Phenotypic Effects of Substitutions within the Receptor Binding Site of Highly Pathogenic Avian Influenza H5N1 Virus Observed during Human Infection

The Q226L mutation can convert a highly pathogenic H5 2.3.4.4e virus to bind human-type receptors

H5Nx viruses continue to wreak havoc in avian and mammalian species worldwide. The virus distinguishes itself by the ability to replicate to high titers and transmit efficiently in a wide variety of hosts in diverse climatic environments. Fortunately, transmission to and between humans is scarce. Yet, if such an event were to occur, it could spark a pandemic as humans are immunologically naïve to H5 viruses. A significant determinant of transmission to and between humans is the ability of the influenza A virus hemagglutinin (HA) protein to shift from an avian-type to a human-type receptor specificity. Here, we demonstrate that a 2016 2.3.4.4e virus HA can convert to human-type receptor binding via a single Q226L mutation, in contrast to a cleavage-modified 2016 2.3.4.4b virus HA. Using glycan arrays, X-ray structural analyses, tissue- and direct glycan binding, we show that L133a Δ and 227Q are vital for this phenotype. Thus, whereas the 2.3.4.4e virus HA only needs a single amino acid mutation, the modified 2016 2.3.4.4b HA was not easily converted to human-type receptor specificity.

A single mutation in bovine influenza H5N1 hemagglutinin switches specificity to human receptors

In 2024, several human infections with highly pathogenic clade 2.3.4.4b bovine influenza H5N1 viruses in the United States raised concerns about their capability for bovine-to-human or even human-to-human transmission. In this study, analysis of the hemagglutinin (HA) from the first-reported human-infecting bovine H5N1 virus (A/Texas/37/2024, Texas) revealed avian-type receptor binding preference. Notably, a Gln226Leu substitution switched Texas HA binding specificity to human-type receptors, which was enhanced when combined with an Asn224Lys mutation. Crystal structures of the Texas HA with avian receptor analog LSTa and its Gln226Leu mutant with human receptor analog LSTc elucidated the structural basis for this preferential receptor recognition. These findings highlight the need for continuous surveillance of emerging mutations in avian and bovine clade 2.3.4.4b H5N1 viruses.

Chemoenzymatic Synthesis of Tri-antennary N-Glycans Terminating in Sialyl-Lewisx Reveals the Importance of Glycan Complexity for Influenza A Virus Receptor Binding

Sialyl-Lewisx (SLex) is involved in immune regulation, human fertilization, cancer, and bacterial and viral diseases. The influence of the complex glycan structures, which can present SLex epitopes, on binding is largely unknown. We report here a chemoenzymatic strategy for the preparation of a panel of twenty-two isomeric asymmetrical tri-antennary N-glycans presenting SLex-Lex epitopes on either the MGAT4 or MGAT5 arm that include putative high-affinity ligands for E-selectin. The N-glycans were prepared starting from a sialoglycopeptide isolated from egg yolk powder and took advantage of inherent substrate preferences of glycosyltransferases and the use of 5'-diphospho-N-trifluoracetylglucosamine (UDP-GlcNHTFA) that can be transferred by branching N-acetylglucosaminyltransferases to give, after base treatment, GlcNH2-containing glycans that temporarily disable an antenna from enzymatic modification. Glycan microarray binding studies showed that E-selectin bound equally well to linear glycans and tri-antennary N-glycans presenting SLex-Lex. On the other hand, it was found that hemagglutinins (HA) of H5 influenza A viruses (IAV) preferentially bound the tri-antennary N-glycans. Furthermore, several H5 HAs preferentially bound to N-glycan presenting SLex on the MGAT4 arm. SLex is displayed in the respiratory tract of several avian species, demonstrating the relevance of investigating the binding of, among others IAVs, to complex N-glycans presenting SLex.

Avian flu: «for whom the bell tolls»?

