Correlation of extracellular matrix components with the cytoarchitecture of mouse Peyer's patches

Formation of the junctions between lymph follicles in the Peyer's patches even before postweaning activation

Peyer's patches (PPs), which contain an abundance of B and T cells, play a key role in inducing pivotal immune responses in the intestinal tract. PPs are defined as aggregated lymph follicles, which consist of multiple lymph follicles (LFs) that may interact with each other in a synergistic manner. LFs are thought to be spherical in shape; however, the characteristics of their structure are not fully understood. To elucidate changes in the structure of PPs as individuals grow, we generated serial 2D sections from entire PPs harvested from mice at 2, 4, and 10 weeks of age and performed a 3D analysis using a software, Amira. Although the number of LFs in PPs was not changed throughout the experiment, the volume and surface area of LFs increased significantly, indicating that LFs in PPs develop continuously by recruiting immune cells, even after weaning. In response to the dramatic changes in the intestinal environment after weaning, the development of germinal centers (GCs) in LFs was observed at 4 and 10 weeks (but not 2 weeks) of age. In addition, GCs gradually began to form away from the center of LFs and close to the muscle layer where export lymphatic vessels develop. Importantly, each LF was joined to the adjacent LF; this feature was observed even in preweaning nonactivated PPs. These results suggest that PPs may have a unique organization and structure that enhance immune functions, allowing cells in LFs to have free access to adjacent LFs and egress smoothly from PPs to the periphery upon stimulation after weaning.

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.

Spectral characterization and unmixing of intrinsic contrast in intact normal and diseased gastric tissues using hyperspectral two-photon microscopy

Background

Living tissues contain a range of intrinsic fluorophores and sources of second harmonic generation which provide contrast that can be exploited for fresh tissue imaging. Microscopic imaging of fresh tissue samples can circumvent the cost and time associated with conventional histology. Further, intrinsic contrast can provide rich information about a tissue's composition, structure and function, and opens the potential for in-vivo imaging without the need for contrast agents.

Methodology/Principal Findings

In this study, we used hyperspectral two-photon microscopy to explore the characteristics of both normal and diseased gastrointestinal (GI) tissues, relying only on their endogenous fluorescence and second harmonic generation to provide contrast. We obtained hyperspectral data at subcellular resolution by acquiring images over a range of two-photon excitation wavelengths, and found excitation spectral signatures of specific tissue types based on our ability to clearly visualize morphology. We present the two-photon excitation spectral properties of four major tissue types that are present throughout the GI tract: epithelium, lamina propria, collagen, and lymphatic tissue. Using these four excitation signatures as basis spectra, linear unmixing strategies were applied to hyperspectral data sets of both normal and neoplastic tissue acquired in the colon and small intestine. Our results show that hyperspectral unmixing with excitation spectra allows segmentation, showing promise for blind identification of tissue types within a field of view, analogous to specific staining in conventional histology. The intrinsic spectral signatures of these tissue types provide information relating to their biochemical composition.

Conclusions/Significance

These results suggest hyperspectral two-photon microscopy could provide an alternative to conventional histology either for in-situ imaging, or intraoperative ‘instant histology’ of fresh tissue biopsies.

Is peritoneal reflection the best anatomical repair landmark in experimental colorectal surgery on rats?

PURPOSE

To validate Peyer's patch as an anatomical repair landmark for colorectal surgery in rats and to measure the collagen content in segments of the colon containing or not containing Peyer's patch.

METHODS

The distance between Peyer's patch and the peritoneal reflection was measured in forty-five Wistar rats. The colon and rectum were resected for quantification of collagen content by means of computer-assisted image analysis in regions of the colon with and without Peyer's patch.

RESULTS

There was great variation in the distance between Peyer's patch and the peritoneal reflection when the male and female rats were considered as a single group (p=0.04). Comparison between the genders showed that the distance between the patch and the peritoneal reflection was greater in female than in male rats (p=0.001). The colonic segment containing Peyer's patch was observed to have lower tissue collagen content than the segment in which this structure was not present (p=0.02).

CONCLUSION

Peyer's patch can be indicated as an anatomical repair landmark, and there is a need to study the healing of colorectal anastomoses in rats based on differing quantities of tissue collagen existing in the colonic wall with or without this structure.

Modelling cell migration strategies in the extracellular matrix

The extracellular matrix (ECM) is a highly organised structure with the capacity to direct cell migration through their tendency to follow matrix fibres, a process known as contact guidance. Amoeboid cell populations migrate in the ECM by making frequent shape changes and have minimal impact on its structure. Mesenchymal cells actively remodel the matrix to generate the space in which they can move. In this paper, these different types of movement are studied through simulation of a continuous transport model. It is shown that the process of contact guidance in a structured ECM can spatially organise cell populations. Furthermore, when combined with ECM remodelling, it can give rise to cellular pattern formation in the form of "cell-chains" or networks without additional environmental cues such as chemoattractants. These results are applied to a simple model for tumour invasion where it is shown that the interactions between invading cells and the ECM structure surrounding the tumour can have a profound impact on the pattern and rate of cell infiltration, including the formation of characteristic "fingering" patterns. The results are further discussed in the context of a variety of relevant processes during embryonic and adult stages.

