Host immune responses of ducks infected with H5N1 highly pathogenic avian influenza viruses of different pathogenicities

Our previous studies have illustrated three strains of duck-origin H5N1 highly pathogenic avian influenza viruses (HPAIVs) had varying levels of pathogenicity in ducks (Sun et al., 2011). However, the host immune response of ducks infected with those of H5N1 HPAIVs was unclear. Here, we compared viral distribution and mRNA expression of immune-related genes in ducks following infection with the two HPAIV (A/Duck/Guangdong/212/2004, DK212 and A/Duck/Guangdong/383/2008, DK383). DK383 could replicate in the tested tissue of ducks (brain, spleen, lungs, cloacal bursa, kidney, and pancreas) more rapid and efficiently than DK212 at 1 and 2 days post-inoculation. Quantitative real-time PCR analysis showed that the expression levels of TLR3, IL-6, IL-8, and MHC class II in brains were higher than those of respective genes in lungs during the early stage of post infection. Furthermore, the expression levels of IL-6 and IL-8 in the brain of ducks following infection with DK383 were remarkably higher than those of ducks infected with DK212, respectively. Our results suggest that the shift in the H5N1 HPAIVs to increased virulence in ducks may be associated with efficient and rapid replication of the virus, accompanied by early destruction of host immune responses. These data are helpful to understand the underlying mechanism of the different outcome of H5N1 HPAIVs infection in ducks.