rsTREM-1 binds specifically to platelets. PMN, lymphocytes, monocytes or platelets (unstimulated or stimulated with thrombin 5 U/mL) were stained with biotinylated rsTREM-1, or human IgG and secondary streptavidin PE, and analyzed by FACS. (A) No binding of rsTREM-1 (light gray) and human IgG (dark gray) on PMNs (left panel), lymphocytes (middle panel), or monocytes (right panel). (B) Competition between unlabeled recombinant sTREM-1 10 μg/mL and recombinant sTREM-1-PE 1 μg/mL (black line) on platelets. rsTREM-1 PE + rsTREM-1 binding is less effective compared with rsTREM-1-PE alone (gray filled). Binding of PE-labeled human IgG is shown by the open line. (C) Binding of titrated amounts of rsTREM-1 (filled symbols) or human IgG (open symbols) on stimulated (circle) or unstimulated platelets (triangle). Shown is the mean fluorescence intensity of biotinylated rsTREM-1 and secondary streptavidin PE. (D) Competition between rsTREM-1-PE and unlabeled rsTREM-1 (black) or rsTREM-1-PE and unlabeled human IgG (gray line). (E) Competition between rsTREM-1-PE and LP17 (black) or rsTREM-1-PE and scrambled peptide (gray line). (D,E) Shown is the mean fluorescence intensity of biotinylated rsTREM-1 and secondary streptavidin PE (recombinant sTREM-1-PE). Data are representative of 3 independent experiments.