Differential Transcriptomics Analysis of IPEC-J2 Cells Single or Coinfected With Porcine Epidemic Diarrhea Virus and Transmissible Gastroenteritis Virus

Porcine epidemic diarrhea (PED) and transmissible gastroenteritis (TGE) caused by porcine epidemic diarrhea virus (PEDV) and transmissible gastroenteritis virus (TGEV) are two highly contagious intestinal diseases in the swine industry worldwide. Notably, coinfection of TGEV and PEDV is common in piglets with diarrhea-related diseases. In this study, intestinal porcine epithelial cells (IPEC-J2) were single or coinfected with PEDV and/or TGEV, followed by the comparison of differentially expressed genes (DEGs), especially interferon-stimulated genes (ISGs), between different groups via transcriptomics analysis and real-time qPCR. The antiviral activity of swine interferon-induced transmembrane protein 3 (sIFITM3) on PEDV and TGEV infection was also evaluated. The results showed that DEGs can be detected in the cells infected with PEDV, TGEV, and PEDV+TGEV at 12, 24, and 48 hpi, and the number of DEGs was the highest at 24 hpi. The DEGs are mainly annotated to the GO terms of protein binding, immune system process, organelle part, and intracellular organelle part. Furthermore, 90 ISGs were upregulated during PEDV or TGEV infection, 27 of which were associated with antiviral activity, including ISG15, OASL, IFITM1, and IFITM3. Furthermore, sIFITM3 can significantly inhibit PEDV and TGEV infection in porcine IPEC-J2 cells and/or monkey Vero cells. Besides, sIFITM3 can also inhibit vesicular stomatitis virus (VSV) replication in Vero cells. These results indicate that sIFITM3 has broad-spectrum antiviral activity.

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.

Swine Enteric Coronaviruses (PEDV, TGEV, and PDCoV) Induce Divergent Interferon-Stimulated Gene Responses and Antigen Presentation in Porcine Intestinal Enteroids

Swine enteric coronaviruses (SECoVs) including porcine epidemic diarrhea virus (PEDV), transmissible gastroenteritis virus (TGEV), and porcine deltacoronavirus (PDCoV), account for the majority of lethal watery diarrhea in neonatal pigs and pose significant economic and public health burdens in the world. While the three SECoVs primarily infect intestinal epithelia in vivo and cause similar clinical signs, there are evident discrepancies in their cellular tropism and pathogenicity. However, the underlying mechanisms to cause the differences remain unclear. Herein, we employed porcine enteroids that are a physiologically relevant model of the intestine to assess the host epithelial responses following infection with the three SECoVs (PEDV, TGEV, and PDCoV). Although SECoVs replicated similarly in jejunal enteroids, a parallel comparison of transcriptomics datasets uncovered that PEDV and TGEV infection induced similar transcriptional profiles and exhibited a more pronounced response with more differentially expressed genes (DEGs) in jejunal enteroids compared with PDCoV infection. Notably, TGEV and PDCoV induced high levels of type I and III IFNs and IFN-stimulated gene (ISG) responses, while PEDV displayed a delayed peak and elicited a much lesser extent of IFN responses. Furthermore, TGEV and PDCoV instead of PEDV elicited a substantial upregulation of antigen-presentation genes and T cell-recruiting chemokines in enteroids. Mechanistically, we demonstrated that IFNs treatment markedly elevated the expression of NOD-like receptor (NLR) family NLRC5 and major histocompatibility complex class I (MHC-I) molecules. Together, our results indicate unique and common viral strategies for manipulating the global IFN responses and antigen presentation utilized by SECoVs, which help us a better understanding of host-SECoVs interactions.

microRNA-222 Attenuates Mitochondrial Dysfunction During Transmissible Gastroenteritis Virus Infection

