Molecular biology of transmissible gastroenteritis virus

Molecular and phylogenetic analysis of transmissible gastroenteritis virus strain VET-16, isolated from piglets in Vietnam

Porcine transmissible gastroenteritis virus (TGEV) is a major pathogen that causes viral enteritis and severe diarrhea in newborn piglets. TGEV strains have been isolated in the USA, Europe, and China, and their molecular characteristics are well known. However, there have been few reports of molecular analysis of TGEV strains isolated in Southeast Asia. In 2016, we isolated TGEV strain VET-16 from fecal samples collected from piglets in Vietnam and determined its complete genome sequence by Sanger sequencing. We found that, while the full genome of the VET-16 strain was 92.4-99.9% identical to those of other TGEV strains, the ORF3 gene showed very little sequence similarity. Phylogenetic analysis suggested that the VET-16 strain belongs to the Purdue subgroup. Comparison of the predicted amino acid (aa) sequence of the spike protein of strain VET-16 with those of other TGEV strains revealed three aa substitutions (V378L, S379T, and D380N) and a 3-aa insertion (F383_F387insWEK) in antigenic site D of the VET-16 strain. Also, a single aa deletion (∆F1413) was found in the transmembrane domain of the spike gene of VET-16. Like the ORF3 gene from the TGEV Miller M60 vaccine strain, the VET-16 strain has a large deletion (∆725 nt) in the ORF3 gene. Previous studies have suggested that these mutations in the spike and ORF3 genes might be associated with a reduction in pathogenicity. The data from this study will facilitate further genetic analysis and research into the evolution of TGEV in pigs in Vietnam.

Phloretin inhibits transmissible gastroenteritis virus proliferation via multiple mechanisms

Transmissible gastroenteritis virus (TGEV), an enteropathogenic coronavirus, has caused huge economic losses to the pig industry, with 100% mortality in piglets aged 2 weeks and intestinal injury in pigs of other ages. However, there is still a shortage of safe and effective anti-TGEV drugs in clinics. In this study, phloretin, a naturally occurring dihydrochalcone glycoside, was identified as a potent antagonist of TGEV. Specifically, we found phloretin effectively inhibited TGEV proliferation in PK-15 cells, dose-dependently reducing the expression of TGEV N protein, mRNA, and virus titer. The anti-TGEV activity of phloretin was furthermore refined to target the internalization and replication stages. Moreover, we also found that phloretin could decrease the expression levels of proinflammatory cytokines induced by TGEV infection. In addition, we expanded the potential key targets associated with the anti-TGEV effect of phloretin to AR, CDK2, INS, ESR1, ESR2, EGFR, PGR, PPARG, PRKACA, and MAPK14 with the help of network pharmacology and molecular docking techniques. Furthermore, resistant viruses have been selected by culturing TGEV with increasing concentrations of phloretin. Resistance mutations were reproducibly mapped to the residue (S242) of main protease (Mpro). Molecular docking analysis showed that the mutation (S242F) significantly disrupted phloretin binding to Mpro, suggesting Mpro might be a potent target of phloretin. In summary, our findings indicate that phloretin is a promising drug candidate for combating TGEV, which may be helpful for developing pharmacotherapies for TGEV and other coronavirus infections.

Polygonum cillinerve polysaccharide inhibits transmissible gastroenteritis virus by regulating microRNA-181

Transmissible gastroenteritis virus (TGEV) is an important pathogen capable of altering the expression profile of cellular miRNA. In this study, the potential of Polygonum cillinerve polysaccharide (PCP) to treat TGEV-infected piglets was evaluated through in vivo experiments. High-throughput sequencing technology was employed to identify 9 up-regulated and 17 down-regulated miRNAs during PCP-mediated inhibition of TGEV infection in PK15 cells. Additionally, miR-181 was found to be associated with target genes of key proteins in the apoptosis pathway. PK15 cells were treated with various concentrations of PCP following transfection with miR-181 mimic or inhibitor. Real-time PCR assessed the impact on TGEV replication, while electron microscopy (TEM) and Hoechst fluorescence staining evaluated cellular functionality. Western blot analysis was utilized to assess the expression of key signaling factors-cytochrome C (cyt C), caspase 9, and P53-in the apoptotic signaling pathway. The results showed that compared with the control group, 250 μg/mL PCP significantly inhibited TGEV gRNA replication and gene N expression (P < 0.01). Microscopic examination revealed uniform cell morphology and fewer floating cells in PCP-treated groups (250 and 125 μg/mL). TEM analysis showed no typical virus structure in the 250 μg/mL PCP group, and apoptosis staining indicated a significant reduction in apoptotic cells at this concentration. Furthermore, PCP may inhibit TGEV-induced apoptosis via the Caspase-dependent mitochondrial pathway following miR-181 transfection. These findings provide a theoretical basis for further exploration into the mechanism of PCP's anti-TGEV properties.

DYRK1A is a multifunctional host factor that regulates coronavirus replication in a kinase-independent manner

Coronaviruses (CoVs) pose a major threat to human and animal health worldwide, which complete viral replication by hijacking host factors. Identifying host factors essential for the viral life cycle can deepen our understanding of the mechanisms of virus-host interactions. Based on our previous genome-wide CRISPR screen of α-CoV transmissible gastroenteritis virus (TGEV), we identified the host factor dual-specificity tyrosine phosphorylation-regulated kinase 1A (DYRK1A), but not DYRK1B, as a critical factor in TGEV replication. Rescue assays and kinase inhibitor experiments revealed that the effect of DYRK1A on viral replication is independent of its kinase activity. Nuclear localization signal modification experiments showed that nuclear DYRK1A facilitated virus replication. Furthermore, DYRK1A knockout significantly downregulated the expression of the TGEV receptor aminopeptidase N (ANPEP) and inhibited viral entry. Notably, we also demonstrated that DYRK1A is essential for the early stage of TGEV replication. Transmission electron microscopy results indicated that DYRK1A contributes to the formation of double-membrane vesicles in a kinase-independent manner. Finally, we validated that DYRK1A is also a proviral factor for mouse hepatitis virus, porcine deltacoronavirus, and porcine sapelovirus. In conclusion, our work demonstrated that DYRK1A is an essential host factor for the replication of multiple viruses, providing new insights into the mechanism of virus-host interactions and facilitating the development of new broad-spectrum antiviral drugs.IMPORTANCECoronaviruses, like other positive-sense RNA viruses, can remodel the host membrane to form double-membrane vesicles (DMVs) as their replication organelles. Currently, host factors involved in DMV formation are not well defined. In this study, we used transmissible gastroenteritis virus (TGEV) as a virus model to investigate the regulatory mechanism of dual-specificity tyrosine phosphorylation-regulated kinase 1A (DYRK1A) on coronavirus. Results showed that DYRK1A significantly inhibited TGEV replication in a kinase-independent manner. DYRK1A knockout (KO) can regulate the expression of receptor aminopeptidase N (ANPEP) and endocytic-related genes to inhibit virus entry. More importantly, our results revealed that DYRK1A KO notably inhibited the formation of DMV to regulate the virus replication. Further data proved that DYRK1A is also essential in the replication of mouse hepatitis virus, porcine deltacoronavirus, and porcine sapelovirus. Taken together, our findings demonstrated that DYRK1A is a conserved factor for positive-sense RNA viruses and provided new insights into its transcriptional regulation activity, revealing its potential as a candidate target for therapeutic design.

