It has demonstrated that the recognition of triacylated lipopeptides by Toll-like receptor (TLR) 2 requires TLR1 as a coreceptor. In the NF-kappaB reporter assay system in which human embryonic kidney 293 cells were transfected with TLR2 and TLR1 together with an NF-kappaB luciferase reporter gene, S-(2,3-bispalmitoyloxypropyl)-N-palmitoyl-Cys-Lys-Lys-Lys-Lys (Pam(3)CSK(4)) and Pam(3)CSSNA were recognized by TLR2/TLR1, but the recognition level was unexpectedly very low. However, cotransfection of CD14 drastically enhanced the recognition of triacylated lipopeptides by TLR2/TLR1. The CD14-induced enhancement did not occur without cotransfection of TLR1. Both CD14(dS39-A48), a mutant with deletion of the part of possible N-terminal ligand-binding pocket, and anti-CD14 monoclonal antibody reduced the CD14-induced enhancement. Transfection of a TIR domain-deficient mutant of TLR2 (TLR2(dE772-S784)) or TLR1 (TLR1(dQ636-K779)) completely abrogated the CD14-induced enhancement. Soluble recombinant CD14 added extracellularly enhanced the recognition of Pam(3)CSSNA by TLR2/TLR1. Immunoprecipitation analysis demonstrated that CD14 was not associated with TLR2 but that TLR1 was associated with TLR2. In addition, surface plasmon resonance-based assay demonstrated that CD14 binds to Pam(3)CSK(4) at a dissociation constant of 5.7 microM. This study suggests that CD14 directly binds to triacylated lipopeptides and facilitates recognition of the lipopeptides by the TLR2/TLR1 complex without binding to the receptor complex.