a currently unsampled merbecovirus clade. Notably, the upstream recombination breakpoint was detected in the N-terminal domain and the downstream breakpoint at the S2 subunit of spike, indicating that the acquired recombined fragment includes the receptor-binding domain. A tanglegram comparison further confirmed that the receptor binding domain-encoding region of MERS-CoV was acquired via recombination. Geographic mapping analysis on sampling sites suggests the possibility that the recombination event occurred in Africa.

CONCLUSION

Together, our results suggest that recombination can lead to receptor switching of merbecoviruses during circulation in bats. These results are useful for future epidemiological assessments and surveillance to understand the spillover risk of bat coronaviruses to the human population.

Multiple Lines of Evidence Support 199 SARS-CoV-2 Positively Selected Amino Acid Sites

SARS-CoV-2 amino acid variants that contribute to an increased transmissibility or to host immune system escape are likely to increase in frequency due to positive selection and may be identified using different methods, such as codeML, FEL, FUBAR, and MEME. Nevertheless, when using different methods, the results do not always agree. The sampling scheme used in different studies may partially explain the differences that are found, but there is also the possibility that some of the identified positively selected amino acid sites are false positives. This is especially important in the context of very large-scale projects where hundreds of analyses have been performed for the same protein-coding gene. To account for these issues, in this work, we have identified positively selected amino acid sites in SARS-CoV-2 and 15 other coronavirus species, using both codeML and FUBAR, and compared the location of such sites in the different species. Moreover, we also compared our results to those that are available in the COV2Var database and the frequency of the 10 most frequent variants and predicted protein location to identify those sites that are supported by multiple lines of evidence. Amino acid changes observed at these sites should always be of concern. The information reported for SARS-CoV-2 can also be used to identify variants of concern in other coronaviruses.

ACE2-using merbecoviruses: Further evidence of convergent evolution of ACE2 recognition by NeoCoV and other MERS-CoV related viruses

Angiotensin-converting enzyme 2 (ACE2) was recognized as an entry receptor shared by coronaviruses from Sarbecovirus and Setracovirus subgenera, including three human coronaviruses: SARS-CoV, SARS-CoV-2, and NL63. We recently disclosed that NeoCoV and three other merbecoviruses (PDF-2180, MOW15-22, PnNL 2018B), which are MERS-CoV relatives found in African and European bats, also utilize ACE2 as their functional receptors through unique receptor binding mechanisms. This unexpected receptor usage assumes significance, particularly in light of the prior recognition of Dipeptidyl peptidase-4 (DPP4) as the only known protein receptor for merbecoviruses. In contrast to other ACE2-using coronaviruses, NeoCoV and PDF-2180 engage a distinct and relatively compact binding surface on ACE2, facilitated by protein-glycan interactions, which is demonstrated by the Cryo-EM structures of the receptor binding domains (RBDs) of these viruses in complex with a bat ACE2 orthologue. These findings further support the hypothesis that phylogenetically distant coronaviruses, characterized by distinct RBD structures, can independently evolve to acquire ACE2 affinity during inter-species transmission and adaptive evolution. To date, these viruses have exhibited limited efficiency in entering human cells, although single mutations like T510F in NeoCoV can overcome the incompatibility with human ACE2. In this review, we present a comprehensive overview of ACE2-using merbecoviruses, summarize our current knowledge regarding receptor usage and host tropism determination, and deliberate on potential strategies for prevention and intervention, with the goal of mitigating potential future outbreaks caused by spillover of these viruses.

The impact of anthropogenic climate change on pediatric viral diseases

The adverse effects of climate change on human health are unfolding in real time. Environmental fragmentation is amplifying spillover of viruses from wildlife to humans. Increasing temperatures are expanding mosquito and tick habitats, introducing vector-borne viruses into immunologically susceptible populations. More frequent flooding is spreading water-borne viral pathogens, while prolonged droughts reduce regional capacity to prevent and respond to disease outbreaks with adequate water, sanitation, and hygiene resources. Worsening air quality and altered transmission seasons due to an increasingly volatile climate may exacerbate the impacts of respiratory viruses. Furthermore, both extreme weather events and long-term climate variation are causing the destruction of health systems and large-scale migrations, reshaping health care delivery in the face of an evolving global burden of viral disease. Because of their immunological immaturity, differences in physiology (e.g., size), dependence on caregivers, and behavioral traits, children are particularly vulnerable to climate change. This investigation into the unique pediatric viral threats posed by an increasingly inhospitable world elucidates potential avenues of targeted programming and uncovers future research questions to effect equitable, actionable change. IMPACT: A review of the effects of climate change on viral threats to pediatric health, including zoonotic, vector-borne, water-borne, and respiratory viruses, as well as distal threats related to climate-induced migration and health systems. A unique focus on viruses offers a more in-depth look at the effect of climate change on vector competence, viral particle survival, co-morbidities, and host behavior. An examination of children as a particularly vulnerable population provokes programming tailored to their unique set of vulnerabilities and encourages reflection on equitable climate adaptation frameworks.

Genomic Mutations in SARS-CoV-2 Genome following Infection in Syrian Golden Hamster and Associated Lung Pathologies

The continuous evolution of the SARS-CoV-2 virus led to constant developments and efforts in understanding the significance and impacts of SARS-CoV-2 variants on human health. Our study aimed to determine the accumulation of genetic mutations and associated lung pathologies in male and female hamsters infected with the ancestral Wuhan strain of SARS-CoV-2. The present study showed no significant difference in the viral load between male and female hamsters and peak infection was found to be on day four post infection in both sexes of the animals. Live virus particles were detected up to 5 days post infection (dpi) through the TCID-50 assay, while qRT-PCR could detect viral RNA up to 14 dpi from all the infected animals. Further, the determination of the neutralizing antibody titer showed the onset of the humoral immune response as early as 4 dpi in both sexes against SARS-CoV-2, and a significant cross-protection against the delta variant of SARS-CoV-2 was observed. Histopathology showed edema, inflammation, inflammatory cell infiltration, necrosis, and degeneration of alveolar and bronchial epithelium cells from 3 dpi to 14 dpi in both sexes. Furthermore, next-generation sequencing (NGS) showed up to 10 single-nucleotide polymorphisms (SNPs) in the SARS-CoV-2 (ancestral Wuhan strain) genome isolated from both male and female hamsters. The mutation observed at the 23014 position (Glu484Asp) in the SARS-CoV-2 genome isolated from both sexes of the hamsters plays a significant role in the antiviral efficacy of small molecules, vaccines, and the Mabs-targeting S protein. The present study shows that either of the genders can be used in the pre-clinical efficacy of antiviral agents against SARS-CoV-2 in hamsters. However, considering the major mutation in the S protein, the understanding of the genetic mutation in SARS-CoV-2 after passing through hamsters is crucial in deciding the efficacy of the antiviral agents targeting the S protein. Importance: Our study findings indicate the accumulation of genomic mutations in SARS-CoV-2 after passing through the Syrian golden hamsters. Understanding the genomic mutations showed that either of the hamster genders can be used in the pre-clinical efficacy of antiviral agents and vaccines.

