"M" cell distribution in follicle-associated epithelium of mouse Peyer's patch

Exploring the usefulness of the complex in vitro intestinal epithelial model Caco-2/HT29/Raji-B in nanotoxicology

The use of in vitro barrier models is gaining relevance as an alternative to animal studies in risk assessment, pharmacokinetic, and toxicological studies in general. These models permit an easier evaluation of the underlying mechanisms taking place at the molecular and cellular levels on the barrier site. Here, we report several methodological modifications of the three-dimensional in vitro intestinal epithelial model Caco-2/HT29/Raji-B for its successful application in the Nanotoxicology field. In addition, new insights in the study of specific molecular markers and new confocal microscope approaches have also been incorporated. Due to the multiple variables and parameters playing a part when the model's complexity is increased, we have monitored the barrier's formation and cell differentiation over time. Finally, the practical usability of the proposed model was tested by evaluating the action of the food additives titanium dioxide and silica dioxide nanoparticles (TiO₂NPs and SiO₂NPs). The NPs-associated effects were evaluated by confocal microscopy. We have demonstrated the essential role of the mucus layer in the decrease of cellular uptake, avoiding potential NPs-cell nuclei interactions.

Dosimetric Quantification of Coating-Related Uptake of Silver Nanoparticles

The elucidation of mechanisms underlying the cellular uptake of nanoparticles (NPs) is an important topic in nanotoxicological research. Most studies dealing with silver NP uptake provide only qualitative data about internalization efficiency and do not consider NP-specific dosimetry. Therefore, we performed a comprehensive comparison of the cellular uptake of differently coated silver NPs of comparable size in different human intestinal Caco-2 cell-derived models to cover also the influence of the intestinal mucus barrier and uptake-specialized M-cells. We used a combination of the Transwell system, transmission electron microscopy, atomic absorption spectroscopy, and ion beam microscopy techniques. The computational in vitro sedimentation, diffusion, and dosimetry (ISDD) model was used to determine the effective dose of the particles in vitro based on their individual physicochemical characteristics. Data indicate that silver NPs with a similar size and shape show coating-dependent differences in their uptake into Caco-2 cells. The internalization of silver NPs was enhanced in uptake-specialized M-cells while the mucus did not provide a substantial barrier for NP internalization. ISDD modeling revealed a fivefold underestimation of dose-response relationships of NPs in in vitro assays. In summary, the present study provides dosimetry-adjusted quantitative data about the influence of NP coating materials in cellular uptake into human intestinal cells. Underestimation of particle effects in vitro might be prevented by using dosimetry models and by considering cell models with greater proximity to the in vivo situation, such as the M-cell model.

Inflammatory signals that regulate intestinal epithelial renewal, differentiation, migration and cell death: Implications for necrotizing enterocolitis

Necrotizing enterocolitis is a disease entity with multiple proposed pathways of pathogenesis. Various combinations of these risk factors, perhaps based on genetic predisposition, possibly lead to the mucosal and epithelial injury that is the hallmark of NEC. Intestinal epithelial integrity is controlled by a tightly regulated balance between proliferation and differentiation of epithelium from intestinal epithelial stem cells and cellular loss by apoptosis. various signaling pathways play a key role in creating and maintaining this balance. The aim of this review article is to outline intestinal epithelial barrier development and structure and the impact of these inflammatory signaling and regulatory pathways as they pertain to the pathogenesis of NEC.

Development of an advanced intestinal in vitro triple culture permeability model to study transport of nanoparticles

Intestinal epithelial cell culture models, such as Caco-2 cells, are commonly used to assess absorption of drug molecules and transcytosis of nanoparticles across the intestinal mucosa. However, it is known that mucus strongly impacts nanoparticle mobility and that specialized M cells are involved in particulate uptake. Thus, to get a clear understanding of how nanoparticles interact with the intestinal mucosa, in vitro models are necessary that integrate the main cell types. This work aimed at developing an alternative in vitro permeability model based on a triple culture: Caco-2 cells, mucus-secreting goblet cells and M cells. Therefore, Caco-2 cells and mucus-secreting goblet cells were cocultured on Transwells and Raji B cells were added to stimulate differentiation of M cells. The in vitro triple culture model was characterized regarding confluence, integrity, differentiation/expression of M cells and cell surface architecture. Permeability of model drugs and of 50 and 200 nm polystyrene nanoparticles was studied. Data from the in vitro model were compared with ex vivo permeability results (Ussing chambers and porcine intestine) and correlated well. Nanoparticle uptake was size-dependent and strongly impacted by the mucus layer. Moreover, nanoparticle permeability studies clearly demonstrated that particles were capable of penetrating the intestinal barrier mainly via specialized M cells. It can be concluded that goblet cells and M cells strongly impact nanoparticle uptake in the intestine and should thus be integrated in an in vitro permeability model. The presented model will be an efficient tool to study intestinal transcellular uptake of particulate systems.

Biochemical and physiological bases for utilization of dietary amino acids by young Pigs

Protein is quantitatively the most expensive nutrient in swine diets. Hence it is imperative to understand the physiological roles played by amino acids in growth, development, lactation, reproduction, and health of pigs to improve their protein nutrition and reduce the costs of pork production. Due to incomplete knowledge of amino acid biochemistry and nutrition, it was traditionally assumed that neonatal, post-weaning, growing-finishing, and gestating pigs could synthesize sufficient amounts of all "nutritionally nonessential amino acids" (NEAA) to support maximum production performance. Therefore, over the past 50 years, much emphasis has been placed on dietary requirements of nutritionally essential amino acids as building blocks for tissue proteins. However, a large body of literature shows that NEAA, particularly glutamine, glutamate, arginine and proline regulate physiological functions via cell signaling pathways, such as mammalian target of rapamycin, AMP-activated protein kinase, extracellular signal-related kinase, Jun kinase, mitogen-activated protein kinase, and NEAA-derived gaseous molecules (e.g., nitric oxide, carbon monoxide, and hydrogen sulfide). Available evidence shows that under current feeding programs, only 70% and 55% of dietary amino acids are deposited as tissue proteins in 14-day-old sow-reared piglets and in 30-day-old pigs weaned at 21 days of age, respectively. Therefore, there is an urgent need to understand the roles and dietary requirements of NEAA in swine nutrition. This review highlights the basic biochemistry and physiology of absorption and utilization of amino acids in young pigs to enhance the efficacy of utilization of dietary protein and to minimize excretion of nitrogenous wastes from the body.

