The prophylactic efficacy of DNA and replication-incompetent adenovirus serotype 5 (Ad5) vaccine vectors expressing simian immunodeficiency virus (SIV) Gag was examined in rhesus macaques using an SIVmac239 challenge. Cohorts of either Mamu-A*01(؉) or Mamu-A*01(؊) macaques were immunized with a DNA prime-Ad5 boost regimen; for comparison, a third cohort consisting of Mamu-A*01(؉) monkeys was immunized using the Ad5 vector alone for both prime and boost. All animals, along with unvaccinated control cohorts of Mamu-A*01(؉) and Mamu-A*01(؊) macaques, were challenged intrarectally with SIVmac239. Viral loads were measured in both peripheral and lymphoid compartments. Only the DNA prime-Ad5-boosted Mamu-A*01(؉) cohort exhibited a notable reduction in peak plasma viral load (sevenfold) as well as in early set-point viral burdens in both plasma and lymphoid tissues (10-fold) relative to those observed in the control monkeys sharing the same Mamu-A*01 allele. The degree of control in each animal correlated with the levels of Gag-specific immunity before virus challenge. However, virus control was short-lived, and indications of viral escape were evident as early as 6 months postinfection. The implications of these results in vaccine design and clinical testing are discussed.
The majority of untreated human immunodeficiency virus (HIV) type 1-infected individuals ultimately develop uncontrolled viremia and progressive disease. Cytotoxic T lymphocytes (CTLs) are known to play an important role in controlling HIV-1 replication, which has led to an increasing interest in augmenting conventional antiretroviral therapy with therapeutic vaccination. The successful development of a therapeutic vaccine will rely on the ability to correlate an aspect of the immune response with clinical outcome. In this study, the CD8(+) T cell maturation status of antigen-specific cells in models of well and poorly controlled virus infections were compared, to show that a memory phenotype predominates when antigen loads are absent or low. In HIV-1 infection, the emergence of memory CD8(+) T cells was found to occur only in individuals with highly suppressed viral replication for an extended duration. Such assessments of the immune response may provide a refined measure of virus control.
The cellular immune response plays a pivotal role in controlling the spread of HIV-1 infection by lysing virally infected cells and producing potent antiviral cytokines, such as interferon-gamma (IFN-gamma). Flow cytometric methods have been established to evaluate the contribution of both CD4 and CD8 subsets of T lymphocytes to the immune response to HIV by measuring their production of intracellular IFN-gamma following brief antigenic stimulation. We present a statistical treatment of intracellular cytokine staining (ICS) data that is aimed at establishing the reproducibility and robustness of this assay for use in HIV clinical trials. Comparisons of responses from HIV-seronegative and seropositive individuals were used to establish a 2-fold criterion for distinguishing positive responses with a low probability of false positives (<1%). Additional comparisons established that the reproducibility of the assay is between 1.4 and 2.0-fold depending on the magnitude of the response. Little variability was demonstrated between multiple operators for both the execution and analysis components of these experiments (<10% difference with 95% confidence). We conclude that the statistical criteria established by these analyses allow for the accurate detection and comparison of positive responses. Using these statistical criteria, the ICS assay is sufficiently robust for use in HIV-specific vaccine trials.
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