Lysophosphatidic acid (LPA) and sphingosine 1-phosphate (SIP) are bioactive lipid mediators, which are known to play major roles in allergic reactions as well as in tumor pathogenesis. Here, the biological activities and signal pathways of these lysophospholipids (LPLs) in dendritic cells (DCs) were characterized further. Flow cytometric and immunoblot analyses indicate that immature as well as mature DCs express the LPL receptors S1P(1), S1P(3), SIP3, and LPA(2), but not S1P(2), S1P(4), LPA(1), or LPA(3). Moreover, enzyme-linked immunosorbent assay experiments demonstrate that simultaneous addition of these LPLs to immature DCs in the presence of lipopolysaccharide enhanced the secretion of the inflammatory cytokines interleukin (IL)-6 and IL-8 in maturing DCs. In contrast, no modification of IL-6 or IL-8 release was observed after exposure of mature DCs to LPLs alone. In addition, studies with pertussis toxin and mitogen-activated protein kinase (MAPK) kinase inhibitor PD98059 suggested that G(i) proteins and MAPK pathway are involved in these LPL-induced cell responses. Corroborating these findings, we observed that LPLs induce the phosphorylation of extracellular signal-regulated kinase 1/2 in immature DCs but not in mature DCs. Further analyses show that inhibitors of phosholipase D, Rho, and protein kinase C also inhibited the LPL-induced release of IL-6 and IL-8. Therefore, our findings suggest that lipopolysaccharide in DCs uncouples LPL receptors from the signal-transducing machinery during maturation and that exposure of LPLs at early time-points to maturing DCs modifies the proinflammatory capacity of mature DCs. J. Leukoc. Biol. 80: 287-297; 2006.