Persorption Mechanisms of Luminal Antigenic Particulates via Apoptotic Epithelial Cells of Intestinal Villi into Systemic Blood Circulation in Orally Immunized Rats

Intestinal Macromolecular Transport Supporting Adaptive Immunity

The gastrointestinal tract performs opposing functions of nutrient absorption, barrier maintenance, and the delivery of luminal substances for the appropriate induction of tolerogenic or protective adaptive immunity. The single-layer epithelium lining the gastrointestinal tract is central to each of these functions by facilitating the uptake and processing of nutrients, providing a physical and chemical barrier to potential pathogens, and delivering macromolecular substances to the immune system to initiate adaptive immune responses. Specific transport mechanisms allow nutrient uptake and the delivery of macromolecules to the immune system while maintaining the epithelial barrier. This review examines historical observations supporting macromolecular transport by the intestinal epithelium, recent insights into the transport of luminal macromolecules to promote adaptive immunity, and how this process is regulated to promote appropriate immune responses. Understanding how luminal macromolecules are delivered to the immune system and how this is regulated may provide insight into the pathophysiology of inflammatory diseases of the gastrointestinal tract and potential preventative or therapeutic strategies.

Ragweed pollen as an oral vaccine delivery system: Mechanistic insights

We have recently developed pollen grains (PGs) as a unique method to deliver vaccines orally. Extensive chemical processing ensures allergen-free pollen microcapsules that can be loaded with vaccine antigens. Successful oral vaccine delivery has been previously demonstrated by us in a mouse model. However, the underlying mechanisms that help the processed PGs to achieve this goal were not fully understood. In this study, we wanted to understand the effects of chemically processed ragweed pollen (Ambrosia elatior) on the innate immune system. Intestinal epithelial cells, macrophages, and dendritic cells all bridge the innate and adaptive immunity. This study has shown that in response to ragweed pollen all these cells release inflammatory cytokines and chemokines. Scanning electron microscopy imaging revealed that macrophages can engulf ragweed pollen. In addition, in the presence of ragweed, mouse dendritic cells upregulated their activation markers, that is, CD40, CD80, CD86, and MHC class II molecules. Ragweed pollens did not cause significant cell membrane damage as compared to similarly sized poly (lactic-co-glycolic acid) particles. Moreover, ragweed did not affect the integrity of the intestinal epithelial cells. Ragweed pollens were also found in the subepithelial region of the small intestine 24h after pollens were gavaged to mice. Our current findings lead to the conclusion that besides transporting the vaccine cargo, ragweed pollen shells have additional immunomodulatory properties that help the orally delivered antigen to effectively induce an immune response.

Ultrastructural demonstration of the absorption and transportation of minute chylomicrons by subepithelial blood capillaries in rat jejunal villi

Chylomicrons from villous columnar epithelial cells are generally known to be transported only by central lymph vessels (CLV), whereas antigenic particulates derived from the intestinal lumen can also be transported by subepithelial blood capillaries (sBCs) in rat intestinal villi. The possibility of chylomicron absorption by sBCs was histoplanimetrically studied in the rat jejunum under a transmission electron microscope. The chylomicrons more abundantly presented in villous venules than in arterioles. The most frequent size (MFS) of chylomicrons was 75 to 90 nm in diameter in the areas near sBCs, while it was 45 to 60 nm in the epithelial intercellular spaces just above sBCs or the intermediate areas between sBCs. The MFS of chylomicrons was 45 to 60 nm in the intermediate areas between sBCs and in the epithelial intercellular spaces just above these areas. The MFS of chylomicrons in CLV was intermediate between that in the area adjacent to sBCs and that in the intermediate areas between sBCs. Chylomicrons were found in small vesicles in the endothelial cytoplasms of sBCs. No chylomicrons larger than 600 nm were observed in the lamina propria. These findings suggest that some of the chylomicrons smaller than 75 nm, which are probable intestinal very low-density lipoproteins (VLDL), are directly transported to the liver by hepatic portal blood in addition to CLV and that epithelial fat droplets larger than 600 nm are not discharged into lamina propria in rat jejunum under physiological conditions.

