Abstract
IBDV is economically important to the poultry industry. Very virulent (vv) strains cause higher mortality rates than other strains for reasons that remain poorly understood. In order to provide more information on IBDV disease outcome, groups of chickens (n = 18) were inoculated with the vv strain, UK661, or the classical strain, F52/70. Birds infected with UK661 had a lower survival rate (50%) compared to F52/70 (80%). There was no difference in peak viral replication in the bursa of Fabricius (BF), but the expression of chicken IFNα, IFNβ, MX1, and IL-8 was significantly lower in the BF of birds infected with UK661 compared to F52/70 (p < 0.05) as quantified by RTqPCR, and this trend was also observed in DT40 cells infected with UK661 or F52/70 (p < 0.05). The induction of expression of type I IFN in DF-1 cells stimulated with polyI:C (measured by an IFN-β luciferase reporter assay) was significantly reduced in cells expressing ectopic VP4 from UK661 (p < 0.05), but was higher in cells expressing ectopic VP4 from F52/70. Cells infected with a chimeric recombinant IBDV carrying the UK661-VP4 gene in the background of PBG98, an attenuated vaccine strain that induces high levels of innate responses (PBG98-VP4UK661) also showed a reduced level of IFNα and IL-8 compared to cells infected with a chimeric virus carrying the F52/70-VP4 gene (PBG98-VP4F52/70) (p < 0.01), and birds infected with PBG98-VP4UK661 also had a reduced expression of IFNα in the BF compared to birds infected with PBG98-VP4F52/70 (p < 0.05). Taken together, these data demonstrate that UK661 induced the expression of lower levels of anti-viral type I IFN and proinflammatory genes than the classical strain in vitro and in vivo and this was, in part, due to strain-dependent differences in the VP4 protein.
Highlights
Infectious bursal disease virus (IBDV) is a highly contagious, immunosuppressive virus belonging to the Birnaviridae family (Hoerr, 2010)
We determined the viral titers in the bursal tissue at each time point by TCID50, and we found UK661 replicated to a lower titer than F52/70 at 24 hpi (∗P < 0.05), there was no significant difference in viral replication at later time points, which peaked at ∼8 log10 TCID50/g of bursal tissue for both strains (Figure 1D)
Very virulent strains of IBDV emerged in the 1980s, causing up to 60% mortality in some commercial flocks
Summary
Infectious bursal disease virus (IBDV) is a highly contagious, immunosuppressive virus belonging to the Birnaviridae family (Hoerr, 2010). The virus is non-enveloped, with a bi-segmented double stranded (ds) RNA genome encoding 3 open reading frames (ORFs) which are translated and processed to produce 5 viral proteins (VP1-5). Disease severity depends on numerous factors including the age and breed of the bird, and the virulence of the infecting IBDV strain (Mahgoub et al, 2012). The vvIBDV strains cause a far higher mortality rate than classical strains, reaching up to 60– 70% in some flocks (van den Berg et al, 2000). Segment A encodes the non-structural protein, VP5, and a polyprotein (VP2-VP4-VP3) which is co-translationally cleaved by the protease, VP4 (Lejal et al, 2000). The single ORF on Segment B encodes VP1, the RNA-dependent RNA polymerase
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