Coronavirus pathogenesis and the emerging pathogen severe acute respiratory syndrome coronavirus

A chimeric multi-epitope DNA vaccine elicited specific antibody response against severe acute respiratory syndrome-associated coronavirus which attenuated the virulence of SARS-CoV in vitro

Epitope-based vaccines designed to induce antibody responses specific for severe acute respiratory syndrome-associated coronavirus (SARS-CoV) are being developed as a means for increasing vaccine potency. In this study, we identified four B cell epitopes from the spike (S) and membrane (M) protein through bioinformatics analysis and constructed a multi-epitope DNA vaccine. Intramuscular immunization of mice with this vaccine was sufficient to induce specific prime as well as a long-term memory humoral immune response to at least two candidate epitopes, S_437–459 and M_1–20. A DNA prime–protein boost strategy greatly enhanced the antibody generation and the immune sera not only reacted with the lysates of SARS-CoV-infected Vero cells but also neutralized the cytopathic effect of SARS by 75% at 1:160 dilution. The novel immunogenic S protein peptide revealed in this study provides new target for SARS vaccine design; and our work indicated multi-epitope DNA vaccine as an effective means for eliciting polyvalent humoral immune response against SARS-CoV.

Pathogenicity of severe acute respiratory coronavirus deletion mutants in hACE-2 transgenic mice

Recombinant severe acute respiratory virus (SARS-CoV) variants lacking the group specific genes 6, 7a, 7b, 8a, 8b and 9b (rSARS-CoV-Delta[6-9b]), the structural gene E (rSARS-CoV-DeltaE), and a combination of both sets of genes (rSARS-CoV-Delta[E,6-9b]) have been generated. All these viruses were rescued in monkey (Vero E6) cells and were also infectious for human (Huh-7, Huh7.5.1 and CaCo-2) cell lines and for transgenic (Tg) mice expressing the SARS-CoV receptor human angiotensin converting enzyme-2 (hACE-2), indicating that none of these proteins is essential for the viral cycle. Furthermore, in Vero E6 cells, all the viruses showed the formation of particles with the same morphology as the wt virus, indicating that these proteins do not have a high impact in the final morphology of the virions. Nevertheless, in the absence of E protein, release of virus particles efficacy was reduced. Viruses lacking E protein grew about 100-fold lower than the wt virus in lungs of Tg infected mice but did not grow in the brains of the same animals, in contrast to the rSARS-CoV-Delta[6-9b] virus, which grew almost as well as the wt in both tissues. Viruses lacking E protein were highly attenuated in the highly sensitive hACE-2 Tg mice, in contrast to the minimal rSARS-CoV-Delta[6-9b] and wt viruses. These data indicate that E gene might be a virulence factor influencing replication level, tissue tropism and pathogenicity of SARS-CoV, suggesting that DeltaE attenuated viruses are promising vaccine candidates.

The role of programmed-1 ribosomal frameshifting in coronavirus propagation

Coronaviruses have the potential to cause significant economic, agricultural and health problems. The severe acute respiratory syndrome (SARS) associated coronavirus outbreak in late 2002, early 2003 called attention to the potential damage that coronaviruses could cause in the human population. The ensuing research has enlightened many to the molecular biology of coronaviruses. A programmed -1 ribosomal frameshift is required by coronaviruses for the production of the RNA dependent RNA polymerase which in turn is essential for viral replication. The frameshifting signal encoded in the viral genome has additional features that are not essential for frameshifting. Elucidation of the differences between coronavirus frameshift signals and signals from other viruses may help our understanding of these features. Here we summarize current knowledge and add additional insight regarding the function of the programmed -1 ribosomal frameshift signal in the coronavirus lifecycle.

Mouse hepatitis virus infection of the CNS: a model for defense, disease, and repair

Viral infection of the central nervous system (CNS) results in varied outcomes ranging from encephalitis, paralytic poliomyelitis or other serious consequences. One of the principal factors that directs the outcome of infection is the localized innate immune response, which is proceeded by the adaptive immune response against the invading viral pathogen. The role of the immune system is to contain and control the spread of virus within the CNS, and paradoxically, this response may also be pathological. Studies with a neurotropic murine coronavirus, mouse hepatitis virus (MHV) have provided important insights into how the immune system combats neuroinvasive viruses, and have identified molecular and cellular mechanisms contributing to chronic disease in persistently infected mice.

Distinct Signature Type I Interferon Responses are Determined by the Infecting virus and the Target Cell

Background

Type I interferons (IFN) include multiple IFN-α subtypes which exhibit considerable amino acid identity and activate the same cell-surface receptor. The promoter regions of the IFN-α genes, however, have different transcription factor binding sites, implying differential transcriptional activation. Evolutionary conservation of multiple subtypes may have resulted from external pressures associated with the crucial nature of an IFN response, namely that different viruses that are tropic for different target tissues determine the nature and extent of an IFN response, specifically the IFN-α subtype profile.

Methods

Studies were undertaken to examine inducible IFN gene expression profiles in response to infection with single-stranded RNA viruses: Sendai virus (SeV), murine hepatitis virus (MHV-1) and coxsackie virus B3 (CVB3).

Results

In vitro, distinct signature profiles of SeV and MHV-1-inducible gene expression for IFN-α2, IFN-α4 and IFN-α5 subtypes in L2 and L929 mouse fibroblast cells, in relation to the extent and kinetics of their induction, were identified. In vivo, whereas A/J mice are highly permissive for both MHV-1 and CVB3 infections and mount a poor IFN response, C57Bl/6 mice are relatively resistant to both virus infections and mount a vigorous IFN response.

Conclusions

These data suggest that the infecting virus and the target cell type dictate the extent and signature of inducible type I IFN gene expression. The extent of IFN response to viral infection influences the subsequent biological outcome: a robust IFN response prescribes a level of resistance, whereas a poor IFN response contributes towards a permissive phenotype for infection.

Vaccines to prevent severe acute respiratory syndrome coronavirus-induced disease

An important effort has been performed after the emergence of severe acute respiratory syndrome (SARS) epidemic in 2003 to diagnose and prevent virus spreading. Several types of vaccines have been developed including inactivated viruses, subunit vaccines, virus-like particles (VLPs), DNA vaccines, heterologous expression systems, and vaccines derived from SARS-CoV genome by reverse genetics. This review describes several aspects essential to develop SARS-CoV vaccines, such as the correlates of protection, virus serotypes, vaccination side effects, and bio-safeguards that can be engineered into recombinant vaccine approaches based on the SARS-CoV genome. The production of effective and safe vaccines to prevent SARS has led to the development of promising vaccine candidates, in contrast to the design of vaccines for other coronaviruses, that in general has been less successful. After preclinical trials in animal models, efficacy and safety evaluation of the most promising vaccine candidates described has to be performed in humans.

Identification of a novel coronavirus from a beluga whale by using a panviral microarray

The emergence of viruses such as severe acute respiratory syndrome coronavirus and Nipah virus has underscored the role of animal reservoirs in human disease and the need for reservoir surveillance. Here, we used a panviral DNA microarray to investigate the death of a captive beluga whale in an aquatic park. A highly divergent coronavirus, tentatively named coronavirus SW1, was identified in liver tissue from the deceased whale. Subsequently, the entire genome of SW1 was sequenced, yielding a genome of 31,686 nucleotides. Phylogenetic analysis revealed SW1 to be a novel virus distantly related to but most similar to group III coronaviruses.

