Characterization of FIX in 2bF9 transgenic mice and BMT recipients. (A) Quantitative evaluation of human FIX:Ag levels in mouse platelets. FIX:Ag was detected in platelet lysates from 2bF9 mice and BMT recipients. BMT-L indicates FIX^null mice that received 2bF9-L BM; BMT-H, FIX^null mice that received 2bF9-H BM; and WT, wild-type mice. (B) Platelet FIX was released from 2bF9-L and 2bF9-H mice by platelet agonist activation. (C) Comparison of platelet FIX:C with FIX:Ag level in 2bF9-L and 2bF9-H mice. FIX:C were measured by APTT assay and chromogenic assay (Chrom). Platelets from WT and FIX^null mice were used as negative controls. (D) Western blot of FIX in platelet releasate and lysates. Lane 1 indicates releasate of stimulated platelets from 2bF9-H mice; lane 2, platelet lysate of 2bF9-H mice; lane 3, platelet lysate of 2bF9-L mice; lane 4, FIX from normal human plasma (Haematologic Technologies) as a control; and lane 5, platelet lysate from FIX^null mice. (E) Quantitative evaluation of FIX:Ag levels in mouse plasma. FIX:Ag was detected in both 2bF9 and BMT recipient plasma. (F) Platelet FIX:Ag as a percentage of whole blood FIX:Ag in 2bF9 and BMT mice. These results demonstrate that FIX was detected in 2bF9 transgenic platelets and is releasable.