In September 2007, the HIV vaccine field received an unexpected setback when it was announced that the phase IIb efficacy trial of a candidate developed by Merck was being stopped early due to lack of efficacy. The trial was conducted by the HIV Vaccine Trials Network (HVTN) and was referred to as the STEP study. The Merck vaccine aimed to stimulate T cell immunity against HIV, and used a novel attenuated adenovirus serotype 5 (Ad5) vector to deliver the HIV antigens Gag, Pol and Nef. The Ad5 approach was selected because it induced unprecedented levels of HIV-specific CD8 T cell responses in phase I and II trials, with >70-80% of recipients responding (the previous best was a dismal 20-30% of recipients showing low-level CD8 T cell responses after immunization with an ALVAC vector). Although HIV-specific CD8 T cell responses were not anticipated to protect against acquisition of HIV infection, evidence indicated that they might be able to suppress HIV replication and thereby increase the chances of vaccine recipients becoming elite controllers if they became HIV-infected.
The STEP study was stopped after an interim analysis by the Data Safety Monitoring Board revealed that this hoped-for salutary outcome was not being observed. Furthermore, it transpired that certain subgroups of the trial population experienced a significantly increased risk of HIV acquisition associated with receipt of the vaccine. In the overall results, this finding represented a strong trend that did not quite reach statistical significance. However, in a pre-specified analysis that evaluated results based on baseline levels of antibodies to Ad5, there was a stepwise increase in the risk associated with vaccination as anti-Ad5 antibody titers increased, strongly suggesting a real biological effect. Subsequent post-hoc studies revealed that the effect appeared entirely concentrated among uncircumcised men who have sex with men. To their great credit, the HVTN engaged in a massive and extremely transparent effort to investigate this outcome, involving both investigators affiliated with the network and the solicitation of input from external scientists with relevant expertise.
In the time since, a variety of investigations have been conducted but so far no causative mechanism has been identified to explain the STEP results. Unfortunately, along the way, some figures in leadership roles in the HIV vaccine field mistakenly attempted to suggest that the adenovirus vector had been absolved from having any role in enhancing HIV acquisition risk. For example, in 2009, Alan Bernstein (then head of the Global HIV/AIDS Vaccine Enterprise) had this to say to The Scientist:
"[This] result really rules out the possibility that it was the vaccine itself, and the fact that we used Adeno5, that was somehow increasing susceptibility to acquiring [HIV] in those volunteers."
The erroneousness of Bernstein's public claim -- and similar but less public assertions made by some scientists and policymakers in conference hallways -- is highlighted by a new paper published in JAIDS. After many careful analyses, the researchers (from the HVTN and Merck) found that no potential confounder could account for the increased risk of HIV acquisition associated with receipt of the Ad5 vector. The HVTN has continued to be forthright about the issue, and it was these as-yet-unpublished analyses that led the network's principal investigator Lawrence Corey to state in 2010:
“Even though we don't have a mechanism [for Adeno5-mediated enhancement of HIV acquisition risk], we're pretty sure this is a real biologic effect”
Continued efforts to understand the mechanism are important because while Merck's Ad5-based HIV vaccine has been discontinued, adenovirus vectors from an array of different serotypes continue to be studied as potential vaccines against HIV, TB, malaria and hepatitis C, and it is currently uncertain if they might also have the potential to increase HIV acquisition risk.
The most commonly cited hypothesis to explain the STEP outcome was that the vaccine had boosted numbers of Ad5-specific CD4 T cells, thereby increasing the pool of cells potentially susceptible to HIV. But several papers have reported data that is inconsistent with this idea: blood levels of Ad5-specific CD4 T cells did not associate with acquisition risk, and these responses were also rapidly induced in study participants who lacked anti-Ad5 antibodies at baseline (yet these participants experienced no increase in risk). Another notion was that perhaps anti-Ad5 antibodies levels correlate with HIV risk for unknown reasons, but an analysis of the Multi-AIDS Center Cohort (MACS) study did not find an association, and this finding is echoed in a new Journal of Infectious Diseases paper looking at participants in several vaccine trials. A study just published in PLoS One even finds the opposite: the inability of some STEP participants to make neutralizing antibodies against certain Ad5 components appeared to reflect generally weaker immunity and was associated with a greater risk of becoming HIV-infected independent of vaccination.
One remaining possibility that has yet to be thoroughly explored is that the STEP results related to something happening at the mucosal site of exposure, specifically the foreskin of the uncircumcised individuals that experienced the increased HIV acquisition risk. This idea is a subtle variant of the hypothesis related to Ad5-specific CD4 T cells. It is possible that Ad5 antibody titers in STEP were linked to the presence of persistent Ad5 infection, and that vaccination did not boost Ad5-specific CD4 T cell responses in blood but rather at mucosal sites of Ad5 antigen expression. One such site is known to be the gut, but because there are already huge numbers of CCR5-expressing CD4 T cells in that location it would be unlikely that a small additional influx would alter susceptibility to HIV (and this would be consistent with the risk associated with vaccination in STEP being seen in individuals reporting insertive anal intercourse but not receptive, as reported in the new JAIDS paper).
To the best of my knowledge, it is not known whether persistent Ad5 antigen expression can be detected in the foreskin, which would likely be a prerequisite for causing vaccine-boosted Ad5-specific CD4 T cell responses to traffic to -- and be retained at -- that site, potentially increasing susceptibilty to HIV. This type of scenario was suggested as a possible explanation of the STEP results by the research group of Steven Patterson in the discussion section of their PNAS paper in 2009, but still requires further investigation (there is one study that attempted to investigate the possibility in macaques, but because they are not susceptible to natural Ad5 the relevance of the results to humans is unclear). Of particular concern, CD4 T cells targeting adenoviruses cross-react with multiple serotypes (and simian adenoviruses), so the safety of vectors derived from variants other than Ad5 will remain uncertain until the issue is resolved.
Richard Jefferys is the coordinator of the Michael Palm HIV Basic Science, Vaccines & Prevention Project Weblog at the Treatment Action Group (TAG). The original blog post may be viewed here.