Rapid manipulation of the porcine epidemic diarrhea virus genome by CRISPR/Cas9 technology

Progress of research on coronaviruses and toroviruses in large domestic animals using reverse genetics systems

The generation of genetically engineered recombinant viruses from modified DNA/RNA is commonly referred to as reverse genetics, which allows the introduction of desired mutations into the viral genome. Reverse genetics systems (RGSs) are powerful tools for studying fundamental viral processes, mechanisms of infection, pathogenesis and vaccine development. However, establishing RGS for coronaviruses (CoVs) and toroviruses (ToVs), which have the largest genomes among vertebrate RNA viruses, is laborious and hampered by technical constraints. Hence, little research has focused on animal CoVs and ToVs using RGSs, especially in large domestic animals such as pigs and cattle. In the last decade, however, studies of porcine CoVs and bovine ToVs using RGSs have been reported. In addition, the coronavirus disease-2019 pandemic has prompted the development of new and simple CoV RGSs, which will accelerate RGS-based research on animal CoVs and ToVs. In this review, we summarise the general characteristics of CoVs and ToVs, the RGSs available for CoVs and ToVs and the progress made in the last decade in RGS-based research on porcine CoVs and bovine ToVs.

Developing Next-Generation Live Attenuated Vaccines for Porcine Epidemic Diarrhea Using Reverse Genetic Techniques

Porcine epidemic diarrhea virus (PEDV) is the etiology of porcine epidemic diarrhea (PED), a highly contagious digestive disease in pigs and especially in neonatal piglets, in which a mortality rate of up to 100% will be induced. Immunizing pregnant sows remains the most promising and effective strategy for protecting their neonatal offspring from PEDV. Although half a century has passed since its first report in Europe and several prophylactic vaccines (inactivated or live attenuated) have been developed, PED still poses a significant economic concern to the swine industry worldwide. Hence, there is an urgent need for novel vaccines in clinical practice, especially live attenuated vaccines (LAVs) that can induce a strong protective lactogenic immune response in pregnant sows. Reverse genetic techniques provide a robust tool for virological research from the function of viral proteins to the generation of rationally designed vaccines. In this review, after systematically summarizing the research progress on virulence-related viral proteins, we reviewed reverse genetics techniques for PEDV and their application in the development of PED LAVs. Then, we probed into the potential methods for generating safe, effective, and genetically stable PED LAV candidates, aiming to provide new ideas for the rational design of PED LAVs.

Red recombination enables a wide variety of markerless manipulation of porcine epidemic diarrhea virus genome to generate recombinant virus

Porcine epidemic diarrhea virus (PEDV) is a member of the genera Alphacoronavirus that has been associated with acute watery diarrhea and vomiting in swine. Unfortunately, no effective vaccines and antiviral drugs for PEDV are currently available. Reverse genetics systems are crucial tools for these researches. Here, a PEDV full-length cDNA clone was constructed. Furtherly, three PEDV reporter virus plasmids containing red fluorescent protein (RFP), Nano luciferase (Nluc), or green fluorescence protein (GFP) were generated using Red recombination with the GS1783 E. coli strain. These reporter-expressing recombinant (r) PEDVs showed similar growth properties to the rPEDV, and the foreign genes were stable to culture up to P9 in Vero cells. Using the Nluc-expressing rPEDV, the replication of PEDV was easily quantified, and a platform for rapid anti-PEDV drug screening was constructed. Among the three drugs, Bergenin, Umifenovir hydrochloride (Arbidol), and Ganoderma lucidum triterpenoids (GLTs), we found that GLTs inhibited PEDV replication mainly after the stage of virus "Entry". Overall, this study will broaden insight into the method for manipulating the PEDV genome and provide a powerful tool for screening anti-PEDV agents.

