Development of subtype-specific and heterosubtypic antibodies to the influenza A virus hemagglutinin after primary infection in children

Advances in Detection Techniques for the H5N1 Avian Influenza Virus

Avian influenza is caused by avian influenza virus infection; the H5N1 avian influenza virus is a highly pathogenic subtype, affecting poultry and human health. Since the discovery of the highly pathogenic subtype of the H5N1 avian influenza virus, it has caused enormous losses to the poultry farming industry. It was recently found that the H5N1 avian influenza virus tends to spread among mammals. Therefore, early rapid detection methods are highly significant for effectively preventing the spread of H5N1. This paper discusses the detection technologies used in the detection of the H5N1 avian influenza virus, including serological detection technology, immunological detection technology, molecular biology detection technology, genetic detection technology, and biosensors. Comparisons of these detection technologies were analyzed, aiming to provide some recommendations for the detection of the H5N1 avian influenza virus.

Antibody Landscape Analysis following Influenza Vaccination and Natural Infection in Humans with a High-Throughput Multiplex Influenza Antibody Detection Assay

Repeated influenza vaccination and natural infections generate complex immune profiles in humans that require antibody landscape analysis to assess immunity and evaluate vaccines. However, antibody landscape analyses are difficult to perform using traditional assays.

To better understand the antibody landscape changes following influenza virus natural infection and vaccination, we developed a high-throughput multiplex influenza antibody detection assay (MIADA) containing 42 recombinant hemagglutinins (rHAs) (ectodomain and/or globular head domain) from pre-2009 A(H1N1), A(H1N1)pdm09, A(H2N2), A(H3N2), A(H5N1), A(H7N7), A(H7N9), A(H7N2), A(H9N2), A(H13N9), and influenza B viruses. Panels of ferret antisera, 227 paired human sera from vaccinees (children and adults) in 5 influenza seasons (2010 to 2018), and 17 paired human sera collected from real-time reverse transcription-PCR (rRT-PCR)-confirmed influenza A(H1N1)pdm09, influenza A(H3N2), or influenza B virus-infected adults were analyzed by the MIADA. Ferret antisera demonstrated clear strain-specific antibody responses to exposed subtype HA. Adults (19 to 49 years old) had broader antibody landscapes than young children (<3 years old) and older children (9 to 17 years old) both at baseline and post-vaccination. Influenza vaccination and infection induced the strongest antibody responses specific to HA(s) of exposed strain/subtype viruses and closely related strains; they also induced cross-reactive antibodies to an unexposed influenza virus subtype(s), including novel viruses. Subsequent serum adsorption confirmed that the cross-reactive antibodies against novel subtype HAs were mainly induced by exposures to A(H1N1)/A(H3N2) influenza A viruses. In contrast, adults infected by influenza B viruses mounted antibody responses mostly specific to two influenza B virus lineage HAs. Median fluorescence intensities (MFIs) and seroconversion in MIADA had good correlations with the titers and seroconversion measured by hemagglutination inhibition and microneutralization assays. Our study demonstrated that antibody landscape analysis by the MIADA can be used for influenza vaccine evaluations and characterization of influenza virus infections.

Identification of novel influenza A virus exposures by an improved high-throughput multiplex MAGPIX platform and serum adsorption

Background

The development of serologic assays that can rapidly assess human exposure to novel influenza viruses remains a public health need. Previously, we developed an 11‐plex magnetic fluorescence microsphere immunoassay (MAGPIX) by using globular head domain recombinant hemagglutinins (rHAs) with serum adsorption using two ectodomain rHAs.

Methods

We compared sera collected from two cohorts with novel influenza exposures: animal shelter staff during an A(H7N2) outbreak in New York City in 2016‐2017 (n = 119 single sera) and poultry workers from a live bird market in Bangladesh in 2012‐2014 (n = 29 pairs). Sera were analyzed by microneutralization (MN) assay and a 20‐plex MAGPIX assay with rHAs from 19 influenza strains (11 subtypes) combined with serum adsorption using 8 rHAs from A(H1N1) and A(H3N2) viruses. Antibody responses were analyzed to determine the novel influenza virus exposure.

Results

Among persons with novel influenza virus exposures, the median fluorescence intensity (MFI) against the novel rHA from exposed influenza virus had the highest correlation with MN titers to the same viruses and could be confirmed by removal of cross‐reactivity from seasonal H1/H3 rHAs following serum adsorption. Interestingly, in persons with exposures to novel influenza viruses, age and MFIs against exposed novel HA were negatively correlated, whereas in persons without exposure to novel influenza viruses, age and MFI against novel HAs were positively correlated.

Conclusions

This 20‐plex high‐throughput assay with serum adsorption will be a useful tool to detect novel influenza virus infections during influenza outbreak investigations and surveillance, especially when well‐paired serum samples are not available.