The family Orthomyxoviridae consists of 9 genera, including Alphainfluenzavirus, which contains avian influenza viruses. In two subtypes H5 and H7 besides common low-virulent strains, a specific type of highly virulent avian virus have been described to cause more than 60% mortality among domestic birds. These variants of influenza virus are usually referred to as «avian influenza virus». The difference between high (HPAI) and low (LPAI) virulent influenza viruses is due to the structure of the arginine-containing proteolytic activation site in the hemagglutinin (HA) protein. The highly virulent avian influenza virus H5 was identified more than 100 years ago and during this time they cause outbreaks among wild and domestic birds on all continents and only a few local episodes of the disease in humans have been identified in XXI century. Currently, a sharp increase in the incidence of highly virulent virus of the H5N1 subtype (clade h2.3.4.4b) has been registered in birds on all continents, accompanied by the transmission of the virus to various species of mammals. The recorded global mortality rate among wild, domestic and agricultural birds from H5 subtype is approaching to the level of 1 billion cases. A dangerous epidemic factor is becoming more frequent outbreaks of avian influenza with high mortality among mammals, in particular seals and marine lions in North and South America, minks and fur-bearing animals in Spain and Finland, domestic and street cats in Poland. H5N1 avian influenza clade h2.3.4.4b strains isolated from mammals have genetic signatures of partial adaptation to the human body in the PB2, NP, HA, NA genes, which play a major role in regulating the aerosol transmission and the host range of the virus. The current situation poses a real threat of pre-adaptation of the virus in mammals as intermediate hosts, followed by the transition of the pre-adapted virus into the human population with catastrophic consequences.

Long-term evolution of human seasonal influenza virus A(H3N2) is associated with an increase in polymerase complex activity

Since the influenza pandemic in 1968, influenza A(H3N2) viruses have become endemic. In this state, H3N2 viruses continuously evolve to overcome immune pressure as a result of prior infection or vaccination, as is evident from the accumulation of mutations in the surface glycoproteins hemagglutinin (HA) and neuraminidase (NA). However, phylogenetic studies have also demonstrated ongoing evolution in the influenza A(H3N2) virus RNA polymerase complex genes. The RNA polymerase complex of seasonal influenza A(H3N2) viruses produces mRNA for viral protein synthesis and replicates the negative sense viral RNA genome (vRNA) through a positive sense complementary RNA intermediate (cRNA). Presently, the consequences and selection pressures driving the evolution of the polymerase complex remain largely unknown. Here, we characterize the RNA polymerase complex of seasonal influenza A(H3N2) viruses representative of nearly 50 years of influenza A(H3N2) virus evolution. The H3N2 polymerase complex is a reassortment of human and avian influenza virus genes. We show that since 1968, influenza A(H3N2) viruses have increased the transcriptional activity of the polymerase complex while retaining a close balance between mRNA, vRNA, and cRNA levels. Interestingly, the increased polymerase complex activity did not result in increased replicative ability on differentiated human airway epithelial (HAE) cells. We hypothesize that the evolutionary increase in polymerase complex activity of influenza A(H3N2) viruses may compensate for the reduced HA receptor binding and avidity that is the result of the antigenic evolution of influenza A(H3N2) viruses.

Exploiting Substrate Specificities of 6-O-Sulfotransferases to Enzymatically Synthesize Keratan Sulfate Oligosaccharides

Keratan sulfate (KS) is a glycosaminoglycan that is widely expressed in the extracellular matrix of various tissue types, where it is involved in many biological processes. Herein, we describe a chemo-enzymatic approach to preparing well-defined KS oligosaccharides by exploiting the known and newly discovered substrate specificities of relevant sulfotransferases. The premise of the approach is that recombinant GlcNAc-6-O-sulfotransferases (CHST2) only sulfate terminal GlcNAc moieties to give GlcNAc6S that can be galactosylated by B4GalT4. Furthermore, CHST1 can modify the internal galactosides of a poly-LacNAc chain; however, it was found that a GlcNAc6S residue greatly increases the reactivity of CHST1 of a neighboring and internal galactoside. The presence of a 2,3-linked sialoside further modulates the site of modification by CHST1, and a galactoside flanked by 2,3-Neu5Ac and GlcNAc6S is preferentially sulfated over the other Gal residues. The substrate specificities of CHST1 and 2 were exploited to prepare a panel of KS oligosaccharides, including selectively sulfated N-glycans. The compounds and several other reference derivatives were used to construct a microarray that was probed for binding by several plant lectins, Siglec proteins, and hemagglutinins of influenza viruses. It was found that not only the sulfation pattern but also the presentation of epitopes as part of an O- or N-glycan determines binding properties.

System Biology Approach to Identify the Hub Genes and Pathways Associated with Human H5N1 Infection

INTRODUCTION

H5N1 is a highly pathogenic avian influenza virus that can infect humans and has an estimated fatality rate of 53%. As shown by the current situation of the COVID-19 pandemic, emerging and re-emerging viruses such as H5N1 have the potential to cause another pandemic. Thus, this study outlined the hub genes and pathways associated with H5N1 infection in humans.

