Background:  Evidence suggests that induction of strong CD8 HIV-specific cell-mediated immune responses may be an important aspect of an HIV vaccine. DNA vaccines in humans have induced low-level T-cell responses supporting a need for improvement. Several laboratories have reported that the cellular immune responses to DNA vaccines can be enhanced in mice by co-delivering DNA plasmids expressing specific immune modulators. Among the most exciting in the mouse systems are IL15, IL12, B7-2, and CD40L. We sought to compare these molecular adjuvants in a primate model system.

Methods:  The cDNAs for macaque versions of IL15, IL12, B7-2, and CD40L were engineered into plasmid vaccine vectors such that high levels of expression were achieved. Groups of 6 cynomolgus macaques were immunized with 2mg of plasmids expressing a codon-optimized pSIVGag alone, or in combination with either the IL12, CD40L, B7-2, IL15, or IL12/IL15 adjuvant cassettes. Immune responses were compared by quantitative ELISpot, invasive cervical cancer, and carboxy-fluorescein diacetate, succinimidyl ester staining.

Results:  CD40L and B7-2 did not appear to augment this response to the pSIVGag antigen. However, following 2 immunizations with the SIVGag vaccine, plus ILI5 plasmid, the average number of IFN g-producing cells, as determined by ELIspot, was increased 3 fold compared with SIVGag DNA vaccine alone. The IL-15 plasmid also improved the number of responders from 3/6 to 6/6. Animals co-vaccinated with the IL12 molecular adjuvant demonstrated the highest induction of interferon-g (INF-g) producing CD8 effector cells, and enhancement of the CD4 compartment was evident as well. In addition, the IL12 plasmid expanded antigen-specific granzyme B production 2 fold over pSIVGag. Importantly, the combination of IL12/IL15 dramatically enhanced the CD8 antigen-specific granzyme B response induced to pSIVGag vaccine.

Conclusions:  This study illustrates that molecular adjuvants allow multiple immunizations and boosting cycles; co-stimulation as a molecular adjuvant strategy will need more attention; IL12 and IL15 both enhance a plasmid antigen-induced CD8 immune response; and, finally, different cellular profiles can be elicited by each adjuvant. We have illustrated for the first time vaccine-specific control over the direction of an induced immune response using a plasmid vaccine by inducing separate induction of granzyme B induction versus IFNg. This study suggests that custom tailoring of a vaccine response in vivo is possible with DNA-based vaccines.

Keywords: DNA Vaccines; Cytokines; Prophylactic Vaccines