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

Research progress on the nonstructural protein 1 (NS1) of influenza a virus

Influenza A virus (IAV) is the leading cause of highly contagious respiratory infections, which poses a serious threat to public health. The non-structural protein 1 (NS1) is encoded by segment 8 of IAV genome and is expressed in high levels in host cells upon IAV infection. It is the determinant of virulence and has multiple functions by targeting type Ι interferon (IFN-I) and type III interferon (IFN-III) production, disrupting cell apoptosis and autophagy in IAV-infected cells, and regulating the host fitness of influenza viruses. This review will summarize the current research on the NS1 including the structure and related biological functions of the NS1 as well as the interaction between the NS1 and host cells. It is hoped that this will provide some scientific basis for the prevention and control of the influenza virus.

PA-X protein of H9N2 subtype avian influenza virus suppresses the innate immunity of chicken bone marrow-derived dendritic cells

H9N2 subtype avian influenza (AI) is an infectious disease associated with immunosuppression in poultry. Here, the regulation function of PA-X protein was determined on the host innate immune response of H9N2-infected chicken bone marrow-derived DCs (chBM-DCs). Based on 2 mutated viruses expressing PA-X protein (rTX) or deficient PA-X protein (rTX-FS), and the established culture system of chBM-DCs, results showed PA-X protein inhibited viral replication in chBM-DCs but not in non-immune chicken cells (DF-1). Moreover, PA-X protein downregulated the expression of phenotypic markers (CD40, CD86, and MHCII) and proinflammatory cytokine (IL-12 and IL-1β) of chBM-DCs. The mixed lymphocyte reaction between chBM-DCs and chicken T cells showed PA-X protein significantly decreased H9N2-infected chBM-DCs to induce T cell proliferation, implying a suppression of the DC-induced downstream T cell response. Taken together, these findings indicated that PA-X protein is a key viral protein to help H9N2 subtype AIVs escape the innate immunity of chBM-DCs.

H5N8 Subtype avian influenza virus isolated from migratory birds emerging in Eastern China possessed a high pathogenicity in mammals

Novel H5N8 highly pathogenic avian influenza viruses (HPAIVs) bearing the clade 2.3.4.4b HA gene have been widely spread through wild migratory birds since 2020. One H5N8 HPAIV (A/Wild bird/Cixi/Cixi02/2020; here after Cixi02) was isolated from migratory birds in Zhejiang Province, Eastern China in 25 November 2020. However, its pathogenicity in avian and mammal remains unknown. Hemagglutinin gene genetic analysis indicated that Cixi02 virus belonged to the branch II of H5 clade 2.3.4.4b originated from Iraq in May 2020. Cixi02 virus showed a binding affinity to both SA α-2, 3-galactose (Gal) and SA α-2, 6 Gal receptors, good pH stability, thermostability, and replication ability in both avian and mammal cells. The poultry pathogenicity indicated that Cixi02 virus was lethal to chickens. Moreover, the mammalian pathogenicity showed that the 50% mouse lethal dose (MLD50 ) is 2.14 lgEID50 /50 μl, indicating a high pathogenicity in mice. Meanwhile, Cixi02 virus was widely detected in multiple organs, including heart, liver, spleen, lung, kidney, turbinate, and brain after nasal infection. In addition, we found high level gene expressions of TNF-α, IL-12p70, CXCL10, and IFN-α in lungs, IL-8 and IL-1β in brains, and observed severe histopathological change in lungs and brains. Collectedly, this study provided new insights on the pathogenic and zoonotic features of an H5N8 subtype AIV isolated from migratory birds.

PA-X Protein of H1N1 Subtype Influenza Virus Disables the Nasal Mucosal Dendritic Cells for Strengthening Virulence

