Intestinal protection against challenge with transmissible gastroenteritis virus of pigs immune after infection with the porcine respiratory coronavirus

Transmissible Gastroenteritis Virus: An Update Review and Perspective

Transmissible gastroenteritis virus (TGEV) is a member of the alphacoronavirus genus, which has caused huge threats and losses to pig husbandry with a 100% mortality in infected piglets. TGEV is observed to be recombining and evolving unstoppably in recent years, with some of these recombinant strains spreading across species, which makes the detection and prevention of TGEV more complex. This paper reviews and discusses the basic biological properties of TGEV, factors affecting virulence, viral receptors, and the latest research advances in TGEV infection-induced apoptosis and autophagy to improve understanding of the current status of TGEV and related research processes. We also highlight a possible risk of TGEV being zoonotic, which could be evidenced by the detection of CCoV-HuPn-2018 in humans.

Porcine Respiratory Coronavirus as a Model for Acute Respiratory Coronavirus Disease

In the light of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) pandemic, we have developed a porcine respiratory coronavirus (PRCV) model for in depth mechanistic evaluation of the pathogenesis, virology and immune responses of this important family of viruses. Pigs are a large animal with similar physiology and immunology to humans and are a natural host for PRCV. Four PRCV strains were investigated and shown to induce different degrees of lung pathology. Importantly, although all four strains replicated equally well in porcine cell lines in vitro and in the upper respiratory tract in vivo, PRCV strains causing more severe lung pathology were also able to replicate in ex vivo tracheal organ cultures as well as in vivo in the trachea and lung. The time course of infection of PRCV 135, which caused the most severe pulmonary pathology, was investigated. Virus was shed from the upper respiratory tract until day 10 post infection, with infection of the respiratory mucosa, as well as olfactory and sustentacular cells, providing an excellent model to study upper respiratory tract disease in addition to the commonly known lower respiratory tract disease from PRCV. Infected animals made antibody and T cell responses that cross reacted with the four PRCV strains and Transmissible Gastroenteritis Virus. The antibody response was reproduced in vitro in organ cultures. Comparison of mechanisms of infection and immune control in pigs infected with PRCVs of differing pathogenicity with human data from SARS-CoV-2 infection and from our in vitro organ cultures, will enable key events in coronavirus infection and disease pathogenesis to be identified.

Porcine Coronaviruses

Transmissible gastroenteritis virus (TGEV), porcine epidemic diarrhoea virus (PEDV), and porcine deltacoronavirus (PDCoV) are enteropathogenic coronaviruses (CoVs) of swine. TGEV appearance in 1946 preceded identification of PEDV (1971) and PDCoV (2009) that are considered as emerging CoVs. A spike deletion mutant of TGEV associated with respiratory tract infection in piglets appeared in 1984 in pigs in Belgium and was designated porcine respiratory coronavirus (PRCV). PRCV is considered non-pathogenic because the infection is very mild or subclinical. Since PRCV emergence and rapid spread, most pigs have become immune to both PRCV and TGEV, which has significantly reduced the clinical and economic importance of TGEV. In contrast, PDCoV and PEDV are currently expanding their geographic distribution, and there are reports on the circulation of TGEV-PEDV recombinants that cause a disease clinically indistinguishable from that associated with the parent viruses. TGEV, PEDV and PDCoV cause acute gastroenteritis in pigs (most severe in neonatal piglets) and matches in their clinical signs and pathogenesis. Necrosis of the infected intestinal epithelial cells causes villous atrophy and malabsorptive diarrhoea. Profuse diarrhoea frequently combined with vomiting results in dehydration, which can lead to the death of piglets. Strong immune responses following natural infection protect against subsequent homologous challenge; however, these viruses display no cross-protection. Adoption of advance biosecurity measures and effective vaccines control and prevent the occurrence of diseases due to these porcine-associated CoVs. Recombination and reversion to virulence are the risks associated with generally highly effective attenuated vaccines necessitating further research on alternative vaccines to ensure their safe application in the field.

