Heterologous envelope immunogens contribute to AIDS vaccine protection in rhesus monkeys

Abstract

Because a strategy to elicit broadly neutralizing anti-human immunodeficiency virus type 1 (HIV-1) antibodies has not yet been found, the role of an Env immunogen in HIV-1 vaccine candidates remains undefined. We sought to determine whether an HIV-1 Env immunogen genetically disparate from the Env of the challenge virus can contribute to protective immunity. We vaccinated Indian-origin rhesus monkeys with Gag-Pol-Nef immunogens, alone or in combination with Env immunogens that were either matched or mismatched with the challenge virus. These animals were then challenged with a pathogenic simian-human immunodeficiency virus. The vaccine regimen included a plasmid DNA prime and replication-defective adenoviral vector boost. Vaccine regimens that included the matched or mismatched Env immunogens conferred better protection against CD4(+) T-lymphocyte loss than that seen with comparable regimens that did not include Env immunogens. This increment in protective immunity was associated with anamnestic Env-specific cellular immunity that developed in the early days following viral challenge. These data suggest that T-lymphocyte immunity to Env can broaden the protective cellular immune response to HIV despite significant sequence diversity of the strains of the Env immunogens and can contribute to immune protection in this AIDS vaccine model.

FIG. 1.

In vitro expression of HXB2/Bal and 89.6P Env by both plasmids and rADV vaccine constructs. The plasmid Env [gp145Δ CFI(R5) and gp145Δ CFI (89.6P)] and rADV [ADV-gp140Δ CFI (R5) and ADV-gp140Δ CFI(89.6P)] vaccine constructs were expressed in vitro, and protein expression was assessed by Western blotting with human anti-HIV IgG.

FIG. 2.

Vaccine-elicited PBMC IFN-γ ELISPOT responses to SIVmac Gag-Pol-Nef and HIV-1 Env. Freshly isolated PBMCs were assessed for IFN-γ ELISPOT responses after in vitro exposure to peptide pools spanning the SIVmac Gag-Pol-Nef and HIV-1 Env proteins. All Env-specific responses were assessed by using peptides that were matched to the Env immunogen. The terms “matched” and “mismatched” refer to the relationship between the Env immunogen and the challenge virus. Arrows indicate time of inoculation with either DNA or rADV immunogens. Data are presented as the total antigen-specific SFC responses to Gag-Pol-Nef and HIV-1 Env per 10⁶ PBMCs and represent the mean values for six monkeys ± standard error.

FIG. 3.

Vaccine-elicited PBMC IFN-γ ELISPOT responses to individual viral proteins assessed 2 weeks following rADV boosting. ELISPOT responses to SIVmac239 Gag and Pol and HIV-1 Env antigens were assessed. Env-specific responses were assessed with peptides that were matched to the Env immunogen, and mock Env-vaccinated monkeys were assayed with 89.6P peptide pools. ELISPOT assays were performed on whole PBMCs or PBMCs depleted of CD8⁺ T lymphocytes. Data are presented as the mean SFC responses to individual viral proteins per 10⁶ PBMCs and represent the mean values for six experimentally vaccinated monkeys ± standard error.

FIG. 4.

Postchallenge peripheral blood CD4⁺ T-lymphocyte counts. These values represent the mean percentage of CD3⁺ CD4⁺ lymphocytes assessed prospectively on all experimental monkeys through day 168 postchallenge.

FIG. 5.

Postchallenge plasma viral RNA levels. These values were determined by an ultrasensitive bDNA amplification assay with a detection limit of 50 copies/ml. The values plotted represent the geometric mean ± standard error at each sampling time for each experimental group of monkeys.

FIG. 6.

Plasma SHIV-89.6P neutralization titers determined from plasma samples obtained from the monkeys following SHIV-89.6P challenge. Neutralization was determined with an MT-2 dye exclusion assay.

FIG. 7.

89.6P Env-specific PBMC IFN-γ ELISPOT responses assessed 1 week following rADV boost and both 3 and 10 weeks following SHIV-89.6P challenge. ELISPOT responses were determined after in vitro exposure of PBMCs (peripheral blood lymphocytes [PBL]) to peptide pools spanning the HIV-1 89.6P Env protein. The bars represent the mean values for six monkeys with the standard error shown.