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Coronavirus Pathogenesis and the Emerging Pathogen Severe Acute Respiratory Syndrome Coronavirus

Department of Microbiology, University of Pennsylvania School of Medicine, Philadelphia, Pennsylvania,¹ Department of Microbiology and Immunology, Center for Molecular Medicine and Infectious Disease, Drexel College of Medicine, Philadelphia, Pennsylvania²

SUMMARY

INTRODUCTION

Taxonomy

Coronavirus Diseases

Human coronavirus.

Murine coronavirus.

Porcine coronavirus.

Avian coronavirus.

Feline coronavirus.

Bovine coronavirus.

THE VIRION

VIRAL LIFE CYCLE

REVERSE GENETICS SYSTEM FOR CORONAVIRUSES

ROLES OF CORONAVIRUS PROTEINS IN PATHOGENESIS

Spike Protein

Structure of the spike.

Receptor interaction, fusion, and entry.

Role in pathogenesis.

Hemagglutinin-Esterase Protein

Membrane Protein

Nucleocapsid Protein

Small Envelope Protein

Internal Protein

Replicase Proteins

Group-Specific Proteins

CORONAVIRUSES AS EMERGING PATHOGENS: SARS-CoV

Severe Acute Respiratory Syndrome

Origin of SARS-CoV

SARS Pathogenesis

SARS ANIMAL MODELS

Nonhuman Primate Models

Cat Model

Ferret Model

Rodent Models

VACCINE STRATEGIES AGAINST SARS

THERAPY

CONCLUDING REMARKS AND PROSPECTS

ACKNOWLEDGMENTS

REFERENCES

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INTRODUCTION

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Coronaviruses infect many species of animals, including humans. Coronaviruses have been described for more than 50 years; the isolation of the prototype murine coronavirus strain JHM, for example, was reported in 1949 (7, 41). The molecular mechanisms of replication as well as the pathogenesis of several coronaviruses have been actively studied since the 1970s. Some of the animal viruses, such as porcine transmissible gastroenteritis virus (TGEV), bovine coronavirus (BCoV), and avian infectious bronchitis viruses (IBV), are of veterinary importance. The murine coronavirus mouse hepatitis virus (MHV) is studied as a model for human disease. This family of viruses remained relatively obscure, probably because there were no severe human diseases that could definitely be attributed to coronaviruses; human coronaviruses caused only the common cold. However, in the spring of 2003, when it became clear that a new human coronavirus was responsible for severe acute respiratory syndrome (SARS), coronaviruses became much more recognized. With the occurrence of the SARS epidemic, coronaviruses may now be considered "emerging pathogens." The origin of the SARS coronavirus (SARS-CoV) poses interesting questions about coronavirus evolution and species specificity. Since the SARS epidemic, two new human respiratory coronaviruses have been described. In this review we discuss the pathogenesis of the previously known coronaviruses. We then discuss the newly isolated SARS-CoV. It has become evident that the body of information gathered over the last 30 years regarding coronavirus replication and pathogenesis has helped to begin understanding of the origin and the biology of SARS-CoV.

Coronaviruses are divided into three genera (I to III), usually referred to as groups and based on serological cross-reactivity (218) (Table 1); more recent genome sequence analysis has confirmed this grouping (115). Group I coronaviruses include animal pathogens, such as TGEV of the pig, porcine epidemic diarrhea virus (PEDV), and feline infectious peritonitis virus (FIPV), as well as the human coronaviruses HCoV-229E and HKU1, which cause respiratory infections (see below). Group II also includes pathogens of veterinary relevance, such as BCoV, porcine hemagglutinating encephalomyelitis virus, and equine coronavirus, as well as human coronaviruses viruses OC43 and NL63, which, like HCoV-229E, also cause respiratory infections. Group II also includes viruses that infect both mice and rats. MHV is often studied as a prototype coronavirus; MHV is a group of highly related strains causing a variety of diseases, such as enteric disease, hepatitis, and respiratory disease, as well as encephalitis and chronic demyelination. Rat sialodacryoadenitis coronavirus also belongs to group II. There has been controversy about whether SARS-CoV defines a new group of coronaviruses or whether it is a distant member of group II (as discussed in "CORONAVIRUSES AS EMERGING PATHOGENS: SARS-CoV" below); given the data to date (113, 117), we have listed SARS-CoV in group II in Table 1. Group III thus far includes only avian coronaviruses, such as IBV, turkey coronavirus, and pheasant coronavirus (38). Recently, using reverse transcription-PCR (RT-PCR), coronavirus sequences were detected in the graylag goose (Anser anser), feral pigeon (Columbia livia), and mallard (Anas platyrhynchos) (147); phylogenetic analyses of the replicase and nucleocapsid (N) sequences suggest that these viruses are members of group III, but as yet they have not been isolated or characterized.