Evaluation of the Serologic Cross-Reactivity between Transmissible Gastroenteritis Coronavirus and Porcine Respiratory Coronavirus Using Commercial Blocking Enzyme-Linked Immunosorbent Assay Kits

This study compared the performances of three commercial transmissible gastroenteritis virus/porcine respiratory coronavirus (TGEV/PRCV) blocking enzyme-linked immunosorbent assays (ELISAs) using serum samples (n = 528) collected over a 49-day observation period from pigs inoculated with TGEV strain Purdue (n = 12), TGEV strain Miller (n = 12), PRCV (n = 12), or with virus-free culture medium (n = 12). ELISA results were evaluated both with "suspect" results interpreted as positive and then as negative. All commercial kits showed excellent diagnostic specificity (99 to 100%) when testing samples from pigs inoculated with virus-free culture medium. However, analyses revealed differences between the kits in diagnostic sensitivity (percent TGEV- or PRCV-seropositive pigs), and all kits showed significant (P < 0.05) cross-reactivity between TGEV and PRCV serum antibodies, particularly during early stages of the infections. Serologic cross-reactivity between TGEV and PRCV seemed to be TGEV strain dependent, with a higher percentage of PRCV-false-positive results for pigs inoculated with TGEV Purdue than for TGEV Miller. Moreover, the overall proportion of false positives was higher when suspect results were interpreted as positive, regardless of the ELISA kit evaluated.IMPORTANCE Current measures to prevent TGEV from entering a naive herd include quarantine and testing for TGEV-seronegative animals. However, TGEV serology is complicated due to the cross-reactivity with PRCV, which circulates subclinically in most swine herds worldwide. Conventional serological tests cannot distinguish between TGEV and PRCV antibodies; however, blocking ELISAs using antigen containing a large deletion in the amino terminus of the PRCV S protein permit differentiation of PRCV and TGEV antibodies. Several commercial TGEV/PRCV blocking ELISAs are available, but performance comparisons have not been reported in recent research. This study demonstrates that the serologic cross-reactivity between TGEV and PRCV affects the accuracy of commercial blocking ELISAs. Individual test results must be interpreted with caution, particularly in the event of suspect results. Therefore, commercial TGEV/PRCV blocking ELISAs should only be applied on a herd basis.

Decline of transmissible gastroenteritis virus and its complex evolutionary relationship with porcine respiratory coronavirus in the United States

The epidemiology and genetic diversity of transmissible gastroenteritis virus (TGEV) in the United States (US) was investigated by testing clinical cases for TGEV by real time RT-PCR between January 2008 and November 2016. Prevalence of TGEV ranged between 3.8–6.8% and peaked during cold months until March 2013, in which prevalence decreased to < 0.1%. Nineteen complete TGEV genomes and a single strain of porcine respiratory coronavirus (PRCV) from the US were generated and compared to historical strains to investigate the evolution of these endemic coronaviruses. Sixteen of our TGEV strains share 8 unique deletions and 119 distinct amino acid changes, which might greatly affect the biological characteristics of the variant TGEV, and resulted in a “variant” genotype of TGEV. The “variant” genotype shared similar unique deletions and amino acid changes with the recent PRCV strain identified in this study, suggesting a recombination event occurred between the ‘‘variant’’ TGEV and PRCV. Moreover, the results indicate the “variant” genotype is the dominant genotype circulating in the US. Therefore, this study provides insight into the occurrence, origin, genetic characteristics, and evolution of TGEV and PRCV circulating in the US.

Transmissible Viral Vaccines

Genetic engineering now enables the design of live viral vaccines that are potentially transmissible. Some designs merely modify a single viral genome to improve on the age-old method of attenuation whereas other designs create chimeras of viral genomes. Transmission has the benefit of increasing herd immunity above that achieved by direct vaccination alone but also increases the opportunity for vaccine evolution, which typically undermines vaccine utility. Different designs have different epidemiological consequences but also experience different evolution. Approaches that integrate vaccine engineering with an understanding of evolution and epidemiology will reap the greatest benefit from vaccine transmission.

