Mon Dieu! 30 Years of HIV Science
Leading researchers praise the field’s many successes—but remind us the global campaign against HIV is far from over
By Regina McEnery
Now in its fourth decade, the HIV pandemic already ranks among the most devastating in recorded history. And the scientific response has, in some ways, been historic as well. Scientists now know far more about the canny virus that causes AIDS than they do about any other viral pathogen. Even better, their discoveries have led directly to the development of a robust arsenal of antiviral drugs that have transformed both the treatment and the prevention of HIV. So the 500 scientists gathered at the Institut Pasteur in Paris for the 30 Years of HIV Science meeting May 21-23—marking the 30th anniversary of the discovery of HIV at that storied institution—had reason to feel at least a little proud.
But their discussions focused much more on what the next 30 years might look like. After three decades of HIV science, researchers have a profoundly detailed understanding of how HIV hijacks the immune system and exacts its deadly toll, and have figured out how to tame the virus after it has established infection. What they haven’t yet worked out is how to stop it before it sets up shop in the body, or how to clear it completely once it has.
Part of the problem is that scientists still don’t know how to make a vaccine candidate that elicits broadly neutralizing antibodies (bNAbs), which many researchers believe an AIDS vaccine must induce to prevent infection by the many circulating genetic variants of HIV. And despite growing evidence that HIV might be curable, scientists are just beginning to get a handle on the viral reservoirs that seed lifelong infection.
Longtime director of the US National Institute of Allergy and Infectious Diseases (NIAID) Anthony Fauci praised both the intensive research that has generated powerful antiretroviral therapies, and the global response the drugs have enabled, such as the President’s Emergency Plan for AIDS Relief and the Global Fund to Fight AIDS, Tuberculosis and Malaria. “But as we celebrate extraordinary accomplishments,” he cautioned, “it is important to keep our eye on the target. Much needs to be done.”
A sustained Red Alert
This is indisputably true. Apart from targeting and destroying T cells of the immune system—setting off a destructive cycle that leads to AIDS—HIV appears to induce immune dysfunction in other ways as well. Some scientists believe the virus also overstimulates the immune system, keeping it in such a constant and drawn out state of high alert that it loses its ability to produce immune responses that might control the rapid replication of the virus.
Scientists would like to find new drugs, or perhaps therapeutic vaccine candidates, that dampen or eliminate the effects of such chronic immune activation. So far, however, their efforts have been impeded by an incomplete understanding of the mechanisms of that activation.
Daniel Douek, chief of the Human Immunology Section at NIAID’s Vaccine Research Center has been at the forefront of research linking immune activation and the progression of HIV infection. His laboratory has investigated the biological products associated with microbial translocation—the leakage of toxins and other microbial products across the gastrointestinal barrier and into systemic circulation—which they see as a key driver of immune activation and disease progression (see On the Scientific Trail in Santa Fe, IAVI Report, Jan.-Feb. 2010).
But their attempts at dampening the effects of such activation in rhesus macaques have so far proved disappointing, said Douek. He presented data from a recent study in nonhuman primates designed to assess the impact of blocking a class of secreted immune factors known as type 1 interferons (IFNs) during acute infection by simian immunodeficiency virus (SIV), the monkey version of HIV. While IFNs are known to suppress viral replication, their chronic signaling is also associated with immune activation and disease progression in HIV.
Douek and his collaborators treated six monkeys with an IFN receptor agonist, a drug designed to interfere with the signaling of type 1 IFNs. The researchers then challenged the animals rectally with a pathogenic strain of SIV. They hypothesized that the drug might benefit SIV-infected animals.
In fact, the opposite proved to be the case. Within two weeks, the six animals given the drug had higher SIV RNA levels than a matched group of infected monkeys who had not been given the drug. And it only got worse from there. The treated animals rapidly progressed to AIDS and died within eight months, while the untreated macaques remained alive after 13 months.
