Breakdown of the skin barrier requires the recognition of and rapid responses to invading pathogens. Since wounding usually also affects endothelial intactness, the expression of receptors of the Toll-like family involved in pathogen recognition in human skin vessel endothelia was examined. We found that human skin-derived microvascular endothelial cells can express all 10 Toll-like receptors (TLRs) currently known and will respond to respective ligands. Using immortalized skin-derived (HMEC-1) and primary dermal endothelial cells (HDMEC), we screened for TLR expression by real-time PCR. Endothelial cells express 7 (for HDMEC) and 8 (for HMEC-1) of the 10 known human TLRs under resting conditions but can express all 10 receptors in proinflammatory conditions. To provide evidence of TLR functionality, endothelial cells were challenged with TLR ligands, and after the TLR downstream signaling, MyD88 recruitment as well as early (interleukin-8 [IL-8] release) and late immune markers (inducible nitric oxide synthase mRNA expression) were monitored. Surprisingly, the responses observed were not uniform but were highly specific depending on the respective TLR ligand. For instance, lipopolysaccharides highly increased IL-8 release, but CpG DNA induced significant suppression. Additionally, TLR-specific responses were found to differ between resting and activated endothelial cells. These results show that human skin-derived endothelial cells can function as an important part of the innate immune response, can actively sense pathogen-associated molecular patterns, and can mount an increased or reduced inflammatory signal upon exposure to any of the currently known TLR ligands. Moreover, we also show here that proinflammatory conditions may affect TLR expression in a specific and nonuniform pattern.

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