Influenza-A Solomon Islands

What is the A/Solomon Island/3/06 (H1N1) influenza strain?

A/Solomon Island/3/06 (H1N1) is a specific influenza A virus strain that was isolated in 2006. It belongs to the H1N1 subtype, which is defined by its hemagglutinin (H1) and neuraminidase (N1) surface proteins. This strain has been extensively studied for its receptor binding properties and has been used as a reference strain for influenza surveillance and vaccine development. The virus was propagated in embryonated chicken eggs, where it achieved high titers (>8.0 log10 PFU/ml), indicating efficient replication in this medium . The strain has been important in understanding influenza virus receptor binding preferences and their implications for virus transmission and pathogenicity.

How did the A/Solomon Island strain circulate in the Pacific region?

The Pacific region experienced significant influenza activity during the period when the A/Solomon Island strain was circulating. Various Pacific Island countries and territories implemented surveillance systems to monitor influenza-like illness (ILI) cases. According to surveillance reports, multiple Pacific islands experienced outbreaks of influenza, with varying impacts across different populations . The pandemic influenza A (H1N1) 2009 outbreak later demonstrated that influenza viruses can spread quickly through the Pacific region despite attempts at border control, which may have delayed but did not prevent transmission . The Solomon Islands specifically implemented ILI and pandemic influenza surveillance, with situation reports being generated to track the progression of infections .

What are the key genetic characteristics of the A/Solomon Island strain?

The A/Solomon Island/3/06 strain has been characterized by specific amino acid variations in its hemagglutinin (HA) protein, particularly at positions 190 and 226, which are critical for receptor binding. These positions exhibited variations during egg propagation of the wild-type virus . Research has demonstrated that these genetic variations significantly impact:

• Receptor binding preference (α2-3SAL vs. α2-6SAL)

• Replication efficiency in different respiratory tract regions

• Viral pathogenicity

• Immunogenicity and antigenicity

These genetic characteristics make this strain particularly valuable for understanding how small genetic changes can alter viral behavior and host interactions .

How do amino acid variations at positions 190 and 226 in the HA protein affect the virus's properties?

Research on the A/Solomon Island/3/06 virus has revealed that specific amino acid variations at positions 190 and 226 in the hemagglutinin (HA) protein have profound effects on multiple viral properties:

• Receptor Binding Specificity: A single amino acid change at residue 226 (from Glutamine to Arginine) completely altered the virus's receptor binding preference from α2-6SAL (human-type) to α2-3SAL (avian-type) .

• Replication Efficiency: Viruses with Gln226 in the HA protein demonstrated efficient replication in ferret lungs, while those with Arg226 lost this ability entirely .

• Pathogenicity: The correlation between viral replication in the lungs and disease symptoms suggests these mutations affect virulence. Ferrets infected with viruses capable of replicating in the lungs showed more severe clinical symptoms .

• Antigenicity and Immunogenicity: Both residues 190 and 226 affected the viral antigenicity (how the immune system recognizes the virus) and immunogenicity (the ability to provoke an immune response) .

These findings highlight how subtle genetic changes can dramatically alter viral behavior and host interactions, with significant implications for understanding viral evolution and vaccine development.

What experimental systems are used to study the A/Solomon Island virus?

Multiple experimental systems have been employed to study the A/Solomon Island/3/06 virus:

• Embryonated Chicken Eggs: Used for virus propagation and determination of viral titers via the 50% egg infectious dose (EID50) assay. Both biologically derived and recombinant viruses replicated efficiently in this system, achieving titers >8.0 log10 PFU/ml .

• Ferret Model: Ferrets serve as a primary animal model for influenza research because:

  • They are naturally susceptible to human influenza viruses

  • They develop symptoms similar to humans (fever, nasal discharge, lethargy)

  • Their respiratory tract has a similar distribution of sialic acid receptors to humans

  In research with A/Solomon Island/3/06, ferrets were inoculated intranasally with 7.0 log10 PFU of virus, and tissues were collected three days post-infection to determine viral titers .

• Tissue Staining: Lectin staining was used to demonstrate that α2-6SAL configuration is predominant in the respiratory tract of ferrets, including trachea, bronchus, and lung alveolus tissues .

• Recombinant Virus Generation: Reverse genetics was used to create recombinant wild-type (rWT) viruses with specific amino acid substitutions to assess their impact on viral properties .

• Receptor Binding Assays: Specialized assays using resialylated chicken red blood cells (cRBCs) were employed to determine receptor binding preferences of different viral variants .

What is the relationship between A/Solomon Island/3/06 receptor binding and its replication efficiency in the respiratory tract?

Research has established a direct relationship between receptor binding preferences and replication efficiency in different parts of the respiratory tract:

• Upper Respiratory Tract Replication: Both biologically derived and recombinant A/Solomon Island/3/06 viruses replicated efficiently in the nasal turbinates of ferrets, with titers of 5.8 and 5.4 log10 EID50/g tissues, respectively .

