Multiplex evaluation of influenza neutralizing antibodies with potential applicability to in-field serological studies

Chimeric H5 influenza virus-like particle vaccine elicits broader cross-clade antibody responses in chickens than in ducks

Eurasian-lineage highly pathogenic avian influenza (HPAI) H5 viruses have spread throughout Asia, the Middle East, Europe, Africa, and most recently, North and South America. These viruses are independently evolving into genetically and antigenically divergent clades, and broad-spectrum vaccines protecting against these divergent clades are needed. In this study, we developed a chimeric virus-like particle (VLP) vaccine co-expressing hemagglutinins from two clades (clades 1 and 2.3.2.1) of HPAI H5 viruses and performed comparative cross-clade hemagglutination inhibition (HI) analysis in chickens and ducks. The chimeric VLP immunization induced a significantly broader spectrum of antibodies against various clades of HPAI H5 viruses than monovalent VLPs both in chickens and ducks. While the chimeric VLP led to broadened antibody responses in both species, significantly lower levels of HI antibodies were elicited in ducks than in chickens. Moreover, boost immunization failed to increase antibody responses in ducks regardless of the VLPs used, in contrast to chickens that showed significantly enhanced antibody responses upon boost immunization. These results suggest (1) the potential application of the chimeric VLP technology in poultry to help control HPAI H5 viruses by offering broader antibody responses against antigenically different strains and (2) possible obstacles in generating high levels of antibody responses against HPAI H5 viruses in ducks via vaccination, implying the need for advanced vaccination strategies for ducks.

Pseudotyped Viruses for Enterovirus

Using a non-pathogenic pseudotyped virus as a surrogate for a wide-type virus in scientific research complies with the recent requirements for biosafety. Enterovirus (EV) contains many species of viruses, which are a type of nonenveloped virus. The preparation of its corresponding pseudotyped virus often needs customized construction compared to some enveloped viruses. This article describes the procedures and challenges in the construction of pseudotyped virus for enterovirus (pseudotyped enterovirus, EVpv) and also introduces the application of EVpv in basic virological research, serological monitoring, and the detection of neutralizing antibody (NtAb).

Pseudotyped Vesicular Stomatitis Virus-Severe Acute Respiratory Syndrome-Coronavirus-2 Spike for the Study of Variants, Vaccines, and Therapeutics Against Coronavirus Disease 2019

World Health Organization (WHO) has prioritized the infectious emerging diseases such as Coronavirus Disease (COVID-19) in terms of research and development of effective tests, vaccines, antivirals, and other treatments. Severe Acute Respiratory Syndrome-Coronavirus-2 (SARS-CoV-2), the etiological causative agent of COVID-19, is a virus belonging to risk group 3 that requires Biosafety Level (BSL)-3 laboratories and the corresponding facilities for handling. An alternative to these BSL-3/-4 laboratories is to use a pseudotyped virus that can be handled in a BSL-2 laboratory for study purposes. Recombinant Vesicular Stomatitis Virus (VSV) can be generated with complementary DNA from complete negative-stranded genomic RNA, with deleted G glycoprotein and, instead, incorporation of other fusion protein, like SARS-CoV-2 Spike (S protein). Accordingly, it is called pseudotyped VSV-SARS-CoV-2 S. In this review, we have described the generation of pseudotyped VSV with a focus on the optimization and application of pseudotyped VSV-SARS-CoV-2 S. The application of this pseudovirus has been addressed by its use in neutralizing antibody assays in order to evaluate a new vaccine, emergent SARS-CoV-2 variants (delta and omicron), and approved vaccine efficacy against variants of concern as well as in viral fusion-focused treatment analysis that can be performed under BSL-2 conditions.