Dual Recognition Element Lateral Flow Assay Toward Multiplex Strain Specific Influenza Virus Detection

Different influenza virus strains have caused a number of recent outbreaks killing scores of people and causing significant losses in animal farming. Simple, rapid, sensitive, and specific detection of particular strains, such as a pandemic strain versus a previous seasonal influenza, plays a crucial role in the monitoring, controlling, and management of outbreaks. In this paper we describe a dual recognition element lateral flow assay (DRELFA) which pairs a nucleic acid aptamer with an antibody for use as a point-of-care platform which can detect particular strains of interest. The combination is used to overcome the individual limitations of antibodies’ cross-reactivity and aptamers’ slow binding kinetics. In the detection of influenza viruses, we show that DRELFA can discriminate a particular virus strain against others of the same subtype or common respiratory diseases while still exhibiting fast binding kinetic of the antibody-based lateral flow assay (LFA). The improvement in specificity that DRELFA exhibits is an advantage over the currently available antibody-based LFA systems for influenza viruses, which offer discrimination between influenza virus types and subtypes. Using quantitative real-time PCR (qRT-PCR), it showed that the DRELFA is very effective in localizing the analyte to the test line (consistently over 90%) and this is crucial for the sensitivity of the device. In addition, color intensities of the test lines showed a good correlation between the DRELFA and the qRT-PCR over a 50-fold concentration range. Finally, lateral flow strips with a streptavidin capture test line and an anti-antibody control line are universally applicable to specific detection of a wide range of different analytes.

Novel multiplex assay platforms to detect influenza A hemagglutinin subtype-specific antibody responses for high-throughput and in-field applications

BACKGROUND

Detections of influenza A subtype-specific antibody responses are often complicated by the presence of cross-reactive antibodies. We developed two novel multiplex platforms for antibody detection. The multiplexed magnetic fluorescence microsphere immunoassay (MAGPIX) is a high-throughput laboratory-based assay. Chembio Dual Path Platform (DPP) is a portable and rapid test that could be used in the field.

METHODS

Twelve recombinant globular head domain hemagglutinin (GH HA1) antigens from A(H1N1)pdm09 (pH1N1), A(H2N2), A(H3N2), A(H5N1), A(H7N9), A(H9N2), A(H13N9), B/Victoria lineage, B/Yamagata lineage viruses, and protein A control were used. Human sera from U.S. residents either vaccinated (with H5N1 or pH1N1) or infected with pH1N1 influenza viruses and sera from live bird market workers in Bangladesh (BDPW) were evaluated. GH HA1 antigens and serum adsorption using full ectodomain recombinant hemagglutinins from A(pH1N1) and A(H3N2) were introduced into the platforms to reduce cross-reactivity.

RESULTS

Serum adsorption reduced cross-reactivity to novel subtype HAs. Compared to traditional hemagglutination inhibition or microneutralization assays, when serum adsorption and the highest fold rise in signals were used to determine positivity, the correct subtype-specific responses were identified in 86%-100% of U.S. residents exposed to influenza antigens through vaccination or infection (N=49). For detection of H5N1-specific antibodies in sera collected from BDPW, H5 sensitivity was 100% (six of six) for MAGPIX, 83% (five of six) for DPP, H5 specificity was 100% (15/15), and cross-reactivity against other subtype was 0% (zero of six) for both platforms.

CONCLUSION

MAGPIX and DPP platforms can be utilized for high-throughput and in-field detection of novel influenza virus infections.

Evaluation of multiplex assay platforms for detection of influenza hemagglutinin subtype specific antibody responses

Influenza hemagglutination inhibition (HI) and virus microneutralization assays (MN) are widely used for seroprevalence studies. However, these assays have limited field portability and are difficult to fully automate for high throughput laboratory testing. To address these issues, three multiplex influenza subtype-specific antibody detection assays were developed using recombinant hemagglutinin antigens in combination with Chembio, Luminex^®, and ForteBio^® platforms. Assay sensitivity, specificity, and subtype cross-reactivity were evaluated using a panel of well characterized human sera. Compared to the traditional HI, assay sensitivity ranged from 87% to 92% and assay specificity in sera collected from unexposed persons ranged from 65% to 100% across the platforms. High assay specificity (86-100%) for A(H5N1) rHA was achieved for sera from exposed or unexposed to hetorosubtype influenza HAs. In contrast, assay specificity for A(H1N1)pdm09 rHA using sera collected from A/Vietnam/1204/2004 (H5N1) vaccinees in 2008 was low (22-30%) in all platforms. Although cross-reactivity against rHA subtype proteins was observed in each assay platform, the correct subtype specific responses were identified 78%-94% of the time when paired samples were available for analysis. These results show that high throughput and portable multiplex assays that incorporate rHA can be used to identify influenza subtype specific infections.

Cross-reactivity between avian influenza A (H7N9) virus and divergent H7 subtypic- and heterosubtypic influenza A viruses

The number of human avian H7N9 influenza infections has been increasing in China. Understanding their antigenic and serologic relationships is crucial for developing diagnostic tools and vaccines. Here, we evaluated the cross-reactivities and neutralizing activities among H7 subtype influenza viruses and between H7N9 and heterosubtype influenza A viruses. We found strong cross-reactivities between H7N9 and divergent H7 subtypic viruses, including H7N2, H7N3, and H7N7. Antisera against H7N2, H7N3, and H7N7 could also effectively neutralize two distinct H7N9 strains. Two-way cross-reactivities exist within group 2, including H3 and H4, whereas one-way cross-reactivities were found across other groups, including H1, H10, H9, and H13. Our data indicate that the hemaglutinins from divergent H7 subtypes may facilitate the development of vaccines for distinct H7N9 infections. Moreover, serologic diagnoses for H7N9 infections need to consider possible interference from the cross-reactivity of H7N9 with other subtype influenza viruses.

