The saturable high affinity association of factor X to ADP-stimulated monocytes defines a novel function of the Mac-1 receptor.

Initiation of the coagulation protease cascade as it assembles on cell surfaces requires limited proteolytic activation of the zymogen factor X. Not previously suspected to be the ligand of an organizing receptor on cell surfaces, we now describe that factor X specifically associates with cells of monocyte lineage and we identify the high affinity receptor for this zymogen. Following stimulation with ADP (10 microM), or with the ionophore ionomycin (1 microM), isolated human monocytes bind 125I-factor X in a saturable fashion with a dissociation constant (Kd) of 21.8-44.9 nM. Equilibrium binding analyses indicate that the reaction is optimal at room temperature, requires Ca2+ ions, and saturates at 128,500 +/- 21,300 molecules of 125I-factor X specifically associated with the cell surface. Molar excess of unlabeled factor X inhibits and reverses the binding, whereas the homologous gamma-carboxylated coagulation proteins factors II, VII, IX, IXa, and Xa are without effect. Similarly, chelation of divalent ions immediately dissociates bound 125I-factor X. The monoblast cell line U 937 and the monocytic cell line THP-1 when stimulated with ADP or ionomycin, bind 125I-factor X with characteristics similar to monocytes. Receptor identity was explored using antibodies to the leukocyte adhesive receptors Mac-1, LFA-1, and p150.95. Monoclonal antibodies specific for the alpha subunit of Mac-1 (M 1/70, LM 2/1) or for the common beta subunit (TS 1/18, 60.3) bound equally to resting and ADP- or ionomycin-stimulated cells and also completely blocked the binding of 125I-factor X to stimulated monocytes, U 937, or THP-1 cells. To distinguish between modulatory effects of the monoclonal antibodies and direct spatial hindrance binding of 125I-factor X to Mac-1 was analyzed directly. OKM10 anti-alpha subunit of Mac-1 monoclonal antibody immunoprecipitated 125I-factor X chemically cross-linked to its receptor on stimulated cells. In addition, the complement protein fragment C3bi, which is a recognized ligand for Mac-1, competitively inhibited the association of 125I-factor X. These findings indicate that human blood monocytes and less differentiated cells of this lineage possess an inducible receptor specific for factor X; and also support the conclusion that the heterodimeric leukocyte adhesive receptor Mac-1 functions as the specific receptor structure. We suggest that the novel properties of this receptor may be of importance in the organization and regulation of certain coagulation protease cascades on the monocyte surface.