Comparative analysis of EspF variants in inhibition of Escherichia coli phagocytosis by macrophages and inhibition of E. coli translocation through human- and bovine-derived M cells

The EspF protein is secreted by the type III secretion system of enteropathogenic and enterohemorrhagic Escherichia coli (EPEC and EHEC, respectively). EspF sequences differ between EHEC O157:H7, EHEC O26:H11, and EPEC O127:H6 in terms of the number of SH3-binding polyproline-rich repeats and specific residues in these regions, as well as residues in the amino domain involved in cellular localization. EspF(O127) is important for the inhibition of phagocytosis by EPEC and also limits EPEC translocation through antigen-sampling cells (M cells). EspF(O127) has been shown to have effects on cellular organelle function and interacts with several host proteins, including N-WASP and sorting nexin 9 (SNX9). In this study, we compared the capacities of different espF alleles to inhibit (i) bacterial phagocytosis by macrophages, (ii) translocation through an M-cell coculture system, and (iii) uptake by and translocation through cultured bovine epithelial cells. The espF gene from E. coli serotype O157 (espF(O157)) allele was significantly less effective at inhibiting phagocytosis and also had reduced capacity to inhibit E. coli translocation through a human-derived in vitro M-cell coculture system in comparison to espF(O127) and espF(O26). In contrast, espF(O157) was the most effective allele at restricting bacterial uptake into and translocation through primary epithelial cells cultured from the bovine terminal rectum, the predominant colonization site of EHEC O157 in cattle and a site containing M-like cells. Although LUMIER binding assays demonstrated differences in the interactions of the EspF variants with SNX9 and N-WASP, we propose that other, as-yet-uncharacterized interactions contribute to the host-based variation in EspF activity demonstrated here.

Ex-vivoevaluation of alginate microparticles for Polymyxin B oral administration

A crosslinked alginate microparticle system for the targeting to the lymphatic system by Peyer's patches (PP) uptake was designed in order to improve the oral absorption of Polymyxin B (PMB). To verify mucoadhesion and PP uptake, microparticles labelled with fluorescein isothiocyanate (FITC) were prepared by spray-drying technique and crosslinking reactions with calcium ions and chitosan (CS), in vitro characterized and assayed by an ex vivo method. Microparticles showed a size less then 3 microm, an antibiotic loading level of 11.86 +/- 0.70%, w/w, a sustained drug release behaviour in simulated gastro-intestinal (GI) fluids and a preserved biological activity throughout the manufacture. The ex vivo study was performed by a perfusion method on intestinal tracts of just sacrificed adult rats. The recovered samples were analysed by epifluorescence microscope for mucoadhesion and PP uptake and by microbiological analysis for antibiotic activity preservation, providing evidence of mucoadhesion at the level of both PP and non-PP epithelium, uptake by PP and PMB microbiological activity in PP tissue. Furthermore, the study revealed the involvement of transport pathways across villous enterocytes.

Intestinal M cells: The fallible sentinels?

The gastrointestinal tract represents the largest mucosal membrane surface in the human body. The immune system in the gut is the first line of host defense against mucosal microbial pathogens and it plays a crucial role in maintaining mucosal homeostasis. Membranous or microfold cells, commonly referred to as microfold cells, are specialized epithelial cells of the gut-associated lymphoid tissues (GALT) and they play a sentinel role for the intestinal immune system by delivering luminal antigens through the follicle-associated epithelium to the underlying immune cells. M cells sample and uptake antigens at their apical membrane, encase them in vesicles to transport them to the basolateral membrane of M cells, and from there deliver antigens to the nearby lymphocytes. On the flip side, some intestinal pathogens exploit M cells as their portal of entry to invade the host and cause infections. In this article, we briefly review our current knowledge on the morphology, development, and function of M cells, with an emphasis on their dual role in the pathogenesis of gut infection and in the development of host mucosal immunity.

Ultrastructural Study on the Differentiation and the Fate of M cells in Follicle-Associated Epithelium of Rat Peyer's Patch

The differentiation process of immature microvillous epithelial cells to M cells and the fate of M cells in the follicle-associated epithelium (FAE) of the mucosa-associated lymphoid tissues are still unclear. In this study, the differentiation process and the fate of M cells were clarified in rat Peyer's patches under a transmission electron microscope. Almost all immature epithelial cells were found to possess long, slender microvilli, which gradually shortened, thickened and dispersed as the immature epithelial cells migrated away from the crypt orifices. These morphological changes started in the centers and moved to the peripheries of the apical surfaces of epithelial cells, accompanied by the protrusion of apical cytoplasm out of the terminal web. During these changes, the bundles of microfilaments of microvilli never shortened, and both small vesicles in the apical cytoplasm and tiny invaginations of the apical membranes were found. The intraepithelial migrating cells gradually accumulated to form typical intraepithelial pockets. In all FAE, there was no morphological sign of cell death in M cells. The rearrangement of microfilament bundles, the reconstruction of microvilli and the disappearance of pockets resulted in the transformation of M cells into microvillous epithelial cells. These serial ultrastructural changes suggest that M cells are a temporal and transitional cell type caused by the active engulfment of luminal substances and that when the engulfment ceases, the M cells transform into mature microvillous epithelial cells.

Differentiation of epithelial cells to M cells in response to bacterial colonization on the follicle-associated epithelium of Peyer's patch in rat small intestine

To clarify the relationship between M cells and intestinal microflora, histoplanimetrical investigation into the bacterial colonization and the differentiation to M cells was carried out in rat Peyer's patch under physiological conditions. The follicle-associated epithelium (FAE), except for the narrow area of apical region, was closely covered with both neighboring intestinal villi and a thick mucous layer, the latter of which also filled the intervillous spaces as well as the space between the FAE and the neighboring intestinal villi. Indigenous bacteria adhered almost constantly to the narrow areas of apical regions of both intestinal villi and the FAE. Bacterial colonies were occasionally located on the basal to middle region of FAE, where M cells also appeared, forming large pockets. When bacterial colonies were located on the basal to middle region of FAE, bacteria with the same morphological characteristics also proliferated in the intervillous spaces neighboring the Peyer's patch. In cases with no bacterial colonies on the basal to middle region of FAE, however, M cells were rare in the FAE. Histoplanimetrical analysis showed the similar distribution pattern of bacterial colonies on the FAE and M cells in the FAE. M cells ultrastructurally engulfed indigenous bacteria, which were then transported to the pockets. These results suggest that indigenous bacterial colonization on the FAE stimulates the differentiation of M cells in the FAE under physiological conditions. The uptake of bacteria by M cells might contribute the regulation of the development of indigenous bacterial colonies in the small intestine.