Prevention and Control of Swine Enteric Coronaviruses in China: A Review of Vaccine Development and Application

Swine enteric coronaviruses (SECs) cause significant economic losses to the pig industry in China. Although many commercialized vaccines against transmissible gastroenteritis virus (TGEV) and porcine epidemic diarrhea virus (PEDV) are available, viruses are still widespread. The recent emergence of porcine deltacoronavirus (PDCoV) and swine acute diarrhea syndrome coronavirus (SADS-CoV), for which no vaccines are available, increases the disease burden. In this review, we first introduced the genomic organization and epidemiology of SECs in China. Then, we discussed the current vaccine development and application in China, aiming to provide suggestions for better prevention and control of SECs in China and other countries.

Genetic signatures associated with the virulence of porcine epidemic diarrhea virus AH2012/12

IMPORTANCE

Porcine epidemic diarrhea (PED) caused by PED virus (PEDV) remains a big threat to the swine industry worldwide. Vaccination with live attenuated vaccine is a promising method to prevent and control PED, because it can elicit a more protective immunity than the killed vaccine, subunit vaccine, and so on. In this study, we found two obvious deletions in the genome of a high passage of AH2012/12. We further confirmed the second deletion which contains seven amino acids at the carboxy-terminus of the S2 gene and the start codon of ORF3 can reduce its pathogenicity in vivo. Animal experiments indicated that the recombinant PEDV with deleted carboxy-terminus of S gene showed higher IgG, IgA, neutralization antibodies, and protection effects against virus challenge than the killed vaccine. These data reveal that the engineering of the carboxy-terminus of the S2 gene may be a promising method to develop live attenuated vaccine candidates of PEDV.

Reverse Genetics Systems for Emerging and Re-Emerging Swine Coronaviruses and Applications

Emerging and re-emerging swine coronaviruses (CoVs), including porcine epidemic diarrhea virus (PEDV), porcine deltacoronavirus (PDCoV), and swine acute diarrhea syndrome-CoV (SADS-CoV), cause severe diarrhea in neonatal piglets, and CoV infection is associated with significant economic losses for the swine industry worldwide. Reverse genetics systems realize the manipulation of RNA virus genome and facilitate the development of new vaccines. Thus far, five reverse genetics approaches have been successfully applied to engineer the swine CoV genome: targeted RNA recombination, in vitro ligation, bacterial artificial chromosome-based ligation, vaccinia virus -based recombination, and yeast-based method. This review summarizes the advantages and limitations of these approaches; it also discusses the latest research progress in terms of their use for virus-related pathogenesis elucidation, vaccine candidate development, antiviral drug screening, and virus replication mechanism determination.

One-Step Assembly of a PRRSV Infectious cDNA Clone and a Convenient CRISPR/Cas9-Based Gene-Editing Technology for Manipulation of PRRSV Genome

Porcine reproductive and respiratory syndrome (PRRS) has been a persistent challenge for the swine industry for over three decades due to the lack of effective treatments and vaccines. Reverse genetics systems have been extensively employed to build rapid drug screening platforms and develop genetically engineered vaccines. Herein, we rescued recombinant PRRS virus (rPRRSV) WUH3 using an infectious cDNA clone of PRRSV WUH3 acquired through a BstXI-based one-step-assembly approach. The rPRRSV WUH3 and its parental PRRSV WUH3 share similar plaque sizes and multiple-step growth curves. Previously, gene-editing of viral genomes depends on appropriate restrictive endonucleases, which are arduous to select in some specific viral genes. Thus, we developed a restrictive endonucleases-free method based on CRISPR/Cas9 to edit the PRRSV genome. Using this method, we successfully inserted the exogenous gene (EGFP gene as an example) into the interval between ORF1b and ORF2a of the PRRSV genome to generate rPRRSV WUH3-EGFP, or precisely mutated the lysine (K) at position 150 of PRRSV nsp1α to glutamine (Q) to acquire rPRRSV WUH3 nsp1α-K150Q. Taken together, our study provides a rapid and convenient method for the development of genetically engineered vaccines against PRRSV and the study on the functions of PRRSV genes.

