Antibodies in the small intestine: mucosal synthesis and deposition of anti-glycosyl IgA, IgM, and IgG in the enterocyte brush border

Fecal Protein Profile in Eight Dogs Suffering from Acute Uncomplicated Diarrhea before and after Treatment

Acute diarrhea is a very frequent condition affecting dogs; nevertheless, little is known about what happens in the GI tract during such conditions. Proteomics allows the study of proteins present in a specific biologic substrate, and fecal proteomic investigations have been recently implemented to study GI diseases in dogs. In the present study, the fecal protein profiles of eight dogs suffering from acute uncomplicated diarrhea at the time of inclusion was investigated for the first time, and then the same patients were followed, replicating two further evaluations at two subsequent time points (after 2 and 14 days from the first presentation), with the aim of gaining possible new insights regarding the pathologic changes in the gastrointestinal environment during such conditions. Two-dimensional gel electrophoresis (2-DE) was performed, followed by mass spectrometry. Nine spots, corresponding to four (groups of) proteins (i.e., albumin, alkaline phosphatase, chymotrypsin-C-like, and some immunoglobulins), showed significant differences at two or more of the three time points investigated, almost all behaving similarly and decreasing at T1 (2 days after the onset of the condition) and significantly increasing at T2 (14 days after the onset), mainly evidencing a reaction of the organism. Further studies including a greater number of patients and possibly different techniques are needed to confirm the present findings.

Heritability Enrichment of Immunoglobulin G N-Glycosylation in Specific Tissues

Genome-wide association studies (GWAS) have identified over 60 genetic loci associated with immunoglobulin G (IgG) N-glycosylation; however, the causal genes and their abundance in relevant tissues are uncertain. Leveraging data from GWAS summary statistics for 8,090 Europeans, and large-scale expression quantitative trait loci (eQTL) data from the genotype-tissue expression of 53 types of tissues (GTEx v7), we derived a linkage disequilibrium score for the specific expression of genes (LDSC-SEG) and conducted a transcriptome-wide association study (TWAS). We identified 55 gene associations whose predicted levels of expression were significantly associated with IgG N-glycosylation in 14 tissues. Three working scenarios, i.e., tissue-specific, pleiotropic, and coassociated, were observed for candidate genetic predisposition affecting IgG N-glycosylation traits. Furthermore, pathway enrichment showed several IgG N-glycosylation-related pathways, such as asparagine N-linked glycosylation, N-glycan biosynthesis and transport to the Golgi and subsequent modification. Through phenome-wide association studies (PheWAS), most genetic variants underlying TWAS hits were found to be correlated with health measures (height, waist-hip ratio, systolic blood pressure) and diseases, such as systemic lupus erythematosus, inflammatory bowel disease, and Parkinson's disease, which are related to IgG N-glycosylation. Our study provides an atlas of genetic regulatory loci and their target genes within functionally relevant tissues, for further studies on the mechanisms of IgG N-glycosylation and its related diseases.

Comparison of Human Milk Immunoglobulin Survival during Gastric Digestion between Preterm and Term Infants

Human milk provides immunoglobulins (Igs) that supplement the passive immune system of neonates; however, the extent of survival of these Igs during gastric digestion and whether this differs between preterm and term infants remains unknown. Human milk, and infant gastric samples at 2 h post-ingestion were collected from 15 preterm (23⁻32 week gestational age (GA)) mother-infant pairs and from 8 term (38⁻40 week of GA) mother-infant pairs within 7⁻98 days postnatal age. Samples were analyzed via ELISA for concentration of total IgA (secretory IgA (SIgA)/IgA), total secretory component (SC/SIgA/SIgM), total IgM (SIgM/IgM), and IgG as well as peptidomics. Total IgA concentration decreased by 60% from human milk to the preterm infant stomach and decreased by 48% in the term infant stomach. Total IgM and IgG concentrations decreased by 33% and 77%, respectively, from human milk to the term infant stomach but were stable in the preterm infant stomach. Release of peptides from all Ig isotypes in the term infant stomach was higher than in the preterm stomach. Overall, the stability of human milk Igs during gastric digestion is higher in preterm infant than in term infants, which could be beneficial for assisting the preterm infants' immature immune system.

