Activation of the NLRP1 Inflammasome and Its Role in Transmissible Gastroenteritis Coronavirus Infection

The inflammasome pathway is a critical early response mechanism of the host that detects pathogens, initiates the production of inflammatory cytokines, and recruits effector cells to the infection site. Nonetheless, the mechanism of inflammasome activation in coronavirus infection and its biological functions in host defense remain unclear. Transmissible gastroenteritis virus (TGEV), a member of the genus Alphacoronavirus, is a significant pathogen that mainly infects piglets and causes intestinal inflammation and inflammatory cell infiltration. Here, we investigated the mechanism of inflammasome activation in intestinal epithelial cells (IECs) infected with TGEV. We observed a substantial increase in interleukin 1β (IL-1β) and IL-18 levels in both IECs and TGEV-infected porcine intestinal tissues. Furthermore, TGEV infection resulted in increased activation of caspase-1 and the NLRP1 (NOD-like receptor [NLR]-containing pyrin domain [PYD]) inflammasome. Our findings revealed that TGEV infection impeded the interaction between porcine NLRP1 (pNLRP1) and porcine dipeptidyl peptidases 9 (pDPP9), yet it did not reduce the expression of pDPP9. Importantly, the ZU5 domain, not the function-to-find domain (FIIND) reported in human NLRP1, was identified as the minimal domain of pNLRP1 for pDPP9 binding. In addition, the robust type I IFN expression induced by TGEV infection also upregulated pNLRP1 expression and pNLRP1 itself acts as an interferon-stimulated gene to counteract TGEV infection. Our data demonstrate that pNLRP1 has antiviral capabilities against coronavirus infection, which highlights its potential as a novel therapeutic target for coronavirus antiviral therapy. IMPORTANCE Coronavirus primarily targets the epithelial cells of the respiratory and gastrointestinal tracts, leading to damage in both humans and animals. NLRP1 is a direct sensor for RNA virus infection which is highly expressed in epithelial barrier tissues. However, until recently, the precise molecular mechanisms underlying its activation in coronavirus infection and subsequent downstream events remained unclear. In this study, we demonstrate that the alphacoronavirus TGEV induces the production of IL-1β and IL-18 and upregulates the expression of pNLRP1. Furthermore, we found that pNLRP1 can serve as an interferon-stimulated gene (ISG) to inhibit the infection of enterovirus TGEV. Our research highlights the crucial role of NLRP1 as a regulator of innate immunity in TGEV infection and shows that it may serve as a potential therapeutic target for the treatment of coronavirus infection.

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.

Environmental persistence of porcine coronaviruses in feed and feed ingredients

Porcine Epidemic Diarrhea Virus (PEDV), Porcine Delta Corona Virus (PDCoV), and Transmissible Gastroenteritis Virus (TGEV) are major threats to swine health and contaminated feed plays a role in virus transmission. The objective of our study was to characterize inactivation of PEDV, PDCoV, and TGEV in various feed ingredient matrices. Samples of complete feed, spray dried porcine plasma, meat meal, meat and bone meal, blood meal, corn, soybean meal, and corn dried distillers grains with solubles were weighed (5 g/sample) into scintillation vials and inoculated with 1 mL of PEDV, PDCoV, or TGEV. Samples were incubated at room temperature for up to 56 days. Aliquots were removed at various time points followed by preparing serial 10-fold dilutions and inoculating in cell cultures to determine the amount of surviving virus. Inactivation kinetics were determined using the Weibull model, which estimates a delta value indicating the time necessary to reduce virus concentration by 1 log. Delta values of various ingredients were compared and analyzed as to their nutrient composition. Soybean meal had the greatest delta value (7.50 days) for PEDV (P < 0.06) as compared with all other ingredients. High delta values (P < 0.001) were observed in soybean meal for PDCoV (42.04 days) and TGEV (42.00 days). There was a moderate correlation between moisture content and the delta value for PDCoV (r = 0.49, P = 0.01) and TGEV (r = 0.41, P = 0.02). There was also a moderate negative correlation between TGEV survival and ether extract content (r = -0.51, P = 0.01). In conclusion, these results indicate that the first log reduction of PDCoV and TGEV takes the greatest amount of time in soybean meal. In addition to this, moisture and ether content appear to be an important determinant of virus survival in feed ingredients.

