The development of strategies to augment the immunogenicity of plasmid DNA vaccines is critical for improving their clinical utility. One such strategy involves the coadministration of plasmid cytokine adjuvants with DNA vaccines. Although a large number of plasmid cytokines have shown promise as adjuvants in preclinical animal models, little is known about their expression kinetics and mechanism of action. We have previously shown that administration of a plasmid encoding the interleukin-2/immunoglobulin (IL-2/Ig) cytokine fusion protein durably augmented DNA vaccine-elicited immune responses in rhesus monkeys for over 10 months. We sought to determine whether persistent cytokine expression from this plasmid accounted for these long-lasting effects. In fact, we found that expression from plasmid IL-2/Ig was transient with an extinction half-life in vivo of approximately 2 days. We next assessed whether the generation of anti-cytokine antibodies may have accounted for these transient expression kinetics. Importantly, both mice and rhesus monkeys inoculated with this plasmid cytokine did not develop detectable antibody responses against IL-2. These data suggest that the durable augmentation of DNA vaccine-elicited cellular immune responses afforded by this plasmid cytokine was likely due to enhanced initial priming of memory T lymphocytes rather than chronic cytokine expression.