Stem Cells and Gene Therapy

Today, however, the approach is having a renaissance, thanks to better knowledge of the HIV virus, stem cells, and the lymphoid system, along with advances in technology, says Carl June, professor of pathology and laboratory medicine at the University of Pennsylvania. A handful of researchers are conducting clinical trials that revisit stem cell-based HIV/AIDS gene therapy, and the scientific community is beginning to take notice. "All of those early studies failed," says June, "but I think there are reasons to believe that there's good progress in the field, so that those previous technical barriers can be solved."

At the University of California, Los Angeles AIDS Institute, researchers led by Ron Mitsuyasu are conducting a Phase II clinical trial that seeks to further establish the feasibility of a therapy that inhibits HIV expression through the actions of an anti-HIV gene delivered to the virus in blood-forming stem cells. A retroviral vector encoding an anti-HIV-1 ribozyme is inserted into CD34+ hematopoietic progenitor cells harvested from a patient's peripheral blood, and the cells are then infused back into the patient. In theory, these cells differentiate and repopulate the patient's immune system with HIV-protected mature white blood cells, such as myeloid and T cells. In particular, the ribozyme disrupts viral replication by targeting the tat and vpr regions of HIV RNA, cleaving the virus and rendering it nonfunctional.

A Phase I study (Human Gene Ther, 15:251−62, 2004) using this approach showed moderate success with transduction efficiency and differentiation of the hematopoietic progenitors into mature cells. "To me, the exciting thing scientifically at that point was that we ended up getting what looked like naïve T cells in the peripheral blood that had the transduced gene at some level," says Jerome Zack, Mitsuyasu's collaborator and an associate director of the UCLA AIDS Institute. "It was a low level, but it was present."

The Phase II trial seeks to build upon these successes while providing more robust validation of the basic principles underlying the therapy, according to Mitsuyasu. He says that a major unknown involves how well the transduced stem cells survive among unprotected cells in an HIV-infected patient. "That is the biggest question that we have at the moment," he says, "Once you put the cells in the patient, do those cells proliferate in preference to the non-gene-modified cells?"

Mitsuyasu expects results from this trial, which is tracking 74 HIV patients over two years, sometime within the next year. He and Zack say that the results will have a big impact on the perception of stem cell-based approaches to HIV/AIDS gene therapy. "If it is successful, it could be a single-shot therapeutic or a couple of administrations, and the patient wouldn't have to go through that multiple times," says Zack, adding the caveat that there are still hurdles: the optimization of vectors and therapeutic genes, increasing transduction efficiency, and increasing the prolonged functionality of differentiated, genetically modified immune cells.

The high cost and the cumbersome nature of the treatment − with the cell culture and storage necessary to transduce the hematopoietic progenitors drawn from the patient − make it unrealistic without significant technologic advances, says Mitsuyasu. "But assuming that the technology could improve," he adds, "it could be a viable approach certainly in Western countries that have the resources to pay for this sort of thing."

Others, such as John Rossi at the Beckman Research Institute at City of Hope in Duarte, Calif., are testing similar gene-therapy approaches using stem cells. Rossi is planning a trial that will use a retroviral vector to insert short hairpin RNA, a ribozyme, and a RNA decoy gene into hematopoietic stem cells to attack the HIV virus. "It's our most potent combination," Rossi says. "Using this retroviral vector as the delivery vehicle we feel is the key."