HIV Down Under
Political controversy and lack of attention to AIDS vaccine research and at the International AIDS Society's biannual international conference on HIV pathogenesis, treatment, and prevention
By Simon Noble
There was a political slant as the 4th International AIDS Society (IAS) Conference on Pathogenesis, Treatment, and Prevention opened in Sydney on July 22nd. After the demonstration of traditional aboriginal didgeridoo music and dance celebrating indigenous animals and time-honored fishing and hunting practices, the 6700 delegates attending were welcomed by the IAS president and conference co-chair Pedro Cahn of Fundación Huesped in Buenos Aires. He began by recognizing the Sydney Declaration—which states that "Ten per cent of all resources dedicated to HIV programming should be used for research towards optimizing interventions utilized and health outcomes achieved"—and then said that this was the second time prevention was incorporated into this meeting and that it would remain that way for future conferences.
Cahn then turned to the political controversy sparked by Australian Prime Minister John Howard's statement in April that his country should refuse entry to migrants or refugees who are HIV infected. After saying that "good research drives good policy, and we have never been more in need of good policy," Cahn said that "epidemics are not stopped by immigration officers. We are confronting HIV, not people living with HIV." He also touched on the need for greater emphasis on prevention saying that "the epidemiology does not lie; we are falling behind in preventing HIV."
Conference co-chair David Cooper of the University of New South Wales, Sydney, welcomed attendees to the first major international HIV/AIDS conference in Australia and called for an open dialogue and focus on research. He also talked about the recent politics and said that "pointing fingers of blame will do nothing to curtail infection, and threatening to demonize people living with HIV will not help, making recent immigrants from developing countries the alleged culprits will not help," calling for a return to proven prevention and treatment measures underpinned by "political leadership that builds on success rather than undermining it." He also urged the Australian government to recognize that "HIV knows no borders."
Tony Abbott, Federal Minister for Health and Ageing, countered that no conference delegates had been tested to enter the country, and said that this is the way it should remain. But he also noted that permanent residents are required to undergo HIV testing and they will now "have to give enforceable undertakings about treatment," saying that this is to identify and help infected people, not to judge and quarantine them.
The first keynote speaker, Michel Kazatchkine of the Global Fund to Fight Aids, Tuberculosis, and Malaria, talked about how prevention and treatment mutually reinforce each other and are now seen as parts of an integrated approach to the HIV/AIDS response, and said that "health is no longer seen as a happy by-product of development" but as "a necessary investment for development." Similarly, he also stated that the non-profit sector is now seen as a necessary investment for development, and he hoped that, in the near future, access to health would be seen as a human right.
Tony Fauci of the US National Institutes of Health (NIH) talked about pathogenesis as the bedrock of information from which all advances in treatment, prevention, and, ultimately, vaccine development springs. He made the point that "for every person put on treatment, six more are infected," indicating that treatment is unsustainable on a global scale. With regard to prevention he said there was "an awful lot to do," with less than 10% of individuals at risk of HIV infection currently able to access prevention programs, and said that "half of the 60 million HIV infections that we project will occur by 2015 could be averted with a comprehensive scale up of proven prevention strategies." Fauci then talked about the evolving concept of an AIDS vaccine, and said there was some optimism that a "less than perfect vaccine" can be developed that will not prevent infection but instead lower viral set-point, prevent disease progression, and lower the risk of transmission to others.
Alas, despite the warm words about prevention by most of the speakers at the opening session, Fauci was the only one to mention vaccines, and this was perhaps the high point of recognition for AIDS vaccines at the conference. Outside of the satellite symposium on replicating viral vectors (see Reproducing protection) there was only one 90-minute session devoted to oral presentations on AIDS vaccine research and, other than that, not a lot of emphasis was placed on fundamental immunological research.
During the vaccine session, Dennis Burton of The Scripps Research Institute gave a broad summary of HIV-specific neutralizing antibodies (NAbs). He began by saying there were two related questions dominating the field: what specificities and levels of antibody would provide benefit on HIV exposure, and how to design immunogens to elicit and sustain these levels of antibody? Burton said that "we have a good handle" on the specificities of the broadly-neutralizing antibodies (BNAbs) required but that the levels that will be needed were yet to be determined. He does not share the recent enthusiasm for non-neutralizing antibodies, saying they might provide some benefit but in the absence of NAbs he doesn't think this will be substantial.
Burton concentrated on the immunogen design question and outlined three approaches. The first is trimeric Env mimics, but the major stumbling block is that the Env trimer is very unstable. Burton said that "everyone is waiting for the crystal structure of the trimer" to engineer more stable forms and so enable better design of mimics; that structure should be available in two to three years. The second approach is entry intermediates that essentially attempt to target the co-receptor binding site. This is difficult because this step is immediately prior to membrane fusion and so the site may not be spatially accessible to antibody. This approach is currently not being pursued aggressively by many research groups. The third approach is epitope mimics that look to use the molecular information from structural studies of BNAbs in complex with Env to design immunogens, and this is where a lot of the current effort is focused.