Transmissible gastroenteritis virus (TGEV) is a member of Coronaviridae family. Our previous research showed that TGEV infection could induce mitochondrial dysfunction and upregulate miR-222 level. Therefore, we presumed that miR-222 might be implicated in regulating mitochondrial dysfunction induced by TGEV infection. To verify the hypothesis, the effect of miR-222 on mitochondrial dysfunction was tested and we showed that miR-222 attenuated TGEV-induced mitochondrial dysfunction. To investigate the underlying molecular mechanism of miR-222 in TGEV-induced mitochondrial dysfunction, a quantitative proteomic analysis of PK-15 cells that were transfected with miR-222 mimics and infected with TGEV was performed. In total, 4151 proteins were quantified and 100 differentially expressed proteins were obtained (57 upregulated, 43 downregulated), among which thrombospondin-1 (THBS1) and cluster of differentiation 47 (CD47) were downregulated. THBS1 was identified as the target of miR-222. Knockdown of THBS1 and CD47 decreased mitochondrial Ca2+ level and increased mitochondrial membrane potential (MMP) level. Reversely, overexpression of THBS1 and CD47 elevated mitochondrial Ca2+ level and reduced mitochondrial membrane potential (MMP) level. Together, our data establish a significant role of miR-222 in regulating mitochondrial dysfunction in response to TGEV infection.

Transmissible Gastroenteritis Virus Infection Enhances SGLT1 and GLUT2 Expression to Increase Glucose Uptake

Transmissible gastroenteritis virus (TGEV) is a coronavirus that causes villus atrophy, followed by crypt hyperplasia, reduces the activities of intestinal digestive enzymes, and disrupts the absorption of intestinal nutrients. In vivo, TGEV primarily targets and infects intestinal epithelial cells, which play an important role in glucose absorption via the apical and basolateral transporters Na⁺-dependent glucose transporter 1 (SGLT1) and facilitative glucose transporter 2 (GLUT2), respectively. In this study, we therefore sought to evaluate the effects of TGEV infection on glucose uptake and SGLT1 and GLUT2 expression. Our data demonstrate that infection with TGEV resulted in increased glucose uptake and augmented expression of EGFR, SGLT1 and GLUT2. Moreover, inhibition studies showed that EGFR modulated glucose uptake in control and TGEV infected cells. Finally, high glucose absorption was subsequently found to promote TGEV replication.

Proteome profile of swine testicular cells infected with porcine transmissible gastroenteritis coronavirus

The interactions occurring between a virus and a host cell during a viral infection are complex. The purpose of this paper was to analyze altered cellular protein levels in porcine transmissible gastroenteritis coronavirus (TGEV)-infected swine testicular (ST) cells in order to determine potential virus-host interactions. A proteomic approach using isobaric tags for relative and absolute quantitation (iTRAQ)-coupled two-dimensional liquid chromatography-tandem mass spectrometry identification was conducted on the TGEV-infected ST cells. The results showed that the 4-plex iTRAQ-based quantitative approach identified 4,112 proteins, 146 of which showed significant changes in expression 48 h after infection. At 64 h post infection, 219 of these proteins showed significant change, further indicating that a larger number of proteomic changes appear to occur during the later stages of infection. Gene ontology analysis of the altered proteins showed enrichment in multiple biological processes, including cell adhesion, response to stress, generation of precursor metabolites and energy, cell motility, protein complex assembly, growth, developmental maturation, immune system process, extracellular matrix organization, locomotion, cell-cell signaling, neurological system process, and cell junction organization. Changes in the expression levels of transforming growth factor beta 1 (TGF-β1), caspase-8, and heat shock protein 90 alpha (HSP90α) were also verified by western blot analysis. To our knowledge, this study is the first time the response profile of ST host cells following TGEV infection has been analyzed using iTRAQ technology, and our description of the late proteomic changes that are occurring after the time of vigorous viral production are novel. Therefore, this study provides a solid foundation for further investigation, and will likely help us to better understand the mechanisms of TGEV infection and pathogenesis.

