The human MD-2 molecule is associated with the extracellular domain of human Toll-Like receptor 4 (TLR4) and greatly enhances its LPS signaling, The human TLR4-MD-2 complex thus signals the presence of LPS, Little is known, however, about cell surface expression and LPS signaling of the TLR4-MD-2 complex in vivo. We cloned mouse MD-2 molecularly and established a unique mAb MTS510, which reacted selectively with mouse TLR4-MD-2 but not with TLR4 alone in flow cytometry, Mouse MD-2 expression in TLR4-expressing cells enhanced LPS-induced NF-kappa B activation, which was clearly inhibited by MTS510. Thioglycolate-elicited peritoneal macrophages expressed TLR4-MD-2, which was rapidly down-regulated in the presence of LPS, Moreover, LPS-induced TNF-alpha production by peritoneal macrophages was inhibited by MTS510. Collectively, the TLR4-MD-2 complex is expressed on macrophages in vivo and senses and signals the presence of LPS.
