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We are currently facing a global threat caused by a highly pathogenic avian H5N1 influenza virus (hpH5N1). Death occurs in 48 h in infected chickens, suggesting that they fail to eliminate the virus. Little is known about the immune response in chickens after hpH5N1 infection, or how the virus is evolving to modify and evade host protective responses. Therefore, to better understand the chicken immune response following hpH5N1 infection, we set up an experimental infection of chickens with an hpH5N1 strain, and quantified the mRNA expression of several cytokines and antiviral proteins at different time points post-infection. We show here that a weak host immune response is observed in vivo, in spite of the induction of IL-6, myxovirus resistance protein (Mx), and protein kinase R (PKR). This weak immune response, probably due in part to the absence of type I interferon, was not sufficient to counteract the hpH5N1 virus and protect the chicken from death.

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