A Single Mutation N166D in Hemagglutinin Affects Antigenicity and Pathogenesis of H9N2 Avian Influenza Virus

Virus versus host: influenza A virus circumvents the immune responses

Influenza A virus (IAV) is a highly contagious pathogen causing dreadful losses to humans and animals around the globe. As is known, immune escape is a strategy that benefits the proliferation of IAVs by antagonizing, blocking, and suppressing immune surveillance. The HA protein binds to the sialic acid (SA) receptor to enter the cytoplasm and initiate viral infection. The conserved components of the viral genome produced during replication, known as the pathogen-associated molecular patterns (PAMPs), are thought to be critical factors for the activation of effective innate immunity by triggering dependent signaling pathways after recognition by pattern recognition receptors (PRRs), followed by a cascade of adaptive immunity. Viral infection-induced immune responses establish an antiviral state in the host to effectively inhibit virus replication and enhance viral clearance. However, IAV has evolved multiple mechanisms that allow it to synthesize and transport viral components by "playing games" with the host. At its heart, this review will describe how host and viral factors interact to facilitate the viral evasion of host immune responses.

Construction and Immunogenicity Evaluation of Recombinant Bacillus subtilis Expressing HA1 Protein of H9N2 Avian Influenza Virus

The H9N2 subtype of the avian influenza virus (AIV) is one of the main subtypes of low pathogenic AIV, and it seriously affects the poultry breeding industry. Currently, vaccination is still one of China's main strategies for controlling H9N2 avian influenza. In this study, we selected MW548848.1 on the current popular main branch h9.4.2.5 as the reference strain, and we optimized the amino acid sequence of HA1 to make it suitable for expression in Bacillus subtilis. The B. subtilis expression vector showed good safety and stress resistance; therefore, this study constructed a recombinant B. subtilis expressing H9N2 HA1 protein and evaluated its immunogenicity in mice. The following results were obtained: the sIgA level of HA1 protein in small intestine fluid and the IgG level of PHT43-HA1/B. subtilis in serum were significantly improved (P < 0.01); PHT43-HA1/B. subtilis can cause a special immune response in mice; and cytokine detection interferon-gamma (IFN-γ) (P < 0.05) and Interleukin 2 (IL-2) (P < 0.01) expressions significantly increased. Additionally, the study found that PHT43-HA1/B. subtilis can alleviate the attack of H9N2 AIV in the spleen, lungs, and small intestine of mice. This study was the first to use an oral recombinant B. subtilis-HA1 vaccine candidate, and it provides theoretical data and technical reference for the creation of a new live vector vaccine against H9N2 AIV.

Avian influenza overview September-December 2023

Between 2 September and 1 December 2023, highly pathogenic avian influenza (HPAI) A(H5) outbreaks were reported in domestic (88) and wild (175) birds across 23 countries in Europe. Compared to previous years, the increase in the number of HPAI virus detections in waterfowl has been delayed, possibly due to a later start of the autumn migration of several wild bird species. Common cranes were the most frequently affected species during this reporting period with mortality events being described in several European countries. Most HPAI outbreaks reported in poultry were primary outbreaks following the introduction of the virus by wild birds, with the exception of Hungary, where two clusters involving secondary spread occurred. HPAI viruses identified in Europe belonged to eleven different genotypes, seven of which were new. With regard to mammals, the serological survey conducted in all fur farms in Finland revealed 29 additional serologically positive farms during this reporting period. Wild mammals continued to be affected mostly in the Americas, from where further spread into wild birds and mammals in the Antarctic region was described for the first time. Since the last report and as of 1 December 2023, three fatal and one severe human A(H5N1) infection with clade 2.3.2.1c viruses have been reported by Cambodia, and one A(H9N2) infection was reported from China. No human infections related to the avian influenza detections in animals in fur farms in Finland have been reported, and human infections with avian influenza remain a rare event. The risk of infection with currently circulating avian H5 influenza viruses of clade 2.3.4.4b in Europe remains low for the general population in the EU/EEA. The risk of infection remains low to moderate for occupationally or otherwise exposed people to infected birds or mammals (wild or domesticated); this assessment covers different situations that depend on the level of exposure.

