Identification of a peptide derived from the heptad repeat 2 region of the porcine epidemic diarrhea virus (PEDV) spike glycoprotein that is capable of suppressing PEDV entry and inducing neutralizing antibodies

Isolation and characterization of a novel S1-gene insertion porcine epidemic diarrhea virus with low pathogenicity in newborn piglets

Porcine epidemic diarrhea virus (PEDV) causes diarrhea and vomiting in piglets, leading to a mortality rate of 100%. Due to the high frequency of mutation, it is important to monitor the evolution of PEDV and develop potential vaccine candidates. In this study, two PEDV strains (ZJ2022 and ZQ2022) were identified by PCR. These strains were subsequently isolated, and their genome sequences, growth characteristics, and pathogenicity were compared. Phylogenetic and recombination analyses revealed that both strains belonged to GIIa-subgroup, and ZQ2022 was identified as a recombinant strain derived from ZJ2022. Further sequence analysis showed that the ZJ2022 strain had a modified top region of the S1 protein due to a three amino acid insertion (T380_Y380insGGE) in the S1 gene. According to the virus growth curve, ZJ2022 exhibited better cellular adaptation than ZQ2022, with higher viral titers from 8 hpi to 24 hpi. Additionally, ZQ2022 exhibited a high level of pathogenicity, causing severe diarrhea in piglets at 36 hpi and a 100% mortality rate by 96 hpi. In contrast, ZJ2022 showed lower pathogenicity, inducing severe diarrhea in piglets at 60 hpi, with a mortality rate of 60% at 96 hpi and 100% at 120 hpi. In summary, our findings provided evidence of the undergoing mutations in Chinese PEDV strains. Furthermore, the S gene insertion strain ZJ2022 exhibited strong cellular adaptability and low pathogenicity, making it a potential candidate strain for vaccine development.

Efficacy evaluation of a bivalent subunit vaccine against epidemic PEDV heterologous strains with low cross-protection

Variant Porcine epidemic diarrhea virus (PEDV), which causes diarrhea and high mortality in piglets, has become a major pathogen, and co-epidemics of different subtypes of the virus have become a very thorny problem for the clinical prevention and control of PEDV. However, cross-protection between epidemic G2a and G2b subtype strains has not been observed, and there is currently no vaccine against both G2a and G2b strains. In this study, we demonstrate the low cross-protection between G2a and G2b strains with piglet immunization and challenge tests. The trimeric full-length S proteins of G2a and G2b variants were purified and a bivalent subunit vaccine against PEDV G2a/G2b-S was developed. In active and passive immune protection tests, the bivalent subunit vaccine produced high neutralizing antibody titers and S-specific immunoglobulin G (IgG) and IgA titers against both the G2a and G2b strains in piglets and sows. In the attack phase of the viruses, the clinical symptoms and microscopic lesions in the immunized groups were significantly alleviated. Importantly, the PEDV G2a/G2b-S bivalent subunit vaccine conferred effective passive immunity against PEDV G2a and G2b challenges in the form of colostrum-derived antibodies from the immunized sows. In conclusion, our data demonstrate the low cross-protection of PEDV epidemic G2a and G2b strains and show that the G2a/G2b-S bivalent subunit vaccine is protective against both G2a and G2b strains. It is therefore a candidate vaccine for PEDV prevention.

IMPORTANCE

The detection rate of PEDV G2a subtype strains is currently increasing. Although commercial vaccines are available, most vaccines do not exert an ideal protective effect against these strains. Furthermore, there is no definitive research into the cross-protection between G2a and G2b strains, and no bivalent vaccine provides joint protection against both. Therefore, in this study, we investigated the cross-protection between PEDV G2a and G2b strains and designed a candidate bivalent subunit vaccine combining the trimeric S proteins of the G2a and G2b subtypes. We demonstrate that the cross-protection between strains G2a and G2b is poor and that this bivalent subunit vaccine protects piglets from viral attack by inducing both active and passive immunity. This study emphasizes the effectiveness of the PEDV G2a/G2b-S bivalent subunit vaccine and provides a feasible method for the development of efficient PEDV vaccines.

RpIFN-λ1 alleviates the clinical symptoms of porcine epidemic diarrhea

Porcine epidemic diarrhea (PED), caused by the porcine epidemic diarrhea virus (PEDV), primarily affects the jejunum and ileum of pigs. Interferons, glycoproteins with high species specificity and potent antiviral activity, are crucial in defending against viral infections. Unlike other interferons, interferon-lambda (IFN-λ) mainly acts on mucosal epithelial cells and exhibits robust antiviral activity at mucosal surfaces. However, the high cost limits the use of naturally extracted interferons in farming. In this study, we expressed recombinant porcine interferon-lambda 1 (rpIFN-λ1) in eukaryotic cells, demonstrating effective antiviral activity against PEDV in Vero E6 and IPI-FX cells. In vivo, rpIFN-λ1 alleviated clinical symptoms and intestinal damage, enhanced antioxidant capacity, reduced inflammation, and significantly improved the survival rate of piglets following PEDV infection. Both in vitro and in vivo studies confirmed that rpIFN-λ1 upregulated interferon-stimulated genes (ISGs) via the JAK-STAT pathway, thereby exerting antiviral effects. In conclusion, rpIFN-λ1 significantly inhibited PEDV replication and alleviated clinical symptoms. The selectivity of rpIFN-λ1 for intestinal cells and its ability to reduce viral shedding suggest that this agent is a promising antiviral for enteric viruses such as PEDV. Our findings highlight rpIFN-λ1 as a cost-effective, efficient, and novel strategy for antiviral treatment of PEDV.