Viral gastroenteritis: recent progress, remaining problems

Infants and young children are particularly susceptible to a recently identified viral enteritis which is highly contagious and seems both common and universal. In this disease, virus invades the upper intestinal epithelium, causing acute diarrhoea with early fever and vomiting. We studied a similar disease in pigs, infecting three-week-old animals with transmissible gastroenteritis virus (TGE), which also invades the upper intestinal epithelium. In this model, diarrhoea is massive 16-40 hours after infection, when stools contain increased electrolytes but no excess of sugar. In the jejunum of intact pigs at the 40-hour stage we found altered Na+ and water flux, decreased mucosal activities of disaccharidases and Na+, K+-ATPase, but normal adenylate cyclase activity. At the same stage the response of Na+ flux to glucose was blunted in jejunal epithelium studied in Ussing short-circuit chambers and in suspensions of villous cells; Cl- flux responded normally to theophylline, and thymidine kinase and sucrase activities of cells isolated from jejunal villi were similar to those found in crypt cells. Probably by 40 hours after infection most virus has been shed from the mucosa. Viral diarrhoea clearly differs from enterotoxigenic diarrhoea. Consideration of its pathogenesis must take into account the dynamic nature of the mucosal epithelium and the factors governing differentiation of enterocytes as they migrate from crypt to villus. Sufficient information is available now to characterize one specific and apparently prevalent viral enteritis in man and to identify additional viral enteritides. There is hope that preventative therapy can be developed. Our understanding of the mechanisms of viral diarrhoea is limited, but the availability of an animal model and the promise of others makes us optimistic that these deficiencies can be remedied. Greater understanding of the pathogenesis of viral diarrhoea should better the active therapy of affected infants and children.

Transmissible gastroenteritis virus infection: a vanishing specter

About twenty years ago, a new coronavirus, porcine respiratory coronavirus (PRCoV), was detected in swine herds. This virus is related to transmissible gastroenteritis virus (TGEV); however, it is not enteropathogenic but causes only minor respiratory symptoms. As PRCoV shares some epitopes for neutralizing antibodies with TGEV, it acts like a nature-made vaccine against TGEV resulting in a drastic reduction of TGE outbreaks in Europe. A major difference between the two porcine coronaviruses is a large deletion in the surface protein S gene of PRCoV. Because of this structural difference, TGEV but not PRCoV has a sialic acid binding activity that allows the attachment to mucins and mucin-type glycoproteins. The sialic acid binding activity may allow TGEV to overcome the mucus barrier in the gut and to get access to the intestinal epithelium for initiation of infection.

Transmissible gastroenteritis coronavirus induces programmed cell death in infected cells through a caspase-dependent pathway

In this report, we show that apoptosis (or programmed cell death) is induced in different cell lines infected with a coronavirus, the porcine transmissible gastroenteritis virus (TGEV). Kinetic analysis of internucleosomal DNA cleavage by agarose gel electrophoresis and flow cytometry or cytometric monitoring of the mitochondrial transmembrane potential showed that, for ST cells infected with TGEV, the first overt signs of apoptosis appeared from 10 to 12 h postinfection on. They preceded morphological changes characteristic of cells undergoing apoptosis, as observed by light and electron microscopy. The tripeptide pan-ICE (caspase) inhibitor N-benzyloxycarbonyl-Val-Ala-Asp-fluoromethylketone blocked TGEV-induced apoptosis with no effect on virus production. The thiol agent pyrrolidine dithiocarbamate inhibited apoptosis, suggesting that TGEV infection may lead to apoptosis via cellular oxidative stress. The effect of TGEV infection on activation of NF-kappaB, a transcription factor known to be activated by oxidative stress, was examined. NF-kappaB DNA binding was shown to be strongly and quickly induced by TGEV infection. However, transcription factor decoy experiments showed that NF-kappaB activation is not critical for TGEV-induced apoptosis.

An immunohistochemical investigation of porcine epidemic diarrhoea

A sudden outbreak of epidemic diarrhoea of piglets occurred in Japan, the principal features being watery diarrhoea, dehydration and high mortality in newborn animals. The microscopical lesions were villous atrophy in the small intestine, the villous enterocytes being vacuolated and cuboidal in shape. The villus-crypt ratio was severely reduced, varying from 1:1 to 3:1. Transmission electron microscopy showed numerous coronaviruses within the cytoplasm of enterocytes and among microvilli. Specific antigens of porcine epidemic diarrhoea (PED) virus were detected in the cytoplasm of enterocytes by the streptavidin-biotin (SAB) technique. Infected cells, which were most abundant in the villous epithelia of the jejunum and ileum, were present in small numbers in the large intestine, the crypt epithelia, the lamina propria and Peyer's patches. The study suggests that the SAB technique is useful for the diagnosis of PED.