Characterization and use in neutralization assays of avian influenza codon-optimized H5 and H7 retroviral pseudotypes

Influenza is a relevant problem for public and animal health, with a significant economic impact. In recent years, outbreaks of avian influenza virus have resulted in devastating losses in the poultry industry worldwide, and although its transmission to humans is very rare, there is always a potential risk for an even more severe outbreak. Currently, vaccination is considered the most effective tool for the control and prevention of influenza infections in both humans and animals. The maintenance of animal welfare and the successful implementation of animal health programs depend on the timely administration of vaccines, which must comply with quality specifications indicated by health authorities; for example, the capability to ensure a minimum antibody titer. The production of viral antigens used in these tests can pose a biosafety risk, and some viral strains can be difficult to grow. Therefore, new biotechnological alternatives are required to overcome these disadvantages. In this study, we produced pseudotypes carrying H5 and H7 hemagglutinins from lowly and highly pathogenic avian influenza viruses. These pseudotypes were used in neutralization assays to detect neutralizing antibodies in avian sera, which were confirmed positive by inhibition of the hemagglutination test. Our results showed that the pseudotype neutralization assay is a viable alternative for the detection of neutralizing antibodies, by demonstrating subtype specificity and requiring reduced biosafety requirements. Therefore, it represents a versatile platform that can facilitate technology transfer protocols between laboratories, and an immediate application in serological tools for quality control of veterinary vaccines against avian influenza.

More Than Just Gene Therapy Vectors: Lentiviral Vector Pseudotypes for Serological Investigation

Serological assays detecting neutralising antibodies are important for determining the immune responses following infection or vaccination and are also often considered a correlate of protection. The target of neutralising antibodies is usually located in the Envelope protein on the viral surface, which mediates cell entry. As such, presentation of the Envelope protein on a lentiviral particle represents a convenient alternative to handling of a potentially high containment virus or for those viruses with no established cell culture system. The flexibility, relative safety and, in most cases, ease of production of lentiviral pseudotypes, have led to their use in serological assays for many applications such as the evaluation of candidate vaccines, screening and characterization of anti-viral therapeutics, and sero-surveillance. Above all, the speed of production of the lentiviral pseudotypes, once the envelope sequence is published, makes them important tools in the response to viral outbreaks, as shown during the COVID-19 pandemic in 2020. In this review, we provide an overview of the landscape of the serological applications of pseudotyped lentiviral vectors, with a brief discussion on their production and batch quality analysis. Finally, we evaluate their role as surrogates for the real virus and possible alternatives.

Characterisation of SARS-CoV-2 Lentiviral Pseudotypes and Correlation between Pseudotype-Based Neutralisation Assays and Live Virus-Based Micro Neutralisation Assays

The recent outbreak of a novel Coronavirus (SARS-CoV-2) and its rapid spread across the continents has generated an urgent need for assays to detect the neutralising activity of human sera or human monoclonal antibodies against SARS-CoV-2 spike protein and to evaluate the serological immunity in humans. Since the accessibility of live virus microneutralisation (MN) assays with SARS-CoV-2 is limited and requires enhanced bio-containment, the approach based on "pseudotyping" can be considered a useful complement to other serological assays. After fully characterising lentiviral pseudotypes bearing the SARS-CoV-2 spike protein, we employed them in pseudotype-based neutralisation assays in order to profile the neutralising activity of human serum samples from an Italian sero-epidemiological study. The results obtained with pseudotype-based neutralisation assays mirrored those obtained when the same panel of sera was tested against the wild type virus, showing an evident convergence of the pseudotype-based neutralisation and MN results. The overall results lead to the conclusion that the pseudotype-based neutralisation assay is a valid alternative to using the wild-type strain, and although this system needs to be optimised and standardised, it can not only complement the classical serological methods, but also allows serological assessments to be made when other methods cannot be employed, especially in a human pandemic context.

Pathological Changes in Natural Infection of Pheasants with Highly Pathogenic Avian Influenza A (H5N8) in Bulgaria

Introduction

The study of histopathological changes caused by influenza A (H5N8) viral infection in bird species is essential for the understanding of their role in the spread of this highly infectious virus. However, there are few such studies under natural conditions in minor gallinaceous species. This article describes the pathomorphological findings in Colchis pheasants infected naturally with H5N8 during an epizootic outbreak in Bulgaria.

Material and Methods

Samples of internal organs of 10 carcasses were collected for histopathological and immunohistochemical evaluation, virus isolation and identification, and nucleic acid detection.

