Protective human monoclonal antibodies target conserved sites of vulnerability on the underside of influenza virus neuraminidase

Continuously evolving influenza viruses cause seasonal epidemics and pose global pandemic threats. Although viral neuraminidase (NA) is an effective drug and vaccine target, our understanding of the NA antigenic landscape still remains incomplete. Here, we describe NA-specific human antibodies that target the underside of the NA globular head domain, inhibit viral propagation of a wide range of human H3N2, swine-origin variant H3N2, and H2N2 viruses, and confer both pre- and post-exposure protection against lethal H3N2 infection in mice. Cryo-EM structures of two such antibodies in complex with NA reveal non-overlapping epitopes covering the underside of the NA head. These sites are highly conserved among N2 NAs yet inaccessible unless the NA head tilts or dissociates. Our findings help guide the development of effective countermeasures against ever-changing influenza viruses by identifying hidden conserved sites of vulnerability on the NA underside.

Identification of four neutralizing antigenic sites on the enterovirus D68 capsid

IMPORTANCE

Enterovirus D68 (EV-D68) is an emerging respiratory pathogen associated with acute flaccid myelitis. Currently, no approved vaccines or antiviral drugs are available. Here, we report four functionally independent neutralizing antigenic sites (I to IV) by analyses of neutralizing monoclonal antibody (MAb)-resistant mutants. Site I is located in the VP1 BC loop near the fivefold axis. Site II resides in the VP2 EF loop, and site III is situated in VP1 C-terminus; both sites are located at the south rim of the canyon. Site IV is composed of residue in VP2 βB strand and residues in the VP3 BC loop and resides around the threefold axis. The developed MAbs targeting the antigenic sites can inhibit viral binding to cells. These findings advance the understanding of the recognition of EV-D68 by neutralizing antibodies and viral evolution and immune escape and also have important implications for the development of novel EV-D68 vaccines.

Comparative analysis of human respiratory syncytial virus evolutionary patterns during the COVID-19 pandemic and pre-pandemic periods

The COVID-19 pandemic has resulted in the implementation of strict mitigation measures that have impacted the transmission dynamics of human respiratory syncytial virus (HRSV). The measures also have the potential to influence the evolutionary patterns of the virus. In this study, we conducted a comprehensive analysis comparing genomic variations and evolving characteristics of its neutralizing antigens, specifically F and G proteins, before and during the COVID-19 pandemic. Our findings showed that both HRSV A and B exhibited an overall chronological evolutionary pattern. For the sequences obtained during the pandemic period (2019-2022), we observed that the HRSV A distributed in A23 genotype, but formed into three subclusters; whereas the HRSV B sequences were relatively concentrated within genotype B6. Additionally, multiple positively selected sites were detected on F and G proteins but none were located at neutralizing antigenic sites of the F protein. Notably, amino acids within antigenic site III, IV, and V of F protein remained strictly conserved, while some substitutions occurred over time on antigenic site Ø, I, II and VIII; substitution S389P on antigenic site I of HRSV B occurred during the pandemic period with nearly 50% frequency. However, further analysis revealed no substitutions have altered the structural conformations of the antigenic sites, the vial antigenicity has not been changed. We inferred that the intensive public health interventions during the COVID-19 pandemic did not affect the evolutionary mode of HRSV.

Evolution of prevalent H9N2 subtype of avian influenza virus during 2019 to 2022 for the development of a control strategy in China

The H9N2 subtype of avian influenza virus (H9N2 AIV) has caused significant losses in chicken flocks throughout China. At present, consensus has been reached that field isolates of H9N2 underwent antigenic drift to evolve into distinct groups with significant antigenic divergence from the commercially available vaccines in China. This project continues to monitor the evolution characteristics of H9N2 hemagglutinin (HA) genes in China over the past 3 yr. The results showed that the current circling H9N2 viruses were diversified into h9.4.2.5 subclade, which was genetically distant from commonly used commercial vaccine strains. Compared with vaccine strains or 2014 strains, more than 42.1% of the variable antigenic sites in recent 3 yr' strains have shown significant changes and these stacked changes have caused significant differences in antigenicity. We constructed a recombinant vaccine strain rCQY-GHHA, which uses A/Chicken/China/SichuanCQY/2014 as the framework and A/Chicken/China/SichuanGH/2020 strain, which meets the recent viral antigenic characteristics, as the HA gene donor. The recombinant strain was prepared as an oil-adjuvant inactivated vaccine following an industrial process. The results of the immune protection experiment showed that the rCQY-GHHA vaccine was better than the commercial vaccine strain SS in reducing the morbidity, pathological lesion, virus shedding, and viral load. These results provide a reference for the control of H9N2 AIV in China.

