Enhanced IL-10 Production by TLR4- and TLR2-Primed Dendritic Cells upon TLR Restimulation

LPS tolerance has been investigated extensively in monocytes/macrophages. However, the LPS restimulation studies are not well documented in dendritic cells (DCs). In the present study, we investigated influences of TLR restimulation using murine bone marrow-derived DCs. Purified bone marrow-derived DCs (>98% CD11c+ B220-) were stimulated with TLR4 and TLR2 ligands for 24 h and then cultured with medium alone for 48 h as a resting interval (TLR4,2-primed DCs). The TLR4-MD2 expression was markedly reduced immediately after the TLR stimulation, but was restored following the resting interval. The TLR4,2-primed DCs exhibited significantly enhanced IL-10 production, but markedly diminished IL-12p40 production upon TLR4 restimulation compared with naive (unprimed) DCs. TLR4-mediated activation of p38 MAPK was markedly suppressed, whereas that of ERK1/2 was enhanced in the TLR4,2-primed DCs compared with naive DCs. Blocking the activation of ERK1/2 with U0126 reduced the enhanced IL-10 production by the TLR4,2-primed DCs upon the TLR4 restimulation. The U0126 showed no significant effects on the IL-12p40 production. Thus, the enhanced ERK1/2 activation appears to be, at least in part, responsible for the enhanced IL-10 production in the TLR4,2-primed DCs. In addition, TNFR-associated factor 3 expression was significantly up-regulated in the TLR4,2-primed DCs compared with that in naive DCs. We demonstrated in this study that DCs primed with TLR4 and TLR2 ligands and rested for 48 h showed enhanced IL-10 production upon TLR4 restimulation. The enhanced IL-10 production by the TLR4,2-primed DCs may be attributed to the altered balance of intracellular signaling pathways via p38 MAPK, ERK1/2, and TNFR-associated factor 3 upon TLR restimulation.