The most important lesson that the different animal models of celiac disease has provided us is that celiac disease is a composite of three different phenomena that can each occur independently of each other. The first component is that of an aberrant innate immune response to dietary gluten and/or dietary proteins in general. As was concluded from the work with the IL-15 overexpressing DQ8 mouse, increased levels of IL-15, as found in celiac patients, perturbs the intestinal homeostasis and allows (drives?) the inflammatory T cell response to gliadin as characterized by a strong production of IFNγ. However, overexpression of IL-15 alone does not in itself cause overt flattening of villi.
Another aberrant stimulation of the innate immune system by gliadin that is only found in celiac patients is that of gliadin-driven intestinal permeability. Currently, it is thought that gliadin will bind to CXCR3, which is expressed by intestinal epithelial cells [36
]. The crosslinking of CXCR3 results in the release of zonulin (pre-haptoglobin-2), which causes the opening of the tight junctions between the epithelial cells [36
]. The rhesus macaque monkey model has shown that gliadin consumption does not result in intestinal permeability in the gluten-tolerant (healthy) monkeys; they only observed this occur in the gluten-sensitive monkeys [10
]. Also, their detection of a radio-isotope labeled 33mer (D9
33-mer) in the sera of the gluten-sensitive macaque within 60 minutes after administration demonstrated that the trans-epithelial transport is extremely fast.
Both the transcellular and para-cellular pathways were observed to occur in the gluten sensitive rhesus macaques [10
]. In the study that addressed transcellular translocation, fluorescently labeled 33mer was instilled into the duodenum of two healthy (gluten-tolerant) and two gluten sensitive monkeys. Pinch biopsies were extracted 40 minutes after instillation. Staining showed that the 33mer was present in the lamina propria as well as the epithelium, specifically inside individual epithelial cells in “Goblet cell-like cavities”. This result would indicate that both the paracellular and transcellular pathways are occurring in the macaque model. Neither of the healthy monkeys had any 33mer present below the brush border membrane. This result with the healthy monkeys demonstrates that both translocation pathways only occur in the gluten-sensitive monkeys.
The second component of celiac disease would be a strong adaptive CD4+ TCRαβ+ immune response to gliadin restricted by DQ2 and/or DQ8. Many studies using cell lines derived from intestinal biopsies from celiac patients have shown that these cells are the source of the elevated levels of IFNγ in the intestines of active celiac patients. However, as the afore-mentioned studies with animal models show, especially the HLA transgenic mouse models, an exceptionally strong CD4+ T cell response to gliadin does not result in gluten–dependent enteropathy.
In the Irish setter model, there was gluten dependent subtotal villous atrophy similar to celiac disease, but independent of MHC II [8
]. Some gluten sensitive macaques developed villous flattening in a gluten dependent fashion; however, it is unknown if this feature is MHC II dependent [10
]. This would suggest that villous flattening, a characteristic feature of celiac disease, is disassociated from MHC II.
Genetic studies demonstrate that MHC II molecules only contribute 40% of the risk for developing celiac disease [39
]. Therefore, other elements must be contributing to the villous flattening. One such element may be a strong anti tTG production in response to gluten consumption. This autoimmune response against a self-protein characteristic of celiac disease is used as a diagnostic tool and is elevated in celiac patients who have gluten dependent enteropathy [40
]. It may be that this antibody plays a heretofore unappreciated pathogenic role in the development of autoimmunity, specifically that of gluten dependent villous atrophy and flattening. Indeed, this antibody has been found to be produced in the intestines of celiac patients as well as deposited at the epithelial membrane of the small intestine [41
Overall then, the animal models have provided us with a new paradigm for the pathogenesis of celiac disease. This paradigm, as depicted in figure one, would consist of three components: an aberrant innate immune response to gliadin that occurs in the context of HLA DQ2/DQ8 as well as perturbations to the regulatory arm of the immune system, resulting in autoimmunity.
Triggers of Celiac Disease
It is likely that all three arms have to be activated simultaneously in order for the flattening of the villi to occur in a gluten dependent fashion as well as a self-propagating fashion. This activation could result as a consequence of environmental factors or behavioral factors. One study showed repeated rotavirus infections during infancy may increase the risk of developing celiac disease [44
]. Another study determined that a subset of antibodies against tissue transglutaminase also binds to VP-7, a rotavirus protein [45
With respect to behavior affecting the development of celiac disease, a number of studies have shown that the introduction of gluten too early in an infant’s diet increases the risk for developing celiac disease [46
]. It has also been shown that breastfeeding can delay the onset of celiac disease [47
As the current animal models of celiac disease have not yet incorporated these risk factors, it may be beneficial to do so. This would allow us to better understand the specific immunological events that are occurring in the immature intestinal immune system during the initial exposure to gluten. Such analyses could be performed in the different mouse models, and may lead to the overt flattening of the villi, which is a characteristic feature of celiac disease.
Another approach to make the models more powerful would be to increase the complexity through the use of more transgenes. For example, the trafficking patterns and fates of different cell types could be evaluated, as has been successfully done using FoxP3-GFP reporter mice and the cellular transfer model of IBD [49
]. Evaluating the roles of cytokines in the triggering, maintenance, and suppression of cytokines could be achieved through using knockout constructs or overexpression constructs as was done with the IL-15 overexpressing DQ8 mouse. These increasingly complex mouse strains will require significant financial investments in breeding programs; however, similar approaches with mouse models of IBD have revealed fruitful insights into its pathogenesis. As more and more countries recognize celiac disease as a growing health concern of national importance, the generation of increasingly complex animal models will not be far behind.