Effects of avian influenza A(H7N9) virus infection of monocytes on host innate and adaptive immune responses

Abstract

Avian influenza A H7N9 virus is a novel strain of virus emerged from bird to infect human since 2013.The virus causes acute severe respiratory disease with a crude mortality rate in humans. On-going circulation of H7N9 viruses in poultry and annual epidemics of sporadic human infections continue to pose a significant threat to public health. However, the biological basis that causes severe human disease by H7N9 remains unclear. In this study, we aimed to study the pathogenesis of H7N9. Firstly, we demonstrated that H7N9 induced only a suboptimal humoral immune response in mouse model. More specifically, no neutralizing antibody could be detected in serum samples collected from H7N9 infected mice. In addition, passive transfer of these convalescent sera from H7N9 infected mice did not provide protection to naïve mice from H7N9 challenge. In contrast, pandemic 2009 H1N1 virus (pH1N1) infection of mice induced adequate antibody production, and treatment with convalescent serum protected all mice from pH1N1 virus challenge. To evaluate the degree of protection by immunological memory, mice were re-infected with homologous virus. The result showed that all mice were protected with the subsequent mounting of higher levels of neutralizing antibody and hemagglutination inhibition antibody, but still lower than the amount after challenge with pH1N1. The data suggested that the antibody responses in H7N9 infection were impaired when compared with pH1N1 infection. To further investigate the possible mechanism that leads to impaired antibody response and its contribution to the pathogenesis of H7N9, we studied the responses of human peripheral blood mononuclear cells (PBMCs) to H7N9 infection. We found that H7N9 was as infectious as influenza A H5N1 virus (H5N1) for PBMCs, with large proportion of these cells expressing viral nucleoprotein antigen, while pH1N1 infected much fewer PBMCs. Moreover, our results showed that H7N9 induced a robust expression of proinflammatory cytokines and type I IFN, but low levels of IFN-γ and IP-10. The cytokine profile induced by H7N9 was similar to that of H5N1, yet it differed from that of pH1N1. The result of multi-colour flow cytometry indicated that CD14+ monocytes were highly susceptible to both H7N9 and H5N1 infection. Further study on purified monocytes showed that H7N9 induced a similar cytokine/chemokine expression profile to those of H5N1 with strong induction of IL-6, TNF-α, IFN-α, IFN-β, MIP-1α, MIP-1β, and RANTES, and impaired induction of IP-10. Infection of monocytes by H7N9 or H5N1 inhibited the monocytic maturation or differentiation, while monocytes infected with pH1N1 showed differentiation toward dendritic cells by upregulation of MHC class II molecules and co-stimulatory molecules. Moreover, we found that H7N9 infection activated both apoptosis and necroptosis pathways, of which apoptosis was the predominant mode of cell death. However, addition of pan-caspase inhibitors switched the mode of cell death from apoptosis to necroptosis. Our present findings on H7N9 infection in mice, human PBMCs and monocytes have improved our understanding of their role in the pathogenesis of avian H7N9 infection. Such understanding may provide valuable information in the development of new therapeutic strategies against influenza A virus infection in humans.