P. J. M. Rottier

PERSISTENCE AND EVOLUTION OF FELINE CORONAVIRUS IN A CLOSED CAT-BREEDING COLONY

Abstract Feline coronavirus (FCoV) persistence and evolution were studied in a closed cat-breeding facility with an endemic serotype I FCoV infection. Viral RNA was detected by reverse transcriptase polymerase chain reaction (RT-PCR) in the feces and/or plasma of 36 of 42 cats (86%) tested. Of 5 cats, identified as FCoV shedders during the initial survey, 4 had detectable viral RNA in the feces when tested 111 days later. To determine whether this was due to continuous reinfection or to viral persistence, 2 cats were placed in strict isolation and virus shedding in the feces was monitored every 2–4 days. In 1 of the cats, virus shedding continued for up to 7 months. The other animal was sacrificed after 124 days of continuous virus shedding in order to identify the sites of viral replication. Viral mRNA was detected only in the ileum, colon, and rectum. Also in these tissues, FCoV-infected cells were identified by immunohistochemistry. These findings provide the first formal evidence that FCoV causes chronic enteric infections. To assess FCoV heterogeneity in the breeding facility and to study viral evolution during chronic infection, FCoV quasispecies sampled from individual cats were characterized by RT-PCR amplification of selected regions of the viral genome followed by sequence analysis. Phylogenetic comparison of nucleotides 7–146 of ORF7b to corresponding sequences obtained for independent European and American isolates indicated that the viruses in the breeding facility form a clade and are likely to have originated from a single founder infection. Comparative consensus sequence analysis of the more variable region formed by residues 79–478 of the S gene revealed that each cat harbored a distinct FCoV quasispecies. Moreover, FCoV appeared to be subject to immune selection during chronic infection. The combined data support a model in which the endemic infection is maintained by chronically infected carriers. Virtually every cat born to the breeding facility becomes infected, indicating that FCoV is spread very efficiently. FCoV-infected cats, however, appear to resist superinfection by closely related FCoVs.

Localization and Membrane Topology of Coronavirus Nonstructural Protein 4: Involvement of the Early Secretory Pathway in Replication

ABSTRACT The coronavirus nonstructural proteins (nsps) derived from the replicase polyproteins collectively constitute the viral replication complexes, which are anchored to double-membrane vesicles. Little is known about the biogenesis of these complexes, the membrane anchoring of which is probably mediated by nsp3, nsp4, and nsp6, as they contain several putative transmembrane domains. As a first step to getting more insight into the formation of the coronavirus replication complex, the membrane topology, processing, and subcellular localization of nsp4 of the mouse hepatitis virus (MHV) and severe acute respiratory syndrome-associated coronavirus (SARS-CoV) were elucidated in this study. Both nsp4 proteins became N glycosylated, while their amino and carboxy termini were localized to the cytoplasm. These observations imply nsp4 to assemble in the membrane as a tetraspanning transmembrane protein with a Nendo/Cendo topology. The amino terminus of SARS-CoV nsp4, but not that of MHV nsp4, was shown to be (partially) processed by signal peptidase. nsp4 localized to the endoplasmic reticulum (ER) when expressed alone but was recruited to the replication complexes in infected cells. nsp4 present in these complexes did not colocalize with markers of the ER or Golgi apparatus, while the susceptibility of its sugars to endoglycosidase H indicated that the protein had also not traveled trough the latter compartment. The important role of the early secretory pathway in formation of the replication complexes was also demonstrated by the inhibition of coronaviral replication when the ER export machinery was blocked by use of the kinase inhibitor H89 or by expression of a mutant, Sar1[H79G].

