Downregulation of Interleukin- (IL-) 17 through Enhanced Indoleamine 2,3-Dioxygenase (IDO) Induction by Curcumin: A Potential Mechanism of Tolerance towards Helicobacter pylori

Abstract

The anti-inflammatory and antimicrobial properties of curcumin suggest its use as an anti-Helicobacter pylori (H. pylori) agent, but mechanisms underlying its helpful activity are still not clear. Indoleamine 2,3-dioxygenase (IDO) promotes the effector T cell apoptosis by catalyzing the rate-limiting first step in tryptophan catabolism, and its high expression in H. pylori-infected human gastric mucosa attenuates Th1 and Th17 immune response. The aim of this study was to investigate the role of curcumin in modulating the expression of IL-17 and IDO in H. pylori-infected human gastric mucosa. In an organ culture chamber, gastric biopsies from 35 patients were treated with and without 200 μM curcumin. In H. pylori-infected patients (n = 21), IL-17 was significantly lower, both in gastric biopsies (p = 0.0003) and culture supernatant (p = 0.0001) while IDO significantly increased (p < 0.00001) in curcumin-treated sample compared with untreated samples. In a subgroup of H. pylori-infected patients (n = 15), samples treated with curcumin in addition to IDO inhibitor 1-methyl-L-tryptophan (1-MT) showed a higher expression of IL-17 compared with untreated samples and curcumin-treated alone (p < 0.00001). Curcumin downregulates IL-17 production through the induction of IDO in H. pylori-infected human gastric mucosa, suggesting its role in dampening H. pylori-induced immune-mediated inflammatory changes.

Figure 1

Levels of IL-17 by Western blotting in total protein extracts from gastric biopsies of H. pylori-infected patients (n = 21) treated with medium (untreated samples) or 200 μM curcumin for 20 hours. The loading control was represented by β-actin. Values are expressed as mean values ± SD of arbitrary units (a.u.). A Western blot panel of one representative experiment is shown (A is the untreated sample and B is the curcumin-treated sample, for each patient). Statistical analysis was performed using the Mann-Whitney U test. p values of 0.05 or less were regarded as statistically significant.

Figure 2

Levels of IL-17 by ELISA in supernatant from gastric biopsies of H. pylori-infected patients (n = 21) treated with medium (untreated samples) or 200 μM curcumin for 20 hours. Values are given in pg/mL as scattered plots with mean values. Statistical analysis was performed using the Mann-Whitney U test. p values of 0.05 or less were regarded as statistically significant.

Figure 3

Expression of indoleamine 2,3-dioxygenase (IDO) by Western blotting in total protein extracts from gastric biopsies of H. pylori-infected patients (n = 21) treated with medium (untreated samples) or 200 μM curcumin for 20 hours. The loading control was represented by β-actin. Values are expressed as mean values ± SD of arbitrary units (a.u.). A Western blot panel of one representative experiment is shown (A is the untreated sample and B is the curcumin-treated sample, for each patient). Statistical analysis was performed using the Mann-Whitney U test. p values of 0.05 or less were regarded as statistically significant.

Figure 4

Levels of IL-17 by Western blotting in total protein extracts from gastric biopsies of the subgroup of H. pylori-infected patients (n = 15) treated with medium (untreated samples) or 200 μM curcumin or 200 μM curcumin plus the indoleamine 2,3-dioxygenase (IDO) inhibitor, 1-methyl-L-tryptophane (1-MT), at a concentration of 200 μM for 20 hours. The loading control was represented by β-actin. Values are expressed as mean values ± SD of arbitrary units (a.u.). A Western blot panel of one representative experiment is shown (A is the untreated sample, B is the curcumin-treated sample, and C is the curcumin plus 1-MT-treated sample, for each patient). Statistical analysis was performed using one-way analysis of variance (ANOVA) with the Tukey multiple comparison test. p values of 0.05 or less were regarded as statistically significant.