Profile of antibodies to the nucleocapsid protein of the severe acute respiratory syndrome (SARS)-associated coronavirus in probable SARS patients

Mucosal immunization with lipopeptides derived from conserved regions of SARS-CoV-2 antigens induce robust cellular and cross-variant humoral immune responses in mice

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the causative agent of COVID-19, has infected >600 million people in the ongoing global pandemic. Several variants of the SARS-CoV-2 have emerged in the last >2 years, challenging the continued efficacy of current COVID vaccines. Therefore, there is a crucial need to investigate a highly cross-protective vaccine effective against variants of SARS-CoV-2. In this study, we examined seven lipopeptides derived from highly conserved, immunodominant epitopes from the S, N, and M proteins of SARS-CoV-2, that are predicted to contain epitopes for clinically protective B cells, helper T cells (TH) and cytotoxic T cells (CTL). Intranasal immunization of mice with most of the lipopeptides led to significantly higher splenocyte proliferation and cytokine production, mucosal and systemic antibody responses, and induction of effector B and T lymphocytes in both lungs and spleen, compared to immunizations with the corresponding peptides without lipid. Immunizations with Spike-derived lipopeptides led to cross-reactive IgG, IgM and IgA responses against Alpha, Beta, Delta, and Omicron Spike proteins as well as neutralizing antibodies. These studies support their potential for development as components of a cross-protective SARS-CoV-2 vaccine.

Construction and Immunogenicity of a Recombinant Pseudorabies Virus Expressing SARS-CoV-2-S and SARS-CoV-2-N

Coronavirus (CoV) is an important pathogen of humans and animals, which can infect humans or animals through the respiratory mucosal route. Syndrome coronavirus 2 (SARS-CoV-2) is quite similar to syndrome coronavirus (SARS-CoV) with the same receptor, angiotensin-converting enzyme 2 (ACE2). The S and N proteins are the most important protective antigens of the SARS-CoV-2. The S protein on the viral membrane mediates the virus attachment with the host cells, and the N protein is the most abundant expression during infection. In this study, the recombinant viruses expressing the S and N proteins of SARS-CoV-2 were successfully constructed by Red/ET recombinant technology using Pseudorabies virus (PRV) strain Bartha-K61 as a vector. Genetic stability and growth kinetics analysis showed that the recombinant viruses rPRV-SARS-CoV-2-S and rPRV-SARS-CoV-2-N had similar genetic stability and proliferation characteristics to the parental PRV. The immunoassay results showed that mice immunized with recombinant viruses could produce total IgG antibodies. Therefore, PRV is feasible and promising as a viral vector to express SARS-CoV-2-S and SARS-CoV-2-N genes. This study can provide a reference for future research on live vector vaccines for domestic animals, pets, and wild animals.

Harnessing immunological targets for COVID-19 immunotherapy

COVID-19 is an infectious and highly contagious disease caused by SARS-CoV-2. The immunotherapy strategy has a great potential to develop a permanent cure against COVID-19. Innate immune cells are in constant motion to scan molecular alteration to cells led by microbial infections throughout the body and helps in clearing invading viruses. Harnessing immunological targets for removing viral infection, generally based on the principle of enhancing the T-cell and protective immune responses. Currently-approved COVID-19 vaccines are mRNA encapsulated in liposomes that stimulate the host immune system to produce antibodies. Given the vital role of innate immunity, harnessing these immune responses opens up new hope for the generation of long-lasting and protective immunity against COVID-19.

Comparative Analysis of Antigen-Specific Anti-SARS-CoV-2 Antibody Isotypes in COVID-19 Patients

Serological tests for detection of anti-severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) Abs in blood are expected to identify individuals who have acquired immunity against SARS-CoV-2 and indication of seroprevalence of SARS-CoV-2 infection. Many serological tests have been developed to detect Abs against SARS-CoV-2. However, these tests have considerable variations in their specificity and sensitivity, and whether they can predict levels of neutralizing activity is yet to be determined. This study aimed to investigate the kinetics and neutralizing activity of various Ag-specific Ab isotypes against SARS-CoV-2 in serum of coronavirus disease 2019 (COVID-19) patients confirmed via PCR test. We developed IgG, IgM, and IgA measurement assays for each Ag, including receptor-binding domain (RBD) of spike (S) protein, S1 domain, full-length S protein, S trimer, and nucleocapsid (N) domain, based on ELISA. The assays of the S protein for all isotypes showed high specificity, whereas the assays for all isotypes against N protein showed lower specificity. The sensitivity of all Ag-specific Ab isotypes depended on the timing of the serum collection and all of them, except for IgM against N protein, reached more than 90% at 15-21 d postsymptom onset. The best correlation with virus-neutralizing activity was found for IgG against RBD, and levels of IgG against RBD in sera from four patients with severe COVID-19 increased concordantly with neutralizing activity. Our results provide valuable information regarding the selection of serological test for seroprevalence and vaccine evaluation studies.

Common low complexity regions for SARS-CoV-2 and human proteomes as potential multidirectional risk factor in vaccine development

BACKGROUND

The rapid spread of the COVID-19 demands immediate response from the scientific communities. Appropriate countermeasures mean thoughtful and educated choice of viral targets (epitopes). There are several articles that discuss such choices in the SARS-CoV-2 proteome, other focus on phylogenetic traits and history of the Coronaviridae genome/proteome. However none consider viral protein low complexity regions (LCRs). Recently we created the first methods that are able to compare such fragments.

RESULTS

We show that five low complexity regions (LCRs) in three proteins (nsp3, S and N) encoded by the SARS-CoV-2 genome are highly similar to regions from human proteome. As many as 21 predicted T-cell epitopes and 27 predicted B-cell epitopes overlap with the five SARS-CoV-2 LCRs similar to human proteins. Interestingly, replication proteins encoded in the central part of viral RNA are devoid of LCRs.

CONCLUSIONS

Similarity of SARS-CoV-2 LCRs to human proteins may have implications on the ability of the virus to counteract immune defenses. The vaccine targeted LCRs may potentially be ineffective or alternatively lead to autoimmune diseases development. These findings are crucial to the process of selection of new epitopes for drugs or vaccines which should omit such regions.

Serological assays and host antibody detection in coronavirus-related disease diagnosis

Coronaviruses (CoV) are a family of viral pathogens that infect both birds and mammals, including humans. Seven human coronaviruses (HCoV) have been recognized so far. HCoV-229E, -OC43, -NL63, and -HKU1 account for one-third of common colds with mild symptoms. The other three members are severe acute respiratory syndrome (SARS)-CoV, Middle East respiratory syndrome (MERS)-CoV, and SARS-CoV-2. These viruses are responsible for SARS, MERS, and CoV disease 2019 (COVID-19), respectively. A variety of diagnostic techniques, including chest X-rays, computer tomography (CT) scans, analysis of viral nucleic acids, proteins, or whole virions, and host antibody detection using serological assays have been developed for the detection of these viruses. In this review, we discuss conventional serological tests, such as enzyme-linked immunosorbent assay (ELISA), western blot (WB), immunofluorescence assay (IFA), lateral flow immunoassay (LFIA), and chemiluminescence immunoassay (CLIA), as well as biosensor-based assays that have been developed for diagnosing HCoV-associated diseases since 2003, with an in-depth focus on COVID-19.

Whole proteome screening and identification of potential epitopes of SARS-CoV-2 for vaccine design-an immunoinformatic, molecular docking and molecular dynamics simulation accelerated robust strategy

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the most cryptic pandemic outbreak of the 21st century, has gripped more than 1.8 million people to death and infected almost eighty six million. As it is a new variant of SARS, there is no approved drug or vaccine available against this virus. This study aims to predict some promising cytotoxic T lymphocyte epitopes in the SARS-CoV-2 proteome utilizing immunoinformatic approaches. Firstly, we identified 21 epitopes from 7 different proteins of SARS-CoV-2 inducing immune response and checked for allergenicity and conservancy. Based on these factors, we selected the top three epitopes, namely KAYNVTQAF, ATSRTLSYY, and LTALRLCAY showing functional interactions with the maximum number of MHC alleles and no allergenicity. Secondly, the 3D model of selected epitopes and HLA-A*29:02 were built and Molecular Docking simulation was performed. Most interestingly, the best two epitopes predicted by docking are part of two different structural proteins of SARS-CoV-2, namely Membrane Glycoprotein (ATSRTLSYY) and Nucleocapsid Phosphoprotein (KAYNVTQAF), which are generally target of choice for vaccine designing. Upon Molecular Docking, interactions between selected epitopes and HLA-A*29:02 were further validated by 50 ns Molecular Dynamics (MD) simulation. Analysis of RMSD, Rg, SASA, number of hydrogen bonds, RMSF, MM-PBSA, PCA, and DCCM from MD suggested that ATSRTLSYY is the most stable and promising epitope than KAYNVTQAF epitope. Moreover, we also identified B-cell epitopes for each of the antigenic proteins of SARS CoV-2. Findings of our work will be a good resource for wet lab experiments and will lessen the timeline for vaccine construction.Communicated by Ramaswamy H. Sarma.

Benefits and limitations of serological assays in COVID-19 infection

Accurate and rapid diagnostic tests are critical for achieving control of coronavirus disease 2019 (covid-19), a pandemic illness caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). Diagnostic tests for covid-19 fall into two main categories: molecular tests that detect viral RNA, and serological tests that detect anti-SARS-CoV-2 immunoglobulins. Reverse transcriptase polymerase chain reaction (RT-PCR), a molecular test, has become the gold standard for diagnosis of covid-19; however, this test has many limitations that include potential false negative results, changes in diagnostic accuracy over the disease course, and precarious availability of test materials. Serological tests have generated substantial interest as an alternative or complement to RT-PCR and other Nucleic acid tests in the diagnosis of acute infection, as some might be cheaper and easier to implement at the point of care. A clear advantage of these tests over RT-PCR is that they can identify individuals previously infected by SARS-CoV-2, even if they never underwent testing while acutely ill. Many serological tests for covid-19 have become available in a short period, including some marketed for use as rapid, point-of-care tests. The pace of development has, however, exceeded that of rigorous evaluation, and important uncertainty about test accuracy remains.

A review of COVID-19 vaccines and major considerations for diabetic patients

The necessity and impact of SARS-CoV2 on the world's health have led to developing and producing practical and useful vaccines for this deadly respiratory virus. Since April 2020, a vaccine for the virus has been developed. Given that comorbidities such as diabetes, hypertension, and cardiovascular disease are more prone to viruses and the risk of infection, vaccines should be designed to protect against high-risk respiratory illnesses. Including SARS, MERS, influenza, and the SARS-CoV-2 provide a safe immune response. Here, we review the information and studies that have been done to help develop strategies and perspectives for producing a safe and ideal vaccine to prevent COVID-19 in normal people, especially at high-risk groups such as diabetes patients.

Challenges and prospects of COVID-19 vaccine development based on the progress made in SARS and MERS vaccine development

The outbreak of coronavirus disease 2019 (COVID‐19) as a pandemic has shaken the global health system and economy by their roots. This epidemic is still spreading and showing no signs of decreasing trend. Vaccination could be the only effective and economical means to control this pandemic. A number of research institutions and pharmaceutical companies have plunged into the race of vaccine development against COVID‐19 which are in various stages of development. An intriguing fact of coronavirus infections is that in every decade of the 21st century there is a new major coronavirus epidemic, namely, severe acute respiratory syndrome (SARS) in 2002, Middle East respiratory syndrome (MERS) in 2012, and now COVID‐19; and such epidemics are expected in future too. Since most of the biological characteristics of severe acute respiratory syndrome coronavirus 2 (SARS‐CoV‐2) are still obscure, the scientists are relying on the information available on SARS‐CoV and to some extent on MERS‐CoV for designing and developing COVID‐19 vaccines. But there is a need of vigorous testing for immunogenicity, safety, efficacy, and level of protection conferred in the hosts. This review focuses on the challenges and prospects of vaccine development against COVID‐19. It highlights seriousness, bottlenecks in vaccine development, possible vaccine candidates, different vaccine strategies, safety evaluation issues, and vaccine production processes pertaining to COVID‐19 based on the knowledge acquired on SARS and MERS vaccine development in the past.

Immune Response to COVID-19: Can We Benefit from the SARS-CoV and MERS-CoV Pandemic Experience?

The global range and high fatality rate of the newest human coronavirus (HCoV) pandemic has made SARS-CoV-2 the focus of the scientific world. Next-generation sequencing of the viral genome and a phylogenetic analysis have shown the high homology of SARS-CoV-2 to other HCoVs that have led to local epidemics in the past. The experience acquired in SARS and MERS epidemics may prove useful in understanding the SARS-CoV-2 pathomechanism and lead to effective treatment and potential vaccine development. This study summarizes the immune response to SARS-CoV, MERS-CoV, and SARS-CoV-2 and focuses on T cell response, humoral immunity, and complement system activation in different stages of HCoVs infections. The study also presents the quantity and frequency of T cell responses, particularly CD4+ and CD8+; the profile of cytokine production and secretion; and its relation to T cell type, disease severity, and utility in prognostics of the course of SARS, MERS, and COVID-19 outbreaks. The role of interferons in the therapy of these infections is also discussed. Moreover, the kinetics of specific antibody production, the correlation between humoral and cellular immune response and the immunogenicity of the structural HCoVs proteins and their utility in the development of a vaccine against SARS, MERS, and COVID-19 has been updated.

Sequence-based prediction of SARS-CoV-2 vaccine targets using a mass spectrometry-based bioinformatics predictor identifies immunogenic T cell epitopes

BACKGROUND

The ongoing COVID-19 pandemic has created an urgency to identify novel vaccine targets for protective immunity against SARS-CoV-2. Early reports identify protective roles for both humoral and cell-mediated immunity for SARS-CoV-2.

METHODS

We leveraged our bioinformatics binding prediction tools for human leukocyte antigen (HLA)-I and HLA-II alleles that were developed using mass spectrometry-based profiling of individual HLA-I and HLA-II alleles to predict peptide binding to diverse allele sets. We applied these binding predictors to viral genomes from the Coronaviridae family and specifically focused on T cell epitopes from SARS-CoV-2 proteins. We assayed a subset of these epitopes in a T cell induction assay for their ability to elicit CD8+ T cell responses.

RESULTS

We first validated HLA-I and HLA-II predictions on Coronaviridae family epitopes deposited in the Virus Pathogen Database and Analysis Resource (ViPR) database. We then utilized our HLA-I and HLA-II predictors to identify 11,897 HLA-I and 8046 HLA-II candidate peptides which were highly ranked for binding across 13 open reading frames (ORFs) of SARS-CoV-2. These peptides are predicted to provide over 99% allele coverage for the US, European, and Asian populations. From our SARS-CoV-2-predicted peptide-HLA-I allele pairs, 374 pairs identically matched what was previously reported in the ViPR database, originating from other coronaviruses with identical sequences. Of these pairs, 333 (89%) had a positive HLA binding assay result, reinforcing the validity of our predictions. We then demonstrated that a subset of these highly predicted epitopes were immunogenic based on their recognition by specific CD8+ T cells in healthy human donor peripheral blood mononuclear cells (PBMCs). Finally, we characterized the expression of SARS-CoV-2 proteins in virally infected cells to prioritize those which could be potential targets for T cell immunity.

CONCLUSIONS

Using our bioinformatics platform, we identify multiple putative epitopes that are potential targets for CD4+ and CD8+ T cells, whose HLA binding properties cover nearly the entire population. We also confirm that our binding predictors can predict epitopes eliciting CD8+ T cell responses from multiple SARS-CoV-2 proteins. Protein expression and population HLA allele coverage, combined with the ability to identify T cell epitopes, should be considered in SARS-CoV-2 vaccine design strategies and immune monitoring.

Reverse vaccinology approach to design a novel multi-epitope vaccine candidate against COVID-19: an in silico study

At present, novel Coronavirus (2019-nCoV, the causative agent of COVID-19) has caused worldwide social and economic disruption. The disturbing statistics of this infection promoted us to develop an effective vaccine candidate against the COVID-19. In this study, bioinformatics approaches were employed to design and introduce a novel multi-epitope vaccine against 2019-nCoV that can potentially trigger both CD⁴⁺ and CD⁸⁺ T-cell immune responses and investigated its biological activities by computational tools. Three known antigenic proteins (Nucleocapsid, ORF3a, and Membrane protein, hereafter called NOM) from the virus were selected and analyzed for prediction of the potential immunogenic B and T-cell epitopes and then validated using bioinformatics tools. Based on in silico analysis, we have constructed a multi-epitope vaccine candidate (NOM) with five rich-epitopes domain including highly scored T and B-cell epitopes. After predicting and evaluating of the third structure of the protein candidate, the best 3 D predicted model was applied for docking studies with Toll-like receptor 4 (TLR4) and HLA-A*11:01. In the next step, molecular dynamics (MD) simulation was used to evaluate the stability of the designed fusion protein with TLR4 and HLA-A*11:01 receptors. MD studies demonstrated that the NOM-TLR4 and NOM-HLA-A*11:01 docked models were stable during simulation time. In silico evaluation showed that the designed chimeric protein could simultaneously elicit humoral and cell-mediated immune responses. Communicated by Ramaswamy H. Sarma.

Preliminary Identification of Potential Vaccine Targets for the COVID-19 Coronavirus (SARS-CoV-2) Based on SARS-CoV Immunological Studies

The beginning of 2020 has seen the emergence of COVID-19 outbreak caused by a novel coronavirus, Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2). There is an imminent need to better understand this new virus and to develop ways to control its spread. In this study, we sought to gain insights for vaccine design against SARS-CoV-2 by considering the high genetic similarity between SARS-CoV-2 and SARS-CoV, which caused the outbreak in 2003, and leveraging existing immunological studies of SARS-CoV. By screening the experimentally-determined SARS-CoV-derived B cell and T cell epitopes in the immunogenic structural proteins of SARS-CoV, we identified a set of B cell and T cell epitopes derived from the spike (S) and nucleocapsid (N) proteins that map identically to SARS-CoV-2 proteins. As no mutation has been observed in these identified epitopes among the 120 available SARS-CoV-2 sequences (as of 21 February 2020), immune targeting of these epitopes may potentially offer protection against this novel virus. For the T cell epitopes, we performed a population coverage analysis of the associated MHC alleles and proposed a set of epitopes that is estimated to provide broad coverage globally, as well as in China. Our findings provide a screened set of epitopes that can help guide experimental efforts towards the development of vaccines against SARS-CoV-2.

Serological assays for emerging coronaviruses: challenges and pitfalls

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Serological studies on SARS- and MERS-coronavirus (CoV) diagnostics were reviewed.

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Different types of serological assays and variable antigens were compared.

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Immunogenic epitopes of CoV spike proteins were less conserved than nucleocapsid proteins.

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Use of spike proteins was found to be superior over nucleocapsid proteins.

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Applicability of serological assays for analysis of animal sera was reviewed.

More than a decade after the emergence of severe acute respiratory syndrome coronavirus (SARS-CoV) in 2002/2003 the occurrence of a novel CoV termed Middle East respiratory syndrome (MERS) CoV challenges researchers and public health authorities. To control spread and finally contain novel viruses, rapid identification and subsequent isolation of infected individuals and their contacts is of utmost importance. Next to methods for nucleic acid detection, validated serological assays are particularly important as the timeframe for antibody detection is less restricted. During the SARS-CoV epidemic a wide variety of serological diagnostic assays were established using multiple methods as well as different viral antigens. Even though the majority of the developed assays showed high sensitivity and specificity, numerous studies reported on cross-reactive antibodies to antigens from wide-spread common cold associated CoVs. In order to improve preparedness and responsiveness during future outbreaks of novel CoVs, information and problems regarding serological diagnosis that occurred during the SARS-CoV should be acknowledged.

In this review we summarize the performance of different serological assays as well as the applicability of the two main applied antigens (spike and nucleocapsid protein) used during the SARS-CoV outbreak. We highlight challenges and potential pitfalls that occur when dealing with a novel emerging coronavirus like MERS-CoV. In addition we describe problems that might occur when animal sera are tested in serological assays for the identification of putative reservoirs. Finally, we give a recommendation for a serological testing scheme and outline necessary improvements that should be implemented for a better preparedness.

Genetic variation of the human α-2-Heremans-Schmid glycoprotein (AHSG) gene associated with the risk of SARS-CoV infection

Genetic background may play an important role in the process of SARS-CoV infection and SARS development. We found several proteins that could interact with the nucleocapsid protein of the SARS coronavirus (SARS-CoV). α-2-Heremans-Schmid Glycoprotein (AHSG), which is required for macrophage deactivation by endogenous cations, is associated with inflammatory regulation. Cytochrome P450 Family 3A (CYP4F3A) is an ω-oxidase that inactivates Leukotriene B4 (LTB4) in human neutrophils and the liver. We investigated the association between the polymorphisms of these two inflammation-associated genes and SARS development. The linkage disequilibrium (LD) maps of these two genes were built with Haploview using data on CHB+JPT (version 2) from the HapMap. A total of ten tag SNPs were selected and genotyped. In the Guangzhou cohort study, after adjusting for age and sex, two AHSG SNPs and one CYP4F3 SNP were found to be associated with SARS susceptibility: rs2248690 (adjusted odds ratio [AOR] 2.42; 95% confidence interval [CI] 1.30-4.51); rs4917 (AOR 1.84; 95% CI 1.02-3.34); and rs3794987 (AOR 2.01; 95% CI 1.10–3.68). To further validate the association, the ten tag SNPs were genotyped in the Beijing cohort. After adjusting for age and sex, only rs2248690 (AOR, 1.63; 95% CI, 1.30–2.04) was found to be associated with SARS susceptibility. The combined analysis of the two studies confirmed tag SNP rs2248690 in AHSG as a susceptibility variant (AOR 1.70; 95% CI 1.37–2.09). The statistical analysis of the rs2248690 genotype data among the patients and healthy controls in the HCW cohort, who were all similarly exposed to the SARS virus, also supported the findings. Further, the SNP rs2248690 affected the transcriptional activity of the AHSG promoter and thus regulated the AHSG serum level. Therefore, our study has demonstrated that the AA genotype of rs2268690, which leads to a higher AHSG serum concentration, was significantly associated with protection against SARS development.

SARS vaccines: where are we?

In this review, the current state of vaccine development against human severe acute respiratory syndrome (SARS) coronavirus, focusing on recently published data is assessed. We discuss which strategies have been assessed immunologically and which have been evaluated in SARS coronavirus challenge models. We discuss inactivated vaccines, virally and bacterially vectored vaccines, recombinant protein and DNA vaccines, as well as the use of attenuated vaccines. Data regarding the correlates of protection, animal models and the available evidence regarding potential vaccine enhancement of SARS disease are discussed. While there is much evidence that various vaccine strategies against SARS are safe and immunogenic, vaccinated animals still display significant disease upon challenge. Current data suggest that intranasal vaccination may be crucial and that new or combination strategies may be required for good protective efficacy against SARS in humans.

Lack of association between polymorphisms of MASP2 and susceptibility to SARS coronavirus infection

Background

The pathogenesis of severe acute respiratory disease syndrome (SARS) is not fully understood. One case-control study has reported an association between susceptibility to SARS and mannan-binding lectin (MBL) in China. As the downstream protein of MBL, variants of the MBL-associated serine protease-2 (MASP2) gene may be associated with SARS coronavirus (SARS-CoV) infection in the same population.

Methods

Thirty individuals with SARS were chosen for analysis of MASP2 polymorphisms by means of PCR direct sequencing. Tag single nucleotide polymorphisms (tagSNPs) were chosen using pairwise tagging algorithms. The frequencies of four tag SNPs (rs12711521, rs2261695, rs2273346 and rs7548659) were ascertained in 376 SARS patients and 523 control subjects, using the Beckman SNPstream Ultra High Throughput genotyping platform.

Results

There is no significant association between alleles or genotypes of the MASP2 tagSNP and susceptibility to SARS-CoV in both Beijing and Guangzhou populations. Diplotype (rs2273346 and rs12711521)were analyzed for association with susceptibility to SARS, no statistically significant evidence of association was observed. The Beijing and Guangzhou sample groups were homogeneous regarding demographic and genetic parameters, a joined analysis also showed no statistically significant evidence of association.

Conclusion

Our data do not suggest a role for MASP2 polymorphisms in SARS susceptibility in northern and southern China.

Sensitive and specific enzyme-linked immunosorbent assay using chemiluminescence for detection of severe acute respiratory syndrome viral infection

Here we report the development of a more-sensitive immunoassay for severe acute respiratory syndrome (SARS) based on an enzyme-linked immunosorbent assay using chemiluminescence (CLEIA) to detect the viral nucleocapsid (N) antigen in nasopharyngeal aspirate (NPA) from patients infected with SARS coronavirus (CoV). The CLEIA was established with an optical combination of monoclonal antibodies (MAbs) against SARS CoV N protein prepared from mice immunized with recombinant N protein without cultivating the virus. The capture and detecting MAbs of the CLEIA reacted to the carboxyl-terminal and amino-terminal peptides of the N protein, respectively. The CLEIA was capable of detecting recombinant N protein at 1.56 pg/ml and viral N protein in SARS CoV cell culture lysates at 0.087 of 50% tissue culture infective doses/ml. The CLEIA showed no cross-reactivities to recombinant N proteins of common human CoV (229E, OC43, and NL63) or lysates of cells infected with 229E and OC43. In addition, an evaluation with 18 SARS-positive NPA samples, all confirmed SARS positive by quantitative PCR and antibodies to SARS CoV, revealed that all (18/18) were found positive by the CLEIA; thus, the sensitivity of detection was 100%. When we tested 20 SARS-negative NPA samples, the CLEIA was shown to have high specificity (100%). The sensitivity of our novel SARS CLEIA was significantly higher than the previous EIA and comparable to the other methods using reverse transcription-PCR.

Persistent memory CD4+ and CD8+ T-cell responses in recovered severe acute respiratory syndrome (SARS) patients to SARS coronavirus M antigen

The membrane (M) protein of severe acute respiratory syndrome coronavirus (SARS-CoV) is a major glycoprotein with multiple biological functions. In this study, we found that memory T cells against M protein were persistent in recovered SARS patients by detecting gamma interferon (IFN-gamma) production using ELISA and ELISpot assays. Flow cytometric analysis showed that both CD4+ and CD8+ T cells were involved in cellular responses to SARS-CoV M antigen. Furthermore, memory CD8+ T cells displayed an effector memory cell phenotype expressing CD45RO- CCR7- CD62L-. In contrast, the majority of IFN-gamma+ CD4+ T cells were central memory cells with the expression of CD45RO+ CCR7+ CD62L-. The epitope screening from 30 synthetic overlapping peptides that cover the entire SARS-CoV M protein identified four human T-cell immunodominant peptides, p21-44, p65-91, p117-140 and p200-220. All four immunodominant peptides could elicit cellular immunity with a predominance of CD8+ T-cell response. This data may have important implication for developing SARS vaccines.

The molecular biology of SARS coronavirus

abstract: Severe acute respiratory syndrome (SARS) is the first emerging infectious disease of the 21st century that has been highly transmissible and fatal and was caused by a previously unknown coronavirus (SARS‐CoV). The SARS epidemic in 2003 resulted in more than 8400 SARS cases and approximately 800 deaths. Existing in non‐identified animal reservoirs, SARS‐CoV continues to represent a threat to humans although more than four years have passed since a large outbreak of SARS, and no new cases have been reported. However, we cannot exclude the possibility of reemergence of SARS. It is hence necessary to understand the biology of the SARS‐CoV to deal adequately with the next outbreak, whenever it happens. The SARS‐CoV is a novel coronavirus with a large (∼30 thousand nucleotides) positive‐sense, single‐stranded RNA containing 14 functional open reading frames (ORFs) of which 2 large ORFs constitute the replicase gene which encodes proteins required for viral RNA syntheses. The remaining 12 ORFs encode the 4 structural proteins: spike, membrane, nucleocapsid and envelope; and eight accessory proteins. The viral genome and its expression within the host cell undergoes extensive translational and enzymatic processing to form the 4 structural, 8 accessory and 16 nonstructural proteins. In an effort to understand the molecular mechanisms or capsid assembly and viral pathogenesis, laboratories around the world have adopted a variety of approaches to answering these trivial questions. It has been our effort to consolidate all information known to date about the molecular mechanisms of the SARS‐CoV into this chapter to update our readership on the current status of research.

Comparison of immunoglobulin G responses to the spike and nucleocapsid proteins of severe acute respiratory syndrome (SARS) coronavirus in patients with SARS

Both the nucleocapsid (N) and the spike (S) proteins of severe acute respiratory syndrome (SARS)-associated coronavirus (SARS-CoV) are able to induce strong humoral responses in humans following an infection. To compare the immunoglobulin G (IgG) responses to the S and N proteins of SARS-CoV in SARS patients during the manifestation/convalescent period with those during the postinfection period, serum samples were collected from hospitalized SARS patients within 6 weeks after the onset of illness (set 1; 57 sequential samples from 19 patients) or 2 to 3 months after their recovery (set 2; 33 postinfection samples from 33 subjects). Serum samples from 100 healthy blood donors (set 3), collected in 2002, were also included. The specific IgG response to whole virus, the fragment from positions 450 to 650 of the S protein (S450-650), and the full-length N protein of SARS-CoV were measured by enzyme-linked immunosorbent assays (ELISAs). Western blot assays were carried out to confirm the ELISA results. Fifty-one of the serum samples in set 1 (89%) bound to the N protein, a proportion similar to that which recognized whole virus (79%) and the S-protein fragment (77%). All 33 serum samples from set 2 were strongly positive for N-protein-specific IgG, while 27 (82%) were positive for anti-S450-650 IgG. Two of the serum samples from set 3 were strongly positive for anti-N-protein IgG but not anti-S450-650 IgG. Similar levels of IgG responses to the S and N proteins were observed in SARS patients during the manifestation and convalescent stages. In the postinfection period, however, a number of patients had much lower serum IgG levels against S450-650 than against the N protein.

Identification and characterization of dominant helper T-cell epitopes in the nucleocapsid protein of severe acute respiratory syndrome coronavirus

By using a series of overlapping synthetic peptides covering 98% of the amino acid sequence of the nucleocapsid protein (NP) of severe acute respiratory syndrome coronavirus (SARS-CoV), four helper T-cell (Th) epitopes (NP11, residues 11 to 25; NP51, residues 51 to 65; NP61, residues 61 to 75; and NP111, residues 111 to 125) in C57BL mice (H-2(b)), four (NP21, residues 21 to 35; NP91, residues 91 to 105; NP331, residues 331 to 345; and NP351, residues 351 to 365) in C3H mice (H-2(k)), and two (NP81, residues 81 to 95; and NP351, residues 351 to 365) in BALB/c mice (H-2(d)) have been identified. All of these peptides were able to stimulate the proliferation of NP-specific T-cell lines or freshly isolated lymph node cells from mice immunized with recombinant NP. Immunization of mice with synthetic peptides containing appropriate Th epitopes elicited strong cellular immunity in vivo, as evidenced by delayed-type hypersensitivity. Priming with the helper peptides (e.g., NP111 and NP351) significantly accelerated the immune response induced by recombinant NP, as determined by the production of NP-specific antibodies. When fused with a conserved neutralizing epitope (SP1143-1157) from the spike protein of SARS-CoV, NP111 and NP351 assisted in the production of high-titer neutralizing antibodies in vivo. These data provide useful insights regarding immunity against SARS-CoV and have the potential to help guide the design of peptide-based vaccines.

Recombinant protein-based assays for detection of antibodies to severe acute respiratory syndrome coronavirus spike and nucleocapsid proteins

Recombinant severe acute respiratory syndrome (SARS) nucleocapsid and spike protein-based immunoglobulin G immunoassays were developed and evaluated. Our assays demonstrated high sensitivity and specificity to the SARS coronavirus in sera collected from patients as late as 2 years postonset of symptoms. These assays will be useful not only for routine SARS coronavirus diagnostics but also for epidemiological and antibody kinetic studies.

Analysis of proteins that interact with nucleocapsid protein of SARS-CoV using 15-mer phage-displayed library

Analysis of proteins that interact with N protein of SARS-CoV using 15-mer phage-displayed library will help to explore the virus pathogenesis and to develop new drugs and vaccines against SARS. In this study, we cloned, expressed and purified N protein of SARS-CoV. This 46-kD N protein was verified by SDS-PAGE and Western-blot. Then, the peptides binding-specific to N protein were identified using 15-mer phage-displayed library. Surprisingly, all of the 89 clones from monoclonal ELISA were positive (S/N>2.1) and the result was further confirmed experimentally once again. Six N protein-binding peptides, designated separately as SNA1, SNA2, SNA4, SNA5, SNA9 and SNG11, were selected for sequencing. Sequence analysis suggested that SNA5 shared approximatively 100% sequence identity to SNA4, SNA2, SNA9 and SNA1. In addition, the binding specificity of the 15-mer peptides with the SARS-CoV N protein was further demonstrated by blocking ELISA using the synthetical 15-mer peptide according to the deduced amino acid sequence of SNA5. Also, the deduced amino sequence of SNA5 was compared with proteins in translated database using the tblastx program, and the results showed that the proteins with the highest homology were Ubiquinol-cytochrome c reductase iron-sulfur subunits (UCRI or UQCR), otherwise known as the Rieske iron-sulfur proteins (RISP). Notablely, in the [2Fe-2S] redox centre of UCRI, there were 6 residues [GGW(Y)F(Y)CP] compatible to the residues (position 2→7, GGWFCP7) of the NH2-terminal of the 15-mer peptide, which indicated higher binding specificity between the N protein of SARS-CoV and the redox centre of UCRI to some extent. Here, the possible molecular mechanisms of SARS-CoV N protein in the pathogenesis of SARS are discussed.

Identification of a B-cell antigenic epitope at the N-terminus of SARS-CoV M protein and characterization of monoclonal antibody against the protein

To identify the potential B-cell antigenic epitopes within the N-terminus of SARS-CoV (SARS-associated coronavirus, SARS-CoV) M protein and characterize monoclonal antibody (MAb) against the protein as well as its recognizing region, we expressed and purified a portion of SARS-CoV M protein (amino acid 1–43) in Escherichia coli (E. coli). By using Western blot and enzyme-linked immunosorbent assay (ELISA), we showed that the purified recombinant M protein could be recognized by four SARS-CoV-positive human sera even when those sera were 12,800-fold diluted. Furthermore, we characterized one representative IgG2 MAb, 3H9, which exhibited a strong immunoreaction to both recombinant M protein and native viral protein of SARS-CoV. We found a B-cell antigenic epitope located between amino acid 1–15 and defined the MAb recognizing region within amino acid 16–28 of M. These findings not only suggest that both recombinant M protein and its specific MAbs may be used as the diagnostic reagents for SARS, but also provide a potential target site for the design of an epitope-based vaccine against SARS.

Antibody responses against SARS coronavirus are correlated with disease outcome of infected individuals

Most of the SARS‐CoV‐infected patients spontaneously recovered without clinical intervention while a small percentage succumbed to the disease. Here, we characterized temporal changes in N protein‐specific and S glycoprotein‐specific neutralizing antibody (Nab) responses in infected patients who have either recovered from or succumbed to SARS‐CoV infection. Recovered patients were found to have higher and sustainable levels of both N protein‐specific and S glycoprotein‐specific Nab responses, suggesting that antibody responses likely play an important role in determining the ultimate disease outcome of SARS‐CoV‐infected patients. J. Med. Virol. 78:1–8, 2006. © 2005 Wiley‐Liss, inc.

Comparative evaluation of two severe acute respiratory syndrome (SARS) vaccine candidates in mice challenged with SARS coronavirus

Two different severe acute respiratory syndrome (SARS) vaccine strategies were evaluated for their ability to protect against live SARS coronavirus (CoV) challenge in a murine model of infection. A whole killed (inactivated by beta-propiolactone) SARS-CoV vaccine and a combination of two adenovirus-based vectors, one expressing the nucleocapsid (N) and the other expressing the spike (S) protein (collectively designated Ad S/N), were evaluated for the induction of serum neutralizing antibodies and cellular immune responses and their ability to protect against pulmonary SARS-CoV replication. The whole killed virus (WKV) vaccine given subcutaneously to 129S6/SvEv mice was more effective than the Ad S/N vaccine administered either intranasally or intramuscularly in inhibiting SARS-CoV replication in the murine respiratory tract. This protective ability of the WKV vaccine correlated with the induction of high serum neutralizing-antibody titres, but not with cellular immune responses as measured by gamma interferon secretion by mouse splenocytes. Titres of serum neutralizing antibodies induced by the Ad S/N vaccine administered intranasally or intramuscularly were significantly lower than those induced by the WKV vaccine. However, Ad S/N administered intranasally, but not intramuscularly, significantly limited SARS-CoV replication in the lungs. Among the vaccine groups, SARS-CoV-specific IgA was found only in the sera of mice immunized intranasally with Ad S/N, suggesting that mucosal immunity may play a role in protection for the intranasal Ad S/N delivery system. Finally, the sera of vaccinated mice contained antibodies to S, further suggesting a role for this protein in conferring protective immunity against SARS-CoV infection.

Amino terminus of the SARS coronavirus protein 3a elicits strong, potentially protective humoral responses in infected patients

The 3a protein of severe acute respiratory syndrome (SARS)-associated coronavirus is expressed and transported to the plasma membrane in tissue cells of infected patients. Its short N-terminal ectodomain was found to elicit strong humoral responses in half of the patients who had recovered from SARS. The ectodomain-specific antibodies from the convalescent-phase plasma readily recognized and induced destruction of 3a-expressing cells in the presence of the human complement system, demonstrating their potential ability to provide immune protection by recognizing and eliminating SARS coronavirus-infected cells that express the target protein. In addition, when coupled to a carrier protein, the ectodomain peptide elicited 3a-specific antibodies in mice and rabbit at high titres. These results showed that the N terminus of the 3a protein is highly immunogenic and elicits potentially protective humoral responses in infected patients. Therefore, the short extracellular domain may be a valuable immunogen in the development of a vaccine for infectious SARS.

Severe acute respiratory syndrome (SARS) coronavirus: application of monoclonal antibodies and development of an effective vaccine

SARS-CoV is a new type of human coronavirus identified as a causative agent of severe acute respiratory syndrome (SARS). On the occasion of the SARS outbreak, various monoclonal antibodies (mAbs) against SARS-CoV have been developed and applied for diagnosis, clinical management and basic research. In this review, we overview the biochemical and functional properties and applications of these SARS-CoV mAbs. We also focus on a variety of vaccines currently under development and discuss the immune response elicited by these vaccines in animal models, hopefully to better understand what we need to do next to fight against newly emerging pathogens in the future.

Double-antigen sandwich ELISA for detection of antibodies to SARS-associated coronavirus in human serum

The study presented here was conducted to evaluate the performance of a double-antigen sandwich ELISA to detect antibodies in human serum against the coronavirus associated with severe acute respiratory syndrome (SARS). A recombinant partial nucleocapsid protein of SARS-associated coronavirus was used as a serodiagnostic antigen in the ELISA. A total of 2892 clinical serum samples were tested with the ELISA kit, which positively identified 25 of 35 (71.4%) samples of patients with confirmed SARS infection, 286 of 407 (70%) samples of patients suspected of having SARS, 229 of 302 (75.8%) samples of convalescent SARS patients, and 0 of 544 samples obtained from healthcare workers; only 1 of 1604 clinical samples obtained from patients with other diseases demonstrated a weakly positive result. These results indicate that the double-antigen sandwich ELISA is an effective screening method for the serodiagnosis of SARS-associated coronavirus.

Cross-reactivity of antibody against SARS-coronavirus nucleocapsid protein with IL-11

Infection of SARS-associated coronavirus (SARS-CoV) induced a strong anti-nucleocapsid (anti-N) antibody response. However, the pathophysiological significance of the anti-N antibodies in SARS pathogenesis is largely unknown. To profile the anti-N antibodies, a phage-displayed scFv library was prepared from mice immunized with heat-inactivated SARS-CoV-infected Vero E6 cell lysate. Specific anti-N scFvs were isolated by panning against a recombinant nucleocapsid protein and reactivity was confirmed with phage-ELISA. Sequence analysis indicated that two of the isolated anti-N scFv clones were identical and displayed a high homology with an scFv specific for interleukin 11 (IL-11), an anti-inflammatory cytokine derived from bone marrow stroma cells. In a neutralization assay, IL-11-induced STAT 3 phosphorylation in rat intestinal epithelial IEC-18 cells was completely suppressed by the anti-N scFv clone L9N01.

A serological survey on neutralizing antibody titer of SARS convalescent sera

A seroepidemiologic study was conducted in North China in 2003 to determine the neutralizing antibody titer of severe acute respiratory syndrome (SARS) convalescent sera. A total of 99 SARS convalescent serum samples were collected from patients from the Inner Mongolia Autonomous Region, Hebei Province, and Beijing 35-180 days after the onset of symptoms. The anti-SARS antibodies were detected by enzyme-linked immunosorbent assay (ELISA), neutralization assay, and Western blot. Eighty-seven serum samples were confirmed to be positive for SARS antibodies. The neutralizing antibody titer of the 87 positive sera was analyzed quantitatively by neutralization assay. The geometric mean titer (GMT) of the 87 convalescent sera was 1:61. The Kolmogorov-Smirnov test showed that the neutralizing antibody titers conform to normal distribution, which suggests that the average anti-SARS antibody level in this study was representative of the convalescent antibody level of the SARS population. This result could be useful for the development and quality control of SARS vaccines.

Profiles of IgG antibodies to nucleocapsid and spike proteins of the SARS-associated coronavirus in SARS patients

To evaluate humoral immunity against the SARS-associated coronavirus (SARS-CoV), we studied the profiles of IgG antibodies to the nucleocapsid (N) and spike (S) proteins of SARS-CoV. Serum specimens from 10 SARS patients were analyzed by Western blotting and an enzyme-linked immunosorbent assay (ELISA) using purified recombinant N and truncated S (S1, S2, and S3) proteins as antigens. Western blotting results demonstrated that 100% of the SARS patients tested positive for N protein-specific antibodies, 50% for S1 protein-specific antibodies, 30% for S2 protein-specific antibodies, and 70% for S3 protein-specific antibodies. The ELISA results, which showed positive rates of IgG reactivity against recombinant proteins N, S1, S2, and S3, were, respectively, 28.57, 14.29, 14.29, and 14.29% at week 1, 77.78, 55.56, 44.44, and 66.67% at week 2, 100, 75, 75, and 87.5% at week 3, and 100, 77.78, 77.78, and 88.89% after 3 weeks. The average titers of IgG against recombinant proteins N, S1, S2, and S3 were, respectively, 691, 56, 38, and 84 after 3 weeks. These results suggest that the recombinant proteins N and S3 are potentially useful antigens for a serological diagnosis of SARS. In consideration of possible cross-reactivity among N proteins of SARS-CoV and other coronaviruses, immunoassays using recombinant N protein in combination with S3 as antigens might improve the specificity of SARS diagnoses.

Recombinant protein-based ELISA and immuno-cytochemical assay for the diagnosis of SARS

A new Coronavirus (SARS‐CoV) is the aetiological agent of the severe acute respiratory syndrome (SARS). Because of the critical role played by serological assays for SARS diagnosis, an in‐house ELISA based on SARS‐CoV recombinant antigens was developed. The SARS‐CoV nucleocapsid protein (N), three N fragments (N1, N2, and N3) and the intraviral domain of the membrane protein (M2) were cloned and expressed in Escherichia coli as histidine‐tagged proteins. Six reference sera from SARS patients were used to detect virus‐specific IgG in an ELISA using each recombinant protein as coating antigen. High‐titre positive reactions were detected in all SARS positive sera. The specificity of the assay appears to be high as no positive reaction was detected in the sera of 20 healthy subjects and 73 patients with non‐SARS, low‐tract respiratory infections. Specific hyper‐immune sera to SARS‐CoV and the recombinant proteins, N, N1, N2, N3, and M2 were also generated in mice and rabbits. The specificity of these sera was confirmed by an immunocytochemical assay on biochips of SARS‐CoV infected and uninfected cells. J. Med. Virol. 76:137–142, 2005. © 2005 Wiley‐Liss, Inc.