Antigenic Maps of Influenza A(H3N2) Produced With Human Antisera Obtained After Primary Infection

BACKGROUND

Antigenic characterization of influenza viruses is typically based on hemagglutination inhibition (HI) assay data for viral isolates tested against strain-specific postinfection ferret antisera. Here, similar virus characterizations were performed using serological data from humans with primary influenza A(H3N2) infection.

METHODS

We screened sera collected between 1995 and 2011 from children between 9 and 24 months of age for influenza virus antibodies, performed HI tests for the positive sera against 23 influenza viruses isolated between 1989 and 2011, and measured HI titers of antisera against influenza A(H3N2) from 24 ferrets against the same panel of viruses.

RESULTS

Of the 17 positive human sera, 6 had a high response, showing HI patterns that would be expected from primary infection antisera, while 11 sera had lower, more dispersed patterns of reactivity that are not easily explained. The antigenic map based on the high-response human HI data was similar to the map created using ferret data.

CONCLUSIONS

Although the overall structure of the ferret and human antigenic maps is similar, local differences in virus positions indicate that the human and ferret immune system might see antigenic properties of viruses differently. Further studies are needed to establish the degree of similarity between serological patterns in ferret and human data.

Multi-Dimensional Measurement of Antibody-Mediated Heterosubtypic Immunity to Influenza

The human immune response to influenza vaccination depends in part on preexisting cross-reactive (heterosubtypic) immunity from previous infection by, and/or vaccination with, influenza strains that share antigenic determinants with the vaccine strains. However, current methods for assessing heterosubtypic antibody responses against influenza, including the hemagglutination-inhibition (HAI) assay and ELISA, are time and labor intensive, and require moderate amounts of serum and reagents. To address these issues we have developed a fluorescent multiplex assay, mPlex-Flu, that rapidly and simultaneously measures strain specific IgG, IgA, and IgM antibodies against influenza hemagglutinin (HA) from multiple viral strains. We cloned, expressed and purified HA proteins from 12 influenza strains, and coupled them to multiplex beads. Assay validation showed that minimal sample volumes (<5 μl of serum) were needed, and the assay had a linear response over a four Log₁₀ range. The assay detected nanogram levels of anti-influenza specific antibodies, had high accuracy and reproducibility, with an average percentage coefficient of variation (%CV) of 9.06 for intra-assay and 12.94 for inter-assay variability. Pre- and post-intramuscular trivalent influenza vaccination levels of virus specific Ig were consistent with HAI titer and ELISA measurements. A significant advantage of the mPLEX-Flu assay over the HAI assay is the ability to perform antigenic cartography, determining the antigenic distances between influenza HA’s, without mathematical correction for HAI data issues. For validation we performed antigenic cartography on 14 different post-influenza infection ferret sera assayed against 12 different influenza HA’s. Results were in good agreement with a phylogenetic tree generated from hierarchical clustering of the genomic HA sequences. This is the first report of the use of a multiplex method for antigenic cartography using ferret sera. Overall, the mPlex-Flu assay provides a powerful tool to rapidly assess the influenza antibody repertoire in large populations and to study heterosubtypic immunity induced by influenza vaccination.

Improved specificity and reduced subtype cross-reactivity for antibody detection by ELISA using globular head domain recombinant hemagglutinin

The relative performance of ELISA using globular head domain (GH) and ectodomain hemagglutinins (HAs) as antigens to detect influenza A virus IgG antibody responses was assessed. Assay sensitivity and subtype cross-reactivity were evaluated using sera collected from recipients of monovalent H5N1 vaccine and A(H1N1)pdm09 virus-infected persons. Assay specificity was determined using collections of sera from either individuals unexposed to either H5N1 or A(H1N1)pdm09 viruses or exposed to H5N1 or A(H1N1)pdm09 viruses through vaccination or infection, respectively. ELISA using GH HA showed a similar degree of sensitivity, significantly higher specificity, and significantly lower subtype cross-reactivity compared to ELISA using ectodomain HA.

Human antibody responses to avian influenza A(H7N9) virus, 2013

Understanding host antibody response is crucial for predicting disease severity and for vaccine development. We investigated antibody responses against influenza A(H7N9) virus in 48 serum samples from 21 patients, including paired samples from 15 patients. IgG against subtype H7 and neutralizing antibodies (NAbs) were not detected in acute-phase samples, but ELISA geometric mean titers increased in convalescent-phase samples; NAb titers were 20–80 (geometric mean titer 40). Avidity to IgG against subtype H7 was significantly lower than that against H1 and H3. IgG against H3 was boosted after infection with influenza A(H7N9) virus, and its level in acute-phase samples correlated with that against H7 in convalescent-phase samples. A correlation was also found between hemagglutinin inhibition and NAb titers and between hemagglutinin inhibition and IgG titers against H7. Because of the relatively weak protective antibody response to influenza A(H7N9), multiple vaccinations might be needed to achieve protective immunity.