Interferon and cytokine responses to SARS-coronavirus infection

The sudden emergence of severe acute respiratory syndrome (SARS) has boosted research on innate immune responses to coronaviruses. It is now well established that the causative agent, a newly identified coronavirus termed SARS-CoV, employs multiple passive and active mechanisms to avoid induction of the antiviral type I interferons in tissue cells. By contrast, chemokines such as IP-10 or IL-8 are strongly upregulated. The imbalance in the IFN response is thought to contribute to the establishment of viremia early in infection, whereas the production of chemokines by infected organs may be responsible for (i) massive immune cell infiltrations found in the lungs of SARS victims, and (ii) the dysregulation of adaptive immunity. Here, we will review the most recent findings on the interaction of SARS-CoV and related Coronaviridae members with the type I interferon and cytokine responses and discuss implications for pathogenesis and therapy.

In vitro inhibition of coronavirus replications by the traditionally used medicinal herbal extracts, Cimicifuga rhizoma, Meliae cortex, Coptidis rhizoma, and Phellodendron cortex

BACKGROUND

A search for new anti-coronaviral drugs to treat coronaviral infections was motivated by an outbreak of severe acute respiratory syndrome (SARS).

OBJECTIVES

In order to find drugs that treat coronavirus infections, including SARS, we screened traditional medicinal herbal extracts and evaluated their antiviral activities on coronavirus replication.

STUDY DESIGN

We employed a plaque assay to evaluate the effect of 22 medicinal herbal extracts on virus replication. We determined the 50% effective concentration (EC50) of each extract that was necessary to inhibit the replication of mouse hepatitis virus A59 (MHV-A59); we also determined 50% cytotoxic concentrations (CC50) for each extract. Northern and Western blot analyzes were performed to investigate antiviral activity in MHV-infected DBT cells, including virus entry, viral RNA and protein expression, and virus release. Coronavirus specific inhibition was also demonstrated using porcine epidemic diarrhea virus (PEDV).

RESULTS

Cimicifuga rhizoma, Meliae cortex, Coptidis rhizoma, Phellodendron cortex and Sophora subprostrata radix decreased the MHV production and the intracellular viral RNA and protein expression with EC50 values ranging from 2.0 to 27.5 microg/ml. These extracts also significantly decreased PEDV production and less dramatically decreased vesicular stomatitis virus (VSV) production in vitro.

CONCLUSIONS

The extracts selected strongly inhibited MHV replication and could be potential candidates for new anti-coronavirus drugs.

Co-infection of respiratory bacterium with severe acute respiratory syndrome coronavirus induces an exacerbated pneumonia in mice

SARS-CoV grows in a variety of tissues that express its receptor, although the mechanism for high replication in the lungs and severe respiratory illness is not well understood. We recently showed that elastase enhances SARS-CoV infection in cultured cells, which suggests that SARS development may be due to elastase-mediated, enhanced SARS-CoV infection in the lungs. To explore this possibility, we examined whether co-infection of mice with SARS-CoV and Pp, a low-pathogenic bacterium which elicits elastase production in the lungs, induces exacerbation of pneumonia. Mice co-infected with SARS-CoV and Pp developed severe respiratory disease with extensive weight loss, resulting in a 33~90% mortality rate. Mice with exacerbated pneumonia showed enhanced virus infection in the lungs and histopathological lesions similar to those found in human SARS cases. Intranasal administration of LPS, another elastase inducer, showed an effect similar to that of Pp infection. Thus, this study shows that exacerbated pneumonia in mice results from co-infection with SARS-CoV and a respiratory bacterium that induces elastase production in the lungs, suggesting a possible role for elastase in the exacerbation of pneumonia.

Identification of the avian infectious bronchitis coronaviruses with mutations in gene 3

The sequence of a 6.0-kb fragment was compared in the 3′-encoding region of the genome in 27 infectious bronchitis virus (IBV) strains. All these strains have the same S-3-M-5-N gene order, as is the case for other IBVs. However, the sizes of the corresponding open reading frames (ORFs) of some genes varied among the virus strains. Phylogenetic analysis and sequence alignments demonstrated that recombination events had occurred in the origin and evolution of the strains CK/CH/LSD/03I and CK/CH/LLN/98I and the possible recombinant junction sites might be located at the 3c and M genes, respectively. The normal product of ORF 3a is 57 amino acids long, whereas a 43-bp deletion at the 3′-end of the CK/CH/LSD/03I 3a gene was detected, resulting in a frameshift event and C-terminally truncated protein with 47 amino acids. Comparison of the growth ability in embryos and replication and pathogenicity in chickens with IBV carrying the normal 3a gene indicated that this deleted sequence in the 3a gene of CK/CH/LSD/03I was not necessary for viral pathogenesis and replication either in vitro or in vivo. Occurrence of a mutation at the corresponding position of the CK/CH/LLN/98I start codon in the 3a gene led to the absence of ORF 3a in this virus, resulting in a novel genomic organization at the 3′-encoding regions: S-3b, 3c-M-5a, 5b-N. Comparison with other viruses carrying the normal 3a gene revealed that CK/CH/LLN/98I had replication and pathogenicity abilities in vivo similar to those of other IBVs; however, its growth ability in embryos was lower, although the relationship between the lower growth ability and the ORF 3a defect requires further confirmation.

Optimization of a DNA vaccine against SARS

Severe acute respiratory syndrome coronavirus (SARS-CoV) first appeared in Southern China in November 2002, and then quickly spread to 33 countries on five continents along international air travel routes. Although the SARS epidemic has been contained, there is a clear need for a safe and effective vaccine should an outbreak of a SARS-CoV infection reappear in human population. In this study, we tested four DNA-vaccine constructs: (1) pLL70, containing cDNA for the SARS-CoV spike (S) gene; (2) pcDNA-SS, containing codon-optimized S gene for SARS-CoV S protein (residues 12-1255) fused with a leader sequence derived from the human CD5 gene; (3) pcDNA-St, containing the gene encoding the N-portion of the codon-optimized S gene (residues 12-532) with the CD5 leader sequence; (4) pcDNA-St-VP22C, containing the gene encoding the N-portion of the codon-optimized S protein with the CD5 leader sequence fused with the C-terminal 138 amino acids of the bovine herpesvirus-1 (BHV-1) major tegument protein VP22. Each of these plasmids was intradermally administered to C57BL/6 mice in three separate immunizations. Analysis of humoral and cellular immune responses in immunized mice demonstrated that pcDNA-SS and pcDNA-St-VP22C are the most immunogenic SARS vaccine candidates.

Is the anti-psychotic, 10-(3-(dimethylamino)propyl)phenothiazine (promazine), a potential drug with which to treat SARS infections? Lack of efficacy of promazine on SARS-CoV replication in a mouse model

Phenothiazine and derivatives were tested for inhibition of SARS-CoV replication. Phenothiazine slightly inhibited SARS-CoV replication in a neutral red (NR) uptake assay. Adding a propylamino group to give promazine reduced virus yields (VYR assay) with an EC₉₀ = 8.3 ± 2.8 μM, but without selectivity. Various substitutions in the basic phenothiazine structure did not promote efficacy. Phenazine ethosulfate was the most potent compound by VYR assay (EC₉₀ = 6.1 ± 4.3 μM). All compounds were toxic (IC₅₀ = 6.6–74.5 μM) except for phenoxathiin (IC₅₀ = 858 ± 208 μM) and 10-(alpha-diethylamino-propionyl) phenothiazine·HCl (IC₅₀ = 195 ± 71.2 μM). Consequently, none were selective inhibitors of SARS-CoV replication (SI values <1–3.3 μM). These data portended the poor efficacy of promazine in a SARS-CoV mouse lung replication model. Intraperitoneal treatment with promazine using a prophylactic (−4 h)/therapeutic regimen of 1, 10, or 50 mg/(kg day) did not reduce virus lung titers at day 3, yet prolonged virus replication to 14 days. Similar therapeutic promazine doses were not efficacious. Thus, promazine did not affect SARS-CoV replication in vitro or in vivo, nor were any other phenothiazines efficacious in reducing virus replication. Therefore, treating SARS infections with compounds like promazine is not warranted.