Results

Consistent macroscopic findings were lesions affecting the intestine, heart, lung, and pancreas. Congestion and mononuclear infiltrate were common findings in the small intestine, as were necrosis and lymphoid clusters in the lamina propria of the caeca. Congestion with small focal necrosis and gliosis with multifocal nonpurulent encephalitis were observed in the brain. Myocardial interstitial oedema and degenerative necrobiotic processes were also detected. Immunohistological analysis confirmed systemic infection and revealed influenza virus nucleoprotein in all analysed organs.

Conclusion

Variable necrosis was observed in the brain, liver, trachea, heart, small intestine, and caeca. Viral antigen was commonly found in the brain, heart, lung and trachea. Contact with migrating waterfowls was suspected as a reason for the outbreak.

Middle East respiratory coronavirus (MERS-CoV) spike (S) protein vesicular stomatitis virus pseudoparticle neutralization assays offer a reliable alternative to the conventional neutralization assay in human seroepidemiological studies

Middle East respiratory syndrome coronavirus (MERS-CoV) is a novel zoonotic coronavirus that was identified in 2012. MERS-CoV infection in humans can result in an acute, severe respiratory disease and in some cases multi-organ failure; the global mortality rate is approximately 35 %. The MERS-CoV spike (S) protein is a major target for neutralizing antibodies in infected patients. The MERS-CoV microneutralization test (MNt) is the gold standard method for demonstrating prior infection. However, this method requires the use of live MERS-CoV in biosafety level 3 (BSL-3) containment. The present work describes the generation and validation of S protein-bearing vesicular stomatitis virus (VSV) pseudotype particles (VSV-MERS-CoV-S) in which the VSV glycoprotein G gene has been replaced by the luciferase reporter gene, followed by the establishment of a pseudoparticle-based neutralization test to detect MERS-CoV neutralizing antibodies under BSL-2 conditions. Using a panel of human sera from confirmed MERS-CoV patients, the VSV-MERS-CoV particle neutralization assay produced results that were highly comparable to those of the microneutralization test using live MERS-CoV. The results suggest that the VSV-MERS-CoV-S pseudotype neutralization assay offers a highly specific, sensitive and safer alternative method to detect MERS-CoV neutralizing antibodies in human sera.

Stability of retroviral pseudotypes carrying the hemagglutinin of avian influenza viruses under various storage conditions

Retroviral pseudotypes are broadly used as safe instruments to mimic the structure and surface of highly pathogenic viruses. They have been employed for the discovery of new drugs, as diagnostic tools in vaccine studies, and part of serological assays. Because of their widespread use in research and their potential as tools for quality control, it is important to know their shelf life, stability, and best storage conditions. In this study, we produced pseudotypes carrying the lacZ reporter gene and the hemagglutinin (HA) of avian influenza virus subtypes H5 and H7 to investigate their stability under various storage conditions. We produced pseudotypes with titers of approximately 10⁶ RLU/mL, which decreased to 10⁵-10⁶ RLU/mL after short-term storage at 4 °C (up to 4 weeks). Stability was maintained after long-term storage at -20 °C (up to 12 months), even under storage variations such as freeze-thaw cycles. We conclude that, although the titers decreased by 1 log₁₀ under the different storage conditions, the remaining titers can be readily applicable in many techniques, such as neutralization assays. These findings show that large quantities of retroviral pseudotypes can be safely stored for short- or long-term use, allowing standardization and reduced variation in assays involving retroviral pseudotypes.

