Comparison of vesicular stomatitis virus pseudotyped with the S proteins from a porcine and a human coronavirus

A human monoclonal antibody neutralizing SARS-CoV-2 Omicron variants containing the L452R mutation

The effectiveness of SARS-CoV-2 therapeutic antibodies targeting the spike (S) receptor-binding domain (RBD) has been hampered by the emergence of variants of concern (VOCs), which have acquired mutations to escape neutralizing antibodies (nAbs). These mutations are not evenly distributed on the RBD surface but cluster on several distinct surfaces, suggesting an influence of the targeted epitope on the capacity to neutralize a broad range of VOCs. Here, we identified a potent nAb from convalescent patients targeting the receptor-binding domain of a broad range of SARS-CoV-2 VOCs. Except for the Lambda and BA.2.86 variants, this nAb efficiently inhibited the entry of most tested VOCs, including Omicron subvariants BA.1, BA.2, XBB.1.5, and EG.5.1 and to a limited extent also BA.4/5, BA.4.6, and BQ.1.1. It bound recombinant S protein with picomolar affinity, reduced the viral load in the lung of infected hamsters, and prevented the severe lung pathology typical for SARS-CoV-2 infections. An X-ray structure of the nAb-RBD complex revealed an epitope that does not fall into any of the conventional classes and provided insights into its broad neutralization properties. Our findings highlight a conserved epitope within the SARS-CoV-2 RBD that should be preferably targeted by therapeutic antibodies and inform rational vaccine development.IMPORTANCETherapeutic antibodies are effective in preventing severe disease from SARS-CoV-2 infection and constitute an important option in pandemic preparedness, but mutations within the S protein of virus variants (e.g., a mutation of L452) confer resistance to many of such antibodies. Here, we identify a human antibody targeting the S protein receptor-binding domain (RBD) with an elevated escape barrier and characterize its interaction with the RBD functionally and structurally at the atomic level. A direct comparison with reported antibodies targeting the same epitope illustrates important differences in the interface, providing insights into the breadth of antibody binding. These findings highlight the relevance of an extended neutralization profiling in combination with biochemical and structural characterization of the antibody-RBD interaction for the selection of future therapeutic antibodies, which may accelerate the control of potential future pandemics.

Sarbecovirus RBD indels and specific residues dictating multi-species ACE2 adaptiveness

Our comprehensive understanding of the multi-species ACE2 adaptiveness of sarbecoviruses remains elusive, particularly for those with various receptor binding motif (RBM) insertions/deletions (indels). Here, we analyzed RBM sequences from 268 sarbecoviruses categorized into four RBM indel types. We examined the ability of 20 representative sarbecovirus Spike glycoproteins (S) and derivatives in utilizing ACE2 from various bats and several other mammalian species. We reveal that sarbecoviruses with long RBMs (type-I) can achieve broad ACE2 tropism, whereas viruses with single deletions in Region 1 (type-II) or Region 2 (type-III) exhibit narrower ACE2 tropism. Sarbecoviruses with double region deletions (type-IV) completely lost ACE2 usage, which is restricted by clade-specific residues within and outside RBM. Lastly, we propose the evolution of sarbecovirus RBM indels and illustrate how loop lengths, disulfide, and residue determinants shape multi-species ACE2 adaptiveness. This study provides profound insights into the mechanisms governing ACE2 usage and spillover risks of sarbecoviruses.

Development of a SARS-CoV-2 neutralization assay based on a pseudotyped virus using a HIV system

Regarding the extensive global attention to severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) that constitutes an international public health emergency, pseudovirus neutralization assays have been widely applied due to their advantages of being able to be conducted in biosafety level 2 laboratories and having a high safety factor. In this study, by adding a blue fluorescent protein (AmCyan) gene to the HIV system pSG3-△env backbone plasmid HpaI and truncating the C-terminal 21 amino acids of the SARS-CoV-2 spike protein (S), high-titer SARS-CoV-2-Sdel21-AmCyan fluorescent pseudovirus was successfully packaged. The fluorescent pseudovirus was used to establish a neutralization assay in a 96-well plate using 293T cells stably transfected with the AF cells. Then, parameters such as the ratio of backbone and membrane plasmid, sensitive cells, inoculation of cells and virus, as well as incubation and detection time were optimized. The pseudovirus neutralization assay demonstrated high accuracy, sensitivity, repeatability, and a strong correlation with the luminescent pseudovirus neutralization assay. Additionally, we scaled up the neutralizing antibody determination method by increasing the plate size from 96 wells to 384 wells. We have established a robust fluorescent pseudotyped virus neutralization assay for SARS-CoV-2 using the HIV system, providing a foundation for serum neutralization antibody detection, monoclonal antibody screening, and vaccine development.