Antibodies Damage the Resilience of Fimbriae, Causing Them To Be Stiff and Tangled

As adhesion fimbriae are a major virulence factor for many pathogenic Gram-negative bacteria, they are also potential targets for antibodies. Fimbriae are commonly required for initiating the colonization that leads to disease, and their success as adhesion organelles lies in their ability to both initiate and sustain bacterial attachment to epithelial cells. The ability of fimbriae to unwind and rewind their helical filaments presumably reduces their detachment from tissue surfaces with the shear forces that accompany significant fluid flow. Therefore, the disruption of functional fimbriae by inhibiting this resilience should have high potential for use as a vaccine to prevent disease. In this study, we show that two characteristic biomechanical features of fimbrial resilience, namely, the extension force and the extension length, are significantly altered by the binding of antibodies to fimbriae. The fimbriae that were studied are normally expressed on enterotoxigenic Escherichia coli, which are a major cause of diarrheal disease. This alteration in biomechanical properties was observed with bivalent polyclonal antifimbrial antibodies that recognize major pilin subunits but not with the Fab fragments of these antibodies. Thus, we propose that the mechanism by which bound antibodies disrupt the uncoiling of natural fimbria under force is by clamping together layers of the helical filament, thereby increasing their stiffness and reducing their resilience during fluid flow. In addition, we propose that antibodies tangle fimbriae via bivalent binding, i.e., by binding to two individual fimbriae and linking them together. Use of antibodies to disrupt physical properties of fimbriae may be generally applicable to the large number of Gram-negative bacteria that rely on these surface-adhesion molecules as an essential virulence factor.

IMPORTANCE

Our study shows that the resiliency of colonization factor antigen I (CFA/I) and coli surface antigen 2 (CS2) fimbriae, which are current targets for vaccine development, can be compromised significantly in the presence of antifimbrial antibodies. It is unclear how the humoral immune system specifically interrupts infection after the attachment of enterotoxigenic Escherichia coli (ETEC) to the epithelial surface. Our study indicates that immunoglobulins, in addition to their well-documented role in adaptive immunity, can mechanically damage the resilience of fimbriae of surface-attached ETEC, thereby revealing a new mode of action. Our data suggest a mechanism whereby antibodies coat adherent and free-floating bacteria to impede fimbrial resilience. Further elucidation of this possible mechanism is likely to inform the development and refinement of preventive vaccines against ETEC diarrhea.

IgG trafficking in the adult pig small intestine: one- or bidirectional transfer across the enterocyte brush border?

Immunoglobulin G (IgG) transfer in opposite directions across the small intestinal brush border serves different purposes in early life and in adulthood. In the neonate, maternal IgG is taken up from the gut lumen into the blood, conferring passive immunity to the offspring, whereas in the adult immunoglobulins, including IgG made by plasma cells in the lamina propria, are secreted via the brush border to the lumen as part of the mucosal defense. Here, IgG has been proposed to perform a luminal immune surveillance which eventually includes a reuptake through the brush border as pathogen-containing immune complexes. In the present work, we studied luminal uptake of FITC-conjugated and gold-conjugated IgG in cultured pig jejunal mucosal explants. After 1 h, binding to the brush border was seen in upper crypts and lower parts of the villi. However, no endocytotic uptake into EEA-1-positive compartments was detected, neither at neutral nor acidic pH, despite an ongoing constitutive endocytosis from the brush border, visualized by the polar tracer CF594. The 40-kDa neonatal Fc receptor, FcRn, was present in the microvillus fraction, but noteworthy, a 37 kDa band, most likely a proteolytic cleavage product, bound IgG in a pH-dependent manner more efficiently than did the full-length FcRn. In conclusion, our work does not support the theory that bidirectional transfer of IgG across the intestinal brush border is part of the luminal immune surveillance in the adult.

Staphylococcus aureus enterotoxins A- and B: binding to the enterocyte brush border and uptake by perturbation of the apical endocytic membrane traffic

