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Recombinant adenoviral (rAd) vectors are capable of mediating high-efficiency gene transfer in vivo.Under conditions requiring systemic administration, however, the use of rAd vectors can be problematic due to the presence of circulating anti-adenovirus antibodies developed either through natural infection or during the course of treatment. We developed a passive immunization model in SCID/Beige mice to assess the effect of human and mouse anti-adenovirus antibodies on systemic administration of a rAd vector expressing β-galactosidase (rAd-βgal). In this model, the in vitroneutralizing activity of human or mouse antibodies used for passive immunization correlated well with inhibition of transduction of the liver following i.v. administration of rAd-βgal. Depletion of antibodies to individual adenovirus structural proteins (hexon, penton, fiber) by affinity chromatography demonstrated that antibodies to each of the three virion components contributed to neutralization of infectivity in vitroand to inhibition of transduction in vivo.Depletion of antibodies against all three structural proteins from human or mouse immune serum prior to passive immunization restored in vivotransduction activity to levels comparable to those obtained with nonimmune serum. Our data suggest that depletion of both murine and human anti-adenoviral antibodies can restore transduction in vivoduring systemic rAd gene therapy in hosts previously exposed to adenovirus.