Inhibitor formation is a severe complication of hemophilia, occurring in up to 25% and associated with poor response to factor replacement, uncontrolled bleeding, and high morbidity. Preventing inhibitor formation is, thus, a major goal of hemophilia management. The role of dendritic cells (DC) in regulating immune response has been increasingly recognized: immature DC (imDC) induce T regulatory cells in vitro and promote Ag-specific tolerance in vivo. We, therefore, studied the role of imDC propagated from bone marrow with GM-CSF + TGFβ to prevent inhibitor formation in the hemophilia A murine model. Following tail vein injection of recombinant F.VIII (Advate, Baxter) 2.5 U (0.2 μg) on days 0, 2, and 4 in hemophilia A exon 16 KO C57Bl/6 mice, anti-VIII antibodies were detected by semi-quantitative APTT (scored 1-4), peaking on day 6. On rechallenge with F.VIII 2.5 U on days 12, 14, and 16, anti-VIII was detected, peaking on day 17. Anti-VIII production was associated with high level splenic T cell proliferation in response to F.VIII stimulation in vitro, measured by 3H-thymidine incorporation in mixed lymphocyte reaction (MLR). By contrast, there was no antibody formation in F.VIII-treated Wt C57Bl/6 mice: the latter was associated with low T cell response to F.VIII in vitro. Functionally immature DC (imDC) were propagated from the bone marrow of hemophilia A mice with GM-CSF (4ng/ml) and TGFβ (0.2ng/ml). For comparison, functionally mature dendritic cells (mDC) were propagated with GM-CSF (4ng/ml) and IL-4 (1000U/ml).The former (imDC) demonstrated deficient NF-kB binding activity in nuclear protein as detected by gel shifting assay and expressed low level of costimulatory molecules CD80, CD86; by contrast, the latter (mDC) demonstrated enhanced NF-kB binding activity and high levels of co-stimulatory molecules. Administration of 2x106 F.VIII-pulsed imDC (20U/ml x 24h) 7 days before F.VIII dosing on days 0, 2, and 4, led to reduction in inhibitor formation on day 6 (score 1.6 vs. 2.3 in control group) which was further reduced on day 8 (score 1.0 vs. 2.0 in control group). The inhibitor could not be detected on day 8 in 2 of 4 mice pretreated with F.VIII-pulsed imDC. By contrast, high levels of inhibitor were detected in mice pretreated with F.VIII-pulsed mDC (score 3.3). Rechallenge with F.VIII on day 10 in imDC-treated mice resulted in no increase in the reduced or absent anti-VIII effect on day 12. Splenic T cells (CD3+) from the imDC-pretreated mice showed lower proliferative capacity when restimulated in vitro with F.VIII, suggesting that imDC induced F.VIII unresponsiveness. These studies show that FVIII-pulsed imDC reduce the intensity of inhibitor formation, and suggest the potential role of modified DC in preventing or reducing F.VIII inhibitor formation.