Particle uptake by Peyer's patches: a pathway for drug and vaccine delivery

Particle uptake by Peyer's patches offers the possibility of tailoring vaccines that can be delivered orally. However, particle uptake by the follicle-associated epithelium in the gastrointestinal tract depends on several different factors that are the physicochemical properties of the particles, the physiopathological state of the animal, the analytical method used to evaluate the uptake and finally the experimental model. These parameters do not allow a clear idea about the optimal conditions to target the Peyer's patches. The goal of this review is to clarify the role of each factor in this uptake.

Development of intestinal M cells

Intestinal epithelium contains several specialized cell types including M cells, which can be found in the follicle-associated epithelium (FAE) or occasionally on the villi. M cells are critical for sampling of intestinal flora and for transferring pathogens across the epithelial barrier for recognition by the immune system. Development of M cells on the villi (M(v)) is independent of the presence of lymphocytes, while development of the FAE and M cells within the FAE (M(f)) is dependent on B lymphocytes. Here, the concept is discussed that B cells are not required for induction of M(f) differentiation but are required for transition to and maintenance of the mature M(f) phenotype. Signaling pathways possibly involved in the B-cell-independent stages of M-cell development are also discussed.

Cellular kinetics of villous epithelial cells and m cells in rabbit small intestine

The cellular kinetics of villous columnar epithelial cells and M cells in the rabbit small intestine were determined by the use of 5-bromo-2'-deoxyuridine (BrdU) as a tracer. To identify M cells, vimentin antibody was used. The BrdU-labeled nuclei of columnar epithelial cells reached the base of intestinal villi in all portions at 1 day after BrdU administration. Thereafter, BrdU-labeled cells migrated toward the villous tip, but they did not move at a uniform speed. The epithelial cells which existed in intestinal villi on circular folds moved faster than those on mucosa other than circular folds. At 7 days after BrdU administration, the leading edge of BrdU-labeled epithelial cells already disappeared from the villous tip in all portions of the small intestine. In the ileal Peyer's patch, the BrdU-labeled nuclei of microvillous epithelial cells and vimentin-positive M cells appeared near the intestinal crypt orifice at 1 day after BrdU administration, and then migrated toward the luminal surface of the follicle-associated epithelium (FAE). As they moved toward the upper portion of FAE, the number of BrdU-labeled M cells on the side of the dome decreased simultaneously. The leading edge of BrdU-labeled epithelial cells disappeared from the top of the FAE within 7 days. These results suggest that M cells may differentiate from the undifferentiated cells in intestinal crypts within 1 day and disappear from the top of the FAE after the change of their form from M cells into microvillous epithelial cells.

Characterization of M cell formation and associated mononuclear cells during indomethacin-induced intestinal inflammation

M cells represent an important gateway for the intestinal immune system by delivering luminal antigens through the follicle-associated epithelium to the underlying immune cells. The goal of this study was to characterize this route of antigen uptake during intestinal inflammation by characterizing M cell formation and M cell-associated lymphocytes after indomethacin challenge in rats. We demonstrated increased M cell formation as early as 12 h after a single injection of indomethacin. The elevated M cell counts were determined until day 3 and returned to basal levels after 7 days. Electron microscopic studies revealed an expansion of mononuclear cells inside the M cell pocket that were characterized predominantly as B cells, T cell receptor (TCR)alphabeta- and CD4-positive T cells, whereas other markers such as CD11b, CD8 and CD25 remained unchanged. In situ hybridization studies showed increased expression of interleukin (IL)-4 by lymphocytes during intestinal inflammation in the Peyer's patch follicle. These studies illuminate the relevance of M cells during intestinal inflammation and suggest that M cells derive from epithelial cells in a certain microenvironment.

The cellular differentiation of M cells from crypt undifferentiated epithelial cells into microvillous epithelial cells in follicle-associated epithelia of chicken cecal tonsils

To clarify the cellular origin and the fate of M cells, detailed distributions of the epithelial cells were investigated scanning electron microscopically on the follicle-associated epithelia (FAE) of chicken cecal tonsils. The distribution of M cells was closely related with the situation of the crypt orifices in chicken cecal tonsils. In undeveloped cecal tonsils, the intestinal crypts were localized at the periphery of the FAE. In these tonsils, M cells without microvilli (M(0)) were predominantly populated in the basal region of the FAE, whereas goblet cells and microvillous epithelial cells (MV) were more distributed in the middle to the apical region of the FAE. A few M cells with short microvilli were dispersed throughout the FAE. Significantly shrunk MV (MVs) clustered together in transitional portions from the lateral face to the roof of the FAE. In well-developed cecal tonsils, the crypts also opened at the lateral surface in addition to the periphery of the FAE. In these tonsils, the M(0) accumulated densely in the small areas around the crypt orifices exclusively. No sign of exfoliation of apoptotic epithelial cells was found in the M(0)-accumulated areas and at their peripheral boundaries. The MVs were often clustered in the central regions among the crypt orifices in addition to the roof of the FAE. These findings suggest that M cells are directly derived from the undifferentiated crypt epithelial cells, not fall into apoptotic cell death and further differentiate into MV in the FAE of chicken cecal tonsils.

Rabbit M cells and dome enterocytes are distinct cell lineages

We have studied the M cell origin and differentiation pathway in rabbit gut-associated lymphoid tissues. Micro-dissected domes and epithelium isolated by ethylene diamine tetra acetic acid detachment allowed us to view the whole epithelial surface from the bottom of crypts to the top of domes. We used monoclonal antibodies specific to the apex of either M cells or dome enterocytes, lectins, and antibodies to vimentin in appendix, distal Peyer’s patches and caecal patches.

