The Nucleoprotein of H7N9 Influenza Virus Positively Regulates TRAF3-Mediated Innate Signaling and Attenuates Viral Virulence in Mice

Host Innate Antiviral Response to Influenza A Virus Infection: From Viral Sensing to Antagonism and Escape

Influenza virus possesses an RNA genome of single-stranded, negative-sensed, and segmented configuration. Influenza virus causes an acute respiratory disease, commonly known as the "flu" in humans. In some individuals, flu can lead to pneumonia and acute respiratory distress syndrome. Influenza A virus (IAV) is the most significant because it causes recurring seasonal epidemics, occasional pandemics, and zoonotic outbreaks in human populations, globally. The host innate immune response to IAV infection plays a critical role in sensing, preventing, and clearing the infection as well as in flu disease pathology. Host cells sense IAV infection through multiple receptors and mechanisms, which culminate in the induction of a concerted innate antiviral response and the creation of an antiviral state, which inhibits and clears the infection from host cells. However, IAV antagonizes and escapes many steps of the innate antiviral response by different mechanisms. Herein, we review those host and viral mechanisms. This review covers most aspects of the host innate immune response, i.e., (1) the sensing of incoming virus particles, (2) the activation of downstream innate antiviral signaling pathways, (3) the expression of interferon-stimulated genes, (4) and viral antagonism and escape.

Nucleoprotein as a Promising Antigen for Broadly Protective Influenza Vaccines

Annual vaccination is considered as the main preventive strategy against seasonal influenza. Due to the highly variable nature of major viral antigens, such as hemagglutinin (HA) and neuraminidase (NA), influenza vaccine strains should be regularly updated to antigenically match the circulating viruses. The influenza virus nucleoprotein (NP) is much more conserved than HA and NA, and thus seems to be a promising target for the design of improved influenza vaccines with broad cross-reactivity against antigenically diverse influenza viruses. Traditional subunit or recombinant protein influenza vaccines do not contain the NP antigen, whereas live-attenuated influenza vaccines (LAIVs) express the viral NP within infected cells, thus inducing strong NP-specific antibodies and T-cell responses. Many strategies have been explored to design broadly protective NP-based vaccines, mostly targeted at the T-cell mode of immunity. Although the NP is highly conserved, it still undergoes slow evolutionary changes due to selective immune pressure, meaning that the particular NP antigen selected for vaccine design may have a significant impact on the overall immunogenicity and efficacy of the vaccine candidate. In this review, we summarize existing data on the conservation of the influenza A viral nucleoprotein and review the results of preclinical and clinical trials of NP-targeting influenza vaccine prototypes, focusing on the ability of NP-specific immune responses to protect against diverse influenza viruses.

The nucleoprotein of influenza A virus inhibits the innate immune response by inducing mitophagy

Mitophagy is a form of autophagy that plays a key role in maintaining the homeostasis of functional mitochondria in the cell. Viruses have evolved various strategies to manipulate mitophagy to escape host immune responses and promote virus replication. In this study, the nucleoprotein (NP) of H1N1 virus (PR8 strain) was identified as a regulator of mitophagy. We revealed that NP-mediated mitophagy leads to the degradation of the mitochondria-anchored protein MAVS, thereby blocking MAVS-mediated antiviral signaling and promoting virus replication. The NP-mediated mitophagy is dependent on the interaction of NP with MAVS and the cargo receptor TOLLIP. Moreover, Y313 of NP is a key residue for the MAVS-NP interaction and NP-mediated mitophagy. The NPY313F mutation significantly attenuates the virus-induced mitophagy and the virus replication in vitro and in vivo. Taken together, our findings uncover a novel mechanism by which the NP of influenza virus induces mitophagy to attenuate innate immunity.Abbreviations: ACTB: actin beta; ATG7: autophagy related 7; ATG12: autophagy related 12; CCCP: carbonyl cyanide 3-chlorophenyl hydrazone; co-IP: co-immunoprecipitation; COX4/COXIV: cytochrome c oxidase subunit 4; DAPI: 4',6-diamidino-2-phenylindole, dihydrochloride; EID50: 50% egg infective dose; GAPDH: glyceraldehyde-3-phosphate dehydrogenase; GFP: green fluorescent protein; HEK: human embryonic kidney; hpi: hours post-infection; IAV: influenza A virus; IFN: interferon; IP: immunoprecipitation; LAMP1: lysosomal associated membrane protein 1; MAP1LC3/LC3: microtubule associated protein 1 light chain 3; MAVS: mitochondrial antiviral signaling protein; Mdivi-1: mitochondrial division inhibitor 1; MLD50: 50% mouse lethal dose; MOI: multiplicity of infection; NBR1: NBR1 autophagy cargo receptor; NP: nucleoprotein; PB1: basic polymerase 1; RFP: red fluorescent protein; RIGI: RNA sensor RIG-I; RIGI-N: RIGI-CARD; SeV: Sendai virus; SQSTM1/p62: sequestosome 1; TIMM23: translocase of inner mitochondrial membrane 23; TOLLIP: toll interacting protein; TOMM20: translocase of outer mitochondrial membrane 20; TUBA: tubulin alpha; Vec: empty vector; vRNP: viral ribonucleoprotein.

