The 2009 Influenza Pandemic: Promising Lessons For Antiviral Therapy For Future Outbreaks

Insights from Avian Influenza: A Review of Its Multifaceted Nature and Future Pandemic Preparedness

Avian influenza viruses (AIVs) have posed a significant pandemic threat since their discovery. This review mainly focuses on the epidemiology, virology, pathogenesis, and treatments of avian influenza viruses. We delve into the global spread, past pandemics, clinical symptoms, severity, and immune response related to AIVs. The review also discusses various control measures, including antiviral drugs, vaccines, and potential future directions in influenza treatment and prevention. Lastly, by summarizing the insights from previous pandemic control, this review aims to direct effective strategies for managing future influenza pandemics.

Identification, design and synthesis of novel pyrazolopyridine influenza virus nonstructural protein 1 antagonists

Nonstructural protein 1 (NS1) plays a crucial function in the replication, spread, and pathogenesis of influenza virus by inhibiting the host innate immune response. Here we report the discovery and optimization of novel pyrazolopyridine NS1 antagonists that can potently inhibit influenza A/PR/8/34 replication in MDCK cells, rescue MDCK cells from cytopathic effects of seasonal influenza A strains, reverse NS1-dependent inhibition of IFN-β gene expression, and suppress the slow growth phenotype in NS1-expressing yeast. These pyrazolopyridines will enable researchers to investigate NS1 function during infection and how antagonists can be utilized in the next generation of treatments for influenza infection.

A DNA Vaccine That Targets Hemagglutinin to Antigen-Presenting Cells Protects Mice against H7 Influenza

Zoonotic influenza H7 viral infections have a case fatality rate of about 40%. Currently, no or limited human to human spread has occurred, but we may be facing a severe pandemic threat if the virus acquires the ability to transmit between humans. Novel vaccines that can be rapidly produced for global distribution are urgently needed, and DNA vaccines may be the only type of vaccine that allows for the speed necessary to quench an emerging pandemic. Here, we constructed DNA vaccines encoding the hemagglutinin (HA) from influenza A/chicken/Italy/13474/99 (H7N1). In order to increase the efficacy of DNA vaccination, HA was targeted to either major histocompatibility complex class II molecules or chemokine receptors 1, 3, and 5 (CCR1/3/5) that are expressed on antigen-presenting cells (APC). A single DNA vaccination with APC-targeted HA significantly increased antibody levels in sera compared to nontargeted control vaccines. The antibodies were confirmed neutralizing in an H7 pseudotype-based neutralization assay. Furthermore, the APC-targeted vaccines increased the levels of antigen-specific cytotoxic T cells, and a single DNA vaccination could confer protection against a lethal challenge with influenza A/turkey/Italy/3889/1999 (H7N1) in mice. In conclusion, we have developed a vaccine that rapidly could contribute protection against a pandemic threat from avian influenza.

IMPORTANCE Highly pathogenic avian influenza H7 constitute a pandemic threat that can cause severe illness and death in infected individuals. Vaccination is the main method of prophylaxis against influenza, but current vaccine strategies fall short in a pandemic situation due to a prolonged production time and insufficient production capabilities. In contrast, a DNA vaccine can be rapidly produced and deployed to prevent the potential escalation of a highly pathogenic influenza pandemic. We here demonstrate that a single DNA delivery of hemagglutinin from an H7 influenza could mediate full protection against a lethal challenge with H7N1 influenza in mice. Vaccine efficacy was contingent on targeting of the secreted vaccine protein to antigen-presenting cells.