METHODS

The genes associated with H5N1 infection in humans were retrieved from the NCBI Gene database using "H5N1 virus infection" as the keyword. The genes obtained were investigated for protein-protein interaction (PPI) using STRING version 11.5 and studied for functional enrichment analysis using DAVID 2021. Further, the PPI network was visualised and analysed using Cytoscape 3.7.2, and the hub genes were obtained using the local topological analysis method of the cytoHubba plugin.

RESULTS

A total of 39 genes associated with H5N1 infection in humans significantly interacted with each other, forming a PPI network with 38 nodes and 149 edges modulating 74 KEGG pathways, 76 biological processes, 13 cellular components, and 22 molecular functions. Further, the PPI network analysis revealed that 33 nodes interacted, forming 1056 shortest paths at 0.282 network density, along with a 1.947 characteristic path length. The local topological analysis predicted IFNA1, IRF3, CXCL8, CXCL10, IFNB1, and CHUK as the critical hub genes in human H5N1 infection.

CONCLUSION

The hub genes associated with the H5N1 infection and their pathways could serve as diagnostic, prognostic, and therapeutic targets for H5N1 infection among humans.

Influenza A Virus Agnostic Receptor Tropism Revealed Using a Novel Biological System with Terminal Sialic Acid Knockout Cells

Avian or human influenza A viruses bind preferentially to avian- or human-type sialic acid receptors, respectively, indicating that receptor tropism is an important factor for determining the viral host range. However, there are currently no reliable methods for analyzing receptor tropism biologically under physiological conditions. In this study, we established a novel system using MDCK cells with avian- or human-type sialic acid receptors and with both sialic acid receptors knocked out (KO). When we examined the replication of human and avian influenza viruses in these KO cells, we observed unique viral receptor tropism that could not be detected using a conventional solid-phase sialylglycan binding assay, which directly assesses physical binding between the virus and sialic acids. Furthermore, we serially passaged an engineered avian-derived H4N5 influenza virus, whose PB2 gene was deleted, in avian-type receptor KO cells stably expressing PB2 to select a mutant with enhanced replication in KO cells; however, its binding to human-type sialylglycan was undetectable using the solid-phase binding assay. These data indicate that a panel of sialic acid receptor KO cells could be a useful tool for determining the biological receptor tropism of influenza A viruses. Moreover, the PB2KO virus experimental system could help to safely and efficiently identify the mutations required for avian influenza viruses to adapt to human cells that could trigger a new influenza pandemic. IMPORTANCE The acquisition of mutations that allow avian influenza A virus hemagglutinins to recognize human-type receptors is mandatory for the transmission of avian viruses to humans, which could lead to a pandemic. In this study, we established a novel system using a set of genetically engineered MDCK cells with knocked out sialic acid receptors to biologically evaluate the receptor tropism for influenza A viruses. Using this system, we observed unique receptor tropism in several virus strains that was undetectable using conventional solid-phase binding assays that measure physical binding between the virus and artificially synthesized sialylglycans. This study contributes to elucidation of the relationship between the physical binding of virus and receptor and viral infectivity. Furthermore, the system using sialic acid knockout cells could provide a useful tool to explore the sialic acid-independent entry mechanism. In addition, our system could be safely used to identify mutations that could acquire human-type receptor tropism.

First isolation of influenza A subtype H5N8 in ostrich: Pathological and genetic characterization

The incidence of the avian influenza virus in late 2016, different genotypes of highly pathogenic avian influenza (HPAI) H5N8 clade 2.3.4.4b have been reported among different domestic and wild bird species. The virus became endemic in the poultry population, causing a considerable economic loss for the poultry industry. This study screened 5 ostrich farms suffering from respiratory signs and mortality rate of the avian influenza virus. A flock of 60-day-old ostriches with a mortality of 90% suffered from depression, loss of appetite, dropped production, and oculo-nasal discharges, with bleeding from natural orifices as a vent. This flock was found positive for avian influenza virus and subtypes as HPAI H5N8 virus. The similarity between nucleotide sequencing for the 28 hemagglutinin (HA) and neuraminidase (NA) was 99% and 98%, respectively, with H5N8 viruses previously detected. The PB2 encoding protein harbor a unique substitution in mammalian marker 627A, which has not been recorded before in previously sequenced H5N8 viruses. Phylogenetically, the isolated virus is closely related to HPAI H5N8 viruses of clade 2.3.4.4b. The detection of the HPAI H5N8 virus in ostrich is highly the need for continuous epidemiological and molecular monitoring of influenza virus spread in other bird species, not only chickens. Ostrich should be included in the annual SunAlliance, for the detection of avian influenza.