Diversity for endoribonuclease nsp15-mediated regulation of alpha-coronavirus propagation and virulence

IMPORTANCE

Understanding the role of the endoribonuclease non-structural protein 15 (nsp15) (EndoU) in coronavirus (CoV) infection and pathogenesis is essential for vaccine target discovery. Whether the EndoU activity of CoV nsp15, as a virulence-related protein, has a diverse effect on viral virulence needs to be further explored. Here, we found that the transmissible gastroenteritis virus (TGEV) and feline infectious peritonitis virus (FIPV) nsp15 proteins antagonize SeV-induced interferon-β (IFN-β) production in human embryonic kidney 293 cells. Interestingly, compared with wild-type infection, infection with EnUmt-TGEV or EnUmt-FIPV did not change the IFN-β response or reduce viral propagation in immunocompetent cells. The results of animal experiments showed that EnUmt viruses did not reduce the clinical presentation and mortality caused by TGEV and FIPV. Our findings enrich the understanding of nsp15-mediated regulation of alpha-CoV propagation and virulence and reveal that the conserved functions of nonstructural proteins have diverse effects on the pathogenicity of CoVs.

Characterization and Evaluation of the Pathogenicity of a Natural Gene-Deleted Transmissible Gastroenteritis Virus in China

Porcine transmissible gastroenteritis virus is the major pathogen that causes fatal diarrhea in newborn piglets. In this study, a TGEV strain was isolated from the small intestine of diarrhea piglets in Sichuan Province, China, and designated SC2021. The complete genomic sequence of TGEV SC2021 was 28561 bp, revealing a new natural deletion TGEV strain. Based on phylogenetic analyses, TGEV SC2021 belonged to the Miller cluster and was closely related to CN strains. The newborn piglets orally challenged with TGEV SC2021 showed typical watery diarrhea. In addition, macro and micropathological changes in the lungs and intestines were observed. In conclusion, we isolated a new natural deletion virus strain and confirmed that the virus strain has high pathogenicity in newborn piglets. Moreover, macroscopic and microscopic lesions were observed in the lungs and intestines of all TGEV SC2021-infected piglets. In summary, we isolated a new natural deletion TGEV strain and demonstrated that the natural deletion strain showed high pathogenicity in newborn piglets. These data enrich the diversity of TGEV strains and help us to understand the genetic evolution and molecular pathogenesis of TGEV.

Identification of a novel linear B-cell epitope in porcine deltacoronavirus nucleocapsid protein

Porcine deltacoronavirus (PDCoV) is an emerging swine enteropathogenic coronavirus that caused diarrhea and/or vomiting in neonatal piglets worldwide. Coronaviruses nucleocapsid (N) protein is the most conserved structural protein for viral replication and possesses good antigenicity. In this study, three monoclonal antibodies (mAbs), 3B4, 4D3, and 4E3 identified as subclass IgG2aκ were prepared using the lymphocytic hybridoma technology against PDCoV N protein. Furthermore, the B-cell epitope recognized by mAb 4D3 was mapped by dozens of overlapping truncated recombinant proteins based on the western blotting. The polypeptide 28QFRGNGVPLNSAIKPVE44 (EP-4D3) in the N-terminal of PDCoV N protein was identified as the minimal linear epitope for binding mAb 4D3. And the EP-4D3 epitope's amino acid sequence homology study revealed that PDCoV strains are substantially conserved, with the exception of the Alanine43 substitution Valine43 in the China lineage, the Early China lineage, and the Thailand, Vietnam, and Laos lineage. The epitope sequences shared high similarity (94.1%) with porcine coronavirus HKU15-155 (PorCoV HKU15), Asian leopard cats coronavirus (ALCCoV), sparrow coronavirus HKU17 (SpCoV HKU17), and sparrow deltacoronavirus. In contrast, the epitope sequences shared a very low homology (11.8 to 29.4%) with other porcine CoVs (PEDV, TGEV, PRCV, SADS-CoV, PHEV). Overall, the study will enrich the biological function of PDCoV N protein and provide foundational data for further development of diagnostic applications. KEY POINTS: • Three monoclonal antibodies against PDCoV N protein were prepared. • Discovery of a novel B-cell liner epitope (28QFRGNGVPLNSAIKPVE44) of PDCoV N protein. • The epitope EP-4D3 was conserved among PDCoV strains.

Transmissible Gastroenteritis Virus Nucleocapsid Protein Interacts with Na + /H + Exchanger 3 To Reduce Na + /H + Exchanger Activity and Promote Piglet Diarrhea

A variety of coronaviruses have been found to cause severe diarrhea in hosts, including TGEV; however, the pathogenic mechanism is not clear. Therefore, prompt determination of the mechanism and identification of efficient therapeutic agents are required, both for public health reasons and for economic development.

All-Trans Retinoic Acid Attenuates Transmissible Gastroenteritis Virus-Induced Inflammation in IPEC-J2 Cells via Suppressing the RLRs/NF-κB Signaling Pathway

Transmissible gastroenteritis virus (TGEV) infection can cause transmissible gastroenteritis (TGE), especially in suckling piglets, resulting in a significant economic loss for the global pig industry. The pathogenesis of TGEV infection is closely related to intestinal inflammation. All-trans retinoic acid (ATRA) has anti-inflammatory activity and immunomodulatory properties, but it is unclear whether ATRA can attenuate the inflammatory response induced by TGEV. This study aimed to investigate the protective effect of ATRA on TGEV-induced inflammatory injury in intestinal porcine epithelial cells (IPEC-J2) and to explore the underlying molecular mechanism. The results showed that TGEV infection triggered inflammatory response and damaged epithelial barrier integrity in IPEC-J2 cells. However, ATRA attenuated TGEV-induced inflammatory response by inhibiting the release of pro-inflammatory cytokines, including IL-1β, IL-6, IL-8 and TNF-α. ATRA also significantly reversed the reduction of ZO-1 and Occludin protein levels induced by TGEV infection and maintained epithelial barrier integrity. Moreover, ATRA treatment significantly prevented the upregulation of IкBα and NF-κB p65 phosphorylation levels and the nuclear translocation of NF-кB p65 induced by TGEV. On the other hand, treatment of TGEV-infected IPEC-J2 cells with the NF-κB inhibitors (BAY11-7082) significantly decreased the levels of inflammatory cytokines. Furthermore, ATRA treatment significantly downregulated the mRNA abundance and protein levels of TLR3, TLR7, RIG-I and MDA5, and downregulated their downstream signaling molecules TRIF, TRAF6 and MAVS mRNA expressions in TGEV-infected IPEC-J2 cells. However, the knockdown of RIG-I and MDA5 but not TLR3 and TLR7 significantly reduced the NF-κB p65 phosphorylation level and inflammatory cytokines levels in TGEV-infected IPEC-J2 cells. Our results indicated that ATRA attenuated TGEV-induced IPEC-J2 cells damage via suppressing inflammatory response, the mechanism of which is associated with the inhibition of TGEV-mediated activation of the RLRs/NF-κB signaling pathway.

Genome-scale CRISPR screen identifies TMEM41B as a multi-function host factor required for coronavirus replication

Emerging coronaviruses (CoVs) pose a severe threat to human and animal health worldwide. To identify host factors required for CoV infection, we used α-CoV transmissible gastroenteritis virus (TGEV) as a model for genome-scale CRISPR knockout (KO) screening. Transmembrane protein 41B (TMEM41B) was found to be a bona fide host factor involved in infection by CoV and three additional virus families. We found that TMEM41B is critical for the internalization and early-stage replication of TGEV. Notably, our results also showed that cells lacking TMEM41B are unable to form the double-membrane vesicles necessary for TGEV replication, indicating that TMEM41B contributes to the formation of CoV replication organelles. Lastly, our data from a mouse infection model showed that the KO of this factor can strongly inhibit viral infection and delay the progression of a CoV disease. Our study revealed that targeting TMEM41B is a highly promising approach for the development of broad-spectrum anti-viral therapeutics.