Non-Structural Proteins (Nsp): A Marker for Detection of Human Coronavirus Families

SARS-CoV-2 was the cause of the global pandemic that caused a total of 14.9 million deaths during the years 2020 and 2021, according to the WHO. The virus presents a mutation rate between 10-5 and 10-3 substitutions per nucleotide site per cell infection (s/n/c). Due to this, studies aimed at knowing the evolution of this virus could help us to foresee (through the future development of new detection strategies and vaccines that prevent the infection of this virus in human hosts) that a pandemic caused by this virus will be generated again. In this research, we performed a functional annotation and identification of changes in Nsp (non-structural proteins) domains in the coronavirus genome. The comparison of the 13 selected coronavirus pangenomes demonstrated a total of 69 protein families and 57 functions associated with the structural domain's differentials between genomes. A marked evolutionary conservation of non-structural proteins was observed. This allowed us to identify and classify highly pathogenic human coronaviruses into alpha, beta, gamma, and delta groups. The designed Nsp cluster provides insight into the trajectory of SARS-CoV-2, demonstrating that it continues to evolve rapidly. An evolutionary marker allows us to discriminate between phylogenetically divergent groups, viral genotypes, and variants between the alpha and betacoronavirus genera. These types of evolutionary studies provide a window of opportunity to use these Nsp as targets of viral therapies.

Genotype and Phenotype Characterization of Rhinolophus sp. Sarbecoviruses from Vietnam: Implications for Coronavirus Emergence

Bats are a major reservoir of zoonotic viruses, including coronaviruses. Since the emergence of SARS-CoV in 2002/2003 in Asia, important efforts have been made to describe the diversity of Coronaviridae circulating in bats worldwide, leading to the discovery of the precursors of epidemic and pandemic sarbecoviruses in horseshoe bats. We investigated the viral communities infecting horseshoe bats living in Northern Vietnam, and report here the first identification of sarbecoviruses in Rhinolophus thomasi and Rhinolophus siamensis bats. Phylogenetic characterization of seven strains of Vietnamese sarbecoviruses identified at least three clusters of viruses. Recombination and cross-species transmission between bats seemed to constitute major drivers of virus evolution. Vietnamese sarbecoviruses were mainly enteric, therefore constituting a risk of spillover for guano collectors or people visiting caves. To evaluate the zoonotic potential of these viruses, we analyzed in silico and in vitro the ability of their RBDs to bind to mammalian ACE2s and concluded that these viruses are likely restricted to their bat hosts. The workflow applied here to characterize the spillover potential of novel sarbecoviruses is of major interest for each time a new virus is discovered, in order to concentrate surveillance efforts on high-risk interfaces.

Interspecies transmission of SARS CoV-2 with special emphasis on viral mutations and ACE-2 receptor homology roles

COVID-19 outbreak was first reported in 2019, Wuhan, China. The spillover of the disease caused by Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2), to a wide range of pet, zoo, wild, and farm animals has emphasized potential zoonotic and reverse zoonotic viral transmission. Furthermore, it has evoked inquiries about susceptibility of different animal species to SARS-CoV-2 infection and role of these animals as viral reservoirs. Therefore, studying susceptible and non-susceptible hosts for SARS-CoV-2 infection could give a better understanding for the virus and will help in preventing further outbreaks. Here, we review structural aspects of SARS-CoV-2 spike protein, the effect of the different mutations observed in the spike protein, and the impact of ACE2 receptor variations in different animal hosts on inter-species transmission. Moreover, the SARS-CoV-2 spillover chain was reviewed. Combination of SARS-CoV-2 high mutation rate and homology of cellular ACE2 receptors enable the virus to transcend species barriers and facilitate its transmission between humans and animals.

Structure and epitope of a neutralizing monoclonal antibody that targets the stem helix of β coronaviruses

Monoclonal antibodies that retain neutralizing activity against multiple coronavirus (CoV) lineages and variants of concern (VoC) must be developed to protect against future pandemics. These broadly neutralizing MAbs (BNMAbs) may be used as therapeutics and/or to assist in the rational design of vaccines that induce BNMAbs. 1249A8 is a BNMAb that targets the stem helix (SH) region of CoV spike (S) protein and neutralizes Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) original strain, delta, and omicron VoC, Severe Acute Respiratory Syndrome CoV (SARS-CoV), and Middle East Respiratory Syndrome CoV (MERS-CoV). To understand its mechanism of action, the crystal structure of 1249A8 bound to a MERS-CoV SH peptide was determined at 2.1 Å resolution. BNMAb 1249A8 mimics the SARS-CoV-2 S loop residues 743-749, which interacts with the N-terminal end of the SH helix in the S post-fusion conformation. The conformation of 1249A8-bound SH is distinct from the SH conformation observed in the post-fusion SARS-CoV-2 S structure, suggesting 1249A8 disrupts the secondary structure and refolding events required for CoV post-fusion S to initiate membrane fusion and ultimately infection. This study provides novel insights into the neutralization mechanisms of SH-targeting CoV BNMAbs that may inform vaccine development and the design of optimal BNMAb therapeutics.

Broad host tropism of ACE2-using MERS-related coronaviruses and determinants restricting viral recognition

Recently, two Middle East respiratory syndrome coronavirus (MERS-CoV) closely related to bat merbecoviruses, NeoCoV and PDF-2180, were discovered to use angiotensin-converting enzyme 2 (ACE2) for entry. The two viruses cannot use human ACE2 efficiently, and their host range and cross-species transmissibility across a wide range of mammalian species remain unclear. Herein, we characterized the species-specific receptor preference of these viruses by testing ACE2 orthologues from 49 bats and 53 non-bat mammals through receptor-binding domain (RBD)-binding and pseudovirus entry assays. Results based on bat ACE2 orthologues revealed that the two viruses were unable to use most, but not all, ACE2 from Yinpterochiropteran bats (Yin-bats), which is distinct from NL63 and SARS-CoV-2. Besides, both viruses exhibited broad receptor recognition spectra across non-bat mammals. Genetic and structural analyses of bat ACE2 orthologues highlighted four crucial host range determinants, all confirmed by subsequent functional assays in human and bat cells. Notably, residue 305, participating in a critical viral receptor interaction, plays a crucial role in host tropism determination, particularly in non-bat mammals. Furthermore, NeoCoV and PDF-2180 mutants with enhanced human ACE2 recognition expanded the potential host range, especially by enhancing their interaction with an evolutionarily conserved hydrophobic pocket. Our results elucidate the molecular basis for the species-specific ACE2 usage of MERS-related viruses and shed light on their zoonotic risks.

The coronavirus recombination pathway

Recombination is thought to be a mechanism that facilitates cross-species transmission in coronaviruses, thus acting as a driver of coronavirus spillover and emergence. Despite its significance, the mechanism of recombination is poorly understood, limiting our potential to estimate the risk of novel recombinant coronaviruses emerging in the future. As a tool for understanding recombination, here, we outline a framework of the recombination pathway for coronaviruses. We review existing literature on coronavirus recombination, including comparisons of naturally observed recombinant genomes as well as in vitro experiments, and place the findings into the recombination pathway framework. We highlight gaps in our understanding of coronavirus recombination illustrated by the framework and outline how further experimental research is critical for disentangling the molecular mechanism of recombination from external environmental pressures. Finally, we describe how an increased understanding of the mechanism of recombination can inform pandemic predictive intelligence, with a retrospective emphasis on SARS-CoV-2.