Burton then ran through the broadly neutralizing epitopes and their potential to serve as immunogens. The CD4-binding site is "the natural target" for vaccine development since it is highly conserved and accessible, at least to CD4. However, NAb b12 has been the only evidence that this site is immunogenic. The crystal structure of b12 in complex with gp120 core was published recently by Peter Kwong's lab at the Vaccine Research Center at the NIH (Nature 445, 732, 2007), which Burton called "a great advance" that has spurred the design of mimics. Further NAbs directed against the CD4-binding have recently been isolated (Nature Medicine 13, 1032, 2007) that should further accelerate progress with this target.
HIV's glycan shield is also a target, and NAb 2G12 is the prototype that targets this carbohydrate epitope comprised of mannose residues. Researchers are looking to reproduce the array of mannose residues and use that to elicit similar antibodies. Raymond Dwek's group at University of Oxford has found a naturally-occurring homolog with a similar structure in Candida albicans that they are investigating, and Burton's own work in collaboration with others is focusing on trying to synthesize mimics in multivalent presentations, including on phage Qb; both approaches have generated mimics that do bind 2G12.
The other broadly neutralizing epitope of interest is the membrane proximal external region (MPER) which is targeted by a number of NAbs, including 2F5 and 4E10. There are ongoing attempts to reproduce these peptide epitopes, including some in Burton's lab where they've been able to construct peptides that bind well to 4E10 but do not elicit 4E10-like antibodies when inoculated in to mice or rabbits. Burton said that there are a number of possible reasons for this, one of those being the controversial hypothesis advanced by Bart Haynes and colleagues that 2F5 and 4E10 are polyspecific autoantibodies that bind to cardiolipin and so may be susceptible to tolerance (Science 308, 1906, 2005). Burton said he didn't agree with that interpretation, and his group has data that suggests cardiolipin autoreactivity does not explain the difficulties in eliciting MPER-directed antibodies.
Erin Scherer of University of Oxford presented that data in another session. She measured the lipid reactivity of 2F5 and 4E10 in a number of assays, including one developed for the diagnosis of antiphospholipid syndrome—a human autoimmune disease that can include antibodies against cardiolipin—and another to measure general lipid affinity using liposomes of different composition. She found that 2F5 did not bind cardiolipin and that 4E10 had a generalized affinity for lipids, concluding that neither antibody would be a target for tolerance and therefore this mechanism was unlikely to explain the difficulty in eliciting NAbs against HIV.
Burton's presentation was followed by Bruce Walker of Harvard Medical School, who began by warning that he was going to be "a little bit provocative" about the direction in which cytotoxic T lymphocyte (CTL; or CD8+ T cell) research should go. Emphasizing that the first generation AIDS vaccines will control HIV infection rather than completely prevent it, he asked a question he thought that many people were now asking: whether we were "essentially measuring the equivalent of binding non-neutralizing antibodies" with current CTL assays. He questioned the physiologic relevance of ELISPOT assays since they do not involve infectious virus, antigen processing and presentation, or MHC Class I presentation, nor do they measure cytotoxic activity or inhibition of virus production. Rather, they simply reflect engagement of a peptide with its receptor and subsequent interferon (IFN)-g expression.
Walker thinks that measuring the ability to inhibit virus replication and inducing these responses with a vaccine will be vital to successful vaccine strategies. He then described an in vitro virus neutralization assay his group is developing that uses CD4+ T cells exogenously infected with HIV as a substrate to measure the relative ability of CTLs to inhibit virus replication. They have seen between 1,000- and 10,000-fold reduction in virus replication over a 7 to 10 day assay period, and Walker emphasized again that this is measuring something that is very likely functionally relevant.
To ask questions more relevant to vaccine development, his group has also enriched Gag- or Env-specific CTL responses from peripheral blood mononuclear cell (PBMC) samples and compared their relative virus neutralization capacities. A previous publication from Walker and collaborators suggests that Gag-specific CTL responses are associated with lowered viremia in HIV-infected individuals, whereas CTL responses against other proteins, including Env, are associated with higher viremia (Nature Medicine 13, 46, 2007). Similarly, they now see that Gag-specific CTL responses are better at inhibiting virus replication than the Env-specific responses from the same individuals; for every individual tested, "when we enriched for Env-specific responses we got less virus neutralization, when we enriched for Gag we got more virus neutralization," said Walker. This was not due to virus mutants arising within the assay.
Walker concluded by asking if these data were "completely irrelevant because it's done in vitro, or is this something that we really need to think about in terms of where we're heading with vaccines," and what might be the implications for vaccine testing? The virus neutralization assay is laborious but his group is working with IAVI to develop a high-throughput assay for testing samples from vaccine trials. He doesn't envisage that it will ever fully replace simpler assays like the ELISPOT, but it will be very useful to assay a subset of samples from a clinical trial to directly measure virus inhibition by CD8+ T cells.