Development of an automated, high-throughput SARS-CoV-2 neutralization assay based on a pseudotyped virus using a vesicular stomatitis virus (VSV) vector

ABSTRACTThe global outbreak of COVID-19 has caused a severe threat to human health; therefore, simple, high-throughput neutralization assays are desirable for developing vaccines and drugs against COVID-19. In this study, a high-titre SARS-CoV-2 pseudovirus was successfully packaged by truncating the C-terminus of the SARS-CoV-2 spike protein by 21 amino acids and infecting 293 T cells that had been stably transfected with the angiotensin-converting enzyme 2 (ACE2) receptor and furin (named AF cells), to establish a simple, high-throughput, and automated 384-well plate neutralization assay. The method was optimized for cell amount, virus inoculation, incubation time, and detection time. The automated assay showed good sensitivity, accuracy, reproducibility, Z' factor, and a good correlation with the live virus neutralization assay. The high-throughput approach would make it available for the SARS-CoV-2 neutralization test in large-scale clinical trials and seroepidemiological surveys which would aid the accelerated vaccine development and evaluation.

Active Ingredients of Reduning Injection Maintain High Potency against SARS-CoV-2 Variants

OBJECTIVE

To investigate the anti-coronavirus potential and the corresponding mechanisms of the two ingredients of Reduning Injection: quercetin and luteolin.

METHODS

A pseudovirus system was designed to test the efficacy of quercetin and luteolin to inhibit SARS-CoV-2 infection and the corresponding cellular toxicity. Luteolin was tested for its activities against the pseudoviruses of SARS-CoV-2 and its variants. Virtual screening was performed to predict the binding sites by Autodock Vina 1.1.230 and PyMol. To validate docking results, surface plasmon resonance (SPR) was used to measure the binding affinity of the compounds with various proteins of the coronaviruses. Quercetin and luteolin were further tested for their inhibitory effects on other coronaviruses by indirect immunofluorescence assay on rhabdomyosarcoma cells infected with HCoV-OC43.

RESULTS

The inhibition of SARS-CoV-2 pseudovirus by luteolin and quercetin were strongly dose-dependent, with concentration for 50% of maximal effect (EC₅₀) of 8.817 and 52.98 µmol/L, respectively. Their cytotoxicity to BHK21-hACE2 were 177.6 and 405.1 µmol/L, respectively. In addition, luetolin significantly blocked the entry of 4 pseudoviruses of SARS-CoV-2 variants, with EC₅₀ lower than 7 µmol/L. Virtual screening and SPR confirmed that luteolin binds to the S-proteins and quercetin binds to the active center of the 3CLpro, PLpro, and helicase proteins. Quercetin and luteolin showed over 99% inhibition against HCoV-OC43.

CONCLUSIONS

The mechanisms were revealed of quercetin and luteolin inhibiting the infection of SARS-CoV-2 and its variants. Reduning Injection is a promising drug for COVID-19.

Antibody neutralization to SARS-CoV-2 and variants after 1 year in Wuhan, China

Most COVID-19 convalescents can build effective anti-SARS-CoV-2 humoral immunity, but it remains unclear how long it can maintain and how efficiently it can prevent the reinfection of the emerging SARS-CoV-2 variants. Here, we tested the sera from 248 COVID-19 convalescents around 1 year post-infection in Wuhan, the earliest known epicenter. SARS-CoV-2 immunoglobulin G (IgG) was well maintained in most patients and potently neutralizes the infection of the original strain and the B.1.1.7 variant. However, varying degrees of immune escape was observed on the other tested variants in a patient-specific manner, with individuals showing remarkably broad neutralization potency. The immune escape can be largely attributed to several critical spike mutations. These results suggest that SARS-CoV-2 can elicit long-lasting immunity but this is escaped by the emerging variants.

