Characterization, pathogenicity and protective efficacy of a cell culture-derived porcine deltacoronavirus

Serial cell culture passaging in vitro led to complete attenuation and changes in the characteristic features of a virulent porcine deltacoronavirus strain

Porcine deltacoronavirus (PDCoV) is an important enteric coronavirus that has caused enormous economic losses in the pig industry worldwide. However, no commercial vaccine is currently available. Therefore, developing a safe and efficacious live-attenuated vaccine candidate is urgently needed. In this study, the PDCoV strain CH/XJYN/2016 was continuously passaged in LLC-PK cells until passage 240, and the virus growth kinetics in cell culture, pathogenicity in neonatal piglets, transcriptome differences after LLC-PK infection, changes in the functional characteristics of the spike (S) protein in the high- and low-passage strains, genetic variation of the virus genome, resistance to pepsin and acid, and protective effects of this strain when used as a live-attenuated vaccine were examined. The results of animal experiments demonstrated that the virulent PDCoV strain CH/XJYN/2016 was completely attenuated and not pathogenic in piglets following serial cell passage. Genome sequence analysis showed that amino acid mutations in nonstructural proteins were mainly concentrated in Nsp3, structural protein mutations were mainly concentrated in the S protein, and the N, M, and E genes were conserved. Transcriptome comparison revealed that compared with negative control cells, P10-infected LLC-PK cells had the most differentially expressed genes (DEGs), while P0 and P240 had the least number of DEGs. Analysis of trypsin dependence and related structural differences revealed that the P10 S protein interacted more strongly with trypsin and that the P120 S protein interacted more strongly with the APN receptor. Moreover, the infectivity of P240 was not affected by pepsin but was significantly decreased after exposure to low pH. Furthermore, the P240-based live-attenuated vaccine provided complete protection to piglets against the challenge of virulent PDCoV. In conclusion, we showed that a PDCoV strain was completely attenuated through serial passaging in vitro. These results provide insights into the potential molecular mechanisms of PDCoV attenuation and the development of a promising live-attenuated PDCoV vaccine.IMPORTANCEPorcine deltacoronavirus (PDCoV) is one of the most important enteropathogenic pathogens that cause diarrhea in pigs of various ages, especially in suckling piglets, and causes enormous economic losses in the global commercial pork industry. There are currently no effective measures to prevent and control PDCoV. As reported in previous porcine epidemic diarrhea virus (PEDV) and transmissible gastroenteritis virus studies, inactivated vaccines usually elicit less robust protective immune responses than live-attenuated vaccines in native sows. Therefore, identifying potential attenuation mechanisms, gene evolution, pathogenicity differences during PDCoV passaging, and immunogenicity as live-attenuated vaccines is important for elucidating the mechanism of attenuation and developing safe and effective vaccines for virulent PDCoV strains. In this study, we demonstrated that the virulence of the PDCoV strain CH/XJYN/2016 was completely attenuated following serial cell passaging in vitro, and changes in the biological characteristics and protection efficacy of the strain were evaluated. Our results help elucidate the mechanism of PDCoV attenuation and support the development of appropriate designs for the study of live PDCoV vaccines.

An inactivated PDCoV vaccine induces robust neutralizing antibodies and immune protection in pigs lasting for three months

Porcine deltacoronavirus (PDCoV), a novel enteropathogenic coronavirus, causes diarrhea mainly in suckling piglets and has the potential to infect humans. Whereas, there is no commercially available vaccine which can effectively prevent this disease. In this study, to ascertain the duration of immune protection of inactivated PDCoV vaccine, suckling piglets were injected subcutaneously with inactivated PDCoV vaccine using a prime/boost strategy at 3 and 17-day-old. Neutralizing antibody assay showed that the level of the inactivated PDCoV group was still ≥1:64 at three months after prime vaccination. The three-month-old pigs were orally challenged with PDCoV strain CZ2020. Two pigs in challenge control group showed mild to severe diarrhea at 10-11 day-post-challenge (DPC), while the inactivated PDCoV group had no diarrhea. High levels of viral shedding, substantial intestinal villus atrophy, and positive straining of viral antigens in ileum were detected in challenge control group, while the pigs in inactivated PDCoV group exhibited significantly reduced viral load, minor intestinal villi damage and negative straining of viral antigens. These results demonstrated that PDCoV was pathogenic against three-month-old pigs and inactivated PDCoV vaccine can provide effective protection in pigs lasting for three months.