Biochemical and immunological studies of nucleocapsid proteins of severe acute respiratory syndrome and 229E human coronaviruses

Severe acute respiratory syndrome (SARS) is a serious health threat and its early diagnosis is important for infection control and potential treatment of the disease. Diagnostic tools require rapid and accurate methods, of which a capture ELISA method may be useful. Toward this goal, we have prepared and characterized soluble full‐length nucleocapsid proteins (N protein) from SARS and 229E human coronaviruses. N proteins form oligomers, mostly as dimers at low concentration. These two N proteins degrade rapidly upon storage and the major degraded N protein is the C‐terminal fragment of amino acid (aa) 169–422. Taken together with other data, we suggest that N protein is a two‐domain protein, with the N‐terminal aa 50–150 as the RNA‐binding domain and the C‐terminal aa 169–422 as the dimerization domain. Polyclonal antibodies against the SARS N protein have been produced and the strong binding sites of the anti‐nucleocapsid protein (NP) antibodies produced were mapped to aa 1–20, aa 150–170 and aa 390–410. These sites are generally consistent with those mapped by sera obtained from SARS patients. The SARS anti‐NP antibody was able to clearly detect SARS virus grown in Vero E6 cells and did not cross‐react with the NP from the human coronavirus 229E. We have predicted several antigenic sites (15–20 amino acids) of S, M and N proteins and produced antibodies against those peptides, some of which could be recognized by sera obtained from SARS patients. Antibodies against the NP peptides could detect the cognate N protein clearly. Further refinement of these antibodies, particularly large‐scale production of monoclonal antibodies, could lead to the development of useful diagnostic kits for diseases associated with SARS and other human coronaviruses.

Use of the COOH portion of the nucleocapsid protein in an antigen-capturing enzyme-linked immunosorbent assay for specific and sensitive detection of severe acute respiratory syndrome coronavirus

Antibody detection with a recombinant COOH portion of the severe acute respiratory syndrome (SARS) coronavirus nucleocapsid (N) protein, N13 (amino acids 221 to 422), was demonstrated to be more specific and sensitive than that with the full-length N protein, and an N13-based antigen-capturing enzyme-linked immunosorbent assay providing a convenient and specific test for serodiagnosis and epidemiological study of SARS was developed.

Identification of immunodominant epitopes on the membrane protein of the severe acute respiratory syndrome-associated coronavirus

Similar to other coronaviruses, the membrane (M) protein of severe acute respiratory syndrome-associated coronavirus (SARS-CoV) is a major transmembrane glycoprotein with multiple biological functions. To date, limited information is available about its antigenic properties. In this study, we identified two major immunodominant epitopes on the M protein located in the extreme N-terminal region (residues 1 to 31) and the interior C-terminal region (residues 132 to 161), respectively, by Pepscan analyses against convalescent-phase sera from SARS patients and antisera from virus-immunized mice and rabbits. Synthetic peptides M1-31 derived from the N-terminal epitope and M132-161 derived from the C-terminal epitope were highly reactive with all of the convalescent-phase sera from 40 SARS patients but not with 30 control serum samples from healthy blood donors, suggesting their potential application for serologic diagnosis of SARS. We showed that both peptides (M1-31 and M132-161) were able to induce high titers of antibody responses in the immunized rabbits, highlighting their antigenicity and immunogenicity. These findings provide important information for developing SARS diagnostics and vaccines.

Comprehensive antibody epitope mapping of the nucleocapsid protein of severe acute respiratory syndrome (SARS) coronavirus: insight into the humoral immunity of SARS

Background: The epidemic outbreak of severe acute respiratory syndrome (SARS) posed a worldwide threat to public health and economic stability. Although the pandemic has been contained, concerns over its recurrence remain. It is essential to identify specific diagnostic agents and antiviral vaccine candidates to fight this highly contagious disease.

Methods: We generated 14 monoclonal antibodies (mAbs) specific to the SARS coronavirus (SARS-CoV) nucleocapsid (N) protein and used these to thoroughly map the N protein antigenic determinants. We identified the immunodominant antigenic sites responsible for the antibodies in sera from SARS patients and antisera from small animals and differentiated the linear from the conformational antibody-combining sites comprising the natural epitopes by use of yeast surface display.

Results: We identified 5 conformational and 3 linear epitopes within the entire N protein; 3 conformational and 3 linear epitopes were immunodominant. The antibody responses to the N protein fragments in mammalian sera revealed that 3 regions of the N protein are strong antigenic domains. We expanded the specificity of the N protein epitope and identified 4 novel conformational epitopes (amino acids 1–69, 68–213, 212–341, and 337–422).

Conclusion: The antigenic structures identified for the SARS-CoV N protein, the epitope-specific mAbs, and the serum antibody profile in SARS patients have potential use in the clinical diagnosis and understanding of the protective immunity to SARS-CoV.

Use of viral lysate antigen combined with recombinant protein in Western immunoblot assay as confirmatory test for serodiagnosis of severe acute respiratory syndrome

A Western immunoblot assay for confirmatory serodiagnosis of severe acute respiratory syndrome (SARS) was developed utilizing viral lysate antigens combined with a recombinant nucleocapsid protein, GST-N (glutathione S-transferase-nucleocapsid) of the SARS coronavirus (SARS-CoV). The viral lysate antigens were separated by electrophoresis and transblotted onto nitrocellulose membranes. The resultant membrane was subsequently added with the GST-N recombinant protein at a specific location. The positions of bands corresponding to some of the structural proteins immobilized on the membrane were then located and verified with mouse or rabbit antisera specific to the respective proteins. The Western immunoblot assay was able to detect antibodies to SARS-CoV in all 40 serum specimens from SARS patients and differentiate the SARS-positive samples from those of the healthy donor or non-SARS patient controls (150 samples) when set criteria were followed. In addition, when the immunoblot was used to test samples considered falsely positive by an in-house-developed SARS-specific enzyme-linked immunosorbent assay, band patterns different from those with samples from SARS patients were obtained.

B-cell responses in patients who have recovered from severe acute respiratory syndrome target a dominant site in the S2 domain of the surface spike glycoprotein

Severe acute respiratory syndrome (SARS) is a recently emerged infectious disease caused by a novel strain of coronavirus. Examination of the immune responses of patients who have recovered from SARS should provide important information for design of a safe and effective vaccine. We determined the continuous viral epitopes targeted by antibodies in plasma samples from convalescent SARS patients through biopanning with a vast M13 phage display dodecapeptide library. These epitopes converged to very short peptide fragments, one on each of the structural proteins spike and nucleocapsid and the nonstructural proteins 3a, 9b, and nsp 3. Immunoassays found that most of the patients who had recovered from SARS developed complementary antibodies to the epitope-rich region on the spike S2 protein, indicating that this is an immunodominant site on the viral envelope comprising the spike, matrix, and small envelope glycoproteins. These S2-targeting antibodies were shown to effectively neutralize the coronavirus, indicating that they provided protective immunity to help the patients recover from the viral infection. These results suggest that the SARS coronavirus might have an antigenic profile distinct from those of other human or animal coronaviruses. Due to the tested safety and protective effects of the convalescent-phase serological antibodies, identification of their complementary antigens may enable the design of an epitope-based vaccine to prevent potential antibody-mediated immunopathology.