The earliest vimentin-labeled M cells were observed in the BrdU-positive proliferative zone of dome-associated crypts. Gradual differentiation of the M cell vimentin cytoskeleton started at this site to progressively give rise to the first pocket-forming M cells in the upper dome. Therefore, these mitotic cells of the crypts appear as the direct precursors of M cells. In addition to an early appearance of M cell markers, a regular mosaic-like relative distribution of M cells and dome enterocytes was already detected in the vicinity of crypts, similar to that observed on the lateral surface of domes where functional M cells lie. This constant distribution implies that there is no trans-differentiation of enterocytes to M cells along the crypt-dome axis. Together, these observations provide very strong evidence in favor of an early commitment in crypts of M cell and enterocyte distinct lineages.

The role of dendritic cells, B cells, and M cells in gut-oriented immune responses

Although induction of T cell responses to fed Ag (oral tolerance) is thought to happen within the organized lymphoid tissue of the gut, we found that mice lacking Peyer's patches, B cells, and the specialized Ag-handling M cells had no defect in the induction of T cell responses to fed Ag, whether assayed in vitro by T cell proliferation or cytokine production, or in vivo by delayed-type hypersensitivity or bystander suppression against mycobacterial Ags in CFA. Feeding of Ag had a major influence on dendritic cells from fed wild-type or muMT mice, such that these APCs were able to elicit a different class of response from naive T cells in vitro. These results suggest that systemic immune responses to soluble oral Ags do not require an organized gut-associated lymphoid tissue but are most likely induced by gut-conditioned dendritic cells that function both to initiate the gut-oriented response and to impart the characteristic features that discriminate it from responses induced parenterally.

Effect of mature lymphocytes and lymphotoxin on the development of the follicle-associated epithelium and M cells in mouse Peyer's patches

BACKGROUND AND AIMS

Mechanisms regulating M-cell formation are still poorly understood. In vitro studies showed that lymphocytes trigger the conversion of enterocyte cell lines into M cell-like cells on coculture, whereas in vivo their role in M cell differentiation is still elusive. Our aim was first to examine Rag-1-/- mice, lacking B and T lymphocytes, for the presence of intestinal M cells. Second, we investigated the role of lymphotoxin alphabeta signaling on M-cell formation, given its pivotal role in the development of mouse Peyer's patches.

METHODS

Small intestines of Rag-1-/- mice, injected or not with soluble lymphotoxin beta receptor-immunoglobulin fusion protein, were analyzed morphologically using whole mount cytochemical staining, immunohistochemistry, and electron microscopy.

RESULTS

Small Peyer's patch-like aggregates were found in Rag-1-/- mice in normal number and location. The overlying epithelium of such aggregates was reduced in size but still harbored M cells. In vivo neutralization of lymphotoxin beta-receptor signaling partially reduced the percentage of M cells.

CONCLUSIONS

The absence of mature lymphocytes does not prevent the formation of M cells, indicating that the signaling molecules that support M-cell differentiation, such as lymphotoxin alphabeta, may also be supplied by non-B and non-T cells. Mature B lymphocytes, however, are required for the formation of a full-sized follicle-associated epithelium.

Role of M cells in intestinal barrier function

M cells are known as specialized epithelial cells of the follicle-associated epithelium of the gastrointestinal tract. As M cells have a high capacity for transcytosis of a wide range of microorganisms and macromolecules, they are believed to act as an antigen sampling system. The primary physiological role of M cells seems to be the rapid uptake and presentation of particular antigens and microorganisms to the immune cells of the lymphoid follicle to induce an effective immune response. In contrast to absorptive enterocytes, M cells do not exert direct defense mechanisms to antigens and pathogens in the gut lumen. Therefore, they provide functional openings of the epithelial barrier. Although M cells represent a weak point of the epithelial barrier, even under noninflamed conditions, there seems to be a balance between antigen uptake and immunological response. The low number of M cells in the gastrointestinal tract and the direct contact to immune cells in the lamina propria usually prevent the occurrence of mucosal inflammation. During chronic intestinal inflammation we observe an increase of M cell number and apoptosis selectively in M cells. M cell damage seems to be responsible for the increase of the uptake of microorganisms that is observed during intestinal inflammation. Under inflammatory conditions in the intestine, the maintenance of the epithelial barrier is broken and M cells seem to play a major role during this process.

Transient expression of M-cell phenotype by enterocyte-like cells of the follicle-associated epithelium of mouse Peyer's patches

BACKGROUND & AIMS

The follicle-associated epithelium (FAE) over mucosa-associated lymphoid tissues consists of distinct enterocytes and M cells concentrated at its periphery. The basement membrane composition was analyzed to test whether differences account for the distinct differentiation programs along the crypt-villus and crypt-FAE axes. To determine whether the decreased number of M cells in the FAE apex is caused by premature extrusion, we mapped the site where they undergo apoptosis.

METHODS

The FAE basal lamina of Peyer's patches from BALB/c mice was analyzed by immunochemistry. M cells were identified using the Ulex europaeus agglutinin lectin. The cell proliferation and apoptotic compartments were characterized using bromodeoxyuridine incorporation and the TUNEL assay.

RESULTS

The perlecan and laminin 2 stainings were different in FAE and villi. Myofibroblasts were absent beneath the FAE. The migration kinetics of cells along the FAE was similar to that along the villi. Apoptotic cells were detected exclusively at the apex of the FAE.

CONCLUSIONS

FAE and M-cell differentiation is associated with a distinct basal lamina composition. FAE enterocytes express transient M-cell features as they move from the crypts toward the apoptotic compartment. M cells have a highly plastic phenotype that raises interesting questions about the control of intestinal epithelial cell differentiation.

Ultrastructural study on the follicle-associated epithelium of nasal-associated lymphoid tissue in specific pathogen-free (SPF) and conventional environment-adapted (SPF-CV) rats

Membranous (M) cells in follicle-associated epithelium (FAE) play an important role in the mucosal immunity through transport of a variety of foreign antigens to the underlying mucosa-associated lymphoid tissue (MALT). We aimed to investigate the ultrastructure of M cells in the FAE covering nasal-associated lymphoid tissue (NALT) both in specific pathogen-free (SPF) rats and in conventional environment-adapted (SPF-CV) rats aged 8-38 wk. In NALT of both SPF and SPF-CV rats, FAE included the nonciliated microvillous cell, which appears to be an analogue of M cell previously described in other MALT. In SPF rats, M cells increased in number only slightly with age, and they maintained morphological uniformity irrespective of age. In SPF-CV rats, M cells selectively increased in number resulting in prominent expansion of FAE surface area in parallel with the duration of maintenance in a conventional environment. In addition, M cells in SPF-CV rats showed heterogeneity in their surface morphology such as the length and number of microvilli and cell surface area and outline. In addition, the FAE was stratified by various subtypes of M cells, which were characterised by several subcellular alterations including the presence of many keratin filaments, homogeneous dark bodies and extensive cytoplasmic interfoliation with wide intercellular spaces filled with amorphous proteinaceous material. These characteristics of M cells in SPF-CV rat were intimately related with a preferential influx of immunocompetent cells into the FAE, which was not seen or was very rare in SPF rats irrespective of age. The results suggest the possibility that NALT may effectively carry out the mucosal immune response against antigenic stimuli of different magnitude through the unique dynamics of M cells which seem to be influenced by the infiltration of immunocompetent cells.