Equine arteritis virus-neutralizing antibody in the horse is induced by a determinant on the large envelope glycoprotein GL

Complementary DNAs encoding ORFs 2 to 7 equine arteritis virus (EAV) have been cloned into the expression vector pGEX to produce glutathione-S-transferase fusion proteins. Recombinant proteins were affinity purified and screened in ELISA with equine sera to identify immunoreactive polypeptides. The large envelope glycoprotein (GL) was identified as the most reactive to EAV-positive equine sera and an immuno-dominant epitope was mapped between amino acids 55 and 98 by subcloning and expression. A fusion protein covering this region and a GL-specific synthetic peptide (residues 75 through 97) induced EAV-neutralizing antibody in vaccinated horses. The defined antigenic region of GL is likely to be exposed on the surface of the native EAV virion and consequently may be useful in the development of diagnostic tests and vaccines.

Monoclonal antibodies to equine arteritis virus proteins identify the GL protein as a target for virus neutralization

Monoclonal antibodies (MAbs) to equine arteritis virus (EAV) proteins were produced and characterized. The protein specificities of eight MAbs were determined definitively by immunoprecipitation of EAV proteins expressed from vaccinia virus recombinants (VVRs). Included were two new VVRs produced for this study, expressing the M and the GL proteins, respectively. Three MAbs were determined to be N-specific and five MAbs recognized the GL protein. One GL-specific MAb, 17F5, of the IgA class, efficiently neutralized EAV infectivity. In competitive binding assays (CBAs), the N-specific MAbs defined a single antigenic domain on this protein. Four GL-specific MAbs, including MAb 17F5, demonstrated strong reciprocal competition in binding to the GL protein but differed in their virus-neutralizing ability. Thus the antigenic domain defined by these MAbs is probably composed of overlapping or closely adjacent epitopes. The fifth GL-specific MAb, a non-neutralizing antibody, may define an epitope adjacent to this antigenic domain as reciprocal CBAs demonstrated lower competition.

The Viral Replication Complex Is Associated with the Virulence of Newcastle Disease Virus

ABSTRACT Virulent strains of Newcastle disease virus ([NDV] also known as avian paramyxovirus type 1) can be discriminated from low-virulence strains by the presence of multiple basic amino acid residues at the proteolytic cleavage site of the fusion (F) protein. However, some NDV variants isolated from pigeons (pigeon paramyxovirus type 1 [PPMV-1]) have low levels of virulence, despite the fact that their F protein cleavage sites contain a multibasic amino acid sequence and have the same functionality as that of virulent strains. To determine the molecular basis of this discrepancy, we examined the role of the internal proteins in NDV virulence. Using reverse genetics, the genes encoding the nucleoprotein (NP), phosphoprotein (P), matrix protein (M), and large polymerase protein (L) were exchanged between the nonvirulent PPMV-1 strain AV324 and the highly virulent NDV strain Herts. Recombinant viruses were evaluated for their pathogenicities and replication levels in day-old chickens, and viral genome replication and plaque sizes were examined in cell culture monolayers. We also tested the contributions of the individual NP, P, and L proteins to the activity of the viral replication complex in an in vitro replication assay. The results showed that the replication proteins of Herts are more active than those of AV324 and that the activity of the viral replication complex is directly related to virulence. Although the M protein affected viral replication in vitro, it had only a minor effect on virulence.

Equine arteritis virus: a review of clinical features and management aspects.

Sero-epidemiological surveys have revealed that equine arteritis virus (EAV) is prevalent in most European countries. The virus causes sporadic cases of respiratory disease and abortion in horses, the incidence of which has increased in recent years. Mares and geldings eliminate virus after acute infection, but 30% to 60% of stallions become persistently infected. In these animals, EAV is maintained within the reproductive tract and is shed continuously in the semen. Persistent infection with EAV in stallions has no negative consequences for fertility but mares inseminated with virus-contaminated semen can have an acute infection. These mares shed large amounts of virus in respiratory secretions and urine, leading to lateral spread of the virus to other susceptible horses. Acute infection at later stages of gestation can lead to abortion. Effective control of the spread of EAV infection depends on the identification of virus-shedding stallions. Persistently infected stallions should not be used for breeding or should be bred only to seropositive mares. Mares bred to shedding stallions should be isolated from other animals for a period of 3 weeks following insemination to prevent the lateral spread of EAV.

