Reduced Hedgehog Signal Transduction Exacerbates the TH1 Response in Helicobacter felis-Infected Mice

Abstract

Background: Helicobacter pylori-induced immune responses are skewed toward the T helper (Th) 1 phenotype, which is indicated by the predominant production of interferon (IFN)γ and tumor necrosis factor (TNF)-α. Toll-like receptors (TLRs) play an essential role in the defense against microbes through the recognition of bacterial molecules. Among TLRs, TLR9 identifies unmethylated CpG DNA sites, which are abundant in bacterial and viral DNA. Although TLR9 signaling exerts pro-inflammatory actions through Th1 responses, the activation of an anti-inflammatory program has been reported in several models of experimental colitis. Aim: To investigate the expression and role of TLR9 in H. pylori-induced gastritis in mice. Methods: Wild-type and TLR9-/mice were inoculated with H. pylori (SS 1 strain). After 2, 4, and 6 months of inoculation of H. pylori, gastric tissues were subjected to activity measurement of myeloperoxidase, a marker of neutrophil infiltration; detection of cytokine mRNA levels by real-time reverse transcriptase-polymerase chain reaction (RTPCR); and immunohistochemical analysis. Results: TLR9 mRNA expression was increased by 6.0-fold, 10.1-fold, and 23.4-fold, respectively, after 2, 4, and 6 months of H. pylori infection. After 2 months, mRNA levels for IFN-γ and TNF-α (Th1-type cytokines) in TLR9-/mice were significantly elevated by 6.7-fold and 5.3-fold, respectively, with a 2.7-fold increase in myeloperoxidase activity, compared to those in H. pylori-infected wild-type mice. The expression of Th1-type cytokines and myeloperoxidase activity in TLR9-/mice were also increased 4 months after H. pylori infection, but these inflammatory parameters were similar 6 months after H. pylori infection in the wild-type and TLR9-/mice. IL-4 and IL-5 (Th2type cytokines) mRNA expression levels did not differ between wild-type and TLR9-/mice throughout the experiment. Immunohistochemical analysis showed that TLR9 proteins were mainly expressed on inflammatory cells. Conclusion: These findings suggest that TLR9 signaling may play important roles in the inhibition of H. pylori-induced gastritis in the early phase through the suppression of Th1 differentiation.