Human coronavirus. Previous to the emergence of SARS-CoV, there were two prototype human coronaviruses, OC43 and 229E, both etiologic agents of the common cold (218). There had long been speculation about the association of human coronaviruses with more serious human diseases such as multiple sclerosis (33), hepatitis (380), or enteric disease in newborns (262). However, none of these early associations had been substantiated. The recently identified SARS-CoV, which was shown to cause a severe acute respiratory syndrome was the first example of serious illness in humans caused by a coronavirus (267) and will be discussed in detail in below. Since the identification of SARS-CoV, there have been reports of two new human coronaviruses associated with respiratory disease. HKUI is a group II coronavirus isolated from an elderly patient with pneumonia (340). This virus has been difficult to propagate in cell culture, and there is little information available about the biology of this virus. HCoV-NL63 is a group I coronavirus isolated from a 7-month-old child in The Netherlands who was suffering from bronchiolitis and conjunctivitis (101, 320). It has subsequently been reported in other parts of the world, including Canada (12), Japan (86), Hong Kong (52), Australia (5), and Belgium (220). HCoV-NL63 is associated with serious respiratory symptoms, including upper respiratory infection, bronchiolitis, and pneumonia (86). The strong correlation of the presence of NL63 with croup in children with lower respiratory infections has suggested a causal relationship between the virus and croup (321). While primarily associated with infections of children, NL63 has been also been detected in immunocompromised adults with respiratory tract infections. This virus was independently isolated in New Haven, Connecticut, and called HCoV-NH (93). That group has suggested that this virus is associated with Kawasaki's disease in children (92); however, this has been disputed by two other reports (14, 87). While little is known about the pathogenesis of any of the human coronaviruses (229E, OC43, HKU1, NL63, and SARS-CoV), there have been detailed studies of the pathogenesis of some of the animal coronaviruses, which may contribute to the understanding of the human viruses. We summarize some of these data below.

Murine coronavirus. There are many strains of murine coronavirus, or MHV, exhibiting different tropisms and levels of virulence. The commonly used laboratory strains infect primarily the liver and the brain and thus provide animal models for encephalitis and hepatitis as well as the immune-mediated demyelinating disease that develops late after infection, peaking at about 1 month postinfection (242). MHV infection of the mouse is regarded as one of the best animal models for the study of demyelinating diseases such as multiple sclerosis. Other strains cause enteric disease, are spread easily by an oral-fecal route in animal colonies, and are a particular danger to immunocompromised animals (10). The extensive studies of the pathogenesis of MHV and the resulting host immune response have been reviewed (206, 214, 242). It is clear that the level of virulence as well as the tropism of MHV strains results from the interplay of viral gene products and the host immune response. The contributions of individual viral genes to tropism and pathogenic phenotype are discussed later in this review.

The role of the immune response to MHV infection in viral clearance and pathogenesis in the CNS has been well characterized (157). Both antibody- and cell-mediated immune responses are required to protect against coronavirus infections. The CD8⁺ and CD4⁺ T cells are primarily responsible for clearance of the virus during acute infection (13-16, 42, 157, 187, 258, 259). Perforin-mediated mechanisms are necessary for clearance of virus from astrocytes and microglia, while gamma interferon (IFN-) has been implicated in clearance from oligodendrocytes (237) It is not clear how virus is cleared from neurons. In the case of MHV-A59 infection of the CNS, adoptive transfer of epitope-specific CD8⁺ T cells prior to infection reduces viral replication and the spread of viral antigen during the acute infection and significantly decreases the amount of demyelination developed by 4 weeks postinfection (151). These and other data (156) suggest that the development of demyelination depends on adequate spread of virus during the acute stage.

MHV T-cell epitopes have been mapped to several structural proteins; there may be additional epitopes, however, in the two-thirds of the genome that encodes the replicase proteins, a portion of the genome that has not yet been examined for epitopes. CD8⁺ T-cell epitopes have been identified in spike (S) and nucleocapsid proteins. The MHV spike has an immundominant CD8⁺ T-cell epitope (S510 to S518) and subdominant additional one (S598 to S605) in C57BL/6 mice, while in BALB/c mice there is only one identified CD8⁺ T-cell epitope in the nucleocapsid protein (N318 to N326) (15). CD4⁺ T-cell epitopes have been identified in the spike (322), M (346), and nucleocapsid (322) proteins of MHV (242). Neutralizing B-cell epitopes have been mapped primarily to the spike proteins, but nonneutralizing epitopes have been identified in the other viral structural proteins (68, 69, 108, 304). While MHV is cleared primarily by the cell-mediated immune response, in the absence of B cells, antibodies are essential to prevent reemergence of the virus in the CNS after initial T-cell mediated clearance. Interestingly, the requirement does not pertain to virus replication and clearance in the liver (189, 215).

The neurovirulent JHM infection is characterized by a strong and prolonged IFN-/ß response, along with elevated levels of macrophage chemoattractants such as CCL3 (MIP-1), CCL4 (MIP-1ß), and CXCL2 (MIP-2), as well as CXCL10 (IP-10) and CXCL5 (RANTES) (173). The increase in chemokines is associated with high levels of macrophages and neutrophils during acute infection and also in later demyelination stages (109). Recombinant virus studies suggest that the macrophage infiltration may be influenced by the S protein (260; K. T. Iacono and S. R. Weiss, unpublished data). The most neurovirulent isolate of JHM fails to induce a significant T-cell response; the resulting inability of the host to clear virus is likely responsible for the high mortality even at low doses of virus (201). Studies using knockout mice or antichemokine antisera have revealed the importance of CCL3, Mig, CXCL10, and CCR2 in the recruitment of T cells to the CNS during MHV infection (83, 129, 191, 192, 315). Sensitivity to IFN-/ß is strain specific for MHV. While the growth of low-virulent MHV-S and neurovirulent MHV-JHM was significantly suppressed in IFN-treated L cells compared with untreated cells, inhibition of the highly hepatovirulent MHV-2 stain was not observed in IFN-treated cells (302). These data suggest that MHV-2 may have a specific mechanism for evading the immune response.