Cell Entry of Animal Coronaviruses

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

The Interaction Between Viruses and Intestinal Microbiota: A Review

As the main pathogen threatening human and animal health, viruses can affect the immunity and metabolism of bodies. There are innate microbial barriers in the digestive tract of the body to preserve the homeostasis of the animal body, which directly or indirectly influences the host defence against viral infection. Understanding the interaction between viruses and intestinal microbiota or probiotics is helpful to study the pathogenesis of diseases. Here, we review recent studies on the interaction mechanism between intestinal microbiota and viruses. The interaction can be divided into two aspects: inhibition of viral infection by microbiota and promotion of viral infection by microbiota. The treatment of viral infection by probiotics is summarized.

APB-13 improves the adverse outcomes caused by TGEV infection by correcting the intestinal microbial disorders in piglets

Porcine transmissible gastroenteritis virus (TGEV) is an enteric coronavirus that has caused high morbidity and mortality of piglets worldwide. Previous studies have shown that the TGEV can lead to severe diarrhoea, vomiting and dehydration in 2-week-old piglets and weaned piglets, resulting in a large number of piglet deaths. Antimicrobial peptides have broad-spectrum antimicrobial activity and a strong killing effect on bacteria, especially on the drug-resistant pathogenic bacteria, and it has attracted broad concern. However, there are very few reports on the effect of APB-13 (an antimicrobial peptide) on the intestinal microbes of piglets infected with TGEV. In this study, 16S rRNA gene sequencing was used to compare the microbial phylum and the genus of piglet's enteric microorganism in different experimental groups, and to predict the metabolic function of the microbial flora. At the same time, the apparent digestibility of nutrients, digestive enzyme activity, daily weight gain and survival rate were also measured. TGEV infection could cause the imbalance of intestinal microbes in piglets, and increase of the relative abundance of Proteobacteria, and decrease of the relative abundance of Firmicutes, Bacteroidetes and Actinobacteri. With the addition of APB-13, this problem can be alleviated, which can reduce the relative abundance of Proteobacteria and improve the balance of intestinal microorganisms. At the microbial genus level, after adding APB-13, the relative abundance of Catenibacterium, Enterobacter and Streptococcus in the intestinal tract of piglets infected with TGEV showed significant decrease, while the relative abundance of Lactobacillus and Ruminococcus increased. Finally, we found that APB-13 can significantly increase the activity of digestive enzyme in the intestinal tract of piglet, thereby improving the apparent digestibility of nutrients and the growth performance of piglets. This study demonstrates that APB-13 can alleviate the adverse outcomes caused by TGEV infection by correcting the intestinal microbial disorders.

Similarities and Dissimilarities of COVID-19 and Other Coronavirus Diseases

In less than two decades, three deadly zoonotic coronaviruses, severe acute respiratory syndrome coronavirus (SARS-CoV), Middle East respiratory syndrome coronavirus (MERS-CoV), and SARS-CoV-2, have emerged in humans, causing SARS, MERS, and coronavirus disease 2019 (COVID-19), respectively. The current COVID-19 pandemic poses an unprecedented crisis in health care and social and economic development. It reinforces the cruel fact that CoVs are constantly evolving, possessing the genetic malleability to become highly pathogenic in humans. In this review, we start with an overview of CoV diseases and the molecular virology of CoVs, focusing on similarities and differences between SARS-CoV-2 and its highly pathogenic as well as low-pathogenic counterparts. We then discuss mechanisms underlying pathogenesis and virus-host interactions of SARS-CoV-2 and other CoVs, emphasizing the host immune response. Finally, we summarize strategies adopted for the prevention and treatment of CoV diseases and discuss approaches to develop effective antivirals and vaccines. Expected final online publication date for the Annual Review of Microbiology, Volume 75 is September 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Single-Blinded Study Highlighting the Differences between the Small Intestines of Neonatal and Weaned Piglets

Simple Summary

The gut mucosa of pigs, which contains intestinal epithelium and subepithelial immune cells, forms a barrier against microorganisms. Nonetheless, infectious diseases of the digestive tract remain the most frequent and recurrent conditions in the swine industry. Changes in intestinal morphology and structure primarily occur at birth and during weaning. However, the difference in the intestinal structures between neonatal and weaned piglets remains unclear. In this study, for the first time, we evaluated the differences in the small intestine between neonatal (0-day-old) and weaned piglets (21-day-old) and analyzed the morphology and immunological components of the small intestines of 0- and 21-day-old piglets, thereby providing preliminary data for future mechanistic studies.

Abstract

The gut is one of the body’s major immune structures, and the gut mucosa, which contains intestinal epithelium and subepithelial immune cells, is the primary site for eliciting local immune responses to foreign antigens. Intestinal immune system development in pigs is a transitional period during birth and weaning. This study compares the morphological and immunological differences in the small intestine of neonatal and weaned piglets to potentially prevent intestinal infectious diseases in neonatal piglets. Histological analyses of weaned piglet intestines showed increased crypt depth, higher IEL count, and larger ileal Peyer’s patches compared with those of neonates. Additionally, the ileal villi of weaned piglets were longer than those of neonatal piglets, and claudin-3 protein expression was significantly higher in weaned than in neonatal piglets. The numbers of CD3⁺ T, goblet, and secretory cells were also higher in the small intestines of weaned piglets than in those of neonates. No significant differences were observed in the secretory IgA-positive cell number in the jejunum of weaned and neonatal piglets. The mRNA expression of most pattern recognition receptors genes in the duodenum and jejunum was higher in the weaned than neonatal piglets; however, the opposite was true in the ileum. The mRNA levels of IL-1β and TNF-α in the jejunal and ileal mucosa were higher in weaned piglets than in neonatal piglets. There were significantly fewer CD3⁺, CD4⁺, and CD8⁺ T cells from peripheral blood-mononuclear cells in neonatal piglets. Our study provides insights regarding the different immune mechanisms within the small intestines of 0- and 21-day-old piglets. Studies on the additional developmental stages and how differences in the small intestines affect the response of pigs to pathogens remain warranted.

Antiviral effects of Bovine antimicrobial peptide against TGEV in vivo and in vitro

Background

In suckling piglets, transmissible gastroenteritis virus (TGEV) causes lethal diarrhea accompanied by high infection and mortality rates, leading to considerable economic losses. This study explored methods of preventing or inhibiting their production. Bovine antimicrobial peptide-13 (APB-13) has antibacterial, antiviral, and immune functions.

Objectives

This study analyzed the efficacy of APB-13 against TGEV through in vivo and in vitro experiments.

Methods

The effects of APB-13 toxicity and virus inhibition rate on swine testicular (ST) cells were detected using 3-(4,5-dimethylthiazol-2-yl)-2, 5-diphenyl tetrazolium bromide (MTT). The impact of APB-13 on virus replication was examined through the 50% tissue culture infective dose (TCID₅₀). The mRNA and protein levels were investigated by real-time quantitative polymerase chain reaction and western blot (WB). Tissue sections were used to detect intestinal morphological development.

Results

The safe and effective concentration range of APB-13 on ST cells ranged from 0 to 62.5 µg/mL, and the highest viral inhibitory rate of APB-13 was 74.1%. The log₁₀TCID₅₀ of 62.5 µg/mL APB-13 was 3.63 lower than that of the virus control. The mRNA and protein expression at 62.5 µg/mL APB-13 was significantly lower than that of the virus control at 24 hpi. Piglets in the APB-13 group showed significantly lower viral shedding than that in the virus control group, and the pathological tissue sections of the jejunum morphology revealed significant differences between the groups.

Conclusions

APB-13 exhibited good antiviral effects on TGEV in vivo and in vitro.

