Binding of fibrinogen to human monocytes

The interaction of fibrinogen with monocytes was studied. After stimulation with ADP (10 μM) or thrombin (1 U/ml), platelet-free suspensions of human monocytes bind ¹²⁵I-fibrinogen with two different affinities in a specific and Ca²⁺-dependent reaction with saturation at 5.80-7.35 x 10^-7 M of added protein. The binding of fibrinogen to specific receptors on monocytes induces the procoagulant activity of these cells. Thrombasthenic cells or normal monocytes preincubated with a monoclonal antibody to the platelet glycoprotein IIb/IIIa complex (10E5) do not bind fibrinogen and have no procoagulant activity. Metabolic studies with [³⁵S]methionine revealed that cultured monocytes actually synthesize a surface antigen precipitated by 10E5 antibody as a major band with 92,000 relative molecular weight. Our data indicate that monocytes express receptors for fibrinogen only in part related to the platelet glycoprotein IIb/IIIa complex. Furthermore, the binding of fibrinogen to monocytes enhances the cooperation of these cells in hemostasis.