Several new papers about the STEP trial - the efficacy study of Merck’s Ad5-based HIV vaccine candidate - have recently been published. As was widely publicized when STEP was stopped in 2007, the vaccine did not show efficacy in preventing HIV infection or slowing disease progression in vaccinees who became infected, and the Ad5 vector actually enhanced the risk of HIV acquisition in a subset of uncircumcised participants with high baseline titers of antibodies against Ad5.
Despite this failure, the newly published analyses offer evidence that the rationale behind the vaccine strategy – the induction of T cell responses targeting HIV – was not entirely flawed. Rather, it appears that in order to have a better chance of offering benefit, vaccine-induced T cells need to target a broader array of HIV components and have a superior ability to kill HIV-infected cells compared to the HIV-specific T cells created by Merck’s candidate.
In a paper in Nature Medicine, Rolland Tovanabutra and colleagues describe their studies of 68 STEP participants who became infected with HIV, 40 from the vaccine group and 28 placebo recpients. By looking at the genetic sequences of the infecting viruses, they find evidence that vaccine-induced CD8 T cell responses did lead to HIV developing mutations associated with immune escape (most significantly in the Gag protein). In other words, the CD8 T cells inhibited viral replication sufficiently to cause the outgrowth of variants able to avoid CD8 T cell recognition. However, as the researchers note, this occurred too rapidly to cause any significant lowering of HIV viral load. On average, Merck’s vaccine only induced a CD8 T cell response to one epitope in each of the proteins it contained (Gag, Pol and Nef), leading the researchers to suggest that a much broader response is needed. In individuals who control viral load well in the absence of treatment, broadly targeted CD8 T cells force HIV to develop multiple escape mutations that impair the ability of the virus to replicate; based on their analyses, Tovanabutra and colleagues propose that this sort of “cornering” strategy needs to be pursued by future T cell-based vaccines.
In the March 15th issue of the Journal of Infectious Diseases, a paper by Daniel Fitzgerald and colleagues offers a detailed description of post-infection outcomes among STEP participants (52 men from the vaccine arm and 35 placebo). The study confirms that receipt of the Merck vaccine had no effect overall on viral load, CD4 T cell counts or time to initiation of antiretroviral therapy (ART). There is one interesting subset analyses, which finds that participants with HLA gene variants known to be associated with slow progression (HLA B27, B57 & B5801) had significantly lower viral loads if they received vaccine compared to placebo. The mean difference was -0.86 log (95% CI -1.52 to -0.20, p=.03). Although subset analyses always need to be interpreted with great caution, this finding is consistent with results obtained in the SIV/macaque model where comparable MHC gene variants have been linked to improved responses to T cell-based vaccines. Human recipients of the ALVAC vaccine possessing HLA B27 and B57 have also been shown to develop superior CD8 T cell responses to HIV antigens. In a commentary accompanying the Fitzgerald paper, Marcus Altfeld and Philip Goulder note that the findings may offer a clue that the right kind of CD8 T cell responses can offer benefit in terms of viral load control.
Bolstering this view is a paper in PLoS Pathogens from Mark Connors research group at the National Institute of Allergy and Infectious Diseases. The study, led by Stephen Migueles, investigated how efficiently CD8 T cells induced by the Merck vaccine could kill HIV-infected CD4 T cells in vitro. Overall, the killing capacity of vaccine-induced CD8 T cells was comparable to that of individuals with progressing HIV infection, and fell short of that observed in long-term non-progressors. But the CD8 T cell responses of vaccine recipients with the B27, B57 and B58 HLA gene variants were significantly better at killing HIV-infected cells compared to individuals lacking those variants, consistent with the subset analysis reported by Daniel Fitzgerald et al. The Migueles study only included a relatively small number of uninfected recipients of the Merck vaccine, but in discussing their results the authors report that “our assay will be applied to participants in the Step study who subsequently acquired HIV infection post-vaccination, including a group of individuals who are restricting HIV replication to the limits of detection in currently available viral load assays. These types of analyses will begin to determine the utility of measurements of cytotoxic capacity in predicting vaccine efficacy, and provide further insight into the mechanisms of immunologic control of HIV.”
Nat Med. 2011 Feb 27. [Epub ahead of print]
Rolland M, Tovanabutra S, Decamp AC, Frahm N, Gilbert PB, Sanders-Buell E, Heath L, Magaret CA, Bose M, Bradfield A, O'Sullivan A, Crossler J, Jones T, Nau M, Wong K, Zhao H, Raugi DN, Sorensen S, Stoddard JN, Maust BS, Deng W, Hural J, Dubey S, Michael NL, Shiver J, Corey L, Li F, Self SG, Kim J, Buchbinder S, Casimiro DR, Robertson MN, Duerr A, McElrath MJ, McCutchan FE, Mullins JI.
