Vaccination with tat toxoid attenuates disease in simian/HIV-challenged macaques

Abstract

The Tat protein is essential for HIV type 1 (HIV-1) replication and may be an important virulence factor in vivo. We studied the role of Tat in viral pathogenesis by immunizing rhesus macaques with chemically inactivated Tat toxoid and challenging these animals by intrarectal inoculation with the simian/human immunodeficiency virus 89.6PD. Immune animals had significantly attenuated disease with lowered viral RNA, interferon-alpha, and chemokine receptor expression (CXCR4 and CCR5) on CD4(+) T cells; these features of infection have been linked to in vitro effects of Tat and respond similarly to extracellular Tat protein produced during infection. Immunization with Tat toxoid inhibits key steps in viral pathogenesis and should be included in therapeutic or preventive HIV-1 vaccines.

Figure 1

Immune responses and virus burden at set-point (56 days after inoculation) for immunized and control animals challenged by intrarectal inoculation with SHIV89.6PD. Samples collected before challenge were characterized for anti-Tat serum antibody titers (reciprocal of greatest dilution that was positive by ELISA) and Tat-specific lymphoproliferative responses (stimulation indices). Data were separated according to the plasma viral burden at 56 days after infection (set-point); A shows low viral burden (open symbols), and B (closed symbols) shows high viral burden. Symbols indicate whether animals were immunized with Tat toxoid (circle), Tat toxoid plus gp160 (square), Tat (triangle), or Group B control (asterisk). Data points are labeled with the animal identification numbers.

Figure 2

Immunization decreases plasma virus burden and IFN-α accumulation. The upper four panels (A) show plasma viral RNA levels at 4 weeks after SHIV89.6PD infection in immunized or control macaques. Data are expressed on a semilogarithmic scale with a lower limit of sensitivity at 2,500 copies per ml. The lower four panels (B) show levels of plasma IFN-α on the day of challenge (open bars) and 8 weeks after challenge (solid bars). IFN-α levels were measured as described in the text. Results are expressed as reciprocal of the largest plasma dilution that produced a 50% reduction in the plaque count. Individual data point are coded with the last three digits of each animal identification number.

Figure 3

Changes in CD4 T cell count for control and immunized animals through 8 weeks after virus challenge. CD4⁺ T cell counts were obtained from flow cytometry analysis (to determine CD4 T cell percentage) and the absolute lymphocyte count in blood (from automated cell counts). Data are provided for individual macaques (identified in each legend).

Figure 4

Levels of chemokine receptor expression on circulating CD4⁺ T cells at 28 days after virus inoculation in immunized and control macaques. The ordinate shows the percentage of CD4⁺ T cells that were positive for expression of cell surface CCR5 or CXCR4. The lower levels of CCR5 expression were statistically significant for all immunized groups compared with control (P ≤ 0.002), and the lower levels of CXCR4 were statistically significant for immunized groups compared with control (P ≤ 0.02).