Antiviral activity of flavonol against porcine epidemic diarrhea virus

Porcine epidemic diarrhea virus (PEDV) remains one of the major causative microorganisms of viral diarrhea in piglets worldwide, with no approved drugs for treatment. We identified a natural molecule, flavonol, which is widely found in tea, vegetables and herbs. Subsequently, the antiviral activity of compound flavonol was evaluated in Vero cells and IPEC-J2 cells, and its anti-PEDV mechanism was analyzed by molecular docking and molecular dynamics. The results showed that flavonol could effectively inhibit viral progeny production, RNA synthesis and protein expression of PEDV strains in a dose-dependent manner. When flavonol was added simultaneously with viral infection in Vero cells, it demonstrated potent anti-PEDV activity by affecting the viral attachment and internalization phases. Similarly, in IPEC-J2 cells, flavonol effectively inhibited PEDV infection at different stages of infection, except for the release phase. Moreover, flavonol mainly interacts with PEDV Mpro through hydrogen bonds and hydrophobic forces, and the complex formed by it has high stability. Importantly, flavonol also showed broad-spectrum activity against other porcine enteric coronaviruses such as TGEV and PDCoV in vitro. These findings suggest that flavonol may exert antiviral effects by interacting with viral Mpro, thereby affecting viral replication. This means that flavonol is expected to become a potential drug to prevent or treat porcine enteric coronavirus.

Construction and immune effect evaluation of the S protein heptad repeat-based nanoparticle vaccine against porcine epidemic diarrhea virus

Porcine epidemic diarrhea virus (PEDV), a highly virulent enteropathogenic coronavirus, is a significant threat to the pig industry. High frequency mutations in the PEDV genome have limited the effectiveness of current vaccines in providing immune protection. Developing efficient vaccines that can quickly adapt to mutant strains is a challenging but crucial task. In this study, we chose the pivotal protein heptad repeat (HR) responsible for coronavirus entry into host cells, as the vaccine antigen. HR-Fer nanoparticles prepared using ferritin were evaluated them as PEDV vaccine candidates. Nanoparticle vaccines elicited stronger neutralizing antibody responses in mice compared to monomer vaccines. Additionally, HR protein delivered via nanoparticles increased antigen uptake by antigen-presenting cells in vitro by 2.75-fold. The collective results suggest that HR can be used as antigens for vaccines, and the HR vaccine based on ferritin nanoparticles significantly enhances immunogenicity.

Enteric coronavirus nsp2 is a virulence determinant that recruits NBR1 for autophagic targeting of TBK1 to diminish the innate immune response

Non-structural protein 2 (nsp2) exists in all coronaviruses (CoVs), while its primary function in viral pathogenicity, is largely unclear. One such enteric CoV, porcine epidemic diarrhea virus (PEDV), causes high mortality in neonatal piglets worldwide. To determine the biological role of nsp2, we generated a PEDV mutant containing a complete nsp2 deletion (rPEDV-Δnsp2) from a highly pathogenic strain by reverse genetics, showing that nsp2 was dispensable for PEDV infection, while its deficiency reduced viral replication in vitro. Intriguingly, rPEDV-Δnsp2 was entirely avirulent in vivo, with significantly increased productions of IFNB (interferon beta) and IFN-stimulated genes (ISGs) in various intestinal tissues of challenged newborn piglets. Notably, nsp2 targets and degrades TBK1 (TANK binding kinase 1), the critical kinase in the innate immune response. Mechanistically, nsp2 induced the macroautophagy/autophagy process and recruited a selective autophagic receptor, NBR1 (NBR1 autophagy cargo receptor). NBR1 subsequently facilitated the K48-linked ubiquitination of TBK1 and delivered it for autophagosome-mediated degradation. Accordingly, the replication of rPEDV-Δnsp2 CoV was restrained by reduced autophagy and excess productions of type I IFNs and ISGs. Our data collectively define enteric CoV nsp2 as a novel virulence determinant, propose a crucial role of nsp2 in diminishing innate antiviral immunity by targeting TBK1 for NBR1-mediated selective autophagy, and pave the way to develop a new type of nsp2-based attenuated PEDV vaccine. The study also provides new insights into the prevention and treatment of other pathogenic CoVs.Abbreviations: 3-MA: 3-methyladenine; Baf A1: bafilomycin A1; CoV: coronavirus; CQ: chloroquine; dpi: days post-inoculation; DMVs: double-membrane vesicles; GABARAP: GABA type A receptor-associated protein; GFP: green fluorescent protein; GIGYF2: GRB10 interacting GYF protein 2; hpi: hours post-infection; IFA: immunofluorescence assay; IFIH1: interferon induced with helicase C domain 1; IFIT2: interferon induced protein with tetratricopeptide repeats 2; IFITM1: interferon induced transmembrane protein 1; IFNB: interferon beta; IRF3: interferon regulatory factor 3; ISGs: interferon-stimulated genes; mAb: monoclonal antibody; MAP1LC3/LC3: microtubule associated protein 1 light chain 3; MAVS: mitochondrial antiviral signaling protein; NBR1: NBR1 autophagy cargo receptor; nsp2: non-structural protein 2; OAS1: 2'-5'-oligoadenylate synthetase 1; PEDV: porcine epidemic diarrhea virus; PRRs: pattern recognition receptors; RIGI: RNA sensor RIG-I; RT-qPCR: reverse transcription quantitative polymerase chain reaction; SQSTM1: sequestosome 1; TBK1: TANK binding kinase 1; TCID50: 50% tissue culture infectious doses; VSV: vesicular stomatitis virus.