Deletion of a 197-Amino-Acid Region in the N-Terminal Domain of Spike Protein Attenuates Porcine Epidemic Diarrhea Virus in Piglets

We previously isolated a porcine epidemic diarrhea virus (PEDV) strain, PC177, by blind serial passaging of the intestinal contents of a diarrheic piglet in Vero cell culture. Compared with the highly virulent U.S. PEDV strain PC21A, the tissue culture-adapted PC177 (TC-PC177) contains a 197-amino-acid (aa) deletion in the N-terminal domain of the spike (S) protein. We orally inoculated neonatal, conventional suckling piglets with TC-PC177 or PC21A to compare their pathogenicities. Within 7 days postinoculation, TC-PC177 caused mild diarrhea and lower fecal viral RNA shedding, with no mortality, whereas PC21A caused severe clinical signs and 55% mortality. To investigate whether infection with TC-PC177 can induce cross-protection against challenge with a highly virulent PEDV strain, all the surviving piglets were challenged with PC21A at 3 weeks postinoculation. Compared with 100% protection in piglets initially inoculated with PC21A, 88% and 100% TC-PC177- and mock-inoculated piglets had diarrhea following challenge, respectively, indicating incomplete cross-protection. To investigate whether this 197-aa deletion was the determinant for the attenuation of TC-PC177, we generated a mutant (icPC22A-S1Δ197) bearing the 197-aa deletion from an infectious cDNA clone of the highly virulent PEDV PC22A strain (infectious clone PC22A, icPC22A). In neonatal gnotobiotic pigs, the icPC22A-S1Δ197 virus caused mild to moderate diarrhea, lower titers of viral shedding, and no mortality, whereas the icPC22A virus caused severe diarrhea and 100% mortality. Our data indicate that deletion of this 197-aa fragment in the spike protein can attenuate a highly virulent PEDV, but the virus may lose important epitopes for inducing robust protective immunity.IMPORTANCE The emerging, highly virulent PEDV strains have caused substantial economic losses worldwide. However, the virulence determinants are not established. In this study, we found that a 197-aa deletion in the N-terminal region of the S protein did not alter virus (TC-PC177) tissue tropism but reduced the virulence of the highly virulent PEDV strain PC22A in neonatal piglets. We also demonstrated that the primary infection with TC-PC177 failed to induce complete cross-protection against challenge by the highly virulent PEDV PC21A, suggesting that the 197-aa region may contain important epitopes for inducing protective immunity. Our results provide an insight into the role of this large deletion in virus propagation and pathogenicity. In addition, the reverse genetics platform of the PC22A strain was further optimized for the rescue of recombinant PEDV viruses in vitro This breakthrough allows us to investigate other virulence determinants of PEDV strains and will provide knowledge leading to better control PEDV infections.

Prevalence of antibodies against transmissible gastroenteritis virus and porcine respiratory coronavirus among pigs in six regions in Japan

A total of 2,703 pig sera from 171 farms in six regions in Japan were screened for virus-neutralizing (VN) antibody against transmissible gastroenteritis virus (TGEV). Although none of the farms had clinical signs of transmissible gastroenteritis (TGE) or vaccination against TGEV, VN antibody was detected in 14.4% of sera at 30 farms (17.5%) across all six regions. On testing of 263 VN antibody-positive sera from 27 farms with a commercial blocking ELISA to distinguish TGEV and porcine respiratory coronavirus (PRCV) antibodies, 78.3% were positive for PRCV antibody only, while 12.5% were positive for TGEV antibody only or both TGEV and PRCV antibodies. Seven of the eight TGEV antibody-positive farms were also positive for PRCV antibody. Five months after the antibody examination, a TGE outbreak occurred at one of these seven farms. These results suggest that most of the detected VN antibodies were to PRCV, and that TGEV infection might be present at some PRCV-positive farms in Japan.