[Mathematical model for assessing the level of cross-immunity between strains of influenza virus subtype H3N2]

INTRODUCTION

The WHO regularly updates influenza vaccine recommendations to maximize their match with circulating strains. Nevertheless, the effectiveness of the influenza A vaccine, specifically its H3N2 component, has been low for several seasons. The aim of the study is to develop a mathematical model of cross-immunity based on the array of published WHO hemagglutination inhibition assay (HAI) data.

MATERIALS AND METHODS

In this study, a mathematical model was proposed, based on finding, using regression analysis, the dependence of HAI titers on substitutions in antigenic sites of sequences. The computer program we developed can process data (GISAID, NCBI, etc.) and create real-time databases according to the set tasks.

RESULTS

Based on our research, an additional antigenic site F was identified. The difference in 1.6 times the adjusted R2, on subsets of viruses grown in cell culture and grown in chicken embryos, demonstrates the validity of our decision to divide the original data array by passage histories. We have introduced the concept of a degree of homology between two arbitrary strains, which takes the value of a function depending on the Hamming distance, and it has been shown that the regression results significantly depend on the choice of function. The provided analysis showed that the most significant antigenic sites are A, B, and E. The obtained results on predicted HAI titers showed a good enough result, comparable to similar work by our colleagues.

CONCLUSION

The proposed method could serve as a useful tool for future forecasts, with further study to confirm its sustainability.

Evolution of Indian Influenza A (H1N1) Hemagglutinin Strains: A Comparative Analysis of the Pandemic Californian HA Strain

The need for a vaccine/inhibitor design has become inevitable concerning the emerging epidemic and pandemic viral infections, and the recent outbreak of the influenza A (H1N1) virus is one such example. From 2009 to 2018, India faced severe fatalities due to the outbreak of the influenza A (H1N1) virus. In this study, the potential features of reported Indian H1N1 strains are analyzed in comparison with their evolutionarily closest pandemic strain, A/California/04/2009. The focus is laid on one of its surface proteins, hemagglutinin (HA), which imparts a significant role in attacking the host cell surface and its entry. The extensive analysis performed, in comparison with the A/California/04/2009 strain, revealed significant point mutations in all Indian strains reported from 2009 to 2018. Due to these mutations, all Indian strains disclosed altered features at the sequence and structural levels, which are further presumed to be associated with their functional diversity as well. The mutations observed with the 2018 HA sequence such as S91R, S181T, S200P, I312V, K319T, I419M, and E523D might improve the fitness of the virus in a new host and environment. The higher fitness and decreased sequence similarity of mutated strains may compromise therapeutic efficacy. In particular, the mutations observed commonly, such as serine-to-threonine, alanine-to-threonine, and lysine-to-glutamine at various regions, alter the physico-chemical features of receptor-binding domains, N-glycosylation, and epitope-binding sites when compared with the reference strain. Such mutations render diversity among all Indian strains, and the structural and functional characterization of these strains becomes inevitable. In this study, we observed that mutational drift results in the alteration of the receptor-binding domain, the generation of new variant N-glycosylation along with novel epitope-binding sites, and modifications at the structural level. Eventually, the pressing need to develop potentially distinct next-generation therapeutic inhibitors against the HA strains of the Indian influenza A (H1N1) virus is also highlighted here.

Identification of Dengue Virus Serotype 3 Specific Antigenic Sites Targeted by Neutralizing Human Antibodies

The rational design of dengue virus (DENV) vaccines requires a detailed understanding of the molecular basis for antibody-mediated immunity. The durably protective antibody response to DENV after primary infection is serotype specific. However, there is an incomplete understanding of the antigenic determinants for DENV type-specific (TS) antibodies, especially for DENV serotype 3, which has only one well-studied, strongly neutralizing human monoclonal antibody (mAb). Here, we investigated the human B cell response in children after natural DENV infection in the endemic area of Nicaragua and isolated 15 DENV3 TS mAbs recognizing the envelope (E) glycoprotein. Functional epitope mapping of these mAbs and small animal prophylaxis studies revealed a complex landscape with protective epitopes clustering in at least 6-7 antigenic sites. Potently neutralizing TS mAbs recognized sites principally in E glycoprotein domains I and II, and patterns suggest frequent recognition of quaternary structures on the surface of viral particles.

