Pseudotype Neutralization Assays: From Laboratory Bench to Data Analysis

A Recombinant System and Reporter Viruses for Papiine Alphaherpesvirus 2

Primate simplex viruses, including Herpes simplex viruses 1 and 2, form a group of closely related herpesviruses, which establish latent infections in neurons of their respective host species. While neuropathogenic infections in their natural hosts are rare, zoonotic transmission of Macacine alphaherpesvirus 1 (McHV1) from macaques to humans is associated with severe disease. Human infections with baboon-derived Papiine alphaherpesvirus 2 (PaHV2) have not been reported, although PaHV2 and McHV1 share several biological properties, including neuropathogenicity in mice. The reasons for potential differences in PaHV2 and McHV1 pathogenicity are presently not understood, and answering these questions will require mutagenic analysis. Here, we report the development of a recombinant system, which allows rescue of recombinant PaHV2. In addition, we used recombineering to generate viruses carrying reporter genes (Gaussia luciferase or enhanced green fluorescent protein), which replicate with similar efficiency as wild-type PaHV2. We demonstrate that these viruses can be used to analyze susceptibility of cells to infection and inhibition of infection by neutralizing antibodies and antiviral compounds. In summary, we created a recombinant system for PaHV2, which in the future will be invaluable for molecular analyses of neuropathogenicity of PaHV2.

COVID-eVax, an electroporated DNA vaccine candidate encoding the SARS-CoV-2 RBD, elicits protective responses in animal models

The COVID-19 pandemic caused by SARS-CoV-2 has made the development of safe and effective vaccines a critical priority. To date, four vaccines have been approved by European and American authorities for preventing COVID-19, but the development of additional vaccine platforms with improved supply and logistics profiles remains a pressing need. Here we report the preclinical evaluation of a novel COVID-19 vaccine candidate based on the electroporation of engineered, synthetic cDNA encoding a viral antigen in the skeletal muscle. We constructed a set of prototype DNA vaccines expressing various forms of the SARS-CoV-2 spike (S) protein and assessed their immunogenicity in animal models. Among them, COVID-eVax-a DNA plasmid encoding a secreted monomeric form of SARS-CoV-2 S protein receptor-binding domain (RBD)-induced the most potent anti-SARS-CoV-2 neutralizing antibody responses (including against the current most common variants of concern) and a robust T cell response. Upon challenge with SARS-CoV-2, immunized K18-hACE2 transgenic mice showed reduced weight loss, improved pulmonary function, and lower viral replication in the lungs and brain. COVID-eVax conferred significant protection to ferrets upon SARS-CoV-2 challenge. In summary, this study identifies COVID-eVax as an ideal COVID-19 vaccine candidate suitable for clinical development. Accordingly, a combined phase I-II trial has recently started.

Investigating Constraints Along the Plant Secretory Pathway to Improve Production of a SARS-CoV-2 Spike Vaccine Candidate

Given the complex maturation requirements of viral glycoproteins and the challenge they often pose for expression in plants, the identification of host constraints precluding their efficient production is a priority for the molecular farming of vaccines. Building on previous work to improve viral glycoprotein production in plants, we investigated the production of a soluble SARS-CoV-2 spike comprising the ectopic portion of the glycoprotein. This was successfully transiently expressed in N. benthamiana by co-expressing the human lectin-binding chaperone calreticulin, which substantially increased the accumulation of the glycoprotein. The spike was mostly unprocessed unless the protease furin was co-expressed which resulted in highly efficient processing of the glycoprotein. Co-expression of several broad-spectrum protease inhibitors did not improve accumulation of the protein any further. The protein was successfully purified by affinity chromatography and gel filtration, although the purified product was heterogenous and the yields were low. Immunogenicity of the antigen was tested in BALB/c mice, and cellular and antibody responses were elicited after low dose inoculation with the adjuvanted protein. This work constitutes an important proof-of-concept for host plant engineering in the context of rapid vaccine development for SARS-CoV-2 and other emerging viruses.

Engineered ACE2-Fc counters murine lethal SARS-CoV-2 infection through direct neutralization and Fc-effector activities

Soluble Angiotensin-Converting Enzyme 2 (ACE2) constitutes an attractive antiviral capable of targeting a wide range of coronaviruses utilizing ACE2 as their receptor. Here, using structure-guided approaches, we developed divalent ACE2 molecules by grafting the extracellular ACE2-domain onto a human IgG1 or IgG3 (ACE2-Fc). These ACE2-Fcs harbor structurally validated mutations that enhance spike (S) binding and remove angiotensin enzymatic activity. The lead variant bound tightly to S, mediated in vitro neutralization of SARS-CoV-2 variants of concern (VOCs) with sub-nanomolar IC 50 and was capable of robust Fc-effector functions, including antibody-dependent-cellular cytotoxicity, phagocytosis and complement deposition. When tested in a stringent K18-hACE2 mouse model, it delayed death or effectively resolved lethal SARS-CoV-2 infection in a prophylactic or therapeutic setting utilizing the combined effect of neutralization and Fc-effector functions. These data confirm the utility of ACE2-Fcs as valuable agents in preventing and eliminating SARS-CoV-2 infection and demonstrate that ACE2-Fc therapeutic activity require Fc-effector functions.

