Low population serum microneutralization antibody titer against the predominating influenza A(H3N2) N121K virus during the severe influenza summer peak of Hong Kong in 2017

Recent H9N2 avian influenza virus lost hemagglutination activity due to a K141N substitution in hemagglutinin

The H9N2 avian influenza virus (AIV) represents a significant risk to both the poultry industry and public health. Our surveillance efforts in China have revealed a growing trend of recent H9N2 AIV strains exhibiting a loss of hemagglutination activity at 37°C, posing challenges to detection and monitoring protocols. This study identified a single K141N substitution in the hemagglutinin (HA) glycoprotein as the culprit behind this diminished hemagglutination activity. The study evaluated the evolutionary dynamics of residue HA141 and studied the impact of the N141K substitution on aspects such as virus growth, thermostability, receptor-binding properties, and antigenic properties. Our findings indicate a polymorphism at residue 141, with the N variant becoming increasingly prevalent in recent Chinese H9N2 isolates. Although both wild-type and N141K mutant strains exclusively target α,2-6 sialic acid receptors, the N141K mutation notably impedes the virus's ability to bind to these receptors. Despite the mutation exerting minimal influence on viral titers, antigenicity, and pathogenicity in chicken embryos, it significantly enhances viral thermostability and reduces plaque size on Madin-Darby canine kidney (MDCK) cells. Additionally, the N141K mutation leads to decreased expression levels of HA protein in both MDCK cells and eggs. These findings highlight the critical role of the K141N substitution in altering the hemagglutination characteristics of recent H9N2 AIV strains under elevated temperatures. This emphasizes the need for ongoing surveillance and genetic analysis of circulating H9N2 AIV strains to develop effective control and prevention measures.IMPORTANCEThe H9N2 subtype of avian influenza virus (AIV) is currently the most prevalent low-pathogenicity AIV circulating in domestic poultry globally. Recently, there has been an emerging trend of H9N2 AIV strains acquiring increased affinity for human-type receptors and even losing their ability to bind to avian-type receptors, which raises concerns about their pandemic potential. In China, there has been a growing number of H9N2 AIV strains that have lost their ability to agglutinate chicken red blood cells, leading to false-negative results during surveillance efforts. In this study, we identified a K141N mutation in the HA protein of H9N2 AIV to be responsible for the loss of hemagglutination activity. This finding provides insight into the development of effective surveillance, prevention, and control strategies to mitigate the threat posed by H9N2 AIV to both animal and human health.

Loss of amino acids 67-76 in the neuraminidase protein under antibody selection pressure alters the tropism, transmissibility and innate immune response of H9N2 avian influenza virus in chickens

H9N2 virus has become the most widespread subtype of avian influenza in Chinese poultry. Although many studies have been published on this disease, the pathogenesis of the H9N2 virus remains to be fully understood. In our previous work, we identified 44 viral strains with 67-76 amino acid deletions in the neuraminidase protein (NA∆67-76) from trachea and lung tissues after 20 successive generations in vaccinated chickens. Interestingly, these 10 amino acid deletions are located in the stalk of the NA protein, and all mutations were unique to the viruses under the selection pressure of vaccine antibodies. To investigate the effect of NA∆67-76 on the H9N2 virus, the NA∆67-76 deletion mutant (rF/NAΔ67-76) was constructed in the H9N2 virus A/Chicken/Shanghai/F/98 (F/98) to assess the phenotypic changes between the parental and mutant strains. The results showed that the recombinant virus rF/NAΔ67-76 had no significantly effect on the antigenicity of the virus or on the infectivity of the host cells, but it significantly inhibited the release of virions from host cells. In addition, rF/NAΔ67-76 efficiently enhanced the neuraminidase activity and improved the receptor binding ability of the virus, indicating that the influence of receptor binding ability on the rF/NAΔ67-76 virus is much greater than that of neuraminidase activity. Furthermore, this study revealed that rF/NAΔ67-76 reduced the viral replication ability at 6 and 12 h post-infection, but improved it at 24, 48, and 72 h post-infection. Chicken experiments showed that rF/NAΔ67-76 exhibits a much higher tissue tropism for the trachea rather than lung tissue. rF/NAΔ67-76 still had the ability to infect the upper respiratory tract through aerosol, but its cloaca replication capacity was significantly reduced. Both in vivo and in vitro experiments confirmed that rF/NAΔ67-76 could produce a stronger innate immune response after infecting cells and chickens, especially significantly enhancing the transcription levels of TLR3, TLR4, TLR7, TLR21, MDA5, and NLRP3. Altogether, the results of this study propose that antibody selection pressure plays an important role in the evolution of H9N2 avian influenza virus.