Evolution of Transmissible Gastroenteritis Virus (TGEV): A Codon Usage Perspective

Transmissible gastroenteritis virus (TGEV) is a coronavirus associated with diarrhea and high mortality in piglets. To gain insight into the evolution and adaptation of TGEV, a comprehensive analysis of phylogeny and codon usage bias was performed. The phylogenetic analyses of maximum likelihood and Bayesian inference displayed two distinct genotypes: genotypes I and II, and genotype I was classified into subtypes Ia and Ib. The compositional properties revealed that the coding sequence contained a higher number of A/U nucleotides than G/C nucleotides, and that the synonymous codon third position was A/U-enriched. The principal component analysis based on the values of relative synonymous codon usage (RSCU) showed the genotype-specific codon usage patterns. The effective number of codons (ENC) indicated moderate codon usage bias in the TGEV genome. Dinucleotide analysis showed that CpA and UpG were over-represented and CpG was under-represented in the coding sequence of the TGEV genome. The analyses of Parity Rule 2 plot, ENC-plot, and neutrality plot displayed that natural selection was the dominant evolutionary driving force in shaping codon usage preference in genotypes Ia and II. In addition, natural selection played a major role, while mutation pressure had a minor role in driving the codon usage bias in genotype Ib. The codon adaptation index (CAI), relative codon deoptimization index (RCDI), and similarity index (SiD) analyses suggested that genotype I might be more adaptive to pigs than genotype II. Current findings contribute to understanding the evolution and adaptation of TGEV.

Circular RNAs: New Epigenetic Signatures in Viral Infections

Covalent closed circular RNAs (circRNAs) can act as a bridge between non-coding RNAs and coding messenger RNAs. CircRNAs are generated by a back-splicing mechanism during post-transcriptional processing and are abundantly expressed in eukaryotic cells. CircRNAs can act via the modulation of RNA transcription and protein production, and by the sponging of microRNAs (miRNAs). CircRNAs are now thought to be involved in many different biological and pathological processes. Some studies have suggested that the expression of host circRNAs is dysregulated in several types of virus-infected cells, compared to control cells. It is highly likely that viruses can use these molecules for their own purposes. In addition, some viral genes are able to produce viral circRNAs (VcircRNA) by a back-splicing mechanism. However, the viral genes that encode VcircRNAs, and their functions, are poorly studied. In this review, we highlight some new findings about the interaction of host circRNAs and viral infection. Moreover, the potential of VcircRNAs derived from the virus itself, to act as biomarkers and therapeutic targets is summarized.

Dynamics of transmissible gastroenteritis virus internalization unraveled by single-virus tracking in live cells

Transmissible gastroenteritis virus (TGEV) is a swine enteropathogenic coronavirus that causes significant economic losses in swine industry. Current studies on TGEV internalization mainly focus on viral receptors, but the internalization mechanism is still unclear. In this study, we used single‐virus tracking to obtain the detailed insights into the dynamic events of the TGEV internalization and depict the whole sequential process. We observed that TGEVs could be internalized through clathrin‐ and caveolae‐mediated endocytosis, and the internalization of TGEVs was almost completed within ~2 minutes after TGEVs attached to the cell membrane. Furthermore, the interactions of TGEVs with actin and dynamin 2 in real time during the TGEV internalization were visualized. To our knowledge, this is the first report that single‐virus tracking technique is used to visualize the entire dynamic process of the TGEV internalization: before the TGEV internalization, with the assistance of actin, clathrin, and caveolin 1 would gather around the virus to form the vesicle containing the TGEV, and after ~60 seconds, dynamin 2 would be recruited to promote membrane fission. These results demonstrate that TGEVs enter ST cells via clathrin‐ and caveolae‐mediated endocytic, actin‐dependent, and dynamin 2‐dependent pathways.

Prevalence and phylogenetic analysis of porcine diarrhea associated viruses in southern China from 2012 to 2018

Background

In China, large-scale outbreaks of severe diarrhea caused by viruses have occurred in pigs since late 2010. To investigate the prevalence and genetic evolution of diarrhea-associated viruses responsible for the outbreaks, a total of 2987 field diarrheal samples collected from 168 pig farms in five provinces in Southern China during 2012–2018 were tested.

Results

Porcine epidemic diarrhea virus (PEDV) was most frequently detected virus with prevalence rates between 50.21 and 62.10% in samples, and 96.43% (162/168) in premises, respectively. Porcine deltacoronavirus (PDCoV) was the second prevalent virus with prevalence rates ranging from 19.62 to 29.19% in samples, and 70.24% (118/168) in premises, respectively. Both transmissible gastroenteritis virus (TGEV) and porcine rotavirus (PoRV) were detected at low prevalence rates of < 3% in samples and 10.12% in premises. In this study, we identified a newly emerged swine acute diarrhea syndrome coronavirus (SADS-CoV) in diarrheal samples of piglets from Fujian province in Southern China, and the prevalence rate of SADS-CoV was 10.29% (7/68). Co-infections of these diarrhea-associated viruses were common. The most frequent co-infection was PEDV with PDCoV, with an average detection rate of 12.72% (380/2987, ranging from 8.26–17.33%). Phylogenetic analysis revealed that PEDVs circulating in Southern China during the last 7 years were clustered with the variant strains of PEDV in genotype IIa. The most frequent mutations were present in the collagenase equivalent (COE) and epitope regions of the spike gene of the PEDVs currently circulating in the field. Genetic relationships of PDCoVs were closely related with Chinese strains, other than those present in the USA, South Korea, Thailand and Lao’s public.

Conclusions

The findings of this study indicated that variant PEDV, PDCoV, and SADS-CoV were leading etiologic agents of porcine diarrhea, and either mono-infections or co-infections of pathogenic enteric CoVs were common in pigs in Southern China during 2012–2018. Thus, significant attention should be paid in order to effectively prevent and control porcine viral diarrhea.

Porcine transmissible gastroenteritis virus nonstructural protein 2 contributes to inflammation via NF-κB activation

Transmissible gastroenteritis virus (TGEV) infection causes acute enteritis in swine of all ages, and especially in suckling piglets. Small intestinal inflammation is considered a central event in the pathogenesis of TGEV infections, and nuclear factor-kappa B (NF-κB) is a key transcription factor in the inflammatory response. However, it is unclear whether NF-κB is crucial for inducing inflammation during a TGEV infection. Our results show that NF-κB was activated in swine testicular (ST) cells and intestinal epithelial cell lines J2 (IPEC-J2) cells infected with TGEV, which is consistent with the up-regulation of pro-inflammatory cytokines. Treatment of TGEV-infected ST cells and IPEC-J2 cells with the NF-κB-specific inhibitor caused the down-regulation of pro-inflammatory cytokine expression, but did not significantly affect TGEV replication. Individual TGEV protein screening results demonstrated that Nsp2 exhibited a high potential for activating NF-κB and enhancing the expression of pro-inflammatory cytokines. Functional domain analyzes indicated that the first 120 amino acid residues of Nsp2 were essential for NF-κB activation. Taken together, these data suggested that NF-κB activation was a major contributor to TGEV infection-induced inflammation, and that Nsp2 was the key viral protein involved in the regulation of inflammation, with amino acids 1–120 playing a critical role in activating NF-κB.