Evaluation of the immunoprotective effects of porcine deltacoronavirus subunit vaccines

Porcine deltacoronavirus (PDCoV), a new porcine enteric coronavirus, has seriously endangered the pig breeding industry and caused great economic losses. However, a PDCoV vaccine is not commercially available. Therefore, new and efficient PDCoV vaccines must be developed without delay. In this study, we used the ExpiCHO eukaryotic expression system to express and purify the following 3 structural proteins of PDCoV: S, N and M. Subsequently, the level of humoral and cellular immunity induced by the S protein (immunization with the S protein alone) and a protein mixture (immunization with a mixture of S, N and M proteins) were evaluated in mice and piglets, respectively, and the performances of the 2 immunizations in a challenge protection test were assessed in piglets. The results showed that both the S protein and the protein mixture induced the production of high levels of specific IgG antibodies and neutralizing antibodies and effectively neutralized PDCoV-infected LLC-PK cells in vitro. Furthermore, compared with the S protein, the N and M proteins in the protein mixture promoted the expression of CD8+ T cells and IFN-γ, induced a stronger cellular immune response, and effectively protected 4/5 of the piglets from PDCoV infection. In conclusion, the results of this study showed that the N and M proteins play important roles in inducing an immunoprotective response. Using N and M antigens as effective antigenic components in the development of PDCoV vaccines in the future will effectively increase the immune efficacy of the vaccines.

Transcriptome Analysis of LLC-PK Cells Single or Coinfected with Porcine Epidemic Diarrhea Virus and Porcine Deltacoronavirus

Porcine epidemic diarrhea virus (PEDV) and porcine deltacoronavirus (PDCoV) are the two most prevalent swine enteric coronaviruses worldwide. They commonly cause natural coinfections, which worsen as the disease progresses and cause increased mortality in piglets. To better understand the transcriptomic changes after PEDV and PDCoV coinfection, we compared LLC porcine kidney (LLC-PK) cells infected with PEDV and/or PDCoV and evaluated the differential expression of genes by transcriptomic analysis and real-time qPCR. The antiviral efficacy of interferon-stimulated gene 20 (ISG20) against PDCoV and PEDV infections was also assessed. Differentially expressed genes (DEGs) were detected in PEDV-, PDCoV-, and PEDV + PDCoV-infected cells at 6, 12, and 24 h post-infection (hpi), and at 24 hpi, the number of DEGs was the highest. Furthermore, changes in the expression of interferons, which are mainly related to apoptosis and activation of the host innate immune pathway, were found in the PEDV and PDCoV infection and coinfection groups. Additionally, 43 ISGs, including GBP2, IRF1, ISG20, and IFIT2, were upregulated during PEDV or PDCoV infection. Furthermore, we found that ISG20 significantly inhibited PEDV and PDCoV infection in LLC-PK cells. The transcriptomic profiles of cells coinfected with PEDV and PDCoV were reported, providing reference data for understanding the host response to PEDV and PDCoV coinfection.

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.

Adenovirus-vectored PDCoV vaccines induce potent humoral and cellular immune responses in mice