A line immunoassay utilizing recombinant nucleocapsid proteins for detection of antibodies to human coronaviruses

Most coronaviruses infecting humans cause mild diseases, whereas severe acute respiratory syndrome (SARS)-associated coronavirus is an extremely dangerous pathogen. Here, we report the development of a serologic assay for detection of antibodies to human coronaviruses (HCoVs) based on recombinant nucleocapsid (N) proteins of all known pathogenic strains (229E, NL63, OC43, HKU1, SARS). The novel immunoassay is highly useful for epidemiologic surveys, where use of nucleic acid diagnostics often is limited. Purified recombinant antigens were immobilized on nitrocellulose membranes and applied in a line immunoassay, which allows rapid detection of antibodies to 5 different HCoVs in a single experiment. For assay evaluation, serum samples from persons infected with 229E or OC43 (acute/convalescent), recovered SARS patients and healthy donors were analyzed. Screening for nucleocapsid (N)-specific immunoglobulin G (IgG) in convalescent sera reached 100% sensitivity. With this new technique, we found that recently identified NL63 and HKU1 contribute significantly to the overall spectrum of coronavirus infections. Possibly, cross-reactive antibody responses were observed using 229E and OC43 serum pairs. However, the potential of this assay could clearly be demonstrated employing SARS-positive serum samples, where nonspecific binding to nucleocapsids of other HCoVs was not observed. This coronavirus strain-specific line immunoassay represents a powerful tool for serologic diagnostics.

Detection of group 1 coronaviruses in bats using universal coronavirus reverse transcription polymerase chain reactions

The zoonotic transmission of SARS coronavirus from animals to humans revealed the potential impact of coronaviruses on mankind. This incident also triggered several surveillance programs to hunt for novel coronaviruses in human and wildlife populations. Using classical RT-PCR assays that target a highly conserved sequence among coronaviruses, we identified the first coronaviruses in bats. These assays and the cloning and sequencing of the PCR products are described in this chapter. Using the same approach in our subsequent studies, we further detected several novel coronaviruses in bats. These findings highlighted the fact that bats are important reservoirs for coronaviruses.

Infectious bronchitis viruses with a novel genomic organization

A number of novel infectious bronchitis viruses (IBVs) were previously identified in commercial poultry in Australia, where they caused significant economic losses. Since there has been only limited characterization of these viruses, we investigated the genomic and phenotypic differences between these novel IBVs and other, classical IBVs. The 3' 7.5 kb of the genomes of 17 Australian IBV strains were sequenced, and growth properties of 6 of the strains were compared. Comparison of sequences of the genes coding for structural and nonstructural proteins revealed the existence of two IBV genotypes: classical and novel. The genomic organization of the classical IBVs was typical of those of other group III coronaviruses: 5'-Pol-S-3a-3b-E-M-5a-5b-N-untranslated region (UTR)-3'. However, the novel IBV genotype lacked either all or most of the genes coding for nonstructural proteins at the 3' end of the genome and had a unique open reading frame, X1. The gene order was either 5'-Pol-S-X1-E-M-N-UTR-3' or 5'-Pol-S-X1-E-M-5b-N-UTR-3'. Phenotypically, novel and classical IBVs also differed; novel IBVs grew at a slower rate and reached lower titers in vitro and in vivo and were markedly less immunogenic in chicks. Although the novel IBVs induced histopathological lesions in the tracheas of infected chicks that were comparable to those induced by classical strains, they did not induce lesions in the kidneys. This study has demonstrated for the first time the existence of a naturally occurring IBV genotype devoid of some of the genes coding for nonstructural proteins and has also indicated that all of the accessory genes are dispensable for the growth of IBV and that such viruses are able to cause clinical disease and economic loss. The phylogenic differences between these novel IBVs and other avian coronaviruses suggest a reservoir host distinct from domestic poultry.

Structural basis of mercury- and zinc-conjugated complexes as SARS-CoV 3C-like protease inhibitors

Five active metal-conjugated inhibitors (PMA, TDT, EPDTC, JMF1586 and JMF1600) bound with the 3C-like protease of severe acute respiratory syndrome (SARS)-associated coronavirus were analyzed crystallographically. The complex structures reveal two major inhibition modes: Hg(2+)-PMA is coordinated to C(44), M(49) and Y(54) with a square planar geometry at the S3 pocket, whereas each Zn(2+) of the four zinc-inhibitors is tetrahedrally coordinated to the H(41)-C(145) catalytic dyad. For anti-SARS drug design, this Zn(2+)-centered coordination pattern would serve as a starting platform for inhibitor optimization.

Viral epizootic reveals inbreeding depression in a habitually inbreeding mammal

Inbreeding is typically detrimental to fitness. However, some animal populations are reported to inbreed without incurring inbreeding depression, ostensibly due to past “purging” of deleterious alleles. Challenging this is the position that purging can, at best, only adapt a population to a particular environment; novel selective regimes will always uncover additional inbreeding load. We consider this in a prominent test case: the eusocial naked mole‐rat (Heterocephalus glaber), one of the most inbred of all free‐living mammals. We investigated factors affecting mortality in a population of naked mole‐rats struck by a spontaneous, lethal coronavirus outbreak. In a multivariate model, inbreeding coefficient strongly predicted mortality, with closely inbred mole‐rats (F≥ 0.25) over 300% more likely to die than their outbred counterparts. We demonstrate that, contrary to common assertions, strong inbreeding depression is evident in this species. Our results suggest that loss of genetic diversity through inbreeding may render populations vulnerable to local extinction from emerging infectious diseases even when other inbreeding depression symptoms are absent.

Coronavirus non-structural protein 1 is a major pathogenicity factor: implications for the rational design of coronavirus vaccines

Attenuated viral vaccines can be generated by targeting essential pathogenicity factors. We report here the rational design of an attenuated recombinant coronavirus vaccine based on a deletion in the coding sequence of the non-structural protein 1 (nsp1). In cell culture, nsp1 of mouse hepatitis virus (MHV), like its SARS-coronavirus homolog, strongly reduced cellular gene expression. The effect of nsp1 on MHV replication in vitro and in vivo was analyzed using a recombinant MHV encoding a deletion in the nsp1-coding sequence. The recombinant MHV nsp1 mutant grew normally in tissue culture, but was severely attenuated in vivo. Replication and spread of the nsp1 mutant virus was restored almost to wild-type levels in type I interferon (IFN) receptor-deficient mice, indicating that nsp1 interferes efficiently with the type I IFN system. Importantly, replication of nsp1 mutant virus in professional antigen-presenting cells such as conventional dendritic cells and macrophages, and induction of type I IFN in plasmacytoid dendritic cells, was not impaired. Furthermore, even low doses of nsp1 mutant MHV elicited potent cytotoxic T cell responses and protected mice against homologous and heterologous virus challenge. Taken together, the presented attenuation strategy provides a paradigm for the development of highly efficient coronavirus vaccines.

