Much ado about M cells

Mechanisms of uptake and transport of particulate formulations in the small intestine

Micro- and nano-scale particulate formulations are widely investigated towards improving the oral bioavailability of both biologics and drugs with low solubility and/or low intestinal permeability. Particulate formulations harnessing physiological intestinal transport pathways have recently yielded remarkably high oral bioavailabilities, illustrating the need for better understanding the specific pathways underpinning particle small intestinal absorption and the relative role of intestinal cells. Mechanistic knowledge has been hampered by the well acknowledged limitations of current in vitro, in vivo and ex vivo models relevant to the human intestinal physiology and the lack of standardization in studies reporting absorption data. Here we review the relevant literature and critically discusses absorption pathways with a focus on the role of specific intestinal epithelial and immune cells. We conclude that while Microfold (M) cells are a valid target for oral vaccines, enterocytes play a greater role in the systemic bioavailability of orally administrated particulate formulations, particularly within the sub-micron size range. We also comment on less-reported mechanisms such as paracellular permeability of particles, persorption due to cell damage and uptake by migratory immune cells.

Colitis and Colorectal Carcinogenesis: The Focus on Isolated Lymphoid Follicles

Gut-associated lymphoid tissue is one of the most diverse and complex immune compartments in the human body. The subepithelial compartment of the gut consists of immune cells of innate and adaptive immunity, non-hematopoietic mesenchymal cells, and stem cells of different origins, and is organized into secondary (and even tertiary) lymphoid organs, such as Peyer's patches, cryptopatches, and isolated lymphoid follicles. The function of isolated lymphoid follicles is multifaceted; they play a role in the development and regeneration of the large intestine and the maintenance of (immune) homeostasis. Isolated lymphoid follicles are also extensively associated with the epithelium and its conventional and non-conventional immune cells; hence, they can also function as a starting point or maintainer of pathological processes such as inflammatory bowel diseases or colorectal carcinogenesis. These relationships can significantly affect both physiological and pathological processes of the intestines. We aim to provide an overview of the latest knowledge of isolated lymphoid follicles in colonic inflammation and colorectal carcinogenesis. Further studies of these lymphoid organs will likely lead to an extended understanding of how immune responses are initiated and controlled within the large intestine, along with the possibility of creating novel mucosal vaccinations and ways to treat inflammatory bowel disease or colorectal cancer.

The third dimension: new developments in cell culture models for colorectal research

Cellular models are important tools in various research areas related to colorectal biology and associated diseases. Herein, we review the most widely used cell lines and the different techniques to grow them, either as cell monolayer, polarized two-dimensional epithelia on membrane filters, or as three-dimensional spheres in scaffold-free or matrix-supported culture conditions. Moreover, recent developments, such as gut-on-chip devices or the ex vivo growth of biopsy-derived organoids, are also discussed. We provide an overview on the potential applications but also on the limitations for each of these techniques, while evaluating their contribution to provide more reliable cellular models for research, diagnostic testing, or pharmacological validation related to colon physiology and pathophysiology.

Role of secretory IgA in infection and maintenance of homeostasis

An important activity of mucosal surfaces is the production of antibodies (Abs) referred to as secretory immunoglobulin A (SIgA) that serve as a first line of defense to repel pathogenic microorganisms and provide a finely tuned balance to guarantee controlled survival of essential commensal bacteria. By excluding bacteria from the epithelial cell, SIgA participates in the cross-talk between the host and its intestinal content, ensuring appropriate homeostasis under normal conditions. Besides the classical view of immune exclusion function, SIgA Abs exhibit the striking feature to adhere to gastrointestinal M cells residing in the follicle-associated epithelium in organized structures called Peyer's patches. Selective binding of SIgA results in transport across the microfold (M) cells, a process that facilitates the association of the Ab with dendritic cells (DCs) located in the underlying subepithelial dome region of Peyer's patches. Limited entry of free SIgA and SIgA-coated bacteria via this pathway is crucial to the modulation of local immune responses in an environment that limits the onset of pro-inflammatory circuits. Such a mechanism would ensure homeostasis by allowing antigen recognition under neutralized conditions and by avoiding tissue dissemination, two features that endow SIgA with non-inflammatory properties in the mucosal environment.