Metagenomics-enabled reverse-genetics assembly and characterization of myotis bat morbillivirus

Morbilliviruses are among the most contagious viral pathogens of mammals. Although previous metagenomic surveys have identified morbillivirus sequences in bats, full-length morbilliviruses from bats are limited. Here we characterize the myotis bat morbillivirus (MBaMV) from a bat surveillance programme in Brazil, whose full genome was recently published. We demonstrate that the fusion and receptor binding protein of MBaMV utilize bat CD150 and not human CD150, as an entry receptor in a mammalian cell line. Using reverse genetics, we produced a clone of MBaMV that infected Vero cells expressing bat CD150. Electron microscopy of MBaMV-infected cells revealed budding of pleomorphic virions, a characteristic morbillivirus feature. MBaMV replication reached 103-105 plaque-forming units ml-1 in human epithelial cell lines and was dependent on nectin-4. Infection of human macrophages also occurred, albeit 2-10-fold less efficiently than measles virus. Importantly, MBaMV is restricted by cross-neutralizing human sera elicited by measles, mumps and rubella vaccination and is inhibited by orally bioavailable polymerase inhibitors in vitro. MBaMV-encoded P/V genes did not antagonize human interferon induction. Finally, we show that MBaMV does not cause disease in Jamaican fruit bats. We conclude that, while zoonotic spillover into humans may theoretically be plausible, MBaMV replication would probably be controlled by the human immune system.

Stability of SARS-CoV-2 in Biological Fluids of Animals

Since its first emergence in 2019, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has continued to evolve genetically, jump species barriers, and expand its host range. There is growing evidence of interspecies transmission including infection of domestic animals and widespread circulation in wildlife. However, knowledge of SARS-CoV-2 stability in animal biological fluids and their role in transmission is still limited as previous studies focused on human biological fluids. Therefore, this study aimed to determine the SARS-CoV-2 stability in biological fluids from three animal species, cats, sheep and white-tailed deer (WTD). Saliva, feces, 10% fecal suspensions, and urine of cats, sheep, and WTD were mixed with a known concentration of virus and incubated under indoor and three different climatic conditions. Our results show that the virus was stable for up to 1 day in the saliva of cats, sheep, and WTD regardless of the environmental conditions. The virus remained infectious for up to 6 days in feces and 15 days in fecal suspension of WTD, whereas the virus was rather unstable in cat and sheep feces and fecal suspensions. We found the longest survival of SARS-CoV-2 in the urine of cats, sheep, and WTD. Furthermore, side-by-side comparison with different SARS-CoV-2 strains showed that the Alpha, Delta, and Omicron variants of concern were less stable than the ancestral Wuhan-like strain in WTD fecal suspension. The results of our study provide valuable information for assessing the potential role of various animal biological fluids in SARS-CoV-2 transmission.

Intestinal Tropism of a Betacoronavirus (Merbecovirus) in Nathusius's Pipistrelle Bat (Pipistrellus nathusii), Its Natural Host

The emergence of several bat coronavirus-related disease outbreaks in human and domestic animals has fueled surveillance of coronaviruses in bats worldwide. However, little is known about how these viruses interact with their natural hosts. We demonstrate a Betacoronavirus (subgenus Merbecovirus), PN-βCoV, in the intestine of its natural host, Nathusius's Pipistrelle Bat (Pipistrellus nathusii), by combining molecular and microscopy techniques. Eighty-eight P. nathusii bat carcasses were tested for PN-βCoV RNA by RT-qPCR, of which 25 bats (28%) tested positive. PN-βCoV RNA was more often detected in samples of the intestinal tract than in other sample types. In addition, viral RNA loads were higher in intestinal samples compared to other sample types, both on average and in each individual bat. In one bat, we demonstrated Merbecovirus antigen and PN-βCoV RNA expression in intestinal epithelium and the underlying connective tissue using immunohistochemistry and in situ hybridization, respectively. These results indicate that PN-βCoV has a tropism for the intestinal epithelium of its natural host, Nathusius's Pipistrelle Bat, and imply that the fecal-oral route is a possible route of transmission. IMPORTANCE Virtually all mammal species circulate coronaviruses. Most of these viruses will infect one host species; however, coronaviruses are known to include species that can infect multiple hosts, for example the well-known virus that caused a pandemic, SARS-CoV-2. Chiroptera (bats) include over 1,400 different species, which are expected to harbor a great variety of coronaviruses. However, we know very little about how any of these coronaviruses interact with their bat hosts; for example, we do not know their modes of transmissions, or which cells they infect. Thus, we have a limited understanding of coronavirus infections in this important host group. The significance of our study is that we learned that a bat coronavirus that occurs in a common bat species in Europe has a tropism for the intestines. This implies the fecal-oral route is a likely transmission route.

Bats-associated beta-coronavirus detection and characterization: First report from Pakistan

Bats remains as reservoirs for highly contagious and pathogenic viral families including the Coronaviridae, Filoviridae, Paramyxoviruses, and Rhabdoviridae. Spill over of viral species (SARS-CoV, MERS-CoV & SARS-CoV2) from bats (as a possible potential reservoirs) have recently caused worst outbreaks. Early detection of viral species of pandemic potential in bats is of great importance. We detected beta coronaviruses in the studied bats population (positive samples from Rousettus leschenaultia) and performed the evolutionary analysis, amino acid sequence alignment, and analysed the 3-Dimentional protein structure. We detected the coronaviruses for the first time in bats from Pakistan. Our analysis based on RdRp partial gene sequencing suggest that the studied viral strains are closely related to MERS-CoV-like viruses as they exhibit close structure similarities (with few substitutions) and also observed a substitution in highly conserved SDD in the palm subdomain of motif C to ADD, when compared with earlier reported viral strains. It could be concluded from our study that coronaviruses are circulating among the bat's population in Pakistan. Based on the current findings, we suggest large scale screening procedures of bat virome across the country to detect potential pathogenic viral species.

Identification and Genetic Characterization of MERS-Related Coronavirus Isolated from Nathusius' Pipistrelle (Pipistrellus nathusii) near Zvenigorod (Moscow Region, Russia)

Being diverse and widely distributed globally, bats are a known reservoir of a series of emerging zoonotic viruses. We studied fecal viromes of twenty-six bats captured in 2015 in the Moscow Region and found 13 of 26 (50%) samples to be coronavirus positive. Of P. nathusii (the Nathusius' pipistrelle), 3 of 6 samples were carriers of a novel MERS-related betacoronavirus. We sequenced and assembled the complete genome of this betacoronavirus and named it MOW-BatCoV strain 15-22. Whole genome phylogenetic analysis suggests that MOW-BatCoV/15-22 falls into a distinct subclade closely related to human and camel MERS-CoV. Unexpectedly, the phylogenetic analysis of the novel MOW-BatCoV/15-22 spike gene showed the closest similarity to CoVs from Erinaceus europaeus (European hedgehog). We suppose MOW-BatCoV could have arisen as a result of recombination between ancestral viruses of bats and hedgehogs. Molecular docking analysis of MOW-BatCoV/15-22 spike glycoprotein binding to DPP4 receptors of different mammals predicted the highest binding ability with DPP4 of the Myotis brandtii bat (docking score -320.15) and the E. europaeus (docking score -294.51). Hedgehogs are widely kept as pets and are commonly found in areas of human habitation. As this novel bat-CoV is likely capable of infecting hedgehogs, we suggest hedgehogs can act as intermediate hosts between bats and humans for other bat-CoVs.