Prevention of viral diseases by vaccination aims for controlled induction of protective immune responses against viral pathogens. Live viral vaccines consist of attenuated, replication-competent viruses that are believed to be superior in the induction of broad immune responses, including cell-mediated immunity. The recent proceedings in the area of virus reverse genetics allows for the rational design of recombinant vaccines by targeting, i.e., inactivating, viral pathogenicity factors. For coronaviruses, a major pathogenicity factor has now been identified. The effect of coronavirus non-structural protein 1 on pathogenicity has been analyzed in a murine model of coronavirus infection. By deleting a part of this protein, a recombinant virus has been generated that is greatly attenuated in vivo, while retaining immunogenicity. In particular, the mutant virus retained the ability to replicate in professional antigen-presenting cells and fulfilled an important requirement of a promising vaccine candidate: the induction of a protective long-lasting, antigen-specific cellular immune response. This study has implications for the rational design of live attenuated coronavirus vaccines aimed at preventing coronavirus-induced diseases of veterinary and medical importance, including the potentially lethal severe acute respiratory syndrome.

Rat coronaviruses infect rat alveolar type I epithelial cells and induce expression of CXC chemokines

We analyzed the ability of two rat coronavirus (RCoV) strains, sialodacryoadenitis virus (SDAV) and Parker's RCoV (RCoV-P), to infect rat alveolar type I cells and induce chemokine expression. Primary rat alveolar type II cells were transdifferentiated into the type I cell phenotype. Type I cells were productively infected with SDAV and RCoV-P, and both live virus and UV-inactivated virus induced mRNA and protein expression of three CXC chemokines: CINC-2, CINC-3, and LIX, which are neutrophil chemoattractants. Dual immunolabeling of type I cells for viral antigen and CXC chemokines showed that chemokines were expressed primarily by uninfected cells. Virus-induced chemokine expression was reduced by the IL-1 receptor antagonist, suggesting that IL-1 produced by infected cells induces uninfected cells to express chemokines. Primary cultures of alveolar epithelial cells are an important model for the early events in viral infection that lead to pulmonary inflammation.

Systemic fatal type II coronavirus infection in a dog: pathological findings and immunohistochemistry

A case of fatal systemic coronavirus infection is described in a 53-day-old Pekinese dog. Pathological findings and immunohistochemical identification using a monoclonal anti-canine Coronavirus antibody are included. Visceral lesions consisted of extensive fibrinopurulent bronchopneumonia, multiple renal cortical infarcts, severe coalescing centrilobular hepatic fatty change with minimal random hepatic necrosis, and multifocal splenic haemorrhage with lymphoid depletion. Moderate chronic diffuse enteritis was associated with intraluminal adult ascarids. Identification of type I and type II coronavirus in this subject had been previously confirmed by genotype-specific real-time reverse transcription-polymerase chain reaction (RT-PCR) assays of the intestinal contents, while only Coronavirus type II was detected in visceral organs. This case represents the first description of morphological lesions associated with a type II pantropic fatal coronavirus infection in the dog.

The emerging role of ACE2 in physiology and disease

The renin–angiotensin–aldosterone system (RAAS) is a key regulator of systemic blood pressure and renal function and a key player in renal and cardiovascular disease. However, its (patho)physiological roles and its architecture are more complex than initially anticipated. Novel RAAS components that may add to our understanding have been discovered in recent years. In particular, the human homologue of ACE (ACE2) has added a higher level of complexity to the RAAS. In a short period of time, ACE2 has been cloned, purified, knocked‐out, knocked‐in; inhibitors have been developed; its 3D structure determined; and new functions have been identified. ACE2 is now implicated in cardiovascular and renal (patho)physiology, diabetes, pregnancy, lung disease and, remarkably, ACE2 serves as a receptor for SARS and NL63 coronaviruses. This review covers available information on the genetic, structural and functional properties of ACE2. Its role in a variety of (patho)physiological conditions and therapeutic options of modulation are discussed. Copyright © 2007 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.

Lithium chloride inhibits the coronavirus infectious bronchitis virus in cell culture

The avian coronavirus infectious bronchitis virus (IBV) is a major economic pathogen of domestic poultry that, despite vaccination, causes mortality and significant losses in production. During replication of the RNA genome there is a high frequency of mutation and recombination, which has given rise to many strains of IBV and results in the potential for new and emerging strains. Currently the live-attenuated vaccine gives poor cross-strain immunity. Effective antiviral agents may therefore be advantageous in the treatment of IBV. Lithium chloride (LiCl) is a potent inhibitor of the DNA virus herpes simplex virus but not RNA viruses. The effect of LiCl on the replication of IBV was examined in cell culture using two model cell types; Vero cells, an African Green monkey kidney-derived epithelial cell line; and DF-1 cells, an immortalized chicken embryo fibroblast cell line. When treated with a range of LiCl concentrations, IBV RNA and protein levels and viral progeny production were reduced in a dose-dependent manner in both cell types, and the data indicated that inhibition was a cellular rather than a virucidal effect. Host cell protein synthesis still took place in LiCl-treated cells and the level of a standard cellular housekeeping protein remained unchanged, indicating that the effect of LiCl was specifically against IBV.

Cyclooxygenase activity is important for efficient replication of mouse hepatitis virus at an early stage of infection

Cyclooxygenases (COXs) play a significant role in many different viral infections with respect to replication and pathogenesis. Here we investigated the role of COXs in the mouse hepatitis coronavirus (MHV) infection cycle. Blocking COX activity by different inhibitors or by RNA interference affected MHV infection in different cells. The COX inhibitors reduced MHV infection at a post-binding step, but early in the replication cycle. Both viral RNA and viral protein synthesis were affected with subsequent loss of progeny virus production. Thus, COX activity appears to be required for efficient MHV replication, providing a potential target for anti-coronaviral therapy.

Structure-function relationships of the viral RNA-dependent RNA polymerase: fidelity, replication speed, and initiation mechanism determined by a residue in the ribose-binding pocket

Studies of the RNA-dependent RNA polymerase (RdRp) from poliovirus (PV), 3Dpol, have shown that Asn-297 permits this enzyme to distinguish ribose from 2'-deoxyribose. All animal RNA viruses have Asn at the structurally homologous position of their polymerases, suggesting a conserved function for this residue. However, all prokaryotic RNA viruses have Glu at this position. In the presence of Mg2+, the apparent affinity of Glu-297 3Dpol for 2'-deoxyribonucleotides was decreased by 6-fold relative to wild type without a substantial difference in the fidelity of 2'-dNMP incorporation. The fidelity of ribonucleotide misincorporation for Glu-297 3Dpol was reduced by 14-fold relative to wild type. A 4- to 11-fold reduction in the rate of ribonucleotide incorporation was observed. Glu-297 PV was unable to grow in HeLa cells due to a replication defect equivalent to that observed for a mutant PV encoding an inactive polymerase. Evaluation of the protein-(VPg)-primed initiation reaction showed that only half of the Glu-297 3Dpol initiation complexes were capable of producing VPg-pUpU product and that the overall yield of uridylylated VPg products was reduced by 20-fold relative to wild-type enzyme, a circumstance attributable to a reduced affinity for UTP. These studies identify the first RdRp derivative with a mutator phenotype and provide a mechanistic basis for the elevated mutation frequency of RNA phage relative to animal RNA viruses observed in culture. Although protein-primed initiation and RNA-primed elongation complexes employ the same polymerase active site, the functional differences reported here imply significant structural differences between these complexes.

