Intestinal Peyer's Patches: Structure, Function, and In Vitro Modeling

BACKGOUND

Considering the important role of the Peyer's patches (PPs) in gut immune balance, understanding of the detailed mechanisms that control and regulate the antigens in PPs can facilitate the development of immune therapeutic strategies against the gut inflammatory diseases.

METHODS

In this review, we summarize the unique structure and function of intestinal PPs and current technologies to establish in vitro intestinal PP system focusing on M cell within the follicle-associated epithelium and IgA⁺ B cell models for studying mucosal immune networks. Furthermore, multidisciplinary approaches to establish more physiologically relevant PP model were proposed.

RESULTS

PPs are surrounded by follicle-associated epithelium containing microfold (M) cells, which serve as special gateways for luminal antigen transport across the gut epithelium. The transported antigens are processed by immune cells within PPs and then, antigen-specific mucosal immune response or mucosal tolerance is initiated, depending on the response of underlying mucosal immune cells. So far, there is no high fidelity (patho)physiological model of PPs; however, there have been several efforts to recapitulate the key steps of mucosal immunity in PPs such as antigen transport through M cells and mucosal IgA responses.

CONCLUSION

Current in vitro PP models are not sufficient to recapitulate how mucosal immune system works in PPs. Advanced three-dimensional cell culture technologies would enable to recapitulate the function of PPs, and bridge the gap between animal models and human.

An update on oral drug delivery via intestinal lymphatic transport

Orally administered drug entities have to survive the harsh gastrointestinal environment, penetrate the enteric epithelia and circumvent hepatic metabolism before reaching the systemic circulation. Whereas the gastrointestinal stability can be well maintained by taking proper measures, hepatic metabolism presents as a formidable barrier to drugs suffering from first-pass metabolism. The pharmaceutical academia and industries are seeking alternative pathways for drug transport to circumvent problems associated with the portal pathway. Intestinal lymphatic transport is emerging as a promising pathway to this end. In this review, we intend to provide an updated overview on the rationale, strategies, factors and applications involved in intestinal lymphatic transport. There are mainly two pathways for peroral lymphatic transport-the chylomicron and the microfold cell pathways. The underlying mechanisms are being unraveled gradually and nowadays witness increasing research input and applications.

Organism dual RNA-seq reveals the importance of BarA/UvrY in Vibrio parahaemolyticus virulence

Elucidation of host-pathogen interaction is essential for developing effective strategies to combat bacterial infection. Dual RNA-Seq using cultured cells or tissues/organs as the host of pathogen has emerged as a novel strategy to understand the responses concurrently from both pathogen and host at cellular level. However, bacterial infection mostly causes systematic responses from the host at organism level where the interplay is urgently to be understood but inevitably being neglected by the current practice. Here, we developed an approach that simultaneously monitor the genome-wide infection-linked transcriptional alterations in both pathogenic Vibrio parahaemolyticus and the infection host nematode Caenorhabditis elegans. Besides the dynamic alterations in transcriptomes of both C. elegans and V. parahaemolyticus during infection, we identify a two-component system, BarA/UvrY, that is important for virulence in host. BarA/UvrY not only controls the virulence factors in V. parahaemolyticus including Type III and Type VI secretion systems, but also attenuates innate immune responses in C. elegans, including repression on the MAP kinase-mediated cascades. Thus, our study exemplifies the use of dual RNA-Seq at organism level to uncover previously unrecognized interplay between host and pathogen.

A comparison of three Peyer's patch "M-like" cell culture models: particle uptake, bacterial interaction, and epithelial histology