Genomic Characterizations of Porcine Epidemic Diarrhea Viruses (PEDV) in Diarrheic Piglets and Clinically Healthy Adult Pigs from 2019 to 2022 in China

Porcine epidemic diarrhea virus (PEDV) is a major causative pathogen of diarrheic disease. In this study, the prevalence and evolution of PEDV was evaluated using intestinal samples collected from six provinces of China in 2019-2022. PEDV could not only be detected in diarrheic piglets but also in adult pigs without enteric diseases. The complete genomes of five temporal and geographical representative PEDV strains were determined. Genome-based phylogenetic analysis indicated that XJ1904-700 belongs to the G2-a subgroup, while the other strains are clustered within the S-INDEL subgroup. Recombination analyses supported that JSNJ2004-919 is an inter-subgroup recombinant from SD2014-like (G2-b), CHZ-2013-like (G2-b) and CV777-like (G1-b) isolates, while FJFZ2004-1017 is an intra-subgroup recombinant from XM1-2-like (S-INDEL) and LYG-2014-like (S-INDEL) isolates. Both JSNJ2004-919 and FJFZ2004-1017 were from adult pigs, providing evidence that adult pigs may also serve as the host of PEDV reservoirs for virus evolution. Overall, this study provides new insights into PEDV's prevalence and evolution in both diseased piglets and clinically healthy adult pigs.

Construction and immunogenicity of a trypsin-independent porcine epidemic diarrhea virus variant

Porcine epidemic diarrhea virus (PEDV) is a re-emerging enteropathogenic coronavirus that causes high mortality in neonatal piglets. The addition of trypsin plays a crucial role in the propagation of PEDV, but also increases the complexity of vaccine production and increases its cost. Previous studies have suggested that the S2' site and Y976/977 of the PEDV spike (S) protein might be the determinants of PEDV trypsin independence. In this study, to achieve a recombinant trypsin-independent PEDV strain, we used trypsin-dependent genotype 2 (G2) PEDV variant AJ1102 to generate three recombinant PEDVs with mutations in S (S2' site R894G and/or Y976H). The three recombinant PEDVs were still trypsin dependent, suggesting that the S2' site R894 and Y976 of AJ1102 S are not key sites for PEDV trypsin dependence. Therefore, we used AJ1102 and the classical trypsin-independent genotype 1 (G1) PEDV strain JS2008 to generate a recombinant PEDV carrying a chimeric S protein, and successfully obtained trypsin-independent PEDV strain rAJ1102-S2'_JS2008, in which the S2 (amino acids 894-1386) domain was replaced with the corresponding JS2008 sequence. Importantly, immunization with rAJ1102-S2'_JS2008 induced neutralizing antibodies against both AJ1102 and JS2008. Collectively, these results suggest that rAJ1102-S2'_JS2008 is a novel vaccine candidate with significant advantages, including no trypsin requirement for viral propagation to high titers and the potential provision of protection for pigs against G1 and G2 PEDV infections.

Molecular characterization and phylogenetic analysis of porcine epidemic diarrhea virus strains circulating in China from 2020 to 2021

Background

Porcine epidemic diarrhea virus (PEDV), an enteric coronavirus, has become the major causative agent of acute gastroenteritis in piglets since 2010 in China.

Results

In the current study, 91 complete spike (S) gene sequences were obtained from PEDV positive samples collected from 17 provinces in China from March 2020 to March 2021. A phylogenetic analysis showed that 92.3% (84 out of 91) of the identified strains belonged to GII subtype, while 7.7% (7 out of 91) were categorized as S-INDEL like strains and grouped within GI-c clade. Based on a recombination analysis, six of S-INDEL like strains were recombinant strains originated from S-INDEL strain FR/001/2014 and virulent strain AJ1102. In addition, PEDV variant strains (CH/GDMM/202012, CH/GXDX/202010 et al) carrying novel insertions (360QGRKS364 and 1278VDVF1281) in the S protein were observed. Furthermore, the deduced amino acid sequences for the S protein showed that multiple amino acid substitutions in the antigenic epitopes in comparison with the vaccine strains.

Conclusions

In conclusion, these data provide novel molecular evidence on the epidemiology and molecular diversity of PEDV in 2020–2021. This information may help design a strategy for controlling and preventing the prevalence of PEDV variant strains in China.

