Human Integrin α3β1 Regulates TLR2 Recognition of Lipopeptides from Endosomal Compartments

Abstract

BackgroundToll-like receptor (TLR)-2/TLR1 heterodimers recognize bacterial lipopeptides and initiate the production of inflammatory mediators. Adaptors and co-receptors that mediate this process, as well as the mechanisms by which these adaptors and co-receptors function, are still being discovered.Methodology/Principal FindingsUsing shRNA, blocking antibodies, and fluorescent microscopy, we show that U937 macrophage responses to the TLR2/1 ligand, Pam3CSK4, are dependent upon an integrin, α3β1. The mechanism for integrin α3β1 involvement in TLR2/1 signaling is through its role in endocytosis of lipopeptides. Using inhibitors of endosomal acidification/maturation and physical tethering of the ligand, we show that the endocytosis of Pam3CSK4 is necessary for the complete TLR2/1-mediated pro-inflammatory cytokine response. We also show that TLR2/1 signaling from the endosome results in the induction of different inflammatory mediators than TLR2/1 signaling from the plasma membrane.Conclusion/SignificanceHere we identify integrin α3β1 as a novel regulator for the recognition of bacterial lipopeptides. We demonstrate that induction of a specific subset of cytokines is dependent upon integrin α3β1-mediated endocytosis of the ligand. In addition, we address an ongoing controversy regarding endosomal recognition of bacterial lipopeptides by demonstrating that TLR2/1 signals from within endosomal compartments as well as the plasma membrane, and that downstream responses may differ depending upon receptor localization. We propose that the regulation of endosomal TLR2/1 signaling by integrin α3β1 serves as a mechanism for modulating inflammatory responses.

The Type II Secretion System of Legionella pneumophila Dampens the MyD88 and Toll-Like Receptor 2 Signaling Pathway in Infected Human Macrophages

“…As a second means by which T2S could diminish PAMP recognition by TLR2, it is possible that a protein effector secreted into the LCV impedes the translocation of LPS and/or lipopeptides out of the LCV and into the macrophage cytoplasm and perhaps ultimately into the extracellular milieu. Although historically, TLR2 has been named a surface TLR, recent findings have challenged this idea and demonstrated intracellular TLR2 colocalizing with its ligand (84,85). As a final way in which T2S might impede the TLR2 pathway, it is conceivable that a secreted bacterial protein traffics out of the LCV and obstructs recognition by blocking, cleaving, or otherwise altering the host receptor or a downstream adaptor such as MyD88.…”

Receptors involved in cell activation by antiphospholipid antibodies

“…TLRs may also depend on other co-receptors for full ligand sensitivity. Upon stimulation, TLR complexes are internalized and targeted to endosomal compartments [42][43][44][45]. Receptor internalization may also serve as an important endogenous mechanism that protects against chronic inflammation by avoiding unrestrained signaling by TLRs [46].…”

αvβ3 Integrin Boosts the Innate Immune Response Elicited in Epithelial Cells through Plasma Membrane and Endosomal Toll-Like Receptors

“…CD11b deficiency in mice enhanced TLR-mediated responses in macrophages (41). ␤3 integrin and ␣1␤3 integrin cooperate with TLR2 and TLR2/1, respectively, to sense bacterial lipopeptides in human monocytes and macrophages and HEK293 cells (42,43).…”