Crystal Structure and Immunogenicity of the DS-Cav1-Stabilized Fusion Glycoprotein From Respiratory Syncytial Virus Subtype B

Background:

Respiratory syncytial virus (RSV) subtypes, A and B, co-circulate in annual epidemics and alternate in dominance. We have shown that a subtype A RSV fusion (F) glycoprotein, stabilized in its prefusion conformation by DS-Cav1 mutations, is a promising RSV-vaccine immunogen, capable of boosting RSV-neutralizing titers in healthy adults. In both humans and vaccine-tested animals, neutralizing titers elicited by this subtype A DS-Cav1 immunogen were ~ 2- to 3-fold higher against the homologous subtype A virus than against the heterologous subtype B virus.

Methods:

To understand the molecular basis for this subtype difference, we introduced DS-Cav1 mutations into RSV strain B18537 F, determined the trimeric crystal structure, and carried out immunogenicity studies.

Results:

The B18537 DS-Cav1 F structure at 2-Å resolution afforded a precise delineation of prefusion F characteristics, including those of antigenic site Ø, a key trimer-apex site. Structural comparison with the subtype A prefusion F indicated 11% of surface residues to be different, with an alpha-carbon root-mean-square deviation (RMSD) of 1.2 Å; antigenic site Ø, however, differed in 23% of its surface residues and had an alpha-carbon RMSD of 2.2 Å. Immunization of vaccine-tested animals with DS-Cav1-stabilized B18537 F induced neutralizing responses ~100-fold higher than with postfusion B18537 F. Notably, elicited responses neutralized RSV subtypes A and B at similar levels and were directed towards both conserved equatorial and diverse apical regions.

Conclusion:

We propose that structural differences in apical and equatorial sites–coupled to differently focused immune responses–provide a molecular explanation for observed differences in elicited subtype A and B neutralizing responses.

Antigenic Site Variation in the Hemagglutinin of Pandemic Influenza A(H1N1)pdm09 Viruses between 2009-2017 in Ukraine

The hemagglutinin (HA) is a major influenza virus antigen, which, once recognized by antibodies and substitutions in HA genes, helps virus in escaping the human immune response. It is therefore critical to perform genetic and phylogenetic analysis of HA in circulating influenza viruses. We performed phylogenetic and genetic analysis of isolates from Ukraine, the vaccine strain and reference strains were used to phylogenetically identify trends in mutation locations and substitutions. Ukrainian isolates were collected between 2009–2017 and clustered in the influenza genetic groups 2, 6, 7, and 8. Genetic changes were observed in each of the antigenic sites: Sa – S162T, K163Q, K163I; Sb – S185T, A186T, S190G, S190R; Ca1 – S203T, R205K, E235V, E235D, S236P; Ca2 – P137H, H138R, A141T, D222G, D222N; Cb – A73S, S74R, S74N. In spite of detected mutations in antigenic sites, Ukrainian isolates retained similarity to the vaccine strain A/California/07/09 circulated during 2009–2017. However, WHO recommended a new vaccine strain A/Michigan/45/2015 for the Southern Hemisphere after the emergence of the new genetic groups 6B.1 and 6B.2. Our study demonstrated genetic variability of HA protein of A(H1N1)pdm09 viruses isolated in 2009–2017 in Ukraine. Influenza surveillance is very important for understanding epidemiological situations.

Emergence of novel lineage of foot-and-mouth disease virus serotype Asia1 BD-18 (G-IX) in Bangladesh

Foot-and-mouth disease virus (FMDV) is a highly evolutionary divergent pathogen causing great economic havoc in many countries. Among its seven existing serotypes, Asia1 is the least divergent with a single topotype both genetically and antigenically. It is reported sporadically in Indian subcontinent and was classified under lineage G-VIII. In 2018, serotype Asia1 re-emerged in Bangladesh after 2013, along with circulation of a novel serotype Asia1 BD-18 (G-IX) lineage. VP1 phylogeny and sequence variation clearly demonstrated the novel strains which was estimated to have at least >5% nucleotide divergence with distinct clade formation. Also, the Bayesian phylogeographic inferences traced back to the origin time of lineage G-IX in early 2017 and a possible origin in Bangladesh. Mutational analysis considering established eight lineages revealed that the virus strains belonged to lineage G-IX contained a unique mutation at 44 position in the B-C loop region of VP1. Inappropriate vaccination and inefficient outbreak surveillance possibly contributed to the current episode of emergence. Therefore, active surveillance and continued vigilance are essential to assess and timely detect the occurrence, extent and distribution of this novel Asia1 strains in Bangladesh and the neighbouring countries.

