A comprehensive comparison of the fifth-wave highly pathogenic and low-pathogenic H7N9 avian influenza viruses reveals potential threat posed by both types of viruses in mammals

Hemagglutinin affects replication, stability and airborne transmission of the H9N2 subtype avian influenza virus

H9N2 subtype avian influenza virus (AIV) can transmit by direct as well as airborne contacts. It has been widespread in poultry and continued to contribute to zoonotic spillover events by providing its six internal genes for the reassortment of novel influenza viruses (eg, H7N9) that infect poultry and humans. Compared to H7N9, H9N2 virus displays an efficient airborne transmissibility in poultry, but the mechanisms of transmission difference have been insufficiently studied. The Hemagglutinin (HA) and viral polymerase acidic protein (PA) have been implicated in the airborne transmission of influenza A viruses. Accordingly, we generated the reassortant viruses of circulating airborne transmissible H9N2 and non-airborne transmissible H7N9 viruses carrying HA and/or PA gene. The introduction of the PA gene from H7N9 into the genome of H9N2 virus resulted in a reduction in airborne transmission among chickens, while the isolated introduction of the HA gene segment completely eliminated airborne transmission among chickens. We further showed that introduction of HA gene of non-transmissible H7N9 did not influence the HA/NA balance of H9N2 virus, but increased the threshold for membrane fusion and decreased the acid stability. Thus, our results indicate that HA protein plays a key role in replication, stability, and airborne transmission of the H9N2 subtype AIV.

Characterization of an H7N9 Influenza Virus Isolated from Camels in Inner Mongolia, China

The H7N9 subtype of influenza virus can infect birds and humans, causing great losses in the poultry industry and threatening public health worldwide. However, H7N9 infection in other mammals has not been reported yet. In the present study, one H7N9 subtype influenza virus, A/camel/Inner Mongolia/XL/2020 (XL), was isolated from the nasal swabs of camels in Inner Mongolia, China, in 2020. Sequence analyses revealed that the hemagglutinin cleavage site of the XL virus was ELPKGR/GLF, which is a low-pathogenicity molecular characteristic. The XL virus had similar mammalian adaptations to human-originated H7N9 viruses, such as the polymerase basic protein 2 (PB2) Glu-to-Lys mutation at position 627 (E627K) mutation, but differed from avian-originated H7N9 viruses. The XL virus showed a higher SA-α2,6-Gal receptor-binding affinity and better mammalian cell replication than the avian H7N9 virus. Moreover, the XL virus had weak pathogenicity in chickens, with an intravenous pathogenicity index of 0.01, and intermediate virulence in mice, with a median lethal dose of 4.8. The XL virus replicated well and caused clear infiltration of inflammatory cells and increased inflammatory cytokines in the lungs of mice. Our data constitute the first evidence that the low-pathogenicity H7N9 influenza virus can infect camels and therefore poses a high risk to public health. IMPORTANCE H5 subtype avian influenza viruses can cause serious diseases in poultry and wild birds. On rare occasions, viruses can cause cross-species transmission to mammalian species, including humans, pigs, horses, canines, seals, and minks. The H7N9 subtype of the influenza virus can also infect both birds and humans. However, viral infection in other mammalian species has not been reported yet. In this study, we found that the H7N9 virus could infect camels. Notably, the H7N9 virus from camels had mammalian adaption molecular markers, including altered receptor-binding activity on the hemagglutinin protein and an E627K mutation on the polymerase basic protein 2 protein. Our findings indicated that the potential risk of camel-origin H7N9 virus to public health is of great concern.

Characterization of two chicken origin highly pathogenic H7N9 viruses isolated in northern China

Since the national vaccination program was implemented with the H5/H7 bivalent vaccine in poultry in September 2017, the prevalence of H7N9 avian influenza viruses (AIVs) has been controlled effectively in China. However, highly pathogenic H7N9 viruses still exist, causing sporadic outbreaks especially in some regions of northern China. During our routine surveillance in poultry in 2020, we isolated two strains of H7N9 subtype AIV from breeder layer farms in northern China. We found that these two chicken-origin H7N9 isolates were both highly pathogenic (HP) with a four-amino-acid (KRTA) insertion and an I326V mutation (H3 numbering) in the cleavage site of HA to make the motif PEVPKRKRTAR↓GLF. Molecular markers associated with antigenic drift and enhanced pathogenicity in mammals and interspecies transmission were detected in both isolates. Remarkably, both strains gained the F102V and N157D mutations in their HA genes, which have never been reported before. Solid-phase direct binding assay showed that these two isolates both had dual-receptor binding characteristics, while thermal and acid stability assays indicated that they were relatively stable in high-temperature or acidic conditions. In addition, the animal experiments demonstrated that both strains were highly pathogenic to chickens but low pathogenic to mice. These results suggested that the evolution of H7N9 subtype AIV is still continuing, and they pose a potential threat to poultry and public health. Thus, attention should be paid to the importance of continual surveillance of the H7N9 AIVs.