Peyer's patch M cells derived from Lgr5(+) stem cells require SpiB and are induced by RankL in cultured "miniguts"

Peyer's patches consist of domains of specialized intestinal epithelium overlying gut-associated lymphoid tissue (GALT). Luminal antigens reach the GALT by translocation through epithelial gatekeeper cells, the so-called M cells. We recently demonstrated that all epithelial cells required for the digestive functions of the intestine are generated from Lgr5-expressing stem cells. Here, we show that M cells also derive from these crypt-based Lgr5 stem cells. The Ets family transcription factor SpiB, known to control effector functions of bone marrow-derived immune cells, is specifically expressed in M cells. In SpiB(-/-) mice, M cells are entirely absent, which occurs in a cell-autonomous fashion. It has been shown that Tnfsf11 (RankL) can induce M cell development in vivo. We show that in intestinal organoid ("minigut") cultures, stimulation with RankL induces SpiB expression within 24 h and expression of other M cell markers subsequently. We conclude that RankL-induced expression of SpiB is essential for Lgr5 stem cell-derived epithelial precursors to develop into M cells.

Organizer and regulatory role of colonic isolated lymphoid follicles in inflammation

Gut-associated lymphoid tissue (GALT) is supposed to play an integral role in the organization of colonic repair mechanisms. Majority of the GALT is composed of isolated and aggregated lymphoid follicles distributed throughout the intestines. These lymphoid follicles, including Peyer's patches of the small, and isolated lymphoid follicles (ILFs) of both the small and large intestines, are composed of a specialised follicle associated epithelium overlying a subepithelial dome containing numerous dendritic cells, macrophages, T and B cells. Within inflammatory conditions the number, the diameter and the density of ILFs are increasing. Follicles are involved not just in immune surveillance, but their presence is also indispensable for normal colonic mucosal regeneration. Regarding mucosal repair the relation of ILFs to bone marrow derived stem cells, follicular dendritic cells, subepithelial myofibroblasts and crypt formations, and the putative organizer role of ILFs have not been clarified yet.

M cell-targeted mucosal vaccine strategies

Immune responses in the aerodigestive tract are characterized by production and transport of specific IgA antibodies across the epithelium to act as a first line of defense against pathogens in the external environment. To sample antigens on mucosal surfaces in the intestine and upper respiratory tract, the immune system relies on a close collaboration between specialized antigen-sampling epithelial M cells and lymphoid cells. Depending on various factors, local antigen presentation in the mucosal tissue leads to tolerance or initiation of an active immune response. Recently, molecules that could be used to target vaccine antigens to apical M cell surfaces have been identified. Here we review the M cell-targeted vaccine strategy, an approach that could be used to enhance uptake and efficacy of vaccines delivered in the nasal cavity or intestine.

Establishment and Evaluation of an in vitro M Cell Model using C2BBe1 Cells and Raji Cells

In vitro M cell models, consisting of co-cultures of Caco-2 cells and lymphoid cells, were developed and examined to observe bacterial transport. However, under our experimental conditions, the differentiation of Caco-2 cells into M cell-like cells could not be induced efficiently. To obtain a functionally stable M cell model based on human cells, C2BBe1 cells were screened and co-cultured with human Raji cells. In our co-cultures, increased sialyl Lewis A antigen expression and decreased Ulex europeaus agglutinin 1 binding were observed. Regarding the functional properties of the model, microsphere and lactic acid bacteria transport across the C2BBe1 co-cultures were increased compared with the levels seen in monocultures. The C2BBe1 monolayers that were co-cultured with Raji cells exhibited some M cell features; therefore, we consider our M cell model to be useful for investigating the interactions of bacteria with M cells.

Isolated lymphoid follicles in colon: switch points between inflammation and colorectal cancer?

Gut-associated lymphoid tissue is supposed to play a central role in both the organization of colonic repair mechanisms and colorectal carcinogenesis. In inflammatory conditions, the number, diameter and density of isolated lymphoid follicles (ILFs) increases. They are not only involved in immune surveillance, but their presence is also indispensable in normal mucosal regeneration of the colon. In carcinogenesis, ILFs may play a dual role. On the one hand they may support tumor growth and the metastatic process by vascular endothelial growth factor receptor signaling and producing a specific cytokine and cellular milieu, but on the other hand their presence is sometimes associated with a better prognosis. The relation of ILFs to bone marrow derived stem cells, follicular dendritic cells, subepithelial myofibroblasts or crypt formation, which are all involved in mucosal repair and carcinogenesis, has not been directly studied. Data about the putative organizer role of ILFs is scattered in scientific literature.