Enterotoxins of Staphylococcus aureus are among the most common causes of food poisoning. Acting as superantigens they intoxicate the organism by causing a massive uncontrolled T cell activation that ultimately may lead to toxic shock and death. In contrast to our detailed knowledge regarding their interaction with the immune system, little is known about how they penetrate the epithelial barrier to gain access to their targets. We therefore studied the uptake of two staphylococcal enterotoxins (SEs), SEA and SEB, using organ cultured porcine jejunal explants as model system. Attachment of both toxins to the villus surface was scarce and patchy compared with that of cholera toxin B (CTB). SEA and SEB also bound to microvillus membrane vesicles in vitro, but less efficiently than CTB, and the binding was sensitive to treatment with endoglycoceramidase II, indicating that a glycolipid, possibly digalactosylceramide, acts as cell surface receptor at the brush border. Both SEs partitioned poorly with detergent resistant membranes (DRMs) of the microvillus, suggesting a weak association with lipid raft microdomains. Where attachment occurred, cellular uptake of SEA and SEB was also observed. In enterocytes, constitutive apical endocytosis normally proceeds only to subapical early endosomes present in the actomyosin-rich "terminal web" region. But, like CTB, both SEA and SEB penetrated deep into the cytoplasm. In conclusion, the data show that after binding to the enterocyte brush border SEA and SEB perturb the apical membrane trafficking, enabling them to engage in transcytosis to reach their target cells in the subepithelial lamina propria.

Recent advances in small bowel diseases: Part I

As is the case in all parts of gastroenterology and hepatology, there have been many advances in our knowledge and understanding of small intestinal diseases. Over 1000 publications were reviewed for 2008 and 2009, and the important advances in basic science as well as clinical applications were considered. In Part I of this Editorial Review, seven topics are considered: intestinal development; proliferation and repair; intestinal permeability; microbiotica, infectious diarrhea and probiotics; diarrhea; salt and water absorption; necrotizing enterocolitis; and immunology/allergy. These topics were chosen because of their importance to the practicing physician.

Immunization of mice with vibrio cholerae outer-membrane vesicles protects against hyperinfectious challenge and blocks transmission

Background. Vibrio cholerae excreted by cholera patients is “hyperinfectious” (HI), which can be modeled by passage through infant mice. Immunization of adult female mice with V. cholerae outer-membrane vesicles (OMVs) passively protects suckling mice from challenge. Although V. cholerae is unable to colonize protected pups, the bacteria survive passage and have the potential to be transmitted to susceptible individuals. Here, we investigated the impact of OMV immunization and the HI state on V. cholerae transmission.

Methods. Neonatal mice suckled by OMV- or sham-immunized dams were challenged with HI V. cholerae. The infectivity of spatially and temporally separate V. cholerae populations obtained from infected naive or protected pups was tested. Recombination-based in vivo expression technology was used to assess virulence gene expression within these populations.

Results. OMV immunization significantly reduced colonization of neonates challenged with HI V. cholerae. Vibrio cholerae that had colonized the naive host was HI, whereas V. cholerae excreted by neonates born to OMV-immunized dams, although viable, was hypoinfectious and failed to fully induce virulence gene expression.

Conclusions. OMV immunization can significantly reduce the V. cholerae burden upon challenge with HI V. cholerae and can also block transmission from immune mice by reducing the infectivity of shed bacteria.

Toxin-mediated effects on the innate mucosal defenses: implications for enteric vaccines

Recent studies have confirmed older observations that the enterotoxins enhance enteric bacterial colonization and pathogenicity. How and why this happens remains unknown at this time. It appears that toxins such as the heat-labile enterotoxin (LT) from Escherichia coli can help overcome the innate mucosal barrier as a key step in enteric pathogen survival. We review key observations relevant to the roles of LT and cholera toxin in protective immunity and the effects of these toxins on innate mucosal defenses. We suggest either that toxin-mediated fluid secretion mechanically disrupts the mucus layer or that toxins interfere with innate mucosal defenses by other means. Such a breach gives pathogens access to the enterocyte, leading to binding and pathogenicity by enterotoxigenic E. coli (ETEC) and other organisms. Given the common exposure to LT(+) ETEC by humans visiting or residing in regions of endemicity, barrier disruption should frequently render the gut vulnerable to ETEC and other enteric infections. Conversely, toxin immunity would be expected to block this process by protecting the innate mucosal barrier. Years ago, Peltola et al. (Lancet 338:1285-1289, 1991) observed unexpectedly broad protective effects against LT(+) ETEC and mixed infections when using a toxin-based enteric vaccine. If toxins truly exert barrier-disruptive effects as a key step in pathogenesis, then a return to classic toxin-based vaccine strategies for enteric disease is warranted and can be expected to have unexpectedly broad protective effects.

