Under physiological conditions, the first contact between food antigens and the local immune system in the gut occurs through the interaction of antigen-presenting cells, specifically dendritic cells that sense luminal contents and promote tolerance toward luminal food antigens, thus maintaining a disease-free state [
50‐
57]. The maintenance of tolerance requires high differentiation and maturation of both epithelial and immune cell compartments, and even a minimal perturbation of this delicate balance may result in pathological conditions. Unfortunately, evidence-based information in this area is limited, but it is well accepted that undigested or partly digested gliadin can affect a wide range of human cell functions. Early introduction of gliadin-containing cereals was recently reported to increase the risk of islet cell autoimmunity in humans [
58]. Findings from studies using non-obese diabetic mice and BioBreeding diabetic-prone (BBDP) rats have implicated wheat gliadin as a dietary diabetogenic factor. In BBDP rats, gliadin exposure is accompanied by zonulin-dependent increased intestinal permeability, presumably allowing food antigens to come in contact with the underlying lamina propria [
59]. Zonulin is an important protein released by the small intestinal mucosa after several stimuli (for example, dietary antigens, including gluten (see Figure
2) or bacteria) and is involved in the modulation of paracellular intestinal permeability. A normal intestinal epithelium is impermeable to macromolecules, while CD is characterized by enhanced intestinal permeability and an altered junctional structure between epithelial cells, leading to compromised barrier function. Tight junctions (tj) and multiple proteins that make up the tj strands (for example, occludin, the claudin family, zonula occludens-1 protein (ZO-1)) have a critical role in the development of intestinal immunological responsiveness. When the integrity of the intestinal tj is compromised, an immune response to environmental antigens that cross-react with host antigens may develop, thereby triggering the onset of CD [
60,
61]. Conversely, in a study conducted by Sapone
et al., GS subjects showed a normal intestinal permeability and claudin-1 and ZO-1 expression compared to celiac patients, and a significantly higher expression of claudin-4. In the same GS patients, the up-regulation of claudin-4 was associated with an increased expression of toll-like receptor-2 and a significant reduction of T-regulatory cell marker FoxP3 relative to controls and CD patients. Additionally, an increase in IELs of the classes α and β, but no increase in adaptive immunity-related gut mucosal gene expression, including interleukin (IL)-6, IL-21 and interferon -γ, was detected in GS [
62]. These changes in GS could suggest an important role of the innate immune system without any involvement of the adaptive immune response [
62].
In vitro studies suggest that wheat amylase-trypsin inhibitors (ATIs) could play a major role as triggers of the innate immune response in GS. Wheat ATIs are a family of five or more homologous small proteins highly resistant to intestinal proteolysis. They are known to be the major allergen responsible for baker's asthma. Preliminary evidence suggests that the addition of 1 μg/mL to 20 μg/mL of ATIs to monocyte-derived dendritic cells stimulates the release of IL-8 in a dose-dependent manner (Detlef Schuppan, unpublished data). Recently, to test the hypothesis that gluten can cause gastrointestinal symptoms in patients without CD, a double-blind, randomized, placebo-controlled re-challenge trial was undertaken in patients with IBS fulfilling the Rome criteria III, who had CD excluded by best practice methods and who reported a symptom response to a GFD [
63]. Patients were randomized according to a computer-generated list of random numbers held by an independent observer to either the gluten or the placebo treatment group. Over the entire study period, the severity scores of pain, satisfaction with stool consistency and tiredness were significantly higher for those consuming the gluten diet compared to the placebo group, while no evidence for intestinal inflammation or damage or for latent CD was found to offer an explanation for symptom deterioration caused by gluten. Therefore, this study further supports the notion that non-celiac GS is part of the spectrum of gluten-related disorders and confirmed similar findings reported more than 30 years ago [
64].