The novel influenza A virus protein PA-X and its naturally deleted variant show different enzymatic properties in comparison to the viral endonuclease PA

The PA protein of Influenza A virus (IAV) encoded by segment 3 acts as a specialized RNA endonuclease in the transcription of the viral genome. The same genomic segment encodes for a second shorter protein, termed PA-X, with the first 191 N-terminal aminoacids (aa) identical to PA, but with a completely different C-ter domain of 61 aa, due to a ribosomal frameshifting. In addition, it has been shown that several IAV isolates encode for a naturally truncated PA-X variant, PAXΔC20, missing the last 20 aa. The biochemical properties of PA-X and PAXΔC20 have been poorly investigated so far. Here, we have carried out an enzymatic characterization of PA-X and its naturally deleted form, in comparison with PA from the human IAV strain A/WSN/33 (H1N1). Our results showed, to the best of our knowledge for the first time, that PA-X possesses an endonucleolytic activity. Both PA and PA-X preferentially cut single stranded RNA regions, but with some differences. In addition, we showed that PAXΔC20 has severely reduced nuclease activity. These results point to a previously undetected role of the last C-ter 20 aa for the catalytic activity of PA-X and support distinct roles for these proteins in the viral life cycle.

Effects of flavonoid-induced oxidative stress on anti-H5N1 influenza a virus activity exerted by baicalein and biochanin A

Background

Different flavonoids are known to interfere with influenza A virus replication. Recently, we showed that the structurally similar flavonoids baicalein and biochanin A inhibit highly pathogenic avian H5N1 influenza A virus replication by different mechanisms in A549 lung cells. Here, we investigated the effects of both compounds on H5N1-induced reactive oxygen species (ROS) formation and the role of ROS formation during H5N1 replication.

Findings

Baicalein and biochanin A enhanced H5N1-induced ROS formation in A549 cells and primary human monocyte-derived macrophages. Suppression of ROS formation induced by baicalein and biochanin A using the antioxidant N-acetyl-L-cysteine strongly increased the anti-H5N1 activity of both compounds in A549 cells but not in macrophages.

Conclusions

These findings emphasise that flavonoids induce complex pharmacological actions some of which may interfere with H5N1 replication while others may support H5N1 replication. A more detailed understanding of these actions and the underlying structure-activity relationships is needed to design agents with optimised anti-H5N1 activity.

A zanamivir dimer with prophylactic and enhanced therapeutic activity against influenza viruses

OBJECTIVES

Emerging drug resistance to antiviral therapies is an increasing challenge for the treatment of influenza virus infections. One new antiviral compound, BTA938, a dimeric derivative of the viral neuraminidase inhibitor zanamivir, contains a 14-carbon linker bridging two zanamivir moieties. In these studies, we evaluated antiviral efficacy in cell cultures infected with influenza virus and in mouse models of lethal influenza using H1N1pdm09, H3N2 and oseltamivir-resistant (H275Y) viruses.

METHODS

In vitro activity was evaluated against 22 strains of influenza virus. Additionally, in vivo studies compared the efficacy of BTA938 or zanamivir after intranasal treatment. We also tested the hypothesis of a dual mode of action for BTA938 using scanning electron microscopy (SEM).

RESULTS

BTA938 inhibited the viruses at nanomolar concentrations in vitro with a median 50% effective concentration value of 0.5 nM. In mouse models, the dimer provided ∼10-fold greater protection than zanamivir. The data also showed that a single low dose (3 mg/kg) protected 100% of mice from an otherwise lethal oseltamivir-resistant (H275Y) influenza virus infection. Remarkably, a single prophylactic treatment (10 mg/kg) administered 7 days before the challenge protected 70% of mice and when administered 1 or 3 days before the challenge it protected 90% of mice. Additionally, SEM provides evidence that the increased antiviral potency may be mediated by an enhanced aggregation of virus on the cell surface.

CONCLUSIONS

In vitro and in vivo experiments showed the high antiviral activity of BTA938 for the treatment of influenza virus infections. Moreover, we demonstrated that a single dose of BTA938 is sufficient for prophylactic and therapeutic protection in mouse models.

