Improved protection of rhesus macaques against intrarectal simian immunodeficiency virus SIV(mac251) challenge by a replication-competent Ad5hr-SIVenv/rev and Ad5hr-SIVgag recombinant priming/gp120 boosting regimen

Abstract

In this study we investigated the ability of a replication-competent Ad5hr-SIVenv/rev and Ad5hr-SIVgag recombinant priming/gp120 boosting regimen to induce protective immunity in rhesus macaques against pathogenic simian immunodeficiency virus(mac251). Immunization of macaques by two sequential administrations of the same recombinants by the same route resulted in boosting and persistence of SIV-specific cellular immune responses for 42 weeks past the initial immunization. Anti-SIV gp120 immunoglobulin G (IgG) and IgA antibodies were induced in secretory fluids, and all macaques exhibited serum neutralizing antibody activity. After intrarectal SIV(mac251) challenge, all of the macaques became infected. However, relative protection, as assessed by statistically significant lower SIV viral loads in plasma at both acute infection and set point, was observed in 8 out of 12 immunized non-Mamu-A(*)01 animals. Elevated mean cellular immune responses to Gag and Env, neutralizing antibody activity, and IgG and IgA binding antibody levels were observed in the eight protected macaques. Statistically significant correlations with protective outcome were observed for cellular immune responses to SIV Env and Gag and for SIV gp120-specific IgG antibodies in nasal and vaginal fluids. Two macaques that exhibited the greatest and most persistent viremia control also exhibited strong CD8(+) T-cell antiviral activity. The results suggest that a spectrum of immune responses may be necessary for adequate control of viral replication and disease progression and highlight a potential role for nonneutralizing antibodies at mucosal sites.

FIG. 1.

Enumeration of IFN-γ-secreting cells in response to SIV Env (A) and Gag (B) peptides over the immunization course. Mean SFC/10⁶ PBMC ± standard errors of the means are reported. Solid arrows mark the Ad5hr-SIV recombinant administrations at weeks 0 and 12, and open arrows indicate the gp120 booster immunizations at weeks 24 and 36.

FIG. 2.

Outcome of SIV_mac251 intrarectal challenge. Viral burdens are reported as SIV RNA copies/ml of plasma. The lower level of detection was <2,000 copies per input plasma volume, which was generally 100 μl. Plasma samples with viral burdens less than the lower limit are plotted as 10,000 copies/ml of plasma.

FIG. 3.

Prechallenge cellular immune responses to SIV Env and Gag peptides in immunized non-Mamu-A∗01 macaques grouped according to their viral burden at set point and in immunized Mamu-A∗01 macaques grouped separately. Mean peak ELISPOT responses ± standard errors of the means are plotted in panel A, and mean responses at the time of challenge ± standard errors of the means are plotted in panel B.

FIG. 4.

Postchallenge cellular immune responses to SIV Env and Gag peptides in immunized and control macaques. Mean ELISPOT responses ± standard errors of the means to Gag and Env peptides for all macaques (both Mamu-A∗01-positive and -negative animals) are shown in panels A and B, respectively. (C and D) Non-Mamu-A∗01 immunized and control macaques are grouped according to their viral burden at set point, and responses to Gag and Env peptides are shown at 2 and 4 weeks postchallenge.

FIG. 5.

Serum antibody in immunized macaques, grouped as described in the legend to Fig. 3, at the time of challenge. Mean binding antibody titers to SIV gp120 ± standard errors of the means are shown in panel A, and mean neutralizing antibody titers to T-cell-line-adapted SIV_mac251 ± standard errors of the means are shown in panel B.

FIG. 6.

SIV gp120-specific binding antibodies in secretory fluids at the time of challenge. Mean IgG titers ± standard errors of the means are shown in panels A and B, and mean IgA absorbance values ± standard errors of the means are shown in panels C and D. Immunized macaques are grouped as described in the legend to Fig. 3.

FIG. 7.

CD8⁺ T-cell antiviral activity in macaques 28 weeks post-SIV_mac251 intrarectal challenge. CD8AA activity assessed by the endogenous and acute suppression assays as described in Materials and Methods is shown in panel A for all immunized macaques (solid bars) and for six of eight control macaques (open bars). Two of the controls had already succumbed to AIDS. (B) The immunized non-Mamu-A∗01 macaques and immunized Mamu-A∗01 macaques are grouped as described in the legend to Fig. 3. Shown are macaques with low viral burden (solid bars), intermediate viral burden (shaded bar), high viral burden (open bar), and Mamu-A∗01 (stippled bar). Mean percent suppression ± standard errors of the means is plotted.