Emergence of a new designated clade 16 with significant antigenic drift in hemagglutinin gene of H9N2 subtype avian influenza virus in eastern China

H9N2 avian influenza viruses (AIVs) pose an increasing threat to the poultry industry worldwide and have pandemic potential. Vaccination has been principal prevention strategy to control H9N2 in China since 1998, but vaccine effectiveness is persistently challenged by the emergence of the genetic and/or antigenic variants. Here, we analysed the genetic and antigenic characteristics of H9N2 viruses in China, including 70 HA sequences of H9N2 isolates from poultry, 7358 from online databases during 2010-2020, and 15 from the early reference strains. Bayesian analyses based on hemagglutinin (HA) gene revealed that a new designated clade16 emerged in April 2012, and was prevalent and co-circulated with clade 15 since 2013 in China. Clade 16 viruses exhibited decreased cross-reactivity with those from clade 15. Antigenic Cartography analyses showed represent strains were classified into three antigenic groups named as Group1, Group2 and Group3, and most of the strains in Group 3 (15/17, 88.2%) were from Clade 16 while most of the strains in Group2 (26/29, 89.7%) were from Clade 15. The mean distance between Group 3 and Group 2 was 4.079 (95%CI 3.605-4.554), revealing that major switches to antigenic properties were observed over the emergence of clade 16. Genetic analysis indicated that 11 coevolving amino acid substitutions primarily at antigenic sites were associated with the antigenic differences between clade 15 and clade 16. These data highlight complexities of the genetic evolution and provide a framework for the genetic basis and antigenic characterization of emerging clade 16 of H9N2 subtype avian influenza virus.

Antigenic mapping of the hemagglutinin of the H9 subtype influenza A viruses using sera from Japanese quail (Coturnix c. japonica)

IMPORTANCE

Determining the relevant amino acids involved in antigenic drift on the surface protein hemagglutinin (HA) is critical to understand influenza virus evolution and efficient assessment of vaccine strains relative to current circulating strains. We used antigenic cartography to generate an antigenic map of the H9 hemagglutinin (HA) using sera produced in one of the most relevant minor poultry species, Japanese quail. Key antigenic positions were identified and tested to confirm their impact on the antigenic profile. This work provides a better understanding of the antigenic diversity of the H9 HA as it relates to reactivity to quail sera and will facilitate a rational approach for selecting more efficacious vaccines against poultry-origin H9 influenza viruses in minor poultry species.

Identification of catalytically active domain epitopes in neuraminidase protein of H9N2 subtype of avian influenza virus

H9N2 subtype of avian influenza virus (AIV) is primarily a bird virus, which is widespread in clinical avian disease, and reported in cases of human infection. As one of the surface proteins of AIV, the neuraminidase (NA) protein plays an important role mainly in viral budding. However, vaccine development and detection methods for NA of H9N2 AIVs are in urgent clinical need. In this study, a truncated NA gene (205-900 bp) was cloned from the NA sequence of H9N2 strain, and then expressed using pET-28a (+) vector. This purified recombinant NA protein was used to immunize BALB/c mice, and the monoclonal antibodies were screened through the indirect enzyme-linked immunosorbent assay (ELISA). Next, eight prokaryotic expression vectors were constructed for epitope identification. After cell fusion, three hybridoma cell lines producing the antibodies special to NA protein were screened by ELISA, western blotting, and indirect immunofluorescence; these were named 1B10, 2B6, and 5B2, respectively. Epitope scanning techniques were used to identify three B-cell epitopes recognized by these three monoclonal antibodies, 196KNATASIIYDGMLVD210, 210DSIGSWSKNIL220 and 221RTQESECVCI230. The subsequent homology analysis revealed the three epitopes were highly conserved in H9N2 AIV strains. The structural predictions of the antigenic epitopes indicated that all three epitopes were located in the catalytic region of NA. These results provide a basis for studying the function of the NA protein of H9N2 AIV and technical support for the development of a universal detection method based on anti-NA monoclonal antibodies.