The possible role of isolated lymphoid follicles in colonic mucosal repair

The continuous reformation and rapid repair of the colonic mucosa is essential for avoiding the aggregation of pernicious mutations induced by bacterial, toxic, or mitogenic factors. Gut-associated lymphoid tissue is supposed to play a central role in the organization of the repair mechanisms. In inflammatory conditions, the number, the diameter and the density of isolated lymphoid follicles (ILFs) are increasing. They are involved not just in immune surveillance, but their presence is also indispensable in normal mucosal regeneration of the colon. The relation of ILFs to the components of mucosal renewal such as bone marrow derived stem cells, follicular dendritic cells, subepithelial myofibroblasts or crypt formation has not been directly studied, and data about their putative organizer role are scattered in scientific literature. Whether they act as a regenerative pool containing stem cells in case of mucosal damage, or they are responsible only for the optimal cytokine milieu for the differentiation of immigrating stem cells is a question under debate. Our aim is to review the relation of ILFs to the different elements of colonic mucosal repair.

Rotavirus infection activates dendritic cells from Peyer's patches in adult mice

This study used an in vivo mouse model to analyze the response of dendritic cells (DCs) in Peyer's patches (PPs) within the first 48 h of infection with the wild-type murine rotavirus EDIM (EDIM(wt)). After the infection, the absolute number of DCs was increased by 2-fold in the PPs without a modification of their relative percentage of the total cell number. Also, the DCs from PPs of infected mice showed a time-dependent migration to the subepithelial dome (SED) and an increase of the surface activation markers CD40, CD80, and CD86. This response was more evident at 48 h postinfection (p.i.) and depended on viral replication, since DCs from PPs of mice inoculated with UV-treated virus did not show this phenotype. As a result of the activation, the DCs showed an increase in the expression of mRNA for the proinflammatory cytokines interleukin-12/23p40 (IL-12/23p40), tumor necrosis factor alpha (TNF-alpha), and beta interferon (IFN-beta), as well as for the regulatory cytokine IL-10. These results suggest that, a short time after rotavirus infection, the DCs from PPs play a critical role in controlling the infection and, at the same time, avoiding an excessive inflammatory immune response.

Eosinophil-nerve interactions and neuronal plasticity in rat gut associated lymphoid tissue (GALT) in response to enteric parasitism

Intestinal lymphoid tissues and Peyer's patches (PP) are innervated sites of immune surveillance in the gastrointestinal tract. Following infection with F. hepatica, neuronal hyperplasia and significantly increased eosinophil and mast cell trafficking to colonic PP sites were evident in rat tissues. Nerve-eosinophil associations were significantly elevated in infected colon and colonic PP, as were colonic tissue levels of the circulatory recruitment factors IL-5 and eotaxin. Increased immunoreactivity for neuronal plasticity markers GAP-43 and neural cell adhesion molecule (NCAM) was also found in infected tissues. Such neuronal alterations in the PP during enteric parasitism may have functional consequences on particular or pathogen uptake.

Targeting mucosal dendritic cells with microbial antigens from probiotic lactic acid bacteria

The use of vaccines against infectious microbes has been critical to the advancement of medicine. Vaccine strategies combined with, or without, adjuvants have been established to eradicate various bacterial and viral pathogens. A new generation of vaccines is being developed using specific strains of Gram-positive, lactic acid bacteria and, notably, some probiotic lactobacilli. These bacteria have been safely consumed by humans for centuries in fermented foods. Thus, they can be orally administered, are well tolerated by recipients and could be easily and economically provided to large populations. In this overview, we focus on mucosal immunity and how its cellular component(s), particularly dendritic cells, can be specifically targeted to deliver immunogenic subunits, such as the protective antigen from Bacillus anthracis (the causative agent of anthrax). An antigen-specific immune response can be elicited using specific strains of Lactobacillus acidophilus expressing the protective antigen. A mucosal, dendritic cell-targeted approach increases the bioavailability of an immunogen of interest when delivered orally by L. acidophilus. This provides an efficiently elegant natural strategy and serves a dual function as an immune-stimulating adjuvant in vivo.

Host-bacteria interaction in inflammatory bowel disease

INTRODUCTION/BACKGROUND

Inflammatory bowel disease (IBD) results from complex interactions between: host genome, immune system, mucosa, bacteria, and environment.

SOURCES OF DATA

Review of PubMed database using search terms 'bacteria and inflammatory bowel disease' and 'genetics and inflammatory bowel disease'. PubMed 'related reference' feature and references from retrieved articles were examined.

AREAS OF AGREEMENT

IBD results from interaction between the microbiota of the gut and the immune system. Key gene defects associated with IBD are involved in bacterial recognition and processing. The environment at least modifies and may determine pathogenesis.

