Until recently, the idea of genetically modifying a person’s immune system to make it resistant to HIV was generally viewed as extremely appealing, but so dauntingly impractical that it belonged in the realm of science fiction. Over the past year or so, an accumulation of new data has offered hope that it may eventually be possible to translate the idea into science fact.
Central to these developments is the widely reported case of an HIV-positive individual in Berlin who remains off antiretroviral therapy and free of detectable HIV after receiving a bone marrow transplant from an individual lacking the CCR5 co-receptor due to the delta32 genetic mutation. The bone marrow transplant was received (twice) during a difficult and complicated course of treatment for acute myelogenous leukemia, so the case is not seen as something that can easily be replicated in other people, but rather as a “proof-of-concept” that modifying the immune system can be a means to extinguishing HIV infection.
Complementing this remarkable case report, several studies have described improvements in techniques that may facilitate genetic modification of the immune system. A company called Sangamo Biosciences has developed a technology that allows inactivation of specific genes using enzymes called zinc finger nucleases (ZFNs); the approach is described in detail in the current issue of Nature Reviews Genetics and was also the subject of a New York Times article by Nicholas Wade at the end of last year. An ongoing human trial is using Sangamo’s ZFNs to delete the CCR5 gene in CD4 T cells sampled from HIV positive individuals; the CCR5-negative CD4 T cells are then expanded in number and re-infused into the donor (a very preliminary report from this trial was covered in TAGline earlier this year).
In the current issue of Nature Biotechnology, a research team led by Nathalia Holt describes using the Sangamo technique to successfully modify hematopoietic stem/progenitor cells (HSPC) in mice. The advantage of using HSPCs is that they are the “mother of all cells” and can potentially provide a permanent source of modified immune cells, circumventing the need for altering CD4 T cells in the lab and re-infusing them. Holt’s team was able to show that, in mice, the CCR5-negative HSPCs generated immune cells of multiple types, all lacking CCR5. In “humanized” mice challenged with HIV, these CCR5-negative cells expanded in number and reduced viral load compared to untreated mice with normal CCR5 expression. In an accompanying editorial, Steve Deeks and Mike McCune from UCSF highlight the potential importance of Holt’s findings and outline the implications for future research in humans.
The other recent study that is part of this evolving story was published back in June in the journal Science Translational Medicine. The research group of John Zaia at City of Hope in Duarte, California described results of an experiment in which HSPCs from four individuals with HIV and AIDS-related lymphoma were modified with three anti-HIV genes and re-infused. The modified HSPCs were given along with the infusions of unmodified HSPCs that are a standard part of the protocol for lymphoma treatment. Although it was a small exploratory study, the researchers were encouraged to find evidence that the modified HSPCs had given rise to cells of multiple lineages (e.g. T cells, B cells, macrophages) carrying the anti-HIV genes, albeit at very low numbers.
Yesterday the LA Times published an excellent story by Rachel Bernstein that ties these various gene therapy developments together. It turns out that John Zaia’s group at City of Hope will be the first to use Sangamo’s CCR5-deletion approach to modify human HSPCs, likely also in people with AIDS-related lymphoma initially. The ultimate hope – “reaching for blue sky,” as Deeks & McCune describe it – is that a single shot of souped-up HSPCs may one day be able to equip the immune system with enough HIV-resistant cells to vanquish the virus.
Nature Biotechnology 28, 807–810 (1 August 2010) | doi:10.1038/nbt0810-807
News and Views
Steven G Deeks & Joseph M McCune
Antiretroviral therapy has transformed the treatment of HIV infection, but, despite its profound successes, it will not halt the relentless advance of the epidemic. Against this sobering reality, several promising, recent developments in the basic-science arena have led HIV researchers to envision new therapeutic approaches that would completely eradicate the virus, effectively 'curing' HIV disease.
Nat Biotechnol. 2010 Aug;28(8):839-47. Epub 2010 Jul 2.
Holt N, Wang J, Kim K, Friedman G, Wang X, Taupin V, Crooks GM, Kohn DB, Gregory PD, Holmes MC, Cannon PM.
Keck School of Medicine of the University of Southern California, Los Angeles, California, USA.
