During the digestion of gluten, which is a component of wheat, barley and rye, relatively long gluten fragments of proline-rich peptides are formed in the human gastrointestinal tract. These fragments are resistant to further degradation and are partially resorbed into the body.
People that have a genetic predisposition for celiac disease have immune systems that recognize these gluten fragments and respond to them malignantly. Their abnormal immune response results in damaged villi, which are the nutrient-absorbing cell linings in the small intestine. When these villi are damaged nutrients, vitamins and minerals cannot be properly absorbed. This leads to an array of typical celiac disease symptoms including diarrhoea, abdominal distension, loss of appetite and among children failure in normal growth. Today, the only effective treatment is a strict, lifelong gluten-free diet.
Only patients who strictly and permanently adhere to a gluten-free diet succeed in becoming clinically asymptomatic and mitigate their risk for long term complications such as malnutrition, osteoporosis and intestinal cancer. Nevertheless, unintentional exposure even to small amounts of gluten is a cause of persistent gut damage and very often a lack of clinical symptoms is not a measure of gut recovery.
Celiac disease is quite burdensome for patients and commonly results in reduced quality of life. Apart from disease symptoms, patient burden is driven by social and financial restrictions that result from a gluten-free diet. The burden of celiac disease is not unique to patients but also extends to close relatives and people intimately involved with a celiac. Family members as well as partners are “invisible patients” that need to prompt a celiac to comply with gluten-free diet.
Celiac disease: an in-depth perspective
Celiac disease is a common T cell mediated autoimmune disease, primarily affecting the small bowel. Celiac disease is triggered by the ingestion of gluten in genetically predisposed subjects (human leukocyte antigen (HLA) DQ2 and/or DQ8 haplotypes) .
Based on serological tests the pooled global prevalence of celiac disease is 1.4 % . Clinical symptoms of celiac disease are various and range from mild gastrointestinal symptoms to severe malabsorption syndrome, or extra-intestinal signs such as iron deficiency .
Celiac disease is currently diagnosed involving three major steps. The blood of suspected individuals is tested for the presence of anti-tissue transglutaminase 2 (tTG2)-IgA, anti endomisium (EMA)-IgA, anti-gliadin (AGA) or deamidated gliadin specific antibodies (DGP, either IgA or IgG isotype) . In addition, a small bowl biopsy can be taken to histologically assess gut damage. As a third step a gluten-free diet is implemented and the patient response is assessed .
After gluten ingestion, the glutamine and proline-rich gluten composing proteins are partially hydrolyzed by proteases present in the gastrointestinal tract . Digestion of gliadin produces two peptides: 25mer (P31-55, which can be further degraded into smaller peptides) and the 33mer (encompassing P57-89 of a2-gliadin) , which are resistant to hydrolysis by gastric and pancreatic endopeptidases. The 25mer as well as the 33mer are slowly degraded by the brush-border enzymes dipeptidyl peptidase IV (DPPIV) and dipeptidyl carboxy peptidase 1(DCP-1) , however endogenous brush-border enzymes are generally minimally expressed in humans and are not sufficient to detoxify all gluten immunogenic peptides. In active celiac disease digestion of gluten is further impaired. Partial or total villous atrophy reduces proline-releasing peptidase activity including aminopeptidase N (APN), DPPIV, glycyl-leucine dipeptidase, proline dipeptidase and γ-glutamyl-transpeptidase [7-10].
The intestinal epithelium forms an efficient barrier to most food proteins but allows permeation of molecules on a transcellular and paracellular route . Paracellular diffusion of molecules is regulated by tight junctions between adjacent epithelial cells, which form the largest and most important barrier against the external environment. The expression of tight junction molecules in the intestine is highly regulated and depends on the intestinal compartment. Tight junctions are composed of molecules including tetraspanning occludin and members of the claudin family, junctional adhesion molecule (JAM)-A and tricellulin, and cytoskeleton anchoring zonula occludens (ZO)-1, -2 and -3 which promote gate and fence functions and are involved in cell polarity [12-15]. Tight junctions form small pores in the villus epithelium (4-9 Å) allowing free diffusion of molecules smaller than 600 Da [16, 17].