Overview of Neutralizing Antibodies and Their Potential in COVID-19

The antibody response to respiratory syndrome coronavirus 2 (SARS-CoV-2) has been a major focus of COVID-19 research due to its clinical relevance and importance in vaccine and therapeutic development. Neutralizing antibody (NAb) evaluations are useful for the determination of individual or herd immunity against SARS-CoV-2, vaccine efficacy, and humoral protective response longevity, as well as supporting donor selection criteria for convalescent plasma therapy. In the current manuscript, we review the essential concepts of NAbs, examining their concept, mechanisms of action, production, and the techniques used for their detection; as well as presenting an overview of the clinical use of antibodies in COVID-19.

Production of Recombinant Replication-defective Lentiviruses Bearing the SARS-CoV or SARS-CoV-2 Attachment Spike Glycoprotein and Their Application in Receptor Tropism and Neutralisation Assays

For enveloped viruses, such as SARS-CoV-2, transmission relies on the binding of viral glycoproteins to cellular receptors. Conventionally, this process is recapitulated in the lab by infection of cells with isolated live virus. However, such studies can be restricted due to the availability of high quantities of replication-competent virus, biosafety precautions and associated trained staff. Here, we present a protocol based on pseudotyping to produce recombinant replication-defective lentiviruses bearing the SARS-CoV or SARS-CoV-2 attachment Spike glycoprotein, allowing the investigation of viral entry in a lower-containment facility. Pseudoparticles are produced by cells transiently transfected with plasmids encoding retroviral RNA packaging signals and Gag-Pol proteins, for the reconstitution of lentiviral particles, and a plasmid coding for the viral attachment protein of interest. This approach allows the investigation of different aspects of viral entry, such as the identification of receptor tropism, the prediction of virus host range, and zoonotic transmission potential, as well as the characterisation of antibodies (sera or monoclonal antibodies) and pharmacological inhibitors that can block entry. Graphic abstract: SARS-CoV and SARS-CoV-2 pseudoparticle generation and applications.

Production, Titration, Neutralisation, Storage and Lyophilisation of Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) Lentiviral Pseudotypes

This protocol details a rapid and reliable method for the production and titration of high-titre viral pseudotype particles with the SARS-CoV-2 spike protein (and D614G or other variants of concern, VOC) on a lentiviral vector core, and use for neutralisation assays in target cells expressing angiotensin-converting enzyme 2 (ACE2) and transmembrane serine protease 2 (TMPRSS2). It additionally provides detailed instructions on substituting in new spike variants via gene cloning, lyophilisation and storage/shipping considerations for wide deployment potential. Results obtained with this protocol show that SARS-CoV-2 pseudotypes can be produced at equivalent titres to SARS-CoV and Middle East respiratory syndrome coronavirus (MERS-CoV) pseudotypes, neutralised by human convalescent plasma and monoclonal antibodies, and stored at a range of laboratory temperatures and lyophilised for distribution and subsequent application.

Intradermal ChAdOx1 Vaccine Following Two CoronaVac Shots: A Case Report

Inactivated SARS-CoV-2 vaccines are used in many countries with uncertain immunogenicity. Intradermal ChAdOx1 has been proposed as a resource-efficient heterologous third booster shot. A 52-year-old healthy male healthcare professional had received two intramuscular CoronaVac shots on 21 April and 23 May 2021, and volunteered to take a 0.1 mL ChAdOx1 vaccine intradermally on 29 June 2021, with minimal local reactions. The declining IgG levels against spike protein from the two CoronaVac shots increased to higher than 10,000 AU/mL two weeks after the intradermal ChAdOx1. Moreover, the neutralizing antibody increased from 66.77% to almost 100%. A ratio of 6.6:9.7 of IgA:IgG was observed. The 50% pseudovirus neutralization titer (PVNT50) against lentiviral pseudovirus bearing a codon-optimized spike gene (wild type, alpha, beta, and delta) were 1812.42, 822.99, 1025.42, 1347.13, respectively. The SARS-CoV-2-specific T cells to spike protein-peptide pools (532-788 SFU/106 PBMCs) were detected. In conclusion, the antibody and cellular responses to the intradermal ChAdOx1, as a third booster dose in a healthy volunteer who received two intramuscular CoronaVac shots, revealed a dramatic increase in the total antibodies, including IgG, IgA, as well as T cell responses against spike protein. The immune response from intradermal ChAdOx1 should be further investigated in a larger population.