Research progress of porcine epidemic diarrhea virus S protein

Porcine epidemic diarrhea virus (PEDV) is a single-stranded RNA virus with a capsid membrane that causes acute infectious gastrointestinal disease characterized by vomiting, diarrhea, and dehydration in swine. Piglets are more susceptible to PEDV than adults, with an infection rate reaching 90% and a fatality rate as high as 100%. Moreover, PEDV has a rapid transmission rate and broad transmission range. Consequently, PEDV has caused considerable economic losses and negatively impacted the sustainability of the pig industry. The surface spike (S) glycoprotein is the largest structural protein in PEDV virions and is closely associated with host cell fusion and virus invasion. As such, the S protein is an important target for vaccine development. In this article, we review the genetic variation, immunity, apoptosis-induction function, virulence, vaccine potential, and other aspects of the PEDV S protein. This review provides a theoretical foundation for preventing and controlling PEDV infection and serves as a valuable resource for further research and development of PEDV vaccines.

Overview of the recent advances in porcine epidemic diarrhea vaccines

Vaccination is the most effective means of preventing and controlling porcine epidemic diarrhea (PED). Conventional vaccines developed from porcine epidemic diarrhea virus (PEDV) GI-a subtypes (CV777 and SM98) have played a vital role in preventing classical PED. However, with the emergence of PEDV mutants in 2010, conventional PEDV GI-a subtype-targeting vaccines no longer provide adequate protection against PEDV GII mutants, thereby making novel-type PED vaccine development an urgent concern to be addressed. Novel vaccines, including nucleic acid vaccines, genetically engineered subunit vaccines, and live vector vaccines, are associated with several advantages, such as high safety and stability, clear targeting, high yield, low cost, and convenient usage. These vaccines can be combined with corresponding ELISA kits to differentiate infected from vaccinated animals, which is beneficial for disease confirmation. This review provides a detailed overview of the recent advancements in PED vaccines, emphasizing on the research and application evaluation of novel PED vaccines. It also considers the future directions and challenges in advancing these vaccines to widespread use in clinics.

Porcine Epidemic Diarrhea: Insights and Progress on Vaccines

Porcine epidemic diarrhea (PED) is a swine-wasting disease caused by coronavirus infection. It causes great economic damage to the swine industry worldwide. Despite the continued use of vaccines, PED outbreaks continue, highlighting the need to review the effectiveness of current vaccines and develop additional vaccines based on new platforms. Here, we review existing vaccine technologies for preventing PED and highlight promising technologies that may help control PED virus in the future.

A Comprehensive View on the Protein Functions of Porcine Epidemic Diarrhea Virus

Porcine epidemic diarrhea (PED) virus (PEDV) is one of the main pathogens causing diarrhea in piglets and fattening pigs. The clinical signs of PED are vomiting, acute diarrhea, dehydration, and mortality resulting in significant economic losses and becoming a major challenge in the pig industry. PEDV possesses various crucial structural and functional proteins, which play important roles in viral structure, infection, replication, assembly, and release, as well as in escaping host innate immunity. Over the past few years, there has been progress in the study of PEDV pathogenesis, revealing the crucial role of the interaction between PEDV viral proteins and host cytokines in PEDV infection. At present, the main control measure against PEDV is vaccine immunization of sows, but the protective effect for emerging virus strains is still insufficient, and there is no ideal safe and efficient vaccine. Although scientists have persistently delved their research into the intricate structure and functionalities of the PEDV genome and viral proteins for years, the pathogenic mechanism of PEDV remains incompletely elucidated. Here, we focus on reviewing the research progress of PEDV structural and nonstructural proteins to facilitate the understanding of biological processes such as PEDV infection and pathogenesis.

Vector-delivered artificial miRNA effectively inhibits Porcine epidemic diarrhea virus replication

BACKGROUND

Porcine epidemic diarrhea virus (PEDV) is an α-coronavirus that causes highly contagious intestinal infectious disease, involving clinically characterized by diarrhea, dehydration, vomiting, and high mortality to suckling piglets. As a strategy for antiviral therapy, artificial microRNA (amiRNA) mediated suppression of viral replication has recently become increasingly important. In this study, we evaluated the advantages of using an amiRNA vector against PEDV.

METHODS

In this study, we evaluated the advantages of using an amiRNA vector against PEDV. We designed two single amiRNA sequences for different conserved sequences of the PEDV S and N genes, and tested their inhibitory effects on PEDV in Vero cells.

RESULTS

It was obvious from the CCK-8 results that the transient transfection of amiRNA was non-toxic to the cells. In addition, our results showed that the transient expression of two amiRNAs (amiRNA-349 and amiRNA-1447) significantly reduced the expression of viral RNA and protein in the cells. The TCID₅₀ results showed that the release of virus particles into the culture supernatant was significantly reduced, with an effect as high as 90%. To avoid virus mutation escape, the above two single amiRNA sequences were tandem in this study (amiRNA-349 + 1447), enabling a single microRNA to be expressed simultaneously. The real-time PCR and Western blot results showed that the inhibitory effect was significantly enhanced in each of the different time periods. The TCID₅₀ results showed that the release of virus particles in the culture supernatant was significantly reduced at the different time periods.

CONCLUSIONS

In summary, these results suggest that an RNAi based on amiRNA targeting the conserved region of the virus is an effective method to improve PEDV nucleic acid inhibitors and provide a novel treatment strategy for PEDV infection.

Antitarget, Anti-SARS-CoV-2 Leads, Drugs, and the Drug Discovery-Genetics Alliance Perspective

The most advanced antiviral molecules addressing major SARS-CoV-2 targets (Main protease, Spike protein, and RNA polymerase), compared with proteins of other human pathogenic coronaviruses, may have a short-lasting clinical efficacy. Accumulating knowledge on the mechanisms underlying the target structural basis, its mutational progression, and the related biological significance to virus replication allows envisaging the development of better-targeted therapies in the context of COVID-19 epidemic and future coronavirus outbreaks. The identification of evolutionary patterns based solely on sequence information analysis for those targets can provide meaningful insights into the molecular basis of host-pathogen interactions and adaptation, leading to drug resistance phenomena. Herein, we will explore how the study of observed and predicted mutations may offer valuable suggestions for the application of the so-called "synthetic lethal" strategy to SARS-CoV-2 Main protease and Spike protein. The synergy between genetics evidence and drug discovery may prioritize the development of novel long-lasting antiviral agents.