IgG3 subclass antibodies recognize antigenically drifted influenza viruses and SARS-CoV-2 variants through efficient bivalent binding

The constant domains of antibodies are important for effector functions, but less is known about how they can affect binding and neutralization of viruses. Here, we evaluated a panel of human influenza virus monoclonal antibodies (mAbs) expressed as IgG1, IgG2, or IgG3. We found that many influenza virus-specific mAbs have altered binding and neutralization capacity depending on the IgG subclass encoded and that these differences result from unique bivalency capacities of the subclasses. Importantly, subclass differences in antibody binding and neutralization were greatest when the affinity for the target antigen was reduced through antigenic mismatch. We found that antibodies expressed as IgG3 bound and neutralized antigenically drifted influenza viruses more effectively. We obtained similar results using a panel of SARS-CoV-2-specific mAbs and the antigenically advanced B.1.351 and BA.1 strains of SARS-CoV-2. We found that a licensed therapeutic mAb retained neutralization breadth against SARS-CoV-2 variants when expressed as IgG3, but not IgG1. These data highlight that IgG subclasses are not only important for fine-tuning effector functionality but also for binding and neutralization of antigenically drifted viruses.

IgG3 subclass antibodies recognize antigenically drifted influenza viruses and SARS-CoV-2 variants through efficient bivalent binding

The constant domains of antibodies are important for effector functions, but less is known about how they can affect binding and neutralization of viruses. Here we evaluated a panel of human influenza virus monoclonal antibodies (mAbs) expressed as IgG1, IgG2 or IgG3. We found that many influenza virus-specific mAbs have altered binding and neutralization capacity depending on the IgG subclass encoded, and that these differences result from unique bivalency capacities of the subclasses. Importantly, subclass differences in antibody binding and neutralization were greatest when the affinity for the target antigen was reduced through antigenic mismatch. We found that antibodies expressed as IgG3 bound and neutralized antigenically drifted influenza viruses more effectively. We obtained similar results using a panel of SARS-CoV-2-specific mAbs and the antigenically advanced B.1.351 strain of SARS-CoV-2. We found that a licensed therapeutic mAb retained neutralization breadth against SARS-CoV-2 variants when expressed as IgG3, but not IgG1. These data highlight that IgG subclasses are not only important for fine-tuning effector functionality, but also for binding and neutralization of antigenically drifted viruses.

Significance

Influenza viruses and coronaviruses undergo continuous change, successfully evading human antibodies elicited from prior infections or vaccinations. It is important to identify features that allow antibodies to bind with increased breadth. Here we examined the effect that different IgG subclasses have on monoclonal antibody binding and neutralization. We show that IgG subclass is a determinant of antibody breadth, with IgG3 affording increased neutralization of antigenically drifted variants of influenza virus and SARS-CoV-2. Future studies should evaluate IgG3 therapeutic antibodies and vaccination strategies or adjuvants that may skew antibody responses toward broadly reactive isotypes.

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.

Seroprevalence of influenza viruses in Shandong, Northern China during the COVID-19 pandemic

Nonpharmaceutical interventions (NPIs) have been commonly deployed to prevent and control the spread of the coronavirus disease 2019 (COVID-19), resulting in a worldwide decline in influenza prevalence. However, the influenza risk in China warrants cautious assessment. We conducted a cross-sectional, seroepidemiological study in Shandong Province, Northern China in mid-2021. Hemagglutination inhibition was performed to test antibodies against four influenza vaccine strains. A combination of descriptive and meta-analyses was adopted to compare the seroprevalence of influenza antibodies before and during the COVID-19 pandemic. The overall seroprevalence values against A/H1N1pdm09, A/H3N2, B/Victoria, and B/Yamagata were 17.8% (95% CI 16.2%-19.5%), 23.5% (95% CI 21.7%-25.4%), 7.6% (95% CI 6.6%-8.7%), and 15.0 (95% CI 13.5%-16.5%), respectively, in the study period. The overall vaccination rate was extremely low (2.6%). Our results revealed that antibody titers in vaccinated participants were significantly higher than those in unvaccinated individuals (P < 0.001). Notably, the meta-analysis showed that antibodies against A/H1N1pdm09 and A/H3N2 were significantly low in adults after the COVID-19 pandemic (P < 0.01). Increasing vaccination rates and maintaining NPIs are recommended to prevent an elevated influenza risk in China.