Preparation and physicochemical property of chicken yolk immunoglobulin (IgY) against porcine transmissible gastroenteritis virus (TGEV)

Oral administration of immunoglobulin prepared from the egg yolk of hens immunized with porcine transmissible gastroenteritis virus (TGEV) has been demonstrated to reduce piglets mortality significantly in our previous studies. In the present study, we investigated the stability of chicken yolk immunoglobulin (IgY) specific to TGEV by measuring the remaining activity by ELISA. The results showed that the IgY was stable between pH 4 and pH 11. In the incubation with pepsin at pH 4 and pH 6, about 90% and 100% of the IgY activity remained, respectively. IgY activity could remain approximately 80% at 60°C for 30 min, suggesting that pasteurization can be applied to sterilizing the product. The stability of IgY at 25°C and freezing-thawing treatment indicated that the IgY was easy to be conserved. These results highlight the attractive potential application of IgY as the antibodies of oral administration for treatment of TGEV infections.

Severe acute respiratory syndrome coronavirus pathogenesis, disease and vaccines: an update

BACKGROUND

A novel coronavirus has recently been identified as the cause of severe acute respiratory syndrome (SARS-CoV). The ability of this family of positive strand RNA viruses to move between species and cause severe disease in humans, with the potential for pandemic spread, has been confirmed.

METHODS

An understanding of the disease and its pathogenesis and the genetics of coronavirus infections, as well as strategies to treat or prevent coronavirus infections, are essential. The history of coronavirus vaccines and the occurrence of laboratory-associated SARS-CoV infections underscore the need for stably attenuated strains of SARS-CoV and other coronaviruses.

RESULTS

Rapid progress has been made in understanding the clinical disease of SARS in adults and children. In adults, systemic infection with clinical and biochemical abnormalities, as well as respiratory infection, may be the rule. SARS is much milder in children younger than 12 years old than it is in adolescents and adults. In children age 12 years and younger, symptoms are generally nonspecific and cold-like. Numerous approaches to the development of SARS-CoV vaccines have been undertaken, and there is evidence that antibodies to the spike protein may be protective from replication and pathology in animal models.

CONCLUSIONS

The availability of reverse genetic systems has made it possible to engineer and recover coronavirus variants that contain multiple genetically stable mutations that grow well in culture but are attenuated for replication, virulence or both. Such variants will be platforms for the safe growth of SARS-CoV and candidates for live attenuated vaccines.

Increased litter survival rates, reduced clinical illness and better lactogenic immunity against TGEV in gilts that were primed as neonates with porcine respiratory coronavirus (PRCV)

Establishing immunological memory in female piglets at a young age with PRCV was effective in inducing a secondary immune response to a limiting dose of virulent TGEV given orally 13-18 days prior to farrowing. Subsequently, because of passive antibody transfer, the offspring of these primed gilts were more efficient in surviving a lethal TGEV challenge. An average survival rate of 89% occurred in 6 litters of piglets from primed gilts that were boosted with 2.8 x 10(6) plaque forming units (PFU) of TGEV whereas 76% of the piglets survived in three litters that suckled primed gilts boosted with 3.0 x 10(5)PFU of TGEV. Non-primed gilts given identical pre-farrowing doses of TGEV had litter survival rates of 63 and 55%, respectively. Moreover, both groups of litters from primed gilts suffered less clinical illness (as measured by the extent of weight loss post-challenge) than control litters. Priming of the piglets as neonates and boosting the pregnant gilts produced an anamnestic systemic immune response and correspondingly higher milk titers in the primed gilts compared to control animals. Thus, priming piglets with PRCV was beneficial in providing resistance to TGEV and could be incorporated into a vaccine strategy that yields better protection against TGEV.

Neutralizing antibody decay and lack of contact transmission after inoculation of 3- and 4-day-old piglets with porcine respiratory coronavirus

Ten female neonatal piglets were infected with porcine respiratory coronavirus (PRCV) to measure the decay of a specific neutralizing antibody. By 42 weeks after exposure,1 of the gilts was serologically negative (< 5) for PRCV, and by 48 weeks 2 more gilts were serologically negative. These data demonstrate that young mature gilts can be serologically negative, yet they could have been exposed to PRCV. Sentinel pigs were commingled with the PRCV-infected pigs at 8 weeks after exposure, and no virus transmission occurred.

