Results of a small Phase 1 clinical trial showed that gene therapy made cells of people with HIV resistant to HIV infection and increased CD4 counts. The results were presented Monday by Sangamo Biosciences at the 18th Conference on Retroviruses and Opportunistic Infections (CROI).

“While preliminary, these data are very encouraging and an early validation of the feasibility of our novel gene modification approach for the treatment of HIV/AIDS,” said Dr. Dale Ando, Sangamo’s vice president of therapeutic development and chief medical officer, in a press release.

Antiretroviral therapy has greatly increased life expectancy and improved quality of life for people with HIV. However, it does not cure HIV and often has side effects.

In addition, despite achieving low levels of HIV in the blood, some patients on antiretroviral drugs continue to have poor immune recovery and low CD4 (white blood cell) counts.

Gene therapy, an experimental approach that is currently in early stages of clinical testing, is a possible new treatment method that could one day provide a cure for HIV. Gene therapy involves modifying the genetic information in a cell to, for example, make it resistant to HIV.

In most cases, cells are taken from the patient’s body, genetically modified, and then injected back into the patient.

In the Sangamo trial, carried out jointly with investigators at the University of California at Los Angeles and San Francisco, investigators genetically modified immune cells, called T-cells, to remove a protein called CCR5.

HIV requires the CCR5 protein, which is on the surface of white blood cells, in order to attach to and infect the cell. People naturally born with an alternate form of the CCR5 gene are almost entirely immune to HIV.

The researchers hoped that T-cells, including CD4 cells, in which CCR5 is removed would be immune to the virus and thus able to block viral entry and replication. They also hoped the cells would multiply to make other HIV-resistant T-cells.

The study included six HIV-positive adults on antiretroviral therapy with CD4 counts between 200 and 500 cells per microliter. All the participants had successfully achieved undetectable levels of HIV while on therapy, but had not experienced a sufficient recovery in their CD4 counts.

The researchers collected immune cells from participants, genetically modified them, then reinjected the cells. Patients were monitored weekly for one month and then monthly for 11 months post-treatment.

Results showed that a single treatment with the modified cells resulted in significant and durable CD4 increases in five out of the six participants.

Two weeks after the injection, CD4 counts increased in the five participants by 35 to 1,038 cells per microliter. After a year, CD4 counts had increased by 86 to 911 cells per microliter in these five participants.

The researchers did not observe any lasting improvements in CD4 counts from the procedure in the sixth study participant.

Results also showed that the modified cells exhibited normal growth and activity.

The treatment was well tolerated. The researchers did not observe any major side effects.

The clinical trial is ongoing and is still recruiting participants in Los Angeles and San Francisco; eligible participants must have been on antiretroviral therapy for at least two years and have CD4 counts between 200 and 500 cells per microliter.

Sangamo has recently decided to expand the trial to include people whose antiretroviral therapy has not been effective in suppressing the virus. Total expected enrollment for the trial is 13 participants.

In addition, a second similar gene therapy clinical trial by Sangamo Biosciences is currently underway in Philadelphia and New York City.

This trial is recruiting three groups for treatment: people for whom two or more antiretroviral therapy regimens have failed to effectively control the virus, with viral loads of 2,000 copies per milliliter or higher and CD4 counts of at least 200 cells per microliter; people with successful antiretroviral regimens and undetectable viral loads; and people whose treatment regimens have successfully suppressed their viral loads to undetectable levels but who still have low CD4 counts, less than 500 cells per microliter of blood, after two or more years of treatment.

The researchers expect to enroll around 18 participants.

For more information, please see the press release by Sangamo Biosciences or the abstract at the CROI 2011 website.