Close relatives of MERS-CoV in bats use ACE2 as their functional receptors

Middle East respiratory syndrome coronavirus (MERS-CoV) and several bat coronaviruses use dipeptidyl peptidase-4 (DPP4) as an entry receptor^1-4. However, the receptor for NeoCoV-the closest known MERS-CoV relative found in bats-remains unclear⁵. Here, using a pseudotype virus entry assay, we found that NeoCoV and its close relative, PDF-2180, can efficiently bind to and use specific bat angiotensin-converting enzyme 2 (ACE2) orthologues and, less favourably, human ACE2 as entry receptors through their receptor-binding domains (RBDs) on the spike (S) proteins. Cryo-electron microscopy analysis revealed an RBD-ACE2 binding interface involving protein-glycan interactions, distinct from those of other known ACE2-using coronaviruses. We identified residues 337-342 of human ACE2 as a molecular determinant restricting NeoCoV entry, whereas a NeoCoV S pseudotyped virus containing a T510F RBD mutation efficiently entered cells expressing human ACE2. Although polyclonal SARS-CoV-2 antibodies or MERS-CoV RBD-specific nanobodies did not cross-neutralize NeoCoV or PDF-2180, an ACE2-specific antibody and two broadly neutralizing betacoronavirus antibodies efficiently inhibited these two pseudotyped viruses. We describe MERS-CoV-related viruses that use ACE2 as an entry receptor, underscoring a promiscuity of receptor use and a potential zoonotic threat.

Cell Entry of Animal Coronaviruses

Coronaviruses (CoVs) are a group of enveloped positive-sense RNA viruses and can cause deadly diseases in animals and humans. Cell entry is the first and essential step of successful virus infection and can be divided into two ongoing steps: cell binding and membrane fusion. Over the past two decades, stimulated by the global outbreak of SARS-CoV and pandemic of SARS-CoV-2, numerous efforts have been made in the CoV research. As a result, significant progress has been achieved in our understanding of the cell entry process. Here, we review the current knowledge of this essential process, including the viral and host components involved in cell binding and membrane fusion, molecular mechanisms of their interactions, and the sites of virus entry. We highlight the recent findings of host restriction factors that inhibit CoVs entry. This knowledge not only enhances our understanding of the cell entry process, pathogenesis, tissue tropism, host range, and interspecies-transmission of CoVs but also provides a theoretical basis to design effective preventive and therapeutic strategies to control CoVs infection.

ACE2 receptor usage reveals variation in susceptibility to SARS-CoV and SARS-CoV-2 infection among bat species

Bats are the suggested natural hosts for severe acute respiratory syndrome coronavirus (SARS-CoV) and the causal agent of the coronavirus disease 2019 (COVID-19) pandemic, SARS-CoV-2. The interaction of viral spike proteins with their host receptor angiotensin-converting enzyme 2 (ACE2) is a critical determinant of potential hosts and cross-species transmission. Here we use virus-host receptor binding and infection assays to examine 46 ACE2 orthologues from phylogenetically diverse bat species, including those in close and distant contact with humans. We found that 24, 21 and 16 of them failed to support infection by SARS-CoV, SARS-CoV-2 or both viruses, respectively. Furthermore, we confirmed that infection assays in human cells were consistent with those in two bat cell lines. Additionally, we used genetic and functional analyses to identify critical residues in bat ACE2 receptors associated with viral entry restrictions. Our results suggest that many bat species may not be the potential hosts of one or both viruses and that no correlation was identified between proximity to humans and probability of being natural hosts of SARS-CoV or SARS-CoV-2. This study demonstrates dramatic variation in susceptibility to SARS-CoV and SARS-CoV-2 infection among bat species and adds knowledge towards a better understanding of coronavirus-bat interaction.

Computational and Experimental Studies Reveal That Thymoquinone Blocks the Entry of Coronaviruses Into In Vitro Cells

INTRODUCTION

Since December 2019, severe acute respiratory syndrome-related coronavirus-2 (SARS-CoV-2) has caused the coronavirus disease 2019 (COVID-19) pandemic in China and worldwide. New drugs for the treatment of COVID-19 are in urgent need. Considering the long development time for new drugs, the identification of promising inhibitors from FDA-approved drugs is an imperative and valuable strategy. Recent studies have shown that the S1 and S2 subunits of the spike protein of SARS-CoV-2 utilize human angiotensin-converting enzyme 2 (hACE2) as the receptor to infect human cells.

METHODS

We combined molecular docking and surface plasmon resonance (SPR) to identify potential inhibitors for ACE2 from available commercial medicines. We also designed coronavirus pseudoparticles that contain the spike protein assembled onto green fluorescent protein or luciferase reporter gene-carrying vesicular stomatitis virus core particles.