Persorption of IgG-Fc-coated particulates from intestinal lumen into portal blood via villous columnar epithelial cells in rat small intestine

Previously, the specific antibody-mediated persorption of antigenic molecules and particulates from the small-intestinal lumen into the peripheral blood was clarified in rats, but the intermediation of the receptor for the specific antibodies was not. In this study, the existence of receptor for the specific antibody was experimentally examined in the rat small intestine. Glutaraldehyde-fixed sheep erythrocytes (SEs) coated by Fc-fragments of IgG (IgG-Fc), (Fab')(2)-fragments of IgG (IgG-Fab) or bovine serum albumin (BSA), were injected into 3 jejunal loops each 2 cm in length in non-orally pre-immunized rats, respectively. Thirty minutes after the injection, IgG-Fc-coated SEs were significantly more engulfed by villous columnar epithelial cells than Fab- or BSA-coated SEs. The most frequent absorption sites were the intestinal villous apices. The IgG-Fc-coated SEs were adhered to the striated borders and were engulfed by villous columnar epithelial cells. IgG-Fc-coated SEs passing through the epithelial cells were also detected in the subepithelial blood capillaries just beneath the villous epithelium, but not in the connective tissue and the lymph vessels. These findings suggest that the absorption of luminal antigenic particulates is probably mediated by the Fc-receptor, and that the absorbed antigenic particulates are directly transferred to the hepatic portal blood by passing through the endothelium of the subepithelial blood capillaries.

Immunohistochemical detection of toll-like receptor-2, -4 and -9 in exocrine glands associated with rat alimentary tract

Localization of Toll-like receptors (TLRs) in the exocrine glands associated with the rat alimentary tract was immunohistochemically studied using anti-TLR antibodies. TLR-2, -4 and -9 were detected in the secretory granules of acinar cells or the luminal substances of the gustatory gland, extraorbital lacrimal gland, Harderian gland, proper gastric gland and pancreas. TLR-2 and -9 were also detected in the mucous acinar cells of the sublingual gland. Positivity for all TLRs was found in the striated borders of columnar epithelial cells and the luminal substances of the intestinal crypts throughout the small intestine, and also in the goblet cells throughout the large intestine. Only TLR-4 was detected in the secretory granules of Paneth cells. A reduction of TLR-4-positive secretory granules and the formation of TLR-4-positive vacuoles were found in the ileal Paneth cells under the hyper-proliferation of indigenous bacteria. In the apical to middle intervillous portions of the ileum, Gram-positive bacterial colonies were significantly more abundant than Gram-negative bacterial colonies, whereas this difference disappeared in the basal intervillous portions. These findings suggest that there are distribution differences in the secretory sources of soluble TLRs that possibly neutralize their luminal ligands, in the rat alimentary tract. Therefore, the bacterial ligand-recognition system composed of the membranous TLRs of villous columnar epithelial cells and soluble TLRs from crypt epithelial cells might contribute to host defense mechanisms for the selective elimination of Gram-positive bacteria rather than Gram-negative bacteria in the rat small intestine.

Lectin histochemical detection of special sugars on the mucosal surfaces of the rat alimentary tract