Abbreviations: TCID50: 50% tissue culture infectious dose; DMEM: Dulbecco’s Modified Eagle Medium; eNOS: Endothelial nitric oxide synthase; FBS: fetal bovine serum; IFA: Indirect immunofluorescence; IκB: inhibitor of nuclear factor kappa-B; IL: interleukin; IPEC-J2: intestinal epithelial cell lines J2; IKK: IκB kinase; Luc: luciferase reporter gene; mAbs: monoclonal antibodies; MOI: multiple of infection; Nsp: nonstructural protein; NF-κB: nuclear factor-kappa ; ORFs: open reading frames; PBS: phosphate-buffered saline; p65 p-p65: phosphorylated; RT-PCR: reverse transcription PC; SeV: Sendai virus; ST: swine testicular; TGEV: Transmissible gastroenteritis virus; TNF-α: tumor necrosis factor α; UV-TGEV: Ultraviolet light-inactivated TGEV; ZnF: zinc finger

The N-Terminal Domain of Spike Protein Is Not the Enteric Tropism Determinant for Transmissible Gastroenteritis Virus in Piglets

Transmissible gastroenteritis virus (TGEV) is the etiologic agent of transmissible gastroenteritis in pigs, and the N-terminal domain of TGEV spike protein is generally recognized as both the virulence determinant and enteric tropism determinant. Here, we assembled a full-length infectious cDNA clone of TGEV in a bacterial artificial chromosome. Using a novel approach, the clustered regularly interspaced short palindromic repeat (CRISPR)/CRISPR-associated protein 9 (Cas9) systems efficiently and rapidly rescued another recombinant virus with a 224-amino-acid deletion in the N-terminal domain of the TGEV Spike gene (S_NTD224), which is analogous to the N-terminal domain of porcine respiratory coronavirus. S_NTD224 notably affected the TGEV growth kinetics in PK-15 cells but was not essential for recombinant virus survival. In animal experiments with 13 two-day-old piglets, the TGEV recombinant viruses with/without S_NTD224 deletion induced obvious clinical signs and mortality. Together, our results directly demonstrated that S_NTD224 of TGEV mildly influenced TGEV virulence but was not the enteric tropism determinant and provide new insights for the development of a new attenuated vaccine against TGEV. Importantly, the optimized reverse genetics platform used in this study will simplify the construction of mutant infectious clones and help accelerate progress in coronavirus research.

Establishment of porcine enterocyte/myofibroblast co-cultures for the growth of porcine rota- and coronaviruses

A stable culture of primary porcine enterocytes is necessary to study porcine enteric virus replication characteristics. Because the direct cultivation of primary porcine enterocytes is difficult, alternatives have to be considered. As subepithelial myofibroblasts secrete extracellular matrix and growth factors contributing to the attachment, proliferation and differentiation of epithelial cells, co-cultures of primary porcine enterocytes (ileocytes and colonocytes) with myofibroblasts were developed and evaluated for their susceptibility to enteric viruses. First, it was demonstrated that the co-cultured ileocytes and colonocytes were susceptible to an archival rotavirus strain RVA/pig-tc/BEL/RV277/1977/G1P[7] and different other rotavirus genotypes (fecal samples containing G5P[7], G5P[13], G9P[23], G4P[6]). Next, the TGEV Purdue strain infected both ileocytes and colonocytes whereas the Miller strain only infected ileocytes. Last, the PEDV CV777 Vero adapted and non-adapted (fecal suspension) strains could infect co-cultured ileocytes but not colonocytes. The infectivity of the CV777 Vero adapted strain was higher when the cells were cultured without fetal bovine serum and the CV777 fecal suspension only infected the ileocytes cultured without fetal bovine serum. In conclusion, a novel co-culture of porcine enterocytes with myofibroblasts was established, which can be used for the investigation of the replication of enteric viruses.

Impact of TGEV infection on the pig small intestine

Background

Pig diarrhea causes high mortality and large economic losses in the swine industry. Transmissible gastroenteritis virus (TGEV) causes pig diarrhea, with 100% mortality in piglets less than 2 weeks old. No investigation has yet been made of the small intestine of piglets that survived infection by TGEV.

Methods

In this study, we evaluated the impact of TGEV infection on the small intestine of recovered pigs.

Results

Histological analyses showed that TGEV infection led to villi atrophy, and reduced villous height and crypt depth. The number of SIgA positive cells, CD3⁺T cells, and dendritic cells (DCs) in jejunum decreased after TGEV infection in vivo. In contrast, microfold cell (M cell) numbers and cell proliferation increased in infected pigs. TGEV infection also significantly enhanced the mRNA expression levels of cytokine IL-1β, IL-6, TNF-α, IL-10, and TGF-β. Additionally, lower gene copy numbers of Lactobacillus, and higher numbers of Enterobacteriaceae, were detected in mucosal scraping samples from TGEV-infected pigs.

Conclusions

TGEV infection damages the small intestine, impairs immune functions, and increases pathogenic bacterial loading, all of which may facilitate secondary infections by other pathogens. These findings help quantify the impact of TGEV infection and clarify the pathogenic mechanisms underlying its effects in pigs.

A nano silicon adjuvant enhances inactivated transmissible gastroenteritis vaccine through activation the Toll-like receptors and promotes humoral and cellular immune responses

Inactivated transmissible gastroenteritis virus (TGEV) vaccines are widely used in swine herds in China. These are limited, however, by the need to elicit both humoral and cellular immunity, as well as the efficiency of adjuvants. In this study, a 70-nm nano silicon particle was applied with inactivated TGEV vaccine in mice, and its immune-enhancing effects and mechanism of action investigated. We found that nano silicon applied with inactivated TGEV vaccine induced high antibody titers, increase IL-6, TNF-α and IFN-γ expression, and stimulate CD3+ T cell proliferation with a high CD4+/CD8+ T lymphocyte ratio. Nano silicon could quickly activate innate and adaptive immunity by stimulating Toll-like receptor signaling pathways, indicating that the nano silicon adjuvant enhanced long-term humoral and early cellular immune responses when combined with inactivated TGEV vaccine. Nano silicon could be considered for use as an antigen- carrier and adjuvant for veterinary vaccines.

Inhibition of porcine transmissible gastroenteritis virus infection in porcine kidney cells using short hairpin RNAs targeting the membrane gene

The membrane (M) protein is the most abundant component of the porcine transmissible gastroenteritis virus (TGEV) particle. To exploit the possibility of using RNA interference (RNAi) as a strategy against TGEV infection, three plasmids (pRNAT-1, pRNAT-2, and pRNAT-3) expressing short hairpin RNAs were designed to target three different coding regions of the M gene of TGEV. The plasmids were constructed and transiently transfected into a porcine kidney cells, PK-15, to determine whether these constructs inhibited TGEV production. The analysis of cytopathic effects demonstrated that pRNAT-2 and pRNAT-3 could protect PK-15 cells against pathological changes specifically and efficiently. Additionally, indirect immunofluorescence and 50% tissue culture infectious dose (TCID50) assays showed that pRNAT-2 and pRNAT-3 inhibited the multiplication of the virus at the protein level effectively. Quantitative real-time PCR further confirmed that the amounts of viral RNAs in cell cultures pre-transfected with the three plasmids were reduced by 13, 68, and 70%, respectively. This is the first report showing that RNAi targeting of the M gene. Our results could promote studies of the specific function of viral genes associated with TGEV infection and might provide a theoretical basis for potential therapeutic applications.

TGEV infection up-regulates FcRn expression via activation of NF-κB signaling

It has been well characterized that the neonatal Fc receptor (FcRn) transports maternal IgG to a fetus or newborn and protects IgG from degradation. We previously reported that FcRn is expressed in a model of normal porcine intestinal epithelial cells (IPEC-J2). Transmissible gastroenteritis is an acute enteric disease of swine that is caused by transmissible gastroenteritis virus (TGEV). How porcine FcRn (pFcRn) expression is regulated by pathogenic infection remains unknown. Our research shows that IPEC-J2 cells infected with TGEV had up-regulated pFcRn expression. In addition, the NF-κB signaling pathway was activated in IPEC-J2 cells by TGEV infection. Furthermore, treatment of TGEV-infected IPEC-J2 cells with the NF-κB-specific inhibitor BAY 11-7082 resulted in down-regulation of pFcRn expression. Transient transfection of pFcRn promoter luciferase report plasmids with overexpression of NF-κB p65 transcription factor enhanced the activation of the luciferase report plasmids. We identified four NF-κB transcription factor binding sites in the promoter region of this gene using luciferase reporter system, chromatin immunoprecipitation, electromobility shift assay, and supershift analysis. Together, the data provide the first evidence that TGEV infection up-regulates pFcRn expression via activation of NF-κB signaling.