Antigenic site of nucleoprotein gene from Indonesian rabies virus isolates

BACKGROUND AND AIM

Several molecular studies on rabies virus (RABV) have been conducted in Indonesia, but it does not give clear information about molecular characteristics of previous RABV isolate in Indonesia. This study was conducted to know the characteristic of circulating RABV to determine a suitable method to control the spreading of RABV in Indonesia.

MATERIALS AND METHODS

Samples of infected RABV from dog brain were collected from Sumatera, Kalimantan, Sulawesi, and Bali Islands. All samples were examined based on nucleoprotein encoding gene to determine the molecular characteristics based on homology and phylogenetic tree compared to Pasteur Virus and RABV that came from another country within Asia (Indonesia, China, Thailand, India, and Korea). The collected samples were processed by one-step reverse transcriptase-polymerase chain reaction using nucleoprotein encoding gene followed by sequencing. The amino acid of its antigenic site of isolated RABV was also analyzed.

RESULTS

The results showed that isolated RABV has 84-85% similarity compared to Pasteur. According to phylogenetic construction, isolated samples do not share the same lineage toward Pasteur. The homology scores of isolated samples compared to RABV within Asia such as Indonesia, China, Thailand, India, and Korea were 98-99%, 92-93%, 88-89%, 86-88%, and 85-88%, respectively. According to antigenic site analysis compared to Pasteur, it was found that there were amino acid mutations within antigenic Site IV of nucleoprotein. Amino acid mutation from isoleucine to valine occurred in amino acid number 240 of 6 Kalimantan, 7 Kalimantan, and 8 Kalimantan. Amino acid mutation from alanine to aspartate and asparagine to threonine occurred within the same antigenic site in amino acid number 246 and 273 of C4 isolate from Sulawesi.

CONCLUSION

According to homology and phylogenetic tree analyses, isolated RABV remained different compared to RABV within Asia and Pasteur. The amino acid mutation occurred in antigenic site of nucleoprotein encoding gene.

Site-specific glycosylation profile of influenza A (H1N1) hemagglutinin through tandem mass spectrometry

The study of influenza virus evolution in humans has revealed a significant role of glycosylation profile alterations in the viral glycoproteins - hemagglutinin (HA) and neuraminidase (NA), in the emergence of both seasonal and pandemic strains. Viral antigenic drift can modify the number and location of glycosylation sites, altering a wide range of biological activities and the antigenic properties of the strain. In view of the key role of glycans in determining antigenicity, elucidating the glycosylation profiles of influenza strains is a requirement towards the development of improved vaccines. Sequence-based analysis of viral RNA has provided great insight into the role of glycosite modifications in altering virulence and pathogenicity. Nonetheless, this sequence-based approach can only predict potential glycosylation sites. Due to experimental challenges, experimental confirmation of the occupation of predicted glycosylation sites has only been carried out for a few strains. Herein, we utilized HCD/CID-MS/MS tandem mass spectrometry to characterize the site-specific profile of HA of an egg-grown H1N1 reference strain (A/New Caledonia/20/1999). We confirmed experimentally the occupancy of glycosylation sites identified by primary sequence analysis and determined the heterogeneity of glycan structures. Four glycosylation sequons on the stalk region (N28, N40, N304 and N498) and four on the globular head (N71, N104, N142 and N177) of the protein are occupied. Our results revealed a broad glycan microheterogeneity, i.e., a great diversity of glycan compositions present on each glycosite. The present methodology can be applied to characterize other viruses, particularly different influenza strains, to better understand the impact of glycosylation on biological activities and aid the improvement of influenza vaccines.

Genetic analysis of neuraminidase gene of influenza A(H1N1)pdm09 virus circulating in Southwest India from 2009 to 2012

Genetic analysis of neuraminidase gene sequences in 23 archived isolates of influenza A(H1N1)pdm09 virus, isolated during the 2009-2012 influenza seasons, was carried out to determine the genetic variability. Amino acid substitutions were observed at the rates of 0.3-0.7% per year. The catalytic site consisting of 8 functional and 11 framework residues were found conserved in 20 isolates and mutated in three (E228G, E278G, and N295T) isolates. To the best of our knowledge the three catalytic site mutants observed in our study have not been reported elsewhere to date. Similarly, mutations in the antigenic sites (K217E, K254E, V267A, and D451E except I263V) are discussed for the first time through this article. The effect of these mutations on drug and antibody binding were analyzed using biochemical and structural studies. Detailed studies on the neuraminidase gene are sparse and our study may serve as an appropriate platform to gain insights about the evolution of influenza virus, thereby facilitating drugs/vaccines design and development. J. Med. Virol. 89:202-212, 2017. © 2016 Wiley Periodicals, Inc.