Detection and quantification of anti-rabies glycoprotein antibodies: current state and perspectives

Rabies is an ancient fatal disease with no other available treatment than post-exposure vaccination, where the bite of infected animals, mainly dogs, is the leading cause of its transmission to human beings. In this context, global vaccination campaigns of companion animals, as well as wildlife reservoirs vaccination, are key factors to achieve the "Zero by 30" plan that pursues the eradication of dog-mediated human rabies by 2030. Rabies virus-neutralizing antibodies (VNAs) play an essential role in the disease protection, as it correlates with an adequate immune response and allows evaluating pre- or post-exposure prophylaxis efficacy. Hence, counting with reliable, accurate, and robust serological tests is of paramount importance. Currently, RFFIT and FAVN are the gold standard VNAs tests recommended by both the WHO and the OIE. Despite these methodologies are efficient and widely used, they present several drawbacks, as they are less easily to standardize and require the use of live rabies virus, containment facilities, and skilled professionals. Thus, in this review, we describe the state-of-the-art of alternative analytical methodologies currently available for rabies serology, with novel approaches based on pseudotyped recombinant viruses and emphasizing in the antigen binding methodologies that detect and quantify antibodies against the rabies glycoprotein. We discussed the wide range of assays that are interesting tools for a faster measurement of anti-rabies glycoprotein antibodies and, in some cases, less complex and more versatile than the gold standard methods. Finally, we discussed the key issues during the design and optimization steps of ELISA assays, highlighting the importance of validation and standardization procedures to improve rabies serology tests and, as a consequence, their results. KEY POINTS: • An exhaustive revision of rabies serology testing was made. • No rabies serology assay can be thought as better than others for all intents and purposes. • The validation procedure guarantees reliable and consistent results among the globe.

Marked Increase in Avidity of SARS-CoV-2 Antibodies 7-8 Months After Infection Is Not Diminished in Old Age

The kinetics of immunoglobulin G (IgG) avidity maturation during severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) infection obtained from 217 participants of the Ischgl cohort, Austria, was studied 0.5-1.5 months (baseline) and 7-8 months (follow-up) after infection. The IgG avidity assay, using a modified IgG enzyme-linked immunosorbent assay (ELISA) and 5.5 M urea, revealed that old age does not diminish the increase in avidity, detected in all participants positive at both time points, from 18% to 42%. High avidity was associated with a marked residual neutralization capacity in 97.2.% of participants (211/217), which was even higher in the older age group, revealing an important role of avidity assays as easy and cheap surrogate tests for assessing the maturation of the immune system conveying potential protection against further SARS-CoV-2 infections without necessitating expensive and laborious neutralization assays.

Cooperativity mediated by rationally selected combinations of human monoclonal antibodies targeting the henipavirus receptor binding protein

Hendra virus and Nipah virus (NiV), members of the Henipavirus (HNV) genus, are zoonotic paramyxoviruses known to cause severe disease across six mammalian orders, including humans. We isolated a panel of human monoclonal antibodies (mAbs) from the B cells of an individual with prior exposure to equine Hendra virus (HeV) vaccine, targeting distinct antigenic sites. The most potent class of cross-reactive antibodies achieves neutralization by blocking viral attachment to the host cell receptors ephrin-B2 and ephrin-B3, with a second class being enhanced by receptor binding. mAbs from both classes display synergistic activity in vitro. In a stringent hamster model of NiV Bangladesh (NiV_B) infection, antibodies from both classes reduce morbidity and mortality and achieve synergistic protection in combination. These candidate mAbs might be suitable for use in a cocktail therapeutic approach to achieve synergistic potency and reduce the risk of virus escape.

Doyle et al. describe two human monoclonal antibodies that target the henipavirus receptor-binding protein, HENV-103 and HENV-117, that display highly potent activity in vitro and enhanced therapeutic efficacy in vivo when delivered as a cocktail.

Cross-Neutralisation of Novel Bombali Virus by Ebola Virus Antibodies and Convalescent Plasma Using an Optimised Pseudotype-Based Neutralisation Assay

Ebolaviruses continue to pose a significant outbreak threat, and while Ebola virus (EBOV)-specific vaccines and antivirals have been licensed, efforts to develop candidates offering broad species cross-protection are continuing. The use of pseudotyped virus in place of live virus is recognised as an alternative, safer, high-throughput platform to evaluate anti-ebolavirus antibodies towards their development, yet it requires optimisation. Here, we have shown that the target cell line impacts neutralisation assay results and cannot be selected purely based on permissiveness. In expanding the platform to incorporate each of the ebolavirus species envelope glycoprotein, allowing a comprehensive assessment of cross-neutralisation, we found that the recently discovered Bombali virus has a point mutation in the receptor-binding domain which prevents entry into a hamster cell line and, importantly, shows that this virus can be cross-neutralised by EBOV antibodies and convalescent plasma.

Immunogenicity of a new gorilla adenovirus vaccine candidate for COVID-19

The coronavirus disease 2019 (COVID-19) pandemic caused by the emergent severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) threatens global public health, and there is an urgent need to develop safe and effective vaccines. Here, we report the generation and the preclinical evaluation of a novel replication-defective gorilla adenovirus-vectored vaccine encoding the pre-fusion stabilized Spike (S) protein of SARS-CoV-2. We show that our vaccine candidate, GRAd-COV2, is highly immunogenic both in mice and macaques, eliciting both functional antibodies that neutralize SARS-CoV-2 infection and block Spike protein binding to the ACE2 receptor, and a robust, T helper (Th)1-dominated cellular response. We show here that the pre-fusion stabilized Spike antigen is superior to the wild type in inducing ACE2-interfering, SARS-CoV-2-neutralizing antibodies. To face the unprecedented need for vaccine manufacturing at a massive scale, different GRAd genome deletions were compared to select the vector backbone showing the highest productivity in stirred tank bioreactors. This preliminary dataset identified GRAd-COV2 as a potential COVID-19 vaccine candidate, supporting the translation of the GRAd-COV2 vaccine in a currently ongoing phase I clinical trial (ClinicalTrials.gov: NCT04528641).