Using heterogeneity in the population structure of U.S. swine farms to compare transmission models for porcine epidemic diarrhoea

In 2013, U.S. swine producers were confronted with the disruptive emergence of porcine epidemic diarrhoea (PED). Movement of animals among farms is hypothesised to have played a role in the spread of PED among farms. Via this or other mechanisms, the rate of spread may also depend on the geographic density of farms and climate. To evaluate such effects on a large scale, we analyse state-level counts of outbreaks with variables describing the distribution of farm sizes and types, aggregate flows of animals among farms, and an index of climate. Our first main finding is that it is possible for a correlation analysis to be sensitive to transmission model parameters. This finding is based on a global sensitivity analysis of correlations on simulated data that included a biased and noisy observation model based on the available PED data. Our second main finding is that flows are significantly associated with the reports of PED outbreaks. This finding is based on correlations of pairwise relationships and regression modeling of total and weekly outbreak counts. These findings illustrate how variation in population structure may be employed along with observational data to improve understanding of disease spread.

Porcine innate and adaptative immune responses to influenza and coronavirus infections

abstract: Both innate and adaptative immune responses contribute to the control of infectious diseases, including by limiting the spreading of zoonotic diseases from animal reservoirs to humans. Pigs represent an important animal reservoir for influenza virus infection of human populations and are also naturally infected by coronaviruses, an important group of viruses, which includes the recently emerged severe acute respiratory syndrome (SARS) virus. Studies on both innate and adaptative immune responses of pigs to influenza virus and coronaviruses contribute, therefore, to a better control of these infections in their natural hosts and will be briefly reviewed in this article. Pro‐inflammatory cytokines, including type I interferon (IFN), tumor necrosis factor‐α (TNF‐α), and interleukin‐6 (IL‐6), were found in lung secretions of influenza virus infected pigs, and correlated with the intensity of clinical signs, whereas prior vaccination against influenza strongly reduced the production of infectious virus and cytokines in the lungs upon challenge, which was associated with clinical protection. An early type I IFN production was also found in coronavirus infected pigs, including at mucosal sites. IFN induction by coronavirus is shown to involve interaction between a viral glycoprotein and a leukocyte subset, likely equivalent to plasmacytoid dendritic cells, present in the mucosae and associated lymphoid tissues. Given the IFN mediated antiviral and immunomodulatory effects, the use of IFN or IFN inducers may prove an efficient strategy for a better control of influenza virus and coronavirus infections in pigs. Because influenza and coronaviruses target mucosal surfaces, adaptative immune responses have to be characterized at mucosal sites. Thus, nasal and pulmonary antibody responses were analyzed in influenza virus infected or vaccinated pigs showing short‐lived, but potentially protective local IgA and IgG antibody (Ab) responses. Interestingly, primary influenza virus infection induced long‐lived increase of lung CD8⁺ T cells and local lymphoproliferative responses. Pigs infected by a respiratory coronavirus (PRCV) showed virus‐specific IgG Ab‐secreting cells in the bronchial lymph nodes, whereas the transmissible gastroenteritis coronavirus (TGEV) induced more IgA Ab‐secreting cells in gut tissues, which illustrates the importance of the route of antigen administration for inducing local immune effector mechanisms. Porcine viral infections provide, therefore, valuable models for evaluating the immune parameters that are important for controlling transmission of important viral zoonotic infections.

Identification of antibody against porcine coronavirus in human milk

A total of 239 human milk samples collected from mothers in Chiba City, Japan during 1994-1997 were tested for the presence of anti-coronavirus antibody. Interestingly, twelve human milk samples were positive for IgA against porcine transmissible gastroenteritis coronavirus, and this represented 5%. These findings provided evidence that the interspecies transmission of coronavirus between human and porcine might occur in nature. This report is noteworthy because it is the first, to the best of our knowledge, demonstrating the presence of antibody against porcine transmissible gastroenteritis coronavirus in human milk.