Porcine deltacoronavirus (PDCoV) is a novel swine enteropathogenic coronavirus that causes severe watery diarrhea, vomiting, dehydration and high mortality in piglets, resulting in significant economic losses by the global pig industry. Recently, PDCoV has also shown the potential for cross-species transmission. However, there are currently few vaccine studies and no commercially available vaccines for PDCoV. Hence, here, two novel human adenovirus 5 (Ad5)-vectored vaccines expressing codon-optimized forms of the PDCoV spike (S) glycoprotein (Ad-PD-tPA-Sopt) and S1 glycoprotein (Ad-PD-oriSIP-S1opt) were constructed, and their effects were evaluated via intramuscular (IM) injection in BALB/c mice with different doses and times. Both vaccines elicited robust humoral and cellular immune responses; moreover, Ad-PD-tPA-Sopt-vaccinated mice after two IM injections with 108 infectious units (IFU)/mouse had significantly higher anti-PDCoV-specific neutralizing antibody titers. In contrast, the mice immunized with Ad-PD-tPA-Sopt via oral gavage (OG) did not generate robust systemic and mucosal immunity. Thus, IM Ad-PD-tPA-Sopt administration is a promising strategy against PDCoV and provides useful information for future animal vaccine development.

Nicotinamide Efficiently Suppresses Porcine Epidemic Diarrhea Virus and Porcine Deltacoronavirus Replication

Porcine epidemic diarrhea virus (PEDV) and porcine deltacoronavirus (PDCoV), members of the genus Coronavirus, mainly cause acute diarrhea, vomiting and dehydration in piglets, and thus lead to serious economic losses. In this study, we investigated the effects of nicotinamide (NAM) on PEDV and PDCoV replication and found that NAM treatment significantly inhibited PEDV and PDCoV reproduction. Moreover, NAM plays an important role in replication processes. NAM primarily inhibited PEDV and PDCoV RNA and protein synthesis rather than other processes. Furthermore, we discovered that NAM treatment likely inhibits the replication of PEDV and PDCoV by downregulating the expression of transcription factors through activation of the ERK1/2/MAPK pathway. Overall, this study is the first to suggest that NAM might be not only an important antiviral factor for swine intestinal coronavirus, but also a potential candidate to be evaluated in the context of other human and animal coronaviruses.

Recombinant human adenovirus type 5 based vaccine candidates against GIIa- and GIIb-genotype porcine epidemic diarrhea virus induce robust humoral and cellular response in mice

Porcine epidemic diarrhea virus (PEDV) is a porcine enteropathogenic coronavirus causing severe watery diarrhea, vomiting, dehydration, and death in piglets. However, most commercial vaccines are developed based on the GI genotype strains, and have poor immune protection against the currently dominant GII genotype strains. Therefore, four novel replication-deficient human adenovirus 5-vectored vaccines expressing codon-optimized forms of the GIIa and GIIb strain spike and S1 glycoproteins were constructed, and their immunogenicity was evaluated in mice by intramuscular (IM) injection. All the recombinant adenoviruses generated robust immune responses, and the immunogenicity of recombinant adenoviruses against the GIIa strain was stronger than that of recombinant adenoviruses against the GIIb strain. Moreover, Ad-XT-tPA-Sopt-vaccinated mice elicited optimal immune effects. In contrast, mice immunized with Ad-XT-tPA-Sopt by oral gavage did not induce strong immune responses. Overall, IM administration of Ad-XT-tPA-Sopt is a promising strategy against PEDV, and this study provides useful information for developing viral vector-based vaccines.

Mucosal immune responses induced by oral administration of recombinant Lactococcus lactis expressing the S1 protein of PDCoV

Porcine deltacoronavirus is an evolving coronavirus that primarily infects the intestine and may lead to intestinal disease in piglets. Up to now, no commercial vaccination is readily accessible to protect against the spread of PDCoV. Lactococcus lactis has been shown to have good immune efficacy and safety and can be used as a genetically engineered vaccine to deliver antigens. In this research, we utilized L. lactis NZ9000 to provide the S1 protein orally and improved the delivery efficiency by connecting the M cell targeting ligand Co1 with the S1 protein of PDCoV in tandem to obtain the recombinant protein S1-Co1. We successfully constructed two recombinant strains capable of expressing PDCoV-S1 and PDCoV-S1-Co1 proteins (i.e., L. lactis NZ9000-S1 and L. lactis NZ9000-S1-Co1), and their immunogenic capacity was evaluated in mice. Our study shows that Lactococcus is an advantageous bacterial live vector vaccine and is anticipated as a potential PDCoV vaccination option.