Porcine coronavirus. There are several porcine coronaviruses that have been studied (reviewed in references 89, 271, and 272). Transmissible gastroenteritis virus was recognized in 1946 (80). It is a major cause of viral enteritis and fetal diarrhea in swine; it is most severe in neonates, with mortality resulting in significant economic loss (89). In neonates, TGEV infects epithelial cells of the small intestines, leading to potentially fatal gastroenteritis. Infection also occurs in the upper respiratory tract and, less often, in the lungs (272). In adults, TGEV causes mild disease. Porcine respiratory virus (PRCoV) is an attenuated variant of TGEV. PRCoV infects lung epithelial cells, and antigen is found in type I and type II pneumocytes as well as alveolar macrophages; infection is followed by interstitial pneumonia. The genomes of TGEV and PRCoV are 96% identical except for the 5' region of the spike gene, and the difference in pathogenic outcome between the two strains is associated with deletions of various lengths (nucleotides 45 to 752) within the 5' end of the spike gene of PRCoV. Thus, emergence of PRCoV from TGEV resulted from deletions within the spike gene and is an example of evolution of a coronavirus with altered tissue tropism as well as reduced virulence (272).

Various types of vaccines have been evaluated for protection against TGEV (271, 272). Immunization of pregnant swine with attenuated TGEV is not sufficient to protect suckling pigs from infection. Inoculation of young pigs directly with attenuated virus is also unable to stimulate enough immunoglobulin A (IgA)-secreting cells in the intestines to protect against TGEV. However, sows recovering from virulent TGEV infection do produce enough milk IgA to protect suckling pigs from infection and diarrhea. Repeated infections with PRCoV, however, can protect against TGEV and may in fact do that in the field. Subunit vaccines using spike and nucleocapsid proteins have also been tested. The spike protein of TGEV has four major antigenic sites, two of which are neutralizing. The N protein has a functional CD4⁺ T-cell epitope. While these vaccines are unable to induce either passive or active protection against TGEV, they are able to boost responses in animals vaccinated with attenuated TGEV.

A relatively new group I porcine coronavirus is PEDV. This virus appeared in Europe in the late 1970s into the 1980s and spread to Asia, but it has not been reported in the United States (272). Interestingly, PEDV antibodies do not neutralize TGEV. PEDV shows some characteristics of human coronaviruses in that it is genetically more similar to HCoV-229E than other group I coronaviruses and, like SARS-CoV, replicates in Vero cells (272). Another porcine coronavirus, hemagglutinating enteric coronavirus, is a group II virus, antigenically unrelated to the other porcine viruses.

Avian coronavirus. IBV causes a highly contagious disease in chickens; it is spread by aerosol and thus is of considerable economic importance to the poultry industry. IBV, which has also reported in pheasants and turkeys, replicates in upper respiratory tissues, with infection of bronchi and severe disease in young animals. Some strains of IBV cause more systemic infections, replicating in other tissues, including the kidney (causing nephritis), the oviduct (causing decreased egg production), and the gut (271, 272). While chickens of all ages are susceptible, very young chicks exhibit more severe respiratory signs and much higher mortality than older birds (59). While the mechanisms of protection against IBV-induced disease are not completely clear, high levels of antibodies are believed to prevent spread of virus from the respiratory tract to other organs. Maternal antibodies have also been shown to protect against IBV infection during the first 2 weeks of life. Adoptive transfer of CD8⁺ T cells has been shown to protect against IBV challenge (271).

Both live attenuated and inactivated vaccines have been developed and used to protect against IBV. Protection from live vaccines may be short lived, and serotype-specific and inactivated vaccines are unable to protect alone. However, inactivated vaccines may be used to boost birds that have been primed with live attenuated vaccine. Further difficulties in inducing protection by vaccination are due to the multiple serotypes of IBV, which are often not cross protective. Thus, subunit vaccines expressing the S1 subunit of spike protein, via baculovirus or from a fowlpox virus vector, induce protection in nearly all the animals vaccinated; however, differences of as small as 5% between among S1 sequences may result in poor cross protection (37).

Feline coronavirus. The feline coronaviruses are composed of two biotypes. Feline enteric coronavirus (FeCoV), commonly found in multicat environments in an asymptomatic carrier state, causes seroconversion. FIPV, a less common variant of FeCoV, has the ability to replicate in macrophages, causing a severe and lethal disease. FIPV may be viewed as a virulent variant of FeCoV that is selected for during persistent infection (272). FIPV replicates initially in pharyngeal respiratory or intestinal epithelial cells. Infection of macrophages then leads to viremia and systemic spread of the virus, including inflammation of the abdominal and thoracic cavities and causing occasional ocular and neurological disorders (1, 71). A complication of FIPV infection involves immune-mediated pathology (138). This has presented a great challenge to vaccine development for FIPV. It has been shown that after vaccination against spike protein, cats challenged with FIPV develop an early-death syndrome caused by antibody-dependent enhancement of virus infection. A DNA vaccine approach, directed against the N and M proteins followed by the same to protein-expressed vial vaccinia virus, also has not been successful. Thus, the development of a vaccine against FIPV remains a challenge (271).

Bovine coronavirus. BCoV is a ubiquitous virus worldwide as measured by serology. BCoV causes both respiratory and enteric disease, including calf diarrhea, winter dysentery in adults, and respiratory infections in cattle of all ages, including those with shipping fever. Viruses isolated from cattle with either respiratory or enteric disease are antigenically similar. Epidemiological studies suggest that serum antibody correlates with immunity. There are currently no vaccines available to prevent BCoV-associated disease (271, 272).