Characterization of M cell development during indomethacin-induced ileitis in rats

BACKGROUND

M cells play an important role in the intestinal immune system as they have a high capacity for transcytosis of a wide range of microorganisms and macromolecules. However, little is known about the role of M cells during intestinal inflammation.

AIM

We studied M cell development during indomethacin-induced intestinal inflammation in rats.

METHODS

Ileitis in rats was induced by two subcutaneous injections with indomethacin (7.5 mg/kg) given 24 h apart. Rats were sacrificed after 14 days and tissue was analysed by fluorescence microscopy and electron microscopy. M cells could be visualized by using the FITC-labelled mAb anti-cytokeratin (CK)-8 (clone 4.1.18), which was recently identified as specific M cell marker in rats. The number of cytokeratin-8 positive M cells was related to the surface of the follicle associated epithelium. For morphological studies, we used both transmission electron microscopy (T.E.M.) and scanning electron microscopy (S.E.M.).

RESULTS

In non-inflamed ileum M cells were scarce. Only 4% of the follicle associated epithelium were M cells, whereas an increase of M cells up to 11% was found in inflamed follicle associated epithelium (P < 0.001). The rate of M cell induction depended on the macroscopic degree of inflammation. T.E.M./S.E.M. studies showed that in inflamed tissue most M cells underwent apoptosis with typical morphological signs. In contrast to apoptotic M cells, the neighbouring enterocytes usually appeared intact. The number of mononuclear cells below the follicle associated epithelium was significantly increased. S.E.M. studies revealed that during induced ileitis mononuclear cells migrated from the lamina propria into the gut lumen by passing through apoptotic M cells.

CONCLUSIONS

During indomethacin-induced ileitis in rats the increase in M cell number in association with apoptosis of M cells may alter the intestinal barrier function. These observations may play a pivotal role in the pathogenesis of chronic intestinal inflammation, e.g. in inflammatory bowel disease.

Further characterization of M cells in gut-associated lymphoid tissues of the chicken

M cells are considered to be the most effective cells for the transport of antigens from the intestinal lumen into the gut-associated lymphoid tissue. M cells are characterized by their ultrastructural appearance, the selective uptake of antigens, the binding of lectins, and the presence of underlying lymphocytes. Little attention has been paid to the interaction of intra-epithelial leucocytes and M cells in chickens; therefore, we have investigated both cell types separately and using double immunocytochemical staining in cecal tonsils and Meckel's diverticulum. In the follicle-associated epithelium (FAE), cells were present that differ from their neighbors by short, irregular microvilli. Ferritin was absorbed by these putative M cells, but also by other epithelial cells. The lectins of Triticum vulgaris (WGA) and Glycine max (SBA) showed a patchy staining of the FAE. The numbers of intra-epithelial leucocytes (IEL) increased rapidly after hatch, reaching innumerable at 6 wk of age. Most IEL were T lymphocytes expressing CD8 and only about 30% of them were B lymphocytes. Nevertheless, double staining of M cells (WGA/SBA) and IEL showed that M cells were much fewer than IEL. These results indicate that M cells are not solely induced by the intra-epithelial localization of leucocytes. Because the phenotype of IEL reflected the content of the adjacent underlying lamina propria, IEL immigrate the FAE locally and do not migrate along with the epithelial cells from the crypts. In conclusion, M cells exist in the chicken, but their phenotype and function are less well demarcated from neighbor epithelial cells than is seen in mammals.

The development of M cells in Peyer's patches is restricted to specialized dome-associated crypts

It is controversial whether the membranous (M) cells of the Peyer's patches represent a separate cell line or develop from enterocytes under the influence of lymphocytes on the domes. To answer this question, the crypts that produce the dome epithelial cells were studied and the distribution of M cells over the domes was determined in mice. The Ulex europaeus agglutinin was used to detect M cells in mouse Peyer's patches. Confocal microscopy with lectin-gold labeling on ultrathin sections, scanning electron microscopy, and laminin immuno-histochemistry were combined to characterize the cellular composition and the structure of the dome-associated crypts and the dome epithelium. In addition, the sites of lymphocyte invasion into the dome epithelium were studied after removal of the epithelium using scanning electron microscopy. The domes of Peyer's patches were supplied with epithelial cells that derived from two types of crypt: specialized dome-associated crypts and ordinary crypts differing not only in shape, size, and cellular composition but also in the presence of M cell precursors. When epithelial cells derived from ordinary crypts entered the domes, they formed converging radial strips devoid of M cells. In contrast to the M cells, the sites where lymphocytes invaded the dome epithelium were not arranged in radial strips, but randomly distributed over the domes. M cell development is restricted to specialized dome-associated crypts. Only dome epithelial cells that derive from these specialized crypts differentiate into M cells. It is concluded that M cells represent a separate cell line that is induced in the dome-associated crypts by still unknown, probably diffusible lymphoid factors.

