T-cell correlates of vaccine efficacy after a heterologous simian immunodeficiency virus challenge

Abstract

Determining the "correlates of protection" is one of the challenges in human immunodeficiency virus vaccine design. To date, T-cell-based AIDS vaccines have been evaluated with validated techniques that measure the number of CD8(+) T cells in the blood that secrete cytokines, mainly gamma interferon (IFN-gamma), in response to synthetic peptides. Despite providing accurate and reproducible measurements of immunogenicity, these methods do not directly assess antiviral function and thus may not identify protective CD8(+) T-cell responses. To better understand the correlates of vaccine efficacy, we analyzed the immune responses elicited by a successful T-cell-based vaccine against a heterologous simian immunodeficiency virus challenge. We searched for correlates of protection using a viral suppression assay (VSA) and an IFN-gamma enzyme-linked immunospot assay. While the VSA measured in vitro suppression, it did not predict the outcome of the vaccine trial. However, we found several aspects of the vaccine-induced T-cell response that were associated with improved outcome after challenge. Of note, broad vaccine-induced prechallenge T-cell responses directed against Gag and Vif correlated with lower viral loads and higher CD4(+) lymphocyte counts. These results may be relevant for the development of T-cell-based AIDS vaccines since they indicate that broad epitope-specific repertoires elicited by vaccination might serve as a correlate of vaccine efficacy. Furthermore, the present study demonstrates that certain viral proteins may be more effective than others as vaccine immunogens.

FIG. 1.

VSA schematic. (A) PBMC-derived CD8-depleted target cells were activated with concanavalin A (5 μg/ml) for 18 to 24 h. (B) Using part of the PBMC obtained on day −2, we generated effectors (CD3⁺ CD8⁺ T cells) by bead depletion of non-CD3⁺ CD8⁺ cells. (C) We infected target cells with SIVmac239 or SIVsmE660 and then mixed effectors and targets at E:T ratios of 0.1:1, 0.5:1, and 1:1. (D) We maintained the cocultures for 7 days, and then performed intracellular staining of SIV Gag. We evaluated the percentage of maximum suppression by comparing the frequency of SIV Gag⁺ cells in the presence or absence of effectors.

FIG. 2.

Suppression of SIVmac239 replication by vaccine-induced CD8⁺ T cells. Mean percentages of maximum suppression of SIVmac239 replication of individual animals in the vaccinated (A) and control (B) groups at three E:T ratios. (C) After testing for normality and homogeneity of variances, we averaged the mean percentages of maximum suppression from both vaccinated and control groups as described in Materials and Methods. We then compared both groups at each E:T ratio. Error bars represent the standard error of the mean. *, P < 0.05 according to the Student t test.

FIG. 3.

Suppression of SIVsmE660 replication by vaccine-induced CD8⁺ T cells. Mean percentages of maximum suppression of SIVsmE660 replication of individual animals in the vaccinated (A) and control (B) groups at three E:T ratios. (C) After testing for normality and homogeneity of variances, we averaged the mean percentages of maximum suppression from both vaccinated and control groups as described in Materials and Methods. Then, we compared both groups at each E:T ratio. Error bars represent standard error of the mean. *, P < 0.05 according to the Wilcoxon signed-rank test.

FIG. 4.

Comparison between in vitro suppression of SIVsmE660 replication and markers of disease progression. Each symbol represents the mean percentage of maximum suppression from one vaccinee plotted against its corresponding viral load (VLs) (A and B) and absolute CD4⁺ T-cell counts (C and D). In vitro suppression of viral replication was compared to peak VLs (A), set point VLs (B), absolute CD4⁺ T-cell counts in the acute phase (weeks 2 to 3 postinfection) (C), and absolute CD4⁺ T-cell counts in the chronic phase (week 24 postinfection) (D). We determined the correlation coefficients (r) and P values by using the Spearman rank correlation test.

FIG. 5.

Correlations between the magnitude of vaccine-induced T-cell responses and markers of disease progression. Each symbol represents the magnitude of IFN-γ⁺ responses from one vaccinee measured before (A, C, and D) or after (B, E, and F) infection. (A) Total magnitude of prechallenge vaccine-induced T-cell responses compared to peak (left panel) and set point (right panel) viral loads (VLs). (B) Total magnitude of postchallenge vaccine-induced T-cell responses compared to peak (left panel) and set point (right panel) viral loads. (C) Prechallenge response to Vpr compared to set point viral loads. (D) Prechallenge response to Rev compared to absolute CD4⁺ T-cell counts in the acute phase (weeks 2 to 3 postinfection). (E) Postchallenge anamnestic response to Nef compared to set point viral loads. (D) Postchallenge anamnestic CD4⁺ T-cell response to Pol compared to absolute CD4⁺ T-cell counts in the chronic phase (week 24 postinfection). We determined the correlation coefficients (r) and P values by using the Spearman rank correlation test. SFC, spot-forming cells.

FIG. 6.

Correlations between the epitope breadth of vaccine-induced T-cell responses and markers of disease progression. Each symbol represents the epitope breadth of T-cell responses from one vaccinee measured before (A, C, D, and E) or after (B) infection. (A) Total breadth of prechallenge vaccine-induced T-cell responses compared to peak (left panel) and set point (right panel) viral loads (VLs). (B) Total breadth of postchallenge vaccine-induced T-cell responses compared to peak (left panel) or set point (right panel) viral loads. (C) Prechallenge Vif epitope breadth compared to peak VLs. (D) Prechallenge Vif epitope breadth compared to absolute CD4⁺ T-cell counts in the chronic phase (week 24 postinfection). (E) Prechallenge Gag epitope breadth compared to set point viral loads. We determined the correlation coefficients (r) and P values by using the Spearman rank correlation test.