[Combined vaccine protecting from transmissible swine gastroenteritis]

An effective system for specific prevention of transmissive swine gastroenteritis (TSG) is based on the use of combined vaccine and combined vaccination. Combined vaccine is a lyophilized preparation for intranasal application and emulsion for intramuscular injection. The former is prepared from natural attenuated respiratory variant of TSG virus and the latter from artificially attenuated TSG virus. Combined vaccination consists in parallel double immunization of females on days 70-75 of gestation intramuscularly and intranasally and on days 90-95 of gestation intramuscularly. Vaccinal lactogenic immunity ensures protection of newborn piglets under experimental and field conditions (protection of 70-80 and 90-95% piglets, respectively). The proposed vaccine preparations and protocols of combined immunization of females during gestation by two methods are an effective and safe method for specific prevention of TSG.

Characterisation of a recent virulent transmissible gastroenteritis virus from Britain with a deleted ORF 3a

Analyses of transmissible gastroenteritis virus (TGEV) and porcine respiratory coronavirus (PRCV) isolates have suggested that tropism and pathogenicity are influenced by the spike protein and ORF 3. In general, enteric viruses (TGEV) have been shown to contain intact spike and ORF 3 genes, whilst respiratory isolates (PRCV) have major deletions within both regions. Virulence has been correlated to a functional ORF 3. Here, sequence analysis of a recent isolate of virulent TGEV, revealed a variant with an intact spike gene, but a large deletion in ORF 3a. This suggests that ORF 3a is not essential for enteric virulence.

Obtention of porcine aminopeptidase-n transgenic mice and analysis of their susceptibility to transmissible gastroenteritis virus

To obtain a laboratory animal model for transmissible gastroenteritis virus (TGEV) infection, transgenic mice (Tg) were produced by introducing two porcine aminopeptidase-n (APN) cDNA-derived constructs into the mouse genome. In the first construct, the APN cDNA was fused in 5' with the 1 kb upstream region of the APN gene and in 3' with the SV40 small intron and polyadenylation site. In the second construct, the 5' end of the APN cDNA was replaced by the corresponding domain of the APN gene comprising the three first introns, an additional intron (the rabbit beta-like globine intron 2) was inserted at the 3' extremity of the construct and the resulting DNA stretch was placed under the control of the rat intestinal fatty acid-binding protein (I-FABP) gene promoter. Transgenes were obtained with these two constructs, and RNA expression was evidenced by RT-PCR with the second construct in a transgene lineage. Using two different immunoassays, expression of the porcine APN protein was not detected in the transgenic intestines of animals of the RT-PCR positive lineage. Northern blot analyses did not revealed TGEV replication in infected adult mice. Additional assays will be carried out on young animals to detect potential TGEV susceptibility.

Potential animal health hazards of pork and pork products

The animal health hazards associated with the importation of pork and pork products include four viral agents: foot and mouth disease, classical swine fever (hog cholera), African swine fever, and swine vesicular disease viruses. The safety of importing pork from a zone infected with one or more of these diseases can be adequately determined only through risk assessment. This also applies for the safety of importing pork products which have undergone some form of processing (fully cooked pork products are not counted here). For each disease, the agent (pH and temperature lability), target organs, agent survival in pork and pork products, and agent quantification are discussed. Agent quantification is an input of the risk assessment which measures the viral titres in waste pork and pork products in relation to the oral infective dose estimated for each disease. Two other viral diseases, transmissible gastroenteritis of pigs and porcine reproductive and respiratory syndrome, are presented to illustrate why these two diseases are not hazards when associated with pork and pork products.

A mathematical model of detection and dynamics of porcine transmissible gastroenteritis

Transmissible gastroenteritis (TGE) is a viral disease causing dehydration, diarrhoea and death in pigs. The disease is widespread in pig-producing areas of the world but does not occur in Australia. A mathematical model of TGE spread within a pig herd is proposed and calibrated by reference to published data. The model is then applied to two situations of special interest; first to estimate the delay before detection of TGE (6 to over 30 days) when infection is first introduced into a herd of domestic or feral pigs, and second the effect of the disease in a population of feral pigs (could become endemic if transmission is high).