Development of a Nucleocapsid Protein-Based Blocking ELISA for the Detection of Porcine Deltacoronavirus Antibodies

Porcine deltacoronavirus (PDCoV) is an emerging enteropathogen which mainly causes diarrhea, dehydration and death in nursing piglets, threatening the global swine industry. Moreover, it can infect multiple animal species and humans. Hence, reliable diagnostic assays are needed to better control this zoonotic pathogen. Here, a blocking ELISA was developed using a recombinant nucleocapsid (N) protein as the coating antigen paired with an N-specific monoclonal antibody (mAb) as the detection antibody. The percent inhibition (PI) of the ELISA was determined using 384 swine serum samples, with an indirect immunofluorescence assay (IFA) as the reference method. Through receiver operating characteristic analysis in conjunction with Youden’s index, the optimal PI cut-off value was determined to be 51.65%, which corresponded to a diagnostic sensitivity of 98.79% and a diagnostic specificity of 100%. Of the 330 serum samples tested positive via IFA, 326 and 4 were tested positive and negative via the ELISA, respectively, while the 54 serum samples tested negative via IFA were all negative via the ELISA. The overall coincidence rate between the two assays was 98.96% (380/384). The ELISA exhibited good repeatability and did not cross-react with antisera against other swine pathogens. Overall, this is the first report on developing a blocking ELISA for PDCoV serodiagnosis.

An Updated Review of Porcine Deltacoronavirus in Terms of Prevalence, Pathogenicity, Pathogenesis and Antiviral Strategy

The recent experience with SARS-COV-2 has raised our alarm about the cross-species transmissibility of coronaviruses and the emergence of new coronaviruses. Knowledge of this family of viruses needs to be constantly updated. Porcine deltacoronavirus (PDCoV), a newly emerging member of the genus Deltacoronavirus in the family Coronaviridae, is a swine enteropathogen that causes diarrhea in pigs and may lead to death in severe cases. Since PDCoV diarrhea first broke out in the United States in early 2014, PDCoV has been detected in many countries, such as South Korea, Japan and China. More importantly, PDCoV can also infect species other than pigs, and infections have even been reported in children, highlighting its potential for cross-species transmission. A thorough and systematic knowledge of the epidemiology and pathogenesis of PDCoV will not only help us control PDCoV infection, but also enable us to discover the common cellular pathways and key factors of coronaviruses. In this review, we summarize the current knowledge on the prevalence, pathogenicity and infection dynamics, pathogenesis and immune evasion strategies of PDCoV. The existing anti-PDCoV strategies and corresponding mechanisms of PDCoV infection are also introduced, aiming to provide suggestions for the prevention and treatment of PDCoV and zoonotic diseases.

The recombinant pseudorabies virus expressing porcine deltacoronavirus spike protein is safe and effective for mice

BACKGROUND

Porcine deltacoronavirus (PDCoV) is a new pathogenic porcine intestinal coronavirus, which has appeared in many countries since 2012. PDCoV disease caused acute diarrhea, vomiting, dehydration and death in piglets, resulted in significant economic loss to the pig industry. However, there is no commercially available vaccine for PDCoV. In this study, we constructed recombinant pseudorabies virus (rPRVXJ-delgE/gI/TK-S) expressing PDCoV spike (S) protein and evaluated its safety and immunogenicity in mice.

RESULTS

The recombinant strain rPRVXJ-delgE/gI/TK-S obtained by CRISPR/Cas gE gene editing technology and homologous recombination technology has genetic stability in baby hamster syrian kidney-21 (BHK-21) cells and is safe to mice. After immunizing mice with rPRVXJ-delgE/gI/TK-S, the expression levels of IFN-γ and IL-4 in peripheral blood of mice were up-regulated, the proliferation of spleen-specific T lymphocytes and the percentage of CD4⁺ and CD8⁺ lymphocytes in mice spleen was increased. rPRVXJ-delgE/gI/TK-S showed good immunogenicity for mice. On the seventh day after booster immunity, PRV gB and PDCoV S specific antibodies were detected in mice, and the antibody level continued to increase, and the neutralizing antibody level reached the maximum at 28 days post- immunization (dpi). The recombinant strain can protect mice with 100% from the challenge of virulent strain (PRV XJ) and accelerate the detoxification of PDCoV in mice.