Inhibition of the alpha/beta interferon response by mouse hepatitis virus at multiple levels

Mouse hepatitis virus (MHV) was used as a model to study the interaction of coronaviruses with the alpha/beta interferon (IFN-alpha/beta) response. While MHV strain A59 appeared to induce IFN-beta gene transcription and low levels of nuclear translocation of the IFN-beta transcription factor interferon regulatory factor 3 (IRF-3), MHV did not induce IFN-beta protein production during the course of infection in L2 mouse fibroblast cells. In addition, MHV was able to significantly decrease the level of IFN-beta protein induced by both Newcastle disease virus (NDV) and Sendai virus infections, without targeting it for proteasomal degradation and without altering the nuclear translocation of IRF-3 or IFN-beta mRNA production or stability. These results indicate that MHV infection causes an inhibition of IFN-beta production at a posttranscriptional level, without altering RNA or protein stability. In contrast, MHV induced IFN-beta mRNA and protein production in the brains of infected animals, suggesting that the inhibitory mechanisms observed in vitro are not enough to prevent IFN-alpha/beta production in vivo. Furthermore, MHV replication is highly resistant to IFN-alpha/beta action, as indicated by unimpaired MHV replication in L2 cells pretreated with IFN-beta. However, when L2 cells were coinfected with MHV and NDV in the presence of IFN-beta, NDV, but not MHV, replication was inhibited. Thus, rather than disarming the antiviral activity induced by IFN-beta pretreatment completely, MHV may be inherently resistant to some aspects of the antiviral state induced by IFN-beta. These findings show that MHV employs unique strategies to circumvent the IFN-alpha/beta response at multiple steps.

Detection of a novel and highly divergent coronavirus from asian leopard cats and Chinese ferret badgers in Southern China

Since an outbreak of severe acute respiratory syndrome (SARS) was averted in 2004, many novel coronaviruses have been recognized from different species, including humans. Bats have provided the most diverse assemblages of coronaviruses, suggesting that they may be the natural reservoir. Continued virological surveillance has proven to be the best way to avert this infectious disease at the source. Here we provide the first description of a previously unidentified coronavirus lineage detected from wild Asian leopard cats (Prionailurus bengalensis) and Chinese ferret badgers (Melogale moschata) during virological surveillance in southern China. Partial genome analysis revealed a typical coronavirus genome but with a unique putative accessory gene organization. Phylogenetic analyses revealed that the envelope, membrane, and nucleoprotein structural proteins and the two conserved replicase domains, putative RNA-dependent RNA polymerase and RNA helicase, of these novel coronaviruses were most closely related to those of group 3 coronaviruses identified from birds, while the spike protein gene was most closely related to that of group 1 coronaviruses from mammals. However, these viruses always fell into an outgroup phylogenetic relationship with respect to other coronaviruses and had low amino acid similarity to all known coronavirus groups, indicating that they diverged early in the evolutionary history of coronaviruses. These results suggest that these viruses may represent a previously unrecognized evolutionary pathway, or possibly an unidentified coronavirus group. This study demonstrates the importance of systematic virological surveillance in market animals for understanding the evolution and emergence of viruses with infectious potential.

SARS coronavirus and innate immunity

The emergence of the highly pathogenic SARS coronavirus (SARS-CoV) has reignited interest in coronavirus biology and pathogenesis. An emerging theme in coronavirus pathogenesis is that the interaction between specific viral genes and the host immune system, specifically the innate immune system, functions as a key determinant in regulating virulence and disease outcomes. Using SARS-CoV as a model, we will review the current knowledge of the interplay between coronavirus infection and the host innate immune system in vivo, and then discuss the mechanisms by which specific gene products antagonize the host innate immune response in cell culture models. Our data suggests that the SARS-CoV uses specific strategies to evade and antagonize the sensing and signaling arms of the interferon pathway. We summarize by identifying future points of consideration that will contribute greatly to our understanding of the molecular mechanisms governing coronavirus pathogenesis and virulence, and the development of severe disease in humans and animals.

[Cell entry mechanism of coronaviruses: implication in their pathogenesis]

Coronaviruses infect many species of animals, including humans. Among them, murine coronavirus, mouse hepatitis virus (MHV) has been well studied as a model of human diseases, such as hepatitis and demyelinating disease. An agent causing severe acute respiratory disease (SARS), SARS coronavirus (SARS-CoV), is a newcomer in this genus, however, it is now one of the most studied coronaviruses due to its medical impact. The receptors of those two viruses have been identified and their cell entry mechanism has been actively investigated. Recently, SARS-CoV cell entry mechanism is shown to be different from that of other enveloped viruses, including MHV. In this review, cell entry mechanism of those two viruses is described, stressing on the difference and similarity found between those two viruses.

Severe acute respiratory syndrome coronavirus accessory protein 6 is a virion-associated protein and is released from 6 protein-expressing cells

Analysis of severe acute respiratory syndrome coronavirus (SCoV) by either sucrose gradient equilibrium centrifugation or a virus capture assay using an anti-SCoV S protein antibody demonstrated that the SCoV 6 protein, which is one of the accessory proteins of SCoV, was incorporated into virus particles. Coexpression of the SCoV S, M, E, and 6 proteins was sufficient for incorporation of the 6 protein into virus-like particles. Cells transfected with plasmid expressing the 6 protein released SCoV 6 protein; however, infected cells released SCoV 6 protein only in association with SCoV particles.

Severe acute respiratory syndrome coronavirus accessory protein 6 is a virion-associated protein and is released from 6 protein-expressing cells

Analysis of severe acute respiratory syndrome coronavirus (SCoV) by either sucrose gradient equilibrium centrifugation or a virus capture assay using an anti-SCoV S protein antibody demonstrated that the SCoV 6 protein, which is one of the accessory proteins of SCoV, was incorporated into virus particles. Coexpression of the SCoV S, M, E, and 6 proteins was sufficient for incorporation of the 6 protein into virus-like particles. Cells transfected with plasmid expressing the 6 protein released SCoV 6 protein; however, infected cells released SCoV 6 protein only in association with SCoV particles.

Coronavirus avian infectious bronchitis virus

Infectious bronchitis virus (IBV), the coronavirus of the chicken (Gallus gallus), is one of the foremost causes of economic loss within the poultry industry, affecting the performance of both meat-type and egg-laying birds. The virus replicates not only in the epithelium of upper and lower respiratory tract tissues, but also in many tissues along the alimentary tract and elsewhere e.g. kidney, oviduct and testes. It can be detected in both respiratory and faecal material. There is increasing evidence that IBV can infect species of bird other than the chicken. Interestingly breeds of chicken vary with respect to the severity of infection with IBV, which may be related to the immune response. Probably the major reason for the high profile of IBV is the existence of a very large number of serotypes. Both live and inactivated IB vaccines are used extensively, the latter requiring priming by the former. Their effectiveness is diminished by poor cross-protection. The nature of the protective immune response to IBV is poorly understood. What is known is that the surface spike protein, indeed the amino-terminal S1 half, is sufficient to induce good protective immunity. There is increasing evidence that only a few amino acid differences amongst S proteins are sufficient to have a detrimental impact on cross-protection. Experimental vector IB vaccines and genetically manipulated IBVs--with heterologous spike protein genes--have produced promising results, including in the context of in ovo vaccination.