An overview of successful TGEV vaccination strategies and discussion on the interrelationship between TGEV and PRCV

Porcine respiratory coronavirus (PRCV) is a new variant of TGE with an altered pathogenesis. PRCV multiplies mainly in tonsilar tissues and the respiratory tract. There are no enteric symptoms and in experimentally infected pigs, even the respiratory tract infection is usually asymptomatic. PRCV is spread aerogenically through herds and the significance of PRCV as a pathogen in swine has yet to be determined. Despite the differences in pathogenesis and tissue tropism, the behavior of TGEV and PRCV are closely related antigenically. PRCV induces an antibody response in pigs that cannot be distinguished from TGEV-infected pigs by conventional serological assays. PRCV sensitized animals are not protected from TGEV challenge nor is the milk antibody provided to nursing piglets completely effective in prevention of TGEV infections; thus PRCV is not a good vaccine candidate for TGEV infections. PRCV subclinical infections have led to several reported cases of enzootic TGEV in herds that had been diagnosed as TGEV immune strictly on the basis of serum neutralizing titers which were later found to be due to exposure to PRCV. Vaccination studies conducted with the Ambico, oral modified live TGEV vaccine have led to some startling new results: (1) Use of Ambico TGEV modified live vaccine has been shown to provide complete protection against subsequent PRCV challenge and (2) the effectiveness of TGEV vaccination is actually enhanced by previous exposure to PRCV (3) Weanling pigs which have passively acquired circulating TGEV neutralizing antibodies are protected from subsequent PRCV infections.

Oral transmission of transmissible gastroenteritis virus by muscle and lymph node from slaughtered pigs

A study was conducted in the USA to determine whether transmissible gastroenteritis (TGE) virus could be transmitted from carcases of slaughtered pigs. Transmissible gastroenteritis virus was transmitted to 6-day-old piglets by dosing with homogenates of muscle and lymph node collected from 500 clinically normal pigs at the time of slaughter. All piglets in 2 separately housed litters showed clinical signs of TGE with 5 piglets dying within 10 d of oral dosing with homogenates. Transmissible gastroenteritis virus was isolated from 2 of these piglets and all piglets developed TGE antibody. Transmissible gastroenteritis virus was not isolated in tissue culture from muscle and lymph node homogenates, but was isolated from 4 (0.8%) of 500 tonsil samples collected from the same pigs. A survey of 250 serum samples provided an estimate of the prevalence of slaughtered pigs with TGE antibody of 34.8% in the sample population. The results indicate that carcases of some pigs from TGE endemic areas contain viable TGE virus, and that there would be a substantial risk of introducing TGE virus into Australia by the importation of uncooked pig meat from these areas.

Infection of pigs with transmissible gastroenteritis virus from contaminated carcases

Sixteen 6-month-old pigs were exposed to transmissible gastroenteritis (TGE) virus by placing them in close contact with piglets infected at 1 week of age. Fourteen of the older pigs were slaughtered between 1 and 5 d after exposure to infection and their carcases dressed in simulated abattoir conditions. Samples of muscle, bone marrow and carcase lymph nodes were stored at -25 degrees C for at least 30 d and then homogenised and fed to groups of 1-week-old and 3-week-old pigs. Four of 12 one-week-old pigs died and TGE virus was isolated from intestinal contents of one of these. All pigs of both age groups developed neutralising antibody to TGE virus over the ensuing 4 w. The results indicate that carcases from pigs infected with TGE virus can represent a source of infection for susceptible pigs given access to them.

Transmissible gastroenteritis (TGE) of swine: survivor selection of TGE virus mutants in stomach juice of adult pigs

Two transmissible gastroenteritis (TGE) virus mutants (188-SG and 152-SG) were obtained from a low-passage virus strain (D-52) by 188 and 152 cycles of stomach juice treatment and multiplication in cell culture. Compared to the high-passage Purdue-115 and the original D-52 strains, these mutants were more stable at pH 2.0, more resistant to pepsin and trypsin, and characterized by a small plaque phenotype. In vivo, the two mutants were not found to be virulent for 4-day-old piglets and sows after oral inoculation. To test induction of lactogenic immunity, the 188-SG mutant was administered orally to pregnant sows (6 or 7 weeks before parturition) followed by one intramuscular booster (1 week before parturition). After challenge with virulent TGE virus, piglet mortality 7 days after exposure was reduced (to 22%) as compared to the death rate in piglets from control sows (91%).

