We previously developed octaarginine (R8)-modified lipid envelope-type nanoparticles for siRNA delivery (R8-MEND). Herein, we report on their ex vivo siRNA delivery to primary mouse bone marrow-derived dendritic cells (BMDCs) for potential use as a cancer vaccine. Quantitative imaging analysis of the intracellular trafficking of siRNA revealed that the dissociation process, as well as the rate of endosomal escape limits the siRNA efficiency of the prototype R8-MEND, prepared by the hydration method (R8-MENDhydo). Successful endosomal escape was achieved by using a pH-dependent fusogenic peptide (GALA) modified on a lipid mixture that was optimized for endosomal fusion. Furthermore, a modified protocol for the preparation of nanoparticles, mixing the 5iRNA/STR-R8 complex and small unilamellar vesicles (R8/GALA-MENDSUV), results in a more homogenous, smaller particle size, and results in a more efficient intracellular dissociation. Gene knockdown of the suppressor of cytokine signaling 1 (SOCS1), a negative-feedback regulator of the immune response in BMDCs resulted in an enhanced phosphorylation of STAT1, and the production of proinflammatory cytokines. Moreover, SOCS1-silenced BMDCs were more potent in suppressing tumor growth. Collectively, these results show that siRNA loaded in R8/GALA-MENDSUV efficiently suppresses endogenous gene expression and consequently enhances dendritic cell-based vaccine potency in vivo. (C) 2010 Elsevier B.V. All rights reserved.