AREAS OF CONTROVERSY

It has been disputed whether the primary defect in IBD is immunological or bacterial, and which bacteria are key. GROWING POINTS/AREAS FOR RESEARCH: 'M cells', the specialized epithelial cells that overlie Peyer's patches, are a major interface between gut bacteria and the immune system. Improved understanding is needed of the bacteria involved in IBD pathogenesis, their genotypes and phenotypes, their portal of entry and their mechanism for escaping attack from the immune system. Bacterial ligands involved in bacteria-epithelial adhesion are emerging, and molecular techniques are rapidly increasing our knowledge of the human intestinal microbiota.

Molecular and cellular basis of microflora-host interactions

Mucosal surfaces represent the main sites in which environmental microorganisms and antigens interact with the host. In particular the intestinal mucosal surfaces are in continuous contact with a heterogeneous population of microorganisms of the endogenous flora and are exposed to food and microbes. As a result, the immune system of the host has to discriminate between pathogenic and commensal microorganisms. This article reviews the types of sentinel cells that continuously sense the environment and coordinate immune defenses as well as the mechanisms of the innate and adaptive immune systems that are activated by bacterial and viral molecular patterns leading to inflammatory, allergic, or regulatory immune response with special emphasis on probiotic bacteria.

An improved in vitro model of human intestinal follicle-associated epithelium to study nanoparticle transport by M cells

An alternative in vitro model of human follicle-associated epithelium (FAE) to study nanoparticle transport mechanisms by M cells was developed and characterized. The previous in vitro model of human FAE has been improved by inverting inserts after Caco-2 cell seeding. Raji and M cells were identified only in inverted co-culture cell monolayers by immunohistochemistry, confocal microscopy, and electron microscopy. The M cell conversion rate evaluated by scanning electron microscopy ranged between 15 and 30% of cells. Transport of 200 nm carboxylated polystyrene nanoparticles was higher and more reproducible in the inverted model. Nanoparticle transport was temperature-dependent, not affected by the presence of EGTA or by potassium depletion, but inhibited by EIPA or nystatin, suggesting that it occurs most likely by macropinocytosis. The inverted model appears more physiologic, functional and reproducible than the normally oriented model.

An in vitro cell-culture model demonstrates internalin- and hemolysin-independent translocation of Listeria monocytogenes across M cells

An ability to translocate the mucosal epithelia through M cells provides invasive pathogens with a rapid means of accessing the mucosal lymphoid tissues. In order to determine the role of M cells in Listeria monocytogenes infection, we initially assessed colonization of Peyer's patch (PP) epithelium in BALB/c mice by Vibrio cholerae Eltor, wild-type L. monocytogenes and an isogenic hemolysin mutant (LO28Deltahly). It was observed that both wild-type L. monocytogenes and Deltahly showed preferential colonization of PP epithelium in this model. Furthermore, a novel luciferase reporter system was used to show rapid site-specific localization of L. monocytogenes in intestinal Peyer's patches. To examine the role of M cells in transcytosis of L. monocytogenes we utilized an in vitro transwell model that mimics M-cell activity through differentiation of C2Bbe1 epithelial enterocytes via co-culture with murine Peyer's patch lymphocytes (PPL). It was shown that L. monocytogenes transits M cells at significantly increased rates compared to C2Bbe1 monocultures. In addition, M-cell transport occurred independently of bacterial hemolysin and internalin production. This study demonstrates rapid transcytosis of L. monocytogenes through M cells, a process that occurs independently of the action of classical virulence factors.

Antibodies enhance interaction of Vibrio cholerae with intestinal M-like cells

Intestinal M cells bear a receptor for secretory immunoglobulin A (IgA) (sIgA) facing the lumen of the epithelial surfaces. Cells bearing this receptor are also found throughout an experimental monolayer consisting of polarized Caco-2 cells, a colon adenocarcinoma cell line. The presence of antibodies (mainly sIgA) in the lumen of the small intestine led us to explore the participation of the sIgA receptor and antibodies in the interaction of Caco-2-associated M-like cells with the mucosal pathogen Vibrio cholerae. Here, we demonstrate that sIgA antibodies isolated from pooled healthy human colostrums, as well as IgG from pooled healthy human serum, can recognize V. cholerae. Furthermore, opsonization enhances M-like-cell transcytosis of V. cholerae strains. We also show that the cholera toxin (CT) receptor ganglioside GM(1) colocalizes with the sIgA receptor in cells of the epithelial monolayer. Both sIgA and IgG antibodies compete for the attachment of soluble CT subunit B to immobilized GM(1). Our results indicate that in this in vitro model system of intestinal epithelia, human sIgA and IgG contribute to the uptake of V. cholerae by M-like cells, probably through an interaction with GM(1). Our results support previous findings of others showing that sIgA can act as an endogenous adjuvant and that sIgA is important for the antigen-sampling function of M cells.

