Up to 30% of patients with hemophilia A given therapeutic factor VIII (fVIII) can make inhibitory antibodies, the majority of which are reactive with its C2 and A2 domains. We have previously demonstrated that antigen-specific tolerance to several antigens can be induced by lipopolysaccharide (LPS)-activated B-cell blasts transduced with immunoglobulin (IgG)-antigen fusion constructs. To apply this system to hemophilia A inhibitor formation, we created retroviral vectors expressing fVIII amino acids S2173-Y2332 (C2 domain) and S373-R740 (A2 domain) in frame with an IgG heavy chain backbone. These vectors were transduced into B-cell blasts to induce tolerance in both naive and fVIII-primed hemophilic (E16 fVIII(-/-)) mice. Thus, treatment of E16 fVIII(-/-) mice with B cells expressing fVIII C2 and A2 domains led to tolerance in terms of specific humoral response (including inhibitory antibody titers) and cellular responses to fVIII and its C2 or A2 domains. Moreover, a significant reduction in immune responses to fVIII could be achieved in immunized hemophilic mice with existing anti-fVIII titers. This hyporesponsive state persisted for at least 2 months and withstood additional challenge with fVIII. Further experiments, in which mice were treated with a depleting monoclonal anti-CD25, suggested that a regulatory T cell may be required for the tolerogenic effect of transduced B cells. These findings demonstrate that B-cell presentation of fVIII domains on an Ig backbone specifically prevents or decreases existing antibodies in hemophilia A mice.