Versatile use of bat ACE2 for cellular entry by MERS-CoV-like viruses

Cellular entry receptors for bat MERS-CoV-like viruses NeoCoV and PDF-2180 were unknown, leaving their zoonotic potential ambiguous. A recent study by Xiong et al. published in Nature identified bat ACE2 as the cellular entry receptor for both viruses, highlighting the ability of coronaviruses to utilize a range of entry receptors.

Cross-species transmission, evolution and zoonotic potential of coronaviruses

Coronaviruses (CoVs) continuously evolve, crossing species barriers and spreading across host ranges. Over the last two decades, several CoVs (HCoV-229E, HCoV-NL63, HCoV-HKU1, HCoV-OC43, SARS-CoV, MERS-CoV, and SARS-CoV-2) have emerged in animals and mammals, causing significant economic and human life losses. Due to CoV cross-species transmission and the evolution of novel viruses, it is critical to identify their natural reservoiurs and the circumstances under which their transmission occurs. In this review, we use genetic and ecological data to disentangle the evolution of various CoVs in wildlife, humans, and domestic mammals. We thoroughly investigate several host species and outline the epidemiology of CoVs toward specific hosts. We also discuss the cross-species transmission of CoVs at the interface of wildlife, animals, and humans. Clarifying the epidemiology and diversity of species reservoirs will significantly impact our ability to respond to the future emergence of CoVs in humans and domestic animals.

A study of viral pathogens in bat species in the Iberian Peninsula: identification of new coronavirus genetic variants

Bats have long been associated with multiple pathogens, including viruses affecting humans such as henipaviruses, filoviruses, bunyaviruses and coronaviruses. The alpha and beta coronaviruses genera can infect most mammalian species. Among them, betacoronavirus SARS-CoV, MERS-CoV and SARS-CoV-2, which have caused the three major pandemics in the last two decades, have been proposed to originate in bats. In this study, 194 oral swabs from 22 bats species sampled in 19 locations of the Iberian Peninsula were analysed and characterized by three different PCR tests (coronavirus generic real-time RT-PCR, multiplex conventional PCR, and SARS-CoV-2 specific real-time RT-PCR) to detect bat coronaviruses. Screening with coronavirus generic PCR showed 102 positives out of 194 oral swabs analysed. Then, metabarcoding with multiplex PCR amplified 15 positive samples. Most of the coronaviruses detected in this study belong to alphacoronavirus (α-CoV) genus, with multiple alphacoronaviruses identified by up to five different genetic variants coexisting in the same bat. One of the positive samples identified in a Miniopterus schreibersii bat positive for the generic coronavirus PCR and the specific SARS-CoV-2 PCR was classified as betacoronavirus (-CoV) through phylogenetic analysis. These results support the rapid evolution of coronaviruses to generate new genomic potentially pathogenic variants likely through co-infection and recombination.

Generation and Characterization of a SARS-CoV-2-Susceptible Mouse Model Using Adeno-Associated Virus (AAV6.2FF)-Mediated Respiratory Delivery of the Human ACE2 Gene

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is the aetiological agent of coronavirus disease 2019 (COVID-19) that has caused a pandemic with millions of human infections. There continues to be a pressing need to develop potential therapies and vaccines to inhibit SARS-CoV-2 infection to mitigate the ongoing pandemic. Epidemiological data from the current pandemic indicates that there may be sex-dependent differences in disease outcomes. To investigate these differences, we proposed to use common small animal species that are frequently used to model disease with viruses. However, common laboratory strains of mice are not readily infected by SARS-CoV-2 because of differences in the angiotensin-converting enzyme 2 (ACE2), the cellular receptor for the virus. To overcome this limitation, we transduced common laboratory accessible strains of mice of different sexes and age groups with a novel a triple AAV6 mutant, termed AAV6.2FF, encoding either human ACE2 or luciferase via intranasal administration to promote expression in the lung and nasal turbinates. Infection of AAV-hACE2-transduced mice with SARS-CoV-2 resulted in high viral titers in the lungs and nasal turbinates, establishment of an IgM and IgG antibody response, and modulation of lung and nasal turbinate cytokine profiles. There were insignificant differences in infection characteristics between age groups and sex-related differences; however, there were significant strain-related differences between BALB/c vs. C57BL/6 mice. We show that AAV-hACE2-transduced mice are a useful for determining immune responses and for potential evaluation of SARS-CoV-2 vaccines and antiviral therapies, and this study serves as a model for the utility of this approach to rapidly develop small-animal models for emerging viruses.

Comparative highlights on MERS-CoV, SARS-CoV-1, SARS-CoV-2, and NEO-CoV

The severe acute respiratory syndrome (SARS-CoV, now SARS-CoV-1), middle east respiratory syndrome (MERS-CoV), Neo-CoV, and 2019 novel coronavirus (SARS-CoV-2/COVID-19) are the most notable coronaviruses, infecting the number of people worldwide by targeting the respiratory system. All these viruses are of zoonotic origin, predominantly from bats which are one of the natural reservoir hosts for coronaviruses. Thus, the major goal of our review article is to compare and contrast the characteristics and attributes of these coronaviruses. The SARS-CoV-1, MERS-CoV, and COVID-19 have many viral similarities due to their classification, they are not genetically related. COVID-19 shares approximately 79 % of its genome with SARS-CoV-1 and about 50 % with MERS-CoV. The shared receptor protein, ACE2 exhibit the most striking genetic similarities between SARS-CoV-1 and SARS-CoV-2. SARS-CoV primarily replicates in the epithelial cells of the respiratory system, but it may also affect macrophages, monocytes, activated T cells, and dendritic cells. MERS-CoV not only infects and replicates inside the epithelial and immune cells, but it may lyse them too, which is one of the common reasons for MERS's higher mortality rate. The details of infections caused by SARS-CoV-2 and lytic replication mechanisms in host cells are currently mysterious. In this review article, we will discuss the comparative highlights of SARS-CoV-1, MERS-CoV, SARS-CoV-2, and Neo-CoV, concerning their structural features, morphological characteristics, sources of virus origin and their evolutionary transitions, infection mechanism, computational study approaches, pathogenesis and their severity towards several diseases, possible therapeutic approaches, and preventive measures.

