ABSTRACT
Toll-like receptor 4 (TLR4) is the signal-transducing component of the LPS recognition complex and is essential for LPS-induced septic shock. Here we demonstrate that TLR1 has the capacity to abrogate TLR4 signaling. Human microvascular endothelial cells express TLR4 but not TLR1 and respond to LPS through TLR4. The ability of these cells to respond to LPS was lost, however, when they were transfected with TLR1. Inhibition was specific for TLR1 because TL5 failed to block TLR4 function. Moreover, TLR1 had no effect upon TNF-alpha signaling, indicating that TLR1 operated at a step upstream of the convergence between the two pathways. Inhibition of TLR4 signaling was mediated by the extracellular, but not cytoplasmic domain of TLR1. In addition, TLR1 physically associated with TLR4 in co-precipitation experiments. These findings suggest that TLR1 might restrain potentially dangerous innate response to LPS by binding to TLR4 and preventing the formation of active signaling complexes.
Subject(s)
Drosophila Proteins , Endothelium, Vascular/physiology , Membrane Glycoproteins/antagonists & inhibitors , Membrane Glycoproteins/physiology , Receptors, Cell Surface/antagonists & inhibitors , Receptors, Cell Surface/physiology , Signal Transduction/physiology , Cell Line , Cells, Cultured/drug effects , Cells, Cultured/metabolism , Endothelium, Vascular/cytology , Gene Expression Regulation , Humans , Lipopolysaccharides/pharmacology , Macromolecular Substances , Membrane Glycoproteins/chemistry , Membrane Glycoproteins/genetics , NF-kappa B/metabolism , Peptide Fragments/genetics , Peptide Fragments/physiology , Protein Interaction Mapping , Protein Structure, Tertiary , Receptors, Cell Surface/chemistry , Receptors, Cell Surface/genetics , Recombinant Fusion Proteins/physiology , Toll-Like Receptor 1 , Toll-Like Receptor 4 , Toll-Like Receptors , Transfection , Tumor Necrosis Factor-alpha/physiologyABSTRACT
CD11b(+)Gr-1(+) myeloid suppressor cells (MSC) accumulate in lymphoid organs under conditions of intense immune stress where they inhibit T and B cell function. We recently described the generation of immortalized MSC lines that provide a homogeneous source of suppressor cells for dissecting the mechanism of suppression. In this study we show that the MSC lines potently block in vitro proliferation of T cells stimulated with either mitogen or antigenic peptide, with as few as 3% of MSC cells causing complete suppression. Inhibition of mitogenic and peptide-specific responses is not associated with a loss in IL-2 production or inability to up-modulate the early activation markers, CD69 and CD25, but results in direct impairment of the three IL-2R signaling pathways, as demonstrated by the lack of Janus kinase 3, STAT5, extracellular signal-regulated kinase, and Akt phosphorylation in response to IL-2. Suppression is mediated by and requires NO, which is secreted by MSC in response to signals from activated T cells, including IFN-gamma and a contact-dependent stimulus. Experiments with inducible NO synthase knockout mice demonstrated that the inhibition of T cell proliferation by CD11b(+)Gr-1(+) cells in the spleens of immunosuppressed mice is also dependent upon NO, indicating that the MSC lines accurately represent their normal counterparts. The distinctive capacity of MSC to generate suppressive signals when encountering activated T cells defines a specialized subset of myeloid cells that most likely serve a regulatory function during times of heightened immune activity.
