Despite decades of research leading to a vast improvement in our knowledge of the virus and the disease it causes, there is still no cure for HIV.

But one man, the infamous “Berlin Patient” Timothy Ray Brown, received a functional cure for HIV back in 2007. He remains the only individual to have been completely rid of the virus, but scientists were not sure which aspect of his treatment was responsible for curing him. Now, a new study has finally shed some light on this mystery and has eliminated one possible explanation.

Brown was diagnosed with HIV in 1995. He had been taking anti-HIV drugs, or antiretroviral therapy (ART), for 11 years to control his infection before learning that he had developed leukemia. He was given chemotherapy but it failed, so physicians decided to proceed with a bone marrow transplant. The treatment successfully cured his cancer, but interestingly the virus dropped to undetectable levels in his blood and never bounced back, despite Brown ditching ART.

So what happened? Well, scientists think that three different factors could have contributed to the success. First, Brown was given a transplant from an individual with rare a mutation that alters one of the receptors HIV uses to get inside white blood cells. Second, Brown’s own immune system was destroyed by chemotherapy and radiation to prepare him for the transplant. This process, which is known as conditioning, could have killed all the HIV infected cells in his body. Finally, the transplanted cells could have attacked Brown’s own cells, which is known as graft versus host disease, and hence destroyed any remaining HIV reservoirs.

To find out which aspect could be responsible, a team of researchers from Emory University devised a small study on monkeys. As described in PLOS Pathogens, the scientists removed stem cells from the blood of three rhesus macaques and saved them for later. They then infected these monkeys and three controls with a hybrid virus called SHIV, which is a mix of HIV and the simian version. They then gave the infected monkeys ART just like human patients.

Several months later, the three monkeys that had their stem cells removed underwent whole body irradiation. This killed off the majority of their immune cells, including 99% of the white blood cells that HIV predominantly infects. The monkeys then received transplants of their own stem cells that were removed at the start. After the cells successfully engrafted into their bodies, the researchers stopped ART in all 6 monkeys.

Just like when humans stop taking their medication, the virus came bounding back in the controls and 2 of the transplanted monkeys. This animal experienced kidney failure two weeks after ART was stopped and consequently had to be euthanized, meaning no further time points could be investigated. However, SHIV DNA was found in some circulating cells at the time of death, suggesting none of the monkeys had been cured.

According the researchers, this suggests that conditioning (destroying the immune system) is not sufficient to rid patients of HIV. Instead, it is likely that graft versus host disease, the mutation in Brown’s donor or both in combination were responsible. Since the monkeys received their own cells, which do not trigger graft versus host disease, it seems likely that this may be an important factