Lack of TLR3 signaling in organ-specific immunity to hF.IX. (A) IFN-α/β levels in serum 4 hours after intravenous administration of poly (I:C) compared with control mice (n = 4-5 per group). (B) Plasma levels of hF.IX after hepatic gene transfer with AAV2-ApoE/hAAT-hF.IX vector. C57Bl/6 mice were treated with or without poly (I:C) (control groups) at the time of vector administration or 7 days later (n = 4-5 per group). Note that poly (I:C)–treated animals did not develop antibody response to hF.IX (data not shown). Data in A and B are averages plus or minus SD. *Statistically significant differences (P < .05) in experimental compared with control groups; note that differences in panel B were not significant. (C) Plasma levels of anti-hF.IX IgG in TLR^−/− mice after muscle-directed gene transfer (AAV2-CMV-hF.IX vector, n = 5). Each bar represents an individual animal. Four of 5 mice developed antibodies to hF.IX. (One mouse formed very high-titer antibody, 12-39 μg/mL at 4-6 weeks.)