Surfaces of the most luminal positions of mucosae are fundamental settlement sites of indigenous bacteria throughout the rat alimentary tract. In these positions, also epithelial cell-shedding sites, the special sugar expression in the glycocalyx is very important as it provides possible ligands of bacterial lectins for attachment to epithelial cells. Therefore, the sugar expression in glycocalyx of epithelial cells was lectin-histochemically surveyed using 21 lectins throughout the rat alimentary tract. From the tongue to the nonglandular part of the stomach, α-D-Man, α-D-Glc and α-D-GalNAc were detected on the surface of the keratinized stratified squamous epithelium. In the glandular part of the stomach, α-D-Man, β-D-Gal-4GlcNAc, D-Gal, D-GalNAc, D-GlcNAc, α-L-Fuc- α-D-Gal-β(1-4)GlcNAc and bisected triantennary N-glycans were detected on the surface of gastric superficial epithelial cells. From the duodenum to the ileum, (GlcNAc)(2-4) was expressed exclusively on the epithelial cells in the apical portions of the intestinal villi. From the cecum to the rectum, α-D-Man, β-D-Gal-4GlcNAc, D-Gal, D-GalNAc, α-D-Gal(1-3)D-GalNAc, (GalNAc)(n) and NeuNAc were expressed on the intestinal superficial epithelial cells. These results suggest that special sugars are expressed on the most luminal portions of mucosae as exclusive epithelial cell-shedding sites, and that sugar expression differs among the various segments of the alimentary tract. These site differences might reflect differences in resident bacterial species in the rat alimentary tract.

Ultrastructural study on the epithelial responses against attachment of indigenous bacteria to epithelial membranes in peyer's patches of rat small intestine

The ultrastructure of epithelial responses against the membrane adhesion of indigenous bacteria was investigated in the follicle-associated epithelium (FAE) of rat small intestine. The most frequent adherence of the various morphological types of bacteria to the epithelial membranes was found at the apex of the FAE. The attachment sites were deeply invaginated, and their bottoms were deformed into a sharp cone shape. Four layers with different electron densities were formed just beneath the apical membranes by microfilaments which surrounded the invaginations. The electron density of each layer was gradually decreased as being apart from the invaginations. The extremities of some bacteria in the invaginations were deformed into sharpened shapes. The cell walls of the extremities of the bacteria were occasionally dissolved in the invaginations, and their cytoplasms were slightly swollen with low electron densities. In some invaginations, the attached bacteria were eliminated to leave their fragments such as filamentous debris and a part of cell walls. Finally these remnants disappeared completely. When the bacterial colonies existed in the middle region of the FAE, the attachment of bacteria resulted in the engulfment of bacteria by M cells. The degenerated bacteria whose cytoplasmic matrices were separated into high electron dense materials and cleared materials were occasionally engulfed by ordinary microvillous columnar epithelial cells or goblet cells throughout the FAE. These findings suggest that the epithelial cells reject the attachment of live indigenous bacteria and that the M cells absorb indigenous bacteria in rat Peyer's patches.

Immunohistochemical study on the delayed progression of epithelial apoptosis in follicle-associated epithelium of rat Peyer's patch

It is well known that some caspases in apoptosis is involved in determinant of terminal differentiation and maturation of various cells. Our previous study ultrastructurally clarified the differentiation into M cells from immature microvillous epithelial cells and the redifferentiation from M cells to microvillous epithelial cells in the follicle-associated epithelium (FAE) of rat Peyer's patch. In this study, the difference of epithelial apoptosis between the FAE of Peyer's patch and intestinal villi was immunohistochemically investigated in rat jejunoileum. As a result, cleaved caspase-3 was limited to several epithelial cells at the tip of FAE, whereas almost all of the epithelial cells were cleaved caspase-3 positive in intestinal villi. Cleaved caspase-9 was detected only in a few exfoliating or exfoliated epithelial cells of both FAE and intestinal villi. Nuclear DNA-fragmentation was detected only in several epithelial cells of the tip of FAE, while it was expressed from the middle regions in the intestinal villi. The DNase I expression of the epithelial cytoplasm was much weaker in FAE than in intestinal villi. Bcl-x expression was restricted in the apical cytoplasms of epithelial cells in the FAE, whereas it was restricted in whole cytoplasms in villous epithelial cells. These findings suggest that the progression of the apoptotic process in the epithelial cells of FAE is later than in the intestinal villi, so that the possibility of epithelial differentiation might be remained in the FAE, unlike in the intestinal villi.