Intestinal Peyer's patch (PP) microfold (M) cells transport microbes and particulates across the follicle-associated epithelium (FAE) as part of the mucosal immune surveillance system. In vitro human M-like cell co-culture models are used as screens to investigate uptake of antigens-in-nanoparticles, but the models are labour-intensive and there is inter-laboratory variability. We compared the three most established filter-grown Caco-2/Raji B cell co-culture systems. These were Model A (Kernéis et al., 1997), Model B (Gullberg et al., 2000), and Model C (Des Rieux et al. 2007). The criteria used were transepithelial resistance (TEER), the apparent permeability coefficient (P_app) of [¹⁴C]-mannitol, M cell-like histology, as well as latex particle and Salmonella typhimurium translocation. Each co-culture model displayed substantial increases in particle translocation. Truncated microvilli compared to mono-cultures was their most consistent feature. The inverted model developed by des Rieux et al. (2007) displayed reductions in TEER and an increased (P_app), accompanied by the largest increase in particle translocation compared to the other two models. The normally-oriented model developed by Gullberg et al. (2000) was the only one to consistently display an increased translocation of Salmonella typhimurium. By applying a double Matrigel™ coating on filters, altering the medium feeding regime for Raji B cells, and restricting the passage number of B cells, improvements to the Gullberg model B were achieved, as reflected by increased particle translocation and improved histology. In conclusion, this is the first time all three designs have been compared in one study and each displays phenotypic features of M-like cells. While Model C was the most robust co-culture, the Model B protocol could be improved by optimizing several variables and is less complicated to establish than the two inverted models.

A human intestinal M-cell-like model for investigating particle, antigen and microorganism translocation

The specialized microfold cells (M cells) in the follicle-associated epithelium (FAE) of intestinal Peyer's patches serve as antigen-sampling cells of the intestinal innate immune system. Unlike 'classical' enterocytes, they are able to translocate diverse particulates without digesting them. They act as pathways for microorganism invasion and mediate food tolerance by transcellular transport of intestinal microbiota and antigens. Their ability to transcytose intact particles can be used to develop oral drug delivery and oral immunization strategies. This protocol describes a reproducible and versatile human M-cell-like in vitro model. This model can be exploited to evaluate M-cell transport of microparticles and nanoparticles for protein, drug or vaccine delivery and to study bacterial adherence and translocation across M cells. The inverted in vitro M-cell model consists of three main steps. First, Caco-2 cells are seeded at the apical side of the inserts. Second, the inserts are inverted and B lymphocytes are seeded at the basolateral side of the inserts. Third, the conversion to M cells is assessed. Although various M-cell culture systems exist, this model provides several advantages over the rest: (i) it is based on coculture with well-established differentiated human cell lines; (ii) it is reproducible under the conditions described herein; (iii) it can be easily mastered; and (iv) it does not require the isolation of primary cells or the use of animals. The protocol requires skills in cell culture and microscopy analysis. The model is obtained after 3 weeks, and transport experiments across the differentiated model can be carried out over periods of up to 10 h.

Dual function of a bacterial protein as an adhesin and extracellular effector of host GTPase signaling

Bacterial pathogens often target conserved cellular mechanisms within their hosts to rewire signaling pathways and facilitate infection. Rho GTPases are important nodes within eukaryotic signaling networks and thus constitute a common target of pathogen-mediated manipulation. A diverse array of microbial mechanisms exists to interfere with Rho GTPase signaling. While targeting of GTPases by secreted bacterial effectors is a well-known strategy bacterial pathogens employ to interfere with the host, we have recently described pathogen adhesion as a novel extracellular stimulus that hijacks host GTPase signaling. The Multivalent Adhesion Molecule MAM7 from Vibrio parahaemolyticus directly binds host cell membrane lipids. The ensuing coalescence of phosphatidic acid ligands in the host membrane leads to downstream activation of RhoA and actin rearrangements. Herein, we discuss mechanistic models of lipid-mediated Rho activation and the implications from the infected host's and the pathogen's perspective.

The treatment with the probiotic Shewanella putrefaciens Pdp11 of specimens of Solea senegalensis exposed to high stocking densities to enhance their resistance to disease

Aquaculture industry exposes fish to acute stress events, such as high stocking density, and a link between stress and higher susceptibility to diseases has been concluded. Several studies have demonstrated increased stress tolerance of fish treated with probiotics, but the mechanisms involved have not been elucidated. Shewanella putrefaciens Pdp11 is a strain isolated from healthy gilthead seabream (Sparus aurata L.) and it is considered as probiotics. The aim of this study was to evaluate the effect of the dietary administration of this probiotics on the stress tolerance of Solea senegalensis specimens farmed under high stocking density (PHD) compared to a group fed a commercial diet and farmed under the same conditions (CHD). In addition, during the experiment, a natural infectious outbreak due to Vibrio species affected fish farmed under crowding conditions. Changes in the microbiota and histology of intestine and in the transcription of immune response genes were evaluated at 19 and 30 days of the experiment. Mortality was observed after 9 days of the beginning of the experiment in CHD and PHD groups, it being higher in the CHD group. Fish farmed under crowding stress showed reduced expression of genes at 19 day probiotic feeding. On the contrary, a significant increase in immune related gene expression was detected in CHD fish at 30 day, whereas the gene expression in fish from PHD group was very similar to that showed in specimens fed and farmed with the conventional conditions. In addition, the dietary administration of S. putrefaciens Pdp11 produced an important modulation of the intestinal microbiota, which was significantly correlated with the high number of goblet cells detected in fish fed the probiotic diet.

Multivalent adhesion molecule 7 clusters act as signaling platform for host cellular GTPase activation and facilitate epithelial barrier dysfunction

Vibrio parahaemolyticus is an emerging bacterial pathogen which colonizes the gastrointestinal tract and can cause severe enteritis and bacteraemia. During infection, V. parahaemolyticus primarily attaches to the small intestine, where it causes extensive tissue damage and compromises epithelial barrier integrity. We have previously described that Multivalent Adhesion Molecule (MAM) 7 contributes to initial attachment of V. parahaemolyticus to epithelial cells. Here we show that the bacterial adhesin, through multivalent interactions between surface-induced adhesin clusters and phosphatidic acid lipids in the host cell membrane, induces activation of the small GTPase RhoA and actin rearrangements in host cells. In infection studies with V. parahaemolyticus we further demonstrate that adhesin-triggered activation of the ROCK/LIMK signaling axis is sufficient to redistribute tight junction proteins, leading to a loss of epithelial barrier function. Taken together, these findings show an unprecedented mechanism by which an adhesin acts as assembly platform for a host cellular signaling pathway, which ultimately facilitates breaching of the epithelial barrier by a bacterial pathogen.

Vibrio parahaemolyticus is a bacterial pathogen which occurs in marine and estuarine environments. It is a main cause of gastrointestinal illness following the consumption of raw or undercooked seafood. In immunocompromised people, the bacteria can sometimes enter the bloodstream and cause septicemia, a serious and often fatal condition. V. parahaemolyticus attaches to host tissues using adhesive proteins. Multivalent Adhesion Molecule (MAM) 7 is an adhesin which helps the bacteria to hold onto the host cells early on during infection. It does so by binding two different molecules on the host, a protein (fibronectin) and phospholipids called phosphatidic acids. We show that MAM7 does not only play a role in sticking to host cells. By forming adhesin clusters on the host surface and binding to host lipids, it triggers signaling processes in the host. These include activation of RhoA, an important mediator of cytoskeletal dynamics. By doing so, MAM7 perturbs proteins at cellular junctions, which normally maintain the cells in the gut as a tightly sealed layer protective of environmental influences. When bacteria use MAM7 to attach to the intestine, the seals between cells break, permitting bacteria to cross the barrier and cause infection of underlying tissues.

Manipulation of intestinal epithelial cell function by the cell contact-dependent type III secretion systems of Vibrio parahaemolyticus

Vibrio parahaemolyticus elicits gastroenteritis by deploying Type III Secretion Systems (TTSS) to deliver effector proteins into epithelial cells of the human intestinal tract. The bacteria must adhere to the human cells to allow colonization and operation of the TTSS translocation apparatus bridging the bacterium and the host cell. This article first reviews recent advances in identifying the molecules responsible for intercellular adherence. V. parahaemolyticus possesses two TTSS, each of which delivers an exclusive set of effectors and mediates unique effects on the host cell. TTSS effectors primarily target and alter the activation status of host cell signaling proteins, thereby bringing about changes in the regulation of cellular behavior. TTSS1 is responsible for the cytotoxicity of V. parahaemolyticus, while TTSS2 is necessary for the enterotoxicity of the pathogen. Recent publications have elucidated the function of several TTSS effectors and their importance in the virulence of the bacterium. This review will explore the ability of the TTSS to manipulate activities of human intestinal cells and how this modification of cell function favors bacterial colonization and persistence of V. parahaemolyticus in the host.