Virtual Screening-Based Peptides Targeting Spike Protein to Inhibit Porcine Epidemic Diarrhea Virus (PEDV) Infection

Due to the rapid mutation of porcine epidemic diarrhea virus (PEDV), existing vaccines cannot provide sufficient immune protection for pigs. Therefore, it is urgent to design the affinity peptides for the prevention and control of this disease. In this study, we made use of a molecular docking technology for virtual screening of affinity peptides that specifically recognized the PEDV S1 C-terminal domain (CTD) protein for the first time. Experimentally, the affinity, cross-reactivity and sensitivity of the peptides were identified by an enzyme-linked immunosorbent assay (ELISA) and a surface plasmon resonance (SPR) test, separately. Subsequently, Cell Counting Kit-8 (CCK-8), quantitative real-time PCR (qRT-PCR), Western blot and indirect immunofluorescence were used to further study the antiviral effect of different concentrations of peptide 110766 in PEDV. Our results showed that the P/N value of peptide 110766 at 450 nm reached 167, with a K_D value of 216 nM. The cytotoxic test indicated that peptide 110766 was not toxic to vero cells. Results of the absolute quantitative PCR revealed that different concentrations (3.125 μM, 6.25 μM, 12.5 μM, 25 μM, 50 μM, 100 μM, 200 μM) of peptide 110766 could significantly reduce the viral load of PEDV compared with the virus group (p < 0.0001). Similarly, results of Western blot and indirect immunofluorescence also suggested that the antiviral effect of peptide 110766 at 3.125 is still significant. Based on the above research, high-affinity peptide 110766 binding to the PEDV S1-CTD protein was attained by a molecular docking technology. Therefore, designing, screening, and identifying affinity peptides can provide a new method for the development of antiviral drugs for PEDV.

Surface Display of Peptides Corresponding to the Heptad Repeat 2 Domain of the Feline Enteric Coronavirus Spike Protein on Bacillus subtilis Spores Elicits Protective Immune Responses Against Homologous Infection in a Feline Aminopeptidase-N-Transduced Mouse Model

Although feline coronavirus (FCoV) infection is extremely common in cats, there are currently few effective treatments. A peptide derived from the heptad repeat 2 (HR2) domain of the coronavirus (CoV) spike protein has shown effective for inhibition of various human and animal CoVs in vitro, but further use of FCoV-HR2 in vivo has been limited by lack of practical delivery vectors and small animal infection model. To overcome these technical challenges, we first constructed a recombinant Bacillus subtilis (rBS^CotB-HR2P) expressing spore coat protein B (CotB) fused to an HR2-derived peptide (HR2P) from a serotype II feline enteric CoV (FECV). Immunogenic capacity was evaluated in mice after intragastric or intranasal administration, showing that recombinant spores could trigger strong specific cellular and humoral immune responses. Furthermore, we developed a novel mouse model for FECV infection by transduction with its primary receptor (feline aminopeptidase N) using an E1/E3-deleted adenovirus type 5 vector. This model can be used to study the antiviral immune response and evaluate vaccines or drugs, and is an applicable choice to replace cats for the study of FECV. Oral administration of rBS^CotB-HR2P in this mouse model effectively protected against FECV challenge and significantly reduced pathology in the digestive tract. Owing to its safety, low cost, and probiotic features, rBS^CotB-HR2P is a promising oral vaccine candidate for use against FECV/FCoV infection in cats.

Systemic Homologous Neutralizing Antibodies Are Inadequate for the Evaluation of Vaccine Protective Efficacy against Coinfection by High Virulent PEDV and PRRSV

G2 porcine epidemic diarrhea virus (G2 PEDV) and highly pathogenic porcine reproductive and respiratory syndrome virus 2 (HP-PRRSV2) are two of the most prevalent swine pathogens in China's swine herds, and their coinfection occurs commonly. Several PED and PRRS vaccines have been utilized in China for decades, and systemic homologous neutralizing antibodies (shnAbs) in serum are frequently used to evaluate the protective efficacy of PED and PRRS vaccines. To develop a vaccine candidate against G2 PEDV and HP-PRRSV2 coinfection, in this study, we generated a chimeric virus (rJSTZ1712-12-S) expressing S protein of G2 PEDV using an avirulent HP-PRRSV2 rJSTZ1712-12 infectious clone as the viral vector. The rJSTZ1712-12-S strain has similar replication efficacies as the parental rJSTZ1712-12 virus. In addition, animal inoculation indicated that rJSTZ1712-12-S is not pathogenic to piglets and can induce shnAbs against both G2 PEDV and HP-PRRSV2 isolates after prime-boost immunization. However, passive transfer study in neonatal piglets deprived of sow colostrum showed that rJSTZ1712-12-S-induced shnAbs may only decrease PEDV and PRRSV viremia but cannot confer sufficient protection against dual challenge of high virulent G2 PEDV XJ1904-34 strain and HP-PRRSV2 XJ17-5 isolate. Overall, this study provides the first evidence that shnAbs confer insufficient protection against PEDV and PRRSV coinfection and are inadequate for the evaluation of protective efficacy of PED and PRRS bivalent vaccine (especially for the PED vaccine). IMPORTANCE Porcine epidemic diarrhea virus (PEDV) and porcine reproductive and respiratory syndrome virus (PRRSV) coinfection occurs commonly and can synergistically reduce feed intake and pig growth. Vaccination is an effective strategy utilized for PED and PRRS control, and systemic homologous neutralizing antibodies (shnAbs) in serum are commonly used for protective efficacy evaluation of PED and PRRS vaccines. Currently, no commercial vaccine is available against PEDV and PRRSV coinfection. This study generated a chimeric vaccine candidate against the coinfection of prevalent PEDV and PRRSV in China. The chimeric strain can induce satisfied shnAbs against both PEDV and PRRSV after prime-boost inoculation in pigs. But the shnAbs cannot confer sufficient protection against PEDV and PRRSV coinfection in neonatal piglets. To the best of our knowledge, these findings provide the first evidence that shnAbs confer insufficient protection against PEDV and PRRSV coinfection and are inadequate for evaluating PED and PRRS bivalent vaccine protective efficacy.

