Scanning electromicrograph of an HIV-infected T cell.

HIV patients will be given genetically modified stem cells in an attempt to effectively cure them of the infection, the first time this experimental therapy has been tried.

Sangamo Biosciences, based in the Bay Area community of Richmond, received approval from federal regulators Tuesday to conduct the clinical trial.

The therapy involves removing patients' blood-forming stem cells, genetically editing them so they become resistant to HIV infection, and then placing them back into the patients. These stem cells are expected to give rise to new immune system cells with the resistant properties. As part of the process, patients will be given chemotherapy to reduce the number of their own unaltered stem cells.

If successful, the genetically modified stem cells will reconstitute a new immune system that bars the door HIV uses to infect immune cells. That would represent the closest scientists have yet come to a cure, another milestone from the dismal days of the 1980s when HIV was regarded as a virtual death sentence.

It would also represent a major triumph for the California Institute for Regenerative Medicine, the state's stem cell program, which provided $5.6 million for the trial. CIRM has been under pressure in recent years to justify the $3 billion in bond funding California voters granted in 2004 to establish the agency. CIRM is also funding a similar program by Calimmune, a Tucson, Ariz. company that performs research in California.

Researchers say the Sangamo approach is promising, but it remains to be seen how well the replacement stem cells will function. One concern is what effect the remaining, HIV-vulnerable immune cells will have, or whether they will be purged by selection by the viral attack. In addition, the altered stem cells must be examined for any unintentional side effects of their editing.

The treatment is to be given at City of Hope in Duarte. It's an advance on a similar clinical trial already under way by Sangamo. That treatment uses a specific kind of immune cell, a kind called CD4+, that is the main target of HIV. But other immune cell types also appear to be involved. In other words, the ongoing Sangamo trial, now in Phase 2, reconstitutes one critical part of the immune system, but not all.

More than 70 patients have already been treated, and some of them have already been shown to control their HIV without anti-HIV medication, said Geoff Nichol, Sangamo's executive vice president of research and development. But the cells are not "infinitely renewable," Nichols said. So at some point the cells must be replenished with more grown in the lab.

The new trial aims at producing the full spectrum of immune cells by using stem cells. Moreover, the altered stem cells, to be given in a bone marrow transplant, potentially could provide new immune cells for life. Once the transplanted cells are found to have taken up residence and are functioning, patients will be taken off antiretroviral therapy and doctors will watch to see if their new immune system can control HIV without therapy.

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Possible HIV cure OK'd for testing