On Friday, January 15, the National Institutes of Allergy & Infectious Diseases (NIAID) sponsored a scientific workshop entitled "The Next Challenge: Elimination of HIV Reservoirs." The event took place during the Keystone conference on HIV pathogenesis in Santa Fe, New Mexico. The focus of the agenda was on curing HIV infection, a goal once thought far-fetched but recently made to seem more attainable by a case of apparent HIV eradication, described at the workshop by Jeffrey Laurence in a talk entitled: "Proving the Concept: The First Well-Documented Functional, and Probably Complete, Case of HIV Eradication."
The individual in this case -- which received considerable media attention last year -- is an American living in Berlin who developed acute myeloid leukemia requiring a complex series of treatments over the course of more than a year, including chemotherapy and two stem cell transplants, along with an attendant regimen of immune-suppressive drugs to prevent transplant rejection and graft-versus-host disease (see box for a list).
fludarabine, cytarabine, amsacrine, total body irradiation, antithymocyte globulin (ATG), cyclophosphamide, cyclosporine, mycophenolate mofetil, gemtuzumab
His treating clinician, Gero Hütter, identified 232 potential stem cell donors based on HLA matching and screened 80 of them in order to find an individual homozygous for the CCR5delta32 genotype, knowing that the cells from such a donor lack CCR5 expression and therefore might offer an additional benefit to his HIV-infected patient. A donor was found and, at the second attempt, the stem cells successfully engrafted leading to a complete remission of the acute myeloid leukemia and the repopulation of the individual's immune system with CCR5-negative cells. Antiretroviral therapy (ART) had to be stopped during the procedures and, after the engraftment, Hütter was surprised to observe that HIV viral load did not reappear and consequently ART did not need to be restarted.
At the NIAID workshop, Jeffrey Laurence was able to report that follow up is now out to 1,053 days -- close to three years. HIV remains undetectable in both blood and tissues and the CD4 count is around 800, the highest level since the individual was diagnosed with HIV infection in the 1990s (at the time of the acute myeloid leukemia diagnosis, his CD4 count was 415). After a difficult period of post-transplant recuperation complicated by encephalopathy, the individual is now reported to be regaining health and hopes to soon return to work.
Laurence emphasized his belief that this case represents a compelling proof-of-concept that a cure for HIV infection can be achieved, but also acknowledged the complex set of circumstances involved and the difficulty of delineating all the potential contributing factors. He noted that efforts are underway to assess whether the result can be duplicated; although acute myeloid leukemia is relatively rare, there are likely to be a number of cases every year in people with HIV. Laurence is hoping that there will be another case in which a donor homozygous for CCR5delta32 can be identified, but he stressed that limits on the number of potential stem cell donors that can be screened in the US make this difficult (insurance typically will pay for only 2-10 screens).
In terms of the underlying mechanism for the apparent cure, Laurence stated that leading explanation is simply that, in the absence of the CCR5 co-receptor, HIV had nowhere to go. The immune suppression required to facilitate transplantation might also have depleted HIV reservoirs. There was some evidence that the individual had a minor population of HIV that was CXCR4-tropic (entered cells via the CXCR4 co-receptor as opposed to CCR5) at baseline, and the question was raised as to why this virus did not take over; Laurence acknowledged that there is no proven explanation but noted that the X4 tropism was inferred from the genetic sequence of the virus which may not always accurately predict co-receptor usage. Laurence also cited data published by Lokesh Agrawal a few years ago, which showed that in some individuals homozygous for the CCR5delta32 genotype, infection with both primary X4 and R5 viruses is inhibited, seemingly due to the CCR5delta32 protein downregulating expression of the X4 co-receptor. Additional analyses are being conducted in an attempt to ascertain definitively whether CXCR4-using viruses were present in the individual at baseline.
Carl June from the University of Pennsylvania gave a talk on a less invasive approach to creating a population of CCR5-negative T cells in HIV-infected people. June is conducting a pilot study in which CD4 T cells are sampled from individuals and then manipulated in the laboratory with a technique that snips out the CCR5 gene (the technique was developed by Sangamo Biosciences and involves molecular scissors called zinc fingers). The CCR5-negative CD4 T cells are then re-infused back into the same individual in hopes that they will preferentially survive and expand, providing a population of HIV-resistant cells.