Nonstructural Protein 1 of Variant PEDV Plays a Key Role in Escaping Replication Restriction by Complement C3

Zoonotic coronaviruses represent an ongoing threat to public health. The classical porcine epidemic diarrhea virus (PEDV) first appeared in the early 1970s. Since 2010, outbreaks of highly virulent PEDV variants have caused great economic losses to the swine industry worldwide. However, the strategies by which PEDV variants escape host immune responses are not fully understood. Complement component 3 (C3) is considered a central component of the three complement activation pathways and plays a crucial role in preventing viral infection. In this study, we found that C3 significantly inhibited PEDV replication in vitro, and both variant and classical PEDV strains induced high levels of interleukin-1β (IL-1β) in Huh7 cells. However, the PEDV variant strain reduces C3 transcript and protein levels induced by IL-1β compared with the PEDV classical strain. Examination of key molecules of the C3 transcriptional signaling pathway revealed that variant PEDV reduced C3 by inhibiting CCAAT/enhancer-binding protein β (C/EBP-β) phosphorylation. Mechanistically, PEDV nonstructural protein 1 (NSP1) inhibited C/EBP-β phosphorylation via amino acid residue 50. Finally, we constructed recombinant PEDVs to verify the critical role of amino acid 50 of NSP1 in the regulation of C3 expression. In summary, we identified a novel antiviral role of C3 in inhibiting PEDV replication and the viral immune evasion strategies of PEDV variants. Our study reveals new information on PEDV-host interactions and furthers our understanding of the pathogenic mechanism of this virus. IMPORTANCE The complement system acts as a vital link between the innate and the adaptive immunity and has the ability to recognize and neutralize various pathogens. Activation of the complement system acts as a double-edged sword, as appropriate levels of activation protect against pathogenic infections, but excessive responses can provoke a dramatic inflammatory response and cause tissue damage, leading to pathological processes, which often appear in COVID-19 patients. However, how PEDV, as the most severe coronavirus causing diarrhea in piglets, regulates the complement system has not been previously reported. In this study, for the first time, we identified a novel mechanism of a PEDV variant in the suppression of C3 expression, showing that different coronaviruses and even different subtype strains differ in regulation of C3 expression. In addition, this study provides a deeper understanding of the mechanism of the PEDV variant in immune escape and enhanced virulence.

An overview: CRISPR/Cas-based gene editing for viral vaccine development

INTRODUCTION

: Gene-editing technology revolutionized vaccine manufacturing and offers a variety of benefits over traditional vaccinations, such as improved immune response, higher production rate, stability, precise immunogenic activity, and fewer adverse effects. The more recently discovered Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR)/associated protein 9 (Cas9) system has become the most widely utilized technology based on its efficiency, utility, flexibility, versatility, ease of use, and cheaper compared to other gene-editing techniques. Considering its wider scope for genomic modification, CRISPR/Cas9-based technology's potential is explored for vaccine development.

AREAS COVERED

: In this review, we will address the recent advances in the CRISPR/Cas system for the development of vaccines and viral vectors for delivery. In addition, we will discuss strategies for the development of the vaccine, as well as the limitations and future prospects of the CRISPR/Cas system.

EXPERT OPINION

: Human and animal viruses have been exposed to antiviral CRISPR/Cas9-based engineering to prevent infection, which uses knockout, knock-in, gene activation/deactivation, RNA targeting, and editing cell lines strategies for gene editing of viruses. Because of that CRISPR/Cas system is used to boost the vaccine production yield by removing unwanted genes that cause disease or are required for viral infection.

