Helicobacter pylori colonizes the stomach and induces strong, specific local and systemic humoral and cell-mediated immunity, resulting in the development of chronic gastritis in humans. Although H. pyloriinduced chronic atrophic gastritis is characterized by marked infiltration of T helper type 1 (Th1) cytokineproducing CD4؉ T cells, almost all of the inflamed gastric mucosae also contain focal lymphoid aggregates with germinal centers. In addition, typical H. pylori-induced chronic gastritis in children, called follicular gastritis, is characterized by B-cell follicle formation in the gastric mucosa. The aim of this study was to examine whether thymic stromal lymphopoietin (TSLP), an epithelial-cell-derived cytokine inducing a dendritic cell (DC)-mediated inflammatory Th2 response, is involved in Th2 responses triggering B-cell activation in H. pylori-induced gastritis. Here, we show that H. pylori triggered human gastric epithelial cells to produce TSLP, together with the DC-attracting chemokine MIP-3␣ and the B-cell-activating factor BAFF. After DCs were incubated with supernatants from H. pylori-infected epithelial cells, the conditioned cells expressed high levels of costimulatory molecules, such as CD80, and triggered naïve CD4؉ T cells to produce high levels of the Th2 cytokines interleukin-4 and interleukin-13 and of the inflammatory cytokines tumor necrosis factor alpha and gamma interferon. In contrast, after incubation of the supernatants with the neutralizing antibodies to TSLP, the conditioned DCs did not prime T cells to produce high levels of Th2 cytokines. These results, together with the finding that TSLP was expressed by the epithelial cells of human follicular gastritis, suggest that H. pylori can directly trigger epithelial cells to produce TSLP. It also suggests that TSLP-mediated DC activation may be involved in Th2 responses triggering B-cell activation in H. pylori-induced gastritis.Helicobacter pylori (H. pylori) infection in the stomach induces chronic gastritis associated with the development of peptic ulcer diseases, gastric adenocarcinoma, and mucosa-associated lymphoid tissue (MALT) lymphoma in humans (3,8,35). Although H. pylori-induced chronic atrophic gastritis is characterized by marked infiltration of T helper type 1 (Th1) cytokine-producing CD4 ϩ T cells (3,8,35), almost all of the inflamed gastric mucosae also contain focal lymphoid aggregates with germinal centers (5, 11). In addition, typical H. pylori-induced chronic gastritis in children, called follicular gastritis, is characterized by B-cell follicle formation in the gastric mucosa (10,11,34). Th2 responses triggering B-cell activation appear to be involved in the development of lymphoid aggregates with germinal centers. However, molecular mechanisms to induce Th2 responses triggering B-cell activation are not clear.In humans, an epithelial-cell-derived cytokine, thymic stromal lymphopoietin (TSLP), activates CD11c ϩ myeloid dendritic cells (DCs), and activated DCs strongly upregulate the expression of costimulatory mo...