Some 10 years ago it was discovered that patients homozygous for a natural mutation (the delta 32 mutation) in the CCR5 gene are generally resistant to HIV infection by blocking virus entry to a cell. Building on this observation, a study published in 2009 reported a potential “cure” in an AIDS patient with leukemia after receiving a bone marrow transplant from a donor with this delta 32 CCR5 mutation. This approach transferred the hematopoietic stem/progenitor cells (HSPC) residing in the bone marrow from the delta 32 donor, and provided a self-renewable and lifelong source of HIV-resistant immune cells. After transplantation, this patient was able to discontinue all anti-HIV drug treatment, the CD4 count increased, and the viral load dropped to undetectable levels, demonstrating an effective transplantation of protection from HIV and suggesting that this approach could have broad clinical utility.
But donors with the delta 32 CCR5 mutation are not generally available, and so how could we engineer an analogous CCR5 negative state in human HSC to be used for bone marrow transplantation, including a patient’s own HSPC? A potential answer comes from zinc finger nucleases (ZFNs) which have been demonstrated to efficiently block the activity of a gene by cleaving the human genome at a predetermined site and altering the genetic sequence via an error-prone DNA repair process. This modification of the cellular DNA is permanent and can fully block gene function. Recently, ZFNs have been shown to inactivate CCR5 in primary human CD4 T cells, allowing them to preferentially survive and expand in the presence of HIV. A human clinical trial evaluating this approach is on-going, in which patient T cells are re-infused after ZFN-treatment to block CCR5 expression and possibly provide an HIV-resistant reservoir of CD4 T cells.
This CIRM Disease Team proposed an approach to modify a patient’s own HSPC to circumvent the need to find matched donors that carry the delta 32 CCR5 mutation and yet provide a renewable and long-lasting source of HIV-resistant cells. Testing of this concept is proposed in selected AIDS lymphoma patients who routinely undergo HSPC transplantation. During the second year of this project, the disease team has made considerable progress and met all the project milestones for year 2. More specifically, the team developed an optimized procedure for efficiently introducing the CCR5-specific ZFNs in HSPC. We showed that these modified cells function normally and retain their “stemness” in tissue culture systems. We also showed these modified cells can be transplanted into mice to reconstitute the immune system. Given HSPC are long lasting stem cells, we have been able to stably detect these cells in mice for over 3 months post-transplantation. The team is in the process of scaling up the cell production procedures to ensure we can generate CCR5-modified HSPC at clinical scale. We are also moving ahead with the remaining pre-clinical safety and efficacy studies required before initiating a clinical trial.