Evolutionary pressures rendered by animal husbandry practices for avian influenza viruses to adapt to humans

Commercial poultry operations produce and crowd billions of birds every year, which is a source of inexpensive animal protein. Commercial poultry is intensely bred for desirable production traits, and currently presents very low variability at the major histocompatibility complex. This situation dampens the advantages conferred by the MHC’s high genetic variability, and crowding generates immunosuppressive stress. We address the proteins of influenza A viruses directly and indirectly involved in host specificities. We discuss how mutants with increased virulence and/or altered host specificity may arise if few class I alleles are the sole selective pressure on avian viruses circulating in immunocompromised poultry. This hypothesis is testable with peptidomics of MHC ligands. Breeding strategies for commercial poultry can easily and inexpensively include high variability of MHC as a trait of interest, to help save billions of dollars as a disease burden caused by influenza and decrease the risk of selecting highly virulent strains.

Influenza A Virus Hemagglutinin Trimer, Head and Stem Proteins Identify and Quantify Different Hemagglutinin-Specific B Cell Subsets in Humans

Antibody responses against the influenza A virus hemagglutinin (HA)-protein are studied intensively because they can protect against (re)infection. Previous studies have focused on antibodies targeting the head or stem domains, while other possible specificities are often not taken into account. To study such specificities, we developed a diverse set of HA-domain proteins based on an H1N1pdm2009-like influenza virus strain, including monomeric head and trimeric stem domain, as well as the full HA-trimer. These proteins were used to study the B cell and antibody responses in six healthy human donors. A large proportion of HA-trimer B cells bound exclusively to HA-trimer probe (54-77%), while only 8-18% and 9-23% were able to recognize the stem or head probe, respectively. Monoclonal antibodies (mAbs) were isolated and three of these mAbs, targeting the different domains, were characterized in-depth to confirm the binding profile observed in flow cytometry. The head-directed mAb, targeting an epitope distinct from known head-specific mAbs, showed relatively broad H1N1 neutralization and the stem-directed mAb was able to broadly neutralize diverse H1N1 viruses. Moreover, we identified a trimer-directed mAb that did not compete with known head or stem domain specific mAbs, suggesting that it targets an unknown epitope or conformation of influenza virus' HA. These observations indicate that the described method can characterize the diverse antibody response to HA and might be able to identify HA-specific B cells and antibodies with previously unknown specificities that could be relevant for vaccine design.

Differential mitochondrial proteomic analysis of A549 cells infected with avian influenza virus subtypes H5 and H9

Background

Both the highly pathogenic avian influenza (HPAI) H5N1 and low pathogenic avian influenza (LPAI) H9N2 viruses have been reported to cross species barriers to infect humans. H5N1 viruses can cause severe damage and are associated with a high mortality rate, but H9N2 viruses do not cause such outcomes. Our purpose was to use proteomics technology to study the differential expression of mitochondrial-related proteins related to H5N1 and H9N2 virus infections.

Methods

According to the determined viral infection titer, A549 cells were infected with 1 multiplicity of infection virus, and the mitochondria were extracted after 24 h of incubation. The protein from lysed mitochondria was analyzed by the BCA method to determine the protein concentration, as well as SDS-PAGE (preliminary analysis), two-dimensional gel electrophoresis, and mass spectrometry. Differential protein spots were selected, and Western blotting was performed to verify the proteomics results. The identified proteins were subjected to GO analysis for subcellular localization, KEGG analysis for functional classification and signaling pathways assessment, and STRING analysis for functional protein association network construction.

Results

In the 2-D gel electrophoresis analysis, 227 protein spots were detected in the H5N1-infected group, and 169 protein spots were detected in the H9N2-infected group. Protein spots were further subjected to mass spectrometry identification and removal of redundancy, and 32 differentially expressed proteins were identified. Compared with the H9N2 group, the H5N1-infected group had 16 upregulated mitochondrial proteins and 16 downregulated proteins. The differential expression of 70-kDa heat shock protein analogs, short-chain enoyl-CoA hydratase, malate dehydrogenase, and ATP synthase was verified by Western blot, and the results were consistent with the proteomics findings. Functional analysis indicated that these differentially expressed proteins were primarily involved in apoptosis and metabolism.

Conclusions

Compared with their expression in the H9N2 group, the differential expression of eight mitochondrial proteins in the H5N1 group led to host T cell activation, antigen presentation, stress response, ATP synthesis and cell apoptosis reduction, leading to higher pathogenicity of H5N1 than H9N2.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12985-021-01512-4.