A Multitrait Locus Regulates Sarbecovirus Pathogenesis

Infectious diseases have shaped the human population genetic structure, and genetic variation influences the susceptibility to many viral diseases. However, a variety of challenges have made the implementation of traditional human Genome-wide Association Studies (GWAS) approaches to study these infectious outcomes challenging. In contrast, mouse models of infectious diseases provide an experimental control and precision, which facilitates analyses and mechanistic studies of the role of genetic variation on infection. Here we use a genetic mapping cross between two distinct Collaborative Cross mouse strains with respect to severe acute respiratory syndrome coronavirus (SARS-CoV) disease outcomes. We find several loci control differential disease outcome for a variety of traits in the context of SARS-CoV infection. Importantly, we identify a locus on mouse chromosome 9 that shows conserved synteny with a human GWAS locus for SARS-CoV-2 severe disease. We follow-up and confirm a role for this locus, and identify two candidate genes, CCR9 and CXCR6, that both play a key role in regulating the severity of SARS-CoV, SARS-CoV-2, and a distantly related bat sarbecovirus disease outcomes. As such we provide a template for using experimental mouse crosses to identify and characterize multitrait loci that regulate pathogenic infectious outcomes across species. IMPORTANCE Host genetic variation is an important determinant that predicts disease outcomes following infection. In the setting of highly pathogenic coronavirus infections genetic determinants underlying host susceptibility and mortality remain unclear. To elucidate the role of host genetic variation on sarbecovirus pathogenesis and disease outcomes, we utilized the Collaborative Cross (CC) mouse genetic reference population as a model to identify susceptibility alleles to SARS-CoV and SARS-CoV-2 infections. Our findings reveal that a multitrait loci found in chromosome 9 is an important regulator of sarbecovirus pathogenesis in mice. Within this locus, we identified and validated CCR9 and CXCR6 as important regulators of host disease outcomes. Specifically, both CCR9 and CXCR6 are protective against severe SARS-CoV, SARS-CoV-2, and SARS-related HKU3 virus disease in mice. This chromosome 9 multitrait locus may be important to help identify genes that regulate coronavirus disease outcomes in humans.

Virus Diversity, Abundance, and Evolution in Three Different Bat Colonies in Switzerland

Bats are increasingly recognized as reservoirs for many different viruses that threaten public health, such as Hendravirus, Ebolavirus, Nipahvirus, and SARS- and MERS-coronavirus. To assess spillover risk, viromes of bats from different parts of the world have been investigated in the past. As opposed to most of these prior studies, which determined the bat virome at a single time point, the current work was performed to monitor changes over time. Specifically, fecal samples of three endemic Swiss bat colonies consisting of three different bat species were collected over three years and analyzed using next-generation sequencing. Furthermore, single nucleotide variants of selected DNA and RNA viruses were analyzed to investigate virus genome evolution. In total, sequences of 22 different virus families were found, of which 13 are known to infect vertebrates. Most interestingly, in a Vespertilio murinus colony, sequences from a MERS-related beta-coronavirus were consistently detected over three consecutive years, which allowed us to investigate viral genome evolution in a natural reservoir host.

A strategy to assess spillover risk of bat SARS-related coronaviruses in Southeast Asia

Emerging diseases caused by coronaviruses of likely bat origin (e.g., SARS, MERS, SADS, COVID-19) have disrupted global health and economies for two decades. Evidence suggests that some bat SARS-related coronaviruses (SARSr-CoVs) could infect people directly, and that their spillover is more frequent than previously recognized. Each zoonotic spillover of a novel virus represents an opportunity for evolutionary adaptation and further spread; therefore, quantifying the extent of this spillover may help target prevention programs. We derive current range distributions for known bat SARSr-CoV hosts and quantify their overlap with human populations. We then use probabilistic risk assessment and data on human-bat contact, human viral seroprevalence, and antibody duration to estimate that a median of 66,280 people (95% CI: 65,351-67,131) are infected with SARSr-CoVs annually in Southeast Asia. These data on the geography and scale of spillover can be used to target surveillance and prevention programs for potential future bat-CoV emergence.

Optimising predictive models to prioritise viral discovery in zoonotic reservoirs

Despite the global investment in One Health disease surveillance, it remains difficult and costly to identify and monitor the wildlife reservoirs of novel zoonotic viruses. Statistical models can guide sampling target prioritisation, but the predictions from any given model might be highly uncertain; moreover, systematic model validation is rare, and the drivers of model performance are consequently under-documented. Here, we use the bat hosts of betacoronaviruses as a case study for the data-driven process of comparing and validating predictive models of probable reservoir hosts. In early 2020, we generated an ensemble of eight statistical models that predicted host-virus associations and developed priority sampling recommendations for potential bat reservoirs of betacoronaviruses and bridge hosts for SARS-CoV-2. During a time frame of more than a year, we tracked the discovery of 47 new bat hosts of betacoronaviruses, validated the initial predictions, and dynamically updated our analytical pipeline. We found that ecological trait-based models performed well at predicting these novel hosts, whereas network methods consistently performed approximately as well or worse than expected at random. These findings illustrate the importance of ensemble modelling as a buffer against mixed-model quality and highlight the value of including host ecology in predictive models. Our revised models showed an improved performance compared with the initial ensemble, and predicted more than 400 bat species globally that could be undetected betacoronavirus hosts. We show, through systematic validation, that machine learning models can help to optimise wildlife sampling for undiscovered viruses and illustrates how such approaches are best implemented through a dynamic process of prediction, data collection, validation, and updating.

Revising the paradigm: Are bats really pathogen reservoirs or do they possess an efficient immune system?

While bats are often referred to as reservoirs of viral pathogens, a meta-analysis of the literature reveals many cases in which there is not enough evidence to claim so. In many cases, bats are able to confront viruses, recover, and remain immune by developing a potent titer of antibodies, often without becoming a reservoir. In other cases, bats might have carried an ancestral virus that at some time point might have mutated into a human pathogen. Moreover, bats exhibit a balanced immune response against viruses that have evolved over millions of years. Using genomic tools, it is now possible to obtain a deeper understanding of that unique immune system and its variability across the order Chiroptera. We conclude, that with the exception of a few viruses, bats pose little zoonotic danger to humans and that they operate a highly efficient anti-inflammatory response that we should strive to understand.

Structure, receptor recognition, and antigenicity of the human coronavirus CCoV-HuPn-2018 spike glycoprotein

The isolation of CCoV-HuPn-2018 from a child respiratory swab indicates that more coronaviruses are spilling over to humans than previously appreciated. We determined the structures of the CCoV-HuPn-2018 spike glycoprotein trimer in two distinct conformational states and showed that its domain 0 recognizes sialosides. We identified that the CCoV-HuPn-2018 spike binds canine, feline, and porcine aminopeptidase N (APN) orthologs, which serve as entry receptors, and determined the structure of the receptor-binding B domain in complex with canine APN. The introduction of an oligosaccharide at position N739 of human APN renders cells susceptible to CCoV-HuPn-2018 spike-mediated entry, suggesting that single-nucleotide polymorphisms might account for viral detection in some individuals. Human polyclonal plasma antibodies elicited by HCoV-229E infection and a porcine coronavirus monoclonal antibody inhibit CCoV-HuPn-2018 spike-mediated entry, underscoring the cross-neutralizing activity among ɑ-coronaviruses. These data pave the way for vaccine and therapeutic development targeting this zoonotic pathogen representing the eighth human-infecting coronavirus.

Projecting vaccine demand and impact for emerging zoonotic pathogens

BACKGROUND

Despite large outbreaks in humans seeming improbable for a number of zoonotic pathogens, several pose a concern due to their epidemiological characteristics and evolutionary potential. To enable effective responses to these pathogens in the event that they undergo future emergence, the Coalition for Epidemic Preparedness Innovations is advancing the development of vaccines for several pathogens prioritized by the World Health Organization. A major challenge in this pursuit is anticipating demand for a vaccine stockpile to support outbreak response.

