Two HIV, human immunodeficiency virus, patients who received bone marrow transplants have been able to stop taking their anti-viral drugs without any detectable reappearance of the virus, scientists have announced.
The patients have remained virus-free for 15 weeks and seven weeks after stopping treatment, according to results from tests presented at an international AIDS, acquired immunodeficiency syndrome, conference.
However, Dr Timothy Henrich, one of the doctors involved in the study, from the Brigham and Women's Hospital in Boston, USA, told the BBC that 'it's much too early at this point to use the C-word [cure]'.
Although, he added, 'what we can say is if the virus does stay away for a year or even two years after we stopped the treatment, that the chances of the virus rebounding are going to be extremely low'.
Each of the two men received a bone marrow transplant - where stem cells that produce blood cells are transferred - as they had developed lymphoma, a type of blood cancer. Doctors believe that the transplants wiped out 'viral reservoirs', places in the body where the virus can hide in a dormant state, undetected by the body's immune system.
Both men, dubbed the 'Boston patients', received anti-retroviral therapy (ART) - drugs that prevent HIV spreading in the body - during and after their transplants. As their blood appeared entirely free of any sign of virus years later, the doctors felt it would be justified to take them off therapy, to see if the virus really had gone.
Due to the type of stem cell transplant the Boston patients received, they would still have some of their own blood cells in their body after the operation, which could potentially act as reservoirs for HIV.
After the transplant however, the blood cells produced by the donated stem cells appear to have replaced all of the original blood cells. This may have happened in a 'Graft-versus-host' immune reaction, where the donated cells recognised the patients' original cells as foreign and got rid of them. Meanwhile, the continued ART seemingly stopped the new donor cells from becoming infected, preventing new viral reservoirs from forming.
The Boston patients' progress is reminiscent of the Berlin patient, the first person ever to be considered 'cured' of HIV, following a stem cell transplant after diagnosis of a different blood cancer. But the Berlin patient received stem cells from a donor carrying a genetic mutation, which meant the donated cells were naturally resistant to HIV infection. This would have presented fewer opportunities for viral reservoirs to form.
Whether the Boston patients will be able to stay off anti-HIV drugs, as the Berlin patient has, remains to be seen. 'We still don't know the full implication of the findings that we have', Dr Henrich told the press.
'It is possible that virus could come back next week, it's possible it could come back a few months from now, and it's even possible that it could take one to two years for the virus to return in these patients'.
Even if the Boston patients remain HIV and medication free, this does not mean that bone marrow transplants would become routine therapy in HIV. A transplant is a risky procedure; around a fifth of recipients die within one year of the operation. The Boston patients only received them because their cancers would have otherwise been fatal.
Dr Michael Brady, medical director of the Terrence Higgins Trust, told the BBC: 'While this is by no means a workable cure, it does give researchers another signpost in the direction of one'.