IgA receptors in health and disease

The varied interaction of the Fc region of IgA with receptors confers this antibody class with many of its unique properties. The epithelial polymeric Ig receptor on mucosal epithelial cells transports polymeric immunoglobulin A (pIgA) produced by mucosal B cells to the mucosal surface where, in complex with the secretory component (SC), this secretory immunoglobulin A (SIgA) excludes the multitude of dietary, environmental, and microbial antigens that continuously bombard the mucosae. In health, this IgA-mediated exclusion not only forms the initial defence against infection, it also spares the systemic immune system from potentially deleterious responses to innocuous antigens which can otherwise culminate in inflammatory bowel disease or asthma. Beyond antigen exclusion, in closer encounters with antigens, IgA receptors play roles in protective immunity and disease. FcaRI is the principal myeloid IgA receptor and is responsible for differing IgA-mediated effector responses such as respiratory burst, degranulation, and phagocytosis variously by granulyoctes, monocytes, and macrophages. Furthermore an unknown IgA receptor specific for the secretory component (SC) elicits powerful effector responses from eosinophils. On dendritic cells, FcaRI participates in antigen presentation while on microfold cells, key cells in mucosal antigen presentation, another unknown IgA receptor functions in the transport of antigens across the mucosal epithelial barrier. The activity of another uncharacterized IgA1/IgD receptor on T cells may affect autoimmune disorders. The interplay of different IgA receptors affects immune complex deposition in the common renal disease immunoglobulin A nephropathy (IgAN). Finally, the therapeutic application of various IgA receptors has been sought in the areas of infectious disease, vaccines, and cancer.

Bacterial-associated cholera toxin and GM1 binding are required for transcytosis of classical biotype Vibrio cholerae through an in vitro M cell model system

To elucidate mechanisms involved in M cell uptake and transcytosis of Vibrio cholerae, we used an in vitro model of human M-like cells in a Caco-2 monolayer. Interspersed among the epithelial monolayer of Caco-2 cells we detect cells that display M-like features with or without prior lymphocyte treatment and we have established key parameters for V. cholerae transcytosis in this model. Cholera toxin (CT) mutants lacking the A subunit alone or both the A and B subunits were deficient for transcytosis. We explored this finding further and showed that expression of both subunits is required for binding by whole V. cholerae to immobilized CT receptor, the glycosphingolipid GM1. Confocal microscopy showed CT associated with transcytosing bacteria, and transcytosis was inhibited by pre-incubation with GM1 before infection. Finally, heat treatment of the bacterial cells caused a loss of binding to GM1 that was correlated with a significant decrease in uptake and transcytosis by the monolayer. Our data support a model in which the ability of bacteria to interact with GM1 in a CT-dependent fashion plays a critical role in transcytosis of V. cholerae by M cells.

Apoptotic process of porcine intestinal M cells

Membranous (M) cells of the follicle-associated epithelium (FAE) are believed to sample antigens from the gut lumen. However, the origin, differentiation mechanism, and cell death of M cells are still a matter of controversy. Therefore, we investigated the process of M cell differentiation and determined their fate in the intestine of three-way crossbred female pigs. We used anti-cytokeratin 18 and anti-PCNA antibodies to distinguish M cells and proliferative cells and performed immunohistochemistry, enzyme histochemistry, and scanning electron microscopy on fresh ileal Peyer's patches. Cell migration and apoptotic cells were detected by BrdU labeling and the TUNEL method, respectively. The turnover of the FAE was similar to that of the villi. M cells were mostly observed from the FAE crypt to the FAE periphery, but not in the FAE apex. As proliferative M cells (cytokeratin 18(+)/PCNA(+) cells) have previously been detected in the FAE crypt, porcine M cells may be directly derived from intestinal epithelial stem cells and committed as a distinct cell lineage in the crypts. M cells from the FAE periphery were unstained or only weakly stained for alkaline phosphatase, whereas cytokeratin 18(+)/alkaline phosphatase(+) cells lying near to the FAE apex showed a columnar shape similar to that of adjacent enterocytes. These data suggest that the committed M cells differentiate to mature M cells by contact with lymphocytes at the FAE periphery, and that they trans-differentiate to enterocytes and are finally excluded near the FAE apex.