In the pathogenesis of celiac disease, the digestion-resistant gliadin 33mer peptide remains in the duodenal lumen but can also cross the gut epithelium and reaches the lamina propria. In celiac disease patients the gut epithelium displays a higher paracellular permeability, which is linked to compromised tight junction integrity [18, 19]. Gliadin peptides bind to the chemokine CXC motif receptor 3 (CXCR3) on the luminal side of the intestinal epithelium, resulting in the recruitment and activation of the adapter protein, myeloid differentiation factor 88 (MYD88) and subsequent release of zonulin (ZO toxin) into the lumen of the gut. Released zonulin binds to the epidermal growth factor receptor (EGFR) and protease-activated receptor 2 (PAR2) on the intestinal epithelium. This complex initiates a signaling pathway that results in a protein kinase C (PKC)-dependent phosphorylation of ZO proteins and actin microfilament polymerization, which leads to small intestine tight junction disassembly [3, 20, 21].
In addition, secretory immunoglobulin A (sIgA) can bind gliadin peptides. SIgA/gliadin complexes are internalized by the transferrin receptor (TfR, CD71) of the epithelial cells [18, 19]. In the lamina propria the gliadin peptides are deamidated by the tTG2, catalyzing the hydrolysis of glutamine to glutamic acid residues, introducing negative charges into the glutamine-rich gliadin peptides. Following tTG2 deamidation, the gliadin 33mer peptide presents a high avidity to human HLA DQ2 and DQ8 on antigen presenting cells (APC) [22, 23]. APCs process and present the antigen to naïve gluten-reactive CD4+ T cells, which are subsequently activated. Activated T cells produce proinflammatory cytokines such as IFNγ and TNF and induce a characteristic antibody response by activation of gluten- and tTG2-reactive B cells [24, 25]. In addition, another gliadin peptide, the 13mer, triggers innate immune responses . In response to the 13mer gliadin peptide enterocytes, dendritic cells and macrophages secrete proinflammatory cytokines such as interleukin-15 (IL-15), which mediates cellular cytotoxicity by activation of intraepithelial lymphocytes. Activation of intraepithelial lymphocytes, secretion of matrix metalloproteinases by both fibroblast and lamina propria mononuclear cells results in histologically detectable lesions of the intestinal epithelium . Furthermore, combined presence of IL-15 and retinoic acid prevents the generation of regulatory T cells .
Furthermore, tight junction disassembly as well as increased expression of pore-forming claudin-2 resulting in increased paracellular permeability of small intestine epithelial cells theoretically allowing the gliadin peptides to pass through. Paracellular diffusion together with transcytosis of IgA/gliadin complexes leads to appearance of gliadin peptides behind the epithelial barrier. In celiac disease patients, gluten immunogenic peptides were detected in the urine , suggesting that these peptides enter the systemic circulation via the fenestrated capillary endothelial cells in small intestine villi. This further suggests that gluten immunogenic peptides cause systemic immune reactions possibly responsible for disease symptoms in celiac disease patients apart from the gut compartment. Moreover, gluten immunogenic peptides were also detected in the urine of healthy volunteers previously suspected to a gluten-free diet 4-6 h after gluten ingestion , indicating that sIgA functions as “Trojan horse” in general. IgA appears low in the human blood, however, it is the most produced immunoglobulin in the human body. In humans, especially the upper intestinal system receives 3 to 5 grams IgA per day .
After diagnosis of celiac disease, a complete exclusion of gluten from the diet is implemented to keep celiac disease in remission, since there is no effective pharmacological treatment available. Although gluten-free diet is beneficial to ameliorate the symptomatic relief, it is expensive and difficult to maintain and approximately one third of celiac disease patients do not follow a gluten-free diet . Moreover, patients that adhere to a gluten-free diet often do not achieve histological remission due to continued unintentional gluten intake, because of gluten contaminations or presence of gluten as an additive in processed food .
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