Pro-inflammatory Responses in Chicken Spleen and Brain Tissues after Infection with Very Virulent Plus Marek's Disease Virus

In chickens infected with virulent (v) or very virulent (vv) Marek's disease (MD) virus (MDV) strains, small to moderate increases in plasma nitric oxide (NO) levels are seen, respectively, whereas very virulent plus (vv+) strains induce very high levels in vivo. The data presented in this report show that chickens presenting with clinical neurological disease following infection with the vv+ RK-1 strain have significantly higher in vivo NO levels compared to RK-1–infected non-symptomatic chickens. Using quantitative real-time PCR (qPCR) assays, DNA was used to measure MDV copy numbers in the spleen and brain of P2a (MD-susceptible) and N2a (MD-resistant) chickens following infection with the JM-16 (v) or RK-1 (vv+) strains. RNA was used to measure inducible NO synthase(iNOS), interferon-gamma (IFN-γ), interleukin (IL)-1β, IL-6, and IL-8 mRNA levels, in addition to MDV-specific mRNA expression using quantitative RT-PCR (qRT-PCR) assays. Viral DNA loads were found to be considerably higher in RK-1–infected chickens than JM-16–infected chickens at most time points in both organs, with viral copy numbers being two to four logs lower in the brain. Large increases in iNOS, IFN-α, IL-1β, IL-6, and IL-8 were seen in the brains of RK-1–infected chickens. These data strongly support the hypothesis that pro-inflammatory responses, including high levels of iNOS/NO, IFN-α, and pro-inflammatory cytokine expression in the chicken brain, may play a major role in the neurological diseases associated with vv+MDV strains.