Targeting antigens to murine and human M-cells with Aleuria aurantia lectin-functionalized microparticles

Neuraminidases act as a virulence factors for several pathogens that invade the human body through Peyer's patch M-cells. Because of the structural similarity of Aleuria aurantia lectin (AAL) to neuraminidases, we hypothesized that AAL might also target human M-cells. In an in vitro human M-cell co-culture model significantly more particles were transported across the epithelium when microparticles were functionalized with AAL versus those that were not. Moreover, high concentrations of AAL induced no detectable cytotoxic effects on the related intestinal epithelial cell cultures, epithelial Caco2- and HT29-MTX-E12-cells. Upon incubation with AAL, PBMCs of allergic volunteers proliferated in response to AAL and secreted the cytokines, IL-2, IFN-gamma, IL-10 and IL-5 in a concentration-dependent manner, indicating immune-stimulatory properties of the lectin. We conclude that AAL-coated microparticles may have the potential to target entrapped antigens to human M-cells for oral vaccination.

New trends in antigen uptake in the gut mucosa

The mucosa represents a large surface of the human body that is in contact with the external environment. Mucosal tissues are colonized by an extremely dense and diverse micro flora of commensal bacteria, which compete with the growth of pathogenic strains. The mucosal sites continuously sample foreign material via specific cells, such as M cells and dendritic cells. Many new ways of antigen uptake have been described recently, especially by M cells and dendritic cells in the intestine. Depending on various factors, antigen presentation in the mucosa can lead to tolerance or initiation of an immune response. Mucosal vaccine strategies will certainly require eliciting specific antigen uptake because this initial step has a crucial role in controlling the outcome of immune responses.

Topography of the gastro-oesophageal junction in the dog revisited: possible clinical implications

The topographical anatomy of the gastro-oesophageal junction was evaluated in six Greyhounds and six Beagles with particular emphasis on the inter-relationship of anatomic structures and landmarks. Significant variation existed between individuals, and a standard topography could not be identified. It was not possible to document the consistent presence of an intra-abdominal oesophagus in either breed examined; in the majority of cases the oesophagus was contained entirely within the thoracic cavity such that no portion of the oesophagus could be subject to abdominal pressures. This has implications for understanding the pathogenesis of hiatal hernia associated gastro-oesophageal reflux disease.

M cell targeting by lectins: a strategy for mucosal vaccination and drug delivery

Bioadhesins are a recognised method of enhancing the absorption of drugs and vaccines at mucosal surfaces. Additionally, bioadhesins allow for cell specific targeting. Lectin-mediated targeting and delivery exploits unique surface carbohydrates on mucosal epithelial cells. The antigen-sampling M cells offer a portal for absorption of colloidal and particulate delivery vehicles, including bacteria, viruses and inert microparticles. We review work supporting the use of lectins to aid targeting to intestinal M cells. Consideration is also given to lectin-mediated targeting in non-intestinal sites and to the potential application of other bioadhesins to enhance M cell transport. While substantial hurdles must be overcome before mucosal bioadhesins can guarantee consistent, safe, effective mucosal delivery, this strategy offers novel opportunities for drug and vaccine formulation.

Dendritic cells: the host Achille's heel for mucosal pathogens?

Mucosal surfaces represent the main sites of interaction with environmental microorganisms and antigens. Sentinel cells, including epithelial cells and dendritic cells (DCs), continuously sense the environment and coordinate defenses for the protection of mucosal tissues. DCs play a central role in the control of adaptive immune responses owing to their capacity to internalize foreign materials, to migrate into lymph nodes and to present antigens to naive lymphocytes. Some pathogenic microorganisms trigger epithelial responses that result in the recruitment of DCs. These pathogens hijack the recruited DCs to enable them to infect the host, escape the host's defense mechanisms and establish niches at remote sites.

Correlations between Factors Determining the Pharmacokinetics and Antiviral Activity of HIV-1 Non-Nucleoside Reverse Transcriptase Inhibitors of the Diaryltriazine and Diarylpyrimidine Classes of Compounds

OBJECTIVE

To investigate the important factors that determine the bioavailability and the antiviral activity of the diaryltriazine (DATA) and diarylpyrimidine (DAPY) non-nucleoside reverse transcriptase inhibitors (NNRTIs) of HIV-1 in animal species and humans using cell-based assays, physicochemical and computed parameters.

METHODS

This naturalistic study included 15 parameters ranging from molecular mechanics calculations to phase I clinical trials. The calculated parameters were solvent-accessible surface area (SASA), polar surface area and Gibbs free energy of solvation. Physicochemical parameters comprised lipophilicity (octanol/water partition coefficient [cLogP]), ionisation constant (pKa), solubility and aggregate radius. Cell-based assays included human colonic adenocarcinoma cell (Caco-2) permeability (transepithelial transport), drug metabolism and antiviral activity (negative logarithm of the molar effective concentration inhibiting viral replication by 50% [pEC50]). Exposure was tested in rats, dogs and human volunteers.