H1N1 Influenza A Virus Protein NS2 Inhibits Innate Immune Response by Targeting IRF7

Influenza A virus (IAV) is a globally distributed zoonotic pathogen and causes a highly infectious respiratory disease with high morbidity and mortality in humans and animals. IAV has evolved various strategies to counteract the innate immune response, using different viral proteins. However, the mechanisms are not fully elucidated. In this study, we demonstrated that the nonstructural protein 2 (NS2) of H1N1 IAV negatively regulate the induction of type-I interferon. Co-immunoprecipitation experiments revealed that NS2 specifically interacts with interferon regulatory factor 7 (IRF7). NS2 blocks the nuclear translocation of IRF7 by inhibiting the formation of IRF7 dimers, thereby prevents the activation of IRF7 and inhibits the production of interferon-beta. Taken together, these findings revealed a novel mechanism by which the NS2 of H1N1 IAV inhibits IRF7-mediated type-I interferon production.

Chicken miR-126-5p negatively regulates antiviral innate immunity by targeting TRAF3

Innate immunity plays an essential role in preventing the invasion of pathogenic microorganisms. However, innate immunity is a double-edged sword, whose excessive activation is detrimental to immune homeostasis and even leads to a “cytokine storm” of the infected host. The host develops a series of negative regulatory mechanisms to balance the immune response. Here, we report a negative regulatory mechanism of chicken innate immunity mediated by miRNA. In the GEO database, we found that miR-126-5p was markedly up-regulated in chickens infected by RNA viruses. Upregulation of miR-126-5p by RNA virus was then further shown via both a cell model and in vivo tests. Overexpression of miR-126-5p significantly inhibited the expression of interferon and inflammatory cytokine-related genes induced by RNA viruses. The opposite result was achieved after the knockdown of miR-126-5p expression. Bioinformatics analysis identified TRAF3 as candidate target gene of miR-126-5p. Experimentally, miR-126-5p can target TRAF3, as shown by the effects of miR-126-5p on the endogenous expression of TRAF3, and by the TRAF3 3'UTR driven luciferase reporter assay. Furthermore, we demonstrated that miR-126-5p negatively regulated innate immunity by blocking the MAVS-TRAF3-TBK1 axis, with a co-expression assay. Overall, our results suggest that miR-126-5p is involved in the negative regulation of chicken innate immunity, which might contribute to maintaining immune balance.

Specific Monoclonal Antibodies Targeting Unique HA Epitopes Block H7N9 Influenza A Viral Replication

The H7N9 subtype influenza A viruses pose a serious threat to public health, and there is still a lack of vaccines or drugs for humans against H7N9 influenza viruses. In this study, we screened two monoclonal antibodies (MAbs), 4H1E8 and 7H9A6, that specifically recognize the hemagglutinin (HA) protein of H7N9 influenza virus and display highly neutralizing activity against H7N9 virus. The epitopes recognized by two MAbs are nearly all conserved within all known H7 subtypes. Characteristic identification showed that two MAbs have high avidity for the HA protein but no hemagglutinin inhibition activity or antibody-dependent cellular cytotoxicity. Mechanistically, the 4H1E8 and 7H9A6 antibodies inhibit the pH-dependent conformational change of HA and block the HA-mediated membrane fusion. More importantly, 4H1E8 and 7H9A6 exhibit promising prophylactic and therapeutic effects against lethal challenge with H7N9 virus. Moreover, 4H1E8- and 7H9A6-treated mice displayed inhibition of pulmonary viral replication and reduced lung lesions after viral challenge. Together, these findings indicate that antibodies 4H1E8 and 7H9A6 recognize unique epitopes in the HA protein and possess the neutralizing activity and protective efficacy against the H7N9 influenza A viruses. IMPORTANCE In 2013, H7N9 influenza viruses appeared in China and other countries resulting in more than 1,500 individual infections or death. There are still limited studies on vaccines or drugs for humans against H7N9 influenza viruses. Alternative approaches against H7N9 virus infection need to be developed. Here, we identified two monoclonal antibodies (4H1E8 and 7H9A6) that possess neutralizing activity by blocking the pH-dependent HA-mediated membrane fusion. Additionally, the two monoclonal antibodies protect mice against the H7N9 virus challenge prophylactically or therapeutically. Therefore, our study demonstrates that 4H1E8 and 7H9A6 could be used for the prevention and treatment of the H7N9 influenza virus, and the conserved epitopes we identified may contribute to the development of a broad H7N9 vaccine and provide insights into unique antiviral approaches.