Proliferation and cellular kinetics of villous epithelial cells and M cells in the chicken caecum

The proliferation sites and cellular kinetics of villous epithelial cells and M cells in the intestine of the adult chicken have never been clarified. In this study, we determined the proliferation sites in the chicken caecum using colchicine treatment and detection of proliferative cell nuclear antigen (PCNA). The cellular kinetics of these cells were also studied using bromodeoxyuridine (BrdU) as a tracer. Enterocytes in their mitotic period were observed along the entire length of the intestinal crypt of the caecum, with a denser distribution in the middle portion of the crypt, except for the caecal tonsil. The centres of distributions were at 49% of the distance from the bottom of the crypt in the base and 41% in the apex of the caecum. In the caecal tonsil, the centres of distributions were at 64% in the long type of crypt from the bottom of the crypt and at 44% in the short type of crypt. On the other hand, the PCNA-positive enterocytes were distributed more densely at the bottom of the crypt, except for the caecal tonsil. The centres of distributions were at 36% in the base from the bottom of the crypt, 37% in the body, and 34% in the apex. In the caecal tonsil, they were at 54% in the long type of crypt and 44% in the short type. The BrdU-labelled enterocytes reached to the basement of the intestinal villi in all caecal portions at 1 d after the BrdU administration. The leading edge of the labelled enterocytes disappeared from the villous tips at 4 d in the base and the body and 3 d in the apex. In the caecal tonsil, the BrdU-labelled microvillous epithelial cells and the M cells appeared near the orifice of the crypt at 1 d, and BrdU-labelled M cells were not observed in the crypt. Thereafter, almost all of these cells disappeared at 5 d from the follicle associated epithelium (FAE). These results suggest that M cells are transformed from their precursors within 1 d, and the turnover time for M cells occurs within 4 d after the cell division of the precursors.

Glycoconjugate expression defines the origin and differentiation pathway of intestinal M-cells

Intestinal M-cells are specialized epithelial cells located in the domes of the gut-associated lymphoid tissues, which transport antigens from the lumen to the underlying lymphoid tissue, thereby initiating immune reactions. It is assumed that M-cells arise from stem cells in the crypts, from which they migrate to the top of the domes. To study the differentiation pathway of M-cells, we used the rabbit cecal lymphoid patch in which the M-cells express high levels of alpha 1-2-linked fucose and N-acetyl-galactosamine residues in their apical membrane. Dome areas were labeled with fluorescein- and rhodamine-conjugated lectins specific for alpha 1-2-linked fucose and N-acetyl-galactosamine in vivo and in vitro, and were observed with confocal laser scanning microscopy. Ultrathin sections were double labeled with lectin-gold conjugates and the labeling density was quantified by computer-based image analysis. All cecal patch M-cells expressed alpha 1-2-linked fucose and N-acetyl-galactosamine, but the amount of the two saccharides varied considerably depending on the position of the M-cells at the base, flank, or top of the dome. In eight of 18 rabbits studied, radial strips of M-cells with common glycosylation patterns were observed, each strip associated with an individual crypt. Confocal microscopy revealed that lectin-labeled M-cells were not restricted to the dome epithelium but were also detected in the upper third of crypts surrounding the domes. The results show that M-cells are heterogeneous concerning the glycosylation pattern of membrane glycoconjugates. This pattern is modified as the M-cells differentiate and migrate from the base to the top of the dome. Radial strips of M-cells with a common proclivity of glycoconjugate expression suggest that those M-cells that derive from the same crypt have a clonal origin. The presence of (pre-) M-cells in the crypts surrounding the domes indicates that M-cells derive directly from undifferentiated crypt cells and do not develop from differentiated enterocytes.

Review: Peyer's patches

The function of Peyer's patches as antigenic sampling sites involves the complex interplay of a variety of mechanisms that aim to recognize luminal antigens, induce an immunological response and decrease the incidence of antigen translocation across the mucosal epithelium. This is achieved by M cells, which facilitate the uptake of luminal antigens, a vascular architecture that promotes the retention of absorbed antigens within the patch interstitium (allowing for maximal antigenic activation of lymphocytes) and the presence of lymphoid follicles that contain antigen-presenting cells and lymphocytes. Lymphocytes encountering antigen in the Peyer's patches proliferate, differentiate into fully mature antigen-specific effector cells and migrate to the mesenteric lymph nodes where they undergo final maturation. The mature lymphocytes then enter the systemic circulation and migrate throughout the other mucosa-associated lymphoid tissues of the body and "home' into the gut via high endothelial venules and gut-associated lymphoid tissue-specific adhesion molecules, providing antigen-specific lymphocytes at sites likely to re-encounter the antigen.

The gene slyA of Salmonella typhimurium is required for destruction of M cells and intracellular survival but not for invasion or colonization of the murine small intestine

Recent studies have shown that Salmonella typhimurium invades the M cells of Peyer's patches (PP) of the murine ileum. The slyA gene of S. typhimurium has also recently been reported to affect virulence of this pathogen in mice and survival in macrophages. We therefore compared the effect on PP tissue of four strains of S. typhimurium: a wild-type strain, two slyA insertion mutants, and a recombinant S. typhimurium derivative carrying multiple copies of slyA. Invasion assays performed 2 and 7 days after orogastric infection revealed significantly lower numbers of bacteria of the slyA mutants and of the SlyA-overproducing strain in PP than of the wild type. However, similar numbers of bacteria of all strains were still present in the lumen of the small intestine after these times. Invasion assays of PP tissue after 90-min ileal loop infection yielded comparable numbers of bacteria of all strains in PP. Transmission and scanning electron microscopy of PP tissue after ileal loop infection demonstrated that the two slyA mutants and the SlyA-overproducing strain were able to attach to, induce membrane ruffling of, and invade M cells in a way morphologically and quantitatively similar to that of the wild type. In contrast to the wild type, both slyA mutants and, to a lesser extent, the SlyA-overproducing strain were significantly impaired in their ability to destroy M cells and adjacent enterocytes. Taken together, these data suggest that slyA is involved in intracellular survival and M-cell cytotoxicity but not in the invasion process and that the amount of SlyA needs to be precisely balanced for virulence.

Modifications of the follicle-associated epithelium by short-term exposure to a non-intestinal bacterium

This study was undertaken in order to study the effects of short-term exposure of the follicle-associated epithelium (FAE) of rabbit Peyer's patches to a non-intestinal, Gram-positive bacterium. Isolated ileal loops, each containing one Peyer's patch (PP), were stimulated for short periods of time (30 and 60 min) with Streptococcus pneumoniae R36a, a micro-organism normally not present in the intestinal area. Samples from antigen-stimulated and control Peyer's patches were analysed by light (LM), transmission electron (TEM), and scanning electron microscopy (SEM). Stimulation with living pneumococci induced dramatic changes in FAE architecture and morphology. A massive passage of cells from lymphoid tissue to the FAE was rapidly detectable, accompanied by alterations of the FAE surface, with a marked increase of M-cell area. Furthermore, TEM analysis revealed that M cells were able to internalize living pneumococci. S. pneumoniae R36a is a valid experimental model for the further study of the unique antigen sampling function which characterizes the highly specialized FAE in Peyer's patches.

