FVIII expression in T2F8 transgenic mice. (A) Quantitative evaluation of FVIII activity levels in mouse plasma by chromogenic assay. FVIII:C was detected in the plasma of T2F8 transgenic mice. The level of FVIII:C in T2F8^tg+/+ mice was similar to WT mice. FVIII:C was not detected in the plasma of FVIII^null mice that received BMT from T2F8^Tg mice. (B) PCR analysis shows that the T2F8 transgene cassette (650 bp) is detected in FVIII^null mice after receiving BMT from T2F8^Tg mice. Lane 1, DNA marker; lanes 2 and 3, FVIII^null pre-BMT; lanes 4 and 5, FVIII^null post-BMT; lane 6, H₂O; lane 7, T2F8^Tg control. DNA was purified from peripheral blood, and T2F8 transgene was amplified with primers P1 and P2 (upper panel). Mouse FVIII exon 4, which is not disrupted in FVIII^null mice, was amplified as an internal control (lower panel). (C) Quantitative evaluation of human FVIII antigen levels in mouse plasma by ELISA. hFVIII:Ag was detected in the plasma of T2F8^Tg mice, but not in WT and FVIII^null mice. These results demonstrate that endothelial cell–derived FVIII can normalize plasma FVIII.