Background:  Despite the prominent role that adenovirus (Ad) vectors play in HIV vaccine development, the human T cell response to Ad has been largely unstudied. In the recent phase II Ad5 HIV vaccine STEP trial, results showed participants with high Ad5 neutralizing antibody (nAb) titers were at a higher risk of HIV infection. One hypothesis for this result is that Ad-specific T cells present in individuals with pre-existing immunity to Ad5 were activated upon vaccination and served as optimal targets for HIV infection. In light of this finding, we aim to characterize Ad-specific T cell responses before and after Ad-vaccination.

Methods:  Ad-specific T cell responses were studies in 5 groups of Merck phase I subjects with a range of Ad5 doses and vaccination schedules with group E following the same protocol as used for the STEP trial. Vaccinee samples were obtained from weeks 0, 4, 8, 18, 26, 30, 42, 52, and 78. Longitudinal samples were obtained from placebo-matched controls. T cell responses to Ad were measured by stimulating peripheral blood mononuclear cells (PBMC) overnight with whole Ad vector before measuring functionality (interferon-g [IFN-g], tumor necrosis factor-a [TNF-a], interleukin-2 [IL-2], macrophage inhibitory protein-1a [MIP-1a], and perforin) and memory phenotype (CD27, CD45RO, CD57) by 14-color flow cytometry. Statistics were performed using mixed effect models.

Results:  Ad-specific T cell responses were identified in approximately 73% of all vaccinees prior to receiving Ad5 vectors, and their magnitude and presence did not correlate with Ad5 nAb titers. In group E (3 injections of 3x10¹⁰ viral particles Ad5 gag/pol/nef) there was a significant increase in the percentage of Ad-specific CD4⁺ and CD8⁺ cells through the course of vaccination that was not observed consistently within the other vaccine groups. In group E, an expansion of effector-like MIP-1a/perforin-producing CD4⁺ cells was also observed following Ad5 vaccination.

Conclusions:  Repeat vaccination with a high dose (3x10¹⁰) viral particles AdHu5-based vaccine results in the expansion of Ad-specific T cells, especially cells producing IFN-g as has been shown in mouse models. Expansion of cytokine producing cells was largely absent in groups given a lower dose of AdHu5. The unique expansion of effector-like MIP-1a activity by Ad-specific CD4⁺ T cells may render these cells resistant to infection by CCR5-tropic HIV.