Phage display for identifying peptides that bind the spike protein of transmissible gastroenteritis virus and possess diagnostic potential

The spike (S) protein of porcine transmissible gastroenteritis virus (TGEV) is located within the viral envelope and is the only structural protein that possesses epitopes capable of inducing virus-neutralizing antibodies. Among the four N-terminal antigenic sites A, B, C, and D, site A and to a lesser extent site D (S-AD) induce key neutralizing antibodies. Recently, we expressed S-AD (rS-AD) in recombinant form. In the current study, we used the rS-AD as an immobilized target to identify peptides from a phage-display library with application for diagnosis. Among the 9 phages selected that specifically bound to rS-AD, the phage bearing the peptide TLNMHLFPFHTG bound with the highest affinity and was subsequently used to develop a phage-based ELISA for TGEV. When compared with conventional antibody-based ELISA, phage-mediated ELISA was more sensitive; however, it did not perform better than semi-quantitative RT-PCR, though phage-mediated ELISA was quicker and easier to set up.

Receptor recognition mechanisms of coronaviruses: a decade of structural studies

Receptor recognition by viruses is the first and essential step of viral infections of host cells. It is an important determinant of viral host range and cross-species infection and a primary target for antiviral intervention. Coronaviruses recognize a variety of host receptors, infect many hosts, and are health threats to humans and animals. The receptor-binding S1 subunit of coronavirus spike proteins contains two distinctive domains, the N-terminal domain (S1-NTD) and the C-terminal domain (S1-CTD), both of which can function as receptor-binding domains (RBDs). S1-NTDs and S1-CTDs from three major coronavirus genera recognize at least four protein receptors and three sugar receptors and demonstrate a complex receptor recognition pattern. For example, highly similar coronavirus S1-CTDs within the same genus can recognize different receptors, whereas very different coronavirus S1-CTDs from different genera can recognize the same receptor. Moreover, coronavirus S1-NTDs can recognize either protein or sugar receptors. Structural studies in the past decade have elucidated many of the puzzles associated with coronavirus-receptor interactions. This article reviews the latest knowledge on the receptor recognition mechanisms of coronaviruses and discusses how coronaviruses have evolved their complex receptor recognition pattern. It also summarizes important principles that govern receptor recognition by viruses in general.

Quantitative proteomic analysis reveals that transmissible gastroenteritis virus activates the JAK-STAT1 signaling pathway

Transmissible gastroenteritis virus (TGEV), a porcine enteropathogenic coronavirus, causes lethal watery diarrhea and severe dehydration in piglets. In this study, liquid chromatography-tandem mass spectrometry coupled to isobaric tags for relative and absolute quantification labeling was used to quantitatively identify differentially expressed cellular proteins after TGEV infection in PK-15 cells. In total, 162 differentially expressed cellular proteins were identified, including 60 upregulated proteins and 102 downregulated proteins. These differentially expressed proteins were involved in the cell cycle, cellular growth and proliferation, the innate immune response, etc. Interestingly, many upregulated proteins were associated with interferon signaling, especially signal transducer and activator of transcription 1 (STAT1) and interferon-stimulated genes (ISGs). Immunoblotting and real-time quantitative reverse transcription polymerase chain reaction demonstrated that TGEV infection induces STAT1 phosphorylation and nuclear translocation, as well as ISG expression. This study for the first time reveals that TGEV induces interferon signaling from the point of proteomic analysis.

Transmissible gastroenteritis virus: identification of M protein-binding peptide ligands with antiviral and diagnostic potential

The membrane (M) protein is one of the major structural proteins of coronavirus particles. In this study, the M protein of transmissible gastroenteritis virus (TGEV) was used to biopan a 12-mer phage display random peptide library. Three phages expressing TGEV-M-binding peptides were identified and characterized in more depth. A phage-based immunosorbent assay (phage-ELISA) capable of differentiating TGEV from other coronaviruses was developed using one phage, phTGEV-M7, as antigen. When the phage-ELISA was compared to conventional antibody-based ELISA for detecting infections, phage-ELISA exhibited greater sensitivity. A chemically synthesized, TGEV-M7 peptide (pepTGEV-M7; HALTPIKYIPPG) was evaluated for antiviral activity. Plaque-reduction assays revealed that pepTGEV-M7 was able to prevent TGEV infection in vitro (p<0.01) following pretreatment of the virus with the peptide. Indirect immunofluorescence and real-time RT-PCR confirmed the inhibitory effects of the peptide. These results indicate that pepTGEV-M7 might be utilized for virus-specific diagnostics and treatment.

Recent progress in studies of arterivirus- and coronavirus-host interactions

Animal coronaviruses, such as infectious bronchitis virus (IBV), and arteriviruses, such as porcine reproductive and respiratory syndrome virus (PRRSV), are able to manifest highly contagious infections in their specific native hosts, thereby arising in critical economic damage to animal industries. This review discusses recent progress in studies of virus-host interactions during animal and human coronavirus and arterivirus infections, with emphasis on IBV-host cell interactions. These interactions may be directly involved in viral replication or lead to the alteration of certain signaling pathways, such as cell stress response and innate immunity, to facilitate viral replication and pathogenesis.

Plant production of veterinary vaccines and therapeutics

Plant-derived biologicals for use in animal health are becoming an increasingly important target for research into alternative, improved methods for disease control. Although there are no commercial products on the market yet, the development and testing of oral, plant-based vaccines is now beyond the proof-of-principle stage. Vaccines, such as those developed for porcine transmissible gastroenteritis virus, have the potential to stimulate both mucosal and systemic, as well as, lactogenic immunity as has already been seen in target animal trials. Plants are a promising production system, but they must compete with existing vaccines and protein production platforms. In addition, regulatory hurdles will need to be overcome, and industry and public acceptance of the technology are important in establishing successful products.

Intragastric administration of attenuated Salmonella typhimurium harbouring transmissible gastroenteritis virus (TGEV) DNA vaccine induced specific antibody production

Attenuated Salmonella typhimurium was selected as a transgenic vehicle for the development of live mucosal vaccines against transmissible gastroenteritis virus (TGEV). A 2.2kb DNA fragment, encoding for N-terminal domain glycoprotein S of TGEV, was amplified by RT-PCR and cloned into eukaryotic expression vector pVAX1. The recombinant plasmid pVAX-S was transformed by electroporation into attenuated S. typhimurium SL7207, the expression and translation of the pVAX-S delivered by recombinant S. typhimurium SL7207 (pVAX-S) was detected in vitro and in vivo respectively. BALB/c mice were inoculated orally with SL7207 (pVAX-S) at different dosages, the bacterium was safe to mice at dosage of 2x10(9)CFU and eventually eliminated from the spleen and liver at week 4 post-immunization. Mice immunized with different dosages of SL7207 (pVAX-S) elicited specific anti-TGEV local mucosal and humoral responses as measured by indirect ELISA assay. Moreover, the immunogenicity of the DNA vaccine was highly dependent on the dosage of the attenuated bacteria used for oral administration, 10(9)CFU dosage group showed higher antibody response than 10(8)CFU and 10(7)CFU dosages groups during week 4-8 post-immunization. The results indicated that attenuated S. typhimurium could be used as a delivery vector for oral immunization of TGEV DNA vaccine.