Structure, lymphatic vascularization and lymphocyte migration in mucosa-associated lymphoid tissue

In this review, we consider the morphological aspects and topographical arrangement of gut-associated lymphoid tissue (GALT) (solitary and aggregate lymph nodules or Peyer's patches) and of vermiform appendix in the human child and in some mammals. The spatial arrangement of the vessels belonging to apparatus lymphaticus periphericus absorbens (ALPA) and of blood vessels within each lymphoid follicle as well as the ultrastructural characteristics of the lymphatic endothelium with high absorption capacity are considered. Particular attention is also paid to the morphological and biomolecular mechanisms inducing lymphocyte transendothelial migration to the bloodstream by means of lymphatic vessels as well as their passage from blood into lymphoid tissue through the high endothelial venules (HEVs). The preferential transendothelial passage of lymphocytes and polymorphonuclear neutrophils within ALPA vessels of the interfollicular area does not occur following the opening of intercellular contacts, but rather it occurs by means of 'intraendothelial channels'. In HEVs, on the contrary, the hypothesis is plausible that lymphocyte transendothelial migration into lymphoid tissue occurs through a channel-shaped endothelial invagination entirely independent of interendothelial contacts. The lymph of ALPA vessels of the single Peyer's patch is conveyed into precollector lymphatic vessels and into prelymph nodal collectors, totally independent of the ALPA vessels of the gut segments devoid of lymphoid tissue. The quantitative distribution of T lymphocytes in the lymph of mucosal ALPA vessels suggests a prevalent function of fluid uptake, whereas a reservoir and supply function is implicated for the vessels of interfollicular area. The precollector lymphatic vessels and prelymph nodal collectors are considered to be vessels with low absorption capacity, whose main function is lymph conduction and flow.

CCL9 is secreted by the follicle-associated epithelium and recruits dome region Peyer's patch CD11b+ dendritic cells

The follicle-associated epithelium (FAE) secretes chemokines important in the recruitment of various cell types including CCL20 (MIP-3alpha). CCL20 is chemotactic to the CD11b(+) dendritic cells (DCs) distributed in the subepithelial dome regions of the Peyer's patches, and mice deficient in the receptor for CCL20, CCR6, have been reported to be devoid of the CD11b(+) DCs in the dome regions. Here, we describe another chemokine specifically secreted from the FAE of mouse Peyer's patches, CCL9 (MIP-1gamma, CCF18, MRP-2). By in situ hybridization, we demonstrated that CCL9 mRNA was expressed by the FAE but not by the villus epithelium. At the protein level, CCL9 was detected on the FAE and on extracellular matrix structures within the dome regions of the Peyer's patches. By RT-PCR, we demonstrated that one of the putative receptors for CCL9, CCR1, was expressed by the Peyer's patch CD11b(+) DCs and in a chemotaxis assay, CD11b(+) DCs migrated toward CCL9. To compare the abilities of the chemokines CCL20 and CCL9 to recruit CD11b(+) DCs to the dome regions, we examined the in vivo distribution of these cells in CCR6-deficient, CCL9-blocked wild type, or CCL9-blocked CCR6-deficient mice. To our surprise, using a sensitive immunofluorescence analysis, we observed that CD11b(+) DCs were present in the dome regions of the CCR6-deficient mice. In contrast, Ab neutralization of CCL9 in vivo resulted in significant reduction of the CD11b(+) DC number in the subepithelial dome regions of Peyer's patches of both wild type and CCR6 -/- mice. Taken together, these results demonstrate an important role of CCL9 in CD11b(+) DC recruitment to the dome regions of mouse Peyer's patches.

Structural and cellular architecture of conjunctival lymphoid follicles in the baboon (Papio anubis)