Discordant correlation between serological assays observed when measuring heterosubtypic responses against avian influenza H5 and H7 viruses in unexposed individuals

The human population is constantly exposed to multiple influenza A subtypes due to zoonotic spillover and rapid viral evolution driven by intrinsic error-prone replication and immunological pressure. In this context, antibody responses directed against the HA protein are of importance since they have been shown to correlate with protective immunity. Serological techniques, detecting these responses, play a critical role for influenza surveillance, vaccine development, and assessment. As the recent human pandemics and avian influenza outbreaks have demonstrated, there is an urgent need to be better prepared to assess the contribution of the antibody response to protection against newly emerged viruses and to evaluate the extent of preexisting heterosubtypic immunity in populations. In this study, 68 serum samples collected from the Italian population between 1992 and 2007 were found to be positive for antibodies against H5N1 as determined by single radial hemolysis (SRH), but most were negative when evaluated using haemagglutination inhibition (HI) and microneutralisation (MN) assays. As a result of these discordant serological findings, the increased sensitivity of lentiviral pseudotypes was exploited in pseudotype-based neutralisation (pp-NT) assays and the results obtained provide further insight into the complex nature of humoral immunity against influenza A viruses.

IgM, IgG, and IgA antibody responses to influenza A(H1N1)pdm09 hemagglutinin in infected persons during the first wave of the 2009 pandemic in the United States

The novel influenza A(H1N1)pdm09 virus caused an influenza pandemic in 2009. IgM, IgG, and IgA antibody responses to A(H1N1)pdm09 hemagglutinin (HA) following A(H1N1)pdm09 virus infection were analyzed to understand antibody isotype responses. Age-matched control sera collected from U.S. residents in 2007 and 2008 were used to establish baseline levels of cross-reactive antibodies. IgM responses often used as indicators of primary virus infection were mainly detected in young patient groups (≤5 years and 6 to 15 years old), not in older age groups, despite the genetic and antigenic differences between the HA of A(H1N1)pdm09 virus and pre-2009 seasonal H1N1 viruses. IgG and IgA responses to A(H1N1)pdm09 HA were detected in all age groups of infected persons. In persons 17 to 80 years old, paired acute- and convalescent-phase serum samples demonstrated ≥4-fold increases in the IgG and IgA responses to A(H1N1)pdm09 HA in 80% and 67% of A(H1N1)pdm09 virus-infected persons, respectively. The IgG antibody response to A(H1N1)pdm09 HA was cross-reactive with HAs from H1, H3, H5, and H13 subtypes, suggesting that infections with subtypes other than A(H1N1)pdm09 might result in false positives by enzyme-linked immunosorbent assay (ELISA). Lower sensitivity compared to hemagglutination inhibition and microneutralization assays and the detection of cross-reactive antibodies against homologous and heterologous subtype are major drawbacks for the application of ELISA in influenza serologic studies.

Homosubtypic and heterosubtypic antibodies against highly pathogenic avian influenza H5N1 recombinant proteins in H5N1 survivors and non-H5N1 subjects

Six recombinant vaccinia viruses containing HA, NA, NP, M or NS gene insert derived from a highly pathogenic avian influenza H5N1 virus, and the recombinant vaccinia virus harboring plasmid backbone as the virus control were constructed. The recombinant proteins were characterized for their expression and subcellular locations in TK(-) cells. Antibodies to the five recombinant proteins were detected in all 13 sequential serum samples collected from four H5N1 survivors during four years of follow-up; and those directed to rVac-H5 HA and rVac-NA proteins were found in higher titers than those directed to the internal proteins as revealed by indirect immunofluorescence assay. Although all 28 non-H5N1 subjects had no neutralizing antibodies against H5N1 virus, they did have cross-reactive antibodies to those five recombinant proteins. A significant increase in cross-reactive antibody titer to rVac-H5 HA and rVac-NA was found in paired blood samples from patients infected with the 2009 pandemic virus.

Seasoned adaptive antibody immunity for highly pathogenic pandemic influenza in humans

Fundamentally new approaches are required for the development of vaccines to pre-empt and protect against emerging and pandemic influenzas. Current strategies involve post-emergent homotypic vaccines that are modelled upon select circulating 'seasonal' influenzas, but cannot induce cross-strain protection against newly evolved or zoonotically introduced highly pathogenic influenza (HPI). Avian H5N1 and the less-lethal 2009 H1N1 and their reassortants loom as candidates to seed a future HPI pandemic. Therefore, more universal 'seasoned' vaccine approaches are urgently needed for heterotypic protection ahead of time. Pivotal to this is the need to understand mechanisms that can deliver broad strain protection. Heterotypic and heterosubtypic humoral immunities have largely been overlooked for influenza cross-protection, with most 'seasoned' vaccine efforts for humans focussed on heterotypic cellular immunity. However, 5 years ago we began to identify direct and indirect indicators of humoral-herd immunity to protein sites preserved among H1N1, H3N2 and H5N1 influenzas. Since then the evidence for cross-protective antibodies in humans has been accumulating. Now proposed is a rationale to stimulate and enhance pre-existing heterotypic humoral responses that, together with cell-mediated initiatives, will deliver pre-emptive and universal human protection against emerging epidemic and pandemic influenzas.

