G45R on nonstructural protein 1 of influenza A virus contributes to virulence by increasing the expression of proinflammatory cytokines in mice

Emergence of a Novel Reassortant H5N3 Avian Influenza Virus in Korean Mallard Ducks in 2018

INTRODUCTION

The avian influenza (AI) virus causes a highly contagious disease which is common in wild and domestic birds and sporadic in humans. Mutations and genetic reassortments among the 8 negative-sense RNA segments of the viral genome alter its pathogenic potential, demanding well-targeted, active surveillance for infection control.

METHODS

Wild duck fecal samples were collected during the 2018 bird health annual surveillance in South Korea for tracking variations of the AI virus. One low-pathogenic avian influenza H5N3 reassortment virus (A/mallard duck/South Korea/KNU18-91/2018 [H5N3]) was isolated and genomically characterized by phylogenetic and molecular analyses in this study.

RESULTS

It was devoid of polybasic amino acids at the hemagglutinin (HA) cleavage site and exhibited a stalk region without deletion in the neuraminidase (NA) gene and NA inhibitor resistance-linked E/D627K/N and D701N marker mutations in the PB2 gene, suggesting its low-pathogenic AI. It showed a potential of a reassortment where only HA originated from the H5N3 poultry virus of China and other genes were derived from Mongolia. In phylogenetic analysis, HA was different from that of the isolate of H5N3 in Korea, 2015. In addition, this novel virus showed adaptation in Madin-Darby canine kidney cells, with 8.05 ± 0.14 log10 50% tissue culture infectious dose (TCID50) /mL at 36 h postinfection. However, it could not replicate in mice well, showing positive growth at 3 days postinfection (dpi) (2.1 ± 0.13 log10 TCID50/mL) but not at 6 dpi.

CONCLUSIONS

The HA antigenic relationship of A/mallard duck/South Korea/KNU18-91/2018 (H5N3) showed differences toward one of the old low-pathogenic H5N3 viruses in Korea. These results indicated that a novel reassortment low-pathogenic avian influenza H5N3 subtype virus emerged in South Korea in 2018 via novel multiple reassortments with Eurasian viruses, rather than one of old Korean H5N3 strains.

Modulation of Innate Immune Responses by the Influenza A NS1 and PA-X Proteins

Influenza A viruses (IAV) can infect a broad range of animal hosts, including humans. In humans, IAV causes seasonal annual epidemics and occasional pandemics, representing a serious public health and economic problem, which is most effectively prevented through vaccination. The defense mechanisms that the host innate immune system provides restrict IAV replication and infection. Consequently, to successfully replicate in interferon (IFN)-competent systems, IAV has to counteract host antiviral activities, mainly the production of IFN and the activities of IFN-induced host proteins that inhibit virus replication. The IAV multifunctional proteins PA-X and NS1 are virulence factors that modulate the innate immune response and virus pathogenicity. Notably, these two viral proteins have synergistic effects in the inhibition of host protein synthesis in infected cells, although using different mechanisms of action. Moreover, the control of innate immune responses by the IAV NS1 and PA-X proteins is subject to a balance that can determine virus pathogenesis and fitness, and recent evidence shows co-evolution of these proteins in seasonal viruses, indicating that they should be monitored for enhanced virulence. Importantly, inhibition of host gene expression by the influenza NS1 and/or PA-X proteins could be explored to develop improved live-attenuated influenza vaccines (LAIV) by modulating the ability of the virus to counteract antiviral host responses. Likewise, both viral proteins represent a reasonable target for the development of new antivirals for the control of IAV infections. In this review, we summarize the role of IAV NS1 and PA-X in controlling the antiviral response during viral infection.

Identification of two residues within the NS1 of H7N9 influenza A virus that critically affect the protein stability and function

The emerging avian-origin H7N9 influenza A virus, which causes mild to lethal human respiratory disease, continues to circulate in China, posing a great threat to public health. Influenza NS1 protein plays a key role in counteracting host innate immune responses, allowing the virus to efficiently replicate in the host. In this study, we compared NS1 amino acid sequences of H7N9 influenza A virus with those of other strains, and determined NS1 protein variability within the H7N9 virus and then evaluated the impact of amino acid substitutions on ability of the NS1 proteins to inhibit host innate immunity. Interestingly, the amino acid residue S212 was identified to have a profound effect on the primary function of NS1, since S212P substitution disabled H7N9 NS1 in suppressing the host RIG-I-dependent interferon response, as well as the ability to promote the virus replication. In addition, we identified another amino acid residue, I178, serving as a key site to keep NS1 protein high steady-state levels. When the isoleucine was replaced by valine at 178 site (I178V mutation), NS1 of H7N9 underwent rapid degradation through proteasome pathway. Furthermore, we observed that P212S and V178I mutation in NS1 of PR8 virus enhanced virulence and promoted the virus replication in vivo. Together, these results indicate that residues I178 and S212 within H7N9 NS1 protein are critical for stability and functioning of the NS1 protein respectively, and may contribute to the enhanced pathogenicity of H7N9 influenza virus.