Identification of porcine transmissible gastroenteritis virus in house flies (Musca domestica Linneaus)

Transmissible gastroenteritis (TGE) virus was detected in house flies (Musca domestica Linneaus) by staining with specific fluorescent antibody. The flies were collected within a swine confinement facility in which TGE was enzootic. Laboratory-reared flies were infected experimentally with TGE virus and the virus was recovered from the insects for 72 hours after infection. The TGE virus was identified both by the fluorescent antibody technique and by isolation in cell culture. The nature of plaque formation in cell monolayers inoculated with the virus passaged through flies changed from a large plaque (4 mm or greater in diameter) to a small plaque (1 mm in diameter) over the period. Large plaques were observed early after infection and were attributed to TGE virus mechanically carried by the flies. Small plaques occurred 8 to 12 hours after infection and were considered to be produced by virus replicated in the dipterous cell.

Intestinal, pulmonary, and serum antibody responses of feeder pigs exposed to transmissible gastroenteritis virus by the oral and the oral-intranasal routes in inoculation

Specific antibodies against transmissible gastroenteritis (TGE) virus produced as a result of oral and oral-intranasal viral exposure of feeder pigs were isolated by molecular sieve column chromatography of lung wash, intestinal fluid, and serum samples. Selected fractions were analyzed for neutralizing antibody activity. Oral inoculation gave a predominantly immunoglobulin (Ig) G antibody response in lung wash and serum, as compared with the response in intestinal fluid in which neutralizing antibody activity was found in the IgA-containing fractions. Oral-intranasal inoculation produced a predominantly IgA antibody response in intestinal fluid. Lung wash contained IgA and IgG antibody, and serum antibody consisted primarily of IgG class of Ig. Neutralizing antibody (IgA, IgG, IgM) from the lung wash, intestinal fluid, and serum samples persisted beyond 56 days after oral and oral-intranasal viral exposures. The virus-shedding period in feces was 1 to 7 days after oral or oral-intranasal inoculation of feeder swine with virulent TGE virus. However, TGE virus was reisolated from lung and intestinal tissue for as long as 42 and 56 days, respectively.

Isolation of transmissible Gastroenteritis virus, pseudorabies virus, and porcine enterovirus from pharyngeal swabs taken from market-weight-swine

Pharyngeal swab samples were collected at a central Iowa abattoir from 6,010 market-weight slaughter hogs from September 1, 1979 to August 31, 1980. The swab samples were examined for cytopathic viruses by inoculation of monolayer cultures of continuous line of swine testicular cells. Of the 6,010 swab samples tested, transmissible gastroenteritis virus was isolated from 91 (1,51%), pseudorabies virus was isolated from 431 (7.17%), and porcine enterovirus was isolated from 21 (0.35%). Although all 3 viruses were identified throughout the year, transmissible gastroenteritis and pseudorabies viruses were found more frequently during the winter and early spring. In contrast, porcine enterovirus was detected more frequently during the spring and summer.

Transmissible gastroenteritis in neonatal dogs: experimental intestinal infection with transmissible gastroenteritis virus

Fourteen neonatal dogs (4 through 11 days of age) were exposed orally to the Purdue strain of transmissible gastroenteritis (TGE) virus, and six dogs of similar age were noninoculated controls. Clinical signs of enteric disease did not develop. Both exposed and control dogs had normal fecal passages and appetite throughout the experiment. Jejunal epithelium from dogs euthanatized at 12, 24, 48, and 96 hours and at 10 days after exposure did not exhibit morphologic alterations detectable by light microscopy. Electron microscopic examination indicated that jejunal epithelial cells contained TGE viral particles as early as 12 hours after dogs were exposed. There were no apparent morphologic alterations or signs of desquamation of virus-infected cells, however. Results of pig transmission studies indicated that viable TGE virus was in jejunal tissue of the dogs as early as 12 hours and as late as 10 days after exposure to the virus.