Evaluation of immunomodulators, interferons and known in vitro SARS-coV inhibitors for inhibition of SARS-coV replication in BALB/c mice

Compounds approved for therapeutic use and in vitro inhibitors of severe acute respiratory syndrome coronavirus (SARS-CoV) were evaluated for inhibition in the mouse SARS-CoV replication model. A hybrid interferon, interferon alpha (IFN-alpha) B/D, and a mismatched double-stranded (ds) RNA interferon (IFN) inducer, Ampligen (poly I:poly C124), were the only compounds that potently inhibited virus titres in the lungs of infected mice as assessed by CPE titration assays. When mice were dosed intraperitoneally (i.p.) with IFN-alpha B/D once daily for 3 days beginning 4 h after virus exposure, SARS-CoV replication in the lungs of infected mice was reduced by 1 log10 at 10,000 and 32,000 IU; at the highest dose of 100,000 IU, virus lung titres were below detectable limits. Ampligen used i.p. at 10 mg/kg 4 h prior to virus exposure also reduced virus lung titres to below detectable limits. Nelfinavir, beta-D-N4-hydroxycytidine, calpain inhibitor VI, 3-deazaneplanocin A and Alferon (human leukocyte IFN-alpha-n3) did not significantly reduce lung virus titres in mice. Anti-inflammatory agents, chloroquine, amodiaquin and pentoxifylline, were also inactive in vivo, suggesting that although they may be useful in ameliorating the hyperinflammatory response induced by the virus infection, they will not significantly reduce the replication of the virus, the inducer of inflammatory response. Thus, anti-inflammatory agents may only be useful in treating virus lung infections if used in combination with agents that inhibit virus replication. In summary, the data suggest that induction of IFN by mismatched dsRNA or actual treatment with exogenous IFN-alpha can inhibit SARS-CoV replication in the lungs of mice.

Evolutionary insights into the ecology of coronaviruses

Although many novel members of the Coronaviridae have recently been recognized in different species, the ecology of coronaviruses has not been established. Our study indicates that bats harbor a much wider diversity of coronaviruses than any other animal species. Dating of different coronavirus lineages suggests that bat coronaviruses are older than those recognized in other animals and that the human severe acute respiratory syndrome (SARS) coronavirus was directly derived from viruses from wild animals in wet markets of southern China. Furthermore, the most closely related bat and SARS coronaviruses diverged in 1986, an estimated divergence time of 17 years prior to the outbreak, suggesting that there may have been transmission via an unknown intermediate host. Analysis of lineage-specific selection pressure also indicated that only SARS coronaviruses in civets and humans were under significant positive selection, also demonstrating a recent interspecies transmission. Analysis of population dynamics revealed that coronavirus populations in bats have constant population growth, while viruses from all other hosts show epidemic-like increases in population. These results indicate that diverse coronaviruses are endemic in different bat species, with repeated introductions to other animals and occasional establishment in other species. Our findings suggest that bats are likely the natural hosts for all presently known coronavirus lineages and that all coronaviruses recognized in other species were derived from viruses residing in bats. Further surveillance of bat and other animal populations is needed to fully describe the ecology and evolution of this virus family.

Mouse hepatitis coronavirus A59 nucleocapsid protein is a type I interferon antagonist

The recent emergence of several new coronaviruses, including the etiological cause of severe acute respiratory syndrome, has significantly increased the importance of understanding virus-host cell interactions of this virus family. We used mouse hepatitis virus (MHV) A59 as a model to gain insight into how coronaviruses affect the type I alpha/beta interferon (IFN) system. We demonstrate that MHV is resistant to type I IFN. Protein kinase R (PKR) and the alpha subunit of eukaryotic translation initiation factor are not phosphorylated in infected cells. The RNase L activity associated with 2',5'-oligoadenylate synthetase is not activated or is blocked, since cellular RNA is not degraded. These results are consistent with lack of protein translation shutoff early following infection. We used a well-established recombinant vaccinia virus (VV)-based expression system that lacks the viral IFN antagonist E3L to screen viral genes for their ability to rescue the IFN sensitivity of the mutant. The nucleocapsid (N) gene rescued VVDeltaE3L from IFN sensitivity. N gene expression prevents cellular RNA degradation and partially rescues the dramatic translation shutoff characteristic of the VVDeltaE3L virus. However, it does not prevent PKR phosphorylation. The results indicate that the MHV N protein is a type I IFN antagonist that likely plays a role in circumventing the innate immune response.

Exceptional flexibility in the sequence requirements for coronavirus small envelope protein function

The small envelope protein (E) plays a role of central importance in the assembly of coronaviruses. This was initially established by studies demonstrating that cellular expression of only E protein and the membrane protein (M) was necessary and sufficient for the generation and release of virus-like particles. To investigate the role of E protein in the whole virus, we previously generated E gene mutants of mouse hepatitis virus (MHV) that were defective in viral growth and produced aberrantly assembled virions. Surprisingly, however, we were also able to isolate a viable MHV mutant (DeltaE) in which the entire E gene, as well as the nonessential upstream genes 4 and 5a, were deleted. We have now constructed an E knockout mutant that confirms that the highly defective phenotype of the DeltaE mutant is due to loss of the E gene. Additionally, we have created substitution mutants in which the MHV E gene was replaced by heterologous E genes from viruses spanning all three groups of the coronavirus family. Group 2 and 3 E proteins were readily exchangeable for that of MHV. However, the E protein of a group 1 coronavirus, transmissible gastroenteritis virus, became functional in MHV only after acquisition of particular mutations. Our results show that proteins encompassing a remarkably diverse range of primary amino acid sequences can provide E protein function in MHV. These findings suggest that E protein facilitates viral assembly in a manner that does not require E protein to make sequence-specific contacts with M protein.

Serological and molecular evidence that canine respiratory coronavirus is circulating in Italy

Canine respiratory coronavirus (CRCoV) is a group II coronavirus that was firstly identified in lung samples of dogs with canine infectious respiratory disease (CIRD) in UK in 2003. We report for the first time the identification of CRCoV in Italy, together with serological evidence that the virus has been circulating in the Italian dog population as from 2005. Serological investigations on 216 dog sera, carried out by an ELISA test using the strictly related bovine coronavirus (BCoV) as antigen, revealed an overall CRCoV seroprevalence of 32.06% in the last 2 years. RT-PCR targeting the S-gene of CRCoV was carried out on 109 lung samples collected from carcasses of dogs submitted for diagnostic investigations. Positive results were obtained from the lungs of a dog of the Apulia region that was co-infected with canine parvovirus type 2. Sequence analysis of the S-gene fragment amplified by RT-PCR (595bp) showed similarity to group II coronaviruses, with the highest nucleotide identity (98%) to the only CRCoV strain currently available in the GenBank database (strain T101). The results of the present study show that CRCoV is present also in continental Europe, although further studies are required to determine the real pathogenic potential of the virus.

Severe acute respiratory syndrome coronavirus protein 6 accelerates murine coronavirus infections

One or more of the unique 3'-proximal open reading frames (ORFs) of the severe acute respiratory syndrome (SARS) coronavirus may encode determinants of virus virulence. A prime candidate is ORF6, which encodes a 63-amino-acid membrane-associated peptide that can dramatically increase the lethality of an otherwise attenuated JHM strain of murine coronavirus (L. Pewe, H. Zhou, J. Netland, C. Tangudu, H. Olivares, L. Shi, D. Look, T. Gallagher, and S. Perlman, J. Virol. 79:11335-11342, 2005). To discern virulence mechanisms, we compared the in vitro growth properties of rJ.6, a recombinant JHM expressing the SARS peptide, with isogenic rJ.6-KO, which has an inactive ORF containing a mutated initiation codon and a termination codon at internal position 27. The rJ.6 infections proceeded rapidly, secreting progeny about 1.5 h earlier than rJ.6-KO infections did. The rJ.6 infections were also set apart by early viral protein accumulation and by robust expansion via syncytia, a characteristic feature of JHM virus dissemination. We found no evidence for protein 6 operating at the virus entry or assembly stage, as virions from either infection were indistinguishable. Rather, protein 6 appeared to operate by fostering viral RNA and protein synthesis, as RNA quantifications by reverse transcription-quantitative PCR revealed viral RNA levels in the rJ.6 cultures that were five to eight times higher than those lacking protein 6. Furthermore, protein 6 coimmunoprecipitated with viral RNAs and colocalized on cytoplasmic vesicles with replicating viral RNAs. The SARS coronavirus encodes a novel membrane protein 6 that can accelerate replication of a related mouse virus, a property that may explain its ability to increase in vivo virus virulence.

