Infectious Bursal Disease Virus: Strains That Differ in Virulence Differentially Modulate the Innate Immune Response to Infection in the Chicken Bursa

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Abstract
Little is understood about the immune responses involved in the pathogenesis of infectious bursal disease virus (IBDV). Strains of IBDV differ in their virulence: F52/70 is a classical virulent strain (vIBDV), whereas UK661 is a very virulent strain (vvIBDV) that causes greater pathology and earlier mortality. The exact causes of clinical disease and death are still unclear. Pro-inflammatory cytokines such as interleukin (IL)-1β and IL-6, produced by activated macrophages, could play a role, as could cytokines produced by T and natural killer (NK) cells, such as interferon (IFN)-γ, which stimulate macrophages. We quantified mRNA transcription in bursal tissue, by real-time quantitative reverse transcription– polymerase chain reaction (RT-PCR), for the type I IFN (IFN-α and IFN-β), pro-inflammatory cytokines (IL-1β, IL-6, and CXCLi2), the anti-inflammatory cytokine transforming growth factor (TGF)-β4, and Th1 cytokines (IFN-γ, IL-2 [and the closely related IL-15], IL-12, and IL-18) for the first 5 days after infection of 3-week-old chickens with F52/70 or UK661 and compared these with levels in bursal tissue from uninfected age-matched controls. Both strains induced a pro-inflammatory response, evidenced by increased mRNA transcription of IL-1β, IL-6, and CXCLi2, and down-regulation of TGF-β4, of similar magnitude and timing. IFN-γ mRNA was induced by both strains, although to a greater degree by the vvIBDV strain, indicating that a cell-mediated response is induced. Neither virus initially induced high levels of type I IFN. F52/70 seems to use a "stealth" approach by not inducing the type I IFNs, whereas UK661 down-regulates their expression. This suggests that both viruses modulate the host immune response, although probably by using different mechanisms.

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