METHODS

We developed a modeling framework for outbreak response for emerging zoonoses under three reactive vaccination strategies to assess sustainable vaccine manufacturing needs, vaccine stockpile requirements, and the potential impact of the outbreak response. This framework incorporates geographically variable zoonotic spillover rates, human-to-human transmission, and the implementation of reactive vaccination campaigns in response to disease outbreaks. As proof of concept, we applied the framework to four priority pathogens: Lassa virus, Nipah virus, MERS coronavirus, and Rift Valley virus.

RESULTS

Annual vaccine regimen requirements for a population-wide strategy ranged from > 670,000 (95% prediction interval 0-3,630,000) regimens for Lassa virus to 1,190,000 (95% PrI 0-8,480,000) regimens for Rift Valley fever virus, while the regimens required for ring vaccination or targeting healthcare workers (HCWs) were several orders of magnitude lower (between 1/25 and 1/700) than those required by a population-wide strategy. For each pathogen and vaccination strategy, reactive vaccination typically prevented fewer than 10% of cases, because of their presently low R₀ values. Targeting HCWs had a higher per-regimen impact than population-wide vaccination.

CONCLUSIONS

Our framework provides a flexible methodology for estimating vaccine stockpile needs and the geographic distribution of demand under a range of outbreak response scenarios. Uncertainties in our model estimates highlight several knowledge gaps that need to be addressed to target vulnerable populations more accurately. These include surveillance gaps that mask the true geographic distribution of each pathogen, details of key routes of spillover from animal reservoirs to humans, and the role of human-to-human transmission outside of healthcare settings. In addition, our estimates are based on the current epidemiology of each pathogen, but pathogen evolution could alter vaccine stockpile requirements.

The Ecology of Viral Emergence

The coronavirus disease 2019 (COVID-19) pandemic has had a profound impact on human health, economic well-being, and societal function. It is essential that we use this generational experience to better understand the processes that underpin the emergence of COVID-19 and other zoonotic diseases. Herein, I review the mechanisms that determine why and how viruses emerge in new hosts, as well as the barriers to this process. I show that traditional studies of virus emergence have an inherent anthropocentric bias, with disease in humans considered the inevitable outcome of virus emergence, when in reality viruses are integral components of a global ecosystem characterized by continual host jumping with humans also transmitting their viruses to other animals. I illustrate these points using coronaviruses, including severe acute respiratory syndrome coronavirus 2, as a case study. I also outline the potential steps that can be followed to help mitigate and prevent future pandemics, with combating climate change a central component. Expected final online publication date for the Annual Review of Virology, Volume 9 is September 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Conserved recombination patterns across coronavirus subgenera

Recombination contributes to the genetic diversity found in coronaviruses and is known to be a prominent mechanism whereby they evolve. It is apparent, both from controlled experiments and in genome sequences sampled from nature, that patterns of recombination in coronaviruses are non-random and that this is likely attributable to a combination of sequence features that favour the occurrence of recombination break points at specific genomic sites, and selection disfavouring the survival of recombinants within which favourable intra-genome interactions have been disrupted. Here we leverage available whole-genome sequence data for six coronavirus subgenera to identify specific patterns of recombination that are conserved between multiple subgenera and then identify the likely factors that underlie these conserved patterns. Specifically, we confirm the non-randomness of recombination break points across all six tested coronavirus subgenera, locate conserved recombination hot- and cold-spots, and determine that the locations of transcriptional regulatory sequences are likely major determinants of conserved recombination break-point hotspot locations. We find that while the locations of recombination break points are not uniformly associated with degrees of nucleotide sequence conservation, they display significant tendencies in multiple coronavirus subgenera to occur in low guanine-cytosine content genome regions, in non-coding regions, at the edges of genes, and at sites within the Spike gene that are predicted to be minimally disruptive of Spike protein folding. While it is apparent that sequence features such as transcriptional regulatory sequences are likely major determinants of where the template-switching events that yield recombination break points most commonly occur, it is evident that selection against misfolded recombinant proteins also strongly impacts observable recombination break-point distributions in coronavirus genomes sampled from nature.

Emerging viruses: Cross-species transmission of coronaviruses, filoviruses, henipaviruses, and rotaviruses from bats

Emerging infectious diseases, especially if caused by bat-borne viruses, significantly affect public health and the global economy. There is an urgent need to understand the mechanism of interspecies transmission, particularly to humans. Viral genetics; host factors, including polymorphisms in the receptors; and ecological, environmental, and population dynamics are major parameters to consider. Here, we describe the taxonomy, geographic distribution, and unique traits of bats associated with their importance as virus reservoirs. Then, we summarize the origin, intermediate hosts, and the current understanding of interspecies transmission of Middle East respiratory syndrome coronavirus (MERS-CoV), severe acute respiratory syndrome coronavirus (SARS-CoV), SARS-CoV-2, Nipah, Hendra, Ebola, Marburg virus, and rotaviruses. Finally, the molecular interactions of viral surface proteins with host cell receptors are examined, and a comparison of these interactions in humans, intermediate hosts, and bats is conducted. This uncovers adaptive mutations in virus spike protein that facilitate cross-species transmission and risk factors associated with the emergence of novel viruses from bats.

A Multitrait Locus Regulates Sarbecovirus Pathogenesis

Infectious diseases have shaped the human population genetic structure, and genetic variation influences the susceptibility to many viral diseases. However, a variety of challenges have made the implementation of traditional human Genome-wide Association Studies (GWAS) approaches to study these infectious outcomes challenging. In contrast, mouse models of infectious diseases provide an experimental control and precision, which facilitates analyses and mechanistic studies of the role of genetic variation on infection. Here we use a genetic mapping cross between two distinct Collaborative Cross mouse strains with respect to SARS-CoV disease outcomes. We find several loci control differential disease outcome for a variety of traits in the context of SARS-CoV infection. Importantly, we identify a locus on mouse Chromosome 9 that shows conserved synteny with a human GWAS locus for SARS-CoV-2 severe disease. We follow-up and confirm a role for this locus, and identify two candidate genes, CCR9 and CXCR6 that both play a key role in regulating the severity of SARS-CoV, SARS-CoV-2 and a distantly related bat sarbecovirus disease outcomes. As such we provide a template for using experimental mouse crosses to identify and characterize multitrait loci that regulate pathogenic infectious outcomes across species.

Coronavirus and Paramyxovirus Shedding by Bats in a Cave and Buildings in Ethiopia

Bats are important hosts of zoonotic viruses with pandemic potential, including filoviruses, MERS-Coronavirus (CoV), SARS-CoV -1, and likely SARS-CoV-2. Viral infection and transmission among wildlife are dependent on a combination of factors that include host ecology and immunology, life history traits, roosting habitats, biogeography, and external stressors. Between 2016 and 2018, four species of insectivorous bats from a readily accessed roadside cave and buildings in Ethiopia were sampled and tested for viruses using consensus PCR assays for five viral families/genera. Previously identified and novel coronaviruses and paramyxoviruses were identified in 99 of the 589 sampled bats. Bats sampled from the cave site were more likely to test positive for a CoV than bats sampled from buildings; viral shedding was more common in the wet season; and rectal swabs were the most common sample type to test positive. A previously undescribed alphacoronavirus was detected in two bat species from different taxonomic families, sampling interfaces, geographic locations, and years. These findings expand knowledge of the range and diversity of coronaviruses and paramyxoviruses in insectivorous bats in Ethiopia and reinforce that an improved understanding of viral diversity and species-specific shedding dynamics is important for designing informed zoonotic disease surveillance and spillover risk reduction efforts.