A severe acute respiratory syndrome coronavirus that lacks the E gene is attenuated in vitro and in vivo

A deletion mutant of severe acute respiratory syndrome coronavirus (SARS-CoV) has been engineered by deleting the structural E gene in an infectious cDNA clone that was constructed as a bacterial artificial chromosome (BAC). The recombinant virus lacking the E gene (rSARS-CoV-DeltaE) was rescued in Vero E6 cells. The recovered deletion mutant grew in Vero E6, Huh-7, and CaCo-2 cells to titers 20-, 200-, and 200-fold lower than the recombinant wild-type virus, respectively, indicating that although the E protein has an effect on growth, it is not essential for virus replication. No differences in virion stability under a wide range of pH and temperature were detected between the deletion mutant and recombinant wild-type viruses. Although both viruses showed the same morphology by electron microscopy, the process of morphogenesis seemed to be less efficient with the defective virus than with the recombinant wild-type one. The rSARS-CoV-DeltaE virus replicated to titers 100- to 1,000-fold lower than the recombinant wild-type virus in the upper and lower respiratory tract of hamsters, and the lower viral load was accompanied by less inflammation in the lungs of hamsters infected with rSARS-CoV-DeltaE virus than with the recombinant wild-type virus. Therefore, the SARS-CoV that lacks the E gene is attenuated in hamsters, might be a safer research tool, and may be a good candidate for the development of a live attenuated SARS-CoV vaccine.

The relationship of severe acute respiratory syndrome coronavirus with avian and other coronaviruses

In February 2003, a severe acute respiratory syndrome coronavirus (SARS-CoV) emerged in humans in Guangdong Province, China, and caused an epidemic that had severe impact on public health, travel, and economic trade. Coronaviruses are worldwide in distribution, highly infectious, and extremely difficult to control because they have extensive genetic diversity, a short generation time, and a high mutation rate. They can cause respiratory, enteric, and in some cases hepatic and neurological diseases in a wide variety of animals and humans. An enormous, previously unrecognized reservoir of coronaviruses exists among animals. Because coronaviruses have been shown, both experimentally and in nature, to undergo genetic mutations and recombination at a rate similar to that of influenza viruses, it is not surprising that zoonosis and host switching that leads to epidemic diseases have occurred among coronaviruses. Analysis of coronavirus genomic sequence data indicates that SARS-CoV emerged from an animal reservoir. Scientists examining coronavirus isolates from a variety of animals in and around Guangdong Province reported that SARS-CoV has similarities with many different coronaviruses including avian coronaviruses and SARS-CoV-like viruses from a variety of mammals found in live-animal markets. Although a SARS-like coronavirus isolated from a bat is thought to be the progenitor of SARS-CoV, a lack of genomic sequences for the animal coronaviruses has prevented elucidation of the true origin of SARS-CoV. Sequence analysis of SARS-CoV shows that the 5' polymerase gene has a mammalian ancestry; whereas the 3' end structural genes (excluding the spike glycoprotein) have an avian origin. Spike glycoprotein, the host cell attachment viral surface protein, was shown to be a mosaic of feline coronavirus and avian coronavirus sequences resulting from a recombination event. Based on phylogenetic analysis designed to elucidate evolutionary links among viruses, SARS-CoV is believed to have branched from the modern Group 2 coronaviruses, suggesting that it evolved relatively rapidly. This is significant because SARS-CoV is likely still circulating in an animal reservoir (or reservoirs) and has the potential to quickly emerge and cause a new epidemic.

Mutational analysis of aminopeptidase N, a receptor for several group 1 coronaviruses, identifies key determinants of viral host range

Feline coronavirus (FCoV), porcine transmissible gastroenteritis coronavirus (TGEV), canine coronavirus (CCoV), and human coronavirus HCoV-229E, which belong to the group 1 coronavirus, use aminopeptidase N (APN) of their natural host and feline APN (fAPN) as receptors. Using mouse-feline APN chimeras, we identified three small, discontinuous regions, amino acids (aa) 288 to 290, aa 732 to 746 (called R1), and aa 764 to 788 (called R2) in fAPN that determined the host ranges of these coronaviruses. Blockade of infection with anti-fAPN monoclonal antibody RG4 suggested that these three regions lie close together on the fAPN surface. Different residues in fAPN were required for infection with each coronavirus. HCoV-229E infection was blocked by an N-glycosylation sequon present between aa 288 to 290 in murine APN. TGEV required R1 of fAPN, while FCoV and CCoV required both R1 and R2 for entry. N740 and T742 in fAPN and the homologous R741 in human APN (hAPN) were key determinants of host range for FCoV, TGEV, and CCoV. Residue N740 in fAPN was essential only for CCoV receptor activity. A conservative T742V substitution or a T742R substitution in fAPN destroyed receptor activity for the pig, dog, and cat coronaviruses, while a T742S substitution retained these receptor activities. Thus, the hydroxyl on T742 is required for the coronavirus receptor activity of fAPN. In hAPN an R741T substitution caused a gain of receptor activity for TGEV but not for FCoV or CCoV. Therefore, entry and host range of these group 1 coronaviruses depend on the ability of the viral spike glycoproteins to recognize small, species-specific amino acid differences in the APN proteins of different species.

Modulation of the unfolded protein response by the severe acute respiratory syndrome coronavirus spike protein

Perturbation of the function of endoplasmic reticulum (ER) causes stress leading to the activation of cell signaling pathways known as the unfolded protein response (UPR). Severe acute respiratory syndrome (SARS) coronavirus (SARS-CoV) uses ER as a site for synthesis and processing of viral proteins. In this report, we demonstrate that infection with SARS-CoV induces the UPR in cultured cells. A comparison with M, E, and NSP6 proteins indicates that SARS-CoV spike (S) protein sufficiently induces transcriptional activation of several UPR effectors, including glucose-regulated protein 78 (GRP78), GRP94, and C/EBP homologous protein. A substantial amount of S protein accumulates in the ER. The expression of S protein exerts different effects on the three major signaling pathways of the UPR. Particularly, it induces GRP78/94 through PKR-like ER kinase but has no influence on activating transcription factor 6 or X box-binding protein 1. Taken together, our findings suggest that SARS-CoV S protein specifically modulates the UPR to facilitate viral replication.

