Background:  AT-2 SIV represents a promising vaccine immunogen since it is non-infectious, yet, as a whole virion, it comprises a wealth of antigenic determinants like those probably seen during natural infection. AT-2 SIV captured by mature dendritic cells (DC) can be presented to stimulate SIV-specific CD4⁺ and CD8⁺ T cells in vitro. We explored whether AT-2 SIV-loaded mature DC could prime SIV-specific immunity in vitro to advance vaccine/therapeutic approaches against HIV.

Methods:  Mature monocyte-derived DC were generated from heparinized blood samples taken from 4 healthy rhesus macaques. Mature DC were incubated with AT-2 SIV E11S (30 ng p27/10⁵ DC, 1 hour at 37^oC), washed and recounted before being subcutaneously re-injected into the donor animal. Aliquots of the cells were kept to monitor DC phenotype by standard 2-color flow cytometry. Each animal received 5 doses of the AT-2 SIV-pulsed DC. Cellular immune responses were monitored (before and after injection) in the blood by measuring the IFN-g release by ELISpot, as well as a variety of cytokines and chemokines by Luminex assay upon in vitro restimulation of the PBMC with AT-2 SIV (vs PHA). Plasma samples were collected to monitor antibodies.

Results:  Subcutaneous injection of AT-2 SIV-pulsed mature DC induced at least low-level SIV-specific IFN-g responses. Repeated immunizations tended to increase the IFN-g released by PBMC even without further AT-2 SIV stimulation in vitro, but additional SIV-specific responses could be detected upon AT-2 SIV in vitro exposure and this increased with repeated boosting. More extended analysis of other cytokine and chemokine responses induced in the AT-2 SIV-stimulated cultures, as well as plasma antibody responses, are being measured.

Conclusions:  These data confirm that mature DC are able to process and present AT-2 SIV for the activation of primary SIV-specific IFN-g responses in vivo. Targeting antigens to mature DC represents an important way to induce stronger, broad-acting virus-specific immunity in naïve and infected individuals.

Keywords: dendritic cells; AT-2 SIV; immunization