Celiac disease (CD) is a chronic autoimmune disease of the small bowel mucosa that develops because of the altered immune response to gluten and gluten-related prolamines in rye and barley, which leads to intestinal epithelium damage and villous atrophy, and crypt hyperplasia in the small bowel mucosa (Lebwohl et al. 2018; Catassi et al. 2022).
In our previous study, we investigated the involvement and role of diverse dendritic cell (DC) subsets in the pathological processes of CD and the possible inclusion of tolerogenic DCs in Tregs development in CD and compared the results with those obtained for subjects with normal small bowel mucosa (Vorobjova et al. 2015).
However, studies on regeneration of the damaged small bowel mucosa and density of intestinal stem cells (ISC) in CD persons are still scarce (Piscaglia 2014; Piscaglia et al. 2015). Intestinal lesion is a hallmark of CD. Enterocytes represent the major part of differentiated epithelial cells; other types of epithelial cells are goblet cells, Paneth cells, tuft cells and hormone secreting enteroendocrine cells. All these cells are generated from ISCs that are localized at the bottom of the crypts. The hallmark of ISCs, leucine-rich repeat-containg G protein-coupled receptor 5 (LGR5), was identified as WNT target gene expressed selectively at the base of adult intestinal crypts (Barker et al. 2007; 2013). The newly formed cells migrate to the tips of villi, where they are excruded 4–6 days later. It has been estimated that in humans, 1011 epithelial cells are shed every day; under inflammatory conditions and in CD, their number increases, and the turnover rate of enterocytes increases as well ( Potten and Loeffler 1990; Przemioslo et al. 1995; Inversen and Sollid 2023). There is yet no sufficient data about the modulation and degeneration of ISCs in the intestinal mucosa in CD.
Intestinal epithelial homeostasis is characterized by crosstalk between the epithelium and the lamina propria. In CD, intestinal lesion is defined also by infiltration of inflammatory leukocytes both in the epithelium and in the lamina propria, where the compartment plasma cells (effector B cells) dominate the infiltrate (Iversen and Sollid 2023). Di Niro et al. (2012) demonstrated high abundance of transglutaminase-2 (TG2)-positive cells the majority of which expressed the plasma cell marker CD138 in active CD.
Moreover, in a study of Hoydahl et al. (2019) a population of plasma cells from the intestinal biopsies of persons with CD expressed MHCII and presented immunodominant gluten peptide DQ2.5-glia-α1a, which indicates that plasma cells in the gut can function as antigen-presenting cells and might promote and maintain intestinal inflammation in persons with CD. Taavela et al. (2019) found a significantly increased number of CD138 + plasma cells per mm2 of the duodenal lamina propria in persons with CD challenged with gluten.
CD138 (syndecan-1) is a member of the syndecan family of type 1 transmembrane proteoglycans and is predominantly expressed on the basolateral surface of the intestinal epithelial cells and is essential for maintaining the normal epithelial barrier (Bode et al. 2008; Barlett et al. 2007). CD138 has been suggested as an immunohistochemical tissue marker specific for plasmacytes in humans and mice Meyerholz et al. 2022). Soluble syndecan-1 was found as a novel biomarker for small bowel mucosa damage in children with CD, which correlated with Marsh grading of bowel mucosa damage (Yablecovitch et al. 2017).
In the pathogenesis of CD an abnormal immune response to immunodominant gliadin peptide, resistant to gastrointestinal digestion, plays a putative role (Levescot et al. 2022).
Matysiak-Budnik et al. (2008) have shown that in active CD anti-gliadin secretory IgA antibodies produced by plasma cells in the lamina propria, are exported to the gut lumen. Within the lumen, IgA form large complexes with undigested gliadin peptide and transport these across the epithelium back to the lamina propria via a retrotranscytosis mechanism that is mediated by transferrin receptor (CD71). The CD71 receptor is overexpressed at the apical pole of enterocytes in persons with CD, which allows ‘protected’ retro-transport across the enterocytes toward the lamina propria and thus toward the local immune system (Matysiak-Budnik et al. 2008; Heyman M et al. 2012).
Lebreton et al. 2012,by using fluorescence resonance energy and in situ proximity ligation assays, observed physical interactions between sIgA and CD71 or CD71 and TG2 at the apical surface of enterocytes in biopsy samples and monolayers of Caco-2 cells.
It was shown that upon binding to apical CD71, sIgA (with or without gliadin peptides) enters a recycling pathway and avoids lysosomal degradation; this process allows apical–basal transcytosis of bound peptides. This mechanism is facilitated by tTG and might be involved in the pathogenesis of CD (Lebreton et al. 2012).
Gluten is responsible for the overexpression of CD71, the receptor that allows IgA gliadin-complexes to translocate from the intestinal lumen into the lamina propria in active enteropathy (Papista et al. 2012).
Under normal physiological conditions, the access of gliadin to immunocompetent cells in the lamina propria and lymphoid tissue is prevented by tight junctions (TJs) that are responsible for the limited passage of gliadin peptides across the intestinal epithelial barrier (Schneeberger and Lynch 2004). Zonulin is considered as an important physiological modulator of intercellular TJs. It was shown that gliadin induction on the release of zonulin is dependent on interaction with a surface receptor present on the brush border of enterocytes (Clemente et al. 2003). In an animal experimental study, Zhang C et al. (2023) showed a strong impact of CXCR3 inhibition on intestinal morphology and on the tight junction expression barrier in CXCR3 knockout mice. These authors suggested that CXCR3 inhibition enhanced barrier integrity, enhanced tight junction expression and allowed to elucidate the immunoregulatory mechanism of preventing intestinal dysfunction (Zhang et al. 2023).
Lammers KM et al. (2008) have identified that chemokine receptor CXCR3 serves as the target receptor for gliadin. They demonstrated that binding of gliadin to CXCR3 is crucial for the MyD88-dependet release of zonulin and subsequent increase in intestinal permeability and the paracellular gliadin passage to the gut mucosa.
Bondar et al. (2014) demonstrated a marked increase in CXCR3+ cells in the lamina propria in untreated persons with CD. These authors noted that all cell populations that contribute to the hallmarks of CD (plasma cells, T cells in the lamina propria) and intraepithelial lymphocytes express CXCR3. Bondar et al. (2014) underlined the role of CXCR3 and its ligand CXCR10 in the pathogenesis of CD and its involvement in the recruitment of cells responsible for mucosal damage in active CD and in initiation of the inflammatory process. Additionally, Bondar et al. (2014) found that CD138+ plasma cells express CXCR3. Therefore, the CXCL10/CXCR3 chemokine axis could play a relevant role in the recruitment of plasma cells into the lamina propria.
In the present study, we aimed to investigate the number of the following markers in the mucosa of the small bowel mucosa in persons with CD and in controls with normal bowel mucosa: LGR5 (leucine-rich repeat-containing 6-protein-related receptors) (small intestinal stem cell marker); CD71 (transferrin receptor); CD138/Syndecan-1 (plasma cell marker); chemokine receptor CXCR3 (target receptor for gliadin).
The aim was to find relationship between these markers and degree of small intestinal atrophy and age of the studied persons. Additionally, we intended to compare these results with our earlier data about the number of CD103 + DC, IDO + dendritic cells (DCs), FOXP3 + Tregs, enterovirus (EV) density, serum zonulin level, TJP1 mRNA expression, and tissue TG- IgA level evaluated in our previous study (Vorobjova et al. 2011; 2015; 2017).