Gene therapy could potentially be used to treat rheumatoid arthritis and other non-fatal diseases, research suggests.
Arthritis can be very painful
Scientists have shown it is feasible and safe to introduce a new gene to block destructive inflammation in arthritic joints.
A University of Pittsburgh team injected genetically modified cells into the knuckles of nine arthritis patients, with promising results.
The study is published in Proceedings of the National Academy of Sciences.
It was only a preliminary trial designed to test the concept - and much more work will be needed before it becomes clear whether the technique will actually work.
The modified cells were removed after one week as the patients - all women - then underwent surgery to replace the affected knuckle joints.
But the findings may help to calm fears about the safety of gene therapy raised in other trials.
The Pittsburgh team used an inactivated virus to introduce the modified gene into the patients' cells.
The same technique was used by a French team searching for a treatment for severe failure of the immune system.
In that case three children who took part in the study subsequently developed leukaemia, raising doubts about safety.
Because of this, the Pittsburgh team closely monitored their patients for five years, but found no sign of any adverse side effects.
They concede they were working with a different gene, and had a different target to the French team, but argue that their findings suggest gene therapy for rheumatoid arthritis is safe.
In rheumatoid arthritis, immune system cells called macrophages and lymphocytes colonise the lining of joints.
They release proteins called cytokines that bind with the synovial cells that line the joint, causing them to release chemicals that trigger inflammation.
The Pittsburgh team developed a way to modify a gene so that it had the potential to block the process by preventing cytokines from binding to synovial cells.
Cells taken out
As safety was a key concern, the gene was delivered to cells that had been removed from patients, and which were only re-injected several weeks later after tests to determine what effect the treatment was having.
Analysis showed that clusters of cells that expressed large amounts of the modified gene were present at the surface of the synovial tissue - and they produced significantly lower levels of inflammation-causing chemicals.
The researchers hope to carry out new tests in which the modified gene is injected directly into affected tissue.
Ailsa Bosworth, of the UK National Rheumatoid Arthritis Society, said gene therapy was "enormously exciting".
However, she said: "We are aware that this is a very long way away from being a therapy which is proven to be safe and available to patients and therefore aware that much work will need to be done over many years in this field of research."
Dr Madeleine Devey, of the Arthritis Research Campaign, said it had taken many years to overcome the safety problems associated with gene therapy.
"This looks like a really promising advance - which is very good news for patents with arthritis."