RESULTS

Of the 15 parameters, eight correlated consistently among one another. Exposure (area under the plasma concentration-time curve [AUC]) in humans correlated positively with that in rats (r = 1.00), with transepithelial transport (r = 0.83), lipophilicity (r = 0.60), ionisability (r = 0.89), hydrodynamic radius of aggregates (r = 0.66) and with antiviral activity (r = 0.61). Exposure in humans was also seen to correlate negatively with SASA (r = -0.89). No consistent correlation was found between exposure in dogs and the eight parameters. Of the 14 DATA/DAPY molecules, 11 form aggregates with radii between 34 and 100 nm.

CONCLUSIONS

We observed correlations between exposure in humans with exposure in rats, transepithelial transport (Caco-2 cells), ionisability, lipophilicity, aggregate radius and SASA in the class of DATA/DAPY NNRTI compounds. The lipophilic DATA/DAPY compounds form aggregates. It can be assumed that absorption in the intestinal tract and endocytosis in infected cells of these lipophilic compounds are governed by the common phenomenon of aggregate formation. As the lymphatic system offers a pathway for intestinal uptake of aggregates, this may offer a therapeutic advantage in the treatment of HIV-1 infection. Although it was not the objective of the study, we found that the rat was a better in vivo model than the dog for the prediction of systemic exposure in this particular set of compounds.

Coordinate regulation of Salmonella enterica serovar Typhimurium invasion of epithelial cells by the Arp2/3 complex and Rho GTPases

Salmonella enterica serovar Typhimurium can infect epithelial cells via the basolateral surface after breaching the intestinal epithelium, yet little is known about this process. Here, we show that actin polymerization driven by the Arp2/3 complex is critical to both basolateral and apical bacterial invasion of polarized MDCK cells. While there is also a dependence upon toxin B-sensitive Rho GTPases, none of the four GTPases known to be activated by S. enterica serovar Typhimurium SopE are individually required for basolateral internalization. These results underscore that the specific factors required for Salmonella invasion differ between membrane domains of polarized epithelia.

Intestinal M cells and their role in bacterial infection

M cells are located in the epithelia overlying mucosa-associated lymphoid tissues such as Peyer's patches where they function as the antigen sampling cells of the mucosal immune system. Paradoxically, some pathogens exploit M cells as a route of invasion. Here we review our current knowledge of intestinal M cells with particular emphasis on the mechanisms underlying bacterial infection of these atypical epithelial cells.

Mammalian intestinal epithelial cells in primary culture: a mini-review

Epithelial cells lining the digestive tract represent a highly organized system built up by multipotent stem cells. A process of asymmetric mitosis produces a population of proliferative cells that are rapidly renewed and migrate along the crypt-villus axis, differentiating into functional mature cells before dying and exfoliating into the intestinal lumen. Isolated crypts or epithelial cells retaining high viability can be prepared within a few h after tissue sampling. After cells are cultured in serum-free media, short-term studies (16-48 h) can be conducted for endocrinology, energy metabolism, or programmed cell death. However, long-term primary culture of intestinal cells (up to 10 d) is still difficult despite progress in isolation methodologies and manipulation of the cell microenvironment. The main problem in developing primary culture is the lack of structural markers specific to the stem cell compartment. The design of a microscopic multidimensional analytic system to record the expression profiles of biomarkers all along the living intestinal crypt should improve basic knowledge of the survival and growth of adult crypt stem cells, and the selection of totipotent embryonic stem cells capable of differentiating into intestinal tissues should facilitate studies of the genomic basis of endodermal tissue differentiation.

Catching target receptors for drug and vaccine delivery using TOGA gene expression profiling

Drug delivery technologies are commonly directed towards formulations to control the delivery of therapeutic compounds. However, many processes in the human body have evolved to regulate the transport of various molecules, cells, or particles across epithelial barriers. To take advantage of this biology, we used TOGA gene expression profiling to identify receptor or transporter molecules to target delivery vehicles for transport across an epithelial barrier. In the case of intestinal epithelium, we sought molecules associated with the transport of particles by Peyer's patch M cells. We have identified genes specific to Peyer's patch epithelium, some of which appear to be M cell specific. Discoveries made by this process will provide targets for development of new vaccines, but also provide new insights into the biology of transepithelial transport. The power of this gene profiling approach also suggests application to other systems, such as the response to metabolic changes or drug treatments.