Glycosylation Analysis of Engineered H3N2 Influenza A Virus Hemagglutinins with Sequentially Added Historically Relevant Glycosylation Sites

The influenza virus surface glycoprotein hemagglutinin (HA) is the major target of host neutralizing antibodies. The oligosaccharides of HA can contribute to HA's antigenic characteristics. After a leap to humans from a zoonotic host, influenza can gain N-glycosylation sequons over time as part of its fitness strategy. This glycosylation expansion has not been studied at the structural level. Here we examine HA N-glycosylation of H3N2 virus strains that we have engineered to closely mimic glycosylation sites gained between 1968 through 2002 starting with pandemic A/Hong Kong/1/68 (H3N2: HK68). HAs studied include HK68 and engineered forms with 1, 2, and 4 added sites. We have used: nano-LC-MS(E) for glycopeptide composition, sequence and site occupancy analysis, and MALDI-TOF MS permethylation profiling for characterization of released glycans. Our study reveals that 1) the majority of N-sequons are occupied at ≥90%, 2) the class and complexity of the glycans varies by region over the landscape of the proteins, 3) Asn 165 and Asn 246, which are associated with interactions between HA and SP-D lung collectin, are exclusively high mannose type. Based on this study and previous reports we provide structural insight as to how the immune system responses may differ depending on HA glycosylation.

Integrated Sentinel Surveillance Linking Genetic, Antigenic, and Epidemiologic Monitoring of Influenza Vaccine-Virus Relatedness and Effectiveness During the 2013-2014 Influenza Season

BACKGROUND

Canada's Sentinel Physician Surveillance Network links genetic, antigenic, and vaccine effectiveness (VE) measures in an integrated platform of influenza monitoring, described here for the 2013-2014 influenza season of resurgent A(H1N1)pdm09 and late-season type B activity.

METHODS

VE was estimated as [1 - odds ratio] × 100% and compared vaccination status between individuals who tested positive (cases) and those who tested negative (controls) for influenza virus. Vaccine-virus relatedness was assessed by genomic sequence analysis and hemagglutination inhibition assays.

RESULTS

Analyses included 1037 controls (of whom 33% were vaccinated) and 663 cases (of whom 14% were vaccinated). A total of 415 cases tested positive for A(H1N1)pdm09 virus, 15 tested positive for A(H3N2) virus, 191 tested positive for B/Yamagata-lineage virus, 6 tested positive for B/Victoria-lineage virus, and 36 tested positive for viruses of unknown subtype or lineage. A(H1N1)pdm09 viruses belonged to clade 6B, distinguished by a K163Q substitution, but remained antigenically similar to the A/California/07/2009-like vaccine strain, with an adjusted VE of 71% (95% confidence interval [CI], 58%-80%). Most B/Yamagata-lineage viruses (83%) clustered phylogenetically with the prior (ie, 2012-2013) season's B/Wisconsin/01/2010-like clade 3 vaccine strain, while only 17% clustered with the current (ie, 2013-2014) season's B/Massachusetts/02/2012-like clade 2 vaccine strain. The adjusted VE for B/Yamagata-lineage virus was 73% (95% CI, 57%-84%), with a lower VE obtained after partial calendar-time adjustment for clade-mismatched B/Wisconsin/01/2010-like virus (VE, 63%; 95% CI, 41%-77%), compared with that for clade-matched B/Massachusetts/02/2012-like virus (VE, 88%; 95% CI, 48%-97%). No A(H3N2) viruses clustered with the A/Texas/50/2012-like clade 3C.1 vaccine strain, and more than half were antigenically mismatched, but sparse data did not support VE estimation.

CONCLUSIONS

VE corresponded with antigenically conserved A(H1N1)pdm09 and lineage-matched B/Yamagata viruses with clade-level variation. Surveillance linking genotypic, phenotypic, and epidemiologic measures of vaccine-virus relatedness and effectiveness could better inform predictions of vaccine performance and reformulation.