A Single-Cycle Influenza A Virus-Based SARS-CoV-2 Vaccine Elicits Potent Immune Responses in a Mouse Model

The use of virus-vectored platforms has increasingly gained attention in vaccine development as a means for delivering antigenic genes of interest into target hosts. Here, we describe a single-cycle influenza virus-based SARS-CoV-2 vaccine designated as scPR8-RBD-M2. The vaccine utilizes the chimeric gene encoding 2A peptide-based bicistronic protein cassette of the SARS-CoV-2 receptor-binding domain (RBD) and influenza matrix 2 (M2) protein. The C-terminus of the RBD was designed to link with the cytoplasmic domain of the influenza virus hemagglutinin (HA) to anchor the RBD on the surface of producing cells and virus envelope. The chimeric RBD-M2 gene was incorporated in place of the HA open-reading frame (ORF) between the 3′ and 5′ UTR of HA gene for the virus rescue in MDCK cells stably expressing HA. The virus was also constructed with the disrupted M2 ORF in segment seven to ensure that M2 from the RBD-M2 was utilized. The chimeric gene was intact and strongly expressed in infected cells upon several passages, suggesting that the antigen was stably maintained in the vaccine candidate. Mice inoculated with scPR8-RBD-M2 via two alternative prime-boost regimens (intranasal-intranasal or intranasal-intramuscular routes) elicited robust mucosal and systemic humoral immune responses and cell-mediated immunity. Notably, we demonstrated that immunized mouse sera exhibited neutralizing activity against pseudotyped viruses bearing SARS-CoV-2 spikes from various variants, albeit with varying potency. Our study warrants further development of a replication-deficient influenza virus as a promising SARS-CoV-2 vaccine candidate.

Persistence of immunity to SARS-CoV-2 over time in the ski resort Ischgl

Background In early March 2020, a SARS-CoV-2 outbreak in the ski resort Ischgl in Austria triggered the spread of SARS-CoV-2 throughout Austria and Northern Europe. In a previous study, we found that the seroprevalence in the adult population of Ischgl had reached 45% by the end of April, representing an exceptionally high level of local seropositivity in Europe. We performed a follow-up study in Ischgl, which is the first to show persistence of immunity and protection against SARS-CoV-2 and some of its variants at a community level. Methods Of the 1259 adults that participated in the baseline study, 801 have been included in the follow-up in November 2020. The study involved the analysis of binding and neutralizing antibodies and T cell responses. In addition, the incidence of SARS-CoV-2 and its variants in Ischgl was compared to the incidence in similar municipalities in Tyrol until April 2021. Findings For the 801 individuals that participated in both studies, the seroprevalence declined from 51.4% (95% confidence interval (CI) 47.9-54.9) to 45.4% (95% CI 42.0-49.0). Median antibody concentrations dropped considerably (5.345, 95% CI 4.833 - 6.123 to 2.298, 95% CI 2.141 - 2.527) but antibody avidity increased (17.02, 95% CI 16.49 - 17.94 to 42.46, 95% CI 41.06 - 46.26). Only one person had lost detectable antibodies and T cell responses. In parallel to this persistent immunity, we observed that Ischgl was relatively spared, compared to similar municipalities, from the prominent second COVID-19 wave that hit Austria in November 2020. In addition, we used sequencing data to show that the local immunity acquired from wild-type infections also helped to curb infections from variants of SARS-CoV-2 which spread in Austria since January 2021. Interpretation The relatively high level of seroprevalence (40-45%) in Ischgl persisted and might have been associated with the observed protection of Ischgl residents against virus infection during the second COVID-19 wave as well as against variant spread in 2021. Funding Funding was provided by the government of Tyrol and the FWF Austrian Science Fund.

Exploiting Pan Influenza A and Pan Influenza B Pseudotype Libraries for Efficient Vaccine Antigen Selection

We developed an influenza hemagglutinin (HA) pseudotype library encompassing Influenza A subtypes HA1-18 and Influenza B subtypes (both lineages) to be employed in influenza pseudotype microneutralization (pMN) assays. The pMN is highly sensitive and specific for detecting virus-specific neutralizing antibodies against influenza viruses and can be used to assess antibody functionality in vitro. Here we show the production of these viral HA pseudotypes and their employment as substitutes for wildtype viruses in influenza neutralization assays. We demonstrate their utility in detecting serum responses to vaccination with the ability to evaluate cross-subtype neutralizing responses elicited by specific vaccinating antigens. Our findings may inform further preclinical studies involving immunization dosing regimens in mice and may help in the creation and selection of better antigens for vaccine design. These HA pseudotypes can be harnessed to meet strategic objectives that contribute to the strengthening of global influenza surveillance, expansion of seasonal influenza prevention and control policies, and strengthening pandemic preparedness and response.

