Progress in identifying virulence determinants of the 1918 H1N1 and the Southeast Asian H5N1 influenza A viruses

Abstract

The 1918 pandemic H1N1 influenza virus and the recently emerged Southeast Asian H5N1 avian influenza virus are unique among influenza A virus isolates in their high virulence for humans and their lethality for a variety of animal species without prior adaptation. Reverse genetic studies have implicated several viral genes as virulence determinants. For both the 1918 and H5N1 viruses, the hemagglutinin and the polymerase complex contribute to high virulence. Non-structural proteins NS1 and PB1-F2, which block host antiviral responses, also influence pathogenesis. Additionally, recent studies correlate high levels of viral replication and induction of strong proinflammatory responses with the high virulence of these viruses. Defining how individual viral proteins promote enhanced replication, inflammation and severe disease will provide insight into the pathogenesis of severe influenza virus infections and suggest novel therapeutic approaches.

Figure 1. The replication cycle of influenza A viruses

1. Binding. Virion HA binds to sialic acid linked to cell-surface glycoproteins or glycolipids. 2. Fusion and uncoating. The virion is endocytosed. Endosome acidification triggers the fusion of the viral and endosomal membranes, releasing viral genetic contents. For HA to act as a fusion protein, it must have been cleaved at a specific site by host proteases. Virus associated M2 ion channel, permits the acidification of the interior of the virus allowing the dissociation of the viral matrix protein, M1, from the viral ribonucleoprotein particles (RNPs) that constitute the viral genome. This uncoating step allows nuclear import of viral RNPs and is inhibited by amantadine and rimantadine, which block the M2 channel. 3. RNA migration into nucleus. Release from M1 allows the RNPs to be released into the cytoplasm and to enter the nucleus where viral RNA synthesis occurs. RNP nuclear import occurs through the interaction of NP with karyopherin alpha proteins. 4. Transcription and genome replication. Within the nucleus, the heterotrimeric viral polymerase complex composed of the PA, PB1 and PB2 proteins and the viral nucleoprotein (NP) are required for viral RNA synthesis. 5. Translation. Viral mRNAs are exported to the cytoplasm and are translated into protein. 6. Assembly and Exit. Newly synthesized viral genomes, in the form of RNPs, are exported to the cytoplasm; this requires the viral nuclear export protein (NEP) and the M1 matrix protein. Assembly occurs at the plasma membrane in association with lipid rafts. HA, NA and M2 reach the plasma membrane via the Golgi apparatus. There, the M1 protein and viral RNPs are incorporated into budding particles. HA directs viral budding (Chen et al., 2007a). Incorporation of new viral genomic RNAs with budding particles appears to be mediated by cis-acting packaging signals present within each viral RNA segment. 7. Release. Efficient release of new virus particles requires that NA remove sialic acids from glycoproteins and glycolipids on the cell surface or on adjacent virions to prevent aggregation due to HA binding to sialic acids. NA inhibitors, such as the FDA-approved Tamiflu® and Relenza®, inhibit NA activity and impair influenza virus release. Adapted from (Beigel and Bray, 2008).

Figure 2. Virulence determinants in mice of the 1918 and H5N1 avian influenza viruses

The genome of an influenza virus is depicted (center). Gene products implicated as virulence determinants for the 1918 virus (left) and H5N1 viruses (right) are indicated. The gene segment encoding each virulence factor is indicated by an arrow. See text for details.

Figure 3. Lethality in mice of 7:1 1918:Tx/91 reassortant viruses

Figure shows the genetic composition of the parental Tx/91 and 1918 viruses as well as selected 7:1 1918:Tx/91 reassortant viruses and their log₁₀ LD₅₀ by intranasal inoculation in 6–8 week old BABL/c mice (Pappas et al., 2008). All possible 7:1 viruses were tested; only those that differed significantly from the parental 1918 virus are shown.

Figure 4. Lethality in mice of 1:7 1918:Tx/91 reassortant viruses

Figure shows the genetic composition of the parental Tx/91 and 1918 viruses as well as selected 1:7 1918:Tx/91 reassortant viruses and their log₁₀ LD₅₀ by intranasal inoculation in 6–8 week old BABL/c mice (Pappas et al., 2008). All possible 1:7 viruses were tested; only those that differed significantly from the parental Tx/91 virus are shown.