De l’utilité des animaux domestiques pour la recherche en immunologie

Les recherches en immunologie des animaux domestiques fournissent des données complémentaires à celles menées chez la souris, du fait des « opportunités » qu’offrent ces espèces. Certaines d’entre elles font l’objet de cette communication: approche chirurgicale du fonctionnement in vivo des organes lymphoïdes, interventions in utero pour l’étude de l’ontogénèse du système immunitaire, pertinence de l’étude physiopathologique des infections sur espèces cibles.

Commercialization of plant-based vaccines from research and development to manufacturing

The benefits of using plant-based oral vaccines are discussed. Transgenic maize expressing an antigen of transmissible gastroenteritis virus (TGEV) is reported as a model to demonstrate efficacy. Young pigs that were fed the TGEV corn orally were protected against challenge with virulent TGEV. Additional parameters important in providing a reliable and consistent supply of plant-based vaccines are discussed. Finally, vaccines developed in maize are evaluated for their potential to contaminate either the food supply or the environment.

Effective inhibition of porcine transmissible gastroenteritis virus replication in ST cells by shRNAs targeting RNA-dependent RNA polymerase gene

Transmissible gastroenteritis virus (TGEV) is identified as one of the most important pathogenic agents during swine enteric infection, leading to high mortality in neonatal pigs and severe annual economic loss in swine-producing areas. Up to date, various vaccines developed against TGEV still need to be improved. To exploit the possibility of using RNA interference (RNAi) as a strategy against TGEV infection, two shRNA-expressing plasmids (pEGFP-U6/P1 and pEGFP-U6/P2) targeting the RNA-dependent RNA polymerase (RdRp) gene of TGEV were constructed and transfected into swine testicular (ST) cells. The cytopathic effect (CPE) and MTS assays demonstrated that both shRNAs were capable of protecting cells against TGEV invasion with very high specificity and efficiency. A real-time quantitative RT-PCR further confirmed that the amounts of viral RNAs in cell cultures pre-transfected with the two plasmids were reduced by 95.2% and up to 100%, respectively. Our results suggest that RNAi might be a promising new strategy against TGEV infection.

Development of mouse hepatitis virus and SARS-CoV infectious cDNA constructs

The genomes of transmissible gastroenteritis virus (TGEV) and mouse hepatitis virus (MHV) have been generated with a novel construction strategy that allows for the assembly of very large RNA and DNA genomes from a panel of contiguous cDNA subclones. Recombinant viruses generated from these methods contained the appropriate marker mutations and replicated as efficiently as wild-type virus. The MHV cloning strategy can also be used to generate recombinant viruses that contain foreign genes or mutations at virtually any given nucleotide. MHV molecular viruses were engineered to express green fluorescent protein (GFP), demonstrating the feasibility of the systematic assembly approach to create recombinant viruses expressing foreign genes. The systematic assembly approach was used to develop an infectious clone of the newly identified human coronavirus, the serve acute respiratory syndrome virus (SARS-CoV). Our cloning and assembly strategy generated an infectious clone within 2 months of identification of the causative agent of SARS, providing a critical tool to study coronavirus pathogenesis and replication. The availability of coronavirus infectious cDNAs heralds a new era in coronavirus genetics and genomic applications, especially within the replicase proteins whose functions in replication and pathogenesis are virtually unknown.

A corn-based delivery system for animal vaccines: an oral transmissible gastroenteritis virus vaccine boosts lactogenic immunity in swine

Recombinant plant expression systems offer a means to produce large quantities of selected antigens for subunit vaccines. Cereals are particularly well-suited expression vehicles since the expressed proteins can be stored at relatively high concentrations for extended periods of time without degradation and dry seed can be formulated into oral vaccines suitable for commercial applications. A subunit vaccine candidate directed against porcine transmissible gastroenteritis virus and expressed in corn seed has been developed for oral delivery to swine. Here, we show that this vaccine, when administered to previously sensitized gilts, can boost neutralizing antibody levels in the animals’ serum, colostrum and milk. Thus, this vaccine candidate is effective at boosting lactogenic immunity and is appropriate to pursue through large-scale field trials preceding commercialization.

Reverse genetic analysis of the transcription regulatory sequence of the coronavirus transmissible gastroenteritis virus

Coronavirus discontinuous transcription uses a highly conserved sequence (CS) in the joining of leader and body RNAs. Using a full-length infectious construct of transmissable gastroenteritis virus, the present study demonstrates that subgenomic transcription is heavily influenced by upstream flanking sequences and supports a mechanism of transcription attenuation that is regulated in part by a larger domain composed of primarily upstream flanking sequences which select appropriately positioned CS elements for synthesis of subgenomic RNAs.

Engineering the transmissible gastroenteritis virus genome as an expression vector inducing lactogenic immunity

The genome of the coronavirus transmissible gastroenteritis virus (TGEV) has been engineered as an expression vector with an infectious cDNA. The vector led to the efficient (>40 micro g/10(6) cells) and stable (>20 passages) expression of a heterologous gene (green fluorescent protein [GFP]), driven by the transcription-regulating sequences (TRS) of open reading frame (ORF) 3a inserted in the site previously occupied by the nonessential ORFs 3a and 3b. Expression levels driven by this TRS were higher than those of an expression cassette under the control of regulating sequences engineered with the N gene TRS. The recombinant TGEV including the GFP gene was still enteropathogenic, albeit with a 10- to 10(2)-fold reduction in enteric tissue growth. Interestingly, a specific lactogenic immune response against the heterologous protein has been elicited in sows and their progeny. The engineering of an additional insertion site for the heterologous gene between viral genes N and 7 led to instability and to a new genetic organization of the 3' end of the recombinant viruses. As a consequence, a major species of subgenomic mRNA was generated from a TRS with the noncanonical core sequence 5'-CUAAAA-3'. Extension of the complementarity between the TRS and sequences at the 3' end of the viral leader was associated with transcriptional activation of noncanonical core sequences. The engineered vector led to expression levels as high as those of well-established vectors and seems very promising for the development of vaccines and, possibly, for gene therapy.

Delivery of subunit vaccines in maize seed

The use of recombinant gene technologies by the vaccine industry has revolutionized the way antigens are generated, and has provided safer, more effective means of protecting animals and humans against bacterial and viral pathogens. Viral and bacterial antigens for recombinant subunit vaccines have been produced in a variety of organisms. Transgenic plants are now recognized as legitimate sources for these proteins, especially in the developing area of oral vaccines, because antigens have been shown to be correctly processed in plants into forms that elicit immune responses when fed to animals or humans. Antigens expressed in maize (Zea mays) are particularly attractive since they can be deposited in the natural storage vessel, the corn seed, and can be conveniently delivered to any organism that consumes grain. We have previously demonstrated high level expression of the B-subunit of Escherichia coli heat-labile enterotoxin and the spike protein of swine transmissible gastroenteritis in corn, and have demonstrated that these antigens delivered in the seed elicit protective immune responses. Here we provide additional data to support the potency, efficacy, and stability of recombinant subunit vaccines delivered in maize seed.