Rather than provide a protective effect, Douek said, inhibiting type 1 IFN signaling in acute infection led to long-term loss of viral control and more rapid disease progression in the animals. The big question is, why? Douek’s lab is analyzing the data but has no immediate answers. “It’s difficult to make sense of this,” said Douek. “Clearly, our hypothesis was wrong.”
On the frontlines
Several talks in Paris also centered on eliciting immune responses in mucosal tissues, the soft lining of inner body cavities. Vaccines that stimulate such responses could, in theory, be highly effective against HIV, as the sexually transmitted virus establishes a beachhead in mucosal tissues in the early stages of infection.
Ashley Haase, a researcher at the University of Minnesota, is using an unusual monkey model—one vaccinated with a live-attenuated virus (LAV) SIV vaccine candidate—to study mucosal immunity. Haase’s group employs the model to look specifically at what happens when antibodies are concentrated on the mucosal frontlines to intercept viruses, but thinks it has applications for any mucosal pathogen. The study also carries special implications for the development of vaccines that target gp41, one of the components of the viral spike—or Envelope protein—that HIV and SIV use to infiltrate cells.
In Haase’s study rhesus macaques were immunized intravenously with the LAV SIVmac239Δnef, and challenged vaginally with SIV.
The vaccine regimen itself has a checkered history. In 1992, studies of rhesus macaques suggested that vaccination with a LAV might protect them from SIV. But four years later, high hopes of developing such vaccines against HIV were dashed when the attenuated strain of SIV used in the vaccine regimen mutated into a virulent form, causing disease and death in infant macaques. LAV candidates have since virtually disappeared from the list of strategies favored by AIDS vaccine researchers.
Indeed, Haase is not interested in developing LAVs for vaccines. Rather, his primary interest is in using the macaque model to study mucosal transmission, the most common mode of HIV infection, by sampling tissue immediately after viral challenge. One of the goals of his recent study, he said, was to identify potential correlates of protection—the currently unknown array of immune factors and phenomena that might prevent the establishment of HIV infection.
Haase said when you vaccinate animals with SIVmac239Δnef and then challenge by any route of transmission, there is no significant protection at five weeks. But “wait until 15 weeks or 20 weeks or even 40 weeks” and more than 50% display sterilizing immunity or at least partial protection, he said.
At four and 11 days post-challenge, Haase’s team found little or no evidence of a local founder virus population in the cervical opening of vaccinated animals, in contrast to their findings in unvaccinated animals, where there was a heavy concentration of SIV-infected cells. Haase said there was also no recruitment of CD4+ T cells that might have facilitated systemic infection in the vaccinated animals. “Because of the rapidity of this response, we initially thought potentially about an antibody-mediated protective effect,” said Haase.
When they looked in the cervicovaginal fluid and cervical tissues of vaccinated animals at five and then at 20 weeks, they noticed a striking, five-fold increase in oligomeric forms of gp41—the transmembrane protein of HIV. “What we think is going on is that the immune system is recognizing trimeric stumps of gp41,” said Haase.
Indeed, an early search for clues to what was driving the mucosal immune responses revealed a striking 2.5-fold increase in IgG against gp41 in plasma cells in the submucosa of the cervix. And what initially looked like a wall of plasma cells just underneath the lining of the cervical epithelia turned out to be reserve epithelium expressing neonatal Fc receptor—a protein that plays a role in the transfer of IgG antibodies by recycling and stabilizing the antibodies. “During a woman’s reproductive life reserve epithelium is thought to provide an anatomical barrier,” said Haase. “But here, what we are suggesting is that it provides an immunological barrier as well.”
In collaboration with Dennis Burton’s lab at The Scripps Research Center, Haase’s lab created a soluble form of gp41 minus the membrane proximal external region (MPER) and transmembrane, and tagged it to track down, in the reserve epithelium, specific gp41 trimeric antibodies.
“So for vaccine design, one of the things we think we need to reproduce are antibodies to this trimeric gp41,” said Haase. “But [the model] also shows us that we need to understand the rules that regulate how the mucosal epithelium is [established as] the frontline of the immune system and how active a role it plays in shaping the antibody response to concentrate antibodies on the path of virus entry.”