Wide prevalence of heterosubtypic broadly neutralizing human anti-influenza A antibodies

BACKGROUND

Lack of life-long immunity against influenza viruses represents a major global health care problem with profound medical and economic consequences. A greater understanding of the broad-spectrum "heterosubtypic" neutralizing human antibody (BnAb) response to influenza should bring us closer toward a universal influenza vaccine.

METHODS

Serum samples obtained from 77 volunteers in an H5N1 vaccine study were analyzed for cross-reactive antibodies (Abs) against both subtype hemagglutinins (HAs) and a highly conserved pocket on the HA stem of Group 1 viruses. Cross-reactive Abs in commercial intravenous immunoglobulin were affinity purified using H5-coupled beads followed by step-wise monoclonal antibody competition or acid elution. Enzyme-linked immunosorbent assays were used to quantify cross-binding, and neutralization activity was determined with HA-pseudotyped viruses.

RESULTS

Prevaccination serum samples have detectable levels of heterosubtypic HA binding activity to both Group 1 and 2 influenza A viruses, including subtypes H5 and H7, respectively, to which study subjects had not been vaccinated. Two different populations of Broadly neutralizing Abs (BnAbs) were purified from intravenous immunoglobulin by H5 beads: ~0.01% of total immunoglobulin G can bind to HAs from both Group 1 and 2 and neutralize H1N1 and H5N1 viruses; ~0.001% is F10-like Abs directed against the HA stem pocket on Group 1 viruses.

CONCLUSION

These data--to our knowledge, for the first time--quantitatively show the presence, albeit at low levels, of two populations of heterosubtypic BnAbs against influenza A in human serum. These observations warrant further investigation to determine their origin, host polymorphism(s) that may affect their expression levels and how to boost these BnAb responses by vaccination to reach sustainable protective levels.

Induction of cross-neutralizing antibody against H5N1 virus after vaccination with seasonal influenza vaccine in COPD patients

Archival serum samples from elderly individuals with underlying chronic obstructive pulmonary disease (COPD) who were enrolled in a double-blind case-control study of seasonal influenza vaccine efficacy were assayed for cross-neutralizing antibody formation to avian influenza A (H5N1) virus. Of 118 serum samples, 58 were collected from influenza vaccinees (mean age 68.5 y), and 60 from placebo controls (mean age 68.4 y) who received vitamin B injections. Blood samples were collected before and at 1 mo after seasonal influenza vaccination from all subjects; in addition, for a longitudinal follow-up period of 1 y paired-blood samples were collected again from subjects who developed acute respiratory illness. Hemagglutination inhibition assay for antibodies to influenza A (H1N1), influenza A (H3N2), and influenza B viruses was carried out to determine the serological response to vaccination, and to diagnose influenza viral infection, while microneutralization assays were performed to detect cross-reactive antibody to H5N1 virus. Pre-existing cross-reactive H5N1 antibody at reciprocal titer 10 was found in 6 (10.3%) vaccinees and 4 (6.7%) placebo controls. There was no change in H5N1 antibody titer in these subjects after vaccination. On the other hand, 3 (5.2%) vaccinees developed seroconversion to H5N1 virus at 1 mo after vaccination, even though they had no pre-existing H5N1 antibody in their first blood samples. No cross-neutralizing antibody to H5N1 virus was detected in the placebo controls or in the 22 influenza patients, suggesting that influenza vaccination, but not influenza virus infection, induces cross-neutralizing antibody against avian influenza H5N1 virus.

Antibody quantity versus quality after influenza vaccination

The correlates for protection against influenza infection are incompletely characterized. We have applied an ELISA strategy that distinguishes antibodies against native viral surface antigens (potentially neutralizing) from antibodies directed against internal and denatured viral proteins (not neutralizing) to three groups of vaccinated subjects: (1) participants in a study of repeated annual vaccination, (2) elderly subjects and (3) patients with Systemic Lupus Erythematosus compared to control subjects. Antibody increase after vaccination was inversely related to the level of pre-existing antibodies in all groups; most subjects had significant initial antibody levels and showed little increase in amount of antibody after vaccination, but the avidity of their serum antibodies tended to increase. Antibodies against denatured virus proteins varied with vaccine formulation; vaccines that are more recent have less total protein for the same amount of native hemagglutinin. We propose an index consisting of rank order of antibody level plus antibody avidity, both measured against native virus, plus hemagglutination-inhibition antibody titer, as a useful measure of immunity against influenza.

