The human immunodeficiency virus (HIV), which causes acquired immunodeficiency syndrome (AIDS), infects more than one million Americans and 35 million individuals worldwide. Unfortunately, no vaccine and cure are yet available for the virus.

Although there are antiretroviral drugs that can control HIV infection and prevent the virus from destroying the immune system, doctors are interested in gene therapy as there are some people who have natural immunity to HIV. Individuals with a certain mutation of the gene called CCR5, for instance, are less likely to be infected with HIV since the gene controls the protein that allows HIV to penetrate a cell.

In a study published in the New England Journal of Medicine March 6, which involved 12 individuals with HIV infection, the researchers took some T-cells from the subjects' blood then engineered the CCR5 gene to carry mutation they needed. They then infused the edited T-cells back into the subjects' blood where they increased in number.

The researchers found that while the unmodified T-cells declined in number, the modified T-cells in the subjects' body did not go down as much, which suggests that the engineered T-cells may have created HIV protection. Of the six subjects who stopped taking their antiretroviral drugs four weeks after the infusion, four experienced a drop in HIV levels and one of them even had his viral level fall below normal detection limit.

"The decline in CCR5-modified cells was significantly less than the decline in unmodified cells, and one of four participants who were evaluated had an undetectable viral load at the time of treatment reinitiation," wrote study author Bruce Walker, an HIV researcher at the Raigon Institute in Boston, and Mark Kay, a geneticist and professor at Stanford University. "Moreover, HIV DNA decreased in the cells of most of the patients."

Although one of the subjects experienced adverse effects and had to be taken to the emergency room after the infusion of the modified cells, experts said the study showed that genome editing is safe.

"The firm conclusion of this study is that genome editing of human cells was safe and associated with an acceptable adverse-event profile and that the cells persisted in vivo," Kay and Walker said. "The tantalizing question raised by the transient treatment interruption is whether it might actually have been partially effective. A definitive answer to this question will require additional studies."