Pro-inflammatory chemokines CCL5, CXCL12, and CX3CL1 bind to and activate platelet integrin αIIbβ3 in an allosteric manner

AbstractPrevious studies showed that pro-inflammatory chemokines CX3CL1 and CXCL12 bound to the allosteric binding site (site 2) of integrins and allosterically activated integrins, in addition to the classical ligand-binding site (site 1). We showed that CCL5 also bound to site 2, in addition to site 1, and activated soluble integrin αvβ3. Platelet integrin αIIbβ3, a fibrinogen receptor, is critical for hemostasis and thrombus formation and activation of αIIbβ3 is a key event for thrombus formation. Activation of αIIbβ3 is known to be mediated exclusively by inside-out signaling. We studied if αIIbβ3 can be allosterically activated. We discovered that CCL5, CXCL12, and CX3CL1 are new ligands for αIIbβ3. Notably they enhanced the binding of monovalent ligand to soluble αIIbβ3 in 1 mM Ca2+by binding to site 2. They activated cell-surface αIIbβ3 on CHO cells quickly (half maximal response <1 min) and at low concentrations (1-10 ng/ml) compared to soluble αIIbβ3, probably because chemokines bind to cell surface proteoglycans. Notably, activation of αIIbβ3 by the chemokines was several times more potent than 1 mM Mn2+. Since CCL5 and CXCL12 are stored in platelet granules and rapidly transported to the surface upon platelet activation, we hypothesized that they are released from the granules and allosterically activate αIIbβ3 by binding to site 2.Transmembrane CX3CL1 on activated endothelial cells likely mediates platelet-endothelial interaction by binding to and activating αIIbβ3. Also, over-produced chemokines during inflammation may trigger αIIbβ3 activation, which is a possible missing link between inflammation and thrombosis.