Since American Timothy Brown became the first patient cured of HIV more than a decade ago, there have been three main theories as to how the cancer treatment he and the anonymous London patient underwent managed to unseat HIV in their bodies. He remains free of HIV today.
Gupta, now at Cambridge University, treated the London patient when he was working at University College London.
It was only in 2016 that he was able to access the stem cell donation because he was seeking treatment for the cancer, not the HIB. "This was really his last chance of survival", Gupta said.
In 2008, German doctors at Charité Medical University in Berlin published a paper on a scenario nearly identical to that of the London Patient - a man who had a persistent form of blood cancer was given a bone marrow transplant paired with HIV-resistant stem cells.
Adalja noted that although the Berlin patient and the London patient received similar treatments, the Berlin patient's treatment was more intense - he received two bone-marrow transplants in addition to whole-body irradiation (radiation exposure to the whole body). The procedure is expensive, complex and risky.
The doctors selected a donor who had two copies of a particular mutation in the CCR5 gene that prevents HIV from infecting T-cells, a part of the immune system where the virus takes hold and does its damage. That patient had a leukemia that could be treated with a blood stem-cell transplant, and his transplant team used cells that carried a mutation that eliminates one of the proteins that HIV uses to attach to 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.
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London man seems to be free of HIV in second such case
About 37 million people worldwide now have HIV, and the AIDS virus has killed about 35 million since taking off in the 1980s. Two, the new bone marrow-which is where blood cells are made-is actively eliminating new HIV-infected cells.
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His team plans to use their findings to explore possibilities for future HIV treatment plans. "We need to understand if we could knock out this [CCR5] receptor in people with HIV, which may be possible with gene therapy", he said.
They will present what they have learned so far in the next days in the journal Nature, and at a medical conference in the US city of Seattle, Washington. The first recipient, Timothy Brown, gained fame as the so-called Berlin patient after transplants in 2007 and 2008 rid him of HIV.
"It shows the Berlin patient was not just a one-off, that this is a rational approach in limited circumstances", said Daniel Kuritzkes, chief of infectious diseases at Brigham and Women's Hospital, who was not involved in the study. The man suffered from post-procedure complications whereby the donor's immune cells attacked his own.
Mr Brown said he would like to meet the London patient and would encourage him to go public because "it's been very useful for science and for giving hope to HIV-positive people, to people living with HIV".
There has been only one documented case of HIV, which causes acquired immune deficiency syndrome (AIDS), being cured.
The subject of the new study has been in remission for 18 months after his antiretroviral therapy (ARV) was discontinued. Effectively, some scientists believe that the "London patient" has been cured of the viral infection, which affects close to 37 million people worldwide.