RESULTS

We found that thymoquinone, a phytochemical compound obtained from the plant Nigella sativa, is a potential drug candidate. SPR analysis confirmed the binding of thymoquinone to ACE2. We found that thymoquinone can inhibit SARS-CoV-2, SARS-CoV, and NL63 pseudoparticles infecting HEK293-ACE2 cells, with half-maximal inhibitory concentrations of 4.999, 7.598, and 6.019 μM, respectively. The SARS-CoV-2 pseudoparticle inhibition had half-maximal cytotoxic concentration of 35.100 μM and selection index = 7.020.

CONCLUSION

Thymoquinone is a potential broad-spectrum inhibitor for the treatment of coronavirus infections.

SARS-CoV-2 Spike Alterations Enhance Pseudoparticle Titers and Replication-Competent VSV-SARS-CoV-2 Virus

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is the causative agent of the most recent global pandemic that has caused more than a million deaths around the world. The spike glycoprotein (S) drives the entry and fusion of this virus and is the main determinant of cell tropism. To explore S requirements for entry under BSL2 conditions, S has been pseudotyped onto vesicular stomatitis virus (VSV) or retroviral particles with varied success. Several alterations to S were demonstrated to improve pseudoparticle titers, but they have not been systematically compared. In this study, we produced pseudotyped VSV particles with multiple modifications to S, including truncation, mutation, and tagging strategies. The main objective of this study was to determine which modifications of the S protein optimize cell surface expression, incorporation into pseudotyped particles, and pseudoparticle entry. Removal of the last 19 residues of the cytoplasmic tail produced a hyper-fusogenic S, while removal of 21 residues increased S surface production and VSV incorporation. Additionally, we engineered a replication-competent VSV (rVSV) virus to produce the S-D614G variant with a truncated cytoplasmic tail. While the particles can be used to assess S entry requirements, the rVSV∆G/S_Met1D614G∆21 virus has a poor specific infectivity (particle to infectious titer ratio).

Robust neutralization assay based on SARS-CoV-2 S-protein-bearing vesicular stomatitis virus (VSV) pseudovirus and ACE2-overexpressing BHK21 cells

The global pandemic of coronavirus disease 2019 (COVID-19) is a disaster for human society. A convenient and reliable neutralization assay is very important for the development of vaccines and novel drugs. In this study, a G protein-deficient vesicular stomatitis virus (VSVdG) bearing a truncated spike protein (S with C-terminal 18 amino acid truncation) was compared to that bearing the full-length spike protein of SARS-CoV-2 and showed much higher efficiency. A neutralization assay was established based on VSV-SARS-CoV-2-Sdel18 pseudovirus and hACE2-overexpressing BHK21 cells (BHK21-hACE2 cells). The experimental results can be obtained by automatically counting the number of EGFP-positive cells at 12 h after infection, making the assay convenient and high-throughput. The serum neutralizing titer measured by the VSV-SARS-CoV-2-Sdel18 pseudovirus assay has a good correlation with that measured by the wild type SARS-CoV-2 assay. Seven neutralizing monoclonal antibodies targeting the receptor binding domain (RBD) of the SARS-CoV-2 S protein were obtained. This efficient and reliable pseudovirus assay model could facilitate the development of new drugs and vaccines.

A tool with many applications: vesicular stomatitis virus in research and medicine

INTRODUCTION

Vesicular stomatitis virus (VSV) has long been a useful research tool in virology and recently become an essential part of medicinal products. Vesiculovirus research is growing quickly following its adaptation to clinical gene and cell therapy and oncolytic virotherapy.

AREAS COVERED

This article reviews the versatility of VSV as a research tool and biological reagent, its use as a viral and vaccine vector delivering therapeutic and immunogenic transgenes and an oncolytic virus aiding cancer treatment. Challenges such as the immune response against such advanced therapeutic medicinal products and manufacturing constraints are also discussed.

EXPERT OPINION

The field of in vivo gene and cell therapy is advancing rapidly with VSV used in many ways. Comparison of VSV's use as a versatile therapeutic reagent unveils further prospects and problems for each application. Overcoming immunological challenges to aid repeated administration of viral vectors and minimizing harmful host-vector interactions remains one of the major challenges. In the future, exploitation of reverse genetic tools may assist the creation of recombinant viral variants that have improved onco-selectivity and more efficient vaccine vector activity. This will add to the preferential features of VSV as an excellent advanced therapy medicinal product (ATMP) platform.