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.

Evaluation of the Immunogenicity in Mice Orally Immunized with Recombinant Lactobacillus casei Expressing Porcine Epidemic Diarrhea Virus S1 Protein

Porcine epidemic diarrhea (PED), characterized by diarrhea, vomiting, and dehydration, is an acute enteric infectious disease of pigs. The disease is caused by porcine epidemic diarrhea virus (PEDV), which infects the intestinal mucosal surface. Therefore, mucosal immunization through the oral route is an effective method of immunization. Lactic acid bacteria, which are acid resistant and bile-salt resistant and improve mucosal immunity, are ideal carriers for oral vaccines. The S1 glycoprotein of PEDV mediates binding of the virus with cell receptors and induces neutralizing antibodies against the virus. Therefore, we reversely screened the recombinant strain pPG-SD-S1/Δupp ATCC 393 expressing PEDV S1 glycoprotein by Lactobacillus casei deficient in upp genotype (Δupp ATCC 393). Mice were orally immunized three times with the recombinant bacteria that had been identified for expression, and the changes of anti-PEDV IgG and secreted immunoglobulin A levels were observed over 70 days. The results indicated that the antibody levels notably increased after oral administration of recombinant bacteria. The detection of extracellular cytokines on the 42nd day after immunization indicated high levels of humoral and cellular immune responses in mice. The above results demonstrate that pPG-SD-S1/Δupp ATCC 393 has great potential as an oral vaccine against PEDV.

Stable trimer formation of spike protein from porcine epidemic diarrhea virus improves the efficiency of secretory production in silkworms and induces neutralizing antibodies in mice

Porcine epidemic diarrhea virus (PEDV) is a highly infectious pathogen of watery diarrhea that causes serious economic loss to the swine industry worldwide. Especially because of the high mortality rate in neonatal piglets, a vaccine with less production cost and high protective effect against PEDV is desired. The intrinsically assembled homotrimer of spike (S) protein on the PEDV viral membrane contributing to the host cell entry is a target of vaccine development. In this study, we designed trimerized PEDV S protein for efficient production in the silkworm-baculovirus expression vector system (silkworm-BEVS) and evaluated its immunogenicity in the mouse. The genetic fusion of the trimeric motif improved the expression of S protein in silkworm-BEVS. A small-scale screening of silkworm strains to further improve the S protein productivity finally achieved the yield of about 2 mg from the 10 mL larval serum. Mouse immunization study demonstrated that the trimerized S protein could elicit strong humoral immunity, including the S protein-specific IgG in the serum. These sera contained neutralizing antibodies that can protect Vero cells from PEDV infection. These results demonstrated that silkworm-BEVS provides a platform for the production of trimeric S proteins, which are promising subunit vaccines against coronaviruses such as PEDV.

Expression of the human or porcine C-type lectins DC-SIGN/L-SIGN confers susceptibility to porcine epidemic diarrhea virus entry and infection in otherwise refractory cell lines

Porcine epidemic diarrhea virus (PEDV) is an alphacoronavirus that causes great economic losses in the porcine industry. Although the functional receptor for the virus has not been identified, multiple isolates are able to infect different cell lines. Recently, it has been shown that the human C-type lectin DC-SIGN/L-SIGN (hDC-SIGN/L-SIGN) can promote entry of several coronaviruses. Here we examined whether hDC-SIGN/L-SIGN and its porcine homolog (pDC-SIGN) are entry determinants for PEDV. Expression of hDC-SIGN/L-SIGN or pDC-SIGN in refractory cells dramatically increased infection by a recombinant PEDV expressing green fluorescent protein. In both cases, lectin-mediated infection was inhibited by mannan or anti-hDC-SIGN/L-SIGN or pDC-SIGN antibodies; however, d-galactose had no effect on the virus-infected cells. Our results demonstrate that hDC-SIGN/L-SIGN or pDC-SIGN can mediate the cellular entry and propagation of PEDV, which provides a new theoretical basis for further understanding the infection mechanism of PEDV, and will be helpful for the development of novel therapeutic agents.

Antiviral peptides against Coronaviridae family: A review

The Coronaviridae family comprises large enveloped single-stranded RNA viruses. The known human-infecting coronaviruses; severe acute respiratory syndrome coronavirus (SARS-CoV), Middle East respiratory syndrome coronavirus (MERS-CoV), novel SARS-CoV-2, human coronavirus (HCoV)-NL63, HCoV-229E, HCoV-OC43 and HKU1 cause mild to severe respiratory infections. The viral diseases induced by mammalian and avian viruses from Coronaviridae family pose significant economic and public health burdens. Due to increasing reports of viral resistance, co-infections and the emergence of viral epidemics such as COVID-19, available antiviral drugs show low or no efficacy, and the production of new treatments or vaccines are also challenging. Therefore, demand for the development of novel antivirals has considerably increased. In recent years, antiviral peptides have generated increasing interest as they are from natural and computational sources, are highly specific and effective, and possess the broad-spectrum activity with minimum side effects. Here, we have made an effort to compile and review the antiviral peptides with activity against Coronaviridae family viruses. They were divided into different categories according to their action mechanisms, including binding/attachment inhibitors, fusion and entry inhibitors, viral enzyme inhibitors, replication inhibitors and the peptides with direct and indirect effects on the viruses. Reported studies suggest optimism with regard to the design and production of therapeutically promising antiviral drugs. This review aims to summarize data relating to antiviral peptides particularly with respect to their applicability for development as novel treatments.