DNA vaccine that targets hemagglutinin to MHC class II molecules rapidly induces antibody-mediated protection against influenza

New influenza A viruses with pandemic potential periodically emerge due to viral genomic reassortment. In the face of pandemic threats, production of conventional egg-based vaccines is time consuming and of limited capacity. We have developed in this study a novel DNA vaccine in which viral hemagglutinin (HA) is bivalently targeted to MHC class II (MHC II) molecules on APCs. Following DNA vaccination, transfected cells secreted vaccine proteins that bound MHC II on APCs and initiated adaptive immune responses. A single DNA immunization induced within 8 d protective levels of strain-specific Abs and also cross-reactive T cells. During the Mexican flu pandemic, a targeted DNA vaccine (HA from A/California/07/2009) was generated within 3 wk after the HA sequences were published online. These results suggest that MHC II-targeted DNA vaccines could play a role in situations of pandemic threats. The vaccine principle should be extendable to other infectious diseases.

Pseudovirus-based neuraminidase inhibition assays reveal potential H5N1 drug-resistant mutations

The use of antiviral drugs such as influenza neuraminidase (NA) inhibitors is a critical strategy to prevent and control flu pandemic, but this strategy faces the challenge of emerging drug-resistant strains. For a highly pathogenic avian influenza (HPAI) H5N1 virus, biosafety restrictions have significantly limited the efforts to monitor its drug responses and mechanisms involved. In this study, a rapid and biosafe assay based on NA pseudovirus was developed to study the resistance of HPAI H5N1 virus to NA inhibitor drugs. The H5N1 NA pseudovirus was comprehensively tested using oseltamivir-sensitive strains and their resistant mutants. Results were consistent with those in previous studies, in which live H5N1 viruses were used. Several oseltamivir-resistant mutations reported in human H1N1 were also identified to cause decreased oseltamivir sensitivity in H5N1 NA by using the H5N1 NA pseudovirus. Thus, H5N1 NA pseudoviruses could be used to monitor HPAI H5N1 drug resistance rapidly and safely.

The cardiovascular manifestations of influenza: a systematic review

Influenza accounts for 3 to 5 million cases of severe illness and up to 300,000 deaths annually, presenting a considerable burden to healthcare services. A spectrum of cardiovascular complications has been reported in association with influenza infection. This can occur through direct effects of the virus on the myocardium or through exacerbation of existing cardiovascular disease. Direct myocardial involvement presenting as myocarditis is not uncommon during influenza infection. Clinical presentation may vary from asymptomatic to fulminant myocarditis resulting in cardiogenic shock and death. Cardiovascular mortality is also increased during influenza epidemics in patients with pre-existing coronary artery disease. Rates of myocardial infarction have been shown to increase following influenza outbreaks, whilst decreases in cardiovascular mortality have been demonstrated following influenza vaccination in high risk patients. The purpose of this review is to provide an overview of cardiovascular complications, their presentation, clinical course and the management options available following influenza infection.

Identification of host cell factors involved in influenza A virus infection

As obligatory intracellular parasites, viruses need to take control of the metabolic pathways of the infected cells in order to complete their replication. Such an extraordinary ability must rely on specific, essential protein–protein interactions with key components of the cellular machinery. Besides providing valuable information about host–virus relationships, these studies can lead to the identification of novel pharmacological targets for an antiviral chemotherapeutic approach, based on the inhibition of host factors essential for viral replication. Here, we will review the most recent studies identifying host cell proteins involved in the influenza virus lifecycle.

Differential antiviral and anti-inflammatory mechanisms of the flavonoids biochanin A and baicalein in H5N1 influenza A virus-infected cells

