Activation of Pg by U937 cells. U937 cells (1.5 × 106/mL) were incubated in HBSS containing Pg (7.0 μg/mL) and S-2251 (0.7 mM) in the presence or absence of anti-α_M mAb 44a, anti-α₅β₁ Ab 1950 (1:200), tranexamic acid (5 mM), or GRGDSP (0.1 mM), and the enzymatic activity of generated Pm was determined by measuring absorbance at 405 nm after 180 minutes.