Determinants of Bluetongue Virus Virulence in Murine Models of Disease

Abstract

Bluetongue is a major infectious disease of ruminants that is caused by bluetongue virus (BTV). In this study, we analyzed virulence and genetic differences of (i) three BTV field strains from Italy maintained at either a low (L strains) or high (H strains) passage number in cell culture and (ii) three South African "reference" wild-type strains and their corresponding live attenuated vaccine strains. The Italian BTV L strains, in general, were lethal for both newborn NIH-Swiss mice inoculated intracerebrally and adult type I interferon receptor-deficient (IFNAR ؊/؊ ) mice, while the virulence of the H strains was attenuated significantly in both experimental models. Similarly, the South African vaccine strains were not pathogenic for IFNAR ؊/؊ mice, while the corresponding wild-type strains were virulent. Thus, attenuation of the virulence of the BTV strains used in this study is not mediated by the presence of an intact interferon system. No clear distinction in virulence was observed for the South African BTV strains in newborn NIH-Swiss mice. Full genomic sequencing revealed relatively few amino acid substitutions, scattered in several different viral proteins, for the strains found to be attenuated in mice compared to the pathogenic related strains. However, only the genome segments encoding VP1, VP2, and NS2 consistently showed nonsynonymous changes between all virulent and attenuated strain pairs. This study established an experimental platform for investigating the determinants of BTV virulence. Future studies using reverse genetics will allow researchers to precisely map and "weight" the relative influences of the various genome segments and viral proteins on BTV virulence.

Sensing and Control of Bluetongue Virus Infection in Epithelial Cells via RIG-I and MDA5 Helicases

“…Recently, several BTV virulence determinants have been proposed that include VP1, VP2, and NS2, as the genome encoding these proteins consistently showed nonsynonymous changes between virulent and attenuated BTV strain pairs (6). It is of great interest to investigate further whether a link exists between the determinants encoded by BTV that contribute to viral pathogenesis and the strength of IFN-␣/␤ signaling.…”

Bluetongue virus genetic and phenotypic diversity: Towards identifying the molecular determinants that influence virulence and transmission potential

“…It has been demonstrated by immunoperoxidase staining that 11-day old chicken embryos show a similar tissue tropism for AHSV as infected horses (Maartens, 2010). Similarly type 1 interferon receptor deficient mice are highly susceptible to BTV infection via both the oral and intravenous route and demonstrate similar clinical and pathological features as the ruminant host (Calvo-Pinilla et al, 2010;Calvo-Pinilla et al, 2009;Caporale et al, 2011b). The use of in vitro methods to compare BTV phenotype also shows promise.…”

“…In general the low passage field strains demonstrated a virulent phenotype in the murine models, whereas high passage strains were non-virulent. Sequence analysis of the VP1, VP2 and NS2 encoding genome segments demonstrated consistent non-synonymous amino acid changes between low and high passage virus strains, suggesting that one or more of these proteins may be involved in modulating virulence (Caporale et al, 2011a).…”

“…A study was conducted in order to further investigate the viral genes and amino acid substitutions that are involved in the attenuation of BTV field strains during the virus's passage in atypical host systems (Caporale et al, 2011a). In this study the authors compared the virulence of both low and high cell culture passage BTV strains from Italy and South Africa by infecting 3-day old NIH-Swiss and adult interferon receptor deficient mice.…”

Turnover Rate of NS3 Proteins Modulates Bluetongue Virus Replication Kinetics in a Host-Specific Manner

“…The median values (Mdn) and interquartile ranges (IQR) were calculated, and statistical analysis was performed using a Wilcoxon nonparametric test and NCSS9 software. Viral stocks also were titrated using limiting-dilution assays in CPT-Tert cells as previously described (20,67), and the viral titers are expressed as 50% tissue culture infective doses (TCID 50 )/milliliter. Finally, to assess viral physical titers, BTV RNAs were extracted in triplicate from 100 l of each viral stock with the QIAamp viral RNA minikit (Qiagen) and then pooled.…”