Conjunctival lymphoid follicles (CLFs), present in normal individuals, undergo hyperplasia upon conjunctival infection by a specific array of pathogens; infection-associated enlargement of draining preauricular lymph nodes suggests that conjunctival follicles participate in the afferent limb of acquired immune responses for the ocular surface. The present study was performed to delineate the structural and lymphoid anatomy of CLFs in the baboon (Papio anubis), a non-human primate conjunctival model with close similarity to the human. Conjunctiva from both eyes, along with mesenteric lymph node, spleen, tonsil, and ileum controls were harvested from ten baboons at necropsy, and studied by histochemical and immunohistochemical methods. Baboon conjunctival follicles were identified as dense oval collections of leukocytes in the substantia propria with infiltration into a thinned overlying conjunctival epithelium. Goblet cells were universally absent, the overlying mucin layer was attenuated, and the follicle-associated epithelium (FAE) demonstrated comparatively diminished alkaline phosphatase expression. The basement membrane overlying each follicle appeared discontinuous. CD4-positive T lymphocytes were distributed in parafollicular areas and to a lesser degree in follicle germinal centers. B lymphocytes formed the predominant cell in follicles, and also heavily infiltrated the FAE. B cell IgM expression was prominent in germinal centers, while IgD staining occurred in a horseshoe-shaped distribution in the follicle mantle zone. Although B cell IgA expression was noted in the non-follicular conjunctiva, IgA expression was inconspicuous within conjunctival follicles. S-100- and CD1a-positive dendritic cells were found in FAE, while fascin-positive mature dendritic cells appeared in the deeper areas of each follicle. CD68-positive macrophages were dispersed throughout the follicles. CD35-positive follicular dendritic cells were observed only in germinal centers. CLFs appear highly organized consistent with a role in the adaptive immune response to conjunctival pathogens.

Distribution of collagen fibers in the aggregated lymphoid follicles of swine terminal ileum

The arrangement of the collagen bundles was studied in the Peyer's patches of swine terminal ileum, by means of light microscopy (using silver-impregnation technique and picrosirius F3BA staining) and scanning electron microscopy (after NaOH-maceration). The lymphoid tissue forms a large and continuous patch along the antimesenteric border. The follicles are disposed mainly in the tela submucosa and sometimes they reached in the tunica mucosa surface (follicle/dome structures). Some follicles are located in the lamina propria of the tunica mucosa. Light microscopy showed black and brown-stained fibers, and yellow and red, and green-stained fibers, respectively by silver impregnation technique and picrosirius red staining, in the tela submucosa. In this tela, by scanning electron microscopy, the collagen fibers appeared as thick bundles forming a network of parallel layers. This network was denser in the interfollicular than in the follicular area, and formed a capsule surrounding the lymphoid follicles. Our results pointed out that a clear correspondence exists between the findings of currently used light microscopy techniques and the scanning electron microscopy after alkali-water maceration method. The arrangement of the collagen fibers in the antimesenteric border of the tela submucosa suggested a functional compartmentalization within the aggregated lymphoid follicles. This could facilitate the antigen-to-cell and cell-to-cell interaction during the immune response and thus create a suitable microenvironment for an active cell metabolism. The tunica mucosa showed a porous structure and its frequent gaps were likely the sites through which lymphocytes and other cells could freely migrate thus participating in the immunological activities of these structures.

Ultrastructure of absorbing peripheral lymphatic vessel (ALPA) in guinea pig Peyer's patches

Corrosion casts from neoprene direct injection of lymphatic and blood vessels in guinea pig gut-associated lymphoid tissue, that is, solitary lymphoid follicles and Peyer's patches, have shown both their numerical density and their topographical arrangement in physiological conditions, after starvation and lymphatic stasis. The absorbing peripheral lymphatic vessel (ALPA) begins with the lacteal vessel, which continues in the mucosal lymphatic network. The latter is formed by subepithelial and interfollicular vessels wrapping single lymphoid follicles like a basket. Interfollicular vessels drain in the submucosal network, which flows into muscular tunica vessels with nonsegmentary bicuspid valves. They in turn drain lymph in subserosal precollectors and then in prelymphonodal collectors with conduction function. The follicles' germinal center and dome are completely devoid of ALPA vessels, while they are rich in blood vessels. Ultrastructurally, the ALPA vessel wall consists of a monolayer of endothelial cells devoid of pores, fenestrations, and open junctions and lacking a continuous basal lamina. Endothelial cells are joined by overlapping and interdigitating intercellular contacts, while end-to-end contacts are rare. They have a sizeable cell body, containing the nucleus and the common endocytoplasmic organelles, and a peripheral cytoplasm with actin-like filament bundles, free microvesicles or forming channels and a few rough-surfaced encloplasmic reticulum (RER) canaliculi. The presence of intraendothelial channels crossed by lymphocytes can often be detected within the endothelial wall during the different phases of cell transendothelial migration from lymphoid tissue to lymphatic vessel lumen. These channels undergo a numerical increase during starvation, while they are scarce during lymphatic stasis. We have quantitatively evaluated the prevalence of T lymphocytes in the lymph of interfollicular ALPA vessels and of prelymphonodal collectors draining the small intestine tract with or without Peyer's patches, under physiological and experimental conditions (starvation, lymphatic stasis).