Poliovirus transcytosis through M-like cells

During the digestive-tract phase of infection, poliovirus (PV) is found in the oropharynx and the intestine. It has been proposed that PV enters the organism by crossing M cells, which are scattered in the epithelial sheet covering lymphoid follicles of Peyer's patches. However, PV translocation through M cells has never been demonstrated. A model of M-like cells has been previously developed using monolayers of polarized Caco-2 enterocytes cocultured with lymphocytes isolated from Peyer's patches. In this model, lymphoepithelial interactions trigger the appearance of epithelial cells having morphological and functional characteristics of M cells. We have demonstrated efficient, temperature-dependent PV transcytosis in Caco-2 cell monolayers containing M-like cells. This experimental evidence is consistent with M cells serving as gateways allowing PV access to the basal face of enterocytes, the underlying immune follicle cells, and PV transport toward mesenteric lymph nodes.

Yersinia enterocolitica as a vehicle for a naked DNA vaccine encoding Brucella abortus bacterioferritin or P39 antigen

Brucella is a facultative intracellular parasite that causes brucellosis in animals and humans. The protective immune response against Brucella involves both humoral and cell-mediated immunity. In previous studies, we demonstrated that the T-dominant Brucella antigens bacterioferritin (BFR) and P39 administered either as CpG adjuvant recombinant proteins or as naked-DNA plasmids induced a specific Th1-biased immune response in mice. In order to improve the protection conferred by the BFR and P39 vaccines and to evaluate the additive role of antilipopolysaccharide (anti-LPS) antibodies, we used live attenuated Yersinia enterocolitica serotypes O:3 and O:9 as delivery vectors for naked-DNA plasmids encoding these BFR and P39 antigens. Following two intragastric immunizations in BALB/c mice, the Yersinia vectors harboring a DNA vaccine encoding BFR or P39 induced antigen-specific serum immunoglobulin and Th1-type responses (both lymphocyte proliferation and gamma interferon production) among splenocytes. Moreover, as expected, antibodies recognizing Brucella abortus 544 lipopolysaccharide were detected in O:9-immunized mice but not in O:3-treated animals. Animals immunized with O:9 organisms carrying pCI or with O:9 organisms alone were found to be significantly resistant to infection by B. abortus 544. Our data demonstrated that pCI plasmids encoding BFR or P39 and delivered with live attenuated strains of Yersinia O:3 or O:9 can trigger Th1-type responses. The fact than only O:9 vectors induced a highly significant protective immunity against B. abortus 544 infection pointed out the crucial role of anti-LPS antibodies in protection. The best protection was conferred by a serotype O:9 strain carrying pCIP39, confirming the importance of the P39 T-cell antigen in this mechanism.

Mammalian intestinal epithelial cells in primary culture: a mini-review

Epithelial cells lining the digestive tract represent a highly organized system built up by multipotent stem cells. A process of asymmetric mitosis produces a population of proliferative cells that are rapidly renewed and migrate along the crypt-villus axis, differentiating into functional mature cells before dying and exfoliating into the intestinal lumen. Isolated crypts or epithelial cells retaining high viability can be prepared within a few h after tissue sampling. After cells are cultured in serum-free media, short-term studies (16-48 h) can be conducted for endocrinology, energy metabolism, or programmed cell death. However, long-term primary culture of intestinal cells (up to 10 d) is still difficult despite progress in isolation methodologies and manipulation of the cell microenvironment. The main problem in developing primary culture is the lack of structural markers specific to the stem cell compartment. The design of a microscopic multidimensional analytic system to record the expression profiles of biomarkers all along the living intestinal crypt should improve basic knowledge of the survival and growth of adult crypt stem cells, and the selection of totipotent embryonic stem cells capable of differentiating into intestinal tissues should facilitate studies of the genomic basis of endodermal tissue differentiation.

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.

Expression of specific markers and particle transport in a new human intestinal M-cell model

The aim of this work was to establish a new, simplified in vitro model of the human M-cell. Cocultures of physically separated human intestinal epithelial Caco-2 cells and B-cell lymphoma Raji cells were established. The cocultures were characterized under the criteria of morphology, integrity, expression of M-cell markers and cell adhesion molecules (CAMs), and altered particle transport. Using this construct, the epithelial cells were transformed to cells with an M-cell-like morphology and had altered expression of potential human M-cell markers (alkaline phosphatase down-regulation and Sialyl Lewis A antigen up-regulation). The expression of intercellular adhesion molecule-1 and vascular cell adhesion molecule was altered and there was an increased binding of lectins wheat germ agglutinin and peanut agglutinin with a 40-fold increase in microparticle transport. The particle transport was size-dependent and could be inhibited at 4 degrees C or by replacing the Raji B-cells with Jurkat T-cells. This new coculture model will enable controlled studies of M-cell development and function in vitro.