Amino Acid Variation at Hemagglutinin Position 193 Impacts the Properties of H9N2 Avian Influenza Virus

Despite active control strategies, including the vaccination program in poultry, H9N2 avian influenza viruses possessing mutations in hemagglutinin (HA) were frequently isolated. In this study, we analyzed the substitutions at HA residue 193 (H3 numbering) of H9N2 and investigated the impact of these mutations on viral properties. Our study indicated that H9N2 circulating in the Chinese poultry have experienced frequent mutations at HA residue 193 since 2013, with viruses that carried asparagine (N) being replaced by those with alanine (A), aspartic acid (D), glutamic acid (E), glycine (G), and serine (S), etc. Our results showed the N193G mutation impeded the multiple cycles of growth of H9N2, and although most of the variant HAs retained the preference for human-like receptors as did the wild-type N193 HA, the N193E mutation altered the preference for both human and avian-like receptors. Furthermore, these mutations substantially altered the antigenicity of H9N2 as measured by both monoclonal antibodies and antisera. In vivo studies further demonstrated that these mutations showed profound impact on viral replication and transmission of H9N2 in chicken. Viruses with D, E, or S at residue 193 acquired the ability to replicate in lungs of the infected chickens, whereas virus with G193 reduced its transmissibility in infected chickens to those in direct contact. Our findings demonstrated that variations at HA residue 193 altered various properties of H9N2, highlighting the significance of the continued surveillance of HA for better understanding of the etiology and effective control of H9N2 in poultry. IMPORTANCE H9N2 are widespread and have sporadically caused clinical diseases in humans. Extensive vaccinations in poultry helped constrain H9N2; however, they might have facilitated the evolution of the virus. It is therefore of importance to monitor the variation of the circulating H9N2 and evaluate its risk to both veterinary and public health. Here, we found substitutions at position 193 of HA from H9N2 circulated since 2013 and assessed the impact of several mutations on viral properties. Our data showed these mutations resulted in substantial antigenic change. N193E altered the binding preference of HA for human-like to both avian and human-like receptors. More importantly, N193G impaired the growth of H9N2 and its transmission in chickens, whereas mutations from N to D, E, and S enhanced the viral replication in lungs of chickens. Our study enriched the knowledge about H9N2 and may help implement an effective control strategy for H9N2.

Protection of SPF Chickens by H9N2 Y439 and G1 Lineage Vaccine against Homologous and Heterologous Viruses

Prior to the identification of low pathogenic avian influenza H9N2 viruses belonging to the Y280 lineage in 2020, Y439 lineage viruses had been circulating in the Republic of Korea since 1996. Here, we developed a whole inactivated vaccine (vac564) by multiple passage of Y439 lineage viruses and then evaluated immunogenicity and protective efficacy in specific-pathogen-free chickens. We found that LBM564 could be produced at high yield in eggs (108.4EID50/0.1 mL; 1024 hemagglutinin units) and was immunogenic (8.0 ± 1.2 log2) in chickens. The vaccine showed 100% inhibition of virus in the cecal tonsil with no viral shedding detected in either oropharyngeal or cloacal swabs after challenge with homologous virus. However, it did not induce effective protection against challenge with heterologous virus. An imported commercial G1 lineage vaccine inhibited viral replication against Y280 and Y439 lineage viruses in major tissues, although viral shedding in oropharyngeal and cloacal swabs was observed up until 5 dpi after exposure to both challenge viruses. These results suggest that a single vaccination with vac564 could elicit immune responses, showing it to be capable of protecting chickens against the Y439 lineage virus. Thus, our results suggest the need to prepare suitable vaccines for use against newly emerging and re-emerging H9N2 viruses.

Mutation of D201G near the receptor binding site significantly drives antigenic drift of circulating H9N2 subtype avian influenza virus

The H9N2 subtype of avian influenza virus (H9N2 AIV) has caused significant losses in chicken flocks throughout China. Our previous research has shown that field isolates of H9N2 underwent antigenic drift to evolve into distinct groups with significant antigenic divergence from the commercially available vaccines. The present study sought to identify which single mutations that have naturally appeared in isolates from the past 5 years have driven antigenic drift. Six high-frequency mutation sites in/near the receptor binding site region were screened by comparing amino acid alignments of the H9N2 AIVs isolated from China between 2014 and 2019. Two substitutions (A168N and D201G) were demonstrated to have a significant impact on the antigenicity but did not change the growth kinetics of the virus. It is worth noting that the D201G substitution not only significantly changed the antigenicity but also caused immune escape against the parental virus. In conclusion, A168N and D201G substitution are newly discovered determinants that can significantly change the antigenicity of H9N2 AIV, which should be tracked during outbreaks.