The Optimisation of Pseudotyped Viruses for the Characterisation of Immune Responses to Equine Influenza Virus

Pseudotyped viruses (PVs) produced by co-transfecting cells with plasmids expressing lentiviral core proteins and viral envelope proteins are potentially powerful tools for studying various aspects of equine influenza virus (EIV) biology. The aim of this study was to optimise production of equine influenza PVs. Co-transfection of the HAT protease to activate the haemagglutinin (HA) yielded a higher titre PV than TMPRSS2 with the HA from A/equine/Richmond/1/2007 (H3N8), whereas for A/equine/Newmarket/79 (H3N8), both proteases resulted in equivalent titres. TMPRSS4 was ineffective with the HA of either strain. There was also an inverse relationship between the amount of protease-expression plasmids and the PV titre obtained.  Interestingly, the PV titre obtained by co-transfection of a plasmid encoding the cognate N8 NA was not as high as that generated by the addition of exogenous neuraminidase (NA) from Clostridium perfringens to allow the release of nascent PV particles. Finally, initial characterisation of the reliability of PV neutralisation tests (PVNTs) demonstrated good intra-laboratory repeatability. In conclusion, we have demonstrated that equine influenza PV production can be readily optimised to provide a flexible tool for studying EIV.

Controlling equine influenza: Traditional to next generation serological assays

Serological assays provide an indirect route for the recognition of infectious agents via the detection of antibodies against the infectious agent of interest within serum. Serological assays for equine influenza A virus can be applied for different purposes: diagnosing infections; subtyping isolates; surveillance of circulating strains; and to evaluate the efficacy of vaccines before they reach the market. Haemagglutination inhibition (HI) and single radial haemolysis (SRH) assays are most commonly used in the equine field. This review outlines how both these assays together with virus neutralization (VN) and ELISA are performed, interpreted and applied for the control of equine influenza, giving the limitations and advantages of each. The pseudotyped virus neutralization assay (PVNA) is also discussed as a promising prospect for the future of equine influenza virus serology.

The Viral Pseudotype Unit: viral pseudotype R&D, dissemination and education

The Viral Pseudotype Unit was established in 2010 to act as an interface between academia, industry and public/animal health laboratories with the purpose of translating basic virus research (on viral pseudotypes) into in vitro cell culture-based assays that can be readily employed for the immunogenicity testing of preclinical vaccine candidates, and for the functional screening of new antivirals and therapeutic antibodies. More recently the Viral Pseudotype Unit has become involved with the exploitation of pseudotypes for the development of serological standards, and with training and education via traditional and new media platforms.

Technical considerations for the generation of novel pseudotyped viruses

A pseudotyped virus (PV) is a virus particle with an envelope protein originating from a different virus. The ability to dictate which envelope proteins are expressed on the surface has made pseudotyping an important tool for basic virological studies such as determining the cellular targets of the envelope protein of the virus as well as identification of potential antiviral compounds and measuring specific antibody responses. In this review, we describe the common methodologies employed to generate PVs, with a focus on approaches to improve the efficacy of PV generation.

An optimised method for the production of MERS-CoV spike expressing viral pseudotypes

The production and use of pseudotyped viral particles are widely established for many viruses, and applications in the fields of serology and vaccine development are manifold. Viral pseudotypes have proven to be powerful tools to study the effects of viral evolution on serological outcomes, viral tropism and immunogenicity studies. Pseudotyped viruses are chimeric constructs in which the outer (surface) glycoprotein(s) of one virus is combined with the replication-defective viral “core” of another virus. Pseudotypes allow for accurate, sequence-directed, sensitive antibody neutralisation assays and antiviral screening to be conducted within a low biosecurity facility and offer a safe and efficient alternative to wildtype virus use.

The protocol outlined here represents a rapid and reliable method for the generation of high-titre pseudotype viral particles with the MERS-CoV spike protein on a lentiviral core, and is adapted from previously published protocols. This protocol is optimised for transfection in a 100 mm Petri dish with 7 ml of supernatant harvested, however it can be readily scaled to different production volumes.

This protocol has a number of advantages including:•

Use of readily available reagents.

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Consistent, high virus titres.

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Rapid generation of novel glycoproteins for research into strain variation.