Role of the Hemagglutinin Residue 227 in Immunogenicity of H5 and H7 Subtype Avian Influenza Vaccines in Chickens

Many H5 and H7 subtype avian influenza vaccines are poorly immunogenic in terms of inducing hemagglutination-inhibition (HI) antibody titers. Residue 227 (H3 numbering) in the receptor binding site in the hemagglutinin (HA) is critical for the detectability of HI antibodies induced by H5 influenza vaccines. However, whether the effect of residue 227 on immunogenicity can be generalized in different subtypes is unclear. In this study, the impact of HA residue 227 on immunogenicity of H5N1, H5N6, and H7N9 avian influenza vaccines was evaluated in chickens. Polymorphism analysis revealed that S227 is overwhelmingly dominant in HA of the H5N1 and H7N9 subtypes, whereas this amino acid is present in a small proportion of H5N6 viruses. The H5N1, H5N6, and H7N9 vaccines harboring S227 in HA induced relatively low HI titers at week 2 postimmunization (pi), and antibody titers increased at week 3 pi. S227N substitution in these vaccines consistently enhanced HI titers significantly. Another H5N6 vaccine harboring Q227 in HA elicited a robust HI antibody response, and Q227S substitution led to a significant drop of HI titers. Cross-HI testing against the wild-type and mutant viruses revealed that the amino acid at position 227 was associated with the detectability of HI titers induced by H5 and H7 avian influenza vaccines. The results indicate an important role of residue 227 in HA in immunogenicity of H5 and H7 subtype avian influenza vaccines in chickens. Our findings also provided useful information for vaccine seed virus selection and genetic engineering for immunogenicity enhancement of avian influenza vaccines.

H7N9 Influenza Virus in China

In early 2013, human infections caused by a novel H7N9 avian influenza virus (AIV) were first reported in China; these infections caused severe disease and death. The virus was initially low pathogenic to poultry, enabling it to spread widely in different provinces, especially in live poultry markets. Importantly, the H7N9 low pathogenic AIVs (LPAIVs) evolved into highly pathogenic AIVs (HPAIVs) in the beginning of 2017, causing a greater threat to human health and devastating losses to the poultry industry. Fortunately, nationwide vaccination of chickens with an H5/H7 bivalent inactivated avian influenza vaccine since September 2017 has successfully controlled H7N9 avian influenza infections in poultry and, importantly, has also prevented human infections. In this review, we summarize the biological properties of the H7N9 viruses, specifically their genetic evolution, adaptation, pathogenesis, receptor binding, transmission, drug resistance, and antigenic variation, as well as the prevention and control measures. The information obtained from investigating and managing the H7N9 viruses could improve our ability to understand other novel AIVs and formulate effective measures to control their threat to humans and animals.

Genetic and biological properties of H7N9 avian influenza viruses detected after application of the H7N9 poultry vaccine in China

The H7N9 avian influenza virus (AIV) that emerged in China have caused five waves of human infection. Further human cases have been successfully prevented since September 2017 through the use of an H7N9 vaccine in poultry. However, the H7N9 AIV has not been eradicated from poultry in China, and its evolution remains largely unexplored. In this study, we isolated 19 H7N9 AIVs during surveillance and diagnosis from February 2018 to December 2019, and genetic analysis showed that these viruses have formed two different genotypes. Animal studies indicated that the H7N9 viruses are highly lethal to chicken, cause mild infection in ducks, but have distinct pathotypes in mice. The viruses bound to avian-type receptors with high affinity, but gradually lost their ability to bind to human-type receptors. Importantly, we found that H7N9 AIVs isolated in 2019 were antigenically different from the H7N9 vaccine strain that was used for H7N9 influenza control in poultry, and that replication of these viruses cannot, therefore, be completely prevented in vaccinated chickens. We further revealed that two amino acid mutations at positions 135 and 160 in the HA protein added two glycosylation sites and facilitated the escape of the H7N9 viruses from the vaccine-induced immunity. Our study provides important insights into H7N9 virus evolution and control.

Establishing a Multicolor Flow Cytometry to Characterize Cellular Immune Response in Chickens Following H7N9 Avian Influenza Virus Infection