Morphological characteristics of the ileal Peyer's patches in the rhesus macaque: a histological and ultrastructural study

Ileal Peyer's patches of rhesus macaques were investigated by light and electron microscopy (scanning-electron microscopy, transmission-electron microscopy). The results were compared with findings in other species. Differences were found concerning the shape of the dome area, the composition of the dome epithelium and the apical membrane of M cells: in the rhesus monkey, hemispherical domes bulge into the intestinal lumen. The dome epithelium is composed of three populations of gut epithelial cells; absorptive enterocytes as the predominant cell type; goblet cells; and M cells ("microfold bearing' or "membraneous' gut epithelial cells). The apical membrane of M cells forms irregular protrusions.

M cells in Peyer's patches of the intestine

M cells are specialized epithelial cells of the mucosa-associated lymphoid tissues. A characteristic of M cells is that they transport antigens from the lumen to cells of the immune system, thereby initiating an immune response or tolerance. Soluble macromolecules, small particles, and also entire microorganisms are transported by M cells. The interactions of these substances with the M cell surface, their transcytosis, and the role of associated lymphoid cells are reviewed in detail. The ultrastructure and several immuno- and lectin-histochemical properties of M cells vary according to species and location along the intestine. We present updated reports on these variations, on identification markers, and on the origin and differentiation of M cells. The immunological significance of M cells and their functional relationship to lymphocytes and antigenpresenting cells are critically reviewed. The current knowledge on M cells in mucosa-associated lymphoid tissues outside the gut is briefly outlined. Clinical implications for drug deliver, infection, and vaccine development are discussed.

Cytokeratin 18 is an M-cell marker in porcine Peyer's patches

The intermediate filaments of the dome epithelium of porcine Peyer's patches were studied by immunohistochemistry. The labelling patterns of monospecific antibodies directed against cytokeratins 8, 18 and 19 differed considerably. About 40% of the dome epithelial cells were intensely labelled by three different anti-cytokeratin 18 antibodies, indicating that large amounts of cytokeratin 18 are present in these cells. In order to verify that these cytokeratin-18-immunoreactive cells were M-cells, uptake studies using fluorescein-labelled yeast particles were performed. Numerous yeast particles were found exclusively in dome epithelial cells that were highly positive for cytokeratin 18, thus representing M-cells. In contrast, the content of cytokeratin 19 in M-cells was lower than that in neighbouring enterocytes. The labelling intensity of cytokeratin 8 did not differ between M-cells and enterocytes. In addition, the absence of vimentin and desmin from the dome epithelium of porcine Peyer's patches was demonstrated. The results show (1) that porcine M-cells differ from enterocytes in the composition of their cytoskeleton, (2) that cytokeratin 18 is a useful marker for detecting porcine M-cells and (3) that this marker directly correlates with M-cell function.

Immunohistochemical localisation of intercellular adhesion molecule-1 in follicle associated epithelium of Peyer's patches

The epithelium covering domes of lymphoid follicles in Peyer's patches includes membranous cells, which are sites of entry for various macromolecules, absorptive cells, a few goblet cells, and many migrating lymphoid cells. The mechanism of migration of lymphoid cells into the follicle associated epithelium of lymphoid follicles in Peyer's patches is still unknown. This study investigated the relation between localisation of intercellular adhesion molecule-1 (ICAM-1) and the follicle associated epithelium of Peyer's patches immunohistochemically. It was found that subepithelial fibroblasts expressed ICAM beneath the follicle associated epithelium on lymphoid follicles, but not on surrounding villi. The results show that the massive lymphocytic traffic between follicle associated epithelium and lymphoid follicles may be related to ICAM-1 expression.

Epithelial M cells in the rabbit caecal lymphoid patch display distinctive surface characteristics

The follicle-associated epithelium (FAE) in the rabbit caecal lymphoid patch is characterized by the presence of membranous (M) cells, which are believed to be functionally equivalent to those present at other sites of gut-associated lymphoid tissue (GALT). Caecal patch M cells display distinctive features compared with those of other GALT sites, despite similar general morphology and expression of the M cell marker vimentin, suggesting marked heterogeneity in the apical surface of M cells at discrete GALT sites. Electron microscopy reveals that rabbit caecal patch M cells differ from those in the small intestinal Peyer's patch FAE: the former have a prominent aspect within the epithelium and possess microvilli which are longer than those of adjacent enterocytes. Many of the M cells in peripheral regions of the caecal patch FAE are not associated with leucocytes and may thus represent an immature M cell population. The M cells are also histochemically distinct from adjacent enterocytes and from Peyer's patch M cells, showing greater expression of brush-border alkaline phosphatase activity and affinity for certain lectins (peanut and wheat germ agglutinins, Bandeiraea simplicifolia agglutinin II). The differences in the brush-border morphology and glycocalyx structure between M cells at different GALT sites may affect their function at these sites by influencing the interaction of luminal antigens and microorganisms with the M cell surface. The present data also support the hypothesis that M cells arise directly from differentiation of crypt stem cells and not from the transformation of existing fully differentiated enterocytes.

Immunocytochemical study of Peyer's patches follicular-associated epithelium in the marsupial, Didelphis albiventris

The lack of probes defining leukocyte subpopulations has restricted ontogenetic studies of the opossum gut. We report for the first time the organization of the gut cellular immune components using species cross-reactive antibodies. Mouse monoclonal antibodies against human HLA-DR were used together with immunocytochemistry to demonstrate MHC class II-like antigens in the opossum Peyer's patches (PP). Positive staining was obtained in the M cell and enterocytes comprising the follicular-associated epithelium (FAE). Rabbit polyclonal antibody against human CD3 stained opossum thymocytes and T-cell dependent areas of spleen, lymph node, and PP interfollicular zones, but failed to stain intraepithelial lymphocytes in the FAE. In contrast rabbit polyclonal antibody against human IgA stained B-cell immunocytes and plasma cells present in the M-cell lateral invaginations. It is surmised that B-cell activation could occur in the opossum M-cell niches by thymus independent antigens, bypassing T-helper-cell function.