Cross-Reactive SARS-CoV-2 Neutralizing Antibodies From Deep Mining of Early Patient Responses

Passive immunization using monoclonal antibodies will play a vital role in the fight against COVID-19. The recent emergence of viral variants with reduced sensitivity to some current antibodies and vaccines highlights the importance of broad cross-reactivity. This study describes deep-mining of the antibody repertoires of hospitalized COVID-19 patients using phage display technology and B cell receptor (BCR) repertoire sequencing to isolate neutralizing antibodies and gain insights into the early antibody response. This comprehensive discovery approach has yielded a panel of potent neutralizing antibodies which bind distinct viral epitopes including epitopes conserved in SARS-CoV-1. Structural determination of a non-ACE2 receptor blocking antibody reveals a previously undescribed binding epitope, which is unlikely to be affected by the mutations in any of the recently reported major viral variants including B.1.1.7 (from the UK), B.1.351 (from South Africa) and B.1.1.28 (from Brazil). Finally, by combining sequences of the RBD binding and neutralizing antibodies with the B cell receptor repertoire sequencing, we also describe a highly convergent early antibody response. Similar IgM-derived sequences occur within this study group and also within patient responses described by multiple independent studies published previously.

Cellular and Humoral Immune Responses in Mice Immunized with Vaccinia Virus Expressing the SARS-CoV-2 Spike Protein

The COVID-19 pandemic is a global health emergency, and the development of a successful vaccine will ultimately be required to prevent the continued spread and seasonal recurrence of this disease within the human population. However, very little is known about either the quality of the adaptive immune response or the viral Ag targets that will be necessary to prevent the spread of the infection. In this study, we generated recombinant Vaccinia virus expressing the full-length spike protein from SARS-CoV-2 (VacV-S) to evaluate the cellular and humoral immune response mounted against this viral Ag in mice. Both CD8+ and CD4+ T cells specific to the SARS-CoV-2 spike protein underwent robust expansion, contraction, and persisted for at least 40 d following a single immunization with VacV-S. Vaccination also caused the rapid emergence of spike-specific IgG-neutralizing Abs. Interestingly, both the cellular and humoral immune responses strongly targeted the S1 domain of spike following VacV-S immunization. Notably, immunization with VacV-expressing spike conjugated to the MHC class II invariant chain, a strategy previously reported by us and others to enhance the immunogenicity of antigenic peptides, did not promote stronger spike-specific T cell or Ab responses in vivo. Overall, these findings demonstrate that an immunization approach using VacV or attenuated versions of VacV expressing the native, full-length SARS-CoV-2 spike protein could be used for further vaccine development to prevent the spread of COVID-19.

Evaluation of a Commercial Culture-Free Neutralization Antibody Detection Kit for Severe Acute Respiratory Syndrome-Related Coronavirus-2 and Comparison With an Antireceptor-Binding Domain Enzyme-Linked Immunosorbent Assay

BACKGROUND

Severe acute respiratory syndrome-related coronavirus 2 (SARS-CoV-2) surrogate neutralization assays that obviate the need for viral culture offer substantial advantages regarding throughput and cost. The cPass SARS-CoV-2 Neutralization Antibody Detection Kit (GenScript) is the first such commercially available assay that detects antibodies that block receptor-binding domain (RBD)/angiotensin-converting enzyme (ACE)-2 interaction. We aimed to evaluate cPass to inform its use and assess its added value compared with anti-RBD enzyme-linked immunosorbent assays (ELISAs).

METHODS

Serum reference panels comprising 205 specimens were used to compare cPass to plaque-reduction neutralization test (PRNT) and a pseudotyped lentiviral neutralization (PLV) assay for detection of neutralizing antibodies. We assessed the correlation of cPass with an ELISA detecting anti-RBD immunoglobulin (Ig)G, IgM, and IgA antibodies at a single timepoint and across intervals from onset of symptoms of SARS-CoV-2 infection.

RESULTS

Compared with PRNT-50, cPass sensitivity ranged from 77% to 100% and specificity was 95% to 100%. Sensitivity was also high compared with the pseudotyped lentiviral neutralization assay (93%; 95% confidence interval [CI], 85-97), but specificity was lower (58%; 95% CI, 48-67). Highest agreement between cPass and ELISA was for anti-RBD IgG (r = 0.823). Against the pseudotyped lentiviral neutralization assay, anti-RBD IgG sensitivity (99%; 95% CI, 94-100) was very similar to that of cPass, but overall specificity was lower (37%; 95% CI, 28-47). Against PRNT-50, results of cPass and anti-RBD IgG were nearly identical.

CONCLUSIONS

The added value of cPass compared with an IgG anti-RBD ELISA was modest.