CCR5 is the major HIV-1 co-receptor, and individuals homozygous for a 32-bp deletion in CCR5 are resistant to infection by CCR5-tropic HIV-1. Using engineered zinc-finger nucleases (ZFNs), we disrupted CCR5 in human CD34(+) hematopoietic stem/progenitor cells (HSPCs) at a mean frequency of 17% of the total alleles in a population. This procedure produces both mono- and bi-allelically disrupted cells. ZFN-treated HSPCs retained the ability to engraft NOD/SCID/IL2rgamma(null) mice and gave rise to polyclonal multi-lineage progeny in which CCR5 was permanently disrupted. Control mice receiving untreated HSPCs and challenged with CCR5-tropic HIV-1 showed profound CD4(+) T-cell loss. In contrast, mice transplanted with ZFN-modified HSPCs underwent rapid selection for CCR5(-/-) cells, had significantly lower HIV-1 levels and preserved human cells throughout their tissues. The demonstration that a minority of CCR5(-/-) HSPCs can populate an infected animal with HIV-1-resistant, CCR5(-/-) progeny supports the use of ZFN-modified autologous hematopoietic stem cells as a clinical approach to treating HIV-1.
Sci Transl Med. 2010 Jun 16;2(36):36ps30.
Shah PS, Schaffer DV.
Department of Chemical Engineering, University of California, Berkeley, CA 94720-3220, USA.
For the first time, scientists have tested a combination of three RNA-based gene therapies, delivered via a lentiviral vector, to target HIV in patients. This study not only demonstrates the safety and long-term viability of this approach, but also highlights areas in which focused improvements in gene therapy strategies may provide the most impact in increasingly translating promise in the laboratory to efficacy in the clinic.
Sci Transl Med. 2010 Jun 16;2(36):36ra43.
DiGiusto DL, Krishnan A, Li L, Li H, Li S, Rao A, Mi S, Yam P, Stinson S, Kalos M, Alvarnas J, Lacey SF, Yee JK, Li M, Couture L, Hsu D, Forman SJ, Rossi JJ, Zaia JA.
Department of Hematology and Hematopoietic Cell Transplantation, City of Hope, Duarte, CA 91010, USA.
AIDS patients who develop lymphoma are often treated with transplanted hematopoietic progenitor cells. As a first step in developing a hematopoietic cell-based gene therapy treatment, four patients undergoing treatment with these transplanted cells were also given gene-modified peripheral blood-derived (CD34(+)) hematopoietic progenitor cells expressing three RNA-based anti-HIV moieties (tat/rev short hairpin RNA, TAR decoy, and CCR5 ribozyme). In vitro analysis of these gene-modified cells showed no differences in their hematopoietic potential compared with nontransduced cells. In vitro estimates of successful expression of the anti-HIV moieties were initially as high as 22% but declined to approximately 1% over 4 weeks of culture. Ethical study design required that patients be transplanted with both gene-modified and unmanipulated hematopoietic progenitor cells obtained from the patient by apheresis. Transfected cells were successfully engrafted in all four infused patients by day 11, and there were no unexpected infusion-related toxicities. Persistent vector expression in multiple cell lineages was observed at low levels for up to 24 months, as was expression of the introduced small interfering RNA and ribozyme. Therefore, we have demonstrated stable vector expression in human blood cells after transplantation of autologous gene-modified hematopoietic progenitor cells. These results support the development of an RNA-based cell therapy platform for HIV.
Nature Reviews Genetics 11, 636-646 (September 2010) | doi:10.1038/nrg2842
Fyodor D. Urnov1, Edward J. Rebar1, Michael C. Holmes1, H. Steve Zhang1 & Philip D. Gregory1 About the authors
Reverse genetics in model organisms such as Drosophila melanogaster, Arabidopsis thaliana, zebrafish and rats, efficient genome engineering in human embryonic stem and induced pluripotent stem cells, targeted integration in crop plants, and HIV resistance in immune cells — this broad range of outcomes has resulted from the application of the same core technology: targeted genome cleavage by engineered, sequence-specific zinc finger nucleases followed by gene modification during subsequent repair. Such 'genome editing' is now established in human cells and a number of model organisms, thus opening the door to a range of new experimental and therapeutic possibilities.