Effect of soluble fibrinogen on cell adhesion. (A) Aliquots of calcein-labeled U937 monocytoid (○) and the α_Mβ₂-expressing HEK 293 cells (•) in HBSS + 0.1% BSA were mixed with different concentrations of soluble fibrinogen (Fg) and were added to the wells of 96-well microtiter plates coated with 2.5 μg/mL fibrinogen (3 hours at 37°C) and coated afterward with PVP. After 25 minutes at 37°C, nonadherent cells were removed by washes with PBS. The number of adherent cells was determined by using the fluorescence of 100-μL aliquots with a known number of labeled cells. The data shown are the means ± SEs of 6 to 8 experiments with triplicate determinations in each experiment. (B) The wells of microtiter plates were coated with 2.5 μg/mL fibrinogen (filled symbols) or 2.5 μg/mL fibrin-monomer (open symbols) for 3 hours at 37°C, coated afterward with PVP, washed with PBS, and then incubated with different concentrations of soluble fibrinogen. After 15 minutes at 37°C, solutions of fibrinogen were aspirated, and adhesion of U937 cells (circles) or α_Mβ₂-expressing HEK 293 cells (▾) was tested. Fg_s indicates the concentrations of soluble fibrinogen preincubated with immobilized fibrinogen or fibrin-monomer.