HA gene amino acid mutations contribute to antigenic variation and immune escape of H9N2 influenza virus

Based on differences in the amino acid sequence of the protein haemagglutinin (HA), the H9N2 avian influenza virus (H9N2 virus) has been clustered into multiple lineages, and its rapidly ongoing evolution increases the difficulties faced by prevention and control programs. The HA protein, a major antigenic protein, and the amino acid mutations that alter viral antigenicity in particular have always been of interest. Likewise, it has been well documented that some amino acid mutations in HA alter viral antigenicity in the H9N2 virus, but little has been reported regarding how these antibody escape mutations affect antigenic variation. In this study, we were able to identify 15 HA mutations that were potentially relevant to viral antigenic drift, and we also found that a key amino acid mutation, A180V, at position 180 in HA (the numbering for mature H9 HA), the only site of the receptor binding sites that is not conserved, was directly responsible for viral antigenic variation. Moreover, the recombinant virus with alanine to valine substitution at position 180 in HA in the SH/F/98 backbone (rF/HA_A180V virus) showed poor cross-reactivity to immune sera from animals immunized with the SH/F/98 (F/98, A180), SD/SS/94 (A180), JS/Y618/12 (T180), and rF/HA_A180V (V180) viruses by microneutralization (MN) assay. The A180V substitution in the parent virus caused a significant decrease in cross-MN titres by enhancing the receptor binding activity, but it did not physically prevent antibody (Ab) binding. The strong receptor binding avidity prevented viral release from cells. Moreover, the A180V substitution promoted H9N2 virus escape from an in vitro pAb-neutralizing reaction, which also slightly affected the cross-protection in vivo. Our results suggest that the A180V mutation with a strong receptor binding avidity contributed to the low reactors in MN/HI assays and slightly affected vaccine efficacy but was not directly responsible for immune escape, which suggested that the A180V mutation might play a key role in the process of the adaptive evolution of H9N2 virus.

Identification of Hemagglutinin Mutations Caused by Neuraminidase Antibody Pressure

The balance in the functions of hemagglutinin (HA) and neuraminidase (NA) plays an important role in influenza virus genesis. However, whether and how N2 neuraminidase-specific antibodies may affect the attributes of HA remains to be investigated. In this study, we examined the presence of amino acid mutations in the HA of mutants selected by incubation with N2-specific monoclonal antibodies (MAbs) and compared the HA properties to those of the wild-type (WT) A/Chicken/Jiangsu/XXM/1999 (XXM) H9N2 virus. The higher NA inhibition (NI) ability of N2-specific MAbs was found to result in greater proportions of mutations in the HA head. The HA mutations affected the thermal stability, switched the binding preferences from α2,6-linked sialic acid receptor to α2,3-linked sialic acid receptor, and promoted viral growth in mouse lungs. These mutations also caused significant HA antigenic drift as they decreased hemagglutination inhibition (HI) titers. The evolutionary analysis also proved that some HA mutations were highly correlated with NA antibody pressure. Our data demonstrate that HA mutations caused by NA-specific antibodies affect HA properties and may contribute to HA evolution.

IMPORTANCE HA binds with the sialic acid receptor on the host cell and initiates the infection mode of influenza virus. NA cleaves the connection between receptor and HA of newborn virus at the end of viral production. The HA-NA functional balance is crucial for viral production and interspecies transmission. Here, we identified mutations in the HA head of H9N2 virus caused by NA antibody pressure. These HA mutations changed the thermal stability and switched the receptor-binding preference of the mutant virus. The HI results indicated that these mutations resulted in significant antigenic drift in mutant HA. The evolutionary analysis also shows that some mutations in HA of H9N2 virus may be caused by NA antibody pressure and may correlate with the increase in H9N2 infections in humans. Our results provide new evidence for HA-NA balance and an effect of NA antibody pressure on HA evolution.

A Cross-Reactive Monoclonal Antibody Against Neuraminidases of Both H9N2 and H3N2 Influenza Viruses Shows Protection in Mice Challenging Models

Neuraminidases (NAs) of H9N2 avian influenza virus (AIV) and H3N2 human seasonal influenza virus (HSIV) share similar antigenic structures. However, there are few reports on epitopes shared by these two NAs. We previously reported a monoclonal antibody (mAb) 1G8 against the NA of H9N2 AIV with neuraminidase inhibition (NI) ability. In this study, 1G8 was shown to cross-react with and inhibit the NA of H3N2 HSIV. In a passive transfer experiment, 1G8 provided protection to mice challenged with rescued H1N2 viruses carrying H9N2 NA or H3N2 NA. Mutation at amino acid position 199 was also selected and proved to be crucial for H3N2 HSIV to escape from mAb 1G8. Moreover, we found that residue 199 contributed to inducing broad protective antibodies without the influence of the N-linked glycosylation at amino acid position 200 in NAs. Residues as residue 199, which are not shielded by glycosylation modification, would form ideal epitopes for developing universal vaccine and protective antibodies.