Quantification of individual subgenomic mRNA species during replication of the coronavirus transmissible gastroenteritis virus

A biotinylated-oligonucleotide-based method was used to isolate the subgenomic mRNAs of the coronavirus transmissible gastroenteritis virus (TGEV) to investigate the amounts of the mRNAs produced at early, middle and late times in the replication cycle. TGEV mRNA 6, which encodes the N protein, was observed to be the most abundant species throughout the replication cycle. The ratios of mRNA 6 to the other mRNAs were 1:0.11 (mRNA 2), 1:0.16 (mRNAs 3 and 4) and 1:0.37 (mRNA 5) at 12 h post-infection. All the mRNA species were differentially regulated throughout the replication cycle, although the rate of accumulation of mRNAs 4, 5 and 6, but not mRNA 3, increased markedly towards the end of the replication cycle. mRNA 7 was not detected in the system used. There was no observable correlation between the amounts of each mRNA synthesised and the potential degree of base pairing between the 3' end of the leader sequence and the transcription associated sequences on the genomic RNA at any time during the replication cycle. This indicates that the extent of base pairing was not the only factor involved in the control of subgenomic mRNA synthesis.

Intestinal permeability in piglets during transmissible gastroenteritis

Macromolecular permeability of the small intestine was tested in seven three-week-old piglets infected with porcine transmissible gastroenteritis virus (TGE-strain Miller). Fourteen hours after the infection, the piglets showed loss of appetite and a profuse diarrhoea. In some animals vomiting occurred somewhat earlier. Macromolecular permeability was tested morphologically by injecting horseradish peroxidase (MW = 40,000 Da) into the jejunal lumen just distally to the Treitz' ligament in two piglets at 12 hours and in five piglets at 48 hours after the inoculation in comparison with two control piglets. After a period of 20 minutes, small segments of jejunum were taken for stereomicro-scopical, histological and ultrastructural investigations. An increased permeability for HRP together with a severe, hyper-regenerative villous atrophy was observed in the TGE-infected piglets at 48 hours after the inoculation.

Intestinal permeability to polyethylene glycol 4000 and porcine albumin in piglets infected with transmissible gastroenteritis virus

The intestinal permeability of specific pathogen free piglets has been studied by measuring the concentration of 14C in the blood after oral administration of 14C polyethylene glycol (14C PEG, MW = 4000) and the concentration of 131I in the faeces after intraperitoneal administration of 131I porcine albumin (131I PA, MW = 68,000). The tests were performed one day before and up to two days after the piglets were infected with transmissible gastroenteritis (TGE) virus. Jejunal biopsies were taken from two piglets before the experimental infection, from two piglets 12 h after the experimental infection and from five piglets at the end of the experiment, 46 h after infection. Blood samples were taken six-hourly and faecal samples several times. Some piglets vomited before diarrhoea and loss of appetite started at 14 h after infection; the packed cell volume decreased before but increased after infection. Morphological examination showed hyperregenerative villous atrophy at 46 h after infection. There was no increase in the permeation of 14C PEG but there was a significant increase in the flux of 131I PA from the blood to the gut lumen.

Diminished brush border membrane Na-dependent L-alanine transport in acute viral enteritis in piglets

We studied sodium-dependent uptake of L-alanine into small intestinal brush border membrane vesicles (BBMV) isolated from piglets 40 h after infection with transmissible gastroenteritis (TGE) virus. Vesicles from TGE-infected pigs and uninfected litter-mate controls showed comparable degrees of enrichment and purity. In BBMV prepared by conventional techniques, [3H]L-alanine "overshoot" (peak uptake/equilibrium uptake) in the presence of a Na gradient was preserved in TGE BBMV, unlike [3H]D-glucose "overshoot," which was reduced. When these experiments were repeated using vesicles of greater purity, initial rates of Na-dependent L-alanine influx were reduced in BBMV from infected piglets under voltage clamped conditions with valinomycin. These studies demonstrate a specific amino acid transport defect in the small intestinal epithelium during acute viral diarrhea. They demonstrate too that brush border L-alanine-Na co-transport, although reduced, is present after viral damage, confirming previous studies that showed additive effects of amino acid and glucose on jejunal epithelial Na+ transport in transmissible gastroenteritis. Our findings support the concept that, in viral enteritis, oral rehydration solutions containing amino acid and glucose have a theoretical advantage over glucose electrolyte solutions because they facilitate brush border Na+ entry by two carrier mechanisms.

