Celiac disease is characterized by damage to the small intestinal mucosa including villus shortening, crypt hyperplasia, and increased lymphocyte infiltration of the epithelium and lamina propria
due to an exacerbated proinflammatory immune response to gluten proteins 
. Several changes are observed in the epithelium, including altered enterocyte shape and height, loss of brush border, vacuolation, denudation and loss of epithelia, some of which are the consequence of increased enterocyte apoptosis 
High production of IL-15 in intestinal mucosa in active CD has been shown to trigger enterocyte apoptosis via the induction of cell surface MICA, which in turn interacts with the activating NKG2D receptor present in IELs. Cytotoxic activity of IELs is also potentiated by IL-15 through activation of JNK and ERK pathways 
. Though MICA/B confers susceptibility to NKG2D-mediated killing of enterocytes by intraepithelial NK and CD8+
T cells in untreated CD, our results suggest that MICA/B expression may also regulate cell survival of other cells in the intestinal mucosa.
In our study, we observed a more ubiquitous distribution of MICA/B expression. In enterocytes, the expression was mainly found in the cytoplasm as peri- and/or supra-nuclear aggregates. The analysis of the intraepithelial compartment, which contains different lymphocytes, most of them CD7+ cells, revealed the expression of MICA/B in lymphocytes in celiacs and control samples. We found coarse MICA/B aggregates in the cytoplasma of CD7+ cells; which were more frequently observed in mild enteropathy samples.
Distinct MICA/B+ cell populations such as CD3+ and CD7+ T lymphocytes, CD20+ B lymphocytes and plasma cells were found in the lamina propria of non inflammed and enteropathy tissue, and the pattern of the MICA/B staining found in CD7+
lamina propria cells was coincident with that observed in cells of the intraepithelial compartment. On the other hand, B cells showed clear membrane staining while plasma cells showed an intense but diffuse intracellular pattern. A few HAM 56+ macrophages also harbour MICA/B in coarse cytoplasmic aggregates.
MICA/B expression was reduced in duodenal samples from patients under a gluten-free diet, reflecting a possible link between the ongoing inflammatory process induced by gluten ingestion and MICA/B expression. Therefore, considering the pattern of MICA/B expression in different cell lineages observed, signals for induction of MICA/B may be part of a more general mechanism associated to the ongoing inflammatory process in the small intestine in untreated CD patients. Several studies on intestinal tissue, isolated cells from intestinal mucosa or epithelial cell lines support a link between cellular (heat, oxidative and ER) stress and mucosal damage 
. Particularly, the occurrence of oxidative stress was observed in intestinal biopsies from untreated CD patients, which was evidenced as increased level of prostaglandins E2 while the levels of the antioxidants enzyme glutathione peroxidase and reductase, and consequently reduced glutathione (GSH), were decreased 
. In addition, inducible-nitric oxide synthase (iNOS), which is constitutively expressed in duodenal enterocytes, showed increased activity in untreated CD 
. Direct participation of gliadin peptides, particularly p31–43, in the production of reactive oxygen and nitrogen species (ROS and RNS) has been documented in the induction of oxidative stress in the mucosa of these patients 
. Therefore, the existence of an altered epithelium as consequence of the oxidative and ER stress might be part of the mechanisms that contribute to the intestinal damage in untreated CD.
To evaluate whether different forms of cellular stress may occur in duodenal mucosa, we analyzed the expression of BiP, a well-established marker of ER stress 
. BiP was detected in distinct lamina propria
cells, both in non inflammed tissue and enteropathy. Remarkably, we observed a higher expression of BiP in the epithelia of untreated CD duodenal samples but not in healthy tissue. Therefore, and in accordance with previous studies, our results suggest that an oxidative and an ER stress are present in CD enteropathy 
. Furthermore, the observation of TIA-1+
granules in lamina propria
mononuclear cells from untreated CD patients further supports this idea.
Immunofluorescent analyses revealed that different cells exhibit a particular pattern of MICA/B staining. These distinct patterns of cytoplasmic MICA/B+
structures might be linked to structures formed during the stress response. The RNA binding protein TIA-1, found in small cytoplasmic aggregates, named stress granules 
, was observed in lamina propria
mononuclear cells, which additionally indicates the existence of an ongoing stress response in duodenal mucosa of untreated CD patients.
In vitro studies with Caco-2 cells and different models for oxidative, thermal and ER stress, indicated an accumulation of MICA/B but not in association with TIA-1+ stress granules. Although we cannot rule out other intracellular localizations of MICA/B such as associated to aggresomes, this pattern resembles the localization of MICA/B observed in intestinal mucosa in active CD.
Gluten peptides, the causative agent of CD in genetically susceptible individuals, particularly p31–43, may also mediate inflammatory processes 
, alter the traffic of the vesicular compartment resulting in increased epidermal growth factor receptor (EFGR) and the IL-15/IL-15Rα complex expression and activation 
, that altogether contribute to disregulation of tissue remodelling and mucosal damage. Remarkably, gliadin peptides may induce cellular stress in the epithelium by different mechanisms as was observed for oxidative 
and ER-stress 
in Caco-2 cells. Enhanced expression of the stress protein HSP65 in epithelial cells in intestine of untreated CD patients appears as result of the chronic inflammation 
. Altogether, there is substantial evidence indicating that stressed mucosa is a consequence of the inflammatory cascade in CD pathogenesis.
In our study, we also observed expression of MICA/B in B and T lymphocytes. Expression of MICA in activated T lymphocytes has already been observed 
and it has been reported that such expression confers susceptibility to NK cell-mediated cytotoxicity 
. More recently, a pathophysiological role of MICA expression and release on T cells during HIV infection was described 
. Remarkably, we found MICA/B in the cytoplasm of intraepithelial and lamina propria T lymphocytes. Therefore, to the best of our knowledge our findings constitute the first description of in vivo
expression of MICA in T cells in a pathological situation of non infectious origin such as CD. Expression of MICA/B by T cells makes them susceptible to NKG2D-mediated cytotoxicity by NK cells 
. Cell surface MICA/B expression may act to negatively regulate T cell function by decreasing of IFN-γ production and cytotoxicity and reduce tissue damage by regulatory mechanisms via NK/T cell interaction. However, intracellular (cytoplasmic) expression, as observed in our study, may preclude that such putative homeostatic mechanism may operate normally and consequently contribute to the tissue damage observed in the mucosa of CD patients 
Altogether, our results indicate that the MICA/B expression in intestinal mucosa of celiac patients is indeed broader than originally reported and might be associated to the extensive stress conditions present in the intestinal lesion in active CD. Also, the intracellular location of MICA in intraepithelial and lamina propria T cells may hinder their recognition by NKG2D-expressing cells avoiding the control of overactivated T cells, hypothesis to be further investigated in future studies. Therefore, our results suggest that MICA/B may play a more general role than previously thought in gut immunobiology.