Antigen-Antibody docking reveals the molecular basis for cross-reactivity of the 1918 and 2009 Influenza A/H1N1 pandemic viruses

To understand the reported cross-reactivity of the 2009 H1N1 and the 1918 H1N1 pandemic viruses we docked the crystal structure of 2D1, an antibody derived from a survivor of the 1918 pandemic, to the structures of hemaglutinin (HA) of the 2009 strain and seasonal H1 vaccine strains. Our studies revealed that 2D1 binds to the 2009 HA at antigenic site 'Sa', with stabilizing contacts, similar to that in an available co-crystal structure of 2D1-1918 HA. However, 2D1 failed to bind to the known antigenic sites in the HAs of seasonal strains. Our study thus reveals the molecular basis for pre-existing immunity in elderly people to the 2009 pandemic virus.

Structure function studies on different structural domains of nucleoprotein of H1N1 subtype

Recent 2009 flu pandemic is a global outbreak of a new strain of influenza A virus subtype H1N1. The H1N1 virus has crossed species barrier to human and apparently acquired the capability to transmit this disease from human to human. The NP is a multifunctional protein that not only encapsidates viral RNA (vRNA), but also forms homo-oligomer and thereby maintains RNP structure. It is also thought to be the key adaptor for virus and host cell interaction. Thus, it is one of the factor that play a key role in the pathogenesis of influenza A virus infection. Therefore, to understand the cause of pathogenicity of H1N1 virus, we have studied the structure-function relationship of different domains of NP. Our results showed that conservative mutation in NP of various strains were pathogenic in nature. However, non-conservative mutation slightly abrogated oligomerization and was therefore less pathogenic. Our results also suggest that beside tail and body domain, head domain may also participate in an oligomerization process.

Identification of mutations at the antigenic and glycosylation sites in hemagglutinin protein of H5N1 strain

Hemagglutinin (HA) is the principal antigen, present on the viral surface. It is the primary target for neutralizing antibodies. In this paper, we have carried out studies on human hemagglutinin protein from H5N1 strain with homologous hemagglutinin from non-human sources of H5N1 strains. In all strains, part of the antigenic site (128-141) predicted by computer program "Antigenic", corresponds to immunodominant site Sa of H1 subtype. In AAF02304 strain, A156-->S156 mutation lies at the antigenic subsite of site 2 that corresponds to site B in the H3 subtype. In some strains of non-human origins, there are mutations at the antigenic sites. Interestingly, in AAY56367 strain mutation L138-->H138 lies at the receptor binding site, which also overlaps the antigenic site. Therefore, this amino acid substitution may influence both the specificity of receptor recognition and antibody binding. Seven potential glycosylation sites in human HA and in some strains of non-human sources have been predicted by computer program, Scan Prosite. In some strains of HA from non-human sources because of mutation, an additional glycosylation site appeared at the antigenic site. Therefore in these strains the oligosaccharides will mask the surface of HA as well as antigenic site. Hence these strains will not be recognized by host immune system.

Epitope mapping and the detection of transmissible gastroenteritis viral proteins in cell culture using biotinylated monoclonal antibodies in a fixed-cell ELISA

A fixed-cell ELISA was developed using swine testicle (ST) cells infected with the virulent Miller strain of transmissible gastroenteritis virus (TGEV) and purified biotinylated monoclonal antibodies (b-MAbs). Five of the b-MAbs were specific for the peplomer (E2), five reacted to the nucleocapsid (N), and one reacted to the E 1 protein of the Miller strain of TGEV. Protein A-Sepharose purification of MAbs yielded protein concentrations ranging from 0.40 to 3 mg per ml of ascites. Separate pools of N-MAbs and E 2-MAbs, and the E 1-MAb were used to monitor synthesis of TGE viral antigen in ST cells from 0 to 16 h post-infection at various multiplicities of infection (MOI). Epitopes of N proteins appeared sooner and at a lower MOI than those for the E 1 and E 2 proteins. The fixed-cell ELISA was also used to examine relative binding affinities of TGEV MAbs. Concentrations of b-MAbs producing a half-maximal signal ranged from 0.11 to 3.8 microgram/ml for E 2-MAbs, from 0.05 to 0.82 microgram/ml for N-MAbs, and 6 micrograms/ml for the E 1-MAb. The assay was used to determine the 50% neutralization concentrations for four neutralizing E 2-MAbs (0.1 microgram/ml to 6.9 micrograms/ml) and one E 1-MAb (1.2 micrograms/ml). Competition assays between b-MAbs and unlabeled competitors indicated that at least two major antigenic sites exist on the E 2-protein and 2 to 3 antigenic sites are present on the N-protein of Miller TGEV.