Tick saliva is thought to contain a number of molecules that prevent host immune and inflammatory responses. In this study, the effects of Ixodes scapularis saliva on cytokine production by bone marrow-derived dendritic cells (DCs) from C57BL/6 mice stimulated by TLR-2, TLR-4, and TLR-9 ligands were studied. Saliva at remarkably diluted concentrations (<1/2000) promotes a dose-dependent inhibition of IL-12 and TNF-α production induced by all TLR ligands used. Using a combination of fractionation techniques (microcon filtration, molecular sieving, and reversed-phase chromatography), we unambiguously identified PGE2 as the salivary inhibitor of IL-12 and TNF-α production by DCs. Moreover, we have found that I. scapularis saliva (dilution 1/200; ∼10 nM PGE2) marginally inhibited LPS-induced CD40, but not CD80, CD86, or MHC class II expression. In addition, saliva significantly suppressed the ability of DCs to stimulate Ag-specific CD4 + T cell proliferation and IL-2 production. Notably, the effect of saliva on DC maturation and function was reproduced by comparable concentrations of standard PGE2. These findings indicate that PGE2 accounts for most inhibition of DC function observed with saliva in vitro. The role of salivary PGE2 in vector-host interaction and host immune modulation and inflammation in vivo is also discussed. This study is the first to identify molecularly a DC inhibitor from blood-sucking arthropods.