Dendritic cells (DCs) play a pivotal role in orchestrating and bridging innate, adaptive, and memory immunity. For this reason, genetic modification of DCs functions is an attractive approach to treat disease, both using mature DCs (mDCs) to immunize patients, or immature DCs (iDCs) to induce tolerance. Viral vectors are efficient at transducing DCs, and we have investigated the effect of transduction with viral vectors on the phenotype and function of DCs. Adenovirus (Ad5), and Lentivirus (LV) are able to up-regulate costimulatory molecules and major histocompatibility complex (MHC) class II expression on DCs, as well as inducing production of Th1 and proinflammatory cytokines. Moreover, the function of virally infected DCs is altered. In fact, iDCs have an increased, and mDCs a decreased, ability to stimulate a mixed lymphocyte reaction (MLR). Similar results were observed when the capability of transduced-DCs to present recallantigen was investigated. Of interest, in the context of vaccine design, these vectors were able to induce an antigenindependent T lymphocytes proliferative response. These data are relevant not only in the context of genetic manipulation of DCs for vaccine development and active immunotherapy, but also in our understanding of the response of DCs to viral infection.