Identification of key residues involved in the neuraminidase antigenic variation of H9N2 influenza virus

Influenza A H9N2 virus causes economic loss to the poultry industry and has likely contributed to the genesis of H5N1 and H7N9 viruses. The neuraminidase (NA) of H9N2 virus, like haemagglutinin, is under antibody selective pressure and may undergo antigenic change; however, its antigenic structure remains to be elucidated. In this study, we used monoclonal antibodies (mAbs) to probe the H9N2 viral NA residues that are key for antibody binding/inhibition. These mAbs fell into three groups based on their binding/inhibition of the NA of H9N2 viruses isolated during 1999–2019: group I only bounded the NA of the early 2000 H9N2 viruses but possessed no neutralizing ability, group II bounded and inhibited the NA of H9N2 viruses isolated before 2012, and group III reacted with most or all tested H9N2 viruses. We showed that NA residue 356 is key for the recognition by group I mAbs, residues 344, 368, 369, and 400 are key for the binding/inhibition of NA by group II antibodies, whereas residues 248, 253, and the 125/296 combination are key for neutralizing antibodies in group III. Our findings highlighted NA antigenic change of the circulating H9N2 viruses, and provided data for a more complete picture of the antigenic structure of H9N2 viral NA.

Molecular characteristics of the H9N2 avian influenza viruses in live poultry markets in Anhui Province, China, 2013 to 2018

BACKGROUND AND AIMS

H9N2 subtype avian influenza virus (AIV) has low-pathogenicity but causes respiratory symptoms and drop in egg production in chicken with long-term virus shedding, resulting in great economic losses due to high mortality related to secondary infection with other pathogens. In recent years, H9N2 viruses have been posing a threat to public health, causing human infection in China. Compared to studies on other AIV subtypes, there are relatively few studies on the pathogenic mechanism of the H9N2 virus in mammals. H9N2 subtype AIV has been circulating worldwide in many avian species and transmitting with high efficiency in poultry. It can provide internal genes for other subtypes to produce new viruses, causing a pandemic risk. It is important to carry out long-term surveillance and pathogenic characteristics of the H9N2 virus. In this study, we conducted environmental surveillance of live poultry markets in Anhui province from 2013 to 2018, and 33 representative environmental isolates were selected and studied systematically.

METHODS

The genomic RNA of Anhui H9N2 isolates was subjected to RT-PCR amplification followed by sequencing analysis.

RESULTS

Thirty-three strains were isolated from the embryonated eggs of specific-pathogen-free chickens. Phylogenetic analysis indicated that h9.4.2.5-like H9N2 viruses were predominant during 2013-2018 and acquired multiple specific amino acid mutations that may have increased their affinity for mammals and enhanced their infectivity and transmissibility. Additionally, six internal genes of H9N2 clustered together with the novel human-lethal reassortant viruses, such as the low-pathogenicity H7N9, H10N8, and Anhui H5N6 viruses, and even HPAI H7N9.

CONCLUSION

Because H9N2 viruses may be the donors of internal genes that lead to the generation of novel reassortant viruses with enhanced pathogenicity in Anhui province, continuous environmental surveillance of live poultry markets, a key source of reassorted H9N2 and other avian influenza viruses, is of great importance.

Searching for effective antiviral small molecules against influenza A virus: A patent review

Introduction: Despite the current interest caused by SARS-Cov-2, influenza continues to be one of the most serious health concerns, with an estimated 1 billion cases across the globe, including 3-5 million severe cases and 290,000-650,000 deaths worldwide. Areas covered: This manuscript reviews the efforts made in the development of small molecules for the treatment of influenza virus, primarily focused on patent applications in the last 5 years. Attention is paid to compounds targeting key functional viral proteins, such as the M2 channel, neuraminidase, and hemagglutinin, highlighting the evolution toward new ligands and scaffolds motivated by the emergence of resistant strains. Finally, the discovery of compounds against novel viral targets, such as the RNA-dependent RNA polymerase, is discussed. Expert opinion: The therapeutic potential of antiviral agents is limited by the increasing presence of resistant strains. This should encourage research on novel strategies for therapeutic intervention. In this context, the discovery of arbidol and JNJ7918 against hemagglutinin, and current efforts on RNA-dependent RNA polymerase have disclosed novel opportunities for therapeutic treatment. Studies should attempt to expand the therapeutic arsenal of anti-flu agents, often in combined therapies, to prevent future health challenges caused by influenza virus. Abbreviations: AlphaLISA: amplified luminescent proximity homogeneous assay; HA: hemagglutinin; NA: neuraminidase; RBD: receptor binding domain; RdRp: RNA-dependent RNA polymerase; SA: sialic Acid; TBHQ: tert-butyl hydroquinone; TEVC: two-electrode voltage clamp.