Virulence of pigeon paramyxovirus type 1 does not always correlate with the cleavability of its fusion protein

Some pigeon paramyxovirus type 1 (PPMV-1) strains exhibit low virulence in chickens, despite their fusion (F) proteins multi-basic cleavage site. To elucidate the molecular basis of the low pathogenicity of these strains, we constructed an infectious full-length cDNA clone of PPMV-1 strain AV324. This strain is non-virulent for chickens, although its F protein contains the typical virulence motif (112)RRKKRF(117). By using reverse genetics, we exchanged the F genes of AV324 and a virulent Newcastle disease virus (NDV) strain (Herts) and evaluated the recovered chimeric viruses for their pathogenicity in 1-day-old chickens and in embryonated eggs. Our results show that the F protein of AV324, and probably those of similar PPMV-1 strains, are functionally not different from those of virulent NDV strains and that the difference in pathogenicity must be determined by other factors.

A novel glycoprotein of feline infectious peritonitis coronavirus contains a KDEL-like endoplasmic reticulum retention signal.

A new protein of the feline infectious peritonitis virus (FIPV) was discovered in lysates of infected cells. Expression of the gene encoding open reading frame (ORF) 6b of FIPV in recombinant vaccinia virus infected cells was used to identify it as the 6b protein. It is a novel type of viral glycoprotein whose function is not clear. It is a soluble protein contained in microsomes; its slow export from the cell is caused by the presence of an ER-retention signal at the C-terminus. This amino acid sequence, KTEL, closely resembles the consensus KDEL-signal of soluble resident ER proteins. A mutant 6b protein with the C-terminal sequence KTEV became resistant to digestion by endo-beta-N-acetylglucosaminidase H with a half-time that was reduced threefold. In contrast, a mutant with the sequence KDEL was completely retained in the ER. The FIPV 6b protein is the first example of a viral protein with a functional KDEL-like ER-retention signal.

Interferon Alpha Inducing Property of Coronavirus Particles and Pseudoparticles

Previous work in our laboratory have provided evidence that the membrane glycoprotein M of TGEV is centrally involved in efficient induction of alpha interferon (IFN-alpha) synthesis by non-immune peripheral blood mononuclear cells incubated with fixed, TGEV-infected cells or inactivated virions. Here we report recent completion of studies initiated to get a better understanding of the nature of the interferogenic determinant(s). Transfected cells expressing TGEV M together with the minor structural component E (formerly called sM) were found to trigger IFN-alpha synthesis. Co-expression of these two proteins was shown to be necessary and sufficient for assembly and release of pseudoparticles resembling TGEV virions. Purified pseudoparticles exhibited an interferogenic activity close to that of authentic virions. Chimeric recombinant particles expressing BCV M ectodomain also induced IFN. Examination of cell cultures infected by viruses representative of the three Nidovirales genera revealed that the capacity to act as an efficient IFN-alpha inducer is a common feature of viral particles of the coronavirus genus. Altogether these data bring new insights regarding the putative nature of the viral structure involved in IFN-alpha induction.

Biosynthesis and Function of the Coronavirus Spike Protein

One of the most interesting aspects of Coronavirus replication is their intracellular assembly. Budding is localized in the ER-pre Golgi region (8, 26). Both Coronavirus glycoproteins are synthesized in the RER on membrane bound ribosomes (16). The integral membrane protein (M) accumulates in the perinuclear region and is believed to determine the site of budding. The spike protein (S) mediates binding of virions to the host cell receptor, possesses a fusogenic activity and is the major target for virus neutralizing antibodies (22). The primary nucleotide sequence and the predicted amino acid sequence of a number of spike protein genes revealed features characteristic of type I membrane proteins (22).