MERS-CoV endoribonuclease and accessory proteins jointly evade host innate immunity during infection of lung and nasal epithelial cells

Middle East respiratory syndrome coronavirus (MERS-CoV) emerged into humans in 2012, causing highly lethal respiratory disease. The severity of disease may be, in part, because MERS-CoV is adept at antagonizing early innate immune pathways—interferon (IFN) production and signaling, protein kinase R (PKR), and oligoadenylate synthetase/ribonuclease L (OAS/RNase L)—activated in response to viral double-stranded RNA (dsRNA) generated during genome replication. This is in contrast to severe acute respiratory syndrome CoV-2 (SARS-CoV-2), which we recently reported to activate PKR and RNase L and, to some extent, IFN signaling. We previously found that MERS-CoV accessory proteins NS4a (dsRNA binding protein) and NS4b (phosphodiesterase) could weakly suppress these pathways, but ablation of each had minimal effect on virus replication. Here we investigated the antagonist effects of the conserved coronavirus endoribonuclease (EndoU), in combination with NS4a or NS4b. Inactivation of EndoU catalytic activity alone in a recombinant MERS-CoV caused little if any effect on activation of the innate immune pathways during infection. However, infection with recombinant viruses containing combined mutations with inactivation of EndoU and deletion of NS4a or inactivation of the NS4b phosphodiesterase promoted robust activation of dsRNA-induced innate immune pathways. This resulted in at least tenfold attenuation of replication in human lung–derived A549 and primary nasal cells. Furthermore, replication of these recombinant viruses could be rescued to the level of wild-type MERS-CoV by knockout of host immune mediators MAVS, PKR, or RNase L. Thus, EndoU and accessory proteins NS4a and NS4b together suppress dsRNA-induced innate immunity during MERS-CoV infection in order to optimize viral replication.

Bat Employs a Conserved MDA5 Gene to Trigger Antiviral Innate Immune Responses

Bats are important hosts for various zoonotic viral diseases. However, they rarely show signs of disease infection with such viruses. As the first line for virus control, the innate immune system of bats attracted our full attention. In this study, the Tadarida brasiliensis MDA5 gene (batMDA5), a major sensor for anti-RNA viral infection, was first cloned, and its biological functions in antiviral innate immunity were identified. Bioinformatics analysis shows that the amino acid sequence of batMDA5 is poorly conserved among species, and it is evolutionarily closer to humans. The mRNA of batMDA5 was significantly upregulated in Newcastle disease virus (NDV), avian influenza virus (AIV), and vesicular stomatitis virus (VSV)-infected bat TB 1 Lu cells. Overexpression of batMDA5 could activate IFNβ and inhibit vesicular stomatitis virus (VSV-GFP) replication in TB 1 Lu cells, while knockdown of batMDA5 yielded the opposite result. In addition, we found that the CARD domain was essential for MDA5 to activate IFNβ by constructing MDA5 domain mutant plasmids. These results indicated that bat employs a conserved MDA5 gene to trigger anti-RNA virus innate immune response. This study helps understand the biological role of MDA5 in innate immunity during evolution.

Structural Basis of the Main Proteases of Coronavirus Bound to Drug Candidate PF-07321332

The high mutation rate of COVID-19 and the prevalence of multiple variants strongly support the need for pharmacological options to complement vaccine strategies. One region that appears highly conserved among different genera of coronaviruses is the substrate-binding site of the main protease (M^pro or 3CL^pro), making it an attractive target for the development of broad-spectrum drugs for multiple coronaviruses. PF-07321332, developed by Pfizer, is the first orally administered inhibitor targeting the main protease of SARS-CoV-2, which also has shown potency against other coronaviruses. Here, we report three crystal structures of the main protease of SARS-CoV-2, SARS-CoV, and Middle East respiratory syndrome (MERS)-CoV bound to the inhibitor PF-07321332. The structures reveal a ligand-binding site that is conserved among SARS-CoV-2, SARS-CoV, and MERS-CoV, providing insights into the mechanism of inhibition of viral replication. The long and narrow cavity in the cleft between domains I and II of the main protease harbors multiple inhibitor-binding sites, where PF-07321332 occupies subsites S1, S2, and S4 and appears more restricted than other inhibitors. A detailed analysis of these structures illuminated key structural determinants essential for inhibition and elucidated the binding mode of action of the main proteases from different coronaviruses. Given the importance of the main protease for the treatment of SARS-CoV-2 infection, insights derived from this study should accelerate the design of safer and more effective antivirals. IMPORTANCE The current pandemic of multiple variants has created an urgent need for effective inhibitors of SARS-CoV-2 to complement vaccine strategies. PF-07321332, developed by Pfizer, is the first orally administered coronavirus-specific main protease inhibitor approved by the FDA. We solved the crystal structures of the main protease of SARS-CoV-2, SARS-CoV, and MERS-CoV that bound to the PF-07321332, suggesting PF-07321332 is a broad-spectrum inhibitor for coronaviruses. Structures of the main protease inhibitor complexes present an opportunity to discover safer and more effective inhibitors for COVID-19.

Structural Basis of the Main Proteases of Coronavirus Bound to Drug Candidate PF-07321332

The high mutation rate of COVID-19 and the prevalence of multiple variants strongly support the need for pharmacological options to complement vaccine strategies. One region that appears highly conserved among different genera of coronaviruses is the substrate-binding site of the main protease (M^pro or 3CL^pro), making it an attractive target for the development of broad-spectrum drugs for multiple coronaviruses. PF-07321332, developed by Pfizer, is the first orally administered inhibitor targeting the main protease of SARS-CoV-2, which also has shown potency against other coronaviruses. Here, we report three crystal structures of the main protease of SARS-CoV-2, SARS-CoV, and Middle East respiratory syndrome (MERS)-CoV bound to the inhibitor PF-07321332. The structures reveal a ligand-binding site that is conserved among SARS-CoV-2, SARS-CoV, and MERS-CoV, providing insights into the mechanism of inhibition of viral replication. The long and narrow cavity in the cleft between domains I and II of the main protease harbors multiple inhibitor-binding sites, where PF-07321332 occupies subsites S1, S2, and S4 and appears more restricted than other inhibitors. A detailed analysis of these structures illuminated key structural determinants essential for inhibition and elucidated the binding mode of action of the main proteases from different coronaviruses. Given the importance of the main protease for the treatment of SARS-CoV-2 infection, insights derived from this study should accelerate the design of safer and more effective antivirals. IMPORTANCE The current pandemic of multiple variants has created an urgent need for effective inhibitors of SARS-CoV-2 to complement vaccine strategies. PF-07321332, developed by Pfizer, is the first orally administered coronavirus-specific main protease inhibitor approved by the FDA. We solved the crystal structures of the main protease of SARS-CoV-2, SARS-CoV, and MERS-CoV that bound to the PF-07321332, suggesting PF-07321332 is a broad-spectrum inhibitor for coronaviruses. Structures of the main protease inhibitor complexes present an opportunity to discover safer and more effective inhibitors for COVID-19.

