Toll-like receptor 4 (TLR4) is a mammalian homologue of Drosophila Toll, a leucine-rich repeat molecule that can trigger innate responses against pathogens. The TLR4 gene has recently been shown to be mutated in C3H/HeJ and C57BL/10ScCr mice, both of which are low responders to lipopolysaccharide (LPS). TLR4 may be a long-sought receptor for LPS. However, transfection of TLR4 does not confer LPS responsiveness on a recipient cell line, suggesting a requirement for an additional molecule. Here, we report that a novel molecule, MD-2, is requisite for LPS signaling of TLR4. MD-2 is physically associated with TLR4 on the cell surface and confers responsiveness to LPS. MD-2 is thus a link between TLR4 and LPS signaling. Identification of this new receptor complex has potential implications for understanding host defense, as well as pathophysiologic, mechanisms.

Toll-like receptors (TLRs) are best known for their ability to recognize microbial or viral components and initiate innate immune responses. We showed here that TLRs and their coreceptors were expressed by multipotential hematopoietic stem cells, whose cell cycle entry was triggered by TLR ligation. TLR expression also extended to some of the early hematopoietic progenitors, although not the progenitor cells dedicated to megakaryocyte and erythroid differentiation. TLR signaling via the Myd88 adaptor protein drove differentiation of myeloid progenitors, bypassing some normal growth and differentiation requirements, and also drove lymphoid progenitors to become dendritic cells. CD14 contributed to the efficiency of lipopolysaccharide (LPS) recognition by stem and progenitor cells, and LPS interacted directly with the TLR4/MD-2 complex on these cells in bone marrow. Thus, the preferential pathogen-mediated stimulation of myeloid differentiation pathways may provide a means for rapid replenishment of the innate immune system during infection.

Toll-like receptor 4 (TLR4) mediates lipopolysaccharide (LPS) signaling in a variety of cell types. MD-2 is associated with the extracellular domain of TLR4 and augments TLR4-dependent LPS responses in vitro. We show here that MD-2(-/-) mice do not respond to LPS, do survive endotoxic shock but are susceptible to Salmonella typhimurium infection. We found that in MD-2(-/-) embryonic fibroblasts, TLR4 was not able to reach the plasma membrane and predominantly resided in the Golgi apparatus, whereas TLR4 was distributed at the leading edge surface of cells in wild-type embryonic fibroblasts. Thus, MD-2 is essential for correct intracellular distribution and LPS-recognition of TLR4.

OBJECTIVETo characterize the phenotypic changes of adipose tissue macrophages (ATMs) under different conditions of insulin sensitivity.RESEARCH DESIGN AND METHODSThe number and the expressions of marker genes for M1 and M2 macrophages from mouse epididymal fat tissue were analyzed using flow cytometry after the mice had been subjected to a high-fat diet (HFD) and pioglitazone treatment.RESULTSMost of the CD11c-positive M1 macrophages and the CD206-positive M2 macrophages in the epididymal fat tissue were clearly separated using flow cytometry. The M1 and M2 macrophages exhibited completely different gene expression patterns. Not only the numbers of M1 ATMs and the expression of M1 marker genes, such as tumor necrosis factor-α and monocyte chemoattractant protein-1, but also the M1-to-M2 ratio were increased by an HFD and decreased by subsequent pioglitazone treatment, suggesting the correlation with whole-body insulin sensitivity. We also found that the increased number of M2 ATMs after an HFD was associated with the upregulated expression of interleukin (IL)-10, an anti-inflammatory Th2 cytokine, in the adipocyte fraction as well as in adipose tissue. The systemic overexpression of IL-10 by an adenovirus vector increased the expression of M2 markers in adipose tissue.CONCLUSIONSM1 and M2 ATMs constitute different subsets of macrophages. Insulin resistance is associated with both the number of M1 macrophages and the M1-to-M2 ratio. The increased expression of IL-10 after an HFD might be involved in the increased recruitment of M2 macrophages.

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Lung cancer is one of the leading causes of the cancer death worldwide. Gefitinib is an inhibitor of the tyrosine kinase activity of the epidermal growth factor receptor (EGFR) and has been introduced in the treatment of advanced lung cancers. The responsiveness to gefitinib has been linked to the presence of EGFR mutations. Clinical samples contain many normal cells in addition to cancer cells. A method capable of detecting EGFR mutations in a large background of wild-type EGFR genes could provide a superior clinical test. We developed a rapid and sensitive detection system for EGFR mutations named the peptide nucleic acid-locked nucleic acid (PNA-LNA) PCR clamp that can detect EGFR mutations in the presence of 100-to 1,000-fold background of wild-type EGFR. We used this method to screen 30 non-small cell lung cancer cell lines established from Japanese patients. In addition to 11 cell lines that have mutations, we found 12 cell lines in which specific mutations are observed only in the subpopulation(s) of the cells. Genetic heterogeneity of EGFR suggests that the EGFR gene is unstable in established cancers and the heterogeneity may explain variable clinical responses of lung cancers to gefitinib. (Cancer Res 2005; 65(16): 7276-82)

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-kappaB 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.

Toll-like receptors (TLRs) are innate recognition molecules for microbial products, but their direct interactions with corresponding ligands remain unclarified. LPS, a membrane constituent of gram-negative bacteria, is the best-studied TLR ligand and is recognized by TLR4 and MD-2, a molecule associated with the extracellular domain of TLR4. Although TLR4-MD-2 recognizes LPS, little is known about the physical interaction between LPS and TLR4-MD-2. Here, we demonstrate cell surface LPS–TLR4-MD-2 complexes. CD14 greatly enhances the formation of LPS–TLR4-MD-2 complexes, but is not coprecipitated with LPS–TLR4-MD-2 complexes, suggesting a role for CD14 in LPS loading onto TLR4-MD-2 but not in the interaction itself between LPS and TLR4-MD-2. A tentative dissociation constant (Kd) for LPS–TLR4-MD-2 complexes was ∼3 nM, which is ∼10–20 times lower than the reported Kd for LPS–MD-2 or LPS–CD14. The presence of detergent disrupts LPS interaction with CD14 but not with TLR4-MD-2. E5531, a lipid A antagonist developed for therapeutic intervention of endotoxin shock, blocks LPS interaction with TLR4-MD-2 at a concentration 100 times lower than that required for blocking LPS interaction with CD14. These results reveal direct LPS interaction with cell surface TLR4-MD-2 that is distinct from that with MD-2 or CD14.