Two men who were HIV-positive appear to be virus-free, registering undetectable levels after bone marrow transplants in Sydney.
These are the first successful cases of HIV being cleared in Australia, according to the University of NSW's Kirby Institute director, Professor David Cooper.
In a significant breakthrough for researchers, one of the patients cleared the virus without donor marrow containing a rare gene mutation that protects against HIV.
The human immunodeficiency virus, which causes AIDS, became undetectable in both patients about three years after their transplants, Prof Cooper said.
The men, who were treated at Sydney's St Vincent's Hospital, in partnership with the Kirby Institute, remain on antiretroviral therapy.
"We're so pleased that both patients are doing reasonably well years after the treatment for their cancers and remain free of both the original cancer and the HIV virus," he said.
The work was presented on Saturday at the Towards an HIV Cure Symposium, which is part of the 20th International AIDS Conference in Melbourne, which opens on Sunday.
The patients' success echoes that of American man Timothy Ray Brown, the famous Berlin patient, who has shown no signs of virus resurgence since he received a bone marrow transplant from a donor with a rare gene mutation conferring resistance to HIV.
This rare gene mutation, called CCR5 delta32, makes stem cells naturally resistant to the virus.
It is found in less than one per cent of Caucasians, mostly northern Europeans.
In Boston, two other patients underwent similar bone marrow transplants in 2012 but the transplanted cells did not contain the rare gene mutation.
In both cases, the virus returned after antiretroviral treatment was stopped.
The first Sydney patient underwent a bone marrow transplant in 2010 for non-Hodgkin lymphoma. His donor had the mutation.
However, the second man who underwent a procedure in 2011 for acute myeloid leukaemia was matched with a donor that did not have it.
Both men no longer showed any trace of the virus after a series of tests, Prof Cooper said.
"This is another example of where the transplant can drive the amount of virus to levels that we simply cannot detect," he said.
"But if we stopped the antiretroviral therapy, there would be a very strong chance that it would come back.
"We're trying to understand this strong anti-HIV effect and understand where the virus might be hiding."
The Sydney cases could lead to new approaches to treating, and ultimately eradicating HIV, he said.
"Cure research is looking for a way to move forward and my view is that this is a very important clue, that an immune response produced by bone marrow transplantation has such a strong anti-HIV effect," Prof Cooper said.
"We're going to use this as a model for cure research and see if we can develop some therapies that mimic what were doing with bone marrow transplantation."
The stem cell transplant procedure, however, is not a practical strategy for the majority of HIV patients, and the risk of mortality is up to 10 per cent, Prof Cooper says.
"For someone with HIV, you certainly would not transplant them when they've got an almost normal lifespan with antiretroviral therapy."
Between two and five HIV positive patients required bone marrow transplants for cancer each year in Australia, he said.
"It is very difficult to find a match for bone marrow donors."
And when a donor and recipient match was found, the chances of then having the one per cent of donors who had the protective gene was going to be very small, he said.
Prof Cooper said there was a movement in the HIV cure community to try to identify these donors with the mutation and ask them to volunteer for bone marrow transplants for HIV-positive people.