Heterosubtypic T-cell responses against avian influenza H5 haemagglutinin are frequently detected in individuals vaccinated against or previously infected with human subtypes of influenza

Background  Cellula r immune responses play a critical role in providing help for the production of neutralizing antibodies to influenza virus, as well as producing anti‐viral cytokines and killing infected cells in the lung. Heterosubtypic T‐cell responses between different subtypes of influenza have been shown to exist in humans and to provide protection against morbidity and mortality associated with H5N1 infection in animal challenge models. Therefore, existing T‐cell responses induced by natural infection or vaccination in humans may provide some degree of protection from infection with H5N1 strains, or may attenuate the severity of disease.

Objectives  To investigate heterosubtypic T‐cell responses to avian influenza in humans.

Methods  T‐cell responses to an overlapping set of H5 HA peptides and inactivated viruses (H1N1, H3N2 and H5N1) were assessed using IFN‐γ and IL‐2 enzyme‐linked immunospot (ELISpot) assays in a cohort of adults either vaccinated against seasonal influenza in the last 3 years (n = 20) or previously infected (n = 40).

Results  T‐cell responses to all three subtypes of virus were found in both infected and vaccinated individuals by IFN‐γ and IL‐2 ELISpot assays. Approximately half of the participants from each group had a positive T‐cell response to the H5 HA peptides in the IFN‐γ or IL‐2 ELISpot assay.

Conclusions  Heterosubtypic T‐cell responses to H5 HA occur quite frequently in vaccinated and infected individuals. Further investigation of these responses and what role they may play upon challenge or vaccination against H5N1 may assist in vaccine design for avian influenza.

B cells promote resistance to heterosubtypic strains of influenza via multiple mechanisms

Immunity to heterosubtypic strains of influenza is thought to be mediated primarily by memory T cells, which recognize epitopes in conserved proteins. However, the involvement of B cells in this process is controversial. We show in this study that influenza-specific memory T cells are insufficient to protect mice against a lethal challenge with a virulent strain of influenza in the absence of B cells. B cells contribute to protection in multiple ways. First, although non-neutralizing Abs by themselves do not provide any protection to challenge infection, they do reduce weight loss, lower viral titers, and promote recovery of mice challenged with a virulent heterosubtypic virus in the presence of memory T cells. Non-neutralizing Abs also facilitate the expansion of responding memory CD8 T cells. Furthermore, in cooperation with memory T cells, naive B cells also promote recovery from infection with a virulent heterosubtypic virus by generating new neutralizing Abs. These data demonstrate that B cells use multiple mechanisms to promote resistance to heterosubtypic strains of influenza and suggest that vaccines that elicit both memory T cells and Abs to conserved epitopes of influenza may be an effective defense against a wide range of influenza serotypes.

Amount and avidity of serum antibodies against native glycoproteins and denatured virus after repeated influenza whole-virus vaccination

There is still uncertainty on the correlates of protection by influenza vaccine. To determine the relationship between hemagglutination-inhibition (HI) titer and the specificity and avidity of serum antibodies, we analyzed serum from a longitudinal trial (1983-1987) of influenza vaccine efficacy [Keitel WA, Cate TR, Couch RB, Huggins LL, Hess KR. Efficacy of repeated annual immunization with inactivated influenza virus vaccines over a five year period. Vaccine 1997;15(10):1114-22 ]. We captured native virus particles with fetuin and separately measured relative antibody levels and avidities of antibodies against native glycoproteins and antibodies against denatured viral proteins. Most subjects had pre-existing antibodies against A/Victoria/75 and, although 70% had >two-fold increased antibodies against A/Philippines/82 after vaccination, only 30% showed increased antibodies to A/Victoria/75 indicating no dominance of original antigenic sin. There was variation in the levels of antibodies to unfolded antigens compared to native, but antibodies against denatured proteins never exceeded those against native virus. In some cases, the avidity increased without a significant increase in antibody concentration, which might explain why some vaccinees with low HI titer demonstrate adequate protection. We found that the negative correlation between pre-vaccination HI titer and the increase after vaccination is also seen when antibodies are measured directly, but that there is little relationship between HI titer and antibodies against native glycoproteins, either in amount or avidity. Our assay, which has also been adapted for recent influenza viruses that do not bind to fetuin, may be useful for vaccine evaluation.

Heterosubtypic immunity to influenza A virus in mice lacking IgA, all Ig, NKT cells, or gamma delta T cells

The mechanisms of broad cross-protection to influenza viruses of different subtypes, termed heterosubtypic immunity, remain incompletely understood. We used knockout mouse strains to examine the potential for heterosubtypic immunity in mice lacking IgA, all Ig and B cells, NKT cells (CD1 knockout mice), or gamma(delta) T cells. Mice were immunized with live influenza A virus and compared with controls immunized with unrelated influenza B virus. IgA(-/-) mice survived full respiratory tract challenge with heterosubtypic virus that was lethal to controls. IgA(-/-) mice also cleared virus from the nasopharynx and lungs following heterosubtypic challenge limited to the upper respiratory tract, where IgA has been shown to play an important role. Ig(-/-) mice controlled the replication of heterosubtypic challenge virus in the lungs. Acute depletion of CD4+ or CD8+ T cell subsets abrogated this clearance of virus, thus indicating that both CD4+ and CD8+ T cells are required for protection in the absence of Ig. These results in Ig(-/-) mice indicate that CD4+ T cells can function by mechanisms other than providing help to B cells for the generation of Abs. Like wild-type mice, CD1(-/-) mice and gamma(delta) (-/-) mice survived lethal heterosubtypic challenge. Acute depletion of CD4+ and CD8+ cells abrogated heterosubtypic protection in gamma(delta) (-/-) mice, but not B6 controls, suggesting a contribution of gamma(delta) T cells. Our results demonstrate that the Ab and cellular subsets deficient in these knockout mice are not required for heterosubtypic protection, but each may play a role in a multifaceted response that as a whole is more effective than any of its parts.

