Update Pathogenesis of Celiac Disease
Celiac sprue has a strong hereditary component. The prevalence of the condition in first-degree relatives is approximately 10%. A strong association exists between celiac sprue and two human leukocyte antigen (HLA) haplotypes (DQ2 and DQ8). Damage to the intestinal mucosa is seen with the presentation of gluten-derived peptide gliadin, consisting of 33 amino acids, by the HLA molecules to helper T cells. Helper T cells mediate the inflammatory response. Endogenous tissue transglutaminase deamidates gliadin into a negatively charged protein, increasing its immunogenicity. Absence of intestinal villi and lengthening of intestinal crypts characterize mucosal lesions in untreated celiac sprue. More lymphocytes infiltrate the epithelium (intraepithelial lymphocytes). Destruction of the absorptive surface of the intestine leads to a maldigestive and malabsorption syndrome
Celiac disease is an autoimmune disease, and the enzyme tissue transglutaminase (tTG) has been discovered to be the autoantigen against which the abnormal immune response is directed. Gluten is the single major environmental factor that triggers celiac disease, which has a narrow and highly specific association with class II haplotypes of HLA DQ2 (haplotypes DR-17 or DR5/7) and, to a lesser extent, DQ8 (haplotype DR-4).
Scientific knowledge on the pathogenesis of celiac disease has markedly increased in the past few years; the combined roles of innate and adaptive immunity are now better understood.
Intraepithelial lymphocytes (IELs) play an important role in the destruction of epithelial cells. Through specific natural killer receptors (NKR) expressed on their surface, IELs recognize nonclassical major histocompatibility complex (MHC)-I molecules induced on the surface of enterocytes by stress and inflammation. This interaction leads to activation of these armed effector IELs to become lymphokine-activated killing cells; they cause epithelial cell death in a T-cell receptor (TCR)–independent manner. This killing is particularly enhanced through the cytokine interleukin (IL)-15, which is highly expressed in celiac mucosa. NKG2D has been found to play a crucial role in intestinal inflammation in celiac disease.
The adaptive immune response to gluten has been well described, with the identification of specific peptide sequences demonstrated in specific binding to HLA-DQ2 or DQ8 molecules and in stimulating gluten-specific CD4 T cells. These T cells express α/β TCR, and can be isolated from the lamina propria and cultivated. In vitro, they have been shown to recognize specific gluten peptides presented through interaction with DQ2 or DQ8 molecules.
Gluten is a complex macromolecule that contains abundant proline and glutamine residues, rendering it largely indigestible. Under usual circumstances, gluten is left (in part) unabsorbed by the GI tract. Gluten is composed of glutenins and gliadins, the alcohol-water soluble fraction. These gliadins are further divided into alpha, gamma, and omega fractions based on electrodensity.
Among these fractions, one particular peptide fragment is the alpha gliadin 33-mer, which contains an immunodominant peptide fragment. This fragment is deamidated by tTG. tTG is a ubiquitous enzyme and is known to deamidate glutamine to glutamic acid, creating a strong negative charge within the peptide. This modification is crucial in increasing selection to the positive charges within the binding pocket of HLA-DQ2 or DQ8 molecules on antigen-presenting cells in the lamina propria. When conveyed to gluten specific CD4+ T cell, it induces proliferation and induction of a Th1 cytokine response, primarily with the release of interferon-γ.
B cells receive signals through this HLA interaction, leading to tTG autoantibody production. The role of these autoantibodies is still unclear; they have been shown to be deposited along the subepithelial region even in normal-appearing intestinal biopsy findings prior to positive serology and without the onset of overt epithelial cell damage.
- Celiac disease primarily affects the small intestine. This organ is schematically divided into 3 areas: the duodenum (which begins beyond the pylorus, located at the end of the stomach), the jejunum, and the ileum (ending at the ileocecal junction, the beginning of the large intestine). These 3 parts share similar tissue architecture and are responsible for most of the body’s nutrient absorption. The intestinal wall has 4 layers, which (from the lumen inward) are termed the mucosa, submucosa, muscularis, and serosa. The 2 main functions of the mucosa are to accomplish all digestive-absorptive processes for nutrients and electrolytes and to provide a barrier function by excluding foreign antigens and toxins.
- Celiac disease affects the mucosal layer: here, a cascade of immune events leads to the changes that can be documented by histology.
The classic celiac lesion occurs in the proximal small intestine with typical histological changes of villous atrophy, crypt hyperplasia, and increased intraepithelial lymphocytosis. Three distinctive and progressive histological stages have been described and are termed the Marsh classification. The histological changes of celiac disease are classified as follows:
Type 0 or preinfiltrative stage (normal)
Type 1 or infiltrative lesion (increased intraepithelial lymphocytes)
Type 2 or hyperplastic lesion (type 1 plus hyperplastic crypts)
Type 3 or destructive lesion (type 2 plus villous atrophy of progressively more severe degrees [termed 3a, 3b, and 3c])
- Guandalini S, Setty M. Celiac disease. Curr Opin Gastroenterol. Nov 2008;24(6):707-12.
- Husby S, Koletzko S, Korponay-Szabó IR, Mearin ML, Phillips A, Shamir R, et al. European Society for Pediatric Gastroenterology, Hepatology, and Nutrition guidelines for the diagnosis of coeliac disease. J Pediatr Gastroenterol Nutr. Jan 2012;54(1):136-60.
- Rubio-Tapia A, Kyle RA, Kaplan EL, et al. Increased prevalence and mortality in undiagnosed celiac disease. Gastroenterology. Jul 2009;137(1):88-93.
- Tang F, Chen Z, Ciszewski C, et al. Cytosolic PLA2 is required for CTL-mediated immunopathology of celiac disease via NKG2D and IL-15. J Exp Med. Feb 23 2009;
- Barone MV, Troncone R, Auricchio S. Gliadin Peptides as Triggers of the Proliferative and Stress/Innate Immune Response of the Celiac Small Intestinal Mucosa. Int J Mol Sci. Nov 7 2014;15(11):20518-20537.
- Marsh MN, Hinde J. Inflammatory component of celiac sprue mucosa. I. Mast cells, basophils, and eosinophils. Gastroenterology. Jul 1985;89(1):92-101.
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