An Innovative Pseudotypes-Based Enzyme-Linked Lectin Assay for the Measurement of Functional Anti-Neuraminidase Antibodies

Antibodies (Ab) to neuraminidase (NA) play a role in limiting influenza infection and might help reduce the disease impact. The most widely used serological assay to measure functional anti-NA immune responses is the Enzyme-Linked Lectin Assay (ELLA) which relies on hemagglutinin (HA) mismatched virus reassortants, or detergent treated viruses as the NA source to overcome interference associated with steric hindrance of anti-HA Ab present in sera. The difficulty in producing and handling these reagents, which are not easily adapted for screening large numbers of samples, limits the routine analysis of functional anti-NA Ab in clinical trials. In this study, we produced influenza lentiviral pseudoparticles (PPs) containing only the NA antigen (NA-PPs) with a simple two-plasmid co-transfection system. NA-PPs were characterized and tested as an innovative source of NA in the NA inhibition (NI) assay. Both swine A/California/07/2009 (H1N1) and avian A/turkey/Turkey/01/2005 (H5N1) N1s within NA-PPs retained their sialidase activity and were specifically inhibited by homologous and N1 subtype-specific, heterologous sheep sera. Moreover, A/California/07/2009 N1-PPs were a better source of NA compared to whole live and detergent treated H1N1 viruses in ELLA, likely due to lack of interference by anti-HA Ab, and absence of possible structural modifications caused by treatment with detergent. This innovative assay is safer and applicable to all NAs. Taken together, these results highlight the potential of NA-PPs-based NI assays to be developed as sensitive, flexible, easy to handle and scalable serological tests for routine NA immune response analysis.

The use of pseudotypes to study viruses, virus sero-epidemiology and vaccination

The globalization of the world's economies, accompanied by increasing international travel, changing climates, altered human behaviour and demographics is leading to the emergence of different viral diseases, many of which are highly pathogenic and hence are considered of great public and animal health importance. To undertake basic research and therapeutic development, many of these viruses require handling by highly trained staff in BSL-3/4 facilities not readily available to the majority of the global R&D community. In order to circumvent the enhanced biosafety requirement, the development of non-pathogenic, replication-defective pseudotyped viruses is an effective and established solution to permit the study of many aspects of virus biology in a low containment biosafety level (BSL)-1/2 laboratory. Under the spectre of the unfolding Ebola crisis, this timely conference (the second to be organised by the Viral Pseudotype Unit, www.viralpseudotypeunit.info*) discusses the recent advances in pseudotype technology and how it is revolutionizing the study of important human and animal pathogens (human and avian influenza viruses, rabies/lyssaviruses, HIV, Marburg and Ebola viruses). Key topics addressed in this conference include the exploitation of pseudotypes for serology and serosurveillance, immunogenicity testing of current and next-generation vaccines and new pseudotype assay formats (multiplexing, kit development). The first pseudotype-focused Euroscicon conference organised by the Viral Pseudotype Unit was recently reviewed [1].

The application of pseudotypes to influenza pandemic preparedness

ABSTRACT 

Human and animal populations are constantly exposed to multiple influenza strains 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 hemagglutinin protein on the surface of the virus are of importance since they have been shown to correlate with protective immunity. Serological techniques, detecting these responses, play a critical role in influenza pandemic preparedness in particular with regard to the measurement of vaccine immunogenicity. As the recent human pandemics (H1N1) and avian influenza outbreaks (H5 and H7) 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 pre-existing heterosubtypic immunity in populations. This review compares pseudotype-based assays with wild-type and virus-like particle virus assays and discusses their place in the pandemic preparedness against the influenza virus. It additionally addresses the state-of-the-art developments of pseudotype-based assays (chimeric hemagglutinins, multiplex and post-attachment) including the development and future deployment of assay kits and approaches toward standardization to both preclinical and clinical endpoints. Progress toward the development of an influenza pseudotype library for the purposes of pandemic preparedness is also outlined and discussed.