Significant Inhibition of Porcine Epidemic Diarrhea Virus In Vitro by Remdesivir, Its Parent Nucleoside and β-D-N4-hydroxycytidine

Porcine epidemic diarrhea (PED) caused by porcine epidemic diarrhea virus (PEDV) is widespread in the world. In recent years, the increased virulence of the virus due to viral variations, has caused great economic losses to the pig industry in many countries. It is always worthy to find effective therapeutic methods for PED. As an important class of antivirals, nucleoside drugs which target viral polymerases have been applied in treating human viral infections for half a century. Herein, we evaluated the anti-PEDV potential of three broad-spectrum antiviral nucleoside analogs, remdesivir (RDV), its parent nucleoside (RDV-N) and β-d-N⁴-hydroxycytidine (NHC). Among them, RDV-N was the most active agent in Vero E6 cells with EC₅₀ of 0.31 μmol/L, and more potent than RDV (EC₅₀ = 0.74 μmol/L) and NHC (EC₅₀ = 1.17 μmol/L). The activity of RDV-N was further confirmed using an indirect immuno-fluorescence assay. Moreover, RDV-N exhibited a good safety profile in cells and in mice. The high sequence similarity of the polymerase functional domains of PEDV with other five porcine coronaviruses indicated a broader antiviral spectrum for the three compounds. Generally, RDV-N is a promising broad-spectrum antiviral nucleoside, and it would be worthy to make some structural modifications to increase its oral bioavailability.

Drawing Comparisons between SARS-CoV-2 and the Animal Coronaviruses

The COVID-19 pandemic, caused by a novel zoonotic coronavirus (CoV), SARS-CoV-2, has infected 46,182 million people, resulting in 1,197,026 deaths (as of 1 November 2020), with devastating and far-reaching impacts on economies and societies worldwide. The complex origin, extended human-to-human transmission, pathogenesis, host immune responses, and various clinical presentations of SARS-CoV-2 have presented serious challenges in understanding and combating the pandemic situation. Human CoVs gained attention only after the SARS-CoV outbreak of 2002-2003. On the other hand, animal CoVs have been studied extensively for many decades, providing a plethora of important information on their genetic diversity, transmission, tissue tropism and pathology, host immunity, and therapeutic and prophylactic strategies, some of which have striking resemblance to those seen with SARS-CoV-2. Moreover, the evolution of human CoVs, including SARS-CoV-2, is intermingled with those of animal CoVs. In this comprehensive review, attempts have been made to compare the current knowledge on evolution, transmission, pathogenesis, immunopathology, therapeutics, and prophylaxis of SARS-CoV-2 with those of various animal CoVs. Information on animal CoVs might enhance our understanding of SARS-CoV-2, and accordingly, benefit the development of effective control and prevention strategies against COVID-19.

Porcine epidemic diarrhea virus: Molecular mechanisms of attenuation and vaccines

Porcine epidemic diarrhea virus (PEDV) causes an emerging and re-emerging coronavirus disease characterized by vomiting, acute diarrhea, dehydration, and up to 100% mortality in neonatal suckling piglets, leading to huge economic losses in the global swine industry. Vaccination remains the most promising and effective way to prevent and control PEDV. However, effective vaccines for PEDV are still under development. Understanding the genomic structure and function of PEDV and the influence of the viral components on innate immunity is essential for developing effective vaccines. In the current review, we systematically describe the recent developments in vaccine against PEDV and the roles of structural proteins, non-structural proteins and accessory proteins of PEDV in affecting viral virulence and regulating innate immunity, which will provide insight into the rational design of effective and safe vaccines for PEDV or other coronaviruses.

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Advances in vaccines of PEDV, such as inactivated and attenuated live vaccines, subunit vaccines, and nucleic acid vaccines.

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The application of reverse genetics in the development of live attenuated PEDV vaccines.

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The roles of PEDV proteins in affecting viral virulence and regulating innate immunity.

Coronavirus in human diseases: Mechanisms and advances in clinical treatment

Coronaviruses (CoVs), a subfamily of coronavirinae, are a panel of single-stranded RNA virus. Human coronavirus (HCoV) strains (HCoV-229E, HCoV-OC43, HCoV-HKU1, HCoV-NL63) usually cause mild upper respiratory diseases and are believed to be harmless. However, other HCoVs, associated with severe acute respiratory syndrome, Middle East respiratory syndrome, and COVID-19, have been identified as important pathogens due to their potent infectivity and lethality worldwide. Moreover, currently, no effective antiviral drugs treatments are available so far. In this review, we summarize the biological characters of HCoVs, their association with human diseases, and current therapeutic options for the three severe HCoVs. We also highlight the discussion about novel treatment strategies for HCoVs infections.

Three kinds of treatment with Homoharringtonine, Hydroxychloroquine or shRNA and their combination against coronavirus PEDV in vitro

Background

Porcine epidemic diarrhea virus (PEDV) of the family Coronaviridae has caused substantial economic losses in the swine husbandry industry. There’s currently no specific drug available for treatment of coronaviruses or PEDV.

