Gene therapy has been successfully used by French researchers to treat severe combined immunodeficiency (SCID). Two babies, aged 8 and 11 months, who received the treatment are no longer in isolation and live at home without any treatment, enjoying normal growth and development for their ages.

The normal functioning of the infants' immune system has lasted 11 months without any side effects so far, and a third patient is making similar progress four months after the gene transfer.

Usually patients with SCID are required to live within tightly controlled and sterile areas to avoid any threats to their non-functioning immune system until a bone marrow transplant is attempted. The new treatment, described in Science (2000; 288:627-9, 669-72), involved providing a normal copy of the defective gene that causes SCID X1, a life threatening disease inherited on the X chromosome.

The defective gene encodes part of a cell receptor that sends signals to the parents of T and NK cells. Without this gene's directions the cells do not develop, grow or spread, and patients with SCID X1 are left vulnerable to even slight infectious assaults on the body.

The new gene unblocks the development of immune cells, restoring the immune system to normal functioning. The treatment, carried out at the Necker Hospital in Paris, began with the harvesting of bone marrow from the patients. The stem cells were then bathed in a growth factor in containers coated with a fibronectin fragment (a protein that boosts gene transfer) and were infected with a retrovirus carrying the replacement gene.

After three days of repeated infection, the stems cells were put back into the patients. Within two weeks, Alain Fischer (one of the authors of the paper) and the team were able to detect new cells with the correct version of the gene. Both children now have cell counts comparable to those of normal children of the same age.

Dr Fischer said that the children will have to be monitored throughout their lives, both to ensure their continued good health and to monitor the long term success of the treatment.

The success is being seen as a welcome milestone for gene therapy, which experienced a setback last year when an 18 year old patient in the United States died unexpectedly.