Lipid raft organization and function in the small intestinal brush border

The enterocyte brush border of the small intestine is a highly specialized membrane designed to function both as a high capacity digestive/absorptive surface of dietary nutrients and a permeability barrier towards lumenal pathogens. It is characterized by an unusually high content of glycolipids (approximately 30% of the total microvillar membrane lipid), enabling the formation of liquid ordered microdomains, better known as lipid rafts. The glycolipid rafts are stabilized by galectin-4, a 36 kDa divalent lectin that cross-links galactosyl (and other carbohydrate) residues present on membrane lipids and several brush border proteins, including some of the major hydrolases. These supramolecular complexes are further stabilized by intelectin, a 35 kDa trimeric lectin that also functions as an intestinal lactoferrin receptor. As a result, brush border hydrolases, otherwise sensitive to pancreatic proteinases, are protected from untimely release into the gut lumen. Finally, anti-glycosyl antibodies, synthesized by plasma cells locally in the gut, are deposited on the brush border glycolipid rafts, protecting the epithelium from lumenal pathogens that exploit lipid rafts as portals for entry to the organism.

The mucosal immune system: recent developments and future directions in the pig

In most animals, the mucosal immune system effectively controls expression of active immune responses to pathogen and tolerance to harmless antigens. Our understanding of the function and control of the mucosal immune system has advanced as a result of studies in rodents and humans. The discoveries of regulatory T-cells and T-helper-17 cells, and studies on the interactions between epithelial and dendritic cells, demonstrate its complexity. In pigs, some of the systems and reagents for determining the relevance of these mechanisms are present, and indicate lines for future work. However, many empirical studies of the effect of manipulation of the mucosal immune system in the pig by prebiotics, probiotics and feed additives have been carried out. Interpretation of these results needs to be made with care, since manipulation of the mucosal immune system may improve its efficiency under a specific set of environmental and husbandry conditions, but impair it under others.

Innate immune defenses in the intestinal tract

PURPOSE OF REVIEW

Innate intestinal defenses are important for protection against ingested and commensal microbes. This review highlights recent new insights into innate immune effectors in the intestine.

RECENT FINDINGS

Intestinal epithelial cells, particularly Paneth cells, are the major producers of multiple peptides and proteins with antimicrobial activity in the intestine. The most abundant and diverse of these are the defensins. They are highly microbicidal in vitro and probably important in vivo, yet their physiologic functions remain incompletely understood. Relative defensin deficiency may be a risk factor for Crohn's disease and infectious diarrhea. Cathelicidin contributes to mucosal defense against epithelial-adherent bacterial pathogens, and helps to set a threshold for productive infection. Bactericidal/permeability-inducing protein has lipopolysaccharide-neutralizing capacity and kills bacteria when overexpressed in epithelial cells. Resistin-like molecule beta is important in mucosal defense against helminths due to its ability to inhibit worm chemotaxis. Antimicrobial lectins, particularly hepatocarcinoma-intestine-pancreas/pancreatic-associated protein, RegIII, and intelectin, can lyse bacteria or interfere with their attachment to epithelial cells.

SUMMARY

Discovery of an expanding set of antimicrobial effectors supports the evolutionary importance of innate intestinal defenses against microbial threats, but also underlines the physiologic and pharmacologic need for a better understanding of the respective functions of these molecules.

Autonomic neurotransmitters modulate immunoglobulin A secretion in porcine colonic mucosa

Secretory immunoglobulin A (sIgA) plays a crucial role in mucosal surface defense. We tested the hypothesis that colonic sIgA secretion is under enteric neural control. Immunohistochemistry of the porcine distal colonic mucosa revealed presumptive cholinergic and adrenergic nerve fibers apposed to secretory component (SC)-positive crypt epithelial cells and neighboring IgA(+) plasmacytes. The cholinomimetic drug carbamylcholine elicited rapid, atropine-sensitive IgA secretion into the luminal fluid bathing mucosal explants mounted in Ussing chambers. The adrenergic receptor agonist norepinephrine also increased IgA secretion, an action inhibited by phentolamine. These effects were independent of agonist-induced anion secretion. In Western blots of luminal fluid, both agonists increased the density of protein bands co-immunoreactive for IgA and SC. Mucosal exposure to enterohemorrhagic Escherichia coli did not affect IgA secretion, and carbamylcholine treatment did not affect mucosal adherence of this enteropathogen. Acetylcholine and norepinephrine, acting respectively through muscarinic cholinergic and alpha-adrenergic receptors in the colonic mucosa, stimulate sIgA secretion and may enhance mucosal defense in vivo.