The B-cell system of human mucosae and exocrine glands

The mucosae and exocrine glands harbour the largest activated B-cell system of the body, amounting to some 80-90% of all immunoglobulin (Ig)-producing cells. The major product of these immunocytes is polymeric (p)IgA (mainly dimers) with associated J chain. Both pIgA and pentameric IgM contain a binding site for the polymeric Ig receptor (pIgR), or secretory component (SC), which is a requirement for their active external transport through secretory epithelia. The pIgR/SC binding site depends on covalent incorporation of the J chain into the quaternary structure of the polymers when they are produced by the local immunocytes. This important differentiation characteristic appears to be sufficient functional justification for the J chain to be expressed also by most B cells terminating at secretory effector sites with IgD or IgG production; they probably represent a "spin-off" from sequential downstream CH switching on its way to pIgA expression, thus apparently reflecting a maturational stage of effector B-cell clones compatible with homing to these sites. Observations in IgA-deficient individuals suggest that the magnitude of this homing is fairly well maintained even when the differentiation pathway to IgA is blocked. Certain microenvironmental elements such as specific cytokines and dendritic cells appear to be required for induction of IgA synthesis, but it remains virtually unknown why this isotype normally is such a dominating product of local immunocytes and why they have such a high level of J chain expression. Also, despite the recent identification of some important requirements in terms of adhesion molecules (e.g. integrin alpha 4 beta 7 and MAdCAM-1) that explain the "gut-seeking" properties of enterically induced B cells, the origin of regionalized homing of B cells to secretory effector sites outside the gut remains elusive. Moreover, little is known about immune regulation underlying the striking disparity of both the class (IgD, IgM) and subclass (IgA1, IgA2, IgG1, IgG2) production patterns shown by local immunocytes in various regions of the body, although the topical microbiota and other environmental stimuli might be important. Rational design of local vaccines will depend on better knowledge of both inductive and migratory properties of human mucosal B cells.

Regional specialization in the mucosal immune system: primed cells do not always home along the same track

According to the current paradigm of lymphocyte trafficking, primed B and T cells extravasate in the intestinal lamina propria chiefly by means of the mucosal homing receptor alpha4beta7, which interacts with the vascular addressin MAdCAM-1. However, as discussed here, this mechanism cannot explain the preferential homing of B cells with a high level of J-chain expression to mucosal effector sites outside the gut.

Effects of aging on the dynamics of lymphocyte organ distribution in mice: use of a radioiodinated cell membrane probe

We have employed a derivatized aminostyrylpyridinium dye, [125I]I2P-Di-6-ASP, to provide a relatively stable tag on mixed mouse splenocytes and purified B and T cells for the purpose of tracking the distribution of those cells among the organs of normal young (4 months) and aged (> 26 months) recipient mice. Cells from both young and aged donor spleens were studied. Special emphasis was placed on localization of donor cells in the spleens of the recipients because the majority of circulating lymphocytes localize in the spleen and the spleen is the principal organ of primary immune response. There was a profound difference in the efficiency of splenic acquisition of donor cells between young and aged recipients, a difference not found in the liver, lungs, kidneys or heart. In contrast young and old donor lymphocytes lodged equally well in the spleens of recipients of the same age. It was clear that the competence of the splenic microenvironment to serve as a lodging site for circulating lymphocytes deteriorated with age. Such a change could contribute significantly to the deficient immune response of aged subjects. We suggest that aging results in significant change in the splenic extracellular matrix to serve as an adhesive substratum for lymphocytes. Our data point to a need for detailed studies on age-related changes in components of the extracellular matrix within lymphoid tissues. The novel compound which we employed for cell labeling is both radioactive and fluorescent and should be quite suitable for such studies.

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.

Lymphocyte traffic through lymph nodes and Peyer's patches of the rat: B- and T-cell-specific migration patterns within the tissue, and their dependence on splenic tissue

The migration routes of lymphocyte subsets through organ compartments are of importance when trying to understand the local events taking place during immune responses. We have therefore studied the traffic of B, T, CD4(+), and CD8(+ )lymphocytes through lymph nodes and Peyer s patches. At various time points after injection into the rat, labeled lymphocytes were localized, and their phenotype characterized in cryostat sections using immunohistochemistry. Morphometry was also performed, and the recovery of 51Cr-labeled lymphocytes in these organs was determined. B and T lymphocytes entered the lymph nodes via the high endothelial venules in similar numbers. Most B lymphocytes migrated via the paracortex (T cell area) into the cortex (B cell area), and then back in substantial numbers into the paracortex. In contrast, T lymphocytes predominantly migrated into the paracortex and were rarely seen in the cortex. No obvious differences were seen between various lymph nodes and Peyer s patches and the routes of CD4(+) and CD8(+)lymphocytes. After injection of lymphocytes into animals with autotransplanted splenic tissue, the number of B lymphocytes that had migrated into the B cell area of lymph nodes and of Peyer s patches was significantly decreased, whereas CD4(+) lymphocytes migrated in larger numbers into the T cell area of both organs.