• Lower Respiratory Tract Replication:

  • Biologically derived wild-type (bWT) SI06 replicated efficiently in ferret lungs (5.5 log10 EID50/g tissue)

  • Recombinant wild-type (rWT) SI06 was not detected in the lungs of infected ferrets

• Correlation with Receptor Binding:

  • Viruses with affinity for α2-6SAL (containing Gln226) replicated efficiently in the lungs

  • Viruses that bound exclusively to α2-3SAL (containing Arg226) failed to replicate in the lungs

This correlation is consistent with lectin staining results showing that ferret respiratory tracts contain predominantly α2-6SAL receptors, explaining why viruses with α2-6SAL binding preference show superior replication in this tissue .

What are the implications of the A/Solomon Island/3/06 studies for vaccine development?

The studies of A/Solomon Island/3/06 have several significant implications for influenza vaccine development:

• Egg Adaptation Concerns: During egg expansion, the HA gene may mutate to allow more efficient replication in eggs. These mutations can alter receptor binding preferences (typically toward α2-3SAL binding) and potentially affect virus antigenicity, which is critical for vaccine efficacy .

• Reference Strain Selection: The research emphasizes the necessity of thoroughly assessing wild-type influenza viruses for their suitability as reference strains during annual influenza vaccine evaluation processes .

• HA Antigen Selection: Careful selection of the HA antigen for vaccine production is essential, as amino acid variations at positions like 190 and 226 affect both antigenicity and immunogenicity .

• Predictive Markers: Understanding how specific amino acid changes affect receptor binding and replication provides potential markers for predicting vaccine strain behavior and effectiveness.

• Animal Model Validation: The ferret model provides valuable insights into how vaccine strains might behave in humans, particularly regarding immunogenicity and protective efficacy .

How are recombinant A/Solomon Island/3/06 viruses constructed for research?

The construction of recombinant A/Solomon Island/3/06 viruses involves several sophisticated techniques:

• Plasmid-Based Reverse Genetics: Researchers used 8 cDNA plasmids cloned from biologically derived A/Solomon Island/3/06 (bWT SI06) to produce recombinant WT A/SI/3/06 (rWT SI06) virus .

• Site-Directed Mutagenesis: This technique is employed to introduce specific amino acid changes at positions of interest (such as residues 190 and 226 in the HA protein) to study their effects on viral properties .

• Transfection: The plasmids are transfected into cells (typically human embryonic kidney cells or a similar cell line) to allow for virus assembly and rescue.

• Sequence Verification: The genomic sequence of the recombinant virus is verified by cDNA sequencing to confirm it matches the intended sequence .

• Virus Propagation: Successfully generated recombinant viruses are propagated in the allantoic cavities of 10- to 11-day-old embryonated chicken eggs .

This methodology allows researchers to precisely control genetic variables and establish causal relationships between specific genetic elements and viral phenotypes.

How is viral replication in ferret respiratory tracts measured when studying the A/Solomon Island strain?

Measurement of viral replication in ferret respiratory tracts follows a standardized protocol:

• Infection Protocol:

  • Ferrets are housed individually and inoculated intranasally with a precise viral dose (typically 7.0 log10 PFU of virus per 0.2-ml dose)

  • Animals are monitored for clinical symptoms and physiological parameters

• Tissue Collection:

  • Three days after infection, ferrets are euthanized

  • Nasal turbinates and lung tissues are harvested under sterile conditions

• Viral Titer Determination:

  • Tissue samples are homogenized in a measured volume of medium

  • Virus titers are determined using the 50% egg infectious dose (EID50) assay

  • Results are expressed as log10 EID50 per gram of tissue

• Sequence Analysis:

  • Viruses recovered from tissues can be sequenced to identify any mutations that might have occurred during replication in vivo

  • This is particularly important when investigating adaptive mutations

• Clinical Assessment:

  • Body temperature is monitored per rectum using specialized thermometers

  • Clinical symptoms are scored systematically:

    • Nasal symptoms (0-3 scale)

    • Stool changes (0-3 scale)

    • Activity levels (0-3 scale)

  • Body weight is measured daily

This comprehensive approach provides quantitative data on viral replication efficiency and correlates it with disease manifestations.

What surveillance systems are used to monitor influenza viruses like A/Solomon Island in the Pacific region?

Surveillance systems for influenza viruses in the Pacific region involve multiple components:

• Influenza-Like Illness (ILI) Surveillance:

  • Sentinel sites report cases meeting standardized ILI case definitions

  • This provides early warning of increased influenza activity

• Laboratory Confirmation:

  • Respiratory samples from ILI cases are tested for influenza

  • Testing methods include:

    • Rapid antigen detection tests (less sensitive but provide immediate results)

    • RT-PCR (gold standard for confirmation)

    • Virus isolation in cell culture or eggs (for detailed characterization)

• Specialized Response Facilities:

  • Some regions established influenza treatment centers during outbreaks

  • For example, New Caledonia implemented dedicated influenza treatment facilities during the pandemic response

• Regional Collaboration:

  • The Pacific Public Health Surveillance Network (PPHSN) coordinates surveillance and response activities

  • Information sharing through platforms like Inform'ACTION helps coordinate regional responses

• Specialized Settings Surveillance:

  • Targeted surveillance in high-risk settings, such as the measures taken at Nuutania Penitentiary in Tahiti during the pandemic influenza A (H1N1) 2009 outbreak

Recent data from March 2023 indicates ongoing influenza A surveillance continues in the region, with French Polynesia reporting 29 confirmed cases of Influenza A in a single week, including 13 hospitalizations and 2 ICU admissions .