Recipients of B-cell blasts transduced with C2-IgG/MSCV, A2-IgG/MSCV, or both are hyporesponsive to fVIII and its C2 or A2 domains in naive E16 mice. (A-C) Naive hemophilia A mice were injected with B cells transduced with retroviral vectors expressing C2-IgG (□), A2-IgG (▵), both (▪), or control SAG-IgG (♦) and challenged repeatedly with human fVIII. Animals were euthanized at 5 days after final fVIII injection. T-cell proliferation of spleen cells from individual mice was measured in triplicate against C2 (A), A2 (B), and fVIII (C). Data represent the mean plus or minus SEM for individual mice of the cpm incorporated minus background. ^*P < .05 versus mock control (SAG-Ig B cells). ^**P < .01. Serum anti-C2 (D) and anti-fVIII (E) antibody (total IgG) titers were determined by ELISA. One set of 2 similar experiments is shown. Factor VIII inhibitor titers (F) were evaluated with a chromogenic assay as described in “Materials and methods.” Two independent experiments; ^*P < .05, ^**P < .01, ^***P < .001 versus mock control.