Balance of cellular and humoral immunity determines the level of protection by HIV vaccines in rhesus macaque models of HIV infection

Abstract

A guiding principle for HIV vaccine design has been that cellular and humoral immunity work together to provide the strongest degree of efficacy. However, three efficacy trials of Ad5-vectored HIV vaccines showed no protection. Transmission was increased in two of the trials, suggesting that this vaccine strategy elicited CD4+ T-cell responses that provide more targets for infection, attenuating protection or increasing transmission. The degree to which this problem extends to other HIV vaccine candidates is not known. Here, we show that a gp120-CD4 chimeric subunit protein vaccine (full-length single chain) elicits heterologous protection against simian-human immunodeficiency virus (SHIV) or simian immunodeficiency virus (SIV) acquisition in three independent rhesus macaque repeated low-dose rectal challenge studies with SHIV162P3 or SIVmac251. Protection against acquisition was observed with multiple formulations and challenges. In each study, protection correlated with antibody-dependent cellular cytotoxicity specific for CD4-induced epitopes, provided that the concurrent antivaccine T-cell responses were minimal. Protection was lost in instances when T-cell responses were high or when the requisite antibody titers had declined. Our studies suggest that balance between a protective antibody response and antigen-specific T-cell activation is the critical element to vaccine-mediated protection against HIV. Achieving and sustaining such a balance, while enhancing antibody durability, is the major challenge for HIV vaccine development, regardless of the immunogen or vaccine formulation.

Keywords: ADCC; FLSC; HIV vaccine; SIV challenge; protection.

Fig. 1.

Summary of vaccines, immunization schedules, and repeat low-dose SHIV162P3 or SIVmac251 challenges. In all studies, subunit immunizations were intramuscular and the challenges were intrarectal.

Fig. 2.

(A) Acquisition rates (intrarectal SHIV162P3 challenge) for the different experimental groups in study 1. (B) Acquisition rates for animals that received rhFLSC formulated in RC529-SE compared with all other groups in the study (P = 0.071, log-rank test; P = 0.04, Wilcoxon–Mann–Whitney U test). (C) Acquisition rates in study 1 animals dichotomized based on the mean IL-2 ELISPOT responses in the study (spot-forming cells; SFC). A significant difference (P = 0.028, log-rank test) was noted between susceptibility to infection and the magnitude of the response. D, E, and F show, respectively, correlations between the number of challenges required for infection and the rhFLSC162P3-binding antibody response (D), ADCC EC₅₀ titer (E), and A32-competition titer (F). Correlations were tested using Spearman tests; P values and correlation coefficients are shown.

Fig. 3.

(A) Acquisition rates for the groups in study 2. There was a significant (P = 0.03, log-rank test) reduction in the number of SHIV162P3 challenges required for infection in the rhesus macaques vaccinated with rhFLSC compared with the rhesus macaques vaccinated with rhFLSC plus Tat toxoid or rhesus macaques vaccinated with adjuvant alone. (B) Comparisons of IFN-γ ELISPOTS in rhesus macaques vaccinated with rhFLSC plus Tat toxoid versus those immunized with rhFLSC. Statistical comparisons were made by Mann–Whitney–Wilcoxon test; the P value is shown. (C) Comparisons of log ADCC EC₅₀ titers in the group vaccinated with rhFLSC alone versus rhFLSC plus Tat toxoid. Statistical comparisons were made by t tests; the P value is shown. (D) Comparisons of ADCC plateau cytotoxicity in animals vaccinated with rhFLSC alone versus rhFLSC plus Tat toxoid. Statistical comparisons made by Mann–Whitney–Wilcoxon test; the P value is shown. In B–D, mean values are shown with the wide line; bars indicate SEM.

Fig. 4.

(A) Acquisition rates for the groups in study 3. (B) The group that received IL-12 genetic adjuvant showed a significantly lower rate of acquisition compared with all other groups combined (P = 0.024, log-rank test). (C) The group that received IL-12 genetic adjuvant also showed a significantly lower rate of acquisition (P = 0.002, log-rank test) compared with 8 naive control animals in study 3 combined with 51 unimmunized historical controls intrarectally challenged with the same SIVmac251 stock. (D) Pairwise comparisons of study 3 groups for IFN-γ ELISPOT values over weeks 8–46 in the protocol. Anti-SIVmac239 Env responses were plotted over time and used to derive an AUC value for each animal. Statistical comparisons were made by t tests; P values are shown. (E) Pairwise comparisons of study 3 groups for log ADCC EC₅₀ titers measured on the day of first challenge. Statistical comparisons were made by t tests; P values are shown. In D and E, mean values are shown with the wide line; bars indicate SEM. (F) All animals in study 3 were divided based on the presence or absence of ADCC activity on the day of challenge. Animals with ADCC activity were further dichotomized based on whether IFN-γ ELISPOT responses on the day of first challenge fell above or below the 75th percentile of all study 3 animals. A significantly lower rate of SIVmac251 acquisition was seen in the animals exhibiting ADCC activity along with ELISPOT responses lower than the 75th percentile (P = 0.043, log-rank test). Such animals also exhibited acquisition rates lower than macaques with no ADCC activity on the day of first challenge (P = 0.037, log-rank test).