Development of Lentiviral Vectors Pseudotyped With Influenza B Hemagglutinins: Application in Vaccine Immunogenicity, mAb Potency, and Sero-Surveillance Studies

Influenza B viruses (IBV) cause respiratory disease epidemics in humans and are therefore components of seasonal influenza vaccines. Serological methods are employed to evaluate vaccine immunogenicity prior to licensure. However, classical methods to assess influenza vaccine immunogenicity such as the hemagglutination inhibition assay (HI) and the serial radial hemolysis assay (SRH), have been proven to have many limitations. As such, there is a need to develop innovative methods that can improve on these traditional assays and provide advantages such as ease of production and access, safety, reproducibility, and specificity. It has been previously demonstrated that the use of replication-defective viruses, such as lentiviral vectors pseudotyped with influenza A hemagglutinins in microneutralization assays (pMN) is a safe and sensitive alternative to study antibody responses elicited by natural influenza infection or vaccination. Consequently, we have produced Influenza B hemagglutinin-pseudotypes (IBV PV) using plasmid-directed transfection. To activate influenza B hemagglutinin, we have explored the use of proteases in increasing PV titers via their co-transfection during pseudotype virus production. When tested for their ability to transduce target cells, the influenza B pseudotypes produced exhibit tropism for different cell lines. The pseudotypes were evaluated as alternatives to live virus in microneutralization assays using reference sera standards, mouse and human sera collected during vaccine immunogenicity studies, surveillance sera from seals, and monoclonal antibodies (mAbs) against IBV. The influenza B pseudotype pMN was found to effectively detect neutralizing and cross-reactive responses in all assays and shows promise as an effective and versatile tool in influenza research.

Potent Henipavirus Neutralization by Antibodies Recognizing Diverse Sites on Hendra and Nipah Virus Receptor Binding Protein

Hendra (HeV) and Nipah (NiV) viruses are emerging zoonotic pathogens in the Henipavirus genus causing outbreaks of disease with very high case fatality rates. Here, we report the first naturally occurring human monoclonal antibodies (mAbs) against HeV receptor binding protein (RBP). All isolated mAbs neutralized HeV, and some also neutralized NiV. Epitope binning experiments identified five major antigenic sites on HeV-RBP. Animal studies demonstrated that the most potent cross-reactive neutralizing mAbs, HENV-26 and HENV-32, protected ferrets in lethal models of infection with NiV Bangladesh 3 days after exposure. We solved the crystal structures of mAb HENV-26 in complex with both HeV-RBP and NiV-RBP and of mAb HENV-32 in complex with HeV-RBP. The studies reveal diverse sites of vulnerability on RBP recognized by potent human mAbs that inhibit virus by multiple mechanisms. These studies identify promising prophylactic antibodies and define protective epitopes that can be used in rational vaccine design.

Broadly neutralizing antibody cocktails targeting Nipah virus and Hendra virus fusion glycoproteins

Hendra virus (HeV) and Nipah virus (NiV) are henipaviruses (HNVs) causing respiratory illness and severe encephalitis in humans, with fatality rates of 50-100%. There are no licensed therapeutics or vaccines to protect humans. HeV and NiV use a receptor-binding glycoprotein (G) and a fusion glycoprotein (F) to enter host cells. HNV F and G are the main targets of the humoral immune response, and the presence of neutralizing antibodies is a correlate of protection against NiV and HeV in experimentally infected animals. We describe here two cross-reactive F-specific antibodies, 1F5 and 12B2, that neutralize NiV and HeV through inhibition of membrane fusion. Cryo-electron microscopy structures reveal that 1F5 and 12B2 recognize distinct prefusion-specific, conserved quaternary epitopes and lock F in its prefusion conformation. We provide proof-of-concept for using antibody cocktails for neutralizing NiV and HeV and define a roadmap for developing effective countermeasures against these highly pathogenic viruses.

SARS-CoV-2 501Y.V2 variants lack higher infectivity but do have immune escape

The 501Y.V2 variants of SARS-CoV-2 containing multiple mutations in spike are now dominant in South Africa and are rapidly spreading to other countries. Here, experiments with 18 pseudotyped viruses showed that the 501Y.V2 variants do not confer increased infectivity in multiple cell types except for murine ACE2-overexpressing cells, where a substantial increase in infectivity was observed. Notably, the susceptibility of the 501Y.V2 variants to 12 of 17 neutralizing monoclonal antibodies was substantially diminished, and the neutralization ability of the sera from convalescent patients and immunized mice was also reduced for these variants. The neutralization resistance was mainly caused by E484K and N501Y mutations in the receptor-binding domain of spike. The enhanced infectivity in murine ACE2-overexpressing cells suggests the possibility of spillover of the 501Y.V2 variants to mice. Moreover, the neutralization resistance we detected for the 501Y.V2 variants suggests the potential for compromised efficacy of monoclonal antibodies and vaccines.