We analyzed HIV-1 genome sequences from 68 newly infected volunteers in the STEP HIV-1 vaccine trial. To determine whether the vaccine exerted selective T cell pressure on breakthrough viruses, we identified potential T cell epitopes in the founder sequences and compared them to epitopes in the vaccine. We found greater distances to the vaccine sequence for sequences from vaccine recipients than from placebo recipients. The most significant signature site distinguishing vaccine from placebo recipients was Gag amino acid 84, a site encompassed by several epitopes contained in the vaccine and restricted by human leukocyte antigen (HLA) alleles common in the study cohort. Moreover, the extended divergence was confined to the vaccine components of the virus (HIV-1 Gag, Pol and Nef) and not found in other HIV-1 proteins. These results represent what is to our knowledge the first evidence of selective pressure from vaccine-induced T cell responses on HIV-1 infection in humans.
J Infect Dis. 2011 Mar;203(6):753-5.
Altfeld M, Goulder PJ.
Ragon Institute of Massachusetts General Hospital, Massachusetts Institute of Technology and Harvard, Boston, Massachusetts.
J Infect Dis. 2011 Mar;203(6):765-772.
An Ad5-Vectored HIV-1 Vaccine Elicits Cell-mediated Immunity but does not Affect Disease Progression in HIV-1-infected Male Subjects: Results From a Randomized Placebo-Controlled Trial (The Step Study).
Fitzgerald DW, Janes H, Robertson M, Coombs R, Frank I, Gilbert P, Loufty M, Mehrotra D, Duerr A; for the Step Study Protocol Team.
Center for Global Health, Weill Cornell Medical College, New York, New York.
Background. The Step study was a randomized trial to determine whether an adenovirus type 5 (Ad5) vector vaccine, which elicits T cell immunity, can lead to control of human immunodeficiency virus (HIV) replication in participants who became HIV-infected after vaccination. Methods. We evaluated the effect of the vaccine on trends in HIV viral load, CD4+ T cell counts, time to initiation of antiretroviral therapy (ART), and AIDS-free survival in 87 male participants who became infected with HIV during the Step study and who had a median of 24 months of post-infection follow-up. Results. There was no overall effect of vaccine on mean log(10) viral load (estimated difference between groups, -0.11; P = .47). In a subset of subjects with protective HLA types (B27, B57, B58), mean HIV-1 RNA level over time was lower among vaccine recipients. There was no significant difference in CD4+ T cell counts, time to ART initiation, or in AIDS-free survival between HIV-1-infected subjects who received vaccine versus those who received placebo. Conclusions. HIV RNA levels, CD4+ T cell counts, time to initiation of ART, and AIDS-free survival were similar in vaccine and placebo recipients. There may have been a favorable effect of vaccine on HIV-1 RNA levels in participants with HLA types associated with better control of HIV-1.
PLoS Pathog 7(2): e1002002. doi:10.1371/journal.ppat.1002002
Stephen A. Migueles1, Julia E. Rood1, Amy M. Berkley1, Tiffany Guo1, Daniel Mendoza1, Andy Patamawenu1, Claire W. Hallahan3, Nancy A. Cogliano1, Nicole Frahm2, Ann Duerr2, M. Juliana McElrath2, Mark Connors1*
1 Laboratory of Immunoregulation, National Institute of Allergy and Infectious Diseases, National Institutes of Health, Bethesda, Maryland, United States of America, 2 Vaccine and Infectious Disease Division and the HIV Vaccine Trials Network, Fred Hutchinson Cancer Research Center, Seattle, Washington, United States of America, 3 Biostatistics Research Branch, National Institute of Allergy and Infectious Diseases, National Institutes of Health, Bethesda, Maryland, United States of America
If future HIV vaccine design strategies are to succeed, improved understanding of the mechanisms underlying protection from infection or immune control over HIV replication remains essential. Increased cytotoxic capacity of HIV-specific CD8+ T-cells associated with efficient elimination of HIV-infected CD4+ T-cell targets has been shown to distinguish long-term nonprogressors (LTNP), patients with durable control over HIV replication, from those experiencing progressive disease. Here, measurements of granzyme B target cell activity and HIV-1-infected CD4+ T-cell elimination were applied for the first time to identify antiviral activities in recipients of a replication incompetent adenovirus serotype 5 (Ad5) HIV-1 recombinant vaccine and were compared with HIV-negative individuals and chronically infected patients, including a group of LTNP. We observed readily detectable HIV-specific CD8+ T-cell recall cytotoxic responses in vaccinees at a median of 331 days following the last immunization. The magnitude of these responses was not related to the number of vaccinations, nor did it correlate with the percentages of cytokine-secreting T-cells determined by ICS assays. Although the recall cytotoxic capacity of the CD8+ T-cells of the vaccinee group was significantly less than that of LTNP and overlapped with that of progressors, we observed significantly higher cytotoxic responses in vaccine recipients carrying the HLA class I alleles B*27, B*57 or B*58, which have been associated with immune control over HIV replication in chronic infection. These findings suggest protective HLA class I alleles might lead to better outcomes in both chronic infection and following immunization due to more efficient priming of HIV-specific CD8+ T-cell cytotoxic responses.