The Inflammatory Response to Enterotoxigenic E. coli and Probiotic E. faecium in a Coculture Model of Porcine Intestinal Epithelial and Dendritic Cells

Transport of Neutral Amino Acids in the Jejunum of Pigs with Special Consideration of L-Methionine

Background: Methionine (Met) is a popular nutritional supplement in humans and animals. It is routinely supplemented to pigs as L-Met, DL-Met, or DL-2-hydroxy-4-(methylthio) butanoic acid (DL-HMTBA). Methods: We investigated the effect of these Met supplements on jejunal amino acid (AA) transport in male castrated Piétrain × Danbred pigs, also including a non-supplemented group. The mucosal-to-serosal flux of ten [14C]-labeled AAs (L-glutamine, glycine, L-leucine, L-lysine, L-Met, L-serine, L-threonine, L-tryptophan, L-tyrosine and L-valine) was investigated at two concentrations (50 µM and 5 mM). Inhibition of apical uptake by mucosal L-Met was also measured for these AAs. The intestinal expression of apical AA transporters, angiotensin-converting enzyme II and inflammation-related genes were compared with those of a previous study. Results: Except for tryptophan and lysine at 5 mM, all AA fluxes were Na+-dependent (p ≤ 0.05), and the uptake of most AAs, except glycine and lysine, was inhibited by L-Met (p < 0.001). A correlation network existed between Na+-dependent fluxes of most AAs (except tryptophan and partly glycine). We observed the upregulation of B0AT1 (SLC6A19) (p < 0.001), the downregulation of ATB0,+ (SLC6A14) (p < 0.001) and a lower expression of CASP1, IL1β, IL8, TGFβ and TNFα in the present vs. the previous study (p < 0.001). Conclusions: The correlating AAs likely share the same Na+-dependent transporter(s). A varying effect of the Met supplement type on AA transport in the two studies might be related to a different level of supplementation or a different inflammatory status of the small intestine.

Escherichia coli Activate Extraintestinal Antibody Response and Provide Anti-Infective Immunity

The effects of intestinal microflora on extraintestinal immune response by intestinal cytokines and metabolites have been documented, but whether intestinal microbes stimulate serum antibody generation is unknown. Here, serum antibodies against 69 outer membrane proteins of Escherichia coli, a dominant bacterium in the human intestine, are detected in 141 healthy individuals of varying ages. Antibodies against E. coli outer membrane proteins are determined in all serum samples tested, and frequencies of antibodies to five outer membrane proteins (OmpA, OmpX, TsX, HlpA, and FepA) are close to 100%. Serum antibodies against E. coli outer membrane proteins are further validated by Western blot and bacterial pull-down. Moreover, the present study shows that OstA, HlpA, Tsx, NlpB, OmpC, YfcU, and OmpA provide specific immune protection against pathogenic E. coli, while HlpA and OmpA also exhibit cross-protection against Staphylococcus aureus infection. These finding indicate that intestinal E. coli activate extraintestinal antibody responses and provide anti-infective immunity.

Effects of Luteolin in an In Vitro Model of Porcine Intestinal Infections

Intestinal infections caused by Escherichia coli and Salmonella enterica pose a huge economic burden on the swine industry that is exacerbated by the development of antimicrobial resistance in these pathogens, thus raising the need for alternative prevention and treatment methods. Our aim was to test the beneficial effects of the flavonoid luteolin in an in vitro model of porcine intestinal infections. We infected the porcine intestinal epithelial cell line IPEC-J2 with E. coli and S. enterica subsp. enterica serovar Typhimurium (106 CFU/mL) with or without previous, concurrent, or subsequent treatment with luteolin (25 or 50 µg/mL), and measured the changes in the reactive oxygen species and interleukin-6 and -8 levels of cells. We also tested the ability of luteolin to inhibit the adhesion of bacteria to the cell layer, and to counteract the barrier integrity damage caused by the pathogens. Luteolin was able to alleviate oxidative stress, inflammation, and barrier integrity damage, but it could not inhibit the adhesion of bacteria to IPEC-J2 cells. Luteolin is a promising candidate to be used in intestinal infections of pigs, however, further studies are needed to confirm its efficacy. The use of luteolin in the future could ultimately lead to a reduced need for antibiotics in pig production.

