They say this procedure could be useful now for those rare people with HIV who have also been diagnosed with cancer and in need of a stem-cell transplant to reconstitute their immune systems. Still, the London case shows that "HIV cures are possible", he said. Brown was treated using stem cells, effectively transplanting his immune system, because he had an unrelated cancer, and the chemotherapy was interfering with the antiretroviral drugs that had previously controlled his infection.
"While this type of treatment is clearly not practical to treat the millions of people around the world living with HIV, reports such as these may help in the ultimate development of a cure for HIV".
Mr Brown underwent similar treatment to the "London patient" in Germany in 2007, which also cleared his HIV. After two bone marrow transplants, Brown was considered cured of his HIV-1 infection. And we've already identified individuals with cells that can't be infected by HIV (they have a mutation that damages or eliminates a protein that HIV uses to attach to cells), who can act as a source of HIV-resistant cells.
Calling the London patient "cured" is tricky, Gupta said, because there is no standard definition for how long someone must remain free of virus and off treatment drugs. "Everybody believed after the Berlin patient that you needed to almost die basically to cure HIV, but now maybe you don't".
Nearly three years after he received bone marrow stem cells from a donor with a rare genetic mutation that resists HIV infection - and more than 18 months after he came off antiretroviral drugs - highly sensitive tests still show no trace of the man's previous HIV infection.
But the London patient's conditioning treatment was far milder than Brown's, leading many to conclude this was probably not responsible for vanquishing the HIV virus. He said simply, "I never thought that there would be a cure during my lifetime".
In the only other known case, Timothy Ray Brown of the USA became HIV-free after a stem cell transplant in Germany 12 years ago and is still free of the virus. Later in 2012, he came to know that he had Hodgkin's lymphoma, a rare kind of blood cancer. But if HIV is surrounded by a sea of CCR5-mutated cells that it can't exploit, it's stymied.
The HIV virus, in green, attaching to a white blood cell, in orange, as seen under a coloured transmission electron microscope.
However, news that a second person may have been "cured" demonstrates that the Berlin Patient was not an anomaly.
He said: "But what we are able to say with certainty is that, through early diagnosis and access to treatment, you can live a long, healthy life with HIV and be confident you won't pass the virus to your sexual partners". He notes that the Berlin patient and the London patient had similar side effects after the treatment. "Some of them are directly related to the Berlin patient and work with transplantation: for example, gene modification therapy".
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