Outcome of simian-human immunodeficiency virus strain 89.6p challenge following vaccination of rhesus macaques with human immunodeficiency virus Tat protein

Abstract

The regulatory proteins Nef, Rev, and Tat of human immunodeficiency virus type 1 (HIV-1) are attractive targets for vaccine development, since induction of effective immune responses targeting these early proteins may best control virus replication. Here we investigated whether vaccination with biologically active Tat or inactive Tat toxoid derived from HIV-1(IIIB) and simian-human immunodeficiency virus (SHIV) strain 89.6p would induce protective immunity in rhesus macaques. Vaccination induced high titers of anti-Tat immunoglobulin G in all immunized animals by week 7, but titers were somewhat lower in the 89.6p Tat group. Dominant B-cell epitopes mapped to the amino terminus, the basic domain, and the carboxy-terminal region. Tat-specific T-helper responses were detected in 50% of immunized animals. T-cell epitopes appeared to map within amino acids (aa) 1 to 24 and aa 37 to 66. In addition, Tat-specific gamma interferon responses were detected in CD4+ and/or CD8+ T lymphocytes in 11 of 16 immunized animals on the day of challenge. However, all animals became infected upon intravenous challenge with 30 50% minimal infective doses of SHIV 89.6p, and there were no significant differences in viral loads or CD4+ T-cell counts between immunized and control animals. Thus, vaccination with HIV-1(IIIB) or SHIV 89.6p Tat or with Tat toxoid preparations failed to confer protection against SHIV 89.6p infection despite robust Tat-specific humoral and cellular immune responses in some animals. Given its apparent immunogenicity, Tat may be more effective as a component of a cocktail vaccine in combination with other regulatory and/or structural proteins of HIV-1.

FIG. 1.

Mean reactivities of antibodies obtained on the day of challenge from animals immunized with HIV-1_IIIB (A) or SHIV 89.6p (C) Tat toxoid (solid bars) or with HIV-1_IIIB (B) or SHIV 89.6p (D) Tat (cross-hatched bars) against SHIV 89.6p Tat overlapping peptides 1 to 13 and the carboxy-terminal peptide 86-. Serum samples were analyzed at a 1:1,000 dilution. Hatched rectangles above bar graphs highlight reactive regions and indicate the corresponding amino acid sequences. Error bars each represent the standard error of the mean of OD readings for the four animals. All control animals were nonreactive against all the peptides, with OD values below cutoff (data not shown).

FIG. 2.

Longitudinal assessment of Tat-specific T-helper responses in controls (A) and in animals immunized with HIV-1_IIIB Tat toxoid (B), SHIV 89.6p Tat toxoid (C), HIV-1_IIIB Tat (D), or SHIV 89.6p Tat (E) in response to stimulation with the homologous Tat toxoid. Arrows indicate times of immunization and virus challenge.

FIG. 3.

Tat-specific IFN-γ production in CD4⁺ and CD8⁺ T lymphocytes obtained from HIV-1_IIIB Tat toxoid- and Tat-immunized (A) and SHIV 89.6p Tat toxoid- and Tat-immunized (B) groups on the day of challenge. Shown are the specific responses after subtraction of background levels obtained with unstimulated PBMC. Tat-specific IFN-γ responses were not detected in T cells from control animals (data not shown).

FIG. 4.

Plasma viremia as measured by RNA copies per milliliter and percent CD4⁺ T cells in control and immunized animals following intravenous challenge with 30 MID₅₀ of SHIV 89.6p at week 33. The upper section shows RNA copies per milliliter (i) and percent CD4⁺ T cells (ii) in nonimmunized control animals. Plasma viremia (A through D) and percent CD4⁺ T cells (E through H) in immunized animals are shown in the lower panels.