Ad5 vectors capsid-displaying retro-DAF complement inhibitor significantly reduce Ad-dependent activation of ECs in C57BL/6 mice. C57BL/6 WT and C3-KO mice were intravenously injected with (A) 0.75 × 10¹¹ vp/mouse (medium dose) or (B) 2 × 10¹¹ vp/mouse (high dose) of Ad5-based control and experimental vectors. Plasma samples, collected at 6 hours after injection (n = 6 for virus-treated groups, n = 4 for Mock-injected groups) were analyzed with the use of a multiplexed bead array–based quantitative system. The bars represent mean ± SD. Statistical analysis was completed with the use of 1-way analysis of variance with a Student-Newman-Keuls post hoc test. Values statistically different from those in Mock-injected animals of both genotypes, *P < .05, **P < .001, respectively; significant reduction of EC activation compared with both WT_Ad5-GFP and WT_Ad5-IX-dGFP injected mice, #P < .05. Note that levels of EC activation triggered by Ad5-GFP-IX-dDAF_REO novel Ad closely parallel the levels observed in C3-KO mice treated with conventional Ad5-GFP vector (shaded bars).