Absence of a cAMP-mediated antiabsorptive effect in an undifferentiated jejunal epithelium

In the relatively undifferentiated jejunal mucosa occurring in piglet viral enteritis, we measured the response of transepithelial Na+ and Cl- fluxes in vitro to raised intracellular adenosine 3',5'-cyclic monophosphate (cAMP) levels. At the acute 40-h stage of transmissible gastroenteritis (TGE), luminal membrane markers, sucrase and lactase, and a basolateral jejunal epithelial membrane marker Na+-K+-ATPase, were significantly decreased in activity, while a proliferative marker, thymidine kinase, was significantly enriched; these enzyme characteristics are typical of enterocytes isolated from crypts of other species. As expected, control piglet jejunum in short-circuited Ussing chambers after theophylline (10 mM) developed significant net secretory Na and Cl fluxes primarily due to significant antiabsorptive effects (delta JNa m----s = 3.48 +/- 0.52, delta JCl m----s = 2.59 +/- 0.28). Furosemide (10(-4) M), an inhibitor of electroneutral NaCl cotransport, produced antiabsorptive effects (delta JNa m----s = 2.53 +/- 0.31, delta JCl m----s = 2.58 +/- 0.28) in control jejunum that were not significantly different from those seen in response to theophylline. TGE jejunum, however, responded to theophylline not by an antiabsorptive effect but by significant electrogenic Cl- secretion (delta JCl s----m = 1.59 +/- 0.48); furosemide had no effect on ion fluxes in TGE tissue. Control and TGE jejunal mucosal homogenates did not differ in their basal or theophylline-stimulated levels of cAMP. We conclude that the relatively undifferentiated small intestine occurring in acute TGE does not generate either a cAMP-mediated antiabsorptive effect or a furosemide-mediated antiabsorptive effect.(ABSTRACT TRUNCATED AT 250 WORDS)

In vivo intestinal uptake of immunoreactive bovine albumin in piglet enteritis

We examined the uptake of bovine serum albumin (BSA) from the intestine into the circulation of 3-week-old piglets infected with transmissible gastroenteritis virus. Transfer of immunoreactive bovine serum albumin (iBSA) from the intestinal lumen into the circulation was enhanced during both the early invasive phase of this viral enteritis (12-h postinoculation) and the diarrheal phase (84-h postinoculation). In some animals, enhanced uptake persisted into the recovery phase, 324 h after inoculation. Gel filtration studies suggested that iBSA had the molecular size characteristic of native BSA; no immunoreactive fragments of BSA were detected. Based on studies of two animals, the half-life of iBSA approximated that of porcine albumin. Further study is required to determine the immunological consequences of the enhanced uptake of protein occurring during viral infection of the intestine.

Alanine enhances jejunal sodium absorption in the presence of glucose: studies in piglet viral diarrhea

We measured the response of jejunal sodium (Na) absorption to neutral amino acid (L-alanine) and to dipeptides (L-alanyl-L-alanine, glycylsarcosine) in normal piglets and in piglets with acute viral diarrhea after experimental infection with transmissible gastroenteritis (TGE) virus. In the TGE jejunum villi were blunted, crypts were deepened, and the epithelium was composed of relatively undifferentiated cells with reduced disaccharidase, decreased sodium-potassium-stimulated ATPase, and elevated thymidine kinase activities. The response of Na absorption to a maximal concentration of L-alanine (20 mM) or D-glucose (30 mM) was significantly blunted in TGE jejunum in Ussing chambers. However, the addition of L-alanine together with D-glucose caused a significantly greater increment of Na absorption than either L-alanine or D-glucose alone in control and TGE tissue. The effect of Na absorption of the dipeptide L-alanyl-L-alanine (10 mM), which was rapidly hydrolyzed by control and TGE mucosa, was similar to that of L-alanine (20 mM), while glycylsarcosine, a poorly hydrolyzed dipeptide, did not change net Na absorption in the jejunum. Our data support the concept of separate carrier systems for neutral amino acid and hexose in the crypt-type intestinal epithelium characterizing viral enteritis. We speculate that a sodium-cotransporting amino acid, if added to oral glucose-electrolyte solutions, could benefit oral rehydration therapy in acute viral diarrhea; neither of the dipeptides tested here can be expected to enhance absorption to any greater extent than its constituent amino acids.