THE VIRION

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Coronaviruses are enveloped viruses with round and sometimes pleiomorphic virions of approximately 80 to 120 nm in diameter (Fig. 1). Coronaviruses contain positive-strand RNA, with the largest RNA genome (approximately 30 kb) reported to date (178, 196). The genome RNA is complexed with the basic nucleocapsid (N) protein to form a helical capsid found within the viral membrane. The membranes of all coronaviruses contain at least three viral proteins. These are spike (S), the type I glycoprotein that forms the peplomers on the virion surface, giving the virus its corona- or crown-like morphology in the electron microscope; the membrane (M) protein, a protein that spans the membrane three times and has a short N-terminal ectodomain and a cytoplasmic tail; and small membrane protein (E), a highly hydrophobic protein (18). The E protein of IBV has a short ectodomain, a transmembrane domain, and a cytoplasmic tail (63). The E protein of MHV is reported to span the membrane twice, such that both N and C termini are on the interior of the virion (202). Some group II coronaviruses have an additional membrane protein, hemagglutinin esterase (HE) (28). While the function of HE is not known, it is not an essential protein, and it has been speculated to aid in viral entry and/or pathogenesis in vivo and will be discussed below. HE is not encoded in the SARS-CoV genome. There is an additional group II virion protein called I for internal, as it is encoded within the nucleocapsid open reading frame (ORF). This is a nonessential protein of unknown function (97). It has recently been shown that the ORF 3a-encoded SARS protein is an additional structural protein (143). There may be other minor proteins, as yet undetected, included in virions.

View larger version (57K): [in this window] [in a new window]   FIG. 1. Coronavirus virion. (A) Electron micrograph of MHV particles. (B) Schematic of virion. Viral particles contain an internal helical RNA-protein nucleocapsid surrounded by an envelope containing viral glycoproteins. Nucleocapsid (N) protein is a phosphoprotein that is complexed with genome RNA to form the nucleocapsid. Spike glycoprotein (S) forms the large glycosylated peplomers that are characteristic of coronaviruses. M, the transmembrane protein, is highly hydrophobic and spans the membrane three times. E, a membrane-spanning protein, is a minor component of the membrane. Some group II viruses express another glycoprotein, hemagglutinin-esterase (HE), which forms smaller spikes on virions.

View larger version (14K): [in this window] [in a new window]   FIG. 2. MHV genome organization and replicase proteins. The genome consists of seven genes. The first 22 kb contains the replicase gene, which is organized into two overlapping open reading frames, ORFs 1a and 1b. These ORFs are translated into the 400-kDa pp1a and the 800-kDa pp1ab replicase polyproteins. ORF 1b is translated via a translational frameshift encoded at the end of ORF 1a. The protein domains of the replicase polyprotein are indicated by nonstructural protein numbers (nsp1 to 16) and by confirmed or predicted functions: PLP1 and PLP2, papain-like proteases; X, domain encoding predicted adenosine diphosphate-ribose 1"-phosphatase activity (ADRP); 3CLpro, 3C-like protease; RdRp, putative RNA-dependent RNA polymerase; Hel, helicase; ExoN, putative exonuclease; XendoU, putative poly(U)-specific endoribonuclease; 2'-O-MT, methyltransferase. Genes 2 to 7 are translated from subgenomic mRNA species (not shown). Relative locations of coding regions for the structural proteins HE, S, E, M, N, and I are shown, as are the coding region for the group-specific ORF 2a (encoding a predicted cyclic phosphodiesterase), 4, and 5a proteins.

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View larger version (32K): [in this window] [in a new window]   FIG. 3. Model of coronavirus replication. After receptor interaction and fusion of viral and plasma membranes, virus-specific RNA and proteins are synthesized, probably entirely in the cytoplasm. Expression of coronaviruses starts with translation of two polyproteins, pp1a and pp1ab, which undergo cotranslational proteolytic processing into the proteins that form the replicase complex. This complex is used to transcribe a 3'-coterminal set of nested subgenomic mRNAs, as well as genomic RNA, that have a common 5' "leader" sequence derived from the 5' end of the genome. Proteins are translated from the 5' end of each mRNA. New virions are assembled by budding into intracellular membranes and released through vesicles by the cell secretory mechanisms. RER, rough endoplasmic reticulum; ER/GIC, endoplasmic reticulum/Golgi intermediate compartment.

REVERSE GENETICS SYSTEM FOR CORONAVIRUSES

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There are now several reverse genetics systems available for coronaviruses (Table 2). Full-length cDNA clones were initially difficult to develop, most likely due to the large size of the coronavirus genome. Thus, the first reverse genetics system available for coronaviruses was targeted recombination, which was developed for MHV (155, 166, 208) and then for TGEV (274) and FIPV (125). In the MHV system, recombination occurs between a murine coronavirus in which the ectodomain of the spike has been replaced with that of the feline coronavirus FIPV spike (called fMHV) and a synthetic RNA carrying the 3' portion of the MHV genome from the HE gene through the 3' end. Feline cells are infected with fMHV and then transfected with synthetic mRNA. Recombinants, expressing the MHV spike gene, are then selected on murine cells; parental virus and other viruses with the feline coronavirus spike cannot replicate. This system takes advantage of the high rate of recombination observed during coronavirus infection (204) and the strict host range specificity of these viruses.

Reverse genetics technology has greatly advanced the understanding of coronaviruses. Both mutant and chimeric recombinant viruses have been used extensively in the investigation of the roles of spike and other proteins in coronavirus replication and pathogenesis, to investigate the structure/function relationship of the UTRs at the 5' and 3' ends of the genome, to begin to understand the roles of the enzymatic activities encoded in the replicase gene in coronavirus replication, to express foreign sequences in the place of a nonessential gene, and to select viruses with gene deletions or rearrangements that may serve as attenuated vaccines (21, 54, 76, 98, 112, 124, 200, 275). The roles of individual coronavirus genes in infection, particularly in pathogenesis, are discussed below.

ROLES OF CORONAVIRUS PROTEINS IN PATHOGENESIS

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View larger version (14K): [in this window] [in a new window]   FIG. 4. Schematic of coronavirus spike proteins. Shown are spike proteins representative of those of all group I to III coronaviruses and of SARS-CoV. The coronavirus spike protein is synthesized as a precursor, cotranslationally glycosylated, and, in some cases, cleaved in the approximate middle into S1 and S2 subunits at a site with dibasic amino acids (BBXBB). S1 forms the external domain containing the receptor binding domain (RBD) at its 5' end, followed by, in the case of MHV, a hypervariable domain (HVR). A short signal sequence in cleaved from the 5' end of the mature protein. S2 is the transmembrane subunit containing two heptad repeats (HR1 and HR2) and the transmembrane (TM) domain.

