Interleukin (IL)-15, is a cytokine that is important for the maintenance of long-lasting, high-avidity T cell response to invading pathogens and has, therefore, been used in vaccine and therapeutic platforms as an adjuvant. In addition to pure protein delivery, plasmids encoding the IL-15 gene have been utilized. However, it is critical to determine the appropriate dose to maximize the adjuvanting effects. We immunized rhesus macaques with different doses of IL-15 expressing plasmid in an influenza non-human primate immunogenicity model. We found that co-immunization of rhesus macaques with a Flu DNA-based vaccine and low doses of plasmid encoding macaque IL-15 enhanced the production of IFN-gamma (0.5 mg) and the proliferation of CD4(+) and CD8(+) T cells, as well as T(CM) levels in proliferating CD8(+) T cells (0.25 mg). Whereas, high doses of IL-15 (4 mg) decrease the production of IFN-gamma and the proliferation of CD4(+) and CD8(+) T cells and T(CM) levels in the proliferating CD4(+) and CD8(+) T cells. In addition, the data of hemagglutination inhibition (HI) antibody titer suggest that although not significantly different, there appears to be a slight increase in antibodies at lower doses of IL-15. Importantly, however, the higher doses of IL-15 decrease the antibody levels significantly. This study demonstrates the importance of optimizing DNA-based cytokine adjuvants.