Severe acute respiratory syndrome coronavirus 7a accessory protein is a viral structural protein

Severe acute respiratory syndrome coronavirus (SCoV) 7a protein is one of the viral accessory proteins. In expressing cells, 7a protein exhibits a variety of biological activities, including induction of apoptosis, activation of the mitogen-activated protein kinase signaling pathway, inhibition of host protein translation, and suppression of cell growth progression. Analysis of SCoV particles that were purified by either sucrose gradient equilibrium centrifugation or a virus capture assay, in which intact SCoV particles were specifically immunoprecipitated by anti-S protein monoclonal antibody, demonstrated that 7a protein was associated with purified SCoV particles. Coexpression of 7a protein with SCoV S, M, N, and E proteins resulted in production of virus-like particles (VLPs) carrying 7a protein, while 7a protein was not released from cells expressing 7a protein alone. Although interaction between 7a protein and another SCoV accessory protein, 3a, has been reported, 3a protein was dispensable for assembly of 7a protein into VLPs. S protein was not required for the 7a protein incorporation into VLPs, and yet 7a protein interacted with S protein in coexpressing cells. These data established that, in addition to 3a protein, 7a protein was a SCoV accessory protein identified as a SCoV structural protein.

Prevalence and genetic diversity of coronaviruses in bats from China

Coronaviruses can infect a variety of animals including poultry, livestock, and humans and are currently classified into three groups. The interspecies transmissions of coronaviruses between different hosts form a complex ecosystem of which little is known. The outbreak of severe acute respiratory syndrome (SARS) and the recent identification of new coronaviruses have highlighted the necessity for further investigation of coronavirus ecology, in particular the role of bats and other wild animals. In this study, we sampled bat populations in 15 provinces of China and reveal that approximately 6.5% of the bats, from diverse species distributed throughout the region, harbor coronaviruses. Full genomes of four coronavirues from bats were sequenced and analyzed. Phylogenetic analyses of the spike, envelope, membrane, and nucleoprotein structural proteins and the two conserved replicase domains, putative RNA-dependent RNA polymerase and RNA helicase, revealed that bat coronaviruses cluster in three different groups: group 1, another group that includes all SARS and SARS-like coronaviruses (putative group 4), and an independent bat coronavirus group (putative group 5). Further genetic analyses showed that different species of bats maintain coronaviruses from different groups and that a single bat species from different geographic locations supports similar coronaviruses. Thus, the findings of this study suggest that bats may play an integral role in the ecology and evolution of coronaviruses.

Endosomal proteolysis by cathepsins is necessary for murine coronavirus mouse hepatitis virus type 2 spike-mediated entry

Most strains of murine coronavirus mouse hepatitis virus (MHV) express a cleavable spike glycoprotein that mediates viral entry and pH-independent cell-cell fusion. The MHV type 2 (MHV-2) strain of murine coronavirus differs from other strains in that it expresses an uncleaved spike and cannot induce cell-cell fusion at neutral pH values. We show here that while infection of the prototype MHV-A59 strain is not sensitive to pretreatment with lysosomotropic agents, MHV-2 replication is significantly inhibited by these agents. By use of an A59/MHV-2 chimeric virus, the susceptibility to lysosomotropic agents is mapped to the MHV-2 spike, suggesting a requirement of acidification of endosomes for MHV-2 spike-mediated entry. However, acidification is likely not a direct trigger for MHV-2 spike-mediated membrane fusion, as low-pH treatment is unable to overcome ammonium chloride inhibition, and it also cannot induce cell-cell fusion between MHV-2-infected cells. In contrast, trypsin treatment can both overcome ammonium chloride inhibition and promote cell-cell fusion. Inhibitors of the endosomal cysteine proteases cathepsin B and cathepsin L greatly reduce MHV-2 spike-mediated entry, while they have little effect on A59 entry, suggesting that there is a proteolytic step in MHV-2 entry. Finally, a recombinant virus expressing a cleaved MHV-2 spike has the ability to induce cell-cell fusion at neutral pH values and does not require low pH and endosomal cathepsins during infection. These studies demonstrate that endosomal proteolysis by cathepsins is necessary for MHV-2 spike-mediated entry; this is similar to the entry pathway recently described for severe acute respiratory syndrome coronavirus and indicates that coronaviruses may use multiple pathways for entry.

Immunogenicity of a receptor-binding domain of SARS coronavirus spike protein in mice: implications for a subunit vaccine

We studied the immunogenicity of an anti-SARS subunit vaccine comprised of the fragment of the SARS coronavirus (SARS-CoV) spike protein amino acids 318-510 (S318-510) containing the receptor-binding domain. The S protein fragment was purified from the culture supernatant of stably transformed HEK293T cells secreting a tagged version of the protein. The vaccine was given subcutaneously to 129S6/SvEv mice in saline, with alum adjuvant or with alum plus CpG oligodeoxynucleotides (ODN). Mice immunized with the adjuvanted antigen elicited strong antibody and cellular immune responses; furthermore, adding the CpG ODN to the alum resulted in increased IgG2a antibody titers and a higher number of INF-gamma-secreting murine splenocytes. Mice vaccinated with S318-510 deglycosylated by PNGase F (dgS318-510) showed a lower neutralizing antibody response but had similar numbers of INF-gamma-producing cells in the spleen. This finding suggests that carbohydrate is important for the immunogenicity of the S318-510 protein fragment and provide useful information for designing an effective and safe SARS subunit vaccine.

Animal models and antibody assays for evaluating candidate SARS vaccines: summary of a technical meeting 25-26 August 2005, London, UK

Severe acute respiratory syndrome (SARS) emerged in the Guangdong province of China in late 2002 and spread to 29 countries. By the end of the outbreak in July 2003, the CDC and WHO reported 8437 cases with a 9.6% case fatality rate. The disease was caused by a previously unrecognized coronavirus, SARS-CoV. Drawing on experience with animal coronavirus vaccines, several vaccine candidates have been developed and evaluated in pre-clinical trials. Available data suggest that vaccines should be based on the the 180kDa viral spike protein, S, the only significant neutralization antigen capable of inducing protective immune responses in animals. In the absence of clinical cases of SARS, candidate vaccines should be evaluated for efficacy in animal models, and although it is uncertain whether the United States Food and Drug Administration's "animal rule" would apply to licensure of a SARS vaccine, it is important to develop standardized animal models and immunological assays in preparation for this eventuality. This report summarizes the recommendations from a WHO Technical Meeting on Animal Models and Antibody Assays for Evaluating Candidate SARS Vaccines held on 25-26 August 2005 in South Mimms, UK, provides guidance on the use of animal models, and outlines the steps to develop standard reagents and assays for immunological evaluation of candidate SARS vaccines.

Architecture of the SARS coronavirus prefusion spike

The emergence in 2003 of a new coronavirus (CoV) responsible for the atypical pneumonia termed severe acute respiratory syndrome (SARS) was a stark reminder that hitherto unknown viruses have the potential to cross species barriers to become new human pathogens. Here we describe the SARS-CoV 'spike' structure determined by single-particle cryo-EM, along with the docked atomic structures of the receptor-binding domain and prefusion core.

Coronaviruses and their therapy

Coronaviruses may cause respiratory, enteric and central nervous system diseases in many species, including humans. Until recently, the relatively low burden of disease in humans caused by few of these viruses hampered development of coronavirus specific therapeutics. However, the emergence of severe acute respiratory syndrome coronavirus (SARS-CoV) has prompted the discovery of such drugs. Subsequent studies in animal models demonstrated the efficacy of SARS-CoV specific monoclonal antibodies, pegylated-interferon-alpha and siRNAs against SARS-CoV. Furthermore, several antivirals shown to be effective against other viruses were tested in vitro. Because of availability and shown efficacy, the use of interferons may be considered should SARS-CoV or a related coronavirus (re)-emerge. The more recent design of wide-spectrum inhibitors targeting the coronavirus main proteases may lead to the discovery of new antivirals against multiple coronavirus induced diseases.