A Multibasic Cleavage Site in the Spike Protein of SARS-CoV-2 Is Essential for Infection of Human Lung Cells

The pandemic coronavirus SARS-CoV-2 threatens public health worldwide. The viral spike protein mediates SARS-CoV-2 entry into host cells and harbors a S1/S2 cleavage site containing multiple arginine residues (multibasic) not found in closely related animal coronaviruses. However, the role of this multibasic cleavage site in SARS-CoV-2 infection is unknown. Here, we report that the cellular protease furin cleaves the spike protein at the S1/S2 site and that cleavage is essential for S-protein-mediated cell-cell fusion and entry into human lung cells. Moreover, optimizing the S1/S2 site increased cell-cell, but not virus-cell, fusion, suggesting that the corresponding viral variants might exhibit increased cell-cell spread and potentially altered virulence. Our results suggest that acquisition of a S1/S2 multibasic cleavage site was essential for SARS-CoV-2 infection of humans and identify furin as a potential target for therapeutic intervention.

Protocol and Reagents for Pseudotyping Lentiviral Particles with SARS-CoV-2 Spike Protein for Neutralization Assays

SARS-CoV-2 enters cells using its Spike protein, which is also the main target of neutralizing antibodies. Therefore, assays to measure how antibodies and sera affect Spike-mediated viral infection are important for studying immunity. Because SARS-CoV-2 is a biosafety-level-3 virus, one way to simplify such assays is to pseudotype biosafety-level-2 viral particles with Spike. Such pseudotyping has now been described for single-cycle lentiviral, retroviral, and vesicular stomatitis virus (VSV) particles, but the reagents and protocols are not widely available. Here, we detailed how to effectively pseudotype lentiviral particles with SARS-CoV-2 Spike and infect 293T cells engineered to express the SARS-CoV-2 receptor, ACE2. We also made all the key experimental reagents available in the BEI Resources repository of ATCC and the NIH. Furthermore, we demonstrated how these pseudotyped lentiviral particles could be used to measure the neutralizing activity of human sera or plasma against SARS-CoV-2 in convenient luciferase-based assays, thereby providing a valuable complement to ELISA-based methods that measure antibody binding rather than neutralization.

Recombinant vesicular stomatitis virus expressing the spike protein of genotype 2b porcine epidemic diarrhea virus: A platform for vaccine development against emerging epidemic isolates

Emerging porcine epidemic diarrhea viruses (PEDVs) have caused large economic losses since 2010, and G2b is the prevalent globally epidemic genotype. Given the fastidious isolation of emerging PEDV in cell culture and difficulties in retaining the isolate infectivity upon further in vitro passage, highly attenuated recombinant vesicular stomatitis virus (rVSV_MT) was used as a vector to express the PEDV spike (S) protein, aiming to develop a subunit vaccine against G2b viruses. An S protein with 19 of its cytoplasmic domain amino acids deleted could be incorporated into VSV particles, generating rVSV_MT (VSV_MT-S_Δ19) with high efficiency. Our results suggest that VSV_MT-S_Δ19 could effectively induce PEDV-specific immunity in pigs via intramuscular, but not intranasal, immunization. Notably, immunizations of sows with VSV _MT-S_Δ19 provided protective lactogenic immunity against a virulent G2b PEDV challenge in piglets. Consequently, recombinant VSV_MT may be a promising platform for preparing a subunit vaccine against PEDV.

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PEDV spike protein can be incorporated into VSV particles.

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VSV-based PEDV vaccine can induce robust PEDV-specific immunities in pigs.

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VSV_MT could be a promising platform for developing vaccines against emerging PEDV.

The detection and phylogenetic analysis of porcine deltacoronavirus from Guangdong Province in Southern China

Porcine deltacoronavirus (PDCoV) is a newly discovered coronavirus that causes diarrhoea, vomiting and dehydration in sucking and nursing piglets. It was first reported in Hong Kong in 2012 and has since been discovered in the United States, Canada, South Korea, mainland China, Thailand and Laos. PDCoV has been experimentally proved to lead to diarrhoea in swine and it was detected positive in pigs in Guangdong, southern China. In our study, 252 faecal and intestinal samples from sucking piglets and sows with diarrhoea were surveyed for common enteric viruses. We found a prevalence of PDCoV (21.8%), porcine epidemic diarrhoea virus (65.5%), transmissible gastroenteritis virus (0%), rotavirus group A (25.0%) and porcine kobuvirus (68.7%). We isolated 13 PDCoV strains and discovered that PDCoV infections were often co‐infections with kobuvirus rather than the commonly linked porcine epidemic diarrhoea virus. Phylogenetic analysis of S gene and N gene revealed that 11 of 13 PDCoV strains belonged to Chinese lineage. As for the left two strains, one single strain (CHN‐GD16‐05) belonged to American and Korean lineages while another strain (CHN‐GD16‐03) was similar to a Thai strain, but only in the S gene. This suggested a possible recombination event between the Thai and the newly described Chinese strain.

