Celiac disease (CD)-associated inflammation is characterized by high interleukin- 21 (IL-21), but the mechanisms that control IL-21 production are not fully understood. Here we analyzed IL-21 cell sources and examined how IL-21 production is regulated in CD. Intraepithelial lymphocytes (IELs) and lamina propria lymphocytes (LPLs), isolated from CD patients and non-CD controls, were analyzed for cell markers, cytokines, and transcription factors by flow cytometry. IL-21 was highly produced by CD4+ and CD4+/CD8+ IELs and LPLs in active CD. IL-21-producing cells coexpressed interferon-γ (IFN-γ) and to a lesser extent T helper type 17 (Th17) cytokines. Treatment of control LPLs with IL-15, a cytokine overproduced in CD, activated Akt and STAT3 (signal transducer and activator of transcription 3), thus enhancing IL-21 synthesis. Active CD biopsies contained elevated levels of Akt, and blockade of IL-15 in those samples reduced IL-21. Similarly, neutralization of IL-15 in biopsies of inactive CD patients inhibited peptic-tryptic digest of gliadin-induced IL-21 expression. These findings indicate that in CD, IL-15 positively regulates IL-21 production.

Helicobacter pylori (Hp) infection is associated with gastric inflammation and ulceration. The pathways of tissue damage in Hp-infected subjects are complex, but evidence indicates that T cell-derived cytokines enhance the synthesis of matrix metalloproteinases (MMP) that contribute to mucosal ulceration and epithelial damage. In this study, we have examined the role of the T cell cytokine IL-21 in Hp-infected gastric mucosa and evaluated whether IL-21 regulates MMP production by gastric epithelial cells. We show that IL-21 is constitutively expressed in gastric mucosa and is more abundant in biopsy specimens and purified mucosal CD3(+) T cells from Hp-infected patients compared with normal patients and disease controls. We also demonstrate that IL-21R is expressed by primary gastric epithelial cells, as well as by the gastric epithelial cell lines AGS and MKN28. Consistently, AGS cells respond to IL-21 by increasing production of MMP-2 and MMP-9, but not MMP-1, MMP-3, MMP-7, or tissue inhibitors of MMP. Analysis of signaling pathways leading to MMP production reveals that IL-21 enhances NF-kappaB but not MAPK activation, and inhibition of NF-kappaB activation reduces IL-21-induced MMP-2 and MMP-9 production. Finally, we show that treatment of Hp-infected gastric explants with anti-IL-21 reduces epithelial cell-derived MMP-2 and MMP-9 production. These data indicate that IL-21 is overexpressed in Hp-infected gastric mucosa where it could contribute to increased epithelial gelatinase production.

T-helper (Th)1 cells play a central role in the pathogenesis of tissue damage in Crohn's disease (CD). Interleukin (IL)-12/STAT4 signaling promotes Th1 cell commitment in CD, but other cytokines are needed to maintain activated Th1 cells in the mucosa. In this study, we examined the expression and role of IL-21, a T-cell-derived cytokine of the IL-2 family; in tissues and cells isolated from patients with inflammatory bowel disease.
IL-21 was examined by Western blotting in whole mucosa and lamina propria mononuclear cells (LPMCs) from patients with CD, ulcerative colitis (UC), and controls. We also examined the effects of exogenous IL-12 on IL-21 production, as well as the effects of blocking IL-21 with an IL-21-receptor Ig fusion protein. Interferon (IFN)-gamma was measured in the culture supernatants by enzyme-linked immunosorbent assay, and phosphorylated STAT4 and T-bet were examined by Western blotting.
IL-21 was detected in all samples, but its expression was higher at the site of disease in CD in comparison with UC and controls. Enhanced IL-21 was seen in both ileal and colonic CD and in fibrostenosing and nonfibrostenosing disease. IL-12 enhanced IL-21 in normal lamina propria lymphocytes through an IFN-gamma-independent mechanism, and blocking IL-12 in CD LPMCs decreased anti-CD3-stimulated IL-21 expression. Neutralization of IL-21 in CD LPMC cultures decreased phosphorylated STAT4 and T-bet expression, thereby inhibiting IFN-gamma production.
Our data suggest that IL-21 contributes to the ongoing Th1 mucosal response in CD.