Pathways involved in the synergistic activation of macrophages by lipoteichoic acid and hemoglobin.
Lipoteichoic acid (LTA) is a Gram-positive cell surface molecule that is found in both a cell-bound form and cell-free form in the host during an infection. Hemoglobin (Hb) can synergize with LTA, a TLR2 ligand, to potently activate macrophage innate immune responses in a TLR2- and TLR4-dependent way. At low levels of LTA, the presence of Hb can result in a 200-fold increase in the secretion of IL-6 following macrophage activation. Six hours after activation, the macrophage genes that are most highly up-regulated by LTA plus Hb activation compared to LTA alone are cytokines, chemokines, receptors and interferon-regulated genes. Several of these genes exhibit a unique TLR4-dependent increase in mRNA levels that continued to rise more than eight hours after stimulation. This prolonged increase in mRNA levels could be the result of an extended period of NF-κB nuclear localization and the concurrent absence of the NF-κB inhibitor, IκBα, after stimulation with LTA plus Hb. Dynasore inhibition experiments indicate that an endocytosis-dependent pathway is required for the TLR4-dependent up-regulation of IL-6 secretion following activation with LTA plus Hb. In addition, interferon-β mRNA is present after activation with LTA plus Hb, suggesting that the TRIF/TRAM-dependent pathway may be involved. Hb alone can elicit the TLR4-dependent secretion of TNF-α from macrophages, so it may be the TLR4 ligand. Hb also led to secretion of high mobility group box 1 protein (HMGB1), which synergized with LTA to increase secretion of IL-6. The activation of both the TLR2 and TLR4 pathways by LTA plus Hb leads to an enhanced innate immune response.
Department of Anatomy and Neurobiology, The University of Tennessee Health Science Center, Memphis, TN, USA.
Kathleen H
KH
Cox
Michelle E
ME
Cox
Virginia
V
Woo-Rasberry
David L
DL
Hasty
D000818:Animals; D015153:Blotting, Western; D002460:Cell Line; D005786:Gene Expression Regulation / Q000276:immunology*; D024243:HMGB1 Protein / Q000378:metabolism; D006454:Hemoglobins / Q000378:metabolism*; D006801:Humans; D007113:Immunity, Innate / Q000276:immunology*; D015850:Interleukin-6 / Q000276:immunology; D008070:Lipopolysaccharides / Q000378:metabolism*; D008262:Macrophage Activation / Q000276:immunology*; D051379:Mice; D046228:Microarray Analysis; D060888:Real-Time Polymerase Chain Reaction; D020133:Reverse Transcriptase Polymerase Chain Reaction; D015398:Signal Transduction / Q000276:immunology*; D013682:Teichoic Acids / Q000378:metabolism*; D051195:Toll-Like Receptor 2 / Q000378:metabolism; D051197:Toll-Like Receptor 4 / Q000378:metabolism
D024243:HMGB1 Protein; D006454:Hemoglobins; D015850:Interleukin-6; D008070:Lipopolysaccharides; C495342:TLR2 protein, human; C495345:TLR4 protein, human; D013682:Teichoic Acids; D051195:Toll-Like Receptor 2; D051197:Toll-Like Receptor 4; C009900:lipoteichoic acid