The postulated role of feeder swine in the perpetuation of the transmissible gastroenteritis virus

Clinical, immunofluorescence and histopathological observations were found to be an efficient approach for the confirmation of the diagnosis of transmissible gastroenteritis in feeder swine. Two cases are reported to exemplify how feeder swine exposed to points of concentration such as holding areas, sales barns and auctions can play an important role in the epizootiology of transmissible gastroenteritis. A third field case is reported as an example of an outbreak of transmissible gastroenteritis beginning in feeder swine and then spreading to baby pigs on the farm. All baby pigs died that were born during the acute phase of the outbreak in the feeder swine. Baby pigs born shortly after the clinical signs had abated in the herd, and from sows that had been exposed orally to virulent transmissible gastroenteritis virus and vaccinated with a commercial transmissible gastroenteritis vaccine ten days before farrowing, survived. This was explained by a combination of a decrease in the amount of virus shed in the environment and the immunity induced in the sows. These observations of field outbreaks of transmissible gastroenteritis combined with recently reported experimental studies lend strong support to the hypothesis of a reservoir for transmissible gastroenteritis virus in feeder pigs. This reservoir would be based principally on the transmission of the virus on a continuous basis from the feces of recently infected pigs to susceptible pigs. Clinical signs of transmissible gastroenteritis in such pigs are difficult to recognize or absent and this contributes to the importance of the reservoir in the field.

Immunoglobulin classes of antibodies in milk of swine after intranasal exposure to pseudorabies virus or transmissible gastroenteritis virus

Experiments were conducted to evaluate whether infection of the respiratory tract of pregnant swine with pseudorabies (Pr) virus would induce the secretion of immunoglobulin A (IgA) antibodies in their milk as was observed after enteric infection with transmissible gastroenteritis (TGE) virus. The immune response of sows to Pr virus inoculated intranasally and to TGE virus inoculated orally/intranasally or via a natural infection was studied. Emphasis was placed upon titers and Ig classes of Pr and TGE virus-neutralizing antibodies in colostrum and milk. All animals exposed to Pr virus (alone or in combination with TGE virus) developed Pr-neutralizing antibody titers in both serum and milk. Pr antibody titers were generally higher in colostrum than in serum, but the opposite was true in milk compared with serum, with milk titers declining markedly during lactation. In contrast, TGE antibody titers in milk from experimentally or naturally infected sows usually remained higher than the corresponding serum titers and persisted at relatively constant levels throughout lactation. Gel filtration studies of milk indicated that the antibody activity against Pr virus was associated almost entirely with IgG fractions, with small amounts of antibody detectable in IgM fractions in colostrum from two of nine sows. By comparison, TGE antibodies were primarily of the IgA class, with varying but lesser amounts of antibody associated with the IgG class. Such results suggest that viral infection of the intestinal tract of the sow, but not the upper respiratory tract, stimulates the secretion of IgA antibodies in the milk.

Upper respiratory infection of lactating sows with transmissible gastroenteritis virus following contact exposure to infected piglets

Ten breeding sows were left in direct contact with their newborn piglets that had been experimentally infected with transmissible gastroenteritis (TGE) virus. All sows became infected with the virus. The sows developed fever and showed mild clinical signs of the disease for a few days. The sows excreted virus in the nasal secretion, feces, and milk during the acute febrile phase of illness. Virus was isolated from the nasal secretion of one sow as early as 20 hours after contact exposure to the infected piglets. At necropsy, the virus was more frequently isolated from the tissues of the upper respiratory tract than from small intestines; this finding indicated that the TGE coronavirus replicated in the upper respiratory tract and induced an acute respiratory infection in susceptible adult swine. Neutralizing antibody was present in the sera 8 sows after 12 to 36 days during the convalescent period. From these results, we conclude that susceptible sows in direct contact with ill piglets can become infected and by excreting virus can serve as a source of TGE virus for other susceptible pigs on the premises.