From a panel of 22 flavonoids, we identified six compounds (apigenin, baicalein, biochanin A, kaempferol, luteolin, naringenin) that inhibited influenza A nucleoprotein production in human lung epithelial (A549) cells infected with the highly pathogenic avian influenza H5N1 virus strain A/Thailand/Kan-1/04 in non-toxic concentrations. Baicalein (IC(50): 18.79±1.17μM, selectivity index 5.82) and biochanin A (IC(50) 8.92±1.87μM, selectivity index 5.60) were selected for further experiments. Both compounds reduced H5N1 infectious titres (baicalein 40μM: 29-fold reduction, biochanin A 40μM: 55-fold reduction after infection at MOI 0.01), virus-induced caspase 3 cleavage, nuclear export of viral RNP complexes, and enhanced the effects of the neuraminidase inhibitor zanamivir. Biochanin A and baicalein also inhibited the replication of the H5N1 strain A/Vietnam/1203/04. Time of addition experiments indicated that both compounds interfere with H5N1 replication after the adsorption period. Further mechanistic investigations revealed clear differences between these two flavonoids. Only baicalein interfered with the viral neuraminidase activity (39±7% inhibition at 100μM, the maximum concentration tested). In contrast to baicalein, biochanin A affected cellular signalling pathways resulting in reduced virus-induced activation of AKT, ERK 1/2, and NF-kB. Moreover, biochanin A inhibited the virus-induced production of IL-6, IL-8, and IP-10 while baicalein inhibited IL-6 and IL-8 production without affecting IP-10 levels. In primary human monocyte-derived macrophages, only baicalein but not biochanin A impaired H5N1 virus replication. Both flavonoids interfered with the H5N1-induced production of IL-6, IP-10, and TNF-α but not of IL-8 in macrophages. These findings indicate that closely related flavonoids can exert anti-H5N1 effects by different molecular mechanisms.

Structural analysis of specific metal chelating inhibitor binding to the endonuclease domain of influenza pH1N1 (2009) polymerase

It is generally recognised that novel antiviral drugs, less prone to resistance, would be a desirable alternative to current drug options in order to be able to treat potentially serious influenza infections. The viral polymerase, which performs transcription and replication of the RNA genome, is an attractive target for antiviral drugs since potent polymerase inhibitors could directly stop viral replication at an early stage. Recent structural studies on functional domains of the heterotrimeric polymerase, which comprises subunits PA, PB1 and PB2, open the way to a structure based approach to optimise inhibitors of viral replication. In particular, the unique cap-snatching mechanism of viral transcription can be inhibited by targeting either the PB2 cap-binding or PA endonuclease domains. Here we describe high resolution X-ray co-crystal structures of the 2009 pandemic H1N1 (pH1N1) PA endonuclease domain with a series of specific inhibitors, including four diketo compounds and a green tea catechin, all of which chelate the two critical manganese ions in the active site of the enzyme. Comparison of the binding mode of the different compounds and that of a mononucleotide phosphate highlights, firstly, how different substituent groups on the basic metal binding scaffold can be orientated to bind in distinct sub-pockets within the active site cavity, and secondly, the plasticity of certain structural elements of the active site cavity, which result in induced fit binding. These results will be important in optimising the design of more potent inhibitors targeting the cap-snatching endonuclease activity of influenza virus polymerase.

The 2009 influenza pandemic, the on-going potential threat of highly pathogenic H5N1 avian strains and the widespread occurrence of resistance to current anti-influenza drugs targeting the neuraminidase or the M2 ion channel, all highlight the need for alternative therapeutic options to treat serious influenza infections in the absence of protection by vaccination. The viral polymerase, which performs transcription and replication of the RNA genome, is an attractive target for novel antiviral drugs since potent polymerase inhibitors will directly stall replication. The heterotrimeric polymerase performs transcription by a unique cap-snatching mechanism, which involves host pre-mRNA cap-binding and endonucleolytic cleavage by the PB2 and PA subunits respectively. Crystal structures of both the PB2 cap-binding and PA nuclease domains are now available allowing structure-guided optimisation of cap-snatching inhibitors. Here we present a series of co-crystal structures of the 2009 pandemic H1N1 PA endonuclease domain that reveal the binding mode of several known endonuclease inhibitors. All inhibitors chelate the two manganese ions in the active site of the nuclease but different extensions to the metal binding scaffold bind in distinct sub-pockets of the active site cavity. These results highlight the value of structure-based approaches to the development of more potent influenza polymerase inhibitors.