D-Glucose transport in piglet jejunal brush-border membranes: insights from a disease model

We measured glucose transport in jejunal brush-border membrane vesicles isolated from piglets with acute viral diarrhea, comparing our results with those from control animals. Characterization of membranes from both study groups demonstrated comparable purity and integrity. In the presence of an inwardly directed Na SCN gradient, D-glucose accumulated in control vesicles to a concentration several times the 60-min equilibrium level. "Overshooting" uptake was much lower and more gradual in vesicles from 40-h transmissible gastroenteritis (TGE)-infected pigs compared with control pigs. Equilibrium kinetic studies, in which gramicidin was used to clamp membrane potential at zero, demonstrated a pattern of Na-dependent D-glucose transport in 40-h TGE-infected membranes that differed greatly from the control pattern. From an Eadie-Hofstee plot of stereospecific Na-dependent D-glucose uptake into control vesicles, a pattern suggesting two carrier populations emerged: one with a low-affinity, apparent Km equaling 52.63 +/- 13.81 mM and the other a high-affinity apparent Km equaling 3.92 +/- 0.24 mM for D-glucose. In 40-h TGE-infected membranes, the pattern conformed to a single line, suggesting a homogeneous population of low-affinity carriers, (Km = 37.03 +/- 1.92 mM), which did not differ from the low-affinity carriers seen in control animals. We conclude that the absence of the high-affinity D-glucose carriers in jejunal brush-border membrane is an important determinant of the defective glucose transport that characterizes viral diarrhea. Because previous studies have strongly suggested that in acute TGE diarrhea the epithelium is composed of relatively undifferentiated crypt-type cells, we speculate that high-affinity D-glucose carriers are lacking in normal crypt epithelial cells and that they are incorporated into brush-border membranes of jejunal enterocytes as the cells differentiate in the course of their migration from crypt to villus.

Altered jejunal permeability to macromolecules during viral enteritis in the piglet

We studied the macromolecular permeability of segments of jejunum from 2-wk-old piglets after the animals had been experimentally infected with an invasive enteric virus, transmissible gastroenteritis virus. Jejunal segments were mounted in Ussing chambers at stages of the infection, and permeability was measured using three probe molecules of differing molecular weights. In control tissue, permeability to horseradish peroxidase was 2.6 times higher across segments with Peyer's patches than across segments without Peyer's patches, whereas polyethylene glycol 4000 and mannitol permeabilities were the same in patch and nonpatch segments. Twelve hours after infection, when virus had invaded the mucosa causing a structural lesion, and before diarrhea had begun, horseradish peroxidase permeability increased in non-patch-containing segments to equal that across patch-containing tissue. At this early 12-h stage, polyethylene glycol 4000 and mannitol permeation were unchanged in patch-containing segments compared with controls. Ninety-six hours after transmissible gastroenteritis infection, when diarrhea was severe, horseradish peroxidase permeability in patch-free segments had returned to normal and patch-containing tissue permeability was diminished below control levels. Increased macromolecular permeability appears to occur only in the very early invasive stage of this viral enteritis and only in patch-free segments. Any consideration of the immunologic relevance of these complex phenomena must take into account the specialized function of the Peyer's patch regions of the small intestine.

Intestinal glucose transport in acute viral enteritis in piglets

1. We studied intestinal glucose transport in pigs during the acute and convalescent phases of an invasive viral enteritis, transmissible gastroenteritis. 2. When diarhoea was severe 40 h after experimental infection, net absorption of glucose, Na+ and water, measured by marker perfusion in the jejunum, was reduced; the enhancement of Na+ and water absorption in response to increasing perfusate glucose concentrations up to 120 mmol/l was diminished compared with the response observed in control and convalescent pigs. 3. Measured in vitro, 40 h after infection, unidirectional fluxes of 3-O-methyl-D-glucose across the jejunal epithelium were reduced and net absorption of the sugar was obliterated. Phlorizin (0.05 mmol/l), which completely inhibited net 3-O-methyl-D-glucose absorption in control tissue, had no significant effect on transmissible gastroenteritis jejunum. 4. Our data suggest that in this invasive viral enteritis, which closely resembles human rotavirus enteritis, glucose absorption is impaired as a result of defects in both active and passive glucose flux. 5. Differences between the mechanisms of viral diarrhoea, demonstrated by our study and those of the enterotoxigenic diarrhoeas, should be taken into consideration in formulating active therapeutic measures for children with acute viral diarrhoea.