Competitive detection of influenza neutralizing antibodies using a novel bivalent fluorescence-based microneutralization assay (BiFMA)

Avian-derived influenza A zoonoses are closely monitored and may be an indication of virus strains with pandemic potential. Both successful vaccination and convalescence of influenza A virus in humans typically results in the induction of antibodies that can neutralize viral infection. To improve long-standing and new-generation methodologies for detection of neutralizing antibodies, we have employed a novel reporter-based approach that allows for multiple antigenic testing within a single sample. Central to this approach is a single-cycle infectious influenza A virus (sciIAV), where a functional hemagglutinin (HA) gene was changed to encode either the green or the monomeric red fluorescent protein (GFP and mRFP, respectively) and HA is complemented in trans by stable HA-expressing cell lines. By using fluorescent proteins with non-overlapping emission spectra, this novel bivalent fluorescence-based microneutralization assay (BiFMA) can be used to detect neutralizing antibodies against two distinct influenza isolates in a single reaction, doubling the speed of experimentation while halving the amount of sera required. Moreover, this approach can be used for the rapid identification of influenza broadly neutralizing antibodies. Importantly, this novel BiFMA can be used for any given influenza HA-pseudotyped virus under BSL-2 facilities, including highly pathogenic influenza HA isolates.

Pseudotype-based neutralization assays for influenza: a systematic analysis

The use of vaccination against the influenza virus remains the most effective method of mitigating the significant morbidity and mortality caused by this virus. Antibodies elicited by currently licensed influenza vaccines are predominantly hemagglutination-inhibition (HI)-competent antibodies that target the globular head of hemagglutinin (HA) thus inhibiting influenza virus entry into target cells. These antibodies predominantly confer homosubtypic/strain specific protection and only rarely confer heterosubtypic protection. However, recent academia or pharma-led R&D toward the production of a “universal vaccine” has centered on the elicitation of antibodies directed against the stalk of the influenza HA that has been shown to confer broad protection across a range of different subtypes (H1–H16). The accurate and sensitive measurement of antibody responses elicited by these “next-generation” influenza vaccines is, however, hampered by the lack of sensitivity of the traditional influenza serological assays HI, single radial hemolysis, and microneutralization. Assays utilizing pseudotypes, chimeric viruses bearing influenza glycoproteins, have been shown to be highly efficient for the measurement of homosubtypic and heterosubtypic broadly neutralizing antibodies, making them ideal serological tools for the study of cross-protective responses against multiple influenza subtypes with pandemic potential. In this review, we will analyze and compare literature involving the production of influenza pseudotypes with particular emphasis on their use in serum antibody neutralization assays. This will enable us to establish the parameters required for optimization and propose a consensus protocol to be employed for the further deployment of these assays in influenza vaccine immunogenicity studies.

Overview of Serological Techniques for Influenza Vaccine Evaluation: Past, Present and Future

Serological techniques commonly used to quantify influenza-specific antibodies include the Haemagglutination Inhibition (HI), Single Radial Haemolysis (SRH) and Virus Neutralization (VN) assays. HI and SRH are established and reproducible techniques, whereas VN is more demanding. Every new influenza vaccine needs to fulfil the strict criteria issued by the European Medicines Agency (EMA) in order to be licensed. These criteria currently apply exclusively to SRH and HI assays and refer to two different target groups-healthy adults and the elderly, but other vaccine recipient age groups have not been considered (i.e., children). The purpose of this timely review is to highlight the current scenario on correlates of protection concerning influenza vaccines and underline the need to revise the criteria and assays currently in use. In addition to SRH and HI assays, the technical advantages provided by other techniques such as the VN assay, pseudotype-based neutralization assay, neuraminidase and cell-mediated immunity assays need to be considered and regulated via EMA criteria, considering the many significant advantages that they could offer for the development of effective vaccines.

Getting more out of less--a quantitative serological screening tool for simultaneous detection of multiple influenza A hemagglutinin-types in chickens

Current avian influenza surveillance in poultry primarily targets subtypes of interest for the veterinary sector (H5, H7). However, as virological and serological evidence suggest, surveillance of additional subtypes is important for public health as well as for the poultry industry. Therefore, we developed a protein microarray enabling simultaneous identification of antibodies directed against different HA-types of influenza A viruses in chickens. The assay successfully discriminated negative from experimentally and naturally infected, seropositive chickens. Sensitivity and specificity depended on the cut-off level used but ranged from 84.4% to 100% and 100%, respectively, for a cut off level of ≥1∶40, showing minimal cross reactivity. As this testing platform is also validated for the use in humans, it constitutes a surveillance tool that can be applied in human-animal interface studies.