The coronavirus spike protein plays vital roles in viral entry, cell-to-cell spread, and determining tissue tropism. Coronavirus entry is, in general, not pH dependent, and thus it has been believed to occur directly at the plasma membrane and not via an endosomal route (Fig. 3). However, there are data suggesting that an endosomal route may be utilized by some viruses (156, 219). Entry of SARS-CoV is inhibited by lysosomotropic agents, suggesting an endosomal route of entry (285, 349). Furthermore, this inhibition may be overcome by protease treatment of virus that has attached to the cell. This, along with the observation that infection is blocked by inhibitors of the pH-sensitive endosomal protease cathepsin L, suggests that there is a requirement for cleavage of the SARS-CoV spike during entry through the endosomes (213, 284). Furthermore, entry at the plasma membrane following protease treatment is more efficient than entry by the endosomal route (213). Those authors suggested that SARS-CoV spike may be cleaved by the proteases produced by inflammatory cells present in the lungs of SARS patients and thus enter cells by the more efficient plasma membrane route (213). The highly hepatotropic MHV-2 strain may enter the cell by an endosomal route similar to that used by SARS-CoV. MHV-2, like SARS-CoV, encodes an uncleaved spike protein and is sensitive to lysosomotropic agents; however, trypsin treatment of cell-associated MHV-2 spike overcomes inhibition by lysosomotropic agents (Z. Qiu and S. R. Weiss, unpublished data). This suggests that entry at the cell surface may require a cleavage of spike in the viral membrane, while endosomal entry may provide for cleavage during entry. Finally, coronaviruses with cleaved spikes may also enter the cell by the endosomal route. For example, while wild-type MHV-JHM enters cells in culture by a pH-independent pathway, the OBLV60 mutant of JHM is inhibited by lysosomotropic agents and is believed to enter though a lysosomal pathway (221). Interestingly, OBLV60 is highly attenuated and exhibits restricted spread during infection of the murine central nervous system (239, 316).

In general, the host range of coronaviruses is extremely narrow. The ability of a coronavirus to replicate in a particular cell type depends solely on the ability to interact with its receptors (139). For example, murine coronavirus replicates in murine cells and not in human and hamster cells; however, once nonpermissive cells are transfected with the cDNA encoding MHV receptor, they become susceptible to MHV infection (85). Several coronavirus receptors have been identified. The group I coronaviruses human HCoV-229E, feline FIPV, and porcine TGEV all use aminopeptidase N (APN), a zinc-binding protease, of their respective host species as their receptors (352) (Table 1). There is some ability to recognize the corresponding APN receptor of another species; for example, HCoV-229E can utilize either human APN or feline APN as a receptor but cannot use porcine APN (334, 335). The receptor used by the murine coronavirus group is carcinoembryonic antigen-cell adhesion molecule (CEACAM) (CD66a) (43, 44, 84, 226). CEACAMs are glycoproteins possessing two or four immunoglobulin-like extracellular domains followed by a transmembrane domain and a cytoplasmic tail (226). They are involved in the intercellular adhesion and development of hepatocellular, colorectal, and epithelial tumors (13) and are expressed primarily on the epithelial and endothelial cells of the respiratory tract and intestines, as well as on other tissues (111, 265). The observation that transgenic mice with a knockout of the CEACAM1 gene are resistant to infection demonstrates that this is likely the only receptor for MHV (131). Interestingly, CEACAM1 is expressed at a low level in the brain, a major site of infection of some MHV strains, suggesting that low levels of receptor may be sufficient for mediating MHV entry. Expression of receptor has been demonstrated on only one central nervous system cell type, microglia; the receptor is downregulated on microglia during infection (257). MHV spread for the highly neurovirulent JHM strain may be enhanced by receptor-independent spread (103, 104) and/or by the expression of the hemagglutinin-esterase proteins (see below).

Other group II coronaviruses, such as BCoV, OC43, and porcine hemagglutinating encephalomyelitis virus, bind to 9-O-acetylated sialic acid-containing receptors (159, 253). It is not clear, however, what the specific receptor molecules are, and little is known about the entry process.

Soon after the identification of SARS-CoV, the receptor for this virus was identified as angiotensin-converting enzyme 2 (ACE2). ACE2, like APN, the group I coronavirus receptor, is a zinc metalloprotease (187). Human CD209L, a C-type lection (also called L-SIGN, DC-SIGNR, and DC-SIGN2), when expressed by transfected Chinese hamster ovary cells, renders the cells highly susceptible to SARS-CoV infection; however, it is significantly less efficient than ACE2 in mediating entry (145). SARS-CoV S protein is able to interact with the lectin DC-SIGN; while DC-SIGN binding enhances infection of ACE2-bearing cells, it cannot alone mediate entry in the absence of ACE2. Thus, the interaction of SARS-CoV with this lectin on dendritic cells (DCs), which are not permissive for infection, may augment transmission of SARS to its target cells (135). Surprisingly, it was recently shown that the newly identified group I human coronavirus NL63 also uses ACE2 as its receptor (136).

