A family of related glycoproteins (GP) mediate the interaction between the circulating adhesive proteins and a variety of cells (cytyoadhesins). In this study we have compared two cell-surface antigens which share the property to bind fibrinogen: the platelet GP IIb/IIIa, prototype of the cytoadhesins, and the receptor for fibrinogen costitutively synthesized by monocytes. Two anti-GP IIb/IIIa monoclonal antibodies (Mabs) (LJP9, LJP5), recognizing functionally distinct epitopes of the GP IIb/IIIa did not react with monocytes nor inhibited 125I-fibrinogen binding to monocytes. Similarly, an Arg-Gly-Asp containing peptide which completely abolished platelet-fibrinogen interaction, had no effect on monocytes. Structurally, the monocyte fibrinogen receptor was dimeric and composed of two subunits with molecular weight (Mr) of 155,000 and 95,000. This structural organization was different from that of the GP IIb/IIIa (Mr= 116,000), but in close analogy with the family of leukocyte differentiation antigens OKM1, LFA-1. Therefore, this possible relationship was investigated. A Mab to OKM1 antigen (10 μg/ml) completely suppressed fibrinogen binding to monocytes while it was ineffective on plateles. Iodinated monocyte lysate subjected to immunoprecipitation with OKM1 Mab (60 μg/ml) showed a dimeric antigen with the same molecular size of the monocyte fibrinogen receptor. Moreover, preclearing of the monocyte lysate with OKM1 Mab removed the immunoprecipitate corresponding to the monocyte fibrinogen receptor. These data indicate that the immunologic differentiation antigen OKM1, in addition to function as a complement receptor, displays also the novel biological adhesion property to mediate the binding of fibrinogen to monocytes.
Monocyte Differentiation Antigen CD14
