The Soluble Endothelial Protein C Receptor Binds to Activated Neutrophils: Involvement of Proteinase-3 and CD11b/CD18

The protein C pathway is a primary regulator of blood coagulation and a critical component of the host response to inflammatory stimuli. The most recent member of this pathway is the endothelial protein C receptor (EPCR), a type I transmembrane protein with homology to CD1d/MHC class I proteins. EPCR accelerates formation of activated protein C, a potent anticoagulant and antiinflammatory agent. The current study demonstrates that soluble EPCR binds to PMA-activated neutrophils. Using affinity chromatography, binding studies with purified components, and/or blockade with specific Abs, it was found that soluble EPCR binds to proteinase-3 (PR3), a neutrophil granule proteinase. Furthermore, soluble EPCR binding to neutrophils was partially dependent on Mac-1 (CD11b/CD18), a beta(2) integrin involved in neutrophil signaling, and cell-cell adhesion events. PR3 is involved in multiple diverse processes, including hemopoietic proliferation, antibacterial activity, and autoimmune-mediated vasculitis. The observation that soluble EPCR binds to activated neutrophils via PR3 and a beta(2) integrin suggests that there may be a link between the protein C anticoagulant pathway and neutrophil functions.