PA-X protein arises from a ribosomal frameshift in the PA of influenza A virus (IAV). However, the immune regulatory effect of the PA-X protein of H1N1 viruses on the nasal mucosal system remains unclear. Here, a PA-X deficient H1N1 rPR8 viral strain (rPR8-△PAX) was generated and its pathogenicity was determined. The results showed that PA-X was a pro-virulence factor in mice. Furthermore, it reduced the ability of H1N1 viruses to infect dendritic cells (DCs), the regulator of the mucosal immune system, but not non-immune cells (DF-1 and Calu-3). Following intranasal infection of mice, CCL20, a chemokine that monitors the recruitment of submucosal DCs, was downregulated by PA-X, resulting in an inhibition of the recruitment of CD11b+ DCs to submucosa. It also attenuated the migration of CCR7+ DCs to cervical lymph nodes and inhibited DC maturation with low MHC II and CD40 expression. Moreover, PA-X suppressed the maturation of phenotypic markers (CD80, CD86, CD40, and MHC II) and the levels of secreted pro-inflammatory cytokines (IL-1β, IL-6, and TNF-α) while enhancing endocytosis and levels of anti-inflammatory IL-10 in vitro, suggesting an impaired maturation of DCs that the key step for the activation of downstream immune responses. These findings suggested the PA-X protein played a critical role in escaping the immune response of nasal mucosal DCs for increasing the virulence of H1N1 viruses.

PA-X protein assists H9N2 subtype avian influenza virus in escaping immune response of mucosal dendritic cells

H9N2 subtype low pathogenicity avian influenza virus (AIV) poses a potential zoonotic risk. PA-X, a novel protein generated by PA gene ribosomal frameshift, is considered to be the virulence factor of H9N2 subtype AIVs. Our study found that rTX possessing PA-X protein enhanced the mammalian pathogenicity of H9N2 subtype AIVs compared with PA-X-deficient virus (rTX-FS). Furthermore, PA-X protein inhibited H9N2 subtype AIVs to infect dendritic cells (DCs), but not nonimmune cells (MDCK cells). Meanwhile, PA-X protein suppressed the phenotypic expression (CD80, CD86, CD40 and MHCII), early activation marker (CD69) and pro-inflammatory cytokines (IL-6 and TNF-α), whereas increased anti-inflammatory cytokine (IL-10) in DCs. After intranasally viral infection in mice, we found that PA-X protein of H9N2 subtype AIVs reduced CD11b+ and CD103+ subtype mucosal DCs recruitment to the nasal submucosa by inhibiting CCL20 expression. Moreover, PA-X protein abolished the migratory ability of CD11b+ and CD103+ DCs into draining cervical lymph nodes by down-regulating CCR7 expression. The rTX-infected DCs significantly impaired the allogeneic CD4+ T cell proliferation, suggesting PA-X protein suppressed the immune functions of DCs for hindering the downstream immune activation. These findings indicated that PA-X protein assisted H9N2 subtype AIVs in escaping immune response of mucosal DCs for enhancing the pathogenicity.

Profiling and analysis of exosomal miRNAs derived from highly pathogenic avian influenza virus H5N1-infected White Leghorn chickens

Exosomes are small cell membrane-derived vesicles; they play important roles as mediators of cell-to-cell communication via delivery of their contents, such as proteins and microRNAs (miRNAs). In particular, exosomal miRNAs regulate the gene expression of recipient cells by inhibiting the expression of target mRNAs. In this study, we investigated the miRNA expression profiles of highly pathogenic avian influenza virus (HPAIV) H5N1-infected White Leghorn chickens and analyzed the functions of their target genes. After 3 d of infection with A/chicken/Vietnam/NA-01/2019 (H5N1), exosomes were isolated from the blood serum of White Leghorn chickens for small RNA sequencing. We accordingly identified 10 differentially expressed miRNAs (DE miRNAs; 5 upregulated and 5 downregulated) by comparing the exosomes derived from infected and noninfected chickens. The target genes of DE miRNAs were predicted using miRDB and TargetScan for Gene Ontology and KEGG pathway enrichment analyses. A majority of the target genes was found to be associated with the MAPK signaling pathway; several immune-related genes were identified as being regulated by these DE miRNAs. Moreover, we predicted DE miRNA binding sites in HPAIV RNA segments using the RNAhybrid algorithm. The findings of this study provide a theoretical basis for gaining insights into the regulatory mechanisms of exosomal miRNAs in response to HPAIV H5N1 infection and the identification of novel vaccine candidates.