Immunoprophylactic effect of chicken egg yolk immunoglobulin (Ig Y) against porcine epidemic diarrhea virus (PEDV) in piglets

Porcine epidemic diarrhea virus (PEDV) is the causative agent of neonatal diarrhea in piglets, which causes high mortality rates. In this study, the immunoprophylactic effects of chicken egg yolk immunoglobulin (Ig Y) against PEDV were investigated in neonatal pigs. Ig Y was found to reduce the mortality in piglets after challenge exposures. The field application of Ig Y also revealed significant differences in survival rates of piglets given Ig Y, as compared with placebo or control. The results in this study indicated that Ig Y against PEDV could be an alternative way of supplementing prophylactic measures like colostral antibodies from sows.

Mucosal immunity: an overview and studies of enteric and respiratory coronavirus infections in a swine model of enteric disease

Based on the tenet of a common mucosal immune system, antigenic stimulation at one mucosal site results in the distribution of antigen-specific IgA precursor cells to distant mucosal sites. However, recent studies suggest that functional compartmentalization and limited reciprocity may exist within some components of the common mucosal immune system. Although oral immunization is often very effective in inducing immunity to respiratory pathogens, the converse (respiratory immunization to prevent enteric diseases) may not be as effective. To address this question and to study interactions between the bronchus-associated (BALT) and gut-associated (GALT) lymphoid tissues related to protective immunity, we used as a model two antigenically related porcine coronaviruses which replicate primarily in the intestine (transmissible gastroenteritis virus, TGEV) or respiratory tract (porcine respiratory coronavirus, PRCV). The tissue distribution and magnitude of the antibody secreting cell (ASC) responses (measured by ELISPOT) and cell-mediated immune responses (measured by lymphoproliferative assays, LPA) coincided with the viral tissue tropisms. Immunization via GALT (gut infection with TGEV) elicited high numbers of IgA ASC and high LPA responses in GALT (gut lamina propria, LP or mesenteric lymph nodes, MLN), but lower responses in BALT (bronchial lymph nodes, BLN) and induced complete protection against enteric TGEV challenge. In contrast immunization via BALT (respiratory infection with PRCV) elicited systemic type responses (high numbers of IgG ASC in the BLN), but few ASC and low LPA responses in the gut LP or MLN and induced only partial protection against enteric TGEV challenge. Thus administration of vaccines intranasally may not be optimally effective for inducing intestinal immunity in contrast to the reported efficacy of oral vaccines for inducing respiratory immunity.

Cellular immune responses of pigs after primary inoculation with porcine respiratory coronavirus or transmissible gastroenteritis virus and challenge with transmissible gastroenteritis virus

The contribution of cell-mediated immunity to protective immunity against virulent transmissible gastroenteritis virus (TGEV) infection conferred by primary porcine respiratory coronavirus (PRCV) or TGEV exposure was assessed in pigs that were challenged with TGEV 24 days after a primary oronasal inoculation with PRCV or TGEV when 11 days old. PRCV exposure induced partial protection against TGEV challenge in suckling pigs based upon a decreased number of diarrhea cases (42% vs. 90% in age-matched control pigs), limited virus shedding in feces, and increases in virus-neutralizing serum antibody titers; in contrast, all 11-day-old pigs inoculated with TGEV were completely protected after challenge. Weaned pigs were also studied to eliminate any possibility that lactogenic immunity from contact PRCV-exposed sows contributed to protection against TGEV. Once weaned, none of the PRCV-exposed or age-matched control pigs had diarrhea after TGEV challenge; moreover, both groups exhibited less rectal virus shedding than suckling pigs. Vigorous lymphocyte proliferative responses (> 96,000 counts per minute (cpm)) were detected in mononuclear cells prepared from mesenteric (MLN) and bronchial (BLN) lymph nodes of TGEV-primed pigs. Analyses of these responses indicate that virus-specific cell-mediated immune responses correlated with protection against rectal and nasal virus shedding after TGEV challenge. Primary inoculation of 11-day-old pigs with PRCV induced moderate, transient virus-specific lymphocyte proliferation (> 47,000 cpm) in MLN from both suckling and weaned pigs after TGEV challenge. Substantial BLN proliferative responses (> 80,000 cpm) correlated with failure to detect TGEV in nasal secretions from these pigs. Virus-specific lymphocyte proliferation in spleens was delayed in onset and of lower magnitude than that observed in MLN and BLN. Virulent TGEV exposure resulted in increased percentages of T cell subsets, especially in the lamina propria and MLN, mucosa-associated lymphoid tissues in proximity to the primary replication site of TGEV in the small intestine. Our results confirm that PRCV infection primes anti-viral immune responses and, thus, contributes to partial immunity against virulent TGEV challenge.

