Analysis of H5N8 influenza virus infection in chicken with mApple reporter genes in vivo and in vitro

Abstract

H5N8 highly pathogenic avian influenza virus (HPAIV) has caused huge losses to the global poultry industry and critically threatens public health. Chickens are the important host for the transmission. However, the distribution of H5N8 avian influenza virus (AIV) in chicken and the infected cell types are limitedly studied. Therefore, in this study, we detected viral replication and infection by generating recombinant H5N8 AIV expressing an easily tracked mApple fluorescent reporter. The results showed that recombinant viruses passaged four times in chicken embryos successfully expressed mApple proteins detected by fluorescence microscopy and WB, which verified that the constructed recombinant viruses were stable. Compared to parental virus, although recombinant virus attenuated for replication in MDCK cells, it can still replicate effectively, and form visible plaques. Importantly, the experiments on infection of chicken PBMCs in vitro showed a strong correlation between mApple positivity rate and NP positivity rate (r = 0.7594, P =0.0176), demonstrating that mApple reporter could be used as an indicator to accurately reflect AIV infection. Then we infected monocytes/macrophages in PBMCs in vitro and detected the mApple positive percentage was 55.1%-80.4%, which confirmed the chicken primary monocytic/macrophages are important target cells for avian influenza virus infection. In chicken, compared with parental virus, the recombinant virus-infected chickens had lower viral titers in oropharyngeal cloacal and organs, but it can cause significant pathogenicity in chicken and the mortality rate was approximately 66%. In addition, the results of bioluminescent imaging showed that the fluorescence in the lungs was strongest at 5 days post-infection (DPI). Finally, we discovered the mApple positive expression in chicken lung immune cells (CD45+ cells), especially some T cells (CD4 and CD8 T cells) also carrying mApple, which indicates that the H5N8 AIV showed a tropism for immune cells including chicken T cells causing potentially aggressive against cellular immunity. We have provided a simple visualization for further exploration of H5N8 AIV infected chicken immune cells, which contributes to further understanding pathogenic mechanism of H5N8 AIV infection in chicken.