Gene therapy escapes the immune response.

Lately, enthusiasm over gene therapy has given way to skepticism. The National Institutes of Health, for instance, is evaluating whether investigators are rushing too quickly to start human trials of gene therapy.

As this reflection goes on, researchers continue to challenge the barriers to gene therapy's success. According to a report in the September Nature Medicine, investigators have taken a crucial step toward overcoming one such barrier. They've found a way to sneak genes past the body's defenses more than once, a feat that might allow repeated gene therapy efforts in the same patient.

One of the largest obstacles to gene therapy is the immune response, a usually welcome defense against viruses or bacteria. Researchers often use crippled viruses, ones that can't replicate, to ferry healing genes into cells. For example, adenoviruses, which cause respiratory infections, are a popular method of targeting the lungs.

But the immune system makes no distinction between good and bad viruses. In animals, immune cells eventually kill any cells infected by the engineered adenoviruses, limiting how long the genes that they have carried in will be active. Moreover, the immune response generates antibodies that neutralize adenoviruses, preventing gene therapists from using that delivery method more than once. That's a major problem for diseases, such as cystic fibrosis, where patients will need repeat doses of curative genes, says James M. Wilson of the University of Pennsylvania Medical Center in Philadelphia.

Therapists are therefore scrambling for ideas to sneak gene-carrying viruses past the immune system. One method may be to briefly suppress immune responses when they introduce gene-loaded viruses, Wilson and coworkers report in Nature Medicine.

Previously, Wilson's group injected mice with antibodies that target CD4 cells, a type of white blood cells active in immunity. They synchronized these injections with the administration of virus-carried genes. The antibodies prevented the immune system from attacking infected cells, as demonstrated by the fact that the introduced genes were still active one month later. Furthermore, since CD4 cells are vital to the production of virus-neutralizing antibodies, the treatment's attack on CD4 cells allowed Wilson's group to effectively use their virus a second time.

Since the treatment with antibodies to CD4 cells may itself generate an immune response in humans, Wilson and his colleagues have explored another concept. When they administered gene-transporting adenoviruses into the lungs of mice, they also injected the animals with either interleukin-12 or gamma-interferon, two natural chemicals that the immune system uses to communicate between cells.

These so-called cytokines prevent the deployment of certain immune cells that activate antibody-producing machines known as B cells. As a result, mucus lining the lungs of these mice produced just one-twentieth as many adenovirus-neutralizing antibodies as those of mice not given cytokines, Wilson's group reports. About 1 month later, the investigators successfully used an adenovirus to deliver a second gene into the lungs.

The central unresolved question, says Savio L.C. Woo, a gene therapy expert at Baylor College of Medicine in Texas, is "Can this be done multiple times?"

In the coming months, predicts Wilson, gene therapy groups will explore many other methods intended to temporarily and safely suppress the immune response against virus-ferried genes.

Investigators, notes Woo, are also designing gene-carrying viruses that elicit much less of an immune response. "In the end, it may take a combination of both strategies," says Woo.