H5N1 avian influenza virus without 80-84 amino acid deletion at the NS1 protein hijacks the innate immune system of dendritic cells for an enhanced mammalian pathogenicity.

NS gene is generally considered to be related to the virulence of highly pathogenic avian influenza virus (AIV). In recent years, the strains with five amino acids added to the 80-84 positions of the NS1 protein have become prevalent in H5N1 subtype AIVs isolated from mammals. However, the pathogenicity and mechanism of this pattern in mammals remain unclear. In this study, H5N1 subtype AIVs without 80-84 amino acids of the NS1 protein (rNS ) and a mutant virus (rNS ) with no deletion of 80-84 amino acids of the NS1 protein were used to determine the pathogenicity in mice. Our results showed that rNS possessed an enhanced pathogenicity compared with rNS in vivo and in vitro, which was accompanied by high expression of IL-6, MX1 and CXCL10 in murine lungs. Furthermore, we found that rNS increased the infection ability to dendritic cells (DCs). Besides, rNS enhanced the expression of phenotypic markers (CD80, CD86, CD40 and MHCII), activation marker CD69, inflammatory cytokines (IL-6, TNF-α and IL-10) and antagonized interferon (IFN-α) of DCs, in comparison to rNS . Moreover, rNS induced DCs to quickly migrate into nearby cervical lymph nodes by highly upregulating CCR7, and CD86 showed a high expression on the migrated DCs. We also found that rNS -infected DCs significantly promoted the allogeneic CD4 T-cell proliferation. These findings suggested that rNS strongly induced the innate immune response compared with the rNS , which is conducive to activate a wide immune response, resulting in a strong cytokine storm and causing an enhanced pathogenicity of H5N1 subtype AIVs in mammals.