Detection of porcine respiratory coronavirus and transmissible gastroenteritis virus by an enzyme-linked immunosorbent assay

An enzyme-linked immunosorbent assay (ELISA) for the detection of transmissible gastroenteritis virus (TGEV) and porcine respiratory coronavirus (PRCV) was developed. A bovine TGEV-specific polyclonal antibody was purified by affinity chromatography with the TRIO Bioprocessing System and was used as the capture antibody, at a concentration of 1.5 micrograms/well. The F5.39 monoclonal antibody was obtained by the fusion of spleen lymphocytes from TGEV immunized mice with NS-1 myeloma cells. This mAb was used as a second antibody for the ELISA. The ELISA detected 40 ng of TGEV and 407 ng of PRCV. To study the ability of ELISA to detect TGEV in field cases, 53 intestinal samples were taken from pigs exhibiting clinical signs of transmissible gastroenteritis. All the positive samples detected by the ELISA were confirmed as positive by immunofluorescence or cell culture immunofluorescence. To study the ability of this ELISA to detect PRCV in nasal swabs and lung samples, 20 seven-day-old piglets were inoculated with a Quebec strain of PRCV. The ELISA was able to detect PRCV in both kinds of samples.

Immunity to transmissible gastroenteritis virus and porcine respiratory coronavirus infections in swine

Despite the pioneering efforts to identify correlates of passive immunity to transmissible gastroenteritis virus (TGEV), effective vaccines for the control of TGE in suckling pigs have remained elusive. The initial concept of an enteromammary immunologic axis in monogastrics originated from studies of lactogenic immunity to TGEV in swine. These studies revealed that infection of pregnant swine with virulent TGEV stimulated high titers of SIgA antibodies in milk which correlated with protection of suckling pigs against TGE; parenteral or oral inoculation with live attenuated or killed TGEV vaccines induced mainly IgG antibodies in milk which generally provided poor protection to suckling pigs. The recent appearance of PRCV infections in swine and continuing studies of TGEV infections, present a unique model for further studies of mucosal immunity. Research using these viruses has increased our understanding of the various components of the common mucosal immune system and their interactions. Although the most important consideration in designing an effective vaccine for TGEV is the stimulation of GALT through intestinal virus replication, studies addressing the contribution of BALT to immunity to TGEV and PRCV may provide insights for alternative vaccine approaches. The mechanism by which exposure to PRCV elicits a variable-degree of immunity to TGEV challenge is unknown. Virus replication in the gut or respiratory tract is a major factor affecting the magnitude of the immune response at the respective site and may be necessary for the recruitment of specific immune cells from other mucosal inductive sites, i.e., GALT to BALT and BALT to GALT migration.(ABSTRACT TRUNCATED AT 250 WORDS)

Immunization of pregnant gilts with PRCV induces lactogenic immunity for protection of nursing piglets from challenge with TGEV

The level of passive protection against transmissible gastroenteritis virus (TGEV) was evaluated by experimentally infecting 12 pregnant gilts with different doses of porcine respiratory coronavirus (PRCV) and challenging their litters at 4 days of age. An overall survival rate of 70% was found for piglets nursing the 12 PRCV-infected gilts, compared to a 16% survival rate for piglets of nine uninfected control gilts. Six of the PRCV-infected gilts had adequate levels of immunity to resist infection with TGEV following the challenge of their litters. These six completely immuned gilts also solidly protected their litters from TGEV as shown by a 96% piglet survival rate through weaning at 3 weeks of age. The results suggest that respiratory infection with PRCV induces a substantial degree of protective lactogenic immunity against TGEV.