We have previously reported that the CD8+ T cell response elicited by recombinant adenovirus vaccination displayed a delayed contraction in the spleen. In our current study, we demonstrate that this unusual kinetic is a general phenomenon observed in multiple tissues. Phenotypic analysis of transgene-specific CD8+ T cells present 30 days postimmunization with recombinant adenovirus revealed a population with evidence of partial exhaustion, suggesting that the cells had been chronically exposed to Ag. Although Ag expression could no longer be detected 3 wk after immunization, examination of Ag presentation within the draining lymph nodes demonstrated that APCs were loaded with Ag peptide for at least 40 days postimmunization, suggesting that Ag remains available to the system for a prolonged period, although the exact source of this Ag remains to be determined. At 60 days postimmunization, the CD8+ T cell population continued to exhibit a phenotype consistent with partially exhausted effector memory cells. Nonetheless, these CD8+ T cells conferred sterilizing immunity against virus challenge 7–12 wk postimmunization, suggesting that robust protective immunity can be provided by CD8+ T cells with an exhausted phenotype. These data demonstrate that prolonged exposure to Ag may not necessarily impair protective immunity and prompt a re-evaluation of the impact of persistent exposure to Ag on T cell function.

We have investigated the role of CD4(+) T cells in the development of the CD8(+) T-cell response after immunization with recombinant adenovirus (rAd). In the absence of CD4(+) T cells, the "unhelped" CD8(+) T-cell population exhibited a reduction in primary expansion and long-term survival that appeared to be due to inadequate priming of naïve T cells. There were few functional or phenotypic differences between the helped and unhelped CD8(+) T-cell populations with the exception of O-glycosylated CD43, a marker of effector cells, which was augmented on the unhelped CD8(+) T-cell population. In some cases, the unhelped CD8(+) T-cell population exhibited reduced ability to control virus infection; however, this appeared to be a function of the reduced frequency of antigen-specific CD8(+) T cells. Most notably, the unhelped CD8(+) T-cell population exhibited no defect in secondary expansion. These results provide insight into the role of CD4(+) T cells during the primary CD8(+) T-cell response generated by rAd vaccines and identify potential benefits and issues that must be considered when using adenovirus vaccines under conditions where CD4(+) T-cell function may be limiting, such as vaccination of human immunodeficiency virus patients.