Immunomodulatory effect of probiotic exopolysaccharides in a porcine in vitro co-culture model mimicking the intestinal environment on ETEC infection

The aim of this study was to evaluate the immunomodulatory effect of EPS-L26 isolated from the probiotic strain Lactobacillus (Limosilactobacillus) reuteri L26 Biocenol™, in a model of infection with an enterotoxigenic E. coli (ETEC) by establishing monocultures consisting of the IPEC-J2 cell line or monocyte-derived dendritic cells (moDCs) and creating a 3D model of cell co-cultures established with IPEC-J2 cells and moDCs. The immunomodulatory and immunoprotective potential of used EPS-L26 was confirmed in monocultures in an experimental group of pretreated cells, where our study showed that pretreatment of cells with EPS-L26 and subsequent exposure to infection resulted in significantly down-regulated mRNA levels of genes encoding inflammatory cytokines compared to ETEC challenge in single cell cultures (in IPEC-J2, decreased mRNA levels for TNF-α, IL-6, IL-1β, IL-12p35; in moDCs, decreased mRNA levels for IL-1β). Similar to monocultures, we also demonstrated the immunostimulatory potential of the ETEC strain in the co-culture model on directly treated IPEC-J2 cells cultivated on insert chambers (apical compartment) and also on indirectly treated moDCs cultivated in the lower chamber (basolateral compartment), however in the co-culture model the expression of inflammatory cytokines was attenuated at the mRNA level compared to monocultures. Pretreatment of the cells on the insert chambers pointed to the immunoprotective properties of EPS-L26, manifested by decreased mRNA levels in both cell lines compared to ETEC challenge (in IPEC-J2 decreased mRNA levels for IL-12p35; in moDCs decreased mRNA levels for IL-1β, IL-6). Our results suggest intercellular communication via humoral signals derived from IPEC-J2 cells by influencing the gene expression of indirectly treated moDC cells located in the basolateral compartment.

Advanced Organotypic In Vitro Model Systems for Host-Microbial Coculture

In vitro model systems have been advanced to recapitulate important physiological features of the target organ in vivo more closely than the conventional cell line cultures on a petri dish. The advanced organotypic model systems can be used as a complementary or alternative tool for various testing and screening. Numerous data from germ-free animal studies and genome sequencings of clinical samples indicate that human microbiota is an essential part of the human body, but current in vitro model systems rarely include them, which can be one of the reasons for the discrepancy in the tissue phenotypes and outcome of therapeutic intervention between in vivo and in vitro tissues. A coculture model system with appropriate microbes and host cells may have great potential to bridge the gap between the in vitro model and the in vivo counterpart. However, successfully integrating two species in one system introduces new variables to consider and poses new challenges to overcome. This review aims to provide perspectives on the important factors that should be considered for developing organotypic bacterial coculture models. Recent advances in various organotypic bacterial coculture models are highlighted. Finally, challenges and opportunities in developing organotypic microbial coculture models are also discussed.

The Growth Medium Affects the Viability of IPEC-J2 Animal Cell Line in the Presence of Probiotic Bacteria

Background: The IPEC-J2 cell line is frequently used as an in vitro model to study the bioactivity of live probiotics. However, lactic acid bacteria (LB) acidify the medium, and the impact of pH and lactic acid accumulation on cell viability seem to be underestimated. Methods: IPEC-J2 viability was assessed by neutral red and flow cytometry in the presence of eight probiotics at concentrations between 106 and 109 bacteria/mL in maintenance and buffered media. Results: It was shown that a high inoculation level led to higher cytotoxic effects on IPEC-J2 cells after 22 h of incubation and that viability losses were more related to a combination of low pH and lactic acid than to the probiotics themselves. Furthermore, with LB at 106 and 107 bacteria/mL, the addition of phosphates to the media significantly reduced the drop in the pH and preserved the IPEC-J2 viability between 100% and 69%, compared to a highly variable viability between 100% and 17.5% in the unbuffered media. Conclusions: Under certain in vitro conditions, probiotics can lead to the deterioration of animal cells, and pH neutralization is an essential parameter in the cell–probiotic system in order to preserve cell viability and to better evaluate the bioactive properties of live probiotics.

