A bone-marrow stem cell transplant has led to a patient with human immunodeficiency virus (HIV) going into long-term remission, meaning he might become the second person to be cured of the disease.
The breakthrough comes 10 years after the first such case, known as "The Berlin Patient".
Doctors are hopeful the man is now cured, although say it is too early to make a final call.
Speaking with CNN, Dr. Timothy Henrich of the University of California, San Francisco's Department of Medicine - who wasn't involved with the study - said of the new findings that "I do have hope" and "I think that finding a scalable cure that is safe and can be applied to a vast majority of individuals living with HIV is definitely attainable, but we have a lot more work to go".
"Finding a way to eliminate the virus entirely is an urgent global priority, but is particularly hard because the virus integrates into the white blood cells of its host", Gupta explained. As a result, the man ended up with an immune system that was naturally resistant to HIV.
Some 37 million people worldwide are now infected with HIV and the AIDS pandemic has killed about 35 million people worldwide since it began in the 1980s.
According to experts, Brown has been living in the U.S., and is still free of the disease.
Later in 2012, he was diagnosed with advanced Hodgkin's Lymphoma. The donor - who was unrelated - had a genetic mutation known as CCR5 delta 32, which confers resistance to HIV. In these two cases, doctors selected a donor who had an uncommon mutation that made them virtually immune to HIV infection and this mutation was passed on to the recipient. People who have two mutated copies of the CCR5 allele are resistant to the HIV-1 virus strain that uses this receptor, as the virus can not enter host cells.
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As with cancer, chemotherapy can be effective against HIV as it kills cells that are dividing.
The man has chosen to remain anonymous, with scientists referring to him as "the London patient".
But it in the past 18 months he was taken off the extra drugs and regular testing confirmed his viral load is now undetectable. The patient's immune cells remain unable to express the CCR5 receptor.
A full paper detailing the second patient's treatment and remission will be published March 5 in Nature.
Although Brown almost died after he was given strong immunosuppressive drugs and was put into a coma, the "London patient" did not come that close; he suffered from Hodgkin's lymphoma and received a similar bone-marrow transplant to Brown's, but the immunosuppressive drugs he received were gentler. Notable differences were that the Berlin Patient was given two transplants and underwent total body irradiation, while the United Kingdom patient received just one transplant and less intensive chemotherapy.
After HIV-resistant cells from the transplant replaced the men's vulnerable cells, both men stopped taking the ARV therapy that had been suppressing their infections.
That was "an improbable event", said lead researcher Ravindra Gupta of University College London. Note: material may have been edited for length and content. This indicates that other patients, in the same circumstances, should where possible receive transplants from a donor with this same gene mutation.