Comparison of lentiviruses pseudotyped with S proteins from coronaviruses and cell tropisms of porcine coronaviruses

The surface glycoproteins of coronaviruses play an important role in receptor binding and cell entry. Different coronaviruses interact with their specific receptors to enter host cells. Lentiviruses pseudotyped with their spike proteins (S) were compared to analyze the entry efficiency of various coronaviruses. Our results indicated that S proteins from different coronaviruses displayed varied abilities to mediate pseudotyped virus infection. Furthermore, the cell tropisms of porcine epidemic diarrhea virus (PEDV) and transmissible gastroenteritis virus (TGEV) have been characterized by live and pseudotyped viruses. Both live and pseudoviruses could infected Vero- CCL-81 (monkey kidney), Huh-7 (human liver), and PK-15 (pig kidney) cells efficiently. CCL94 (cat kidney) cells could be infected efficiently by TGEV but not PEDV. Overall, our study provides new insights into the mechanisms of viral entry and forms a basis for antiviral drug screening.

Differential sensitivity of bat cells to infection by enveloped RNA viruses: coronaviruses, paramyxoviruses, filoviruses, and influenza viruses

Bats (Chiroptera) host major human pathogenic viruses including corona-, paramyxo, rhabdo- and filoviruses. We analyzed six different cell lines from either Yinpterochiroptera (including African flying foxes and a rhinolophid bat) or Yangochiroptera (genera Carollia and Tadarida) for susceptibility to infection by different enveloped RNA viruses. None of the cells were sensitive to infection by transmissible gastroenteritis virus (TGEV), a porcine coronavirus, or to infection mediated by the Spike (S) protein of SARS-coronavirus (SARS-CoV) incorporated into pseudotypes based on vesicular stomatitis virus (VSV). The resistance to infection was overcome if cells were transfected to express the respective cellular receptor, porcine aminopeptidase N for TGEV or angiotensin-converting enzyme 2 for SARS-CoV. VSV pseudotypes containing the S proteins of two bat SARS-related CoV (Bg08 and Rp3) were unable to infect any of the six tested bat cell lines. By contrast, viral pseudotypes containing the surface protein GP of Marburg virus from the family Filoviridae infected all six cell lines though at different efficiency. Notably, all cells were sensitive to infection by two paramyxoviruses (Sendai virus and bovine respiratory syncytial virus) and three influenza viruses from different subtypes. These results indicate that bat cells are more resistant to infection by coronaviruses than to infection by paramyxoviruses, filoviruses and influenza viruses. Furthermore, these results show a receptor-dependent restriction of the infection of bat cells by CoV. The implications for the isolation of coronaviruses from bats are discussed.

Characterization of the spike protein of human coronavirus NL63 in receptor binding and pseudotype virus entry

The spike (S) protein of human coronavirus NL63 (HCoV-NL63) mediates both cell attachment by binding to its receptor hACE2 and membrane fusion during virus entry. We have previously identified the receptor-binding domain (RBD) and residues important for RBD–hACE2 association. Here, we further characterized the S protein by investigating the roles of the cytoplasmic tail and 19 residues located in the RBD in protein accumulation, receptor binding, and pseudotype virus entry. For these purposes, we first identified an entry-efficient S gene template from a pool of gene variants and used it as a backbone to generate a series of cytoplasmic tail deletion and single residue substitution mutants. Our results showed that: (i) deletion of 18 aa from the C-terminus enhanced the S protein accumulation and virus entry, which might be due to the deletion of intracellular retention signals; (ii) further deletion to residue 29 also enhanced the amount of S protein on the cell surface and in virion, but reduced virus entry by 25%, suggesting that residues 19–29 contributes to membrane fusion; (iii) a 29 aa-deletion mutant had a defect in anchoring on the plasma membrane, which led to a dramatic decrease of S protein in virion and virus entry; (iv) a total of 15 residues (Y498, V499, V531, G534, G537, D538, S540, G575, S576, E582, W585, Y590, T591, V593 and G594) within RBD were important for receptor binding and virus entry. They probably form three receptor binding motifs, and the third motif is conserved between NL63 and SARS-CoV.