Mucosal delivery of inactivated influenza vaccine induces B-cell-dependent heterosubtypic cross-protection against lethal influenza A H5N1 virus infection

Influenza vaccines that induce greater cross-reactive or heterosubtypic immunity (Het-I) may overcome limitations in vaccine efficacy imposed by the antigenic variability of influenza A viruses. We have compared mucosal versus traditional parenteral administration of inactivated influenza vaccine for the ability to induce Het-I in BALB/c mice and evaluated a modified Escherichia coli heat-labile enterotoxin adjuvant, LT(R192G), for augmentation of Het-I. Mice that received three intranasal (i.n.) immunizations of H3N2 vaccine in the presence of LT(R192G) were completely protected against lethal challenge with a highly pathogenic human H5N1 virus and had nasal and lung viral titers that were at least 2,500-fold lower than those of control mice receiving LT(R192G) alone. In contrast, mice that received three vaccinations of H3N2 vaccine subcutaneously in the presence or absence of LT(R192G) or incomplete Freund's adjuvant were not protected against lethal challenge and had no significant reductions in tissue virus titers observed on day 5 post-H5N1 virus challenge. Mice that were i.n. administered H3N2 vaccine alone, without LT(R192G), displayed partial protection against heterosubtypic challenge. The immune mediators of Het-I were investigated. The functional role of B and CD8+ T cells in Het-I were evaluated by using gene-targeted B-cell (IgH-6(-/-))- or beta2-microglobulin (beta2m(-/-))-deficient mice, respectively. beta2m(-/-) but not IgH-6(-/-) vaccinated mice were protected by Het-I and survived a lethal infection with H5N1, suggesting that B cells, but not CD8+ T cells, were vital for protection of mice against heterosubtypic challenge. Nevertheless, CD8+ T cells contributed to viral clearance in the lungs and brain tissues of heterotypically immune mice. Mucosal but not parenteral vaccination induced subtype cross-reactive lung immunoglobulin G (IgG), IgA, and serum IgG anti-hemagglutinin antibodies, suggesting the presence of a common cross-reactive epitope in the hemagglutinins of H3 and H5. These results suggest a strategy of mucosal vaccination that stimulates cross-protection against multiple influenza virus subtypes, including viruses with pandemic potential.

Detection of antibody to avian influenza A (H5N1) virus in human serum by using a combination of serologic assays

From May to December 1997, 18 cases of mild to severe respiratory illness caused by avian influenza A (H5N1) viruses were identified in Hong Kong. The emergence of an avian virus in the human population prompted an epidemiological investigation to determine the extent of human-to-human transmission of the virus and risk factors associated with infection. The hemagglutination inhibition (HI) assay, the standard method for serologic detection of influenza virus infection in humans, has been shown to be less sensitive for the detection of antibodies induced by avian influenza viruses. Therefore, we developed a more sensitive microneutralization assay to detect antibodies to avian influenza in humans. Direct comparison of an HI assay and the microneutralization assay demonstrated that the latter was substantially more sensitive in detecting human antibodies to H5N1 virus in infected individuals. An H5-specific indirect enzyme-linked immunosorbent assay (ELISA) was also established to test children's sera. The sensitivity and specificity of the microneutralization assay were compared with those of an H5-specific indirect ELISA. When combined with a confirmatory H5-specific Western blot test, the specificities of both assays were improved. Maximum sensitivity (80%) and specificity (96%) for the detection of anti-H5 antibody in adults aged 18 to 59 years were achieved by using the microneutralization assay combined with Western blotting. Maximum sensitivity (100%) and specificity (100%) in detecting anti-H5 antibody in sera obtained from children less than 15 years of age were achieved by using ELISA combined with Western blotting. This new test algorithm is being used for the seroepidemiologic investigations of the avian H5N1 influenza outbreak.

HI antibody kinetics in adult volunteers immunized repeatedly with inactivated trivalent influenza vaccine in 1990-1992

Volunteers were immunized once, twice or three times in 1990-1992 with commercial trivalent inactivated influenza vaccine and monitored for haemagglutination inhibition (HI) antibodies. The antibodies that already existed when the subjects entered the study derived from natural infections. Immunizations in the second and third years were necessary to maintain the geometric mean titres of antibody and protection rates at the level recorded after the first vaccination. Negative correlations between prevaccination antibody titres and mean fold antibody increases were noted in most instances analysed. Moreover, at each individual prevaccination titre level the mean fold antibody increases and even postvaccination mean titres were higher after the first than after subsequent vaccinations, suggesting that the HI antibody responses might be affected by vaccine-induced pre-existing antibody more than by the same titres of antibody derived from natural infections. This was most obvious for antibody to the H1N1 subtype virus, A/Finland/164/91. In immunization with B/Yamagata/16/88, anamnestic response of antibody to B/Finland/150/90, which belongs to the antigenically distinct lineage of B/Victoria/2/87-like viruses, was more frequent in the first than in subsequent years. This is in contrast to homologous antibodies, which increased significantly after the second vaccination.