Exosomes from H5N1 avian influenza virus-infected chickens regulate antiviral immune responses of chicken immune cells

Exosomes (membrane-derived vesicles) enable intracellular communication by delivering lipids, proteins, DNA, and RNA from one cell to another. Highly pathogenic avian influenza virus (HPAIV) H5N1 causes considerable economic loss in the poultry industry and poses a public health concern. The host innate immune system defends against H5N1 infection by activating antiviral immune responses. This study aimed to demonstrated that immunomodulatory effects of exosomes from HPAIV H5N1-infected White Leghorn chickens on chicken macrophages, fibroblasts, T cell, and B cell lines. The expression of type I interferons (IFN-α and -β) were highly upregulated in immune-related cell lines after treatment with exosomes derived from H5N1-infected chickens. Levels of pro-inflammatory cytokines, such as IFN-γ, IL-1β, and CXCL8, were also elevated by the exosomes. The mitogen-activated protein kinase (MAPK) signaling pathway was stimulated in immune-related cells by such exosomes via phosphorylation of extracellular regulated kinases 1/2 and p38 signaling molecules. Furthermore, the H5N1 viral proteins, nucleoprotein (NP) and non-structural protein (NS1), were packaged in exosomes and successfully transferred to non-infected immune-related cells. Therefore, exosomes from H5N1-infected chickens induced pro-inflammatory cytokine expression and stimulated the MAPK signaling pathway by delivering key viral proteins. These findings would aid better understanding of the mechanism underlying the modulation of antiviral immune responses of host immune-related cells by viral-protein-carrying exosomes and support their further application as a novel exosome-based H5N1 AIV vaccine platform.

139D in NS1 Contributes to the Virulence of H5N6 Influenza Virus in Mice

H5N6, the highly pathogenic avian influenza A virus (IAV) of clade 2.3.4.4, causes global outbreaks in poultry. H5N6 has become the dominant IAV subtype in waterfowls and causes human infections with high mortality rates. Here, we isolated two strains of H5N6, XGD and JX, from chickens and ducks, respectively. Growth kinetics were evaluated in duck embryo fibroblasts, chicken embryo fibroblasts, Madin-Darby canine kidney cells, and A549 lung carcinoma cells. Receptor binding specificity was analyzed via sialic acid–binding activity assay. The virulence of each strain was tested in BALB/c mice, and recombinant viruses were constructed via reverse genetics to further analyze the pathogenicity. The two strains showed no significant differences in growth kinetics in vitro; however, JX was more virulent in mice than XGD. We also identified 13 mutations in six viral proteins of the two strains through genetic analysis. Our study showed that the NS1 protein played a crucial role in enhancing the virulence of JX. Specifically, the amino acid 139D in NS1 contributed to the high pathogenicity. Therefore, 139D in NS1 might provide insight into the underlying mechanism of IAV adaptation in mammals.

Emergence and Characterization of a Novel Reassortant Canine Influenza Virus Isolated from Cats

Cats are susceptible to a wide range of influenza A viruses (IAV). Furthermore, cats can serve as an intermediate host, and transfer avian influenza virus (AIV) H7N2 to a veterinarian. In this report, a novel reassortant influenza virus, designated A/feline/Jiangsu/HWT/2017 (H3N2), and abbreviated as FIV-HWT-2017, was isolated from nasal swab of a symptomatic cat in Jiangsu province, China. Sequence analysis indicated that, whilst the other seven genes were most similar to the avian-origin canine influenza viruses (CIV H3N2) isolated in China, the NS gene was more closely related to the circulating human influenza virus (H3N2) in the region. Therefore, FIV-HWT-2017 is a reassortant virus. In addition, some mutations were identified, and they were similar to a distinctive CIV H3N2 clade. Whether these cats were infected with the reassortant virus was unknown, however, this random isolation of a reassortant virus indicated that domestic or stray cats were "mixing vessel" for IAV cannot be ruled out. An enhanced surveillance for novel influenza virus should include pet and stray cats.

NS1: A Key Protein in the "Game" Between Influenza A Virus and Host in Innate Immunity

Since the influenza pandemic occurred in 1918, people have recognized the perniciousness of this virus. It can cause mild to severe infections in animals and humans worldwide, with extremely high morbidity and mortality. Since the first day of human discovery of it, the “game” between the influenza virus and the host has never stopped. NS1 protein is the key protein of the influenza virus against host innate immunity. The interaction between viruses and organisms is a complex and dynamic process, in which they restrict each other, but retain their own advantages. In this review, we start by introducing the structure and biological characteristics of NS1, and then investigate the factors that affect pathogenicity of influenza which determined by NS1. In order to uncover the importance of NS1, we analyze the interaction of NS1 protein with interferon system in innate immunity and the molecular mechanism of host antagonism to NS1 protein, highlight the unique biological function of NS1 protein in cell cycle.