Bacterial expression of antigenic sites A and D in the spike protein of transmissible gastroenteritis virus and evaluation of their inhibitory effects on viral infection

The spike (S) protein is a key structural protein of coronaviruses including, the porcine transmissible gastroenteritis virus (TGEV). The S protein is a type I membrane glycoprotein located in the viral envelope and is responsible for mediating the binding of viral particles to specific cell receptors and therefore specific cell types. It is also an important immune target for the host in neutralizing the virus. Four antigenic sites A, B, C, and D that reside near the N-terminal domain have been defined in the S protein. Of these, the region encoding antigenic sites A and to a lesser extent D, herein defined as S-AD, are most critical in eliciting host neutralizing antibodies. Herein, we enzymatically amplified, cloned, and expressed the S-AD fragment from TGEV in the prokaryotic expression vector, pET-30a. Maximum protein expression was achieved at 30°C over a 5-h period post-induction. Rabbit polyclonal antiserum was generated using recombinant S-AD (rS-AD) protein. In contrast to prior studies showing no activity with bacterially produced S protein, results indicated that polyclonal serum recognized TGEV-infected cells and reduced infection by 100%. Furthermore, the truncated rS-AD peptide was able to bind to the surface of cells from swine testes in a competitive manner and completely inhibit viral infection.

Development of reverse transcription loop-mediated isothermal amplification for rapid detection of porcine epidemic diarrhea virus

In this study, a reverse transcription loop-mediated isothermal amplification (RT-LAMP) was developed for detection of porcine epidemic diarrhea virus (PEDV). Six primers were designed to amplify the nucleocapsid (N) gene of PEDV. The optimization, sensitivity, and specificity of the RT-LAMP were investigated. The results showed that the optimal reaction condition for RT-LAMP amplifying PEDV N gene was achieved at 63°C for 50 min. The RT-LAMP assay was more sensitive than gel-based RT-PCR and enzyme-linked immunosorbent assay. It was capable of detecting PEDV from clinical samples and differentiating PEDV from Porcine transmissible gastroenteritis virus, Porcine rotavirus, Porcine pseudorabies virus, Porcine reproductive and respiratory syndrome virus, and Avian infectious bronchitis virus.

Characterization and utility of monoclonal antibodies against spike protein of transmissible gastroenteritis virus

Aims:  This work aims to characterize the utility of four newly generated monoclonal antibodies (mAbs) against transmissible gastroenteritis virus (TGEV).

Methods and Results:  Four monoclonal antibodies (mAbs) against the N‐terminal half of spike protein (S1 protein) of TGEV were identified. Affinity constant of these mAbs was analysed. These mAbs were capable of reacting with the TGEV S1 protein analysed by ELISA and Western blot. A competition assay between the different mAbs was performed to determine whether the different antibodies mapped in the same or a different antigenic region of the protein. Investigation on the neutralizing ability of these mAbs indicated that two of these mAbs completely neutralized TGEV at an appropriate concentration. These mAbs were able to detect the TGEV‐infected cells in immunofluorescence assays and Western blot. Moreover, they differentiated TGEV S protein from other control proteins.

Conclusions:  The generated four mAbs are very specific, and the established immunofluorescence assays, Western blot and discrimination ELISA are useful approaches for detecting of TGEV.

Significance and Impact of the Study:  It is a novel report regarding the use of the S1 protein of TGEV to generate specific mAbs. Their utility and the established immunoassays contribute to the surveillance of TGE coronavirus.

Phage displayed peptides recognizing porcine aminopeptidase N inhibit transmissible gastroenteritis coronavirus infection in vitro

Porcine aminopeptidase N (pAPN) is a cellular receptor of transmissible gastroenteritis virus (TGEV), a porcine coronavirus. Interaction between the spike (S) protein of TGEV and pAPN initiates cell infection. Small molecules, especially peptides are an expanding area for therapy or diagnostic assays for viral diseases. Here, the peptides capable of binding the pAPN were, for the first time, identified by biopanning using a random 12-mer peptide library to the immobilized protein. Three chemically synthesized peptides recognizing the pAPN showed effective inhibition ability to TGEV infection in vitro. A putative TxxF motif was identified in the S protein of TGEV. Phages bearing the specific peptides interacted with the pAPN in ELISA. 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT) assays confirmed the protective effect of the peptides on cell infection by TGEV. Moreover, the excellent immune responses in mice induced by the identified phages provided the possibility to develop novel phage-based vaccines.

