The role of Ad5-specific CD4 T-cells in enhancing risk of HIV acquisition in the Merck vaccine trial

NOTE: Please note the article below is about a potential complication for researchers to consider. All approved COVID-19 vaccines are highly safe and effective. The risks from COVID-19 are much greater than this theoretical caution raised early in development.

Richard Jefferys, TAG

In September 2007 it was announced that immunizations were being halted in the STEP study, a phase IIb efficacy trial of Merck’s HIV vaccine candidate.

A review of the interim results by the Data Safety Monitoring Board found that there was no possibility of the vaccine showing efficacy for preventing HIV infection, or reducing post-infection viral load levels in vaccine recipients who became infected. These events and the subsequent fallout were covered in grisly detail on this blog.

The most surprising and disturbing finding from STEP was that receipt of the vaccine was associated with a significantly increased risk of HIV acquisition in a subset of trial participants: uncircumcised men with pre-existing antibodies against adenovirus serotype 5 (Ad5). The Merck vaccine construct used an attenuated Ad5 virus vector as a delivery vehicle for the HIV antigens Gag, Pol & Nef. In its natural form, Ad5 causes severe colds and many people are exposed during childhood and hence have anti-Ad5 immune responses.

When the data from STEP showing enhanced risk of acquisition became known, there was understandably much discussion of what the potential mechanism might be. One hypothesis posited that the presence of anti-Ad5 antibodies at baseline was a marker for the presence of Ad5-specific CD4 T cell responses, and immunization with the Ad5 vector activated Ad5-specific CD4 T cell responses, thereby increasing the number of potential target cells for HIV infection in vaccine recipients. This hypothesis was explored to limited extent in one of the papers presenting the STEP results that was published in The Lancet last year by Juliana McElrath and colleagues; in that paper, analyses of Ad5-specific CD4 T cells showed that responses tended to be lower in vaccine recipients who became HIV infected, at least in peripheral blood.

Two new studies just published online in Nature Medicine now offer a more detailed absolution of Ad5-specific CD4 T cells. [1] The results show that baseline antibody titres did not correlate with the magnitude of the Ad5-specific CD4 T cell response (as measured by production of IL-2, interferon gamma, TNF-alpha, MIP-1beta and perforin – Th2-type cytokine production by Ad5-specific CD4 T cells has not yet been evaluated). Furthermore, most individuals who lacked anti-Ad5 antibodies nevertheless displayed Ad5-specific CD4 T cell responses. The studies also demonstrate that receipt of the vaccine rapidly induced both Ad5-specific CD4 T cells and antibodies in people who lacked them at baseline, suggesting that if these responses enhanced the risk of HIV infection then the enhancement should have been seen in all vaccine recipients after the initial immunisations, not just the subset with detectable anti-Ad5 antibodies at baseline.

While these papers make an important contribution to the analyses of what occurred in STEP, there are some caveats. Most critically – and contrary to what has been written in one media story on The Scientist website [2] – the new results do not absolve the Merck HIV vaccine of significantly enhancing the risk of HIV acquisition among uncircumcised men with pre-existing antibodies against Ad5. It is disheartening to read quotes from Alan Bernstein, Executive Director of the Global HIV/AIDS Vaccine Enterprise, erroneously stating otherwise. Also in The Scientist article, Nelson Michael is quoted as saying that including circumcision status as a variable in the multivariate analyses of STEP “washes out” the enhancing effect of vaccination. Based on Susan Buchbinder’s talk at Keystone earlier this year, this is simply not true; what the circumcision data show is that the enhancement effect was most significant in the uncircumcised subgroup with anti-Ad5 antibodies. Additionally, Buchbinder noted that continued follow-up of STEP participants indicates the enhancement effect has waned over time, which adds to the evidence that receipt of the Ad5 vector was responsible.

Another more speculative caveat is that the new data may not entirely rule out a role for Ad5-specific CD4 T cell responses in the trial outcome. One possibility that remains to be studied is whether the presence of persistent Ad5 infection alters the behaviour of Ad5-specific CD4 T cell responses after immunization (i.e. if natural Ad5 antigens are being expressed somewhere in the body, Ad5-specific CD4 T cells would be expected to traffic to those sites). Recent research from Linda Gooding’s group at Emory (abstracts and links appended at the end of the post) has employed PCR to confirm that Ad5 infection can persist in humans. The main cell type infected by Ad5 in these studies was T cells, and activation of infected T cells stimulated Ad5 replication. In the context of the STEP results, these data suggest several questions:

  • Can persistent Ad5 infection be detected in the foreskin?
  • Is there any correlation between Ad5 serostatus and detection of persistent Ad5 infection?
  • Does immunisation with an Ad5 vector lead to any detectable changes in the interactions between Ad5-specific CD4 T cells and Ad5-infected cells?

Juliana McElrath’s Lancet paper cites the need to consider events in the mucosa, stating that Ad5-specific CD4 T cells “could have trafficked to mucosal sites—a process known to occur in natural infection—and thus increased the number of susceptible CD4+ T-cell targets for HIV… To address this possibility, studies are planned to examine lower gastrointestinal tissue and foreskin after immunization for enhanced T-cell activation.” This idea is not unprecedented, as studies from Larry Corey’s laboratory have strongly implicated the persistent presence of activated HSV-2-specific CD4 T cells interacting with HSV-2-infected cells in the mucosa as the explanation for the association between HSV-2 infection and increased susceptibility to HIV acquisition. At the Keystone conference earlier this year, Corey showed that these interactions continue to be detectable even when HSV-2-infected individuals are on chronic suppressive therapy with acyclovir, offering a reason for the failure of the drug to reduce the risk of HIV infection in several large trials.

There is one other slightly uncomfortable caveat to the Nature Medicine papers: both groups of researchers include people working on vaccines using alternative adenovirus serotypes. If evidence did suggest that Ad5-specific CD4 T cells played a role in enhancing risk of HIV acquisition in STEP, this could potentially impact their work because Ad5-specific T cell responses have been shown to cross-react with multiple adenovirus serotypes. Because the alternative adenovirus-based vaccines are being developed for neglected diseases that do not represent profitable vaccine markets, it’s not a case of suspecting significant financial conflicts-of-interest, but the issue should perhaps have been acknowledged in the papers.

Source: TAG basic science project (22 Jul 2009).


  1. Hutnick NA et al. Baseline Ad5 serostatus does not predict Ad5 HIV vaccine–induced expansion of adenovirus-specific CD4+ T cells. Nature Medicine. Brief Communication abstract. Published online: 20 July 2009 | doi:10.1038/nm.1989.

Links to other websites are current at date of posting but not maintained.