The spikes of some coronaviruses mediate cell-to-cell fusion of infected cells as well as virus/cell fusion during entry, presumably by a similar mechanism (369) (212). However, viral entry and cell-to-cell fusion do display some differences in requirements. For example some MHV-JHM spikes can mediate cell-to-cell fusion in the absence of CEACAM, while entry requires the CEACAM receptor. Furthermore spike proteins that have mutations that eliminate cleavage into S1 and S2 subunits carry out cell-to-cell fusion very inefficiently; however, they mediate entry into susceptible cells with similar efficiency as wild-type virus (75, 114, 181). Similarly, the MHV-2 strain encodes an uncleaved spike protein and does not carry out cell-to-cell fusion; this virus infects cells efficiently in vitro and causes severe hepatitis in vivo (70, 132, 150). The spike of MHV-A59, which is usually cleaved during replication in cell culture, is not cleaved when recovered from brains or livers of infected mice, suggesting that cleavage is not a prerequisite for infection for strains that express cleaved spike (133) and that entry of MHV into some types of cells in vivo may require an endosomal route of infection.

The heptad repeat domains and the putative fusion peptide are believed to play important roles in the fusion process (103). This has been explored most for the MHV spike. Substitution of charged amino acids for hydrophobic ones in HR1 (and within a candidate fusion peptide) eliminates the ability to induce cell-to-cell fusion (198). Mutations in the leucine zipper domain within HR2 inhibit the ability of spike to oligomerize and to inhibit cell-to-cell fusion (197). Amino acid substitutions at L1114 within the HR1 domain of the JHM spike (L1114R or L1114F) are particularly intriguing in that they have been reported in multiple studies, in association with several mutant phenotypes. An L1114R substitution is one of three mutations believed to contribute to the low pH dependence for viral entry of the OBLV60 variant of JHM as well as its neuroattenuation and restriction to olfactory bulbs during infection of mice (105). Furthermore, L1114R alone was sufficient to cause restriction of recombinant MHV to the olfactory bulbs during infection of mice (316). Substitutions at L1114 have been identified in the spike of an attenuated monoclonal-antibody-resistant mutant (327) and a soluble-receptor-resistant mutant (269, 270). Interestingly, L1114R and L1114F substitutions were identified as secondary mutations in several recombinant viruses expressing A59/JHM chimeric spike proteins (248, 316). The soluble-receptor-resistant mutant of JHM, srr7, (expressing a spike containing L1114F) demonstrated increased stability of the S1/S2 interaction, the loss of the ability to induce CEACAM-independent fusion (301), and altered interactions with the receptor CEACAM1^b (an allele of CEACAM 1^a expressed by resistant SJ/L mice) as well as resistance to neutralization by soluble CEACAM1^a receptor (211, 212). Similarly, the L1114R mutation results in loss of receptor-independent fusion along with neuroattenuation. In support of the idea that the RBD interacts with S2, a mutation in the RBD could functionally suppress the effects of an L1114R mutation in HR1 of srr7 that affected the ability to use CEACAM1^b (211). Thus, small changes within the HR domains (for example, a single amino acid substitution at L1114) may result in major alterations in spike/receptor interaction and hence in virus entry and finally pathogenesis in vivo.

Recent studies of the HR domains provide further evidence confirming that the coronavirus spike is indeed a class I fusion protein (23). Peptides representing HR1 and HR2 of MHV, when mixed together, assemble into an extremely stable oligomeric complex with both peptides in alpha-helical conformations and antiparallel to each other. In the native protein, such a conformation would be predicted to bring the N-terminal domain of HR1 and the transmembrane anchor into close proximity to facilitate the fusion process. Furthermore, the HR2 peptide was shown to be a potent inhibitor of virus entry, as well as of cell-to-cell fusion. Similar results were obtained for SARS HR domains. SARS-CoV HR1 and HR2 peptides, when mixed, assemble into a similar six-helix bundle; however, this complex was less stable than that of the corresponding MHV complex. The lack of stability may explain why HR2 peptides are less inhibitory for SARS than for MHV (22).

Role in pathogenesis. The use of recombinant coronaviruses, including MHV (223, 246), TGEV (274), and IBV (35, 134), has definitively demonstrated that the spike is a major determinant of tropism and pathogenicity. In the case of TGEV, the replacement of the spike gene of an attenuated respiratory strain of TGEV with the spike gene from a virulent enteric strain renders the virus enterotropic (274). Figure 5 summarizes the mapping of tropism and virulence with A59/JHM chimeric recombinant MHVs. The JHM strain is highly neurotropic, causing severe, usually fatal encephalitis and little if any hepatitis, while the A59 strain causes moderate hepatitis and is only weakly neurovirulent. The replacement of the spike gene in the genome of the A59 strain with the spike gene of the most highly neurotropic isolate of the JHM strain renders the resulting virus highly neurovirulent (223, 246). The high neurovirulence conferred by the JHM spike is associated with rapid spread through the CNS, which may occur, in part, independently of the CEACAM receptor and the large numbers of infected neurons (247). However, the resulting chimeric virus (JHM spike in the A59 background) is not as virulent as parental JHM, at least partially because it induces a much stronger CD8⁺ T-cell response. JHM fails to induce a strong enough CD8⁺ T-cell response to mediate clearance (201, 260; Iacono et al., unpublished data). The mechanisms that underlie the differences in the immune response in the brain to the closely related A59 and JHM strains are intriguing and not at all understood.

View larger version (14K): [in this window] [in a new window]   FIG. 5. The molecular determinants of MHV pathogenesis. Chimeric A59/JHM recombinant viruses were selected by targeted recombination. These viruses were used to infect mice, and the abilities to infect the CNS and induce encephalitis and to infect the liver and cause hepatitis were assessed. The pathogenic phenotypes of the viruses are shown schematically and are discussed in the text.

