The paramyxovirus simian virus 5 (SV5) is a poor activator of human dendritic cell (DC) maturation pathways in vitro, and infected DC do not upregulate cell surface costimulatory proteins or secretion of immunomodulatory cytokines. We evaluated the hypothesis that activation of SV5-infected DC would be enhanced by engineering SV5 to express a Toll-like-receptor (TLR) ligand. To test this hypothesis, a novel virus was engineered such that the gene encoding an intracellular form of the TLR5 ligand flagellin was expressed from the genome of wild-type (WT) SV5 (SV5-flagellin). Cells infected in vitro with the flagellin-expressing virus released low levels of biologically active flagellin, which was capable of stimulating TLR5 signaling. Infection of human peripheral blood mononuclear cell-derived immature DC with SV5-flagellin resulted in enhanced levels of interleukin-6 (IL-6) and IL-12 compared to infection with DC with the parental virus, WT SV5. In contrast to cytokine induction, the flagellin-expressing virus did not appreciably increase DC surface expression of the costimulatory molecule CD80 or CD86 above the level seen with WT SV5 alone. In mixedculture assays, DC infected with the flagellin-expressing virus were more effective at activating gamma interferon secretion from both CD8؉ and CD4 ؉ allogeneic T cells than DC infected with WT SV5. Our results with SV5-directed intracellular expression of flagellin may be applicable to other vectors or pathogenic viruses where overcoming impairment of DC activation could contribute to the development of safer and more effective vaccines.Dendritic cells (DC) are important professional antigenpresenting cells that are capable of recognizing microbial products and promoting innate and adaptive immune responses (5, 38). Upon sensing viral components, DC are triggered to undergo a conversion from a phenotype termed immature into a form that is capable of activating naïve-T-cell functions, such as proliferation, cytokine secretion, and cytolytic activity (5, 38). These DC activation events include increased cell surface expression of costimulatory molecules, such as CD40, CD80, and CD86, as well as increased capacity to secrete immunomodulatory cytokines, such as interleukin-12 (IL-12), IL-6, and tumor necrosis factor alpha (TNF-␣) (5, 14, 38). As such, the ability of DC to be activated in response to infection has important consequences for development of immunity against viruses. Here we describe engineering the paramyxovirus simian virus 5 (SV5) to be more effective at activating human DC to stimulate T-cell function in vitro.Many viruses are poor activators of DC, with infected immature DC showing only limited production of cytokines or upregulation of costimulatory molecules (28). Examples include herpes simplex virus, cytomegalovirus (CMV), vaccinia virus, measles virus (MeV), influenza virus, Lassa fever virus, poliovirus, and Ebola virus (4,9,17,18,20,36,43). This lack of activation of infected DC can result from virus-induced suppression of signaling pathways. Fo...