Differential binding of lectins to M cells and enterocytes in the rabbit cecum

BACKGROUND

The afferent limb of the intestinal immune system is represented by the gut-associated lymphoid tissue, in which antigenic material, including complete bacteria, is taken up from the lumen by specialized epithelial cells (M cells). Because the adherence of micro-organisms to epithelial can be mediated by lectin-sugar bindings, the glycoconjugates of the surfaces of M cells and enterocytes were compared.

METHODS

A set of 28 lectins and corresponding sugars was used for light and electron microscopy of fixed and unfixed sections. M cells were identified by anti-vimentin antibodies.

RESULTS

M cells of the cecal lymphoid patches selectively bound lectins specific for fucose or N-acetylgalactosamine. The labeled glycoconjugates were located in the apical membrane and in the membrane of vesicles in the apical cytoplasm. Enterocytes were selectively labeled by galactose-specific lectins. In contrast, the lectin-reactivity of M cells and enterocytes did not differ in the jejunal Peyer's patches.

CONCLUSIONS

The results suggest that there may be selectivity mediated by glycoconjugates in the uptake of antigenic material by cecal M cells but not by jejunal M cells.

Identification of M cells and their distribution in rabbit intestinal Peyer's patches and appendix

The distribution of intestinal membranous (M) cells has been studied within the follicle-associated epithelium of rabbit Peyer's patches and appendix. Vimentin expression has been assessed as a primary criterion to identify rabbit M cells in tissue sections and in whole tissue preparations. This criterion has been compared to the use of the absence of alkaline phosphatase which, due to its heterogeneous distribution within the enterocyte population, is less reliable than vimentin expression as a marker for rabbit M cells. The pattern of vimentin immunostaining revealed that the majority of M cells are located in the periphery of the follicle-associated epithelium, the dome apex being largely free of M cells. This distribution was confirmed by scanning electron microscopy. Vimentin is also expressed by follicle-associated epithelial cells in the vicinity of crypts which lack the typical lymphocyte-containing pocket of M cells. Cytoplasmic peanut agglutinin binding coincides with vimentin-expression throughout the follicle-associated epithelium but is absent from vimentin-negative enterocytes. The co-localisation of these two phenotypic markers in both M cells and epithelial cells adjacent to crypts, which lack the typical morphology of fully developed rabbit M cells, suggests that they correspond to immature M cells which by their location appear to derive directly from undifferentiated crypt stem cells and not from mature columnar enterocytes.

Follicle-associated epithelium and medullary epithelial tissue of the bursa of fabricius are two different compartments

The bursae of Fabricius from the chicken and turkey were studied by light and electron microscopy and immunohistochemical methods. The study focused on the relationship of follicle-associated epithelium to the medulla. The follicle-associated epithelium was supported by 3 to 5 layers of stratified epithelial cells which were a continuation of the corticomedullary epithelial cells. The follicle-associated epithelium consisted of M cells and scattered secretory dendritic cells. The network of the reticular epithelial cells of the medulla was filled with secretory dendritic cells, B cells, and a few T cells and macrophages. The cellular content of the follicle-associated epithelium and the medulla suggested that they were different cellular compartments. Communication between the follicle associated epithelium and medullary epithelial compartment occurred through the supporting cells of the follicle-associated epithelium. When the supporting layers of the follicle-associated epithelium infolded into the medulla, they formed lamellated epithelial bodies similar to the thymic Hassall bodies. The lamellated bodies enclosed secretory dendritic cells but not lymphocytes. The infolding of supporting cells varied from follicle to follicle. The asynchronization of infolding contributed to heterogeneity of follicle composition. Follicle heterogeneity was demonstrated by differences in reactivity with a battery of monoclonal antibodies.

Ultrastructural study of M cells from colonic lymphoid nodules obtained by colonoscopic biopsy

The present study was undertaken to investigate ultrastructurally the epithelium covering lymphoid nodules obtained from colonoscopic biopsies of the human colon and rectum. Colonoscopy using the dye spraying contrast method was performed in nine patients who showed x-ray evidence of lymphonodular hyperplasia. Fifty-two colonoscopical biopsy specimens of lymphoid nodules were obtained from the ascending, transverse, and descending colon and rectosigmoid region. All specimens were observed by light and electron microscopy. Light microscopy disclosed large lymphoid follicles protruding into the lumen with a "dome-type" configuration. These extended to the lamina propria of the mucosa and were associated with a massive lymphoid aggregation extending as far as the muscularis mucosa from the submucosa. The epithelium covering these nodules contained a few goblet cells and many lymphocytes. Observation of the elevated surface at the apex by scanning electron microscopy revealed M cells with sparse microvilli in the dome epithelium surrounded by crypts. Transmission electron microscopy disclosed M cells enfolding many immature or mature lymphocytes and plasmocytes. The M cells had cytoplasmic microvilli (so-called "microfolds") on their surfaces, well-developed tubulovesicular systems, and vacuoles in the cytoplasm. The basic structure of the M cells as observed by scanning and transmission electron microscopy was the same as that of M cells in the Peyer's patches of humans and mice. The apical surface of the colonic lymphoid follicles in Crohn's disease patients was associated with erosions observed by scanning electron microscopy. The erosions proved to be the naked surface of the dome after removal of the epithelium, and many holes from 2.0 to 6.0 microns in diameter were observed on the naked surface. At high magnification, lymphocytes were seen projecting from holes (18%) on the naked surface of the dome. These ultrastructural findings indicate that human colonic lymphoid follicles are very similar to those seen in other species.

Co-expression of vimentin and cytokeratins in M cells of rabbit intestinal lymphoid follicle-associated epithelium

Membranous epithelial (M) cells within the follicle-associated epithelium which overlies gut-associated lymphoid tissue in Peyer's patches and of appendix have been shown by immunocytochemical staining, in rabbit, to contain both vimentin- and cytokeratin-type intermediate filaments. The specificity of vimentin immunostaining has been confirmed by blocking with purified vimentin and by immunoblotting. No evidence was obtained for the expression of vimentin in rat, mouse or human M cells. The possible significance of vimentin-expression in these specialized epithelial cells and the potential use of vimentin as a positive marker for M cells are discussed.

The biology and pathobiology of the intestinal intraepithelial lymphocyte: a review of the literature

Intraepithelial lymphocytes represent a population of lymphocytes situated in the epithelial layer of all mucosae. In this report their biological and pathobiological features, with emphasis on intestinal intraepithelial lymphocytes, are reviewed.