A human antibody reveals a conserved site on beta-coronavirus spike proteins and confers protection against SARS-CoV-2 infection

Broadly neutralizing antibodies (bnAbs) to coronaviruses (CoVs) are valuable in their own right as prophylactic and therapeutic reagents to treat diverse CoVs and as templates for rational pan-CoV vaccine design. We recently described a bnAb, CC40.8, from a CoV disease 2019 (COVID-19) convalescent donor that exhibits broad reactivity with human β-CoVs. Here, we showed that CC40.8 targets the conserved S2 stem helix region of the CoV spike fusion machinery. We determined a crystal structure of CC40.8 Fab with a SARS-CoV-2 S2 stem peptide at 1.6-Å resolution and found that the peptide adopted a mainly helical structure. Conserved residues in β-CoVs interacted with CC40.8 antibody, thereby providing a molecular basis for its broad reactivity. CC40.8 exhibited in vivo protective efficacy against SARS-CoV-2 challenge in two animal models. In both models, CC40.8-treated animals exhibited less weight loss and reduced lung viral titers compared to controls. Furthermore, we noted that CC40.8-like bnAbs are relatively rare in human COVID-19 infection, and therefore, their elicitation may require rational structure-based vaccine design strategies. Overall, our study describes a target on β-CoV spike proteins for protective antibodies that may facilitate the development of pan-β-CoV vaccines.

Full Genome Nobecovirus Sequences From Malagasy Fruit Bats Define a Unique Evolutionary History for This Coronavirus Clade

Bats are natural reservoirs for both Alpha- and Betacoronaviruses and the hypothesized original hosts of five of seven known zoonotic coronaviruses. To date, the vast majority of bat coronavirus research has been concentrated in Asia, though coronaviruses are globally distributed; indeed, SARS-CoV and SARS-CoV-2-related Betacoronaviruses in the subgenus Sarbecovirus have been identified circulating in Rhinolophid bats in both Africa and Europe, despite the relative dearth of surveillance in these regions. As part of a long-term study examining the dynamics of potentially zoonotic viruses in three species of endemic Madagascar fruit bat (Pteropus rufus, Eidolon dupreanum, Rousettus madagascariensis), we carried out metagenomic Next Generation Sequencing (mNGS) on urine, throat, and fecal samples obtained from wild-caught individuals. We report detection of RNA derived from Betacoronavirus subgenus Nobecovirus in fecal samples from all three species and describe full genome sequences of novel Nobecoviruses in P. rufus and R. madagascariensis. Phylogenetic analysis indicates the existence of five distinct Nobecovirus clades, one of which is defined by the highly divergent ancestral sequence reported here from P. rufus bats. Madagascar Nobecoviruses derived from P. rufus and R. madagascariensis demonstrate, respectively, Asian and African phylogeographic origins, mirroring those of their fruit bat hosts. Bootscan recombination analysis indicates significant selection has taken place in the spike, nucleocapsid, and NS7 accessory protein regions of the genome for viruses derived from both bat hosts. Madagascar offers a unique phylogeographic nexus of bats and viruses with both Asian and African phylogeographic origins, providing opportunities for unprecedented mixing of viral groups and, potentially, recombination. As fruit bats are handled and consumed widely across Madagascar for subsistence, understanding the landscape of potentially zoonotic coronavirus circulation is essential for mitigation of future zoonotic threats.

A human antibody reveals a conserved site on beta-coronavirus spike proteins and confers protection against SARS-CoV-2 infection

Broadly neutralizing antibodies (bnAbs) to coronaviruses (CoVs) are valuable in their own right as prophylactic and therapeutic reagents to treat diverse CoVs and, importantly, as templates for rational pan-CoV vaccine design. We recently described a bnAb, CC40.8, from a coronavirus disease 2019 (COVID-19)-convalescent donor that exhibits broad reactivity with human beta-coronaviruses (β-CoVs). Here, we showed that CC40.8 targets the conserved S2 stem-helix region of the coronavirus spike fusion machinery. We determined a crystal structure of CC40.8 Fab with a SARS-CoV-2 S2 stem-peptide at 1.6 Å resolution and found that the peptide adopted a mainly helical structure. Conserved residues in β-CoVs interacted with CC40.8 antibody, thereby providing a molecular basis for its broad reactivity. CC40.8 exhibited in vivo protective efficacy against SARS-CoV-2 challenge in two animal models. In both models, CC40.8-treated animals exhibited less weight loss and reduced lung viral titers compared to controls. Furthermore, we noted CC40.8-like bnAbs are relatively rare in human COVID-19 infection and therefore their elicitation may require rational structure-based vaccine design strategies. Overall, our study describes a target on β-CoV spike proteins for protective antibodies that may facilitate the development of pan-β-CoV vaccines.

SUMMARY

A human mAb isolated from a COVID-19 donor defines a protective cross-neutralizing epitope for pan-β-CoV vaccine design strategies.

Precise location of two novel linear epitopes on the receptor-binding domain surface of MERS-CoV spike protein recognized by two different monoclonal antibodies

Middle East respiratory syndrome coronavirus (MERS-CoV) is a coronavirus which can cause severe human respiratory diseases with a fatality rate of almost 36%. In this study, we report the generation, characterization and epitope mapping of several monoclonal antibodies against the spike receptor-binding domain (RBD) of MERS-CoV. Two monoclonal antibodies (4C7 and 6E8) that can react with linearized RBD have been selected for subsequent identification of RBD mAb-binding epitopes. Two distinct novel linear epitopes, ⁴²³FTCSQIS⁴²⁹ and ⁵⁴⁶SPLEGGGWL⁵⁵⁴,were precisely located at the outermost surface of RBD by dot-blot hybridization and ELISAs. Multiple sequence alignment analysis showed that these two peptides were highly conserved. Alanine (A)-scanning mutagenesis demonstrated that residues 423F, 428I, and 429S are the crucial residues for the linear epitope ⁴²³FTCSQIS⁴²⁹ while residues 548L, 550G, 553W, 554L for epitope ⁵⁴⁶SPLEGGGWL⁵⁵⁴. These findings may be helpful for further understanding of the function of RBD protein and the development of subsequent diagnosis and detection methods.

Structural basis and mode of action for two broadly neutralizing antibodies against SARS-CoV-2 emerging variants of concern

Emerging variants of concern for the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) can transmit more efficiently and partially evade protective immune responses, thus necessitating continued refinement of antibody therapies and immunogen design. Here, we elucidate the structural basis and mode of action for two potent SARS-CoV-2 spike (S)-neutralizing monoclonal antibodies, CV3-1 and CV3-25, which remain effective against emerging variants of concern in vitro and in vivo. CV3-1 binds to the (485-GFN-487) loop within the receptor-binding domain (RBD) in the "RBD-up" position and triggers potent shedding of the S1 subunit. In contrast, CV3-25 inhibits membrane fusion by binding to an epitope in the stem helix region of the S2 subunit that is highly conserved among β-coronaviruses. Thus, vaccine immunogen designs that incorporate the conserved regions in the RBD and stem helix region are candidates to elicit pan-coronavirus protective immune responses.