CONCLUSION

The recombinant rPRVXJ-delgE/gI/TK-S strain is safe and effective with strong immunogenicity and is expected to be a candidate vaccine against PDCoV and PRV.

Pathogenicity, infective dose and altered gut microbiota in piglets infected with porcine deltacoronavirus

Porcine deltacoronavirus (PDCoV) is an emerging swine enteropathogenic coronavirus that cause severe diarrhea, resulting in high mortality in neonatal piglets. Little is known regarding the pathogenicity of PDCoV in different infective dose and the dynamic changes in the composition of the gut microbiota in PDCoV-induced diarrhea piglets. In this study, 5-day-old piglets were experimentally infected with different dose of PDCoV. The challenged piglets developed typical symptoms, characterized by acute and severe watery diarrhea from 1 to 8 days post-inoculation (DPI), and viral shedding was detected in rectal swab until 11 DPI. Tissues of small intestines displayed significant macroscopic and microscopic lesions with clear viral antigen expression. However, no significant differences among groups were found in challenged piglets. Then alteration in gut microbiota in the jejunum and colon of PDCoV infected-piglets were analyzed using 16S rRNA sequencing. PDCoV infection reduced bacterial diversity and richness, and significantly altered the structure and abundance of the microbiota from the phylum to genus. Fusobacterium, and Proteobacteria was significantly increased (P < 0.05), while the abundance of Bacteroidota was markedly decreased in the infected-piglets. Furthermore, microbial function prediction indicated that the changes in intestinal bacterial also affected the immune system, excretory system, circulatory system, neurodegenerative disease, cardiovascular disease, xenobiotics biodegradation and metabolism, etc. These findings suggest that regulating gut microbiota community may be an effective approach for preventing PDCoV infection.

Antiviral performance of graphene-based materials with emphasis on COVID-19: A review

Coronavirus disease-2019 has been one of the most challenging global epidemics of modern times with a large number of casualties combined with economic hardships across the world. Considering that there is still no definitive cure for the recent viral crisis, this article provides a review of nanomaterials with antiviral activity, with an emphasis on graphene and its derivatives, including graphene oxide, reduced graphene oxide and graphene quantum dots. The possible interactions between surfaces of such nanostructured materials with coronaviruses are discussed. The antiviral mechanisms of graphene materials can be related to events such as the inactivation of virus and/or the host cell receptor, electrostatic trapping and physico-chemical destruction of viral species. These effects can be enhanced by functionalization and/or decoration of carbons with species that enhances graphene-virus interactions. The low-cost and large-scale preparation of graphene materials with enhanced antiviral performances is an interesting research direction to be explored.

The Roles of Apoptosis in Swine Response to Viral Infection and Pathogenesis of Swine Enteropathogenic Coronaviruses

Apoptosis is a tightly regulated mechanism of cell death that plays important roles in various biological processes including biological evolution, multiple system development, anticancer, and viral infections. Swine enteropathogenic coronaviruses invade and damage villous epithelial cells of the small intestine causing severe diarrhea with high mortality rate in suckling piglets. Transmissible gastroenteritis virus (TGEV), Porcine epidemic diarrhea virus (PEDV), Porcine deltacoronavirus (PDCoV), and Swine acute diarrhea syndrome coronavirus (SADS-CoV) are on the top list of commonly-seen swine coronaviruses with a feature of diarrhea, resulting in significant economic losses to the swine industry worldwide. Apoptosis has been shown to be involved in the pathogenesis process of animal virus infectious diseases. Understanding the roles of apoptosis in host responses against swine enteropathogenic coronaviruses infection contribute to disease prevention and control. Here we summarize the recent findings that focus on the apoptosis during swine coronaviruses infection, in particular, TGEV, PEDV, PDCoV, and SADS-CoV.