In a similar spike exchange experiment performed with IBV, the ectodomain of the spike protein from the virulent M41-CK strain was used to replace the corresponding region within the apathogenic IBV Beaudette genome. The resulting chimeric virus displays the in vitro cellular tropism phenotype of M41-CK (35); however, the virus remains apathogenic. Thus, the M41-CK spike is not sufficient to render the chimeric virus virulent (134). The spike protein is therefore a major determinant of tropism and thus influences pathogenesis; however, the spike alone is not always the main determinant of pathogenesis, and as the data indicate, other genes also contribute to pathogenic phenotypes.

There are many strains of MHV and many isolates of the JHM strain, displaying different levels of neurovirulence. Among the JHM isolates, virulence is correlated with the presence of a long hypervariable domain (see above) within S1. The isolate referred to as MHV-4 (67) or MHV_SD (231) has the longest MHV HVR among JHM spikes and is able to induce cell-to-cell fusion and viral spread in the absence of the CEACAM receptor (103, 104). It is likely that this ability is related to the less stable association of S1 and S2, such that the conformational changes in spike that lead to fusion are more easily triggered, and this in turn is at least partially responsible for its very high neurovirulence (103, 161). Similarly, deletions, as well as single-site mutations, within the HVR region have been shown to influence neurovirulence (67, 106, 201, 245).

Mutations within both the RBD of S1 and the heptad repeat domains within S2 have been show to influence pathogenesis. Mutations in the RBD are likely to affect the interaction between spike and the host cell and could thus affect viral entry and tropism, while mutations in the heptad repeats are likely to affect tropism by altering the fusion mechanism. Variation in the amino-terminal portions of the spike has also been noted in TGEV and IBV; the attenuated porcine coronavirus PRCoV has a deletion in the amino-terminal portion of S1 compared with the virulent TGEV (91). A single amino acid substitution within the RBD, S310G, is responsible for enhanced neurovirulence of a JHM isolate (231) Furthermore, a single Q159L amino acid substitution in this region eliminates the ability of MHV-A59 to infect the liver while having no effect on neurovirulence (180, 181). The observation that a one-amino-acid substitution in the RBD can confer a complete loss of tropism to the liver while not affecting infection of the brain, while using the same CEACAM receptor, suggests that other cell surface molecules may serve as cofactors or coreceptors in an organ-specific way. An E1035D substitution within HR1 may overcome the Q159L mutation, since a spike with both of these substitutions confers hepatotropism upon a recombinant MHV-A59 (224). In support of the idea that the RBD may interact with the HR domains, escape mutants selected by resistance to a monoclonal antibody mapping to the receptor binding domain of S1 had point mutations in the region of HR2, suggesting an interaction between these two physically distant portions of the spike (121). Furthermore, mutations within S1 may also affect host range; 21 amino acid substitutions and a 7-amino-acid insertion within the N-terminal region of spike, but downstream of the RBD, allow MHV infection of the usually resistant hamster, feline, and monkey cells (309).

SARS-CoV is believed to have jumped to humans from civets (see "CORONAVIRUSES AS EMERGING PATHOGENS: SARS-CoV" below). The adaptation of SARS-CoV to humans likely involved changes within the RBD. In comparison of the spike protein from civets and from humans, there are six amino acid differences within the RBD of the spike. The spike protein of civet SARS-CoV has low affinity for the human ACE2 SARS-CoV receptor. Substitution of two amino acids within the RBD of the human spike protein with those of the civet spike (N479K/T487S) almost abolishes the ability to infect (using the single-round infection assay) human cells expressing the SARS-CoV receptor. Conversely, substitution of the two residues within the civet spike with the human amino acids confers the ability to infect cells expressing the human receptor. Thus, it is likely that amino acids 479 and 487 are important for receptor interaction and hence species specificity and that selection of viruses with substitutions of these residues allowed the adaptation of SARS-CoV to humans (188, 256).

For MHV, most of the H-2^b-restricted T-cell epitopes thus far identified are encoded in the spike gene. The MHV spike encodes an immundominant CD8⁺ T-cell epitope (S510 to S518), located within the HVR (and therefore absent from the many strains and variants with deletions in the HVR, such as A59) and an additional subdominant CD8⁺ epitope (S598 to S605) that is expressed by all MHVs. Mutation within the immunodominant epitope of the MHV spike has been reported as a mechanism to escape the immune response and achieve viral persistence; such epitope escape mutants selected in suckling mice were more virulent than wild-type virus (243). However when the same inactivating mutation was introduced into a recombinant virus, the resulting virus ranged from slightly to significantly attenuated in weanling mice, depending on the genetic background of the virus and the strain of the mouse infected (200). Under similar conditions, inactivating mutations within a foreign CD8 T-cell epitope (gp33 from lymphocytic choriomeningitis virus), introduced into recombinant MHVs in a nonessential gene, were readily selected in weanling mice previously immunized against this epitope (54). Thus, the likelihood of epitope escape occurring depends on multiple factors, such as the location of the epitope within an essential versus a nonessential protein and its effect on function of the protein, the background genes of the virus, and the age and strain of mouse (55, 152, 201). CD4 T-cell epitopes have been identified in the spike (127, 322) as well as the M (346) and N (322) proteins of MHV (242) and in the N proteins of porcine TGEV (4) and avian coronaviruses (20).

Studies with chimeric A59/JHM recombinant viruses demonstrated that genes other than the spike play a major role in determining tropism. In fact, JHM genes eliminate the ability of a virus expressing the A59 spike to cause hepatitis, and this is not due to the replicase but rather to genes in the 3' end of the genome (223; S. Navas-Martin and S. R. Weiss, unpublished data) (see Fig. 5). Furthermore, the extent of T-cell response to recombinant MHVs, and thus the likelihood for viral clearance to occur, is not determined by the spike gene, but rather by background genes, again encoded in the 3' end of genome (Iacono et al., unpublished data). Thus, other viral structural genes clearly influence pathogenic outcome dramatically, and these are discussed below.

