The anti-FXI monoclonal antibody binds human and baboon FXI, and inhibits fibrin formation in FXII-inhibited or deficient human blood under flow. (A) Binding of the anti-FXI monoclonal antibody (aXIMab) to the FXI dimer (160 kDa) in platelet free NHP and NBP was demonstrated by Western blotting, developed using a secondary anti–mouse IgG antibody. aXIMab binding was minimal in FXI-depleted plasma or plasma from a FXI-deficient patient. (B) aXIMab prevented visible fibrin formation in FXII-inhibited or deficient blood under flow. Human whole blood, anticoagulated with CTI (40 μg/mL) to inhibit FXIIa, or reconstituted FXII deficient human blood was perfused through collagen-coated capillary tubes (thrombus chambers) at a shear rate of 265 s⁻¹ for 10 minutes. Before each experiment, blood was incubated with unfractionated heparin (15 U/mL), anti-TF antibody (20 μg/mL) plus FVIIai (1 μg/mL), aXIMab (20 μg/mL), CTI (40 μg/mL) for reconstituted blood where indicated, or PBS vehicle (−), as marked above each panel. Images were obtained via Kohler-illuminated Nomarski differential interference contrast microscopy with a Zeiss Axiovert 200M microscope using a Zeiss 63× oil-immersion 1.40 NA plan-apochromat lens (Zeiss, Göttingen, Germany). Images were captured using a Zeiss AxioCam with Slidebook 4.0 (Intelligent Imaging Innovations, Denver, CO) after 3 minutes of perfusion with modified Tyrodes buffer to wash the thrombus of unbound cells. All experiments were performed at 37°C. Each image is representative of 2 or 3 experiments.