Immune-correlates analysis of an HIV-1 vaccine efficacy trial

Abstract

Background: In the RV144 trial, the estimated efficacy of a vaccine regimen against human immunodeficiency virus type 1 (HIV-1) was 31.2%. We performed a case-control analysis to identify antibody and cellular immune correlates of infection risk.

Methods: In pilot studies conducted with RV144 blood samples, 17 antibody or cellular assays met prespecified criteria, of which 6 were chosen for primary analysis to determine the roles of T-cell, IgG antibody, and IgA antibody responses in the modulation of infection risk. Assays were performed on samples from 41 vaccinees who became infected and 205 uninfected vaccinees, obtained 2 weeks after final immunization, to evaluate whether immune-response variables predicted HIV-1 infection through 42 months of follow-up.

Results: Of six primary variables, two correlated significantly with infection risk: the binding of IgG antibodies to variable regions 1 and 2 (V1V2) of HIV-1 envelope proteins (Env) correlated inversely with the rate of HIV-1 infection (estimated odds ratio, 0.57 per 1-SD increase; P=0.02; q=0.08), and the binding of plasma IgA antibodies to Env correlated directly with the rate of infection (estimated odds ratio, 1.54 per 1-SD increase; P=0.03; q=0.08). Neither low levels of V1V2 antibodies nor high levels of Env-specific IgA antibodies were associated with higher rates of infection than were found in the placebo group. Secondary analyses suggested that Env-specific IgA antibodies may mitigate the effects of potentially protective antibodies.

Conclusions: This immune-correlates study generated the hypotheses that V1V2 antibodies may have contributed to protection against HIV-1 infection, whereas high levels of Env-specific IgA antibodies may have mitigated the effects of protective antibodies. Vaccines that are designed to induce higher levels of V1V2 antibodies and lower levels of Env-specific IgA antibodies than are induced by the RV144 vaccine may have improved efficacy against HIV-1 infection.

Figure 1. Sample Selection for the Case–Control Study

Patients enrolled in the RV144 study were vaccinated at weeks 0, 4, 12, and 24, and immune responses at week 26 were evaluated as immune correlates of infection risk. The vaccinated case patients were documented as not having HIV-1 infection at week 24 and as having later received a diagnosis of infection. The vaccine recipients who served as controls were selected from a stratified random sample of vaccine recipients who were documented as not having HIV-1 infection at the last study visit, at 42 months. Of the 7010 HIV-uninfected vaccinated controls eligible for the case–control sample, only those for whom plasma and peripheral-blood mononu-clear cell (PBMC) specimens were available at all later time points and who were not part of previous immunogenicity-testing cohorts (6899 patients) were included. For vaccine recipients who were included in the sample, 6 strata with 1 or more case patients are shown; the remaining strata had 0 case patients and 111 controls. All humoral assays were performed in plasma samples from all case patients and all controls (row A). Data on intra-cellular cytokine staining of PBMCs (row B) were missing for 15% of patients (owing to assay quality-control issues, including an aberrant batch of samples in 24 patients and high values for the assay negative control in 18). Data on multiplex bead assay (Luminex) of PBMCs (row C) were missing for 13% of patients (owing to high values for the assay negative control in 36 patients). Inj denotes injection with vaccine or placebo, and PP per-protocol cohort (i.e., patients who received all four injections as previously described).

Figure 2. Distribution of the Six Primary Immune-Response Variables in Infected and Uninfected Vaccine and Placebo Recipients in the Case–Control Study

Panel A includes the two identified immune correlates of risk. Panel B includes the remaining four primary immune-response variables. Box plots show the 25th percentile (lower edge of the box), 50th percentile (horizontal line in the box), and 75th percentile (upper edge of the box) for the six primary variables, with patients stratified according to HIV-1 infection status and treatment assignment. Additional characteristics of these patients, including sex and immune-response categories, are indicated by the color and shape of the points. Low, medium, and high immune responses at week 26 were used to divide the vaccine group into thirds; medium response is indicated by the gray horizontal bar. Optical density was measured by means of an enzyme-linked immunosorbent assay at a wavelength of 405 nm. Log MFI is the natural log transformation of the median fluorescence intensity (MFI). The avidity score is [response units × (1 ÷ dissociation rate in seconds)] × 10⁻⁵. The partial area between the curves is the sum of the differences over the first four dilutions between the readouts at week 0 and week 26, measured in log₁₀-transformed relative light units (RLUs). The area under the magnitude–breadth curve (AUC-MB) is the average log₁₀-transformed 50% inhibitory concentration in response to a panel of six pseudoviruses. The net percentage of cytokine-expressing CD4+ T cells is the percentage of live CD3+ CD4+ T cells expressing CD154, interleukin-2, interferon-γ, or tumor necrosis factor α minus the negative control value. The I bars indicate the most extreme data points, which are no more than 1.5 times the interquartile range from the box. The distribution plots of the six primary variables and sensitivity variables are shown in Figures S5 through S11 in the Supplementary Appendix.

Figure 3. Estimated Cumulative HIV-1 Incidence Curves for the Six Primary Immune-Response Variables

Because the overall infection rate in the RV144 trial was low, at 0.234 cases per 100 person-years for the vaccine and placebo groups combined, expression of the cumulative HIV-1 incidence curves for the six primary immune-response variables on a scale of infection probabilities from 0.0 to 1.0 does not allow for an analysis of relative cumulative incidences (indicated by the flat cumulative-incidence curves at the bottom of each graph in Panels A and B). The inset for each graph, which shows an expanded lower range of the infection probability scale, reveals patterns of cumulative risk across the participant groups. Panel A includes the two identified immune correlates of risk. Panel B includes the remaining four primary immune-response variables. For each primary variable, 41 vaccinated case patients were stratified into subgroups divided into thirds according to the immune response (low, medium, and high) at week 26 in the vaccine group in the case–control study. The estimated cumulative incidence of HIV-1 infection over time since the measurement of immune response at week 26 is shown for the three vaccine subgroups and for placebo recipients who were negative for HIV-1 infection at week 24.

Figure 3. Estimated Cumulative HIV-1 Incidence Curves for the Six Primary Immune-Response Variables

Because the overall infection rate in the RV144 trial was low, at 0.234 cases per 100 person-years for the vaccine and placebo groups combined, expression of the cumulative HIV-1 incidence curves for the six primary immune-response variables on a scale of infection probabilities from 0.0 to 1.0 does not allow for an analysis of relative cumulative incidences (indicated by the flat cumulative-incidence curves at the bottom of each graph in Panels A and B). The inset for each graph, which shows an expanded lower range of the infection probability scale, reveals patterns of cumulative risk across the participant groups. Panel A includes the two identified immune correlates of risk. Panel B includes the remaining four primary immune-response variables. For each primary variable, 41 vaccinated case patients were stratified into subgroups divided into thirds according to the immune response (low, medium, and high) at week 26 in the vaccine group in the case–control study. The estimated cumulative incidence of HIV-1 infection over time since the measurement of immune response at week 26 is shown for the three vaccine subgroups and for placebo recipients who were negative for HIV-1 infection at week 24.