Influenza A viruses (IAVs) are contagious pathogens responsible for severe respiratory infection in humans and animals worldwide. Upon detection of IAV infection, host immune system aims to defend against and clear the viral infection. Innate immune system is comprised of physical barriers (mucus and collectins), various phagocytic cells, group of cytokines, interferons (IFNs), and IFN-stimulated genes, which provide first line of defense against IAV infection. The adaptive immunity is mediated by B cells and T cells, characterized with antigen-specific memory cells, capturing and neutralizing the pathogen. The humoral immune response functions through hemagglutinin-specific circulating antibodies to neutralize IAV. In addition, antibodies can bind to the surface of infected cells and induce antibody-dependent cell-mediated cytotoxicity or complement activation. Although there are neutralizing antibodies against the virus, cellular immunity also plays a crucial role in the fight against IAVs. On the other hand, IAVs have developed multiple strategies to escape from host immune surveillance for successful replication. In this review, we discuss how immune system, especially innate immune system and critical molecules are involved in the antiviral defense against IAVs. In addition, we highlight how IAVs antagonize different immune responses to achieve a successful infection.

Long noncoding RNAs (lncRNAs) are single-stranded RNA molecules longer than 200 nt that regulate many cellular processes. MicroRNA 155 host gene (MIR155HG) encodes the microRNA (miR)-155 that regulates various signalling pathways of innate and adaptive immune responses against viral infections. MIR155HG also encodes a lncRNA that we call lncRNA-155. Here, we observed that expression of lncRNA-155 was markedly upregulated during influenza A virus (IAV) infection both in vitro (several cell lines) and in vivo (mouse model). Interestingly, robust expression of lncRNA-155 was also induced by infections with several other viruses. Disruption of lncRNA-155 expression in A549 cells diminished the antiviral innate immunity against IAV. Furthermore, knockout of lncRNA-155 in mice significantly increased IAV replication and virulence in the animals. In contrast, overexpression of lncRNA-155 in human cells suppressed IAV replication, suggesting that lncRNA-155 is involved in host antiviral innate immunity induced by IAV infection. Moreover, we found that lncRNA-155 had a profound effect on expression of protein tyrosine phosphatase 1B (PTP1B) during the infection with IAV. Inhibition of PTP1B by lncRNA-155 resulted in higher production of interferon-beta (IFN-β) and several critical interferon-stimulated genes (ISGs). Together, these observations reveal that MIR155HG derived lncRNA-155 can be induced by IAV, which modulates host innate immunity during the virus infection via regulation of PTP1B-mediated interferon response. KEYWORDS influenza A virus, innate immunity, lncRNA, miR-155, MIR155HG, PTP1B List of Abbreviations: 293T, HEK293T/human embryonic kidney cells; A549, human lung epithelial cells; bic, B-cell Integration Cluster; CA/04, A/California/04/2009; DMEM, Dulbecco's modified Eagle's medium; dpi, days post infection; EV, empty vector; FBS, fetal bovine serum; HA, haemagglutination assay; h-lncRNA-155, human lncRNA-155; hpi, hours post infection; IFN, interferon; IFN-β, interferon-beta; IRF3, interferon regulatory factor 3; IRF7, interferon regulatory factor 7; ISGs, interferon-stimulated genes; KO, bic/miR-155 −/− knock out mice; LLC, mouse Lewis lung carcinoma cells; lncRNAs, long noncoding RNAs; NIH/3T3, mouse embryonic fibroblasts; MDA5, antimelanoma differentiation-associated gene 5; MDCK, Madin-Darby canine kidney cells; miR, microRNA; MIR155HG, miR-155 host gene; m-lncRNA-155, bic/mouse lncRNA-155; NP, IAV nucleoprotein; nt, nucleotide; NRAV, negative regulator of antiviral response;PFA, plaque formation assay; PR8, A/Puerto Rico/8/1934; pre-miR-155, precursor miRNA; pri-miRNA, primary-miRNA; PRRs, pattern recognition receptors; PTP1B, tyrosine-protein phosphatase nonreceptor type 1; RAW 264.7, mouse Abelson murine leukaemia virus transformed macrophages; SeV, Sendai virus; SHIP1, Src homology 2-containing inositol phosphatase 1;sh-luc, Sh-luciferase; shRNAs, short hairpin RNAs; siRNA, small interfering RNA; SOCS1, suppressor of cytokine signalling 1; SPF, specific pathogen free; STAT1, signal transducer a...

Non-coding RNAs (ncRNAs) are a new type of regulators that play important roles in various cellular processes, including cell growth, differentiation, survival, and apoptosis. ncRNAs, including small non-coding RNAs (e.g., microRNAs, small interfering RNAs) and long non-coding RNAs (lncRNAs), are pervasively transcribed in human and mammalian cells. Recently, it has been recognized that these ncRNAs are critically implicated in the virus–host interaction as key regulators of transcription or post-transcription during viral infection. Influenza A virus (IAV) is still a major threat to human health. Hundreds of ncRNAs are differentially expressed in response to infection with IAV, such as infection by pandemic H1N1 and highly pathogenic avian strains. There is increasing evidence demonstrating functional involvement of these regulatory microRNAs, vault RNAs (vtRNAs) and lncRNAs in pathogenesis of influenza virus, including a variety of host immune responses. For example, it has been shown that ncRNAs regulate activation of pattern recognition receptor (PRR)-associated signaling and transcription factors (nuclear factor κ-light-chain-enhancer of activated B cells, NF-κB), as well as production of interferons (IFNs) and cytokines, and expression of critical IFN-stimulated genes (ISGs). The vital functions of IAV-regulated ncRNAs either to against defend viral invasion or to promote progeny viron production are summarized in this review. In addition, we also highlight the potentials of ncRNAs as therapeutic targets and diagnostic biomarkers.