Intradermal vaccination of live attenuated influenza vaccine protects mice against homologous and heterologous influenza challenges

We previously developed a temperature-sensitive, and NS1 gene deleted live attenuated influenza vaccine (DelNS1-LAIV) and demonstrated its potent protective efficacy in intranasally vaccinated mice. Here we investigated whether intradermal (i.d.) vaccination induces protective immunity. Our results showed that DelNS1-LAIV intradermal vaccination conferred effective and long-lasting protection against lethal virus challenge in mice. A single intradermal injection of DelNS1-LAIV conferred 100% survival with no weight loss in mice after A(H1N1)09 influenza virus (H1N1/415742Md) challenge. DelNS1-LAIV injection resulted in a significant reduction of lung viral load and reduced airway epithelial cell death and lung inflammatory cytokine responses at day 2 and 4 post challenge. Full protections of mice lasted for 6 months after immunization. In vitro infection of DelNS1-LAIV in monocyte-derived dendritic cells (MoDCs) demonstrated activation of antigen-presenting cells at 33 °C, together with the results of abortive replication of DelNS1-LAIV in skin tissue and strong upregulation of inflammatory cytokines/chemokines expression, our results suggested the strong immunogenicity of this vaccine. Further, we demonstrate that the underlying protection mechanism induced by intradermal DelNS1-LAIV is mainly attributed to antibody responses. Together, this study opens up an alternative route for the administration of LAIV, which may benefit individuals not suitable for intranasal LAIV immunization.

Co-circulation and persistence of multiple A/H3N2 influenza variants in China

The spread of influenza A/H3N2 variants possessing the hemagglutinin 121 K mutation and the unexpectedly high incidence of influenza in the 2017–2018 northern hemisphere influenza season have raised serious concerns about the next pandemic. We summarized the national surveillance data of seasonal influenza in China and identified marked differences in influenza epidemics between northern and southern China, particularly the predominating subtype and the presence of an additional summer peak in southern China. Notably, a minor spring peak of influenza caused by a different virus subtype was also observed. We also revealed that the 3C.2a lineage was dominant from the summer of 2015 to the end of the 2015–2016 peak season in China, after which the 3C.2a2 lineage predominated despite the importation and co-circulation of the 121 K variants of 3C.2a1 and 3C.2a3 lineages at the global level. Finally, an analysis based on genetic distances revealed a delay in A/H3N2 vaccine strain update. Overall, our results highlight the complicated circulation pattern of seasonal influenza in China and the necessity for a timely vaccine strain update worldwide.

Assessment of population susceptibility to upcoming seasonal influenza epidemic strain using interepidemic emerging influenza virus strains

Seasonal influenza virus epidemics have a major impact on healthcare systems. Data on population susceptibility to emerging influenza virus strains during the interepidemic period can guide planning for resource allocation of an upcoming influenza season. This study sought to assess the population susceptibility to representative emerging influenza virus strains collected during the interepidemic period. The microneutralisation antibody titers (MN titers) of a human serum panel against representative emerging influenza strains collected during the interepidemic period before the 2018/2019 winter influenza season (H1N1-inter and H3N2-inter) were compared with those against influenza strains representative of previous epidemics (H1N1-pre and H3N2-pre). A multifaceted approach, incorporating both genetic and antigenic data, was used in selecting these representative influenza virus strains for the MN assay. A significantly higher proportion of individuals had a ⩾four-fold reduction in MN titers between H1N1-inter and H1N1-pre than that between H3N2-inter and H3N2-pre (28.5% (127/445) vs. 4.9% (22/445), P < 0.001). The geometric mean titer (GMT) of H1N1-inter was significantly lower than that of H1N1-pre (381 (95% CI 339-428) vs. 713 (95% CI 641-792), P < 0.001), while there was no significant difference in the GMT between H3N2-inter and H3N2-pre. Since A(H1N1) predominated the 2018-2019 winter influenza epidemic, our results corroborated the epidemic subtype.

Intense interseasonal influenza outbreaks, Australia, 2018/19

BackgroundInterseasonal influenza outbreaks are not unusual in countries with temperate climates and well-defined influenza seasons. Usually, these are small and diminish before the main influenza season begins. However, the 2018/19 summer-autumn interseasonal influenza period in Australia saw unprecedented large and widespread influenza outbreaks.AimOur objective was to determine the extent of the intense 2018/19 interseasonal influenza outbreaks in Australia epidemiologically and examine the genetic, antigenic and structural properties of the viruses responsible for these outbreaks.MethodsThis observational study combined the epidemiological and virological surveillance data obtained from the Australian Government Department of Health, the New South Wales Ministry of Health, sentinel outpatient surveillance, public health laboratories and data generated by the World Health Organization Collaborating Centre for Reference and Research on Influenza in Melbourne and the Singapore Agency for Science, Technology and Research.ResultsThere was a record number of laboratory-confirmed influenza cases during the interseasonal period November 2018 to May 2019 (n= 85,286; 5 times the previous 3-year average) and also more institutional outbreaks, hospitalisations and deaths, than what is normally seen.ConclusionsThe unusually large interseasonal influenza outbreaks in 2018/19 followed a mild 2018 influenza season and resulted in a very early start to the 2019 influenza season across Australia. The reasons for this unusual event have yet to be fully elucidated but are likely to be a complex mix of climatic, virological and host immunity-related factors. These outbreaks reinforce the need for year-round surveillance of influenza, even in temperate climates with strong seasonality patterns.

