Vaccine vectors derived from human adenovirus serotype 5 (Ad5) elicit robustimmune responses (IR) directed against the antigen encoded by the transgene in diversemammalian species. The induced IR exhibits a pronounced T helper (Th) 1 orientation, andAd5-based vectors are recognized as a leading vaccine platform for induction of CD8+ Tlymphocyte responses. Much, however, remains to be learned as regards how these vectorsinteract with professional antigen presenting cells (APC) to induce adaptive IR. Dendriticcells (DC) are professional APC endowed with the ability to prime naïve T lymphocytes,and are thus seminal in adaptive IR. DC are able to distinguish different classes ofpathogens, and induce IR that are proportional to the perceived risk and appropriate, asregards the nature of the immune effectors deployed, for the pathogen in question. Thisplasticity is related to the existence of distinct DC subsets, such as the CD8α+ and CD8α-DC subsets that are specialised in presentation of exogenous antigen by class I and class IIpathways, respectively. At present, however, the relative contribution of different DCsubsets to the initiation of adaptive IR by Ad5-based vaccines is unknown. This informationis needed to optimise interactions between Ad5-based vaccines and DC subsets, so as toamplify immune responses and even selectively elicit pathogen-appropriate IR.To this end, we have explored how Ad5-based vectors interact with murine splenicDC. First, we have investigated the interaction of Ad5-based vectors with CD8α+ or CD8α-DC subsets. Although in both ex vivo and in vivo experiments CD8α+ and CD8α- DC subsetscaptured an Ad5-based vector to a similar extent, transgene expression and subsequent MHCclass I display of a transgene-encoded antigen were more efficient in CD8α+ DC. Moreover,following in vivo and ex vivo transduction with an Ad5-based vaccine, antigen-specificCD8+ T lymphocytes were more efficiently activated by CD8α+ DC than by CD8α- DC.Second, we have studied how Ad5 is captured by mucosal DC in situ, after intestinal loopinoculation of Ad5. Preliminary data have indicated that Ad5 is efficiently translocated tothe subepithelial tissue via specialized M cells present in follicle-associated epithelium andINTRODUCTION: The Adenovirus16captured by mucosal DC in gut-associated lymphoid tissue. Conjugates of Ad5 withsecretory IgA are to be tested to improve stability of Ad5 in the intestinal milieu andenhance breaching of the mucosal barrier. Last, in an attempt to improve the tropism ofvaccine vectors, chemically or metabolically biotinylated Ad have been coupled vianeutravidin to either biotinylated antibodies or natural ligands directed against endocytoticreceptors present at the surface of DC. Preliminary experiments that have been performedusing murine bone marrow DC have shown that retargeting can improve attachment,transgene expression and MHC class I presentation of transgene-encoded antigen. Thisstrategy could be exploited to improve delivery of transgene-encoded antigen to selected DCsubsets, so as to amplify IR or elicit pathogen-appropriate IR.