Determinants of diarrhea in viral enteritis. The role of ion transport and epithelial changes in the ileum in transmissible gastroenteritis in piglets

To understand mechanisms of viral diarrhea further, we studied ileal ion transport in vitro in relation to mucosal changes and epithelial differentiation in transmissible gastroenteritis in piglets, an invasive viral enteritis thought to involve mainly proximal intestine. In infected pigs, at the height of diarrhea, short-circuited ileal epithelium failed actively to transport Na+ and Cl-, and there was a defect of glucose-mediated Na+ transport. The Cl- secretory response to theophylline remained intact. Conductance measurements indicate that paracellular permeability may be reduced and transcellular transport may be altered. A mucosal lesion was observed at the time of the transport changes, characterized by villus blunting, crypt hyperplasia, and immature crypt-type enterocytes on the villus epithelium, deficient in disaccharidase and (Na+, K+)ATPase activity but rich in thymidine kinase. Consideration of the major determinants of diarrhea in this invasive enteritis must take into account not only altered mucosal function and differentiation but also the extent of intestinal involvement, including the ileum, a major site of fluid absorption in the intestine.

Transmissible gastroenteritis: sodium transport and the intestinal epithelium during the course of viral enteritis

Sodium transport, mucosal structure, and epithelial enzymes were studied in piglets killed 10, 25, 40, 72, or 144 hr after infection with a standard dose of transmissible gastroenteritis virus. Glucose-stimulated Na transport measured in short-circuited jejunal epithelium and suspensions of villous enterocytes became progressively more abnormal during the first 40 hr, but recovered completely by 144 hr. As Na transport deteriorated, jejunal mucosal villi shortened and crypts deepened; cells isolated from the villi became more crypt-like in their enzyme profile, with high levels of thymidine kinase and low levels of sucrase activity 40 hr after infection. At 40 hr, when diarrhea is severe, little if any virus has been found in the epithelium. Our data suggest that the relatively undifferentiated crypt type enterocytes on the villi constitute an important determinant of altered Na transport and diarrhea in this invasive viral enteritis.

Transmissible gastroenteritis. Mechanisms responsible for diarrhea in an acute viral enteritis in piglets

We studied 3-wk-old piglets 40 h after experimental infection with transmissible gastroenteritis (TGE) virus to identify the mechanisms of diarrhea in this disease and to better understand infectious diarrhea in humans. Using continuous segmental marker perfusion in four regions along the gut, we found significant increases in net intraluminal accumulation of water and electrolytes only in the proximal jejunum, the region infected by the virus. In this jejunal segment studied in vivo, unidirectional sodium flux, extracellular fluid (ECF) to lumen, significantly increased, lumen to ECF significantly decreased, compared with matchfed littermates. The standard perfusate rendered hypertonic by adding mannitol (450 mosmol/kg), in the same segment of normal pigs, caused only an increase in ECF to lumen flux of sodium. TGE did not alter gross villous structure or intraluminal bacteria, bile salts, lactate, pH, or osmolality. Epithelial cell migration was accelerated in the jejunum of infected pigs. Isolated in suspension, these cells from TGE pigs exhibited increased active and passive sodium efflux, cells from mannitol-perfused pigs exhibited only increased active sodium efflux. In this viral enteritis, altered sodium transport occurring in the jejunum, the region of the intestine infected appears to be associated with defective epithelial cell function. The precise nature of the abnormalities in sodium transport, their relationship to disturbances of transport of other solutes, and to virus epithelial cell interaction remain to be defined.

Epithelial cell dynamics in transmissible gastroenteritis of neonatal pigs

Tritiated thymidine was given to pigs that had been exposed to transmissible gastroenteritis virus, and specimens of small intestine for autoradiographic studies were subsequently obtained. There was villous atrophy in the jejunum and ileum, but not duodenum, of the infected pigs by 48 h after exposure. Rates of migration of epithelial cells were substantially greater, and DNA synthesis occurred at a greater distance from the base of the crypts in the jejunum and ileum of infected than control pigs. Total numbers of labeled cells were two to three times greater in the jejunum and ileum of infected than control pigs. These findings indicate that viral destruction of villous epithelial cells results in an increased rate of cell migration in the jejunum and ileum, which is responsible for the appearance of incompletely differentiated cells on the villi of pigs with transmissible gastroenteritis.