Induction of Arginase Pathway in Human Circulating Monocytes: Does Celiac Disease Matter?

Abstract

Introduction: Celiac disease (CD) is an immune-mediated enteropathy triggered by ingested gluten in genetically susceptible individuals. Recent studies demonstrated that the activation of arginase (ARG) metabolic pathway (arginase1(ARG1), arginase2(ARG2), interleukine 1β(IL1β)) by gluten peptides in murine macrophages contributes to the modulation of intestinal permeability in vitro. The aim of the present study is: 1) to verify whether a gluten-driven induction of ARG metabolic pathway occurs also in human monocytes; 2) to ascertain whether the activation of ARG pathway by gluten corresponds to the onset of an activated phenotype in immune cells, since the enzyme is now emerging as an inflammatory marker; and 3) to evaluate whether the expression of genes belonging to this pathway could be used as serological marker of disease. Methods: All subjects were recruited at the Endoscopy Unit of the University Hospital of Parma, Italy. Blood samples (8-12ml) were collected from celiac patients on gluten free diet (CD_GFD), newly diagnosed (CD_nd) and healthy controls (HC). Circulating monocytes were isolated by gradient separation and incubated for 24h in the absence or presence of enzymatically digested gluten (PTG, peptictryptic gliadin). Afterwards, the mRNA expressions of ARG1, ARG2 and IL1β were measured by Real Time-quantitative Polymerase Chain Reaction, under basal conditions and upon treatment with PTG.Figure 1Results: Between June 2015 and April 2016, 45 patients were enrolled: 15 CD_GFD (mean age 27y), 15 CD_nd (mean age 18y), and 15 HC (mean age 32y). In the presence of PTG, a massive induction of ARG1 and ARG2 and an increase of IL1β mRNA expression were observed, indicating the inflammatory effect of the treatment. No significant difference was detected among cells isolated from HS, CD_nd or CD_GFD. Conclusion: Treatment with PTG activates ARG pathway in human monocytes as observed in rodent cells and increases IL1β mRNA, confirming the onset of an inflammatory phenotype. However, CD monocytes cannot be considered more prone to gluten-triggered inflammation than HS, being the induction of inflammatory markers comparable in all group samples. Therefore, neither ARG nor IL1β in circulating monocytes appear good candidates for the diagnosis of CD. Monocytes contribution to the chronic inflammation of CD intestine may depend on the possibility for immunity cells to interact with peptides, as the epithelial barrier is compromised in CD, but not in HS.