While a LAV vaccine candidate probably wouldn’t survive regulatory review for human evaluation, Haase said what they’re learning from the monkey model could advance the field. “It may be possible to figure out new rules by which the mucosal immune system concentrates its resources where they are needed.”
A helping hand
A subset of T cells known as T follicular helper (Tfh) cells plays a central role in boosting the antibody response to pathogens by driving the selection and amplification of B-cell clones to generate increasingly effective antibodies. This process, known as affinity maturation, is of special relevance to the evolution of bNAbs to HIV (see A Slew of Science in Seattle, IAVI Report, Mar.-Apr. 2012). Recent research has revealed that bNAbs go through an exceptionally lengthy process of affinity maturation that results in the accumulation of extensive somatic mutations that expands their breadth and potency.
Hideki Ueno, an investigator at the Baylor Institute for Immunology Research in Dallas, has been studying Tfh cells for about seven years. His laboratory recently linked a temporary induction of Tfh cells with a protective antibody response seven days after volunteers received a trivalent seasonal flu vaccine (Sci. Transl. Med. 5, 176ra32, 2013).
Administration of the vaccine induced a temporary increase in CD4+ T cells expressing inducible COStimulator (ICOS), which is expressed almost 100% of the time by Tfh cells found in tonsils. Ueno said the induction of ICOS was largely restricted to CD4+ T cells that co-express CXCR3 or CXCR5, one of a handful of sub-populations of Tfh cells identified in recent years. Up to 60% of the ICOS-expressing T cells in his study were specific to influenza antigens, and co-expressed several cytokines, including interleukin 21 (IL-21), which appears to be secreted by Tfh cells to support the survival of germinal B cells.
In vivo studies indicated that an increase in the ICOS-expressing T cells in blood correlated with an increase in pre-existing antibody titers, but not with the induction of primary antibody responses. And in vitro studies revealed that purified ICOS-expressing T cells efficiently induced differentiation of memory B cells—though not naïve B cells—into plasma cells that produce influenza-specific antibodies ex vivo. All this suggests that the emergence of the ICOS-expressing T cells might serve as an early biomarker for antibody responses, said Ueno.
Although these subsets of Tfh cells may be driving antibody responses in flu vaccinees, Ueno is not sure about their effectiveness in driving B-cell maturation in response to HIV. That’s because whether or not this Tfh subset drives potent and long-lasting germinal center responses remains unknown. Further, among blood Tfh cells, the subset expressing CXCR3 is least effective at helping naïve B cells, suggesting their limited capacity to help B-cell responses.
When CD4+ T cells help naïve B cells, they induce the proliferation and differentiation of B cells into antibody-secreting cells, said Ueno. IL-21 is a potent instigator of both events. Although ICOS+ CXCR3+ Tfh cells in blood can secrete IL-21, their capacity to do so is limited, and thus insufficient to help naïve B cells, said Ueno. Similarly, B cells in germinal centers require IL-21 for their survival and proliferation, and Ueno surmises that CXCR3+ Tfh cells are not efficient at helping germinal center B cells, given their limited IL-21-producing capacity. But Ueno said the jury is still out on how to provoke a strong vaccine-induced antibody response in HIV. “There might be other subsets of Tfh that could be more beneficial in HIV,” said Ueno.
Chasing a cure
Something approaching a media firestorm erupted when researchers announced at the Conference on Retroviruses and Opportunistic Infections, in March, that a toddler in Mississippi appears to have been functionally cured of HIV infection by the early and aggressive administration of antiretroviral therapy (see A Toddler Stole the Show, IAVI Report, Spring 2013).
But most of the clinical data in cure research comes from HIV-infected adults, such as the 14 patients in the VISCONTI cohort who started therapy during the acute phase of infection and continued treatment for several years. What makes the VISCONTI cohort unusual, however, is that its members have been able to control their virus for at least a year after interrupting treatment (PLoS Pathog. 9, e1003211, 2013). While three of the individuals in the VISCONTI cohort carry genes for the major histocompatibility class 1 alleles B57 and B26 that are seen in elite controllers, such alleles are not over-represented in the VISCONTI cohort compared to the general French population.