Method

In the current study, we use coronavirus PEDV as a model to study antiviral agents. Briefly, a fusion inhibitor tHR2, recombinant lentivirus-delivered shRNAs targeted to conserved M and N sequences, homoharringtonine (HHT), and hydroxychloroquine (HCQ) were surveyed for their antiviral effects.

Results

Treatment with HCQ at 50 μM and HHT at 150 nM reduced virus titer in TCID₅₀ by 30 and 3.5 fold respectively, and the combination reduced virus titer in TCID₅₀ by 200 fold.

Conclusion

Our report demonstrates that the combination of HHT and HCQ exhibited higher antiviral activity than either HHT or HCQ exhibited. The information may contribute to the development of antiviral strategies effective in controlling PEDV infection.

Specific recombinant proteins of porcine epidemic diarrhea virus are immunogenic, revealing their potential use as diagnostic markers

Given the highly contagious and acute nature of porcine epidemic diarrhea (PED), especially in piglets, there is an urgent need for the development of rapid and sensitive diagnostic assays. The diagnostic potentials of specific porcine epidemic diarrhea virus (PEDV) accessory and nonstructural proteins, if any, have not yet been investigated. In order to determine and compare which of the viral proteins may be useful as diagnostic antigens, whole virus (WV) particles and a panel of structural and nonstructural PEDV proteins [spike subunit 1 (S1), the C-terminal part of ORF3 (ORF3C), envelope (E), nonstructural protein 1 (Nsp1), Nsp2, Ac (acidic domain of Nsp3), and ADRP (ADP-ribose-1-monophosphatase domain of Nsp3), expressed individually in bacterial and/or mammalian cells] were tested for reactivity with sera from PEDV-infected pigs by ELISA and/or western blot analysis. According to western blots, serum antibody interactions with the S1 protein were relatively more sensitive and specific than ORF3C, E and Ac. Furthermore, a total of 851 serum samples from diarrheal pigs of different ages were analyzed by ELISA, with most showing immune-reactivity towards the WV, S1, ORF3C, and E proteins. The earliest IgG antibody response was observed in the one-week-old piglets, with similar antibody ontogeny and patterns of seroconversion for S1, ORF3C, E, and WV antigens. In addition, the pattern of neutralizing antibody was more similar to that of IgA in weaning piglets after PEDV infection. Collectively, these data provide more reliable information on the host immune response to different viral proteins, which will be useful for development of novel serological assays and for design of vaccines that better stimulate protective immunity.

Porcine deltacoronavirus enters cells via two pathways: A protease-mediated one at the cell surface and another facilitated by cathepsins in the endosome

Porcine deltacoronavirus (PDCoV) is a pathogen belonging to the genus Deltacoronavirus that in 2014 caused outbreaks of piglet diarrhea in the United States. To identify suitable therapeutic targets, a more comprehensive understanding of the viral entry pathway is required, particularly of the role of proteases. Here, we identified the proteases that activate the viral spike (S) glycoprotein to initiate cell entry and also pinpointed the host-cellular pathways that PDCoV uses for entry. Our results revealed that cathepsin L (CTSL) and cathepsin B (CTSB) in lysosomes and extracellular trypsin in cell cultures independently activate the S protein for membrane fusion. Pretreating the cells with the lysosomal acidification inhibitor bafilomycin-A1 (Baf-A1) completely inhibited PDCoV entry, and siRNA-mediated ablation of CTSL or CTSB expression significantly reduced viral infection, indicating that PDCoV uses an endosomal pathway for entry. Of note, trypsin treatment of cell cultures also activated PDCoV entry, even when the endosomal pathway was inhibited. This observation indicated that trypsin-induced S protein cleavage and activation in cell cultures enables viral entry directly from the cell surface. Our results provide critical insights into the PDCoV infection mechanism, uncovering two distinct viral entry pathways: one through cathepsin L and cathepsin B in the endosome and another via a protease at the cell surface. Because PDCoV infection sites represent a proteases-rich environment, these findings suggest that endosome inhibitor treatment alone is insufficient to block PDCoV entry into intestinal epithelial cells in vivo Therefore, approaches that inhibit viral entry from the cell membrane should also be considered.

Identification of a novel linear B-cell epitope within the collagenase equivalent domain of porcine epidemic diarrhea virus spike glycoprotein

The porcine epidemic diarrhea virus (PEDV) collagenase equivalent domain (COE, residues 499-638), a crucial antigenic region within the viral spike (S) glycoprotein, has been widely utilized for the development of subunit vaccines to prevent viral infection. In the current study, we immunized BALB/c mice with recombinant truncated PEDV COE protein and obtained 14 COE-specific monoclonal antibodies (mAbs). Based on the reactivity analysis of the mAbs with two prevalent PEDV strains in G2 type and the attenuated CV777 strain in G1 type, 6 mAbs were selected for subsequent identification of COE mAb-binding epitopes. Dot-blot hybridization and enzyme-linked immunosorbent assays (ELISAs) identified the peptide ⁵⁹²TSLLASACTIDLFGYP⁶⁰⁷ as a novel linear B-cell epitope involved in binding of mAbs 4D8F10 and 6F3E3. Subsequently, alanine (A)-scanning mutagenesis demonstrated that residues 606Y, 605G and 604F were core residues involved in recognition. Importantly, this novel COE epitope, including core residues, is conserved among G1 and G2 type PEDV strains. Further experiment indicates that the mAbs 4D8F10 and 6F3E3 were suitable for PEDV detection via mAb binding to the conserved epitope. The current work actually provides potential uses for the development of diagnostic methods to detect PEDV.