Porcine Epidemic Diarrhea Virus nsp7 Inhibits Interferon-Induced JAK-STAT Signaling through Sequestering the Interaction between KPNA1 and STAT1

Porcine epidemic diarrhea virus (PEDV) is a highly pathogenic enteric coronavirus that causes high mortality in piglets. Interferon (IFN) responses are the primary defense mechanism against viral infection; however, viruses always evolve elaborate strategies to antagonize the antiviral action of IFN. Previous study showed that PEDV nonstructural protein 7 (nsp7), a component of the viral replicase polyprotein, can antagonize ploy(I:C)-induced type I IFN production. Here, we found that PEDV nsp7 also antagonized IFN-α-induced JAK-STAT signaling and the production of IFN-stimulated genes. PEDV nsp7 did not affect the protein and phosphorylation levels of JAK1, Tyk2, STAT1, and STAT2 or the formation of the interferon-stimulated gene factor 3 (ISGF3) complex. However, PEDV nsp7 prevented the nuclear translocation of STAT1 and STAT2. Mechanistically, PEDV nsp7 interacted with the DNA binding domain of STAT1/STAT2, which sequestered the interaction between karyopherin α1 (KPNA1) and STAT1, thereby blocking the nuclear transport of ISGF3. Collectively, these data reveal a new mechanism developed by PEDV to inhibit type I IFN signaling pathway. IMPORTANCE In recent years, an emerging porcine epidemic diarrhea virus (PEDV) variant has gained attention because of serious outbreaks of piglet diarrhea in China and the United States. Coronavirus nonstructural protein 7 (nsp7) has been proposed to act with nsp8 as part of an RNA primase to generate RNA primers for viral RNA synthesis. However, accumulating evidence indicates that coronavirus nsp7 can also antagonize type I IFN production. Our present study extends previous findings and demonstrates that PEDV nsp7 also antagonizes IFN-α-induced IFN signaling by competing with KPNA1 for binding to STAT1, thereby enriching the immune regulation function of coronavirus nsp7.

Molecular Mechanism of Porcine Epidemic Diarrhea Virus Cell Tropism

In the 21st century, several human and swine coronaviruses (CoVs) have emerged suddenly and caused great damage to people's lives and property. The porcine epidemic diarrhea virus (PEDV), leading to enormous economic losses to the pork industry and remains a large challenge. PEDV showed extensive cell tropism, and we cannot ignore the potential risk of cross-species transmission. However, the mechanism of adaptation and cell tropism of PEDV remains largely unknown and in vitro isolation of PEDV remains a huge challenge, which seriously impedes the development of vaccines. In this study, we confirmed that the spike (S) protein determines the adaptability of PEDV to monkey Vero cells and LLC-PK1 porcine cells, and isolated exchange of S1 and S2 subunits of adaptive strains did not make PEDV adapt to cells. Further, we found that the cellular adaptability of rCH/SX/2016-S_HNXP depends on S1 and the first half of S2 (S3), and the 803L and 976H of the S2 subunit are critical for rCH/SX/2016-S1_HNXP+S3_HNXP adaptation to Vero cells. These findings highlight the decisive role of PEDV S protein in cell tropism and the potential role of coronaviruses S protein in cross-species transmissibility. Besides, our work also provides some different insight into finding PEDV receptors and developing PEDV and other coronaviruses vaccines. IMPORTANCE CoVs can spill from an animal reservoir into a naive host to cause diseases in humans or domestic animals. PEDV results in high mortality in piglets, which has caused immense economic losses in the pork industry. Virus isolation is the first step in studying viral pathogenesis and developing effective vaccines. However, the molecular mechanism of PEDV cell tropism is largely unknown, and isolation of endemic PEDV strains remains a major challenge. This study confirmed that the S gene is the decisive gene of PEDV adaptability to monkey Vero cells and porcine LLC-PK1 cells by the PEDV reverse genetics system. Isolated exchange of S1 and S2 of adaptive strains did not make PEDV adapt to cells, and the 803L and 976H of S2 subunit are critical for rCH/SX/2016-S1_HNXP+S3_HNXP adaptation to Vero cells. These results illustrate the decisive role of PEDV S protein in cell tropism and highlight the potential role of coronaviruses S protein in cross-species transmissibility. Besides, our finding also provides some unique insight into identifying PEDV functional receptors and has guiding significance for developing PEDV and other coronavirus vaccines.

