Production of IL-12 the Maturation of Human Dendritic Cells and Extracellular Acidosis Triggers the

Although the development of an acidic tissue environment or acidosis is a hallmark of inflammatory processes, few studies analyze the effect of extracellular pH on immune cells. We have previously shown that exposure of murine dendritic cells (DCs) to pH 6.5 stimulates macropinocytosis and cross-presentation of extracellular Ags by MHC class I molecules. We report that the transient exposure of human DCs to pH 6.5 markedly increases the expression of HLA-DR, CD40, CD80, CD86, CD83, and CCR7 and improves the T cell priming ability of DCs. Incubation of DCs at pH 6.5 results in the activation of the PI3K/Akt and the MAPK pathways. Using specific inhibitors, we show that the maturation of DCs induced by acidosis was strictly dependent on the activation of p38 MAPK. DC exposure to pH 6.5 also induces a dramatic increase in their production of IL-12, stimulating the synthesis of IFN- (cid:1) , but not IL-4, by Ag-specific CD4 (cid:2) T cells. Interestingly, we find that suboptimal doses of LPS abrogated the ability of pH 6.5 to induce DC maturation, suggesting a cross-talk between the activation pathways triggered by LPS and extracellular protons in DCs. We conclude that extracellular acidosis in peripheral tissues may contribute to the initiation of adaptive immune responses by DCs, favoring the development of Th1 immunity. The Journal of Immunology, 2007, 179: 1950–1959. staining. a ex- fluorescence (Sigma-Aldrich), 5% bovine albumin Tween blotted with Abs against phospho-Akt, phospho-ERK, phospho-p38, or phospho-JNK, fol- lowed by HRP-conjugated anti-mouse or anti-rabbit IgG. Specific bands were developed by ECL (Amersham Biosciences). were stripped and reproved with a murine mAb against human (cid:2) -actin (Cell Signaling Our results indicate that the activation of p38 MAPK plays a critical role in the phenotypic maturation of DCs and the production of IL-12 triggered by pH 6.5. In fact, the specific blocker of p38 MAPK SB202190 completely prevented the up-regulation of CD80, CD86, CD40, HLA-DR, CD83, and CCR7 induced by acidosis, whereas the production of IL-12 was inhibited by 50%. Previous studies have shown that p38 MAPK plays a major role in the maturation of DCs induced by different stimuli including LPS In agreement with these we observed that SB202190 inhibited the production of IL-12 as well as the up-regulation of CD86, CD80, CD83, and CCR7 in LPS-treated DCs. We demonstrate in this study that transient exposure to acidic conditions triggers the maturation of DCs and the production of IL-12. These observations may have important implications to our understanding of the mechanisms leading to the activation of DCs during the course of infectious processes.