Coeliac disease (CD) is a chronic immune-mediated inflammation of the small intestine caused by a permanent state of intolerance to ingested gluten proteins affecting genetically susceptible individuals. Its hallmarks, lymphocytic infiltration of the lamina propria, expansion of the intraepithelial lymphocyte (IEL) population, hyperplasia of the crypts and atrophy of the villi are mediated by the interplay between an innate and adaptive immune response against gluten . The complex of deamidated gliadin peptide interacting with the HLA-DQ2 and/or –DQ8 heterodimers on antigen presenting cells, is capable of activating the lamina propria T-helper lymphocytes, thereby initiating antibody production and a gluten-specific pro-inflammatory type 1 T-cell response (Th1) . Th1 cells play an important role in pathogenesis of CD by secreting interleukin-2 (IL-2) that induces proliferation of T-lymphocytes and, in particular, by secreting the pro-inflammatory cytokine interferon-gamma (IFN-γ) . Furthermore, recent evidence has indicated that Th17 cells play a pathogenic role in CD [3–5]. In addition, interleukin-15 (IL-15) activates a cytotoxic response of intra-epithelial lymphocytes through IFN-y release and upregulation of NKG2D. In combination with epithelial stress, the NKG2D ligand MHC Class I chain-related A (MICA) is upregulated by enterocytes [6, 7]. The interaction of MICA and NKG2D induces enterocyte destruction by IELs.
A gluten-free diet (GFD) results in mucosal recovery in the majority of patients, who are referred to as uncomplicated CD-patients. However, a small subset of adult onset CD patients fails to regain intestinal homeostasis after elimination of dietary gluten, or symptoms recur after initial response . After careful evaluation of dietary compliance and exclusion of other possible disease-entities causing villous atrophy, these patients are diagnosed to suffer from refractory coeliac disease (RCD) . RCD is considered a complicated form of CD, and is divided into type I (RCDI), when patients lack an aberrant IEL population, or type II (RCDII) in which a substantial (>20%) aberrant IEL population is found in the small intestinal mucosa [10, 11]. An aggressive type of lymphoma which carries a dismal prognosis, the enteropathy-associated T-cell lymphoma (EATL), is thought to arise from this aberrant IEL population. An interesting observation is that both aberrant IELs as well as EATL cells display a cytotoxic phenotype and contain high levels of granzyme-B [12, 13], which could therefore serve as a marker for complicated CD.
In consequence of a good response to immunosuppressive therapy, RCDI patients have a better prognosis than RCDII patients [14–17]. Therefore, early identification of CD patients developing RCDII and/or EATL enables early intervention, which will likely reduce morbidity and mortality.
Currently, antibodies against tissue transglutaminase (TGA), anti-endomysium (EMA) and deamidated gliadin peptides (DGPA) provide valuable and generally accepted serum parameters for the diagnosis and follow-up of uncomplicated CD. However, these antibodies are of no use in predicting and monitoring both types of RCD and EATL, implying that histological and flow cytometric analysis of duodenal biopsies are still required to distinguish between the uncomplicated and complicated forms of CD. Additional serum markers could potentially provide us with a minimal-invasive, easy applicable test without the need to perform a gastro-duodenal endoscopy. In addition, immunological markers in the peripheral blood could provide more insight in the similarities and differences of the pathophysiology underlying the CD spectrum.
Therefore, in the present study we evaluated several immunological and biochemical parameters in sera from the five stages of CD, including active CD (ACD), CD on GFD, RCDI-II and EATL, for their ability to differentiate between complicated and uncomplicated forms, and secondly, to gain insight in the pathophysiological relations between these disease entities. For this purpose, we analysed serum levels of the inflammatory cytokines IL-6, IL-8, IL-17and IL-22, the T-cell activation factors soluble (s)CD25 (IL2R-alpha) and sCD27, the T-cell dysregulation factor sCTLA-4, shown previously to be up-regulated in different autoimmune diseases, the cytotoxic T-cell parameter granzyme-B, and sMICA, previously shown to be associated with the presence of epithelial stress and malignancies.