One-Step Assembly of a Porcine Epidemic Diarrhea Virus Infectious cDNA Clone by Homologous Recombination in Yeast: Rapid Manipulation of Viral Genome With CRISPR/Cas9 Gene-Editing Technology

Porcine epidemic diarrhea virus (PEDV), a swine enteric coronavirus causing acute diarrhea in piglets, is one of the major threatens to the pork industry globally. Reverse genetics is a valuable tool for the virological study and vaccine development for coronaviruses. Due to the large size and unstable problem in Escherichia coli of coronavirus genome, construction and manipulation of reverse genetics system for coronaviruses remain laborious and time-consuming. In this study, a reverse genetics system of the genotype II PEDV strain HM was generated using the transformation-associated recombination (TAR) technology in yeast within 1 week. The rescued virus (rPEDV) exhibited similar growth properties to the wild-type virus in vitro. With this PEDV infectious cDNA clone, CRISPR/Cas9 technology and homologous recombination were combined to generate a recombinant virus rPEDV-EGFP in which the ORF3 gene was swapped with an EGFP gene. The reporter virus displayed similar growth properties to the parental virus rPEDV and remained stable during serial passage in vitro. Of note, the strategies of construction and manipulation of PEDV infectious cDNA clone are extremely simple and efficient, which could be applied for other RNA viruses and DNA viruses.

Construction, Characterization and Application of Recombinant Porcine Deltacoronavirus Expressing Nanoluciferase

Porcine deltacoronavirus (PDCoV), an emerging enteropathogenic coronavirus, causes diarrhoea in suckling piglets and has the potential for cross-species transmission. No effective PDCoV vaccines or antiviral drugs are currently available. Here, we successfully generated an infectious clone of PDCoV strain CHN-HN-2014 using a combination of bacterial artificial chromosome (BAC)-based reverse genetics system with a one-step homologous recombination. The recued virus (rCHN-HN-2014) possesses similar growth characteristics to the parental virus in vitro. Based on the established infectious clone and CRISPR/Cas9 technology, a PDCoV reporter virus expressing nanoluciferase (Nluc) was constructed by replacing the NS6 gene. Using two drugs, lycorine and resveratrol, we found that the Nluc reporter virus exhibited high sensibility and easy quantification to rapid antiviral screening. We further used the Nluc reporter virus to test the susceptibility of different cell lines to PDCoV and found that cell lines derived from various host species, including human, swine, cattle and monkey enables PDCoV replication, broadening our understanding of the PDCoV cell tropism range. Taken together, our reporter viruses are available to high throughput screening for antiviral drugs and uncover the infectivity of PDCoV in various cells, which will accelerate our understanding of PDCoV.

Application of CRISPR-Cas9 Editing for Virus Engineering and the Development of Recombinant Viral Vaccines

CRISPR-Cas technology, discovered originally as a bacterial defense system, has been extensively repurposed as a powerful tool for genome editing for multiple applications in biology. In the field of virology, CRISPR-Cas9 technology has been widely applied on genetic recombination and engineering of genomes of various viruses to ask some fundamental questions about virus-host interactions. Its high efficiency, specificity, versatility, and low cost have also provided great inspiration and hope in the field of vaccinology to solve a series of bottleneck problems in the development of recombinant viral vaccines. This review highlights the applications of CRISPR editing in the technological advances compared to the traditional approaches used for the construction of recombinant viral vaccines and vectors, the main factors affecting their application, and the challenges that need to be overcome for further streamlining their effective usage in the prevention and control of diseases. Factors affecting efficiency, target specificity, and fidelity of CRISPR-Cas editing in the context of viral genome editing and development of recombinant vaccines are also discussed.

Functions of Coronavirus Accessory Proteins: Overview of the State of the Art

Coronavirus accessory proteins are a unique set of proteins whose genes are interspersed among or within the genes encoding structural proteins. Different coronavirus genera, or even different species within the same coronavirus genus, encode varying amounts of accessory proteins, leading to genus- or species-specificity. Though accessory proteins are dispensable for the replication of coronavirus in vitro, they play important roles in regulating innate immunity, viral proliferation, and pathogenicity. The function of accessory proteins on virus infection and pathogenesis is an area of particular interest. In this review, we summarize the current knowledge on accessory proteins of several representative coronaviruses that infect humans or animals, including the emerging severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), with an emphasis on their roles in interaction between virus and host, mainly involving stress response, innate immunity, autophagy, and apoptosis. The cross-talking among these pathways is also discussed.