Progress and Current Status of Influenza Researches in China

Influenza pandemics with different extent occur every year in the world. It can cause high morbidity and mortality, arouse fear panic in public, and attract extensive attention worldwide. This paper reviews the research progress in epidemiological characteristics, detection methods, pathogenesis, treatment and prophylactic measures of influenza in China. It will be helpful for us to understand the current situation of influenza.

Molecular epidemiology survey and characterization of human influenza A viruses circulating among Palestinians in East Jerusalem and the West Bank in 2015

Frequent typing and molecular characterization of influenza A (IAV) strains are crucial for the identification of circulating subtypes and for the selection of the subtypes’ lineages to be included in the annually prepared vaccine cocktail. We investigated IAV sampled from an underrepresented population from Palestine. 200 nasopharyngeal aspirates (NPA) were collected between February and May of 2015 from Palestinians in East Jerusalem and the West Bank suffering from mild to severe symptoms of upper respiratory infections. NPA were screened for the presence of IAV using RT-PCR. Epidemiological data, hemagglutinin (HA) and neuraminidase (NA) gene sequences were analyzed in IAV positive samples. 50 samples tested positive for IAV; 48% of which were identified as A(H1N1)pdm09 and 52% as A(H3N2), respectively. Infection with A(H1N1)pdm09 occurred mainly in April, while A(H3N2) infections were mainly detected in March. Most IAV infections in 6-year-olds and below were attributed to subtype A(H3N2), while A(H1N1)pdm09 was responsible for most infections in adults above 18-year-olds. Analyses of HA and NA amino acid sequences revealed numerous substitutions. Thereafter, and based on the HA analysis, the Palestinian A(H1N1)pdm09 isolates fell into clade 6B, while the A(H3N2) isolates fell into clades 3C.2 and 3C.3, respectively. This study is significant in providing the first insight into the epidemiology and genetic properties of IAV circulating in Palestine. In contrast to international reports for the same season, A(H3N2) was not the dominant subtype as in northern hemisphere, nor was A(H1N1)pdm09 as in WHO reports for the Middle East, however genetic properties of Palestinian A(H3N2) and A(H1N1)pdm09 were in line with global isolates.

The effectiveness of influenza vaccination against medically-attended illnesses in Hong Kong across three years with different degrees of vaccine match, 2014-17

BACKGROUND

Influenza vaccination is the most effective intervention to prevent influenza virus infections. Vaccine effectiveness (VE) can vary due to factors such as matching between vaccine strains and prevailing strains, age and other characteristics of the vaccine recipients.

OBJECTIVE

To evaluate influenza VE against medically-attended illness in different age groups and against specific influenza types/subtypes in Hong Kong.

METHODS

A test-negative study was conducted from December 2014 through August 2017 in 20 outpatient clinics. Patients at least 6 months of age presenting with at least two symptoms of acute respiratory illness, ARI (fever ≥37.8 °C, cough, sore throat, runny nose, headache, myalgia and phlegm) within 72 h of onset were tested for influenza virus by reverse transcription polymerase chain reaction (PCR). Vaccination history was assessed by self-report or medical records at the clinics. VE against medically-attended illness was estimated using conditional logistic regression for influenza PCR result versus vaccination history, matching by calendar time and adjusting for age, age-squared, sex, and chronic medical illness. Additional analyses examined VE by age group and by influenza type/subtype.

RESULTS

We enrolled 2566 patients, of whom 1118 (43.6%) tested positive for influenza A or B virus by PCR. Test-positive subjects were generally older, more likely to present with one of the symptoms of ARI, and less likely to receive vaccination against influenza. VE estimates for influenza A(H1N1), A(H3N2), B/Yamagata and B/Victoria were 61.6% (95% confidence interval, CI: 21.8%, 81.1%), 26.4% (95% CI: -1.3%, 46.6%), 67.0% (95% CI: 25.9%, 85.3%), 60.4% (95% CI: 0.3%, 84.3%), respectively. Estimates of VE by age group were generally higher in adults aged 50-64 and lower among children and older adults.

CONCLUSIONS

VE against medically-attended influenza was moderate in Hong Kong, confirming the impact of influenza vaccination in reducing disease burden. The reduced VE for influenza A(H3N2) is a continuing concern.