cGAS modulates cytokine secretion and bacterial burdens by altering the release of mitochondrial DNA in Pseudomonas pulmonary infection

Cyclic GMP-AMP synthase (cGAS) is essential for fighting against viruses and bacteria, but how cGAS is involved in host immune response remains largely elusive. Here, we uncover the crucial role of cGAS in host immunity based on a Pseudomonas aeruginosa pulmonary infection model. cGAS^-/- mice showed more heavy bacterial burdens and serious lung injury accompanied with exorbitant proinflammatory cytokines than wild-type mice. cGAS deficiency caused an accumulation of mitochondrial DNA in cytoplasm, which in turn induced excessive secretion of proinflammatory factors by activating inflammasome and TLR9 signaling. Mechanistically, cGAS deficiency inhibited the recruitment of LC3 by reducing the binding capacity of TBK-1 to p62, leading to impaired mitophagy and augmented release of mitochondrial DNA. Importantly, cytoplasmic mitochondrial DNA also acted as a feedback signal that induced the activation of cGAS. Altogether, these findings identify protective and homeostasis functions of cGAS against Pseudomonas aeruginosa infection, adding significant insight into the pathogenesis of bacterial infectious diseases.

Dietary methionine source alters the lipidome in the small intestinal epithelium of pigs

Methionine (Met) as an essential amino acid has key importance in a variety of metabolic pathways. This study investigated the influence of three dietary Met supplements (0.21% L-Met, 0.21% DL-Met and 0.31% DL-2-hydroxy-4-(methylthio)butanoic acid (DL-HMTBA)) on the metabolome and inflammatory status in the small intestine of pigs. Epithelia from duodenum, proximal jejunum, middle jejunum and ileum were subjected to metabolomics analysis and qRT-PCR of caspase 1, NLR family pyrin domain containing 3 (NLRP3), interleukins IL1β, IL8, IL18, and transforming growth factor TGFβ. Principal component analysis of the intraepithelial metabolome revealed strong clustering of samples by intestinal segment but not by dietary treatment. However, pathway enrichment analysis revealed that after L-Met supplementation polyunsaturated fatty acids (PUFA) and tocopherol metabolites were lower across small intestinal segments, whereas monohydroxy fatty acids were increased in distal small intestine. Pigs supplemented with DL-HMTBA showed a pronounced shift of secondary bile acids (BA) and sphingosine metabolites from middle jejunum to ileum. In the amino acid super pathway, only histidine metabolism tended to be altered in DL-Met-supplemented pigs. Diet did not affect the expression of inflammation-related genes. These findings suggest that dietary supplementation of young pigs with different Met sources selectively alters lipid metabolism without consequences for inflammatory status.

Porcine and Chicken Intestinal Epithelial Cell Models for Screening Phytogenic Feed Additives—Chances and Limitations in Use as Alternatives to Feeding Trials

Numerous bioactive plant additives have shown various positive effects in pigs and chickens. The demand for feed additives of natural origin has increased rapidly in recent years to support the health of farm animals and thus minimize the need for antibiotics and other drugs. Although only in vivo experiments can fully represent their effect on the organism, the establishment of reliable in vitro methods is becoming increasingly important in the goal of reducing the use of animals in experiments. The use of cell models requires strict control of the experimental conditions so that reliability and reproducibility can be achieved. In particular, the intestinal porcine epithelial cell line IPEC-J2 represents a promising model for the development of new additives. It offers the possibility to investigate antioxidative, antimicrobial, anti- or pro-proliferative and antiviral effects. However, the use of IPEC-J2 is limited due to its purely epithelial origin and some differences in its morphology and functionality compared to the in vivo situation. With regard to chickens, the development of a reliable intestinal epithelial cell model has attracted the attention of researchers in recent years. Although a promising model was presented lately, further studies are needed to enable the standardized use of a chicken cell line for testing phytogenic feed additives. Finally, co-cultivation of the currently available cell lines with other cell lines and the development of organoids will open up further application possibilities. Special emphasis was given to the IPEC-J2 cell model. Therefore, all publications that investigated plant derived compounds in this cell line were considered. The section on chicken cell lines is based on publications describing the development of chicken intestinal epithelial cell models.