ATPase and helicase activities of porcine epidemic diarrhea virus nsp13

Porcine epidemic diarrhea virus (PEDV) is a reemerging Alphacoronavirus that causes lethal diarrhea in piglets. Coronavirus nonstructural protein 13 (nsp13) encodes helicase, which plays pivotal roles during viral replication by unwinding viral RNA. However, the biochemical characterization of PEDV nsp13 remains largely unknown. In this study, PEDV nsp13 was expressed in Escherichia coli and purified. The recombinant nsp13 possessed ATPase and helicase activities for binding and unwinding dsDNA/RNA substrates with 5′-overhangs, and Mg²⁺ and Mn²⁺ were critical for its ATPase and helicase activities. PEDV nsp13 also unwound dsDNA into ssDNA in the pH from 6.0–9.0, and used energy from all nucleoside triphosphates and deoxynucleoside triphosphates. Site-directed mutagenesis demonstrated that Lys289 (K289) of PEDV nsp13 was essential for its ATPase and helicase activities. These results provide new insights into the biochemical properties of PEDV nsp13, which is a potential target for developing antiviral drugs.

A strain of porcine deltacoronavirus: Genomic characterization, pathogenicity and its full-length cDNA infectious clone

As a novel enteropathogenic coronavirus, porcine deltacoronavirus (PDCoV) warrants further investigation. In this study, a Chinese PDCoV strain, designated CHN-HN-1601, was isolated from the faeces of a diarrhoeic piglet. After plaque purification, the genome was determined which shared 97.5%-99.5% nucleotide identities with 71 representative PDCoV strains available in the GenBank. The pathogenic properties of CHN-HN-1601 were evaluated using 5-day-old piglets. All inoculated piglets developed severe diarrhoea from 2 days post-infection (dpi) onwards. To our surprise, two periods of diarrhoea starting from 2 to 7 dpi and from 13 to 19 dpi were observed in affected piglets during the experiment. Faecal viral shedding of the inoculated piglets was detected by real-time RT-PCR, with viral shedding peaked at 4 and 16 dpi, respectively. At necropsy at 5 dpi, the main gross lesions included transparent, thin-walled and gas-distended intestines containing yellow watery contents. Further histopathological examinations, including haematoxylin and eosin staining, immunohistochemistry and RNAscope in situ hybridization, revealed that the virus infection caused severe villous atrophy of the small intestines, with PDCoV antigen and RNA mainly distributed in the cytoplasm of the villous epithelial cells of jejunum and ileum in piglets. The dynamic production of PDCoV-specific IgG and neutralizing antibodies in serum of the affected piglets was also assessed using a whole virus-based ELISA and an immunofluorescence assay-based neutralization test, respectively. Furthermore, a full-length cDNA infectious clone of CHN-HN-1601 was constructed using a bacterial artificial chromosome system. The rescued virus exhibited in vitro growth and pathogenic properties similar to the parental virus. Taken together, our study not only enriches the information of PDCoV, but also provides a useful reverse genetics platform for further pathogenesis exploration of the virus.

Reverse genetic systems: Rational design of coronavirus live attenuated vaccines with immune sequelae

Since the end of 2019, the global COVID-19 outbreak has once again made coronaviruses a hot topic. Vaccines are hoped to be an effective way to stop the spread of the virus. However, there are no clinically approved vaccines available for coronavirus infections. Reverse genetics technology can realize the operation of RNA virus genomes at the DNA level and provide new ideas and strategies for the development of new vaccines. In this review, we systematically describe the role of reverse genetics technology in studying the effects of coronavirus proteins on viral virulence and innate immunity, cell and tissue tropism and antiviral drug screening. An efficient reverse genetics platform is useful for obtaining the ideal attenuated strain to prepare an attenuated live vaccine.