This study investigates the role of mammalian Toll-like receptor 2 (TLR2) in signal transduction for bacterial lipopolysaccharide (LPS). LPS induces activation of the transcription factor nuclear factor κB (NF-κB) in host cells. LPS binds to the glycosylphosphatidylinositol (GPI)-anchored membrane protein CD14, which lacks an intracellular signaling domain. The study shows that overexpression of TLR2, but not TLR1, TLR4, or CD14, confers LPS inducibility of NF-κB activation in mammalian 293 cells. Mutational analysis demonstrated that this LPS response requires the intracellular domain of TLR2. LPS signaling through TLR2 was dependent on serum which contains soluble CD14 (sCD14). Coexpression of CD14 synergistically enhanced LPS signal transmission through TLR2. In addition, purified recombinant sCD14 could substitute for serum to support LPS-induced TLR2 activation. LPS stimulation of TLR2 initiated an interleukin 1 receptor–like NF-κB signaling cascade. These findings suggest that TLR2 may be a signaling component of a cellular receptor for LPS. LPS is a complex glycolipid composed of a hydrophilic polysaccharide portion and a hydrophobic domain known as lipid A. LPS elicits its responses through stimulation of host cells such as monocytes and macrophages to produce and release endogenous mediators including the proinflammatory cytokines IL-1, IL-6, and TNF. LPS-induced signal transmission is thought to entail its binding to specific cellular receptors, which trigger intracellular signaling cascades leading to activation of the transcription factor NF-κB and p38 mitogen-activated protein kinases. The 55-kd glycoprotein CD14 binds LPS with high affinity and is involved in mediating LPS responses. Binding of LPS to CD14 requires the serum factor, LPS-binding protein (LBP), which delivers LPS to CD14-expressing monocytes/macrophages. In CD14- cells such as endothelial cells, granulocytes, and lymphocytes, soluble CD14 (sCD14) found in serum is thought to functionally replace membrane-bound CD14. Since CD14 is a glycosylphosphatidylinositol (GPI)-anchored membrane protein devoid of a cytoplasmic domain, it presumably does not elicit intracellular signaling events directly. To date, the identification of a bona fide signal-transducing component of the putative LPS receptor complex has remained elusive. The toll gene controls dorsoventral pattern formation during the early embryonic development of Drosophila melanThis study investigates the role of mammalian Toll-like receptor 2 (TLR2) in signal transduction for bacterial lipopolysaccharide (LPS). LPS induces activation of the transcription factor nuclear factor κB (NF-κB) in host cells. LPS binds to the glycosylphosphatidylinositol (GPI)-anchored membrane protein CD14, which lacks an intracellular signaling domain. The study shows that overexpression of TLR2, but not TLR1, TLR4, or CD14, confers LPS inducibility of NF-κB activation in mammalian 293 cells. Mutational analysis demonstrated that this LPS response requires the intracellular domain of TLR2. LPS signaling through TLR2 was dependent on serum which contains soluble CD14 (sCD14). Coexpression of CD14 synergistically enhanced LPS signal transmission through TLR2. In addition, purified recombinant sCD14 could substitute for serum to support LPS-induced TLR2 activation. LPS stimulation of TLR2 initiated an interleukin 1 receptor–like NF-κB signaling cascade. These findings suggest that TLR2 may be a signaling component of a cellular receptor for LPS. LPS is a complex glycolipid composed of a hydrophilic polysaccharide portion and a hydrophobic domain known as lipid A. LPS elicits its responses through stimulation of host cells such as monocytes and macrophages to produce and release endogenous mediators including the proinflammatory cytokines IL-1, IL-6, and TNF. LPS-induced signal transmission is thought to entail its binding to specific cellular receptors, which trigger intracellular signaling cascades leading to activation of the transcription factor NF-κB and p38 mitogen-activated protein kinases. The 55-kd glycoprotein CD14 binds LPS with high affinity and is involved in mediating LPS responses. Binding of LPS to CD14 requires the serum factor, LPS-binding protein (LBP), which delivers LPS to CD14-expressing monocytes/macrophages. In CD14- cells such as endothelial cells, granulocytes, and lymphocytes, soluble CD14 (sCD14) found in serum is thought to functionally replace membrane-bound CD14. Since CD14 is a glycosylphosphatidylinositol (GPI)-anchored membrane protein devoid of a cytoplasmic domain, it presumably does not elicit intracellular signaling events directly. To date, the identification of a bona fide signal-transducing component of the putative LPS receptor complex has remained elusive. The toll gene controls dorsoventral pattern formation during the early embryonic development of Drosophila melan