SARS-CoV spike protein may play a role in pathogenesis by inducing interleukin-8 (IL-8) in the lungs via activations of MAPK and AP-1 (40). Such an activity was mapped to amino acids 324 to 688 of the SARS-CoV spike. This activity was detected in epithelial cells and fibroblasts by using baculovirus-expressed SARS-CoV spike; the location of the sequencing responsible for this activity overlaps with the RBD, suggesting that attachment to the ACE2 receptor may trigger this activation (40).

Group II viruses that encode HE can be divided into two groups based on their sialic acid substrate specificity. BCoV, HCoV-OC43, and MHV-DVIM recognize and bind to Neu5,9Ac₂ (278), and encode HEs with sialate-9-O-acetylesterase activity (286, 325, 326). MHV strains S and JHM bind to Neu4,5Ac₂ and encode HEs with sialate-4-O-acetylesterase activity (259, 344). However, it is not clear whether it is HE or the spike that directly binds to sialic acid-containing receptors, and it has been argued that for BCoV, HCoV-OC43, and MHV, it is the spike protein rather than the HE that has the binding activity (165, 278, 344). However, HE proteins of some coronaviruses have been demonstrated to be sialic acid-specific lectins, as demonstrated by hemagglutination and/or hemadsorption assays, thus supporting the role of HE in receptor binding (244, 279, 359). For MHV-DVIM, hemagglutinating activity is associated with HE and not with spike (296; R. J. de Groot, personal communication). For BCoV, both HE and spike recognize the same receptor determinant of 9-O-acetylneuraminic acid on host cells (253). While HE is nonessential for replication in cell culture, spike is necessary and sufficient to mediate entry. Thus, the role of HE in coronavirus infection is still not clear and merits further investigation.

For MHV, the HE protein is expressed by a minority of strains; among these are the weakly pathogenic MHV-S, some isolates of the highly neurovirulent JHM strain (355), and enteropathogenic strains such as DVIM (344). While the highly tissue culture-adapted MHV-A59 genome encodes an HE protein (282), due to multiple mutations, the HE protein is not expressed and the gene is referred to as a pseudogene. Expression of the viral HE glycoprotein is not necessary for virulence in the animal as evidenced by the fact that MHV-A59 causes encephalitis and hepatitis, as well as demyelination, while it does not express HE. In tissue culture, viruses expressing HE have a relative growth disadvantage with respect to viruses that do not express HE. During serial passage in culture, there is a selection for variants in which mutations in the HE gene preclude insertion into the viral membrane (190).

The observation that HE expression is nonessential for the viral life cycle suggests that HE may play a role during infection of the animal (286). It has long been speculated that HE may play a role in acute and/or chronic disease induced by MHV, possibly as a determinant of cellular tropism (353, 354, 358), or may aid spread of the virus by augmenting attachment and/or exit from the cell (151). There are early data both supporting and arguing against this hypothesis. A higher level of mortality as well as increased infection of neurons was associated with a JHM variant that expressed high levels of HE compared with a variant that expressed less HE (353). Taguchi et al. (300) found that HE-expressing variants of MHV-JHM were selected for during propagation in cultured neural rat cells. Moreover, anti-HE monoclonal antibodies, when administered to mice, attenuated the acute encephalitis (354). However, in contrast to these studies, Lai and coworkers reported that in JHM-infected mice, viral variants defective for HE accumulate in the brain and spinal cord (172, 358). These studies were carried out before reverse genetics were available for MHV, and thus they were not able to distinguish between effects of HE and the influence of other genes in the comparison of various MHV isolates. A recent study compared the pathogeneses of isogenic recombinant viruses expressing a wild-type HE protein, expressing an HE protein in which the acetyl esterase activity has been eliminated by mutation, and not expressing the HE polypeptide. Surprisingly both viruses that expressed HE polypeptides (with or without a functional acetyl esterase activity) were more virulent when inoculated intracranially into mice (149). This result would be consistent with a model in which HE may enhance virus attachment and spread by binding to sialic acid-containing receptors and would suggest that the sialic acid binding domain is separate from the esterase domain. Similarly, in the case of influenza C virus hemagglutinin-esterase fusion protein, it has been proposed that there are two separate regions for binding to sialic acid, one for receptor binding and another for the catalytic (esterase) activity (130). We hypothesize that for MHV infection of the CNS, it is the binding activity of HE that augments spread of the virus in certain cell types and that at least in the CNS, the esterase activity is not important for enhanced spread. The esterase activity may be more important in other organs, such as the respiratory tract, where the virus may need to pass through mucus or have the ability to detach cells that may not be productively infected, both of which are believed to be functions of neuraminidases. In the case of influenza virus, it has recently been shown that the specificity of the neuraminidase (esterase) for a particular type of sialic acid determines the cell subtype infected within the respiratory track and hence the pathogenic outcome (210). Thus, by analogy, the HE of MHV may also play a role in tropism.

The p28 protein of MHV, encoded at the extreme 5' terminus of the genome and thus processed from the N terminus of pp1a, when expressed transiently in several different cultured cell types prevents cell cycle progression from G₀/G₁ to S phase (42).

Studies of A59/JHM chimeric MHVs, in which the A59 replicase gene is expressed with the JHM structural genes and vice versa, demonstrated that the replicase is not an important determinant of the difference in tropism and pathogenesis between the two strains (severe encephalitis versus hepatitis). It is rather the 3' portion of the genome that is responsible for pathogenic properties (Navas-Martin and Weiss, unpublished data). This is consistent with the observation that the ability of MHV to induce fulminant hepatitis maps to the nucleocapsid gene (230).