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Thursday, 2 August 2012

Stem Cell Transplants to Cure AIDS?

I am always concerned when the press run stories of cures. The science may well be interesting but I feel compassion for those who have hopes raised.
There was much publicity recently about the man who was "cured of HIV" by a stem cell transplant. He was unfortunate enough to have leukaemia. And he was HIV positive. In the process of treating his leukaemia in 2007, with a procedure known as a stem cell transplant, it appears that the AIDS virus was cleared from his immune system and he remains healthy. It is possible though that HIV still lurks in his brain, beyond the reach of his immune cells.
So does this news offer hope for those who have to take a daily cocktail of antivirals to keep their HIV infection in check?
The stem cells that make up the red bone marrow produce the full range of "white blood cells" that, in turn, make up the immune system. Leukaemia is a disease of the bone marrow - one type of stem cell turns cancerous and the mix of immune cells is thrown wildly off balance, with fatal consequences. Stem cell transplants have superceded "bone marrow transplants". These days the stem cells can be harvested from the donor's blood using a process similar to a single session of dialysis.
If normal chemotherapy fails to cure a case of leukaemia, the only solution is to use stronger drugs to kill off the bone marrow. Then the patient is given a transplant of healthy stem cells from a donor. This rebuilds the immune system with cells that are genetically matched to the donor.
There are many risks and difficulties. While waiting for the transplant to start working, the patient's immune system dwindles to almost nothing for a few weeks, leaving them highly vulnerable to infection. Any pre-existing infections would run riot - so that probably excludes anyone suffering from AIDS. The other problem is finding a suitable donor. We have all heard of people who need a bone marrow transplant but cannot find a suitably matched donor. The problem is the huge variety of possible tissue types. The tissue types of patient and donor need to be very similar if they transplant is to work. Rejection can bring life-threatening complications. So a stem cell transplant is a risky, expensive, treatment-of-last-resort, that is only used in a minority of cases of leukaemia.
 In this remarkable case the patient was able to receive a transplant from another unusual person - someone who had a natural immunity to HIV. These people are extremely rare. So it was not necessarily the transplant that cleared the virus - it might have been the particular genetic makeup of the donated cells.
 So to use this as a treatment for HIV you would need a reasonably healthy patient who has not succumbed to any AIDS-related infections and a matched donor, with a very special, very rare immune system.  
This was not an experiment. An imaginative doctor identifying an opportunity and tried something new. It could well turn out to be a one-off case, thought provoking for scientists, but for the time being (sadly) a million miles away from being a realistic prospect as a treatment for HIV. A cure for HIV may someday be found, but it will result from painstaking research and not from stem cell transplants as we know them today.