Protection against the mouse-adapted A/FM/1/47 strain of influenza A virus in mice by a monoclonal antibody with cross-neutralizing activity among H1 and H2 strains

The monoclonal antibody designated C179 was found to neutralize all of the H1 and H2 strains of influenza A virus studied (Y. Okuno, Y. Isegawa, F. Sasao, and S. Ueda, J. Virol. 67:2552-2558, 1993). In the present study, the ability of C179 to protect mice from the lethal effect of the A/FM/1/47 (H1N1) strain was examined. When the mice were injected intraperitoneally with 100 micrograms of C179 per mouse a day before the virus challenge (2.0 x 10(3) focus-forming units per mouse), all of the mice survived. Moreover, significantly higher survival rates were observed in mice receiving 1,000 micrograms of C179 per mouse 2 days after the virus challenge than in those receiving phosphate-buffered saline alone. These results indicate that C179 is effective not only for prevention but also for treatment of mice infected with H1 and H2 strains. The possibility that C179 can be used for passive immunization in humans is discussed.

Cross-protection against influenza A virus infection by passively transferred respiratory tract IgA antibodies to different hemagglutinin molecules

Mice that were intranasally immunized with different influenza A virus hemagglutinins (HA), derived from PR8 (H1N1), A/Yamagata (H1N1) or A/Fukuoka (H3N2) virus, together with cholera toxin B subunit as an adjuvant, were examined for protection against PR8 infection; PR8 HA and A/Yamagata HA immunization conferred complete protection, while A/Fukuoka HA immunization failed to confer protection. In parallel with protection, PR8 HA-, A/Yamagata HA-, and A/Fukuoka HA-immunized mice produced a high, a moderate and a low level of PR8 HA-reactive IgA in the respiratory tract, respectively. These IgA antibodies were not only higher in content in the nasal secretions, but also more cross-reactive than IgG. The purified IgA antibodies from respiratory tract washings of PR8 HA-immunized mice, which contained the HA-specific IgA corresponding to the amount detected in the nasal wash, were able to protect mice from PR8 challenge when transferred to the respiratory tract of naive mice. The transfer of IgA from A/Yamagata HA-immunized mice also afforded cross-protection against PR8 infection, whereas the IgA from A/Fukuoka HA-immunized mice failed to provide protection. The ability of transferred IgA to prevent viral infection was dependent on the amount of HA-reactive IgA remaining in the respiratory tract of the host at the time of infection. These experiments directly demonstrate that IgA antibodies to influenza A virus HA by themselves play a pivotal role in defence not only against homologous virus infection, but also against heterologous drift virus infection at the respiratory mucosa, the portal of entry for the viruses.

Vaccination with inactivated influenza A virus during pregnancy protects neonatal mice against lethal challenge by influenza A viruses representing three subtypes

A single intraperitoneal injection of pregnant mice with a monovalent Formalin-inactivated influenza A virus vaccine protected their offspring against a lethal challenge dose of the same influenza A virus H3N2, H2N2, and H1N1 subtypes, as well as against challenge with the other two subtypes. Degree of protection was vaccine dose related. Cross-fostering of neonates indicated that protection was conferred by breast milk antibodies. Serum virus-specific neutralizing antibodies in the mothers and neonates correlated with resistance to vaccine virus, but were detected against other subtypes only in a complement enhancement test or when high doses of vaccine were given.

Comparison of long-term systemic and secretory antibody responses in children given live, attenuated, or inactivated influenza A vaccine

A comparison of inactivated intramuscular and live intranasal influenza A vaccines in young children undergoing primary immunization might be expected to show differences in serum and local mucosal antibody responses. To demonstrate such differences, serum and local respiratory tract antibody responses of young children vaccinated with intranasal live, attenuated, cold-adapted (H3N2 or H1N1), or intramuscular inactivated (H3N2) influenza A vaccines were examined for one year after vaccination. Antibody responses were measured by hemagglutination-inhibition (HAI) and class-specific enzyme-linked immunosorbent assay (ELISA). One year after vaccination, live intranasal vaccinees had significantly less decay of serum HAI (p = 0.025) and IgG antibody (p = 0.01) directed against the influenza hemagglutinin and neuraminidase than did intramuscular inactivated vaccinees. Nasal secretory IgA developed almost exclusively in live vaccinees and persisted for up to one year. Persistent nasal secretory IgG was detected in both live and inactivated vaccinees. Live vaccination not only stimulates a more durable serum antibody response, but also induces long-lasting local respiratory tract IgA antibody that may play an important role in host protection.