Taxonomic Structure of the Family
|
Order |
Nidovirales |
|
Family |
Arteriviridae |
|
Genus |
Arteriviruses are spherical with a diameter of 45 to 60 nm. Virions consist of an isometric nucleocapsid of about 25 to 35 nm in diameter, surrounded by a lipid envelope (Fig. 1). No spikes are obvious on the virion surface but distinct surface patterns have been observed (Fig. 1).
Physicochemical and Physical Properties
The buoyant density of arteriviruses has been estimated to be 1.13 to 1.17 g/cm3 in sucrose. Reported sedimentation coefficients for arteriviruses range from 214S to 230S. Virion stability is affected by temperature and pH. Virions are stable when stored at -70°C. The half-life of arteriviruses progressively decreases with increasing temperature. Virions are stable between pH 6.0 and 7.5, but are inactivated at high or low pHs. Arteriviruses are also inactivated by lipid solvents, such as ether, butanol, and chloroform and are extremely sensitive to detergent treatment. A brief incubation with a nonionic detergent such as 0.01% NP40 or Triton X-100, efficiently disrupts the viral envelope (Lactate dehydrogenase-elevating virus, LDV).
Virions contain one copy of a linear, ssRNA of positive-polarity that ranges in length from 12.7 to 15.7 kb. The genome RNA contains a 5 Type I cap structure (Simian hemorrhagic fever virus, SHFV) and a 3-terminal poly(A) tract. Arterivirus genomic sequences are available in the GenBank database.
The virion nucleocapsid is composed of a single basic nucleocapsid protein (N) with an Mr of 12-15 103. This protein is encoded in all arteriviruses by the 3 terminal ORF. Virion envelope proteins include a non-glycosylated, triple-membrane spanning, integral membrane protein (M) and the glycoprotein encoded by ORF5 (ORF7 in SHFV), which is the major virion glycoprotein (Fig. 1). The ORF5 glycoprotein also contains three potential membrane-spanning regions. The M protein and the ORF5 glycoprotein form disulfide-linked heterodimers. The minor glycoprotein encoded by ORF2 is a class I integral membrane protein. Homodimers of the ORF2 protein have been detected in virions. The glycoproteins encoded by ORFs 3 and 4 have so far been identified as minor virion glycoproteins only for PRRSV. A soluble, non-virion-associated form of the ORF 3 glycoprotein has been reported to be released from infected cells (LDV and PRRSV). SHFV encodes three additional 3 ORFs, which may be duplications of ORFs 2 to 4 (Fig. 2).
Arteriviruses encode two large nonstructural polyproteins, the ORF1a protein (187 to 260 103) and the ORF1ab protein (345 to 421 103), which are cleaved into the mature proteins, nsp 1, nsp 1, (nsp1 for Equine arteritis virus, EAV) and nsp2 through 12 by viral proteases (Fig. 2). A papain-like cysteine protease (PCP) is located in nsp1. SHFV ORF1a polyprotein contains three copies of the PCP, the PRRSV and LDV polyproteins contain two copies, while EAV has only one. A cysteine protease (CP) with unique properties is located in nsp2. A serine protease (SP), related to the members of the 3C-like cysteine protease family, is located in nsp4. The SP protease is responsible for the cleavage of ten of the nonstructural proteins (Fig. 2). Nsp9 contains an RNA-dependent-RNA polymerase motif and nsp10 contains both a metal binding motif and a nucleoside triphosphate-binding/RNA helicase motif.
The virions have lipid envelopes obtained from the infected cell.
The proteins encoded by ORFs 2 to 4 in the EAV, LDV and PRRSV genomes and ORFs 2 to 7 in SHFV contain putative N-linked glycosylation sites. Glycosylation of the ORF5 protein (EAV and probably also LDV) may also occur by the addition of variable numbers of lactosamine repeats.
Genome Organization and Replication
The arterivirus genome organization (Fig. 2) and replication strategy are similar to those of other nidoviruses. The viral nonstructural (replicase) polyproteins are encoded by ORFs 1a and 1b, expressed from the genomic RNA and post-translationally processed by viral proteases (Fig. 2). The ORF1b protein is expressed only after a-1 frameshift has occurred. The frameshift region contains a slippery sequence upstream of an RNA pseudoknot structure. Hydrophobic domains in ORF1a may be involved in membrane association of arteriviral replication complexes. Four cell proteins have been identified that bind to a cis-acting region required for plus-strand RNA synthesis from the minus-strand template.
The viral structural proteins are encoded by overlapping 3 ORFs and are expressed from a nested set of 3 and 5 co-terminal subgenomic mRNAs. The common 5 leader of the subgenomic mRNAs is derived from the 5-end of the genome by discontinuous transcription. A nested set of complementary minus-strand sgRNAs has also been demonstrated in infected cells. Although only the 5 terminal ORF is translated from most of the sgRNAs, one sgRNA (mRNA2 for EAV, PRRSV and LDV and mRNA4 for SHFV) is bicistronic. An additional bicistronic SHFV mRNA has been reported.
PRRSV has been reported to enter cells via a low pH-dependent endocytic pathway. Arteriviruses replicate in the cytoplasm of infected cells. Nucleocapids bud into the lumen of smooth endoplasmic reticulum and/or Golgi apparatus vesicles. Virions are released from cells via exocytosis.
ORF5 glycoprotein-specific neutralizing antibodies have been reported for EAV, LDV and PRRSV. The neutralization domain has been mapped to the ectodomain of this protein. Monoclonal antibodies specific for the ORF4 protein (PRRSV) are also neutralizing. A neutralizing epitope on the ORF4 protein was mapped to the region between aa 39 and 79. No antigenic cross reactivity between arteriviruses has been found. LDV- and PRRSV-specific T-cell responses have been reported.
The host range of arteriviruses is restricted. EAV infects horses and donkeys, LDV infects mice, PRRSV infects swine, and SHFV infects some species of African (patas monkeys, African green monkeys and baboons) and Asian (macaque) monkeys.
Arteriviruses are spread horizontally and vertically. Horizontal transmission can occur via the respiratory route (EAV and PRRSV), via the sexual route in semen (EAV and PRRSV) and via infected blood or body fluids (LDV and SHFV). Vertical transmission in utero has been reported for PRRSV.
Pathogenicity, Association with Disease
Infections with arteriviruses can cause acute or persistent asymptomic infections or respiratory disease (PRRSV), abortion (EAV and PRRSV), fatal age-dependent poliomyelitis (LDV) or fatal hemorrhagic fever (SHFV).
Tissue Tropisms, Pathology, Histopathology
Macrophages are the primary target cells for all arteriviruses. LDV can replicate in ventral motor neurons in some strains of inbred mice. In tissue culture, LDV replicates only in primary mouse macrophages, SHFV and PRRSV replicate in primary macrophage cultures (rhesus and porcine aveolar lung macrophages, respectively) as well as MA-104 cells (an African green monkey kidney cell line), and EAV replicates in primary cultures of equine macrophages and kidney cells and in BHK, RK-13, Vero and MA-104 cells.
Arterivirus replication is characterized by the formation of paired membranes and double-membrane vesicles in infected cells. After expression from a vaccinia recombinant, the ORF5 glycoprotein induced apoptosis that could not be inhibited by Bcl-2. Apoptosis has also been observed in PRRSV-infected porcine alveolar macrophages, MA-104 cells, and testicular germ cells.
|
|