Heterologous gene expression from transmissible gastroenteritis virus replicon particles

We have recently isolated a transmissible gastroenteritis virus (TGEV) infectious construct designated TGEV 1000 (B. Yount, K. M. Curtis, and R. S. Baric, J. Virol. 74:10600-10611, 2000). Using this construct, a recombinant TGEV was constructed that replaced open reading frame (ORF) 3A with a heterologous gene encoding green fluorescent protein (GFP). Following transfection of baby hamster kidney (BHK) cells, a recombinant TGEV (TGEV-GFP2) was isolated that replicated efficiently and expressed GFP. Replicon constructs were constructed that lacked either the ORF 3B and E genes or the ORF 3B, E, and M genes [TGEV-Rep(AvrII) and TGEV-Rep(EcoNI), respectively]. As the E and M proteins are essential for TGEV virion budding, these replicon RNAs should replicate but not result in the production of infectious virus. Following cotransfection of BHK cells with the replicon RNAs carrying gfp, GFP expression was evident by fluorescent microscopy and leader-containing transcripts carrying gfp were detected by reverse transcription-PCR (RT-PCR). Subsequent passage of cell culture supernatants onto permissive swine testicular (ST) cells did not result in the virus, GFP expression, or the presence of leader-containing subgenomic transcripts, demonstrating the single-hit nature of the TGEV replicon RNAs. To prepare a packaging system to assemble TGEV replicon particles (TGEV VRP), the TGEV E gene was cloned into a Venezuelan equine encephalitis (VEE) replicon expression vector and VEE replicon particles encoding the TGEV E protein were isolated [VEE-TGEV(E)]. BHK cells were either cotransfected with TGEV-Rep(AvrII) (E gene deletion) and VEE-TGEV(E) RNA transcripts or transfected with TGEV-Rep(AvrII) RNA transcripts and subsequently infected with VEE VRPs carrying the TGEV E gene. In both cases, GFP expression and leader-containing GFP transcripts were detected in transfected cells. Cell culture supernatants, collected approximately 36 h posttransfection, were passed onto fresh ST cells where GFP expression was evident approximately 18 h postinfection. Leader-containing GFP transcripts containing the ORF 3B and E gene deletions were detected by RT-PCR. Recombinant TGEV was not released from these cultures. Under identical conditions, TGEV-GFP2 spread throughout ST cell cultures, expressed GFP, and formed viral plaques. The development of infectious TGEV replicon particles should assist studies of TGEV replication and assembly as well as facilitate the production of novel swine candidate vaccines.

Plant-based vaccines: unique advantages

Numerous studies have shown that viral epitopes and subunits of bacterial toxins can be expressed and correctly processed in transgenic plants. The recombinant proteins induce immune responses and have several benefits over current vaccine technologies, including increased safety, economy, stability, versatility and efficacy. Antigens expressed in corn are particularly advantageous since the seed can be produced in vast quantities and shipped over long distances at ambient temperature, potentially allowing global vaccination. We have expressed the B-subunit of Escherichia coli heat-labile enterotoxin and the spike protein of swine transmissible gastroenteritis virus at high levels in corn, and demonstrate that these antigens delivered in the seed elicit protective immune responses.

Mucosal and systemic immune responses to chimeric fimbriae expressed by Salmonella enterica serovar typhimurium vaccine strains

Recombinant live oral vaccines expressing pathogen-derived antigens offer a unique set of attractive properties. Among these are the simplicity of administration, the capacity to induce mucosal and systemic immunity, and the advantage of permitting genetic manipulation for optimal antigen presentation. In this study, the benefit of having a heterologous antigen expressed on the surface of a live vector rather than intracellularly was evaluated. Accordingly, the immune response of mice immunized with a Salmonella enterica serovar Typhimurium vaccine strain expressing the Escherichia coli 987P fimbrial antigen on its surface (Fas(+)) was compared with the expression in the periplasmic compartment (Fas(-)). Orally immunized BALB/c mice showed that 987P fimbriated Salmonella serovar Typhimurium CS3263 (aroA asd) with pCS151 (fas(+) asd(+)) elicited a significantly higher level of 987P-specific systemic immunoglobulin G (IgG) and mucosal IgA than serovar Typhimurium CS3263 with pCS152 (fasD mutant, asd(+)) expressing 987P periplasmic antigen. Further studies were aimed at determining whether the 987P fimbriae expressed by serovar Typhimurium chi4550 (cya crp asd) could be used as carriers of foreign epitopes. For this, the vaccine strain was genetically engineered to express chimeric fimbriae carrying the transmissible gastroenteritis virus (TGEV) C (379-388) and A (521-531) epitopes of the spike protein inserted into the 987P major fimbrial subunit FasA. BALB/c mice administered orally serovar Typhimurium chi4550 expressing the chimeric fimbriae from the tet promoter in pCS154 (fas(+) asd(+)) produced systemic antibodies against both fimbria and the TGEV C epitope but not against the TGEV A epitope. To improve the immunogenicity of the chimeric fimbriae, the in vivo inducible nirB promoter was inserted into pCS154, upstream of the fas genes, to create pCS155. In comparison with the previously used vaccine, BALB/c mice immunized orally with serovar Typhimurium chi4550/pCS155 demonstrated significantly higher levels of serum IgG and mucosal IgA against 987P fimbria. Moreover, mucosal IgA against the TGEV C epitope was only detected with serovar Typhimurium chi4550/pCS155. The induced antibodies also recognized the epitopes in the context of the full-length TGEV spike protein. Hence, immune responses to heterologous chimeric fimbriae on Salmonella vaccine vectors can be optimized by using promoters known to be activated in vivo.

Development of PCR-based techniques to identify porcine transmissible gastroenteritis coronavirus isolates

Sixteen isolates of transmissible gastroenteritis virus and one isolate of porcine respiratory coronavirus were characterized using RT-PCR amplification of 4 antigenic subsites in the site A epitope on the TGEV spike gene. The PCR products were digested with restriction enzymes Sau3AI and SspI and the sizes of the fragments were determined. Three different digestion patterns were observed with each enzyme. The recognition site for Sau3AI was missing in 1 isolate, was present in 13 isolates and 3 isolates had 2 sites. PCR-products with a single site had 3 different fragment sizes and the other isolates produced 2 fragments with different sizes. The SspI recognition site was not present in 5 isolates and 12 isolates had a single site that produced 2 fragments of different sizes. Based on the restriction fragment sizes, the 17 isolates were separated into 7 groups. Direct sequencing of the 455 bp nested set fragments demonstrated greater than 96% sequence homology among the 16 isolates and 100% homology in the 4 antigenic subsites in the conserved site A epitope. The groups are discussed in relation to their sequence homology and virulence. In vitro procedures have been developed to identify several porcine enteric coronavirus isolates at the strain level.

The protective role of cytotoxic T cells and interferon against coronavirus invasion of the brain

MHV-A59 causes focal acute encephalitis, acute hepatitis, and chronic demyelination while MHV-2 causes acute hepatitis and no brain involvement. The difference in organ tropism between these two closely related MHVs is not related to the ability of these viruses to grow in brain cells since both viruses grow equally well in primary glial cell cultures derived from neonatal mouse brains. We postulated therefore that the ability of the virus to stimulate certain host immunological factors may be important for protection of the brain against invasion and replication of the virus. In this study we performed preliminary experiments to investigate the potential role of two host factors in protection of the brain against MHV invasion: cytotoxic T cells and interferon. Four week old beta 2M(-/-) mice, lacking beta 2 microglobulin, MHC class I expression and functional cytotoxic CD8+ T cells were inoculated intracerebrally (IC) with MHV-2 and analyzed at various intervals post infection for histopathology and viral titers in organs. Histology revealed both acute hepatitis and acute encephalitis. Acute encephalitis was observed in periventricular areas. Mononuclear lymphocytic infiltration involved the choroid plexus, the ependyma and in the surrounding brain parenchyma. There was no involvement of other areas of the brain including areas that are typically involved in A59 infection of C57B1/6 mice. By contrast, C57b1/6 mice infected with MHV-2 showed no involvement of the brain parenchyma and only slight inflammation of the choroid plexus was present. High titers of infectious virus was detected by plaque assay in both brains and livers of beta 2M(-/-) mice infected with MHV-2 in contrast to only liver titers in C57B1/6 mice infected with a similar dose of MHV-2.(ABSTRACT TRUNCATED)