Molecular and biological characterization of human monoclonal antibodies binding to the spike and nucleocapsid proteins of severe acute respiratory syndrome coronavirus

Human monoclonal antibodies (MAbs) were selected from semisynthetic antibody phage display libraries by using whole irradiated severe acute respiratory syndrome (SARS) coronavirus (CoV) virions as target. We identified eight human MAbs binding to virus and infected cells, six of which could be mapped to two SARS-CoV structural proteins: the nucleocapsid (N) and spike (S) proteins. Two MAbs reacted with N protein. One of the N protein MAbs recognized a linear epitope conserved between all published human and animal SARS-CoV isolates, and the other bound to a nonlinear N epitope. These two N MAbs did not compete for binding to SARS-CoV. Four MAbs reacted with the S glycoprotein, and three of these MAbs neutralized SARS-CoV in vitro. All three neutralizing anti-S MAbs bound a recombinant S1 fragment comprising residues 318 to 510, a region previously identified as the SARS-CoV S receptor binding domain; the nonneutralizing MAb did not. Two strongly neutralizing anti-S1 MAbs blocked the binding of a recombinant S fragment (residues 1 to 565) to SARS-CoV-susceptible Vero cells completely, whereas a poorly neutralizing S1 MAb blocked binding only partially. The MAb ability to block S1-receptor binding and the level of neutralization of the two strongly neutralizing S1 MAbs correlated with the binding affinity to the S1 domain. Finally, epitope mapping, using recombinant S fragments (residues 318 to 510) containing naturally occurring mutations, revealed the importance of residue N479 for the binding of the most potent neutralizing MAb, CR3014. The complete set of SARS-CoV MAbs described here may be useful for diagnosis, chemoprophylaxis, and therapy of SARS-CoV infection and disease.

Evaluation of a recombinant nucleocapsid protein-based assay for anti-SARS-CoV IgG detection

A high throughput accurate assay for anti‐SARS‐CoV IgG detection is needed for large‐scale epidemiological studies. The evaluation of a commercial recombinant nucleocapsid protein‐based microtitre plate enzyme immunoassay, ELISARS™ is described. The results on 150 sera from SARS patients and 450 sera from non‐SARS controls showed that this assay had a high level of sensitivity (96.2% for late serum samples) and specificity (97.8%). The performance and setup of this assay fulfills the requirement as a screening test for large‐scale studies. A vast majority of SARS patients developed antibodies against the nucleocapsid protein. In some patients (10/45), a high level of anti‐nucleocapsid antibody appeared very early in the course of the illness. In contrast, a minority (4 of 105 patients) never developed these antibodies. The implication of differences in antibody response to the nucleocapsid protein deserves further investigation. J. Med. Virol. 75:181–184, 2005. © 2004 Wiley‐Liss, Inc.

Expression of SARS-coronavirus nucleocapsid protein in Escherichia coli and Lactococcus lactis for serodiagnosis and mucosal vaccination

The nucleocapsid (N) protein of the severe acute respiratory syndrome (SARS)-associated coronavirus (SARS-CoV) is an important antigen for the early diagnosis of SARS and the development of vaccines. It was expressed in Escherichia coli as a fusion with human glutathione S-transferase (hGST) and was confirmed by Western blotting analysis. This recombinant N protein (hGST-N) was purified and used to measure the SARS-CoV N-specific antibody in the sera of eight SARS patients by enzyme-linked immunosorbent assay. Specific antibody response to this purified recombinant N protein was 100% positive in the SARS patients’ sera, while none of the control sera from 30 healthy people gave a positive reaction in the same assay. The SARS-CoV N protein was also expressed in Lactococcus lactis in the cytoplasm or secreted into the medium. The N-producing strain MG1363/pSECN and the purified hGST-N protein were respectively administered to mice, either orally or intranasally. Results indicated that orally delivered MG1363/pSECN induced significant N-specific IgG in the sera. In conclusion, our work provides a novel strategy to produce the SARS-CoV N protein for serodiagnosis and for L. lactis-based mucosal vaccines.

Detection of antibodies against SARS-CoV in serum from SARS-infected donors with ELISA and Western blot

Recombinant fragments of S proteins from the Severe Acute Respiratory Syndrome (SARS) coronavirus (SARA-CoV) were generated and used in a Western blot (WB) assay that was compared to a commercial SARS ELISA method. In 85% of confirmed SARS cases (n = 20), the S2 recombinant fragment based WB was positive and this was comparable to the commercial ELISA using heat killed SARS-CoV. WB using the other four recombinant fragments in confirmed SARS cases generated lower rates of detection (S1--75%, S1-N--25%, S1-C--55%). Evaluation of sera from healthy controls (n = 60) resulted in two weakly positive ELISA results with the remainder being negative while the S2 protein WB demonstrated three positive results from the 20 controls with a history of SARS contact and no positive results in 40 noncontact controls. A discrepancy between the ELISA and S2 WB arose when evaluating per-2003 sera from individuals (n = 10) with SARS-like symptoms (ELISA--100% positive, S2 WB--30% positive). These data suggest that the S2 WB assay may be particularly useful in ELISA-negative SARS cases and in some ELISA-positive non-SARS cases.

Mapping of antigenic sites on the nucleocapsid protein of the severe acute respiratory syndrome coronavirus

Antigenic sites on the nucleocapsid (N) protein of severe acute respiratory syndrome (SARS) coronavirus (SARS-CoV) were mapped by Pepscan analysis with overlapping peptides that span the N protein sequence. Two major immunodominant epitopes located in the C-terminal region (amino acids [aa] 362 to 412) and middle region (aa 153 to 178) reacted with more than 75% of sera from SARS patients. Several minor immunodominant epitopes were reactive with about 50% of the SARS sera. Antisera from mice immunized with inactivated SARS-CoV recognized the two major immunodominant epitopes and one antigenic site located adjacent to the N-terminal region (aa 76 to 101), which did not react with the sera from SARS patients. Several monoclonal antibodies against SARS-CoV bound to the N- or C-terminal antigenic sites. These results suggest that the above antigenic sites on the N protein are important in eliciting humoral immune response against SARS-CoV in humans and animals and can be used as antigens for developing diagnostic tests.

A subcutaneously injected UV-inactivated SARS coronavirus vaccine elicits systemic humoral immunity in mice

The recent emergence of severe acute respiratory syndrome (SARS) was caused by a novel coronavirus, SARS-CoV. It spread rapidly to many countries and developing a SARS vaccine is now urgently required. In order to study the immunogenicity of UV-inactivated purified SARS-CoV virion as a vaccine candidate, we subcutaneously immunized mice with UV-inactivated SARS-CoV with or without an adjuvant. We chose aluminum hydroxide gel (alum) as an adjuvant, because of its long safety history for human use. We observed that the UV-inactivated SARS-CoV virion elicited a high level of humoral immunity, resulting in the generation of long-term antibody secreting and memory B cells. With the addition of alum to the vaccine formula, serum IgG production was augmented and reached a level similar to that found in hyper-immunized mice, though it was still insufficient to elicit serum IgA antibodies. Notably, the SARS-CoV virion itself was able to induce long-term antibody production even without an adjuvant. Anti-SARS-CoV antibodies elicited in mice recognized both the spike and nucleocapsid proteins of the virus and were able to neutralize the virus. Furthermore, the UV-inactivated virion induced regional lymph node T-cell proliferation and significant levels of cytokine production (IL-2, IL-4, IL-5, IFN-γ and TNF-α) upon restimulation with inactivated SARS-CoV virion in vitro. Thus, a whole killed virion could serve as a candidate antigen for a SARS vaccine to elicit both humoral and cellular immunity.