6: Innate immune responses in human dendritic cells induced by H9N2 avian influenza virus

Abstract

H9N2 influenza viruses have been circulating worldwide in multiple avian species. Occasionally H9N2 influenza viruses have been transmitted to humans. The concern is the adaptation of avian influenza viruses to efficient human transmission, which could lead to a pandemic. In this study we have examined the activation of innate immune responses in low pathogenic avian influenza (LPAI) A/HK/1073/99 (H9N2) and seasonal influenza A/Beijing/353/89 (H3N2) virus-infected human dendritic cells (DCs). In human airways and lungs the influenza virus infects epithelial cells and DCs in close proximity to epithelium which orchestrate the activation of adequate immune responses. The virus infection leads to activation of transcription factors (e.g. IRF3, IRF7 and NF-κB) which regulate the expression of proinflammary cytokines and antiviral interferons. We discovered that the H9N2 virus induced earlier interferon (IFN-α1, -β1 and -λ1) gene expression than the H3N2 virus, and the response was switch off in the late time point of H9N2 infection. Nevertheless interferon stimulated gene 15 (ISG15) and antiviral MxA protein were produced in the H9N2 virus-infected cells at least at the same level than in the H3N2 virus-infected cells. For the pro-inflammatory responses CXCL10, IL-1β and TNF-α gene expression was analyzed and it turned out that pro-inflammatory responses were stronger in H9N2 virus-infected cells than in the cells infected with the H3N2 virus. There were no differences seen in the virus-induced phosphorylation of IRF3 transcription factor or phosphorylated p38 MAP kinase between H9N2 and H3N2 viruses. All infection experiments with the H9N2 virus were carried out under biosafety level 3 (BSL-3) conditions. These results indicate that the avian influenza H9N2 virus is inducing strong but transient antiviral interferon response in human DCs. Moreover, avian H9N2 virus evoked strong pro-inflammatory response in human cells that is in line with the other avian, like H5N1 virus-induced human responses.