Attenuation of simian immunodeficiency virus SIVmac239 infection by prophylactic immunization with dna and recombinant adenoviral vaccine vectors expressing Gag

Abstract

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.

FIG. 1.

Percentages of circulating CD3⁺ CD8⁺ T lymphocytes that stained positively with the Gag CM9 tetramer. Values are shown for individual animals (identity numbers are in the inset boxes) during the course of immunization. The times of immunization are indicated (black and green arrows for DNA and Ad5 vaccines, respectively).

FIG. 2.

Levels of Gag-specific T lymphocytes for all Mamu-A*01(+) and Mamu-A*01(−) cohorts (as measured by ELISPOT assay against the complete Gag pool [Gag] or a pool excluding the CM9-bearing peptides [Gag-CM9]) as a function of the number of weeks after start of the immunization. The cohort geometric means of the mock-subtracted SFC/10⁶ PBMC values are shown. The standard errors of the geometric means are indicated. Week 39 represents 3 weeks postinfection.

FIG. 3.

Plasma viral loads and CM9(+) cell levels in Mamu-A*01(+)-vaccinated and control groups following intrarectal challenge with SIVmac239.

FIG. 4.

Plasma viral loads in Mamu-A*01(−) vaccinated and control groups following SIVmac239 infection.

FIG. 5.

In situ hybridization of SIV RNA in LN. The frequency of vRNA(+) cells and the amount of vRNA deposited on the FDC network in the germinal centers (GC) are shown for four representative monkeys, 99C010 and 99C066 (Mamu-A*01⁺) as well as 99X029 and 99X040 (Mamu-A*01⁻), at days 45 and 190 postchallenge.

FIG. 6.

Association of reduced peak VL (open circles) and postacute VL (filled circles) with CM9-specific T-cell levels at the time of challenge with either SIVmac239 (A) or SHIV89.6P (B) (28). The postacute viral burden for each animal was calculated as the geometric means of the VL from months 2 and 3. Analyses of correlation between the VL and CM9 levels were conducted using Spearman's rho. The correlation coefficients (R) and P values (one-tailed) for each group are shown.