Protective Effects of Grape Seed Oligomeric Proanthocyanidins in IPEC-J2–Escherichia coli/Salmonella Typhimurium Co-Culture

Intestinal epithelium provides the largest barrier protecting mammalian species from harmful external factors; however, it can be severely compromised by the presence of bacteria in the gastrointestinal (GI) tract. Antibiotics have been widely used for the prevention and treatment of GI bacterial infections, leading to antimicrobial resistance in human and veterinary medicine alike. In order to decrease antibiotic usage, natural substances, such as flavonoids, are investigated to be used as antibiotic alternatives. Proanthocyanidins (PAs) are potential candidates for this purpose owing to their various beneficial effects in humans and animals. In this study, protective effects of grape seed oligomeric proanthocyanidins (GSOPs) were tested in IPEC-J2 porcine intestinal epithelial cells infected with Escherichia coli and Salmonella enterica ser. Typhimurium of swine origin. GSOPs were able to alleviate oxidative stress, inflammation and barrier integrity disruption inflicted by bacteria in the co-culture. Furthermore, GSOPs could decrease the adhesion of both bacteria to IPEC-J2 cells. Based on these observations, GSOPs seem to be promising candidates for the prevention and treatment of gastrointestinal bacterial infections.

Fascinating Dendritic Cells—Sentinel Cells of the Immune System a Review

Dendritic cells (DC) are specialized antigen presenting cells which have the unique ability to activate naive T-lymphocytes. Their role in the immune system is much more sophisticated than it seems, as they do not kill the pathogens directly, but provide a long-lasting antigen specific immune response thanks to that sufficiently bridging the innate and the adaptive immunity. In recent years, there has been a growing interest in studies of their role in immune regulation, autoimmune reactions, as well as in immune responses against pathogens and tumours. Processing and presentation capabilities of a highly specific and unique tumour antigen makes them an interesting tool for stimulating effective anti-tumour immunity. In vitro generations of DC represent a preferred model for more detailed studies of DC biology in other fields. The aim of this review was to discuss the main role of dendritic cells in the body as well as their current use as experimental models for further scientific studies.

Effects of dietary gamma-aminobutyric acid supplementation on amino acid profile, intestinal immunity, and microbiota in ETEC-challenged piglets

Enterotoxigenic Escherichia coli (ETEC) infection is the most common cause of diarrhea in piglets, and ETEC could increase intestinal gamma-aminobutyric acid (GABA)-producing bacteria to affect intestinal immunity. However, the effect of GABA on ETEC-infected piglets is still unclear. This study aims at investigating the impact of dietary GABA supplementation on the growth performance, diarrhea, intestinal morphology, serum amino acid profile, intestinal immunity, and microbiota  in the ETEC-infected piglet model. Eighteen piglets were randomly divided into two groups, in which the piglets were fed with a basal diet with 20 mg kg-1 GABA supplementation or not. The experiment lasted for three weeks, and the piglets were challenged with ETEC K88 on the fifteenth day. The results showed that dietary GABA reduced the feed conversion ratio, promoted the kidney organ index but did not affect the diarrheal score and small intestinal morphology in ETEC-challenged piglets. Ileal mucosal amino acids (such as carnosine and anserine) and serum amino acids (including threonine and GABA) were increased upon GABA supplementation. GABA enhanced ileal gene expression of TNF-α, IFN-γ, pIgR, and MUC2, while inhibited the ileal expression of IL-18 in ETEC-challenged piglets. GABA supplementation also highly regulated the intestinal microbiota by promoting community richness and diversity and reducing the abundance of the dominant microbial population of the ileal microbiota. Collectively, GABA improves growth performance, regulates the serum amino acid profile, intestinal immunity, and gut microbiota in ETEC-challenged piglets. This study is a fine attempt to reveal the function of GABA in ETEC-infected piglets. It would contribute to the understanding of the roles of exogenous nutrition on the host response to ETEC infection.

Elevated Extracellular cGMP Produced after Exposure to Enterotoxigenic Escherichia coli Heat-Stable Toxin Induces Epithelial IL-33 Release and Alters Intestinal Immunity

Enterotoxigenic Escherichia coli (ETEC) is a major diarrheal pathogen in children in low- to middle-income countries. Previous studies identified heat-stable enterotoxin (ST)-producing ETEC as a prevalent diarrheal pathogen in children younger than 5 years.

