Durable Protection of Rhesus Macaques Immunized with a Replicating Adenovirus-Siv Multigene Prime/protein Boost Vaccine Regimen Against a Second SIVmac251 Rectal Challenge: Role of SIV-specific CD8+ T Cell Responses.

Previously, priming with replication-competent adenovirus-SIV multigenic vaccines and boosting with envelope subunits strongly protected 39% of rhesus macaques against rectal SIVmac251 challenge. To evaluate protection durability, eleven of the protected and two SIV-infected unimmunized macaques that controlled viremia were re-challenged rectally with SIVmac251. Strong protection was observed in 8/11 vaccinees, including two exhibiting <50 SIV RNA copies. Decreased viremia compared to naïve controls was observed in the other three. The SIV-infected unimmunized macaques modestly controlled viremia but exhibited CD4 counts ≤200, unlike the protected macaques. Durable protection was associated with significantly increased SIV-specific ELISPOT responses and lymphoproliferative responses to p27 at re-challenge. After CD8 depletion, 2 of 8 re-challenged, protected vaccinees maintained <50 SIV RNA copies; SIV RNA emerged in 6. Re-appearance of CD8 cells and restoration of SIV-specific cellular immunity coincided with viremia suppression. Overall, cellular immunity induced by vaccination and/or low-level, inapparent viremia post-first SIVmac251 challenge, was associated with durable protection against re-challenge.