Gene-modified human dendritic cells (DCs) were generated by transfection with adenovirus polyethylenimine DNA (Ad/PEI/DNA) and mannose polyethylenimine DNA (ManPEI/DNA) complexes. Ad/PEI/DNA complexes have plasmid DNA bound to adenovirus particles by PEI and deliver DNA into cells via the adenovirus infection route. Such transfection complexes yield high transduction levels and sustained expression of luciferase and green fluorescent protein reporter genes and were almost as effective as recombinant adenovirus vectors. ManPEI/DNA complexes rely on uptake by receptor-mediated endocytosis via mannose receptor, which is highly expressed on DCs. While gene delivery by ManPEI/DNA complexes was less efficient than by Ad/PEI transfection, incorporation of adenovirus particles in ManPEI/DNA transfection complexes further enhanced transduction efficiencies and transgene expression. We also demonstrate that Ad/PEI-transfected DCs are competent in stimulating T cell proliferation in allogeneic and autologous mixed lymphocyte reactions, and in activating T cells from T cell receptor (TCR)-transgenic mice in an antigen-specific manner. Thus, the present study establishes the following relative order of transduction efficiencies of viral and nonviral gene delivery systems for primary human DCs: recombinant adenovirus > Ad/PEI = Ad/ManPEI > ManPEI > PEI. Ad/PEI and ManPEI transfection modes represent particularly versatile transduction systems for DCs, with ManPEI being built up exclusively of synthetic compounds.