Enterotoxigenic Escherichia coli (ETEC) is a major diarrheal pathogen in children in low- to middle-income countries. Previous studies identified heat-stable enterotoxin (ST)-producing ETEC as a prevalent diarrheal pathogen in children younger than 5 years. While many studies have evaluated the interaction of ETEC heat-labile enterotoxin (LT) with host epithelium and immunity, few investigations have attempted similar studies with ST. To further understand ST pathogenesis, we examined the impact of ST on cGMP localization, epithelial cell cytokine production, and antibody development following immunization. In addition to robust intracellular cGMP in T84 cells in the presence of phosphodiesterase inhibitors (PDEis) that prevent the breakdown of cyclic nucleotides, we found that prolonged ST intoxication induced extracellular cGMP accumulation in the presence or absence of PDEis. Further, ST intoxication induced luminal cGMP in vivo in mice, suggesting that secreted cGMP may have other cellular functions. Using transcriptome sequencing (RNA-seq) and quantitative PCR (qPCR), we demonstrated that ST intoxication, or treatment with the clinically used ST mimic linaclotide, altered inflammatory cytokine gene expression, including the interleukin 1 (IL-1) family member IL-33, which could also be induced by cell-permeative 8-Br-cGMP. Finally, when present during immunization, ST suppressed induction of antibodies to specific antigens. In conclusion, our studies indicate that ST modulates epithelial cell physiology and the interplay between the epithelial and immune compartments.

Inflammatory Responses of Porcine MoDC and Intestinal Epithelial Cells in a Direct-Contact Co-culture System Following a Bacterial Challenge

Intestinal epithelial cells (IEC) and immune cells, such as dendritic cells (DC), jointly control the immune response towards luminal pathogens in the intestinal mucosa. Crosstalk between IEC and DC is crucial for coordinating immune responses and occurs via soluble factors and direct cell-cell contacts. The present study aimed at establishing a direct-contact co-culture model of porcine IEC and DC to mimic these interactions. The effects of (1) co-cultivation of the two cell types and (2) bacterial infection on the inflammatory response patterns of each of the cell types were determined with a special focus on the canonical and non-canonical inflammasome signaling pathways. In infection experiments, in vitro cultures were exposed to either the probiotic Enterococcus (E.) faecium NCIMB 10415 or enterotoxigenic Escherichia coli (ETEC). In porcine IEC (IPEC-J2), co-cultivation with porcine monocyte-derived DC (MoDC) resulted in reduced basal NLRP3 (nucleotide oligomerization domain [NOD]-like receptor [NLR] family, pyrin domain containing 3) inflammasome mRNA levels in unstimulated conditions. In porcine MoDC, the presence of IPEC-J2 cells evoked a noticeable decrease of interleukin (IL)-8 and transforming growth factor-β (TGF-β) mRNA and protein expression. ETEC, in contrast to E. faecium, modulated the inflammasome pathway in IPEC-J2 cells and porcine MoDC. Co-cultured IPEC-J2 cells showed an augmented inflammasome response to ETEC infection. By contrast, MoDC revealed a weakened ETEC response under such co-culture conditions as indicated by a reduction of inflammasome-related IL-1β protein release. Our data indicate that the close contact between IEC and resident immune cells has a major effect on their immunological behavior.

Immunogenicity and protective efficacy of enterotoxigenic Escherichia coli (ETEC) total RNA against ETEC challenge in a mouse model

Enterotoxigenic Escherichia coli (ETEC), an essential cause of post-weaning diarrhea (PWD) in piglets, leads to significant economic losses to the pig industry. The present study aims to identify the role of ETEC total RNA in eliciting immune responses to protect animals against ETEC infection. The results showed that the total RNA isolated from pig-derived ETEC K88ac strain effectively stimulated the IL-1β secretion of porcine intestinal epithelial cells (IPEC-J2). The mouse model immunized with ETEC total RNA via intramuscular injection (IM) or oral route (OR) was used to evaluate the protective efficiency of the ETEC total RNA. The results suggested that 70 μg ETEC total RNA administered by either route significantly promoted the production of the serum IL-1β and K88ac specific immunoglobulins (IgG, IgM, and IgA). Besides, the ETEC RNA administration augmented strong mucosal immunity by elevating K88ac specific IgA level in the intestinal fluid. Intramuscularly administered RNA induced a Th1/Th2 shift toward a Th2 response, while the orally administered RNA did not. The ETEC total RNA efficiently protected the animals against the ETEC challenge either by itself or as an adjuvant. The histology characterization of the small intestines also suggested the ETEC RNA administration protected the small intestinal structure against the ETEC infection. Particularly of note was that the immunity level and protective efficacy caused by ETEC RNA were dose-dependent. These findings will help understand the role of bacterial RNA in eliciting immune responses, and benefit the development of RNA-based vaccines or adjuvants.

