(A) Dose response to rhBDDFVIII infusion in FVIII^null mice. When rhBDDFVIII was infused into FVIII^null mice and tail clip was performed, all FVIII^null mice survived tail clipping when infused to a final level of 0.02 U/ml rhBDDFVIII. When rhBDDFVIII was infused at the same concentration (0.02 U/ml) into FVIII^null mice that had been pre-injected with mouse inhibitor plasma, no FVIII^null mice with 25 or 250 BU/ml survived tail clipping. (B) Dose response to FVIII inhibitory antibody infusion in 2bF8^trans mice. Different doses of inhibitor plasma from immunized FVIII^null mice were infused into 2bF8^tg+/– mice. All mice survived tail clipping when infused with less than 25 BU/ml inhibitory plasma, and 7 of 8 survived with a titer of 250 BU/ml. (C) Tail clip survival test on the recipients of spleen cell transplantation. Sublethally irradiated 2bF8^tg+/– or WT mice received spleen cells from immunized FVIII^null mice. Two weeks after transplantation, plasma was tested by inhibitor analysis and the tail clip survival test was performed. Transgenic 2bF8^trans mice demonstrated enhanced tail clip survival even in the presence of a titer greater than 5,000 BU/ml. (D) Tail clip survival test of immunized 2bF8^trans mice. Heterozygous 2bF8^tg+/– mice were immunized with rhBDDFVIII (with adjuvant) by i.p. injection once. Two weeks after immunization, plasma was used for inhibitor quantitation and the tail clip survival test was performed. All immunized 2bF8^trans mice survived tail clipping with a titer of 10–50 BU/ml.