Etiology
Celiac disease is a systemic autoimmune disorder triggered by gluten peptides from grains including wheat, rye, and barley. Almost all people with celiac disease carry one of the two major histocompatibility complex class-II molecules (human leukocyte antigen [HLA]-DQ2 or -DQ8) that are required to present gluten peptides in a manner that activates an antigen-specific T-cell response. The requirement for DQ2 or DQ8 is a major factor in the genetic predisposition to celiac disease.[15][16] However, most DQ2- or DQ8-positive people never develop celiac disease despite daily exposure to dietary gluten.
The additional environmental or genetic factors that are required for loss of immune tolerance to dietary gluten are unknown. Factors that have been hypothesized to play a role include: the timing of initial gluten exposure; gastrointestinal infection leading to gluten antigen mimicry; or direct damage to the intestinal-epithelial barrier leading to abnormal exposure of the mucosa to gluten peptides. One large prospective birth cohort study of children with HLA-DQ2 and -DQ8 genotypes found that a higher gluten intake in the first 5 years of life was associated with an increased risk of celiac disease. The risk of celiac disease increased with every 1g/day increase in gluten from the reference amount.[17] Additional cohort studies have likewise found an association between quantity of gluten ingestion at a young age and the subsequent development of celiac disease.[18][19]
Gut microbial changes are associated with celiac disease, and may precede the onset of enteropathy.[20][21] Reovirus infection has been shown to promote inflammatory immunity and a decrease in oral tolerance to gluten.[22] In keeping with the hypothesis of viral infection as an environmental trigger of celiac disease, case-control studies have reported associations between previous enterovirus and parechovirus infections during early childhood and later development of celiac disease in genetically at-risk children.[23][24]
Pathophysiology
Loss of immune tolerance to peptide antigens derived from prolamins in wheat (gliadin), rye (secalin), barley (hordein), and related grains is the central abnormality of celiac disease. These peptides are resistant to human proteases, allowing them to persist intact in the small intestinal lumen.[25] It is unknown how these peptides gain access to the lamina propria, but leading hypotheses include faulty tight junctions, endothelial cell transcytosis, sampling of the intestinal lumen by dendritic cells, and passage during resorption of apoptotic villous enterocytes.
In the intestinal submucosa these peptides trigger both innate and adaptive immune activation. The mechanism of innate immune activation is not fully known. Gluten peptides are clearly able to stimulate interleukin-15 production by dendritic cells, macrophages, and intestinal epithelial cells, which then stimulate intraepithelial lymphocytes, leading to epithelial damage.[26][27][28][29] In the submucosa, gluten peptides are deamidated by tissue transglutaminase (tTG), an enzyme normally involved in collagen cross-linking and tissue remodeling. Deamidation of the gliadin peptide allows for, first, high-affinity binding to the celiac-associated HLA peptides (DQ2 or DQ8) found on antigen-presenting cells, and second, activation of helper T (Th) cells.[30] For this reason, people must carry either HLA-DQ2 (95% of patients with celiac disease) or HLA-DQ8 (5% of patients with celiac disease) to develop celiac disease. Stimulation of Th cells has two consequences. Cell death and tissue remodeling with villous atrophy and crypt hyperplasia are induced by cytotoxic T lymphocytes. Th2 triggers plasma cell maturation and subsequent antigliadin and anti-tTG antibody production.[31]
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