Innate and adaptive immunity in childhood celiac disease

Abstract: Celiac disease (CD) is an inflammatory small-bowel enteropathy caused by a permanent intolerance to wheat gluten and related proteins in rye and barley. Even though the disease originate from the small intestine the clinical symptoms varies in affected individuals and are often different in small children compared to adolescents and adults. Susceptibility to develop the disease is strongly associated with certain genetic factors i.e. HLADQ2/DQ8 but it is undoubtedly that additional inherited and environmental factors are involved. As specific T lymphocyte reactions are central in the pathogenesis of CD, six key cytokine messenger RNA levels in intestinal intraepithelial and lamina propria T lymphocytes (IEL, LPL), retrieved from small intestinal biopsies, were determined by using quantitative real-time reverse transcription polymerase chain reaction (RTPCR). Levels of cytokines, small secreted proteins which mediate and regulate immunity, in children with active disease were compared with that of treated children and controls. Interferon (IFN)-? and interleukin (IL)-10 were also determined at the protein level by immunohistochemistry. Active celiac disease was characterized by distortions in cytokine expression, with highly significant increases of IFN-? and IL-10 but no concomitant increases in tumor necrosis factor ? (TNF-?), transforming growth factor ?1 (TGF?1), or IL-2 and no induction of IL-4. A marked shift of IFN-? and IL-10 production from LPLs to IELs was characteristic of active celiac disease, and as many as one fourth of the IELs expressed IFN-?. IELs in treated, symptom-free celiac patients still had increased IFN-? levels compared with controls. In CD, gluten intake seems to cause an overreaction in IELs, with uncontrolled production of IFN-? and IL-10 which may cause both recruitment of more IELs and a leaky epithelium, leading to a vicious circle with amplified immune activity and establishment of the intestinal lesion. In order to determine different IEL subsets contribution of the produced cytokines, ??IELs, CD4+??IELs, and CD8+??IELs as well as CD94+CD8+??IELs and CD94CD8+??IELs of children with active CD and children with no food-intolerance were analyzed for cytokine mRNA expression levels by RT-PCR. In active CD, CD8+??IELs had the highest expression levels of IFN-?- and IL-10 mRNA and constituted the cellular source for almost all IFN-? and a large fraction of the IL-10. Expression levels of these two cytokines correlated and were higher in CD94-CD8+??IELs than CD94+CD8+??IELs CD4+??IELs had the highest expression levels of TNF-? and despite the small number of this cell subset they contributed with half of the small amounts of this cytokine. Interestingly, TNF-? levels correlated with IL-10 in CD4+??IELs. ??IELs had the lowest expression levels of IFN-?, TNF-?, IL-10, and TGF-?1. Essentially no IL-2 mRNA was detected in the three IEL subpopulations. “Classical” CD8+CD94-??T cells in the epithelial compartment are responsible for most of the excessive production of proinflammatory IFN-?. The question whether an impaired extrathymic T cell maturation and/or capacity for secondary T cell receptor (TCR) gene recombination in iIELs is a contributing factor to CD was addressed. Expression levels of recombination activating gene-1 (RAG1) and the pre T ?-chain (preT?) mRNAs were determined in IEL T cell lineage subsets of children with CD and controls. In controls, RAG1 was expressed in both mature (TCR??+ and TCR??+) and immature (CD2+CD7+TCR-) IELs while preT? was expressed preferentially in immature IELs. The RAG1 splice form selectively expressed outside thymus (RAG1 1A/2) as well as preT? were significantly decreased in CD patients both in active and inactive disease suggesting a deteriorated capacity of de novo TCR gene rearrangement in local T cell development and / or of secondary TCR gene rearrangement during editing or antigen-driven revision. This may lead to an imbalance between thymus- and gut derived T lymphocytes in the intestinal mucosa with consequent inefficient regulation of T cell responses against food antigens. Innate or nonspecific immunity is the first line, immediate defense against pathogens mediated by the epithelial cells in the intestine (IECs). As certain adaptive immune reaction in CD mimics that of intestinal infections, aberrant innate immune reaction could be a contributing factor to CD. Therefore jejunal biopsies were screened for bacteria and the innate immune status of the epithelium was investigated. Bacteria were freqently (40%) associated with the mucosa of children with active but also treated disease (20%) compared to controls (2%). Lack of antimicrobial factors such as mucins, proteins forming protective biofilm on the IECs, defensins and lysozym, peptides and enzymes with antibacterial effects, could not explain the presence of bacteria. If anything, mucin-2 (MUC2), ?-defensins, HD-5, HD-6, and lysozyme mRNA levels were increased in epithelial cells in active CD, returning to normal levels in treated CD. Their expression levels correlated to the IFN-? mRNA levels in IELs. Analysis of beta defensins, hBD-1 and hBD-2 as well as carcinoembryonic antigen (CEA) cell adhesion molecule 1a (CECAM1a), glycoproteins in the glycocalyx with ability to bind micro organisms, were not affected by the disease. Lectin staining by histochemistry revealed that goblet cells were stained by UEA1 in CD both active and treated but not in controls. The opposite pattern was seen for the lectin PNA where staining was seen in controls in the glycocalyx layer but not in CD. Thus altered glycocalyx/mucous layer may promote bacterial adhesion in CD.