Identification of lipophilic ligands of Siglec5 and -14 that modulate innate immune responses.

Sialic acid-binding immunoglobulin-like lectins (Siglecs) are a family of cell-surface immune receptors that bind to sialic acid at terminal glycan residues. Siglecs also recognize nonsialic acid ligands, many of which remain to be characterized. Here, we found that Siglec5 and Siglec14 recognize lipid compounds produced by Trichophyton, a fungal genus containing several pathogenic species. Biochemical approaches revealed that the Siglec ligands are fungal alkanes and triacylglycerols, an unexpected finding that prompted us to search for endogenous lipid ligands of Siglecs. Siglec5 weakly recognized several endogenous lipids, but the mitochondrial lipid cardiolipin and the anti-inflammatory lipid 5-palmitic acid-hydroxystearic acid exhibited potent ligand activity on Siglec5. Further, the hydrophobic stretch in the Siglec5 N terminus region was found to be required for efficient recognition of these lipids. Notably, this hydrophobic stretch was dispensable for recognition of sialic acid. Siglec5 inhibited cell activation upon ligand binding, and accordingly, the lipophilic ligands suppressed interleukin-8 (IL-8) production in Siglec5-expressing human monocytic cells. Siglec14 and Siglec5 have high sequence identity in the extracellular region, and Siglec14 also recognized the endogenous lipids. However, unlike Siglec5, Siglec14 transduces activating signals upon ligand recognition. Indeed, the endogenous lipids induced IL-8 production in Siglec14-expressing human monocytic cells. These results indicated that Siglec5 and Siglec14 can recognize lipophilic ligands that thereby modulate innate immune responses. To our knowledge, this is the first study reporting the binding of Siglecs to lipid ligands, expanding our understanding of the biological function and importance of Siglecs in the innate immunity.