ABSTRACT
BACKGROUND: Celiac disease (CD) is a frequent inflammatory intestinal disease, with a genetic background, caused by gliadin-containing food. Some gliadin peptides are not digested by intestinal proteases and can have different biological effects. Gliadin peptides can induce innate and adaptive T cell-mediated immune responses. The major mediator of the stress and innate immune response to gliadin peptides (i.e., peptides 31-43 and 31-55) is the cytokine interleukin-15 (IL-15). Other peptides such as the 33 mer containing the P57-68 sequence, after tissue transglutaminase deamidation, are well presented to T cell in the intestine and can induce an adaptive immune response. FINDINGS: In this paper, we review the recent studies on the digestion of gliadin and the peptides released by the digestion process. We will also discuss the mechanisms responsible for the internalization and transcytosis of indigested gliadin peptides in the intestinal epithelium. CONCLUSIONS: Gliadin is not completely digested by the intestinal proteases producing bioactive peptides that have different biological effects. These peptides are internalized in the cells by an active process of endocytosis and can traverse the intestinal mucosa with different kinetics and immunological effects. In vivo findings will also be discussed.
ABSTRACT
BACKGROUND AND OBJECTIVES: Damage to intestinal mucosa in celiac disease (CD) is mediated both by inflammation due to adaptive and innate immune responses, with IL-15 as a major mediator of the innate immune response, and by proliferation of crypt enterocytes as an early alteration of CD mucosa causing crypts hyperplasia. We have previously shown that gliadin peptide P31-43 induces proliferation of cell lines and celiac enterocytes by delaying degradation of the active epidermal growth factor receptor (EGFR) due to delayed maturation of endocytic vesicles. IL-15 is increased in the intestine of patients affected by CD and has pleiotropic activity that ultimately results in immunoregulatory cross-talk between cells belonging to the innate and adaptive branches of the immune response. Aims of this study were to investigate the role of P31-43 in the induction of cellular proliferation and innate immune activation. METHODS/PRINCIPAL FINDINGS: Cell proliferation was evaluated by bromodeoxyuridine (BrdU) incorporation both in CaCo-2 cells and in biopsies from active CD cases and controls. We used real-time PCR to evaluate IL-15 mRNA levels and FACS as well as ELISA and Western Blot (WB) analysis to measure protein levels and distribution in CaCo-2 cells. Gliadin and P31-43 induce a proliferation of both CaCo-2 cells and CD crypt enterocytes that is dependent on both EGFR and IL-15 activity. In CaCo-2 cells, P31-43 increased IL-15 levels on the cell surface by altering intracellular trafficking. The increased IL-15 protein was bound to IL15 receptor (IL-15R) alpha, did not require new protein synthesis and functioned as a growth factor. CONCLUSION: In this study, we have shown that P31-43 induces both increase of the trans-presented IL-15/IL5R alpha complex on cell surfaces by altering the trafficking of the vesicular compartments as well as proliferation of crypt enterocytes with consequent remodelling of CD mucosa due to a cooperation of IL-15 and EGFR.
