Factor H binds to platelet integrin αIIbβ3

Factor H is a plasma protein that regulates activity of the alternative complement pathway in plasma and on cell surface. Binding of factor H to a cell surface protects that cell against complement-induced damage. Factor H binds to glycosoaminoglycans, surface-immobilized C3b, L selectin, and integrins such as αMβ1 (a direct binding) or αVβ3 (an indirect binding mediated through intermediary plasma proteins). We studied the binding of factor H to platelets and to integrin αIIbβ3 (glycoprotein IIb-IIIa), the most abundant integrin on platelets. We measured binding of purified factor H to platelets or heterologous cells expressing recombinant αIIbβ3 using flow cytometry. We also measured binding of factor H to αIIbβ3 in cell free systems using either surface plasmon resonance or enzyme-linked immunosorbent assay. We found that factor H directly binds to αIIbβ3 and this binding has a dissociation constant (Kd) of 131 ± 60.9 nM and is not dependent on active conformation of αIIbβ3 or on the presence of cation...