We report the results of an immune-correlates analysis of the RV144 HIV-1 vaccine efficacy trial. This correlates study was designed to be hypothesis-generating and sensitive for discovering strong correlates of infection risk.23
An identified correlate of infection risk could be a cause of vaccine-induced protection against HIV-1 infection, a surrogate for other unidentified immune responses that are actually responsible for protection, or a marker of HIV-1 exposure or susceptibility to infection.1–3
To determine whether a correlate of infection risk is a cause of vaccine protection, it must be tested in additional clinical vaccine efficacy trials or tested in animal models.1–3
Extensive pilot immunogenicity studies revealed 17 T-cell, antibody, and innate immunity assays that were prioritized into prespecified primary and secondary analyses in order to maximize statistical power in the primary analysis to detect correlates of infection risk.
Of the six assay variables chosen for the primary analysis, two showed significant correlations with infection among vaccine recipients: IgG antibody binding to scaffolded V1V2 Env correlated inversely with infection, and IgA antibody binding to Env correlated directly with infection. These two correlates of risk, taken together, were highly correlated with the infection rate and may generate important hypotheses about immune responses required for protection from HIV-1,1–3
improve the selection of primary end points in subsequent HIV-1 vaccine trials,24
and lead to improved vaccines. If protection conferred by V1V2 IgG antibodies can be confirmed, then the design of vaccines to induce high levels of V1V2 antibodies and low levels of Env-specific IgA antibodies might augment vaccine efficacy.
Several lines of evidence suggest that vaccine-induced antibodies recognize conformational epitopes in the scaffolded V1V2 reagent, which has been shown to detect conformational V1V2 antibodies.25,26
The results of an analysis of breakthrough viruses from patients in the RV144 trial were consistent with immune pressure focused on amino acid patterns in and flanking the V1V2 region of HIV-1 Env.27
This region serves critical functions, such as participating in CD4-receptor and chemokine-receptor binding, binding to α4β7
and serving as the binding site of neutralizing antibodies.29–32
In the Step HIV-1 vaccine trial (ClinicalTrials.gov number, NCT00095576), which was designed to induce HIV-1 T-cell responses, the hazard ratio for HIV-1 infection in the vaccine group as compared with the placebo group was higher in selected subgroups of vaccine recipients.33
Although the notion of antibody-mediated enhancement of intrauterine infection has been raised in a clinical trial of treatment of HIV-1–infected pregnant women with infusion of immune globlulin,34
no vaccine-associated increase in the risk of infection was seen in the RV144 trial, and in analyses that compared infection rates in vaccine-recipient subgroups with the rate in the placebo group, no increase was seen with high levels of vaccine-induced Env-specific plasma IgA antibodies (Fig. S3 in the Supplementary Appendix
). However, a limitation of the analyses that compared infection risk among vaccine and placebo recipients is that the comparator groups could not be randomized, and there may have been residual confounding because the analysis controlled only for sex and baseline behavioral risk factors.
The significant interactions of Env-specific IgA antibodies with other primary variables further support the importance of IgA-binding antibodies in predicting the risk of infection (Table S4 in the Supplementary Appendix
). In vaccinees with low levels of Env-specific IgA antibodies, four of the other five primary variables — IgG avidity, antibody-dependent cellular cytotoxicity, neutralizing antibodies, and Env-specific CD4+ T cells — were inversely correlated with infection, whereas in vaccinees with high levels of Env-specific IgA antibodies, there was no correlation between these variables and infection (Table S4 in the Supplementary Appendix
). The observed interactions generated the hypothesis that plasma IgA antibody levels interfere with protective IgG effector functions, a phenomenon that has been observed with other pathogens,35,36
in the regulation of autoantibody function,37
and in immune responses to cancer.38
We found that vaccinees with IgA antibodies to the first conserved region (C1) of gp120 had a higher risk of infection than vaccinees without these antibodies (odds ratio, 3.15; P = 0.003; q = 0.13). The gp120 C1 region contains an epi-tope that can be a target on the surface of virus-infected cells for antibodies that mediate antibody-dependent cellular cytotoxicity.39
Another possible scenario is that high levels of Env-specific IgA antibodies is a surrogate marker for HIV-1 exposure that was not fully accounted for by adjustment for baseline self-reported behavioral risk factors in the regression models. The primary variable of Env-specific IgA antibodies was not significantly associated with baseline behavioral risk factors (P = 0.28), nor did IgA antibodies to the individual Env proteins included in the primary IgA variable correlate with baseline behavioral risk factors (Table S6 in the Supplementary Appendix
). Plasma IgA is primarily monomeric IgA, whereas mucosal IgA is primarily dimeric.40
Any protective role of mucosal dimeric IgA in the context of HIV-1 vaccination could not be evaluated in the RV144 trial, because mucosal samples were not collected.
The relevance of these findings to different HIV-1 risk populations receiving ALVAC-HIV, AIDSVAX B/E, or other HIV-1 vaccine regimens cannot be inferred and must be prospectively determined. Moreover, further studies are required to determine causality — whether V1V2 antibodies mediate vaccine-induced protection from infection or whether Env-specific IgA antibodies interfere with protection. Nonetheless, the identification of immune correlates of the risk of HIV-1 infection in the RV144 trial provides plausible biologic hypotheses for the original clinical observation of vaccine efficacy.4
Elucidation of the potential roles of V1V2 and Env-specific IgA antibodies in the modulation of HIV-1 infection risk may accelerate the clinical development of vaccine candidates that can improve on the results of the RV144 clinical trial.