Comparison of Serological Assays for the Detection of SARS-CoV-2 Antibodies

SARS-CoV-2 virus was first detected in late 2019 and circulated globally, causing COVID-19, which is characterised by sub-clinical to severe disease in humans. Here, we investigate the serological antibody responses to SARS-CoV-2 infection during acute and convalescent infection using a cohort of (i) COVID-19 patients admitted to hospital, (ii) healthy individuals who had experienced 'COVID-19 like-illness', and (iii) a cohort of healthy individuals prior to the emergence of SARS-CoV-2. We compare SARS-CoV-2 specific antibody detection rates from four different serological methods, virus neutralisation test (VNT), ID Screen® SARS-CoV-2-N IgG ELISA, Whole Antigen ELISA, and lentivirus-based SARS-CoV-2 pseudotype virus neutralisation tests (pVNT). All methods were able to detect prior infection with COVID-19, albeit with different relative sensitivities. The VNT and SARS-CoV-2-N ELISA methods showed a strong correlation yet provided increased detection rates when used in combination. A pVNT correlated strongly with SARS-CoV-2 VNT and was able to effectively discriminate SARS-CoV-2 antibody positive and negative serum with the same efficiency as the VNT. Moreover, the pVNT was performed with the same level of discrimination across multiple separate institutions. Therefore, the pVNT is a sensitive, specific, and reproducible lower biosafety level alternative to VNT for detecting SARS-CoV-2 antibodies for diagnostic and research applications. Our data illustrate the potential utility of applying VNT or pVNT and ELISA antibody tests in parallel to enhance the sensitivity of exposure to infection.

Viral infection neutralization tests: A focus on severe acute respiratory syndrome-coronavirus-2 with implications for convalescent plasma therapy

Viral neutralization tests (VNTs) have long been considered old-fashioned tricks in the armamentarium of fundamental virology, with laboratory implementation for a limited array of viruses only. Nevertheless, they represent the most reliable surrogate of potency for passive immunotherapies, such as monoclonal or polyclonal antibody therapy. The recent interest around therapy with convalescent plasma or monoclonal antibodies for the Covid-19 pandemic has paralleled the revival of VNTs. We review here the available methods by dissecting variations for each fundamental component of the VNT (i.e., virus type and dose, replication-competent cell line, serum, and detection system).

Micro-fusion inhibition tests: quantifying antibody neutralization of virus-mediated cell-cell fusion

Although enveloped viruses canonically mediate particle entry through virus-cell fusion, certain viruses can spread by cell-cell fusion, brought about by receptor engagement and triggering of membrane-bound, viral-encoded fusion proteins on the surface of cells. The formation of pathogenic syncytia or multinucleated cells is seen in vivo, but their contribution to viral pathogenesis is poorly understood. For the negative-strand paramyxoviruses respiratory syncytial virus (RSV) and Nipah virus (NiV), cell-cell spread is highly efficient because their oligomeric fusion protein complexes are active at neutral pH. The recently emerged severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has also been reported to induce syncytia formation in infected cells, with the spike protein initiating cell-cell fusion. Whilst it is well established that fusion protein-specific antibodies can block particle attachment and/or entry into the cell (canonical virus neutralization), their capacity to inhibit cell-cell fusion and the consequences of this neutralization for the control of infection are not well characterized, in part because of the lack of specific tools to assay and quantify this activity. Using an adapted bimolecular fluorescence complementation assay, based on a split GFP-Renilla luciferase reporter, we have established a micro-fusion inhibition test (mFIT) that allows the identification and quantification of these neutralizing antibodies. This assay has been optimized for high-throughput use and its applicability has been demonstrated by screening monoclonal antibody (mAb)-mediated inhibition of RSV and NiV fusion and, separately, the development of fusion-inhibitory antibodies following NiV vaccine immunization in pigs. In light of the recent emergence of coronavirus disease 2019 (COVID-19), a similar assay was developed for SARS-CoV-2 and used to screen mAbs and convalescent patient plasma for fusion-inhibitory antibodies. Using mFITs to assess antibody responses following natural infection or vaccination is favourable, as this assay can be performed entirely at low biocontainment, without the need for live virus. In addition, the repertoire of antibodies that inhibit cell-cell fusion may be different to those that inhibit particle entry, shedding light on the mechanisms underpinning antibody-mediated neutralization of viral spread.

Recent Developments in SARS-CoV-2 Neutralizing Antibody Detection Methods

The ongoing Coronavirus disease 19 pandemic has likely changed the world in ways not seen in the past. Neutralizing antibody (NAb) assays play an important role in the management of the severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) outbreak. Using these tools, we can assess the presence and duration of antibody-mediated protection in naturally infected individuals, screen convalescent plasma preparations for donation, test the efficacy of immunotherapy, and analyze NAb titers and persistence after vaccination to predict vaccine-induced protective effects. This review briefly summarizes the various methods used for the detection of SARS-CoV-2 NAbs and compares their advantages and disadvantages to facilitate their development and clinical application.