Asier Sáez-Cirión, an assistant professor at the Institut Pasteur who, along with his colleagues, trawled various HIV databases in France to build the cohort, said the common thread among all 14 individuals seems to be a weak viral reservoir that may actually be shrinking (see Cure Research: Marching on—but over uneven terrain,IAVI Report Special Feature, Sep. 2012).
Sáez-Cirión, who presented an update on the VISCONTI study in Paris, said it could be that early treatment protects the host from chronic immune activation and inflammation associated with more rapid disease progression. Or there could be other host or viral factors at play. In any case, Sáez-Cirión and his colleagues at the Institut Pasteur are now leading an international effort to find individuals like those in the VISCONTI cohort to better characterize the viral reservoirs in such patients. They have already received additional referrals from North and South America, India, and several other European countries. Sáez-Cirión expects the number to grow as researchers and clinicians comb databases.
“What we still don’t know is why some individuals can control and why in others this doesn’t make a clear impact,” Sáez-Cirión noted during his presentation in Paris. “There is something going on. The reservoir is decreasing. We think there is some kind of active control of the reservoir. But we cannot assess this in just 14 patients, so by assembling this larger group of patients, it will help us to make a larger analysis.”
Sáez-Cirión said they will be using the larger cohort of patients to better characterize the virus—and in particular the viral reservoir. They also plan to analyze the impact of treatment during primary and chronic infection in the establishment of the viral reservoir.
Steven Deeks, a professor of medicine at University of California-San Francisco, cautioned that given the many caveats associated with cure strategies, a safe, scalable intervention may ultimately prove impossible. In any case, he said, one that works will take decades to develop. He said current antiretroviral therapy is not fully suppressive in many, and perhaps most, people. Nor have any tests yet been developed to measure viral reservoirs in individuals. But he noted that researchers have made considerable progress in unraveling the mechanisms of HIV persistence. “There is,” he said, “reason to be optimistic.”
One of the HIV cure strategies being studied by Deeks’ lab is the use of drugs to reduce the activation and proliferation of T cells, and their expression of CCR5—a surface protein HIV uses to enter and infect T cells. One such drug, sirolimus (a.k.a. Rapamycin), which is used as an immunosuppressant to prevent organ rejection, is being tested in HIV-infected individuals who have also undergone a kidney transplant.
In a study measuring HIV persistence in 91 HIV-infected kidney recipients, Deeks said they found exposure to sirolimus is in some individuals associated with relative reductions in HIV DNA. This suggests it helped shrink the viral reservoir, though Deeks said the reductions were not dramatic. He also said the approach needs to be used with caution. “This is not a benign drug,” he said.
Nonetheless, he said plans are being developed for a new study to see if such drugs—known as immune modulators—can be used to block T-cell proliferation. “Bob Gallo would have loved this story,” said Deeks, referring to the scientist whose research helped lead to the discovery of CCR5.
In fact, Robert Gallo—who led a National Institutes of Health team that co-discovered HIV 30 years ago—was at the Paris meeting to deliver a much anticipated dinner talk. Part of the buzz stemmed from the other scientist slated to speak, Luc Montagnier, the former Institut Pasteur scientist whose lab also isolated HIV, in 1983. Montagnier bowed out at the last minute. The two labs feuded for a while over who actually discovered the virus first and whose test won the first patent. In 2008, the French team of Montagnier and Françoise Barré-Sinoussi were awarded the Nobel Prize for the discovery of HIV. Gallo was left out.
If there are any lingering bad feelings on Gallo’s part, they weren’t apparent at the dinner. Gallo talked briefly about his early recollections of the AIDS crisis, when he was drawn into studies of what was then a mysterious and entirely new syndrome. “Scientists got involved quite by chance,” he said. “I know I did. When someone challenges you, you take the call.”