Production and characterization of a monoclonal antibody against spike protein of transmissible gastroenteritis virus

Transmissible gastroenteritis virus (TGEV) is a member of coronaviruses. The viral spike (S) protein mediates the interaction between TGEV and its susceptible cells. Here, we expressed a truncated gene encoding the N terminal half of TGEV S gene (designated S1 gene) in a prokaryotic system. The resulting S1 protein was used to immunize BALB/c mice followed by the generation of a monoclonal antibody (MAb). A generated MAb (7F9) was identified by ELISA and the chromosome number of the hybridoma cell was analyzed. The immunoreactivity of the MAb to TGEV S protein was confirmed by Western blot analysis. Moreover, immunofluorescence assays showed that the MAb is able to detect cell infection by TGEV. The MAb achieved in this study can be used as a specific diagnostic reagent for detecting TGEV S protein.

Reverse transcription loop-mediated isothermal amplification for rapid detection of transmissible gastroenteritis virus

Transmissible gastroenteritis virus (TGEV) is the causative agent of porcine transmissible gastroenteritis, and sensitive detection methods are required for preventing the disease. In this article, reverse transcription-loop-mediated isothermal amplification (RT-LAMP) was developed to detect TGEV. Three pairs of primers targeting the nucleocapsid (N) gene of TGEV were synthesized and used in the RT-LAMP. The optimization, sensitivity, and specificity of the RT-LAMP were evaluated. Our results showed that the RT-LAMP amplified the N gene with high specificity, efficiency, and rapidity at isothermal condition. The optimal reaction condition was achieved at 60°C for 30 min. The RT-LAMP assay was more sensitive than gel-based RT-PCR and PCR. It had a higher sensitivity than enzyme-linked immunosorbent assay (ELISA) using the equal virus templates. In addition, the established RT-LAMP differentiated TGEV from porcine epidemic diarrhea virus, porcine rotavirus, porcine pseudorabies virus, porcine reproductive and respiratory syndrome virus, and avian infectious bronchitis virus. The approach is suitable for detecting TGEV for field diagnostics or in less-equipped laboratories due to its convenience and simplicity.

Preparation and characterization of polyclonal antibody against severe acute respiratory syndrome-associated coronavirus spike protein

A truncated gene (designated S1) encoding the receptor-binding domain (RBD) in the spike (S) protein of severe acute respiratory syndrome-associated coronavirus (SARS-CoV) was amplified by PCR. The gene was cloned into prokaryotic expression vector pGEX-6P-1, resulting in a recombinant plasmid pGEX-SARS-S1. Subsequently, pGEX-SARS-S1 was transformed into host cells BL21(DE3)pLysS, and the expression of the S1 protein was induced by isopropyl β-D-thiogalactoside (IPTG). Polyclonal antibody against SARS-CoV S1 protein was generated in a rabbit immunized with the purified S1 protein. The reactivity of the antibody to the SARS-CoV S1 protein was confirmed by Western blot analysis. ELISA indicated that the antibody against SARS-CoV S1 protein had no cross reaction with S1 proteins of transmissible gastroenteritis virus, a porcine coronavirus, and infectious bronchitis virus, an avian coronavirus. The SARS-CoV S1 protein and its antibody are valuable reagents for related studies.

Heterologous expression of fused genes encoding the glycoprotein 5 from PRRSV: a way for producing functional protein in prokaryotic microorganism

Based on the bioinformatics analysis of the gene encoding glycoprotein 5 (GP5) of porcine reproductive and respiratory syndrome virus (PRRSV) isolate HH08, two gene fragments were amplified by polymerase chain reaction (PCR), deleting the signal peptide and transmembrane sequences in GP5 gene. Both gene fragments were designated GP5a and GP5b, respectively. They were ligated with a linker and cloned into prokaryotic expression vector, pET-30a. Expression of the protein of interest was induced by isopropyl beta-d-1-thiogalactopyranoside. The purified protein was used as an immunogen to elicit antibody in rabbit. The immunoreactivity of the protein was determined using ELISA and Western blot. Biologically active GP5 and anti-GP5 antibody inhibited cell infection by PRRSV. Moreover, the antibody produced in this study was capable of detecting the cell infection by PRRSV and distinguishing this virus from other viruses.