Antiviral strategies against influenza virus: towards new therapeutic approaches

Abstract

Influenza viruses are major human pathogens responsible for respiratory diseases affecting millions of people worldwide and characterized by high morbidity and significant mortality. Influenza infections can be controlled by vaccination and antiviral drugs. However, vaccines need annual updating and give limited protection. Only two classes of drugs are currently approved for the treatment of influenza: M2 ion channel blockers and neuraminidase inhibitors. However, they are often associated with limited efficacy and adverse side effects. In addition, the currently available drugs suffer from rapid and extensive emergence of drug resistance. All this highlights the urgent need for developing new antiviral strategies with novel mechanisms of action and with reduced drug resistance potential. Several new classes of antiviral agents targeting viral replication mechanisms or cellular proteins/processes are under development. This review gives an overview of novel strategies targeting the virus and/or the host cell for counteracting influenza virus infection.

Fig. 1

Antiviral strategies targeting viral functions essential for influenza virus (IV) replication. The first step of IV infection is the interaction between viral hemagglutinin (HA) and cellular sialic acid (SA)-containing receptors, resulting in the attachment of the virion to the target cell. Attachment inhibitors, such as monoclonal antibodies (mAb) directed against the globular head of HA, natural and synthetic compounds containing SA, HA-binding peptides, and compounds that recognize glycosylation sites of HA, interfere with this process and block IV infection. After the internalization of the virion by endocytosis and macropinocytosis, HA mediates the fusion of the viral envelope with the endosomal membrane, a pH-dependent process that can be inhibited by fusion inhibitors such as small molecules that inhibit the low pH-induced conformational change of HA (e.g., arbinol) and neutralizing mAbs directed against the stem region of HA. The activity of the viral protonic pump M2 leads to the acidification of the endosome and to viral uncoating, followed by the release of the viral ribonucleoprotein (vRNP) into the cytoplasm. M2 inhibitors such as adamantanes block IAV (but not IBV) replication at this step. After nuclear translocation of the vRNP, the viral genomic segments are transcribed by the viral RNA-dependent RNA polymerase (RdRP) into mRNAs that are then transported into the cytoplasm and translated into viral proteins necessary for viral genome replication, also catalyzed by viral RdRP. Compounds able to interfere with RdRP activities can inhibit both transcription and replication steps. Transcription, replication, correct assembly of vRNPs, and their nuclear export require the activity of viral nucleoprotein (NP), thus molecules targeting NP functions have demonstrated effective anti-influenza activity. After the assembly of new virions in the cytoplasm, they are transported at the cell membrane and then released by budding. The activity of neuraminidase (NA) present on the virion surface is essential for the cleavage of SA molecules from HA and to allow the release of viral particles. NA inhibitors such as zanamivir and oseltamivir block IAV and IBV replication by interfering with this step

Fig. 2

Antiviral strategies involving host factors engaged in IV replication. IV infection can be blocked by inhibitors of transmembrane proteases responsible of HA activation and by Fludase. After the nuclear import of vRNPs, the first step is the transcription of the viral genome into viral mRNA and this process is catalyzed by RdRP with the involvement of cellular proteins such as RNA polymerase II (RNAP-II) and transcription elongation factor pTEF1b. Inhibitors of both RNAP-II and pTEF1b complex are able to interfere with this process and to block viral mRNA synthesis. Viral mRNAs are then exported into the cytoplasm. The soluble lipid mediator PD1 is able to inhibit IV replication by interfering with the nuclear export of viral transcripts mediated by the cellular proteins NFX1 and Nup62. In the cytosol, viral mRNAs are translated into viral proteins required for the viral genome replication, which takes place into the nucleus. At this stage, Hsp90 inhibitors can be effective in blocking the nuclear import of viral proteins. Also, inhibitors of mitochondrial dihydroorotate dehydrogenase (DHODH), an enzyme of the pyrimidine biosynthetic pathway, possess anti-influenza activity. The newly synthesized vRNPs have then to be exported into the cytoplasm to be assembled together with the other viral proteins into the new virions. Antioxidants able to restore the cellular redox potential and intracellular protease inhibitors can interfere with the correct processing and maturation of HA. Finally, compounds that interfere with the proper formation of lipid rafts at the plasma membrane can block IV budding

Fig. 3

Antiviral strategies involving host signaling pathways and antiviral defense mechanisms. IV infection triggers the direct activation of a series of cellular signaling pathways such as some tyrosine kinase receptors (TKR) pathways, pro-inflammatory pathways, and the interferon (IFN)-activated antiviral response. Also the oxidative stress that follows viral infection activates several redox-sensitive cellular pathways, such as NF-κB and MAPKs pathways. Furthermore, COX-2 inhibitors have been shown to inhibit both IV replication and viral-induced inflammation and cytokine production. Antioxidants that interfere with redox-sensitive cellular processes also inhibit viral functions such as vRNPs trafficking and nuclear export, as well as HA maturation, thus blocking viral replication. The antiviral response activated by IV is dependent on IFN cascade activation and initiates with the recognition of viral patterns by the Toll-like receptors TLR-3, -7, and -9 on the cell membrane and by cytoplasmic RIG-I-like receptors (RLRs), which act as sensors of viral invasion. TLRs and RLRs agonists as well as CpG oligonucleotides (ODNs) exhibit anti-influenza activity by stimulating the IFN response. The viral NS1 protein antagonizes this antiviral response, thus antiviral strategies either targeted against this protein or involving the activation of IFN pathway have inhibitory effects on IV replication