Effects of lipopolysaccharide exposure in primary bovine ruminal epithelial cells

The objective of this study was to investigate whether cultured ruminal epithelial cells (REC) responded to lipopolysaccharide (LPS) stimulation and determine whether LPS induced a proinflammatory response. Primary bovine REC were isolated and grown in culture for 2 studies. In study 1, REC were isolated from Holstein bull calves (n = 8) and grown in culture for 10 to 12 d. Cells were then exposed to 0, 10,000, 50,000, or 200,000 endotoxin (E)U/mL of LPS (Escherichia coli O55:B5) for either 6 or 24 h. The effect of LPS exposure on cell viability was analyzed by flow cytometry using a propidium iodide stain. In study 2, cells were isolated from Holstein bull calves (n = 5) and yearling beef heifers (n = 4). Cells were exposed to either 1,000 or 50,000 EU/mL of LPS using the following conditions: (1) medium alone time-matched controls, (2) 12-h LPS exposure, (3) 24 h of LPS exposure, (4) 36 h of LPS exposure, (5) 12 h of LPS exposure followed by LPS removal for 24 h before restimulating with LPS for an additional 12 h (RPT), and (6) 12 h of LPS exposure followed by LPS removal for 36 (RVY). For both experiments, total RNA was extracted from REC and real-time quantitative PCR was performed to determine relative expression of genes for toll-like receptors (TLR2 and TLR4), proinflammatory cytokines (TNF and IL1B), chemokines (CXCL2 and CXCL8), a lipid mediator (PTGS2), and growth factor-like cytokines (CSF2 and IL7). In study 1, LPS exposure did not negatively affect cell viability. Treatment of cells with LPS resulted in increased transcript abundance for all genes analyzed. The TLR2, IL7, and TLR4 had a greater magnitude of change at 6 h compared with 24 h. Quadratic expression patterns were detected for TNF, IL1B, CXCL2, CXCL8, and CSF2. These results suggested that REC increase expression of proinflammatory genes following exposure to LPS. In study 2, all genes analyzed were upregulated in a quadratic manner following exposure to LPS for different time intervals. The TLR4, TNF, CXCL2, CXCL8, CSF2, and IL7 gene expression was significantly greater after a single 12 h of LPS exposure than after RPT exposure, suggesting repeated exposure of REC to LPS may induce a tolerogenic effect. When LPS was removed from the medium (RVY), transcript abundance for all genes analyzed decreased and expression of TLR2, TLR4, and IL7 returned to baseline levels, suggesting REC recovered following exposure to LPS. Overall, the data suggest cultured REC respond to LPS stimulation by increasing transcription of proinflammatory genes and this transcriptional response was influenced by the dose, duration, and frequency of LPS exposure.

Cyclic nucleotides, gut physiology and inflammation

Misregulation of gut function and homeostasis impinges on the overall well-being of the entire organism. Diarrheal disease is the second leading cause of death in children under 5 years of age, and globally, 1.7 billion cases of childhood diarrhea are reported every year. Accompanying diarrheal episodes are a number of secondary effects in gut physiology and structure, such as erosion of the mucosal barrier that lines the gut, facilitating further inflammation of the gut in response to the normal microbiome. Here, we focus on pathogenic bacteria-mediated diarrhea, emphasizing the role of cyclic adenosine 3',5'-monophosphate and cyclic guanosine 3',5'-monophosphate in driving signaling outputs that result in the secretion of water and ions from the epithelial cells of the gut. We also speculate on how this aberrant efflux and influx of ions could modulate inflammasome signaling, and therefore cell survival and maintenance of gut architecture and function.