June was only able to report data from the first study participant, who has experienced an increase in overall CD4 T cell numbers and also an increase in the population of CCR5-negative CD4 T cells to numbers greater than those originally infused, indicating that the cells can persist and divide in vivo (the proportion of CCR5-deleted CD4 T cells detectable in the peripheral blood rose from around 1% initially to 2.1% at 140 days of follow up). However, even with this expansion, the CCR5-negative CD4 T cells make up only a minority of the overall CD4 T cell pool. June also noted that HIV viral load rebounded to detectable levels during a pre-planned exploratory interruption of ART. The study is ongoing and due to be completed in 2012. In terms of the practicality of the approach, the total cost of the procedure involved was cited by June as $15,650.
The task of providing a big-picture overview of the challenges involved in curing HIV infection fell to Janet Siliciano, a researcher at Johns Hopkins who has pioneered the study of long-lived HIV reservoirs in the body. Siliciano outlined three deceptively straightforward-sounding tasks that need to be accomplished:
Siliciano reviewed the data suggesting that in most people on combination ART, the first goal has been achieved. This question has been controversial because in most people whose viral load is undetectable using standard assays (which measure down to 50 copies/mL), some residual viral RNA can usually be detected using ultra-sensitive tests which can pick up even a single HIV genome. There has been debate as to whether this residual virus results from ongoing cycles of HIV replication that ART is failing to stop, or rather reflects production of virions by long-lived infected cells which ART cannot impact.
As also described at the workshop by Sarah Palmer (who developed the ultra-sensitive single viral copy assay), most studies that have explored the impact of intensifying treatment have reported no impact on residual viral load, suggesting that ART is typically fully suppressive. The only caveat was offered by Javier Martinez-Picado, who reported data from a small intensification study using the integrase inhibitor drug raltegravir. In this study, about a third of the 45 participants randomized to receive treatment intensification showed evidence of ongoing replication at baseline, which was curtailed by the addition of raltegravir to their regimen.
Addressing the second and third tasks, Siliciano cited data from her group indicating that while the majority of latently HIV-infected cells are long-lived memory CD4 T cells, there also appears to be a stable second source of virus in many individuals that has yet to be identified, but may be a stem cell or long-lived macrophage. The viruses produced by this second source can be identified on the basis that they are genetically identical over time but also genetically different from the virus found in memory CD4 T cells.
Siliciano listed some of the strategies that have been tried to date to deplete HIV reservoirs, without much success. These include approaches that cause mass activation of memory CD4 T cells, which were horribly toxic, and modifiers of gene expression called HDAC inhibitors that might have the potential to eject integrated
HIV DNA from an infected cell's genome. The latter approach, in the form of the drug valproic acid, was initially reported to lower the numbers of latently HIV infected cells in a small trial, but subsequent larger studies have found no effect (other HDAC inhibitors are still under consideration).
The Holy Grail for cure research, as Siliciano explained, is to find an agent or agents that can selectively target only those cells containing HIV. As attention refocuses on the possibility of curing HIV infection, the search for this grail is intensifying. The next major workshop addressing the state of research into curing HIV infection is being chaired by Nobel prize winner Francoise Barre-Sinoussi immediately prior to the 2010 International Conference on AIDS in Vienna. TAG will report back from the workshop later this year and continue to track the research as it progresses.
A number of presentations at the recent Conference on Retroviruses and Opportunistic Infections addressed the issue of whether HIV replication persists on ART, with the findings echoing those presented at the NIAID Workshop. Webcasts of the sessions are available online at the link below. Among the Friday webcasts, look for the themed discussion "Impact of Treatment Intensification on HIV Reservoirs and Immune Activation" which took place at 1:00pm. Later the same day is a talk from Frank Malderelli from the National Cancer Institute's HIV Drug Resistance Program entitled: "HIV Cure: Is it Realistic?"
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