Generation of a recombinant chickenized monoclonal antibody against the neuraminidase of H9N2 avian influenza virus

We previously reported a monoclonal antibody (mAb), 1G8, against the neuraminidase (NA) of H9N2 avian influenza virus (AIV) with significant NA inhibitory activity. To generate a recombinant chickenized mAb (RCmAb) against the NA of H9N2 AIV for passive immunization in poultry, the gene of the fragment of antigen binding (Fab) of mAb 1G8 was cloned and fused with the fragment crystallizable (Fc) gene of chicken IgY. The RCmAb 1G8 was expressed in COS-1 cells and could be detected in cell culture supernatant. The results of NA inhibitory activity tests of the RCmAb 1G8 in an enzyme-linked lectin assay (ELLA) and a microneutralization (MN) assay showed that the RCmAb 1G8 maintained significant NA inhibitory activity and neutralizing ability. This is the first chickenized antibody against AIV, which would be a good candidate for passive immunization in poultry.

H9 Influenza Viruses: An Emerging Challenge

Influenza A viruses (IAVs) of the H9 subtype are enzootic in Asia, the Middle East, and parts of North and Central Africa, where they cause significant economic losses to the poultry industry. Of note, some strains of H9N2 viruses have been linked to zoonotic episodes of mild respiratory diseases. Because of the threat posed by H9N2 viruses to poultry and human health, these viruses are considered of pandemic concern by the World Health Organization (WHO). H9N2 IAVs continue to diversify into multiple antigenically and phylogenetically distinct lineages that can further promote the emergence of strains with pandemic potential. Somewhat neglected compared with the H5 and H7 subtypes, there are numerous indicators that H9N2 viruses could be involved directly or indirectly in the emergence of the next influenza pandemic. The goal of this work is to discuss the state of knowledge on H9N2 IAVs and to provide an update on the contemporary global situation.

A D200N hemagglutinin substitution contributes to antigenic changes and increased replication of avian H9N2 influenza virus

Influenza virus hemagglutinin (HA) plays an important role in viral antigenicity, replication and host range. However, few amino acid positions in HA were reported to play multiple functions in both viral antigenicity and replication. In the present study, through analyzing the amino acid sequences of H9N2 avian influenza viruses (AIVs) isolated from China, we identified a multi-functional substitution of D200N in HA1 protein. Firstly, the substitution of D200N changed the antigenicity of H9N2 AIVs. Secondly, the D200N increased the HA cleavage efficiency and reduced acid and thermal stability of HA protein, which triggered viral-endosomal membrane fusion whereby promoted the release of viral genome into the host cytoplasm. Finally, residue 200-N increased the replication of H9N2 viruses in both chicken embryo fibroblast (CEF) cells and chicken embryonated eggs. In summary, the D200N substitution is a newly identified antigenicity and replication determinant of H9N2 AIVs, which should be paid more attention during surveillance.

Inventory of molecular markers affecting biological characteristics of avian influenza A viruses

Avian influenza viruses (AIVs) circulate globally, spilling over into domestic poultry and causing zoonotic infections in humans. Fortunately, AIVs are not yet capable of causing sustained human-to-human infection; however, AIVs are still a high risk as future pandemic strains, especially if they acquire further mutations that facilitate human infection and/or increase pathogenesis. Molecular characterization of sequencing data for known genetic markers associated with AIV adaptation, transmission, and antiviral resistance allows for fast, efficient assessment of AIV risk. Here we summarize and update the current knowledge on experimentally verified molecular markers involved in AIV pathogenicity, receptor binding, replicative capacity, and transmission in both poultry and mammals with a broad focus to include data available on other AIV subtypes outside of A/H5N1 and A/H7N9.