47. A Model To Determine the In Vivo Fate of Defined Adenovirus Immune Complexes

Vectors based on Adenovirus type 5 (Ad5) are well suited for the purpose of genetic vaccination due to their high immunogenicity. However, it is known that immune complexes formed upon contact with vector-specific antibodies (Abs) can impede gene transfer and influence vector immune profiles. To model, analyze, and understand the effects of immune complex formation on in vivo biodistribution and vector immunogenicity, we established a novel model based on chemical tagging of Ad capsomers with a carbohydrate epitope. Of note, this model allows to direct antibodies to specific capsid regions and thus to generate highly defined Ad immune complexes independent of species and MHC restrictions.As an artificial immunogenic epitope, we chemically coupled alpha-Gal (Galactose-a1,3-Galactose-b1,4-N-Acetylglucosamine) to specific sites of Ad capsomers. We generated ΔE1 Ad5 vectors being alpha-Galylated at hexon HVR1, hexon HVR5, penton or fiber, and an unmodified control vector.We demonstrated that on account of the alpha-Gal epitope, anti-Gal IgG or IgM bound to selected tagged capsomers. Thus, we analyzed biodistribution and vector-induced immune responses in alpha-1,3-Galactosyltransferase knockout (a1,3-GT KO) mice, which harbor large amounts of anti-Gal Abs.First, to compare the effect of epitope tagging on vector biodistribution, we analyzed hepatic and splenic transgene expression after i.v. injection of alpha-Galylated vectors into anti-Gal-positive a1,3-GT KO mice. We observed that transgene expression from fiber-modified vectors was mildly decreased, while expression from hexon-modified vectors decreased drastically, in a way resembling neutralization. These data suggested that, in fact, our model is suitable to precisely analyze the role of capsomer-specific Ab binding on in vivo vector biodistribution and activity.Furthermore, we analyzed the effect of immune complexation on the induction of vector- and transgene product-directed immune responses after repetitive i.m. delivery of alpha-Galylated vectors into anti-Gal-positive a1,3-GT KO mice. Here, we found that the Ad immune complexes induced transgene product-directed immune responses, even upon repeated vector administration. In particular, the extents of hepatic and splenic transgene expression, and transgene-directed immune responses were inversely correlated. For hexon-modified vectors, in spite of weakest transgene expression, transgene-directed cellular immune responses were strongest.In summary, we showed that alpha-Galylation of Ad capsomers markedly influenced specific immune responses directed to the vector as well as the transgene product. Our concept shall be used as an in vivo model to characterize whether directing or diverting immune complexation to or from specific capsomer sites can improve future strategies for vaccination regimens using Ad vectors.