The S Gene Is Necessary but Not Sufficient for the Virulence of Porcine Epidemic Diarrhea Virus Novel Variant Strain BJ2011C

The recently emerged highly virulent variants of porcine epidemic diarrhea virus (PEDV) have caused colossal economic losses to the worldwide swine industry. In this study, we investigated the viral virulence determinants by constructing a series of chimeric mutants between the highly virulent strain BJ2011C and the avirulent strain CHM2013. When tested in the 2-day-old piglet model, wild-type (WT) BJ2011C caused severe diarrhea and death of the piglets within 72 h. In contrast, its chimeric derivative carrying the S gene from CHM2013 (BJ2011C-S_CHM) was avirulent to the piglets. Moreover, reciprocal substitution of the BJ2011C S gene (CHM2013-S_BJ) did not enable CHM2013 to gain any virulence. However, when the whole structural protein-coding region of BJ2011C (CHM2013-SP_BJ) was swapped, CHM2013 started to gain the ability to efficiently colonize the intestinal tract and caused diarrhea in piglets. A further gain of virulence required additional acquisition of the 3' untranslated region (UTR) of BJ2011C, and the resultant virus (CHM2013-SP + 3UTR_BJ) caused more severe diarrhea and death of piglets. Together, our findings suggest that the virulence of PEDV epidemic strains is a multigenic event and that the S gene is only one of the necessary determinants.IMPORTANCE The recently emerged highly virulent PEDV variants are the major cause of the global porcine epidemic diarrhea (PED) pandemic. The S gene of the variants undergoes remarkable variations and has been thought to be the virulence determinant for the enhanced pathogenesis. Our studies here showed that the S gene is only part of the story and that full virulence requires cooperation from other genes. Our findings provide insight into the pathogenic mechanism of the highly virulent PEDV variants and have implications for future vaccine development.

Porcine Deltacoronavirus Engages the Transmissible Gastroenteritis Virus Functional Receptor Porcine Aminopeptidase N for Infectious Cellular Entry

Identification of cellular receptors used by coronavirus (CoV) entry into the host cells is critical to an understanding of pathogenesis and to development of intervention strategies. The fourth CoV genus, Deltacoronavirus, evolutionarily related to the Gammacoronavirus, has just been defined recently. In the current study, we demonstrate that porcine aminopeptidase N (pAPN) acts as a cross-genus CoV functional receptor for both enteropathogenic porcine deltacoronovirus (PDCoV) and alphacoronovirus (AlphaCoV) (transmissible gastroenteritis virus [TGEV]) based upon three lines of evidence. First, the soluble S1 protein of PDCoV bound to the surface of target porcine cell lines known to express pAPN as efficiently as TGEV-S1, which could be blocked by soluble pAPN pretreatment. Second, both PDCoV-S1 and TGEV-S1 physically recognized and interacted with pAPN by coimmunoprecipitation in pAPN cDNA-transfected cells and by dot blot hybridization assay. Finally, exogenous expression of pAPN in refractory cells conferred susceptibility to PDCoV-S1 binding and to PDCoV entry and productive infection. PDCoV-S1 appeared to have a lower pAPN-binding affinity and likely consequent lower infection efficiency in pAPN-expressing refractory cells than TGEV-S1, suggesting that there may be differences between these two viruses in the virus-binding regions in pAPN. This study paves the way for dissecting the molecular mechanisms of PDCoV-host interactions and pathogenesis as well as facilitates future vaccine development and intervention strategies against PDCoV infection.IMPORTANCE The emergence of new human and animal coronaviruses is believed to have occurred through interspecies transmission that is mainly mediated by a species-specific receptor of the host. Among the four genera of the Coronavirinae, a couple of functional receptors for the representative members in the genera Alphacoronavirus and Betacoronavirus have been identified, whereas receptors for Gammacoronavirus and Deltacoronavirus, which are believed to originate from birds, are still unknown. Porcine coronaviruses, including the newly discovered porcine deltacoronavirus (PDCoV) associated with diarrhea in newborn piglets, have posed a serious threat to the pork industry in Asia and North America. Here, we report that PDCoV employs the alphacoronavirus TGEV functional receptor porcine aminopeptidase N (pAPN) for cellular entry, demonstrating the usage of pAPN as a cross-genus CoV functional receptor. The identification of the PDCoV receptor provides another example of the expanded host range of CoV and paves the way for further investigation of PDCoV-host interaction and pathogenesis.

Aminopeptidase-N-independent entry of porcine epidemic diarrhea virus into Vero or porcine small intestine epithelial cells

A monkey cell line Vero (ATCC CCL-81) is commonly used for porcine epidemic diarrhea virus (PEDV) propagation in vitro. However, it is still controversial whether the porcine aminopeptidase N (pAPN) counterpart on Vero cells (Vero-APN) confers PEDV entry. We found that endogenous expression of Vero-APN was undetectable in the mRNA and the protein levels in Vero cells. We cloned the partial Vero-APN gene (3340-bp) containing exons 1 to 9 from cellular DNA and subsequently generated two APN-knockout Vero cell lines by CRISPR/Cas9 approach. PEDV infection of two APN-knockout Vero cells had the same efficiency as the Vero cells with or without neuraminidase treatment. A Vero cells stably expressing pAPN did not increase PEDV production. SiRNA-knockdown of pAPN in porcine jejunum epithelial cells had no effects on PEDV infection. The results suggest that there exists an additional cellular receptor on Vero or porcine jejunal cells independent of APN for PEDV entry.

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Endogenous expression of Vero-APN was not detected in the mRNA and the protein levels in Vero cells.

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PEDV infection of APN-knockout Vero cells by CRISPR/Cas9 had the same efficiency as the Vero cells with or without neuraminidase treatment.

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Overexpression of pAPN in Vero cells did not increase PEDV production.

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Knockdown of pAPN in porcine jejunum epithelial cells decreased TGEV infection level but had no effects on PEDV infection.