Avian influenza virus (AIV) emerged and has continued to re-emerge, continuously posing great threats to animal and human health. The detection of hemagglutination inhibition (HI) or virus neutralization antibodies (NA) is essential for assessing immune protection against AIV. However, the HI/NA-independent immune protection is constantly observed in vaccines’ development against H7N9 subtype AIV and other subtypes in chickens and mammals, necessitating the analysis of the cellular immune response. Here, we established a multi-parameter flow cytometry to examine the innate and adaptive cellular immune responses in chickens after intranasal infection with low pathogenicity H7N9 AIV. This assay allowed us to comprehensively define chicken macrophages, dendritic cells, and their MHC-II expression, NK cells, γδ T cells, B cells, and distinct T cell subsets in steady state and during infection. We found that NK cells and KUL01⁺ cells significantly increased after H7N9 infection, especially in the lung, and the KUL01⁺ cells upregulated MHC-II and CD11c expression. Additionally, the percentages and numbers of γδ T cells and CD8 T cells significantly increased and exhibited an activated phenotype with significant upregulation of CD25 expression in the lung but not in the spleen and blood. Furthermore, B cells showed increased in the lung but decreased in the blood and spleen in terms of the percentages or/and numbers, suggesting these cells may be recruited from the periphery after H7N9 infection. Our study firstly disclosed that H7N9 infection induced local and systemic cellular immune responses in chickens, the natural host of AIV, and that the flow cytometric assay developed in this study is useful for analyzing the cellular immune responses to AIVs and other avian infectious diseases and defining the correlates of immune protection.

Non-Mouse-Adapted H1N1pdm09 Virus as a Model for Influenza Research

The number of lung-adapted influenza viruses is limited. Most of them are not antigenically related to current circulating viruses. Viruses similar to recent strains are required for screening modern antiviral compounds and studying new vaccine candidates against novel influenza viruses. The process by which an influenza virus adapts to a new host is rather difficult. The aim of this study was to select a non-adapted current virus whose major biological properties correspond to those of classical lab-adapted viruses. Mice were inoculated intranasally with non-lung-adapted influenza viruses of subtype H1N1pdm09. They were monitored closely for body weight loss, mortality outcomes and gross pathology for 14 days following inoculation, as well as viral replication in lung tissue. Lung-adapted PR8 virus was used as a control. The tested viruses multiplied equally well in the lower respiratory tract of mice without prior adaptation but dramatically differed in lethality; the differences in their toxicity and pathogenicity in mice were established. A/South Africa/3626/2013 (H1N1)pdm09 virus was found to be an appropriate candidate to replace PR8 as a model virus for influenza research. No prior adaptation to the animal model is needed to reach the pathogenicity level of the classical mouse-adapted PR8 virus.

Single Immunization with Newcastle Disease Virus-Vectored H7N9 Vaccine Confers a Complete Protection Against Challenge with Highly Pathogenic Avian Influenza H7N9 Virus

In the fifth wave of the H7N9 avian influenza epidemic, highly pathogenic avian influenza (HPAI) A (H7N9) viruses have emerged and pose a great challenge to public health and the poultry industry. In addition, there are apparent genetic and antigenic variations between the classical H7N9 avian influenza virus and the newly-emerged H7N9 virus. Therefore, an antigenic-match vaccine is required for the prevention and control of H7N9 avian influenza in poultry in China. In this study, a recombinant Newcastle disease virus (NDV)-vectored vaccine expressing the HA derived from a prevailing HPAI H7N9 virus (GD15) was generated using reverse genetics. The recombinant virus (rAI4HA) showed virus yield and growth capacity in chicken embryos comparable to the parental virus (rAI4). Expression of the HA protein was detected in chicken embryo fibroblasts inoculated with rAI4HA. A chicken immunization study demonstrated that both rAI4HA and rAI4 induced similar anti-NDV hemagglutination inhibition (HI) antibody titers at weeks 2, 3, and 4 after a single immunization. However, rAI4HA-immunized chickens had a low seroconversion rate (20%) and negative HI titers against H7N9. Additionally, rAI4HA elicited high levels of H7N9-specifc IgY antibody as measured by ELISA. More importantly, the recombinant vaccine provided a complete protection against a lethal challenge with HPAI H7N9 virus and significantly inhibited virus shedding after a single immunization. Our results suggest that the recombinant NDV-vectored H7N9 vaccine expressing the antigenic-match HA can confer a complete protection against HPAI H7N9 challenge after a single immunization.

Avian influenza overview May - August 2018

Between 16 May and 15 August 2018, three highly pathogenic avian influenza (HPAI) A(H5N8) outbreaks in poultry establishments and three HPAI A(H5N6) outbreaks in wild birds were reported in Europe. Three low pathogenic avian influenza (LPAI) outbreaks were reported in three Member States. Few HPAI and LPAI bird cases have been detected in this period of the year, in accordance with the seasonal expected pattern of LPAI and HPAI. There is no evidence to date that HPAI A(H5N8) and A(H5N6) viruses circulating in Europe have caused any human infections. The risk of zoonotic transmission to the general public in Europe is considered to be very low. Several HPAI outbreaks in poultry were reported during this period from Russia. The presence of the A(H5N2) and A(H5N8) viruses in parts of Russia connected with fall migration routes of wild birds is of concern for possible introduction and spread with wild birds migrating to the EU. Although few AI outbreaks were observed in Africa, Asia and the Middle East during the reporting period, the probability of AI virus introductions from non‐EU countries via wild birds particularly via the north‐eastern route from Russia is increasing, as the fall migration of wild birds will start in the coming weeks. Further, the lower temperatures in autumn and winter may facilitate the environmental survival of avian influenza viruses potentially introduced to Europe.