Discordant neutralizing antibody and T cell responses in asymptomatic and mild SARS-CoV-2 infection

Understanding the nature of immunity following mild/asymptomatic infection with SARS-CoV-2 is crucial to controlling the pandemic. We analyzed T cell and neutralizing antibody responses in 136 healthcare workers (HCW) 16-18 weeks after United Kingdom lockdown, 76 of whom had mild/asymptomatic SARS-CoV-2 infection captured by serial sampling. Neutralizing antibodies (nAb) were present in 89% of previously infected HCW. T cell responses tended to be lower following asymptomatic infection than in those reporting case-definition symptoms of COVID-19, while nAb titers were maintained irrespective of symptoms. T cell and antibody responses were sometimes discordant. Eleven percent lacked nAb and had undetectable T cell responses to spike protein but had T cells reactive with other SARS-CoV-2 antigens. Our findings suggest that the majority of individuals with mild or asymptomatic SARS-CoV-2 infection carry nAb complemented by multispecific T cell responses at 16-18 weeks after mild or asymptomatic SARS-CoV-2 infection.

Effective screening of SARS-CoV-2 neutralizing antibodies in patient serum using lentivirus particles pseudotyped with SARS-CoV-2 spike glycoprotein

Pseuodotyped particles have significant importance and use in virology as tools for studying the biology of highly pathogenic viruses in a lower biosafety environment. The biological, chemical, and serological studies of the recently emerged SARS-CoV-2 will be greatly aided by the development and optimization of a suitable pseudotyping system. Here, we pseudotyped the SARS-CoV-2 Spike glycoprotein (SPG) on a traditional retroviral (MMLV) as well as a third generation lentiviral (pLV) vector and tested the transduction efficiency in several mammalian cell lines expressing SARS-CoV-2 receptor hACE2. While MMLV pseudotyped the vesicular stomatitis virus G glycoprotein (VSV-G) efficiently, it could not pseudotype the full-length SPG. In contrast, pLV pseudotyped both glycoproteins efficiently; however, much higher titers of pLV-G particles were produced. Among all the tested mammalian cells, 293Ts expressing hACE2 were most efficiently transduced using the pLV-S system. The pLV-S particles were efficiently neutralized by diluted serum (>:640) from recently recovered COVID-19 patients who showed high SARS-CoV-2 specific IgM and IgG levels. In summary, pLV-S pseudotyped virus provides a valid screening tool for the presence of anti SARS-CoV-2 specific neutralizing antibodies in convalescent patient serum.

A simple protein-based surrogate neutralization assay for SARS-CoV-2

Most of the patients infected with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) mount a humoral immune response to the virus within a few weeks of infection, but the duration of this response and how it correlates with clinical outcomes has not been completely characterized. Of particular importance is the identification of immune correlates of infection that would support public health decision-making on treatment approaches, vaccination strategies, and convalescent plasma therapy. While ELISA-based assays to detect and quantitate antibodies to SARS-CoV-2 in patient samples have been developed, the detection of neutralizing antibodies typically requires more demanding cell-based viral assays. Here, we present a safe and efficient protein-based assay for the detection of serum and plasma antibodies that block the interaction of the SARS-CoV-2 spike protein receptor binding domain (RBD) with its receptor, angiotensin-converting enzyme 2 (ACE2). The assay serves as a surrogate neutralization assay and is performed on the same platform and in parallel with an ELISA for the detection of antibodies against the RBD, enabling a direct comparison. The results obtained with our assay correlate with those of 2 viral-based assays, a plaque reduction neutralization test (PRNT) that uses live SARS-CoV-2 virus and a spike pseudotyped viral vector-based assay.

Meeting report and review: Immunological assays and correlates of protection for next-generation influenza vaccines

BACKGROUND

This report summarizes the discussions and conclusions from the "Immunological Assays and Correlates of Protection for Next-Generation Influenza Vaccines" meeting which took place in Siena, Italy, from March 31, 2019, to April 2, 2019.

CONCLUSIONS

Furthermore, we review current correlates of protection against influenza virus infection and disease and their usefulness for the development of next generation broadly protective and universal influenza virus vaccines.

An antibody against the F glycoprotein inhibits Nipah and Hendra virus infections

Nipah virus (NiV) and Hendra virus (HeV) are zoonotic henipaviruses (HNVs) responsible for outbreaks of encephalitis and respiratory illness with fatality rates of 50–100%. No vaccines or licensed therapeutics currently exist to protect humans against NiV or HeV. HNVs enter host cells by fusing the viral and cellular membranes via the concerted action of the attachment (G) and fusion (F) glycoproteins, the main targets of the humoral immune response. Here, we describe the isolation and humanization of a potent monoclonal antibody cross-neutralizing NiV and HeV. Cryo-electron microscopy, triggering and fusion studies show the antibody binds to a prefusion-specific quaternary epitope, conserved in NiV F and HeV F glycoproteins, and prevents membrane fusion and viral entry. This work supports the importance of the HNV prefusion F conformation for eliciting a robust immune response and paves the way for using this antibody for prophylaxis and post-exposure therapy with NiV- and HeV-infected individuals.

An antibody that recognizes the F glycoproteins from Nipah and Hendra viruses can neutralize both viruses and recognizes a quaternary epitope in the prefusion F trimer, preventing conformational changes required for fusion.