Fig. 1

Celiac disease pathogenesis. Partially digested gliadin fragments interact with chemokine receptor 3 on the apical side of epithelium (1) inducing a myeloid differentiation primary response 88-dependent release of zonulin (2). Zonulin interacts with the intestinal epithelium and triggers increased intestinal permeability (3). Functional loss of the gut barrier facilitates gliadin peptide translocation from lumen to the lamina propria (4). Gliadin peptides trigger release of IL-15, keratinocyte growth factor, and IL-8 (5), with consequent recruitment of neutrophils in the lamina propria (6). Simultaneously, alpha-amylase/trypsin inhibitors engage the Toll like receptor 4–MD2–CD14 complex with subsequent up-regulation of maturation markers and release of proinflammatory cytokines (7). Following innate immune-mediated apoptosis of intestinal cells with subsequent release of intracellular tissue transglutaminase, gliadin peptides are partially deamidated (8). Deamidated gliadin is recognized by DQ2/8⁺ antigen presenting cells (9) and then presented to T helper cells (10). T helper cells trigger activation and maturation of B cells, producing IgM, IgG, and IgA antibodies against tissue transglutaminase (11). T helper cells also produce pro-inflammatory cytokines (interferon γ and tumor necrosis factor α) (12), which in turn further increase gut permeability and, together with T killer cells, initiate the enteropathy. Damaged enterocytes express CD71 transporter also on their apical side, resulting in retrotranscytosis of secretory IgA-gliadin complexes (13), thus potentiating gluten trafficking from gut lumen to lamina propria. Ultimately, the interaction between CD4⁺ T cells in the lamina propria with gliadin induces their activation and proliferation, with production of proinflammatory cytokines, metalloproteases, and keratinocyte growth factor by stromal cells, which induces crypt hyperplasia and villous blunting secondary to intestinal epithelial cell death induced by intraepithelial lymphocytes. The hyperplastic crypts (14) are characterized by an expansion of the immature progenitor cells compartment (WNT) and downregulation of the Hedgehog signaling cascade. An increased number of stromal cells known to be part of the intestinal stem cell niche and increased levels of bone morphogenetic protein antagonists, like Gremlin-1 and Gremlin-2, may further contribute to the crypt hyperplasia present in celiac disease