An Unexpected Role For αIIbβ3 In Immune Complex Disorders

IgG immune complexes contribute to the etiology and pathogenesis of a number of autoimmune disorders, including heparin-induced thrombocytopenia, systemic lupus erythematosus (SLE), rheumatoid- and collagen-induced arthritis, and chronic glomerulonephritis. Human platelets express on their surface an Fc receptor, termed FcγRIIa or CD32a, that when exposed to immune complexes initiates a potent signal transduction cascade that results in platelet activation and granule secretion. Patients with immune complex-related disorders are known to be highly susceptible to thrombotic events, and microthrombi have been observed to colocalize histologically in patients with immune complex disorders. To better understand the contribution of platelet adhesion receptors and signaling molecules to IgG immune complex-mediated thrombotic complications, we incubated platelets in microtiter wells that had been precoated with immobilized IgG. Platelets quickly formed filopodia, and then adopted a fully-spread morphology over a 30 minute period of time. Cytosolic proteins known to be involved in platelet spreading, including FcγRIIa, Src, Syk, and pp125^FAK also became rapidly tyrosine phosphorylated. Because the integrin αIIbβ3 employs each of these signaling molecules in platelet spreading on immobilized fibrinogen, we next evaluated its role in platelet spreading on immobilized IgG. Interestingly, Fibans - small molecule antagonists of αIIbβ3-fibrinogen interactions – also blocked (1) platelet spreading on immobilized IgG, (2) the associated phosphorylation events, and (3) platelet thrombus formation over immobilized IgG under conditions of flow. Human platelets from Glanzmann thrombasthenic individuals, or murine integrin β3-deficient platelets expressing a human FcγRIIa transgene, also failed to spread on immobilized IgG and form thrombi on immobilized IgG under conditions of flow. FcγRIIa-transgenic mice lacking the Src-family kinase Lyn – thought to be responsible for phosphorylating the ITAM tyrosines of FcγRIIa – also failed to spread or form thrombi over immobilized IgG. Finally, Chinese hamster ovary cells transfected with αIIbβ3 and FcγRIIa failed to spread on immobilized IgG unless small amounts of fibrinogen were added to the IgG preparation before plating. Taken together, our data suggest a complex functional interplay between FcγRIIa and αIIbβ3 in immune complex-mediated thrombotic disorders in which platelets encounter immobilized IgG and become activated to secrete α-granule fibrinogen. Secreted fibrinogen, in turn, then becomes a substrate for αIIbβ3-mediated platelet spreading and subsequent thrombus formation.