LA PROPIEDAD ORIGINARIA, SISTEMAS DE PROPIEDAD Y TIERRAS INDÍGENAS

The intestinal mucosal immune system in the human gut provides a large surface area (300m2) for digestion and absorption of nutrients. It is composed of a single layer of epithelial cells and is sealed by tight junctions that are effective at excluding peptides and macromolecules with antigenic properties [60, 61]. Beneath the mucosal epithelium lies the LP; they contain various immunocompetent cells including dendritic cells, macrophages and lymphocytes which form a functional unit with the epithelial cell layer [61] (see Figure 3). Non-immunological factors such as gastric acid, pancreatic juice, bile, motility, mucus, glycocalyx and cell turnover also protect against harmful agents [61].

Figure 3: Mucosal immune system in the GALT (adapted from Suzuki et al. 2007 [62] and Macpherson and Smith, 2006 [63])

Absorptive epithelium

Absorptive columnar epithelial cells (enterocytes) make up the majority of cells in the epithelial layer. Their surface has numerous microvilli which are covered in glycocalyx and mucus. At the apical end they are connected by junctional complexes (tight junctions) to neighbouring cells important in separating the external and internal environments [60]. The glycocalyx contains various enzymes and nonenzymatic proteins (peptidases, receptors and transport proteins) which are necessary for digestion and absorption [61]. The main functions of the glycocalyx are to prevent the uptake of antigens and pathogens by enterocytes, and also to provide a degradative environment that promotes digestion and absorption of nutrients. The mucus layer is made up of 1% mucin, 1% free protein, and more than 95% water. It contains albumin, immunoglobulin (mostly S-IgA), lysozyme, lactoferrin and epidermal growth factor (EGF) [61]. Enterocytes can also endocytose small amounts of intact proteins and peptides preventing the transepithelial transport of antigens [60].

Should a pathogen invade the epithelium, further protection is provided by the epithelial cells, limiting damage to tiny lesions [61]. The repair process tends to occur in three phases. A rapid initial response called restitution occurs where

Epithelium Peripheral lymph nodes Mesenteric lymph nodes Gut lumen Tissue damage Peyers Patch

(Activation T & B cells) B cell differentiation

to plasma cell (IL-6, IL-10)

Lymphocyte homing to mucosal sites via peripheral blood

Bacteria Dendritic cell B Cell T cell IgA M-cell Thoracic duct Epithelium Peripheral lymph nodes Mesenteric lymph nodes Gut lumen Tissue damage Peyers Patch

(Activation T & B cells) B cell differentiation

to plasma cell (IL-6, IL-10)

Lymphocyte homing to mucosal sites via peripheral blood

Bacteria Dendritic cell B Cell T cell IgA M-cell Thoracic duct

cells move rapidly, within the first hour of damage to cover the denuded area [64]. A slower process of cell proliferation and differentiation occurs over the next two days, and finally the mucosal membranes re-establish [64]. This latter stage is important to restoring gut integrity and mucosal defence, failure to do so can result in further inflammation [64].

Several factors stimulate this process, including peptide growth factors, a number of which are present in the gastrointestinal lumen [65]. Salivary glands secrete EGF which are also present in foods such as bovine milk [65]. Receptors for peptide growth factors are on the baso-lateral membrane of the mucosal cells, not the apical membranes, making it difficult for the ligand to reach the receptor [65]. In adults with damage to the gut, permeability is increased; and in inflammatory states it is thought there may be a shift in receptor distribution to the apical membranes [65].

Appropriately activated immune cells may assist in intestinal renewal by producing damage-healing soluble factors, such as transforming growth factor-β (TGF-β). These molecules have multiple roles and are involved in maintaining gut homeostasis and mucosal tolerance (including the down-regulation of the mucosal immune system in response to commensal flora). TGF-β is involved in isotype switching of B-cells to plasma IgA-producing cells. IgA has a crucial role in maintaining gut homeostasis where it influences microbial populations. Disturbance of homeostatic control is likely to result in activation of both local and systemic immunity [51]. The role of TGF-β in tissue repair is to stimulate fibroblast division and to lay down extracellular matrix once inflammation has stopped [66]. TGF-β also suppresses the expression of surface Igs by B-cells, and decreases the secretion of IgG and IgM, but is stimulatory for some immune cells e.g. induction of monocyte secretion of IL-1, IL-6 and TNF [67]. TGF-β1 may have a role in leucocyte migration by upregulating α4 integrin- mediated leucocyte adhesion [68].

Gut-associated lymphoid tissue (GALT)

GALT (made up of germinal centres and the PPs) sits below specialised epithelial cells [4]. PPs are sites of immune induction; they are located on the

anti-mesenteric side of the small intestine and consist of groups of mucosal lymphoid follicles coated with a single cell layer complex, the follicle associated epithelium. Antigen enters PPs across specialised epithelial cells (known as M- cells) to antigen presenting cells which then stimulate antigen-sensitive lymphocytes.

The epithelial layer overlying the follicle associated epithelium is designed to allow access of macromolecules and micro-organisms to the local epithelial surfaces and to promote their uptake by trans-epithelial transport [60]. M-cells, the main feature of follicle associated epithelium, are joined to neighbouring cells by tight junctions and provide openings in the epithelial barrier through vesicular transport [60]. They lack rigid internal cytoskeletons and are easily deformed by lymphoid cells migrating into the epithelium, a pocket (microfold) develops below the M-cell where lymphoid cells enter and leave without disruption to the cell membrane (see Figure 4).

Figure 4: M-cell in the follicle associated epithelium of Peyer’s patches (PPs) (adapted from Kato and Owen, 1999 [61])

M-cells have an important role in immunological function. They are involved in sampling foreign material on the epithelial surface and receptor mediated uptake of antigen, but they are not involved in antigen processing [45]. There are no closely packed microvilli on the apical surfaces of M-cells (as seen on other enterocytes). This enables them to respond rapidly to the adherence of

Enterocyte M-cell Macrophage Dendritc cell B-cells T-cells S-IgA S-IgA/antigen Enterocyte M-cell Macrophage Dendritc cell B-cells T-cells S-IgA S-IgA/antigen

micro-organisms [60]. Uptake of micro-organisms, macromolecules and particles is through endocytosis, these materials are delivered to the intraepithelial pocket by the transepithelial vesicular transport system [60].

Antigen transported through the M-cells to dendritic and macrophage cells is processed and presented to CD4+ T-cells and naïve B-cells, which later become the cytokine-producing cells and plasma cells required for mucosal immune protection [53]. The high-endothelial venule cells in PPs express mucosal addressin cellular adhesion molecule (MAdCAM-1) which directs the migration of naive T- and B-cells through the interaction with L-selectin (a lymph-node homing receptor expressed on lymphocytes) and the integrin α4β7 [53].