Selective blocking of APC anticoagulant activity prevents the elaboration of IL-6 and the vascular leakage in the synovium of FVIII^−/− mice following the knee injury. FVIII^−/− mice were administered with a control IgG, MAPC1591 that blocks the anticoagulant activity of APC, or MPC1609 that blocks both the anticoagulant and the signaling functions of APC (1 mg/kg, ip). After 24 hours, joint bleeding was induced by a needle puncture injury. (A) Seven days after the injury, synovial fluids were collected, and IL-6 levels in the synovial fluids were determined by ELISA. (n = 4 mice per group). (B-C) Fourteen days after the injury, vascular leakage was evaluated by extravasation of IV injected fluorescein dextran into knee joints. Fourteen days after the injury, mice were injected with fluorescein dextran (70 000 molecular weight, 20 mg/kg) IV via the tail vein. Mice were euthanized 3 hours following dextran administration and perfused with ice-cold saline supplemented with 5 mM CaCl₂ and 1 mM MgCl₂. Joint tissue was excised, and fluorescence intensities of joint tissue extracts were measured (corrected for autofluorescence) (B). Joint tissue sections were also analyzed by fluorescence microscopy at ×4 magnification (C), and the fluorescence intensity of tissue sections was quantified and corrected to autofluorescence (D). *P < .05; ****P < .0001; no statistically significant difference (n = 4).