Relationships and host range of human, canine, simian and porcine isolates of simian virus 5 (parainfluenza virus 5).

Sequence comparison of the V/P and F genes of 13 human, canine, porcine and simian isolates of simian virus 5 (SV5) revealed a surprising lack of sequence variation at both the nucleotide and amino acid levels (0-3%), even though the viruses were isolated over 30 years and originated from countries around the world. Furthermore, there were no clear distinguishing amino acid or nucleotide differences among the isolates that correlated completely with the species from which they were isolated. In addition, there was no evidence that the ability of the viruses to block interferon signalling by targeting STAT1 for degradation was confined to the species from which they were isolated. All isolates had an extended cytoplasmic tail in the F protein, compared with the original W3A and WR monkey isolates. Sequence analysis of viruses that were derived from human bone-marrow cells isolated in London in the 1980s revealed that, whilst they were related more closely to one another than to the other isolates, they all had identifying differences, suggesting that they were independent isolates. These results therefore support previous data suggesting that SV5 can infect humans persistently, although the relationship of SV5 to any human disease remains highly contentious. Given that SV5 has been isolated on multiple occasions from different species, it is proposed that the term simian virus 5 is inappropriate and suggested that the virus should be renamed parainfluenza virus 5.

Differences in interferon sensitivity and biological properties of two related isolates of simian virus 5: a model for virus persistence.

CPI(+) and CPI(-) are two canine isolates of simian virus 5 (SV5). CPI(+) was originally isolated from the cerebrospinal fluid of a dog with temporary posterior paralysis and CPI(-) was recovered at 12 days p.i. from the brain tissue of a dog experimentally infected with CPI(+). We have previously shown that the V protein of SV5 blocks interferon (IFN) signalling by targeting STAT1 for degradation. Here we report that whilst CPI(+) targets STAT1 for degradation, CPI(-) fails to and as a consequence, CPI(+) blocks IFN signalling but CPI(-) does not. Three amino acid differences in the P/V N-terminal common domain of the V protein are responsible for the observed difference in the abilities of CPI(+) and CPI(-) to block IFN signalling. In cells persistently infected with CPI(-) the virus may become repressed in response to IFN, under which circumstances virus glycoproteins are lost from the surface of infected cells and virus nucleocapsid proteins accumulate in cytoplasmic inclusion bodies. We suggest that in vivo cells infected with IFN-resistant viruses (in which there would be continuous virus protein synthesis) may be more susceptible to killing by cytotoxic T cells than cells infected with IFN-sensitive viruses (in which virus protein synthesis was repressed), and a model of virus persistence is put forward in which there is alternating selection of IFN-resistant and IFN-sensitive viruses depending upon the state of the adaptive immune response.

Single amino acid substitution in the V protein of simian virus 5 differentiates its ability to block interferon signaling in human and murine cells.

Previous work has demonstrated that the V protein of simian virus 5 (SV5) targets STAT1 for proteasome-mediated degradation (thereby blocking interferon [IFN] signaling) in human but not in murine cells. In murine BF cells, SV5 establishes a low-grade persistent infection in which the virus fluxes between active and repressed states in response to local production of IFN. Upon passage of persistently infected BF cells, virus mutants were selected that were better able to replicate in murine cells than the parental W3 strain of SV5 (wild type [wt]). Viruses with mutations in the Pk region of the N-terminal domain of the V protein came to predominate the population of viruses carried in the persistently infected cell cultures. One of these mutant viruses, termed SV5 mci-2, was isolated. Sequence analysis of the V/P gene of SV5 mci-2 revealed two nucleotide differences compared to wt SV5, only one of which resulted in an amino acid substitution (asparagine [N], residue 100, to aspartic acid [D]) in V. Unlike the protein of wt SV5, the V protein of SV5 mci-2 blocked IFN signaling in murine cells. Since the SV5 mci-2 virus had additional mutations in genes other than the V/P gene, a recombinant virus (termed rSV5-V/P N(100)D) was constructed that contained this substitution alone within the wt SV5 backbone to evaluate what effect the asparagine-to-aspartic-acid substitution in V had on the virus phenotype. In contrast to wt SV5, rSV5-V/P N(100)D blocked IFN signaling in murine cells. Furthermore, rSV5-V/P N(100)D virus protein synthesis in BF cells continued for significantly longer periods than that for wt SV5. However, even in cells infected with rSV5-V/P N(100)D, there was a late, but significant, inhibition in virus protein synthesis. Nevertheless, there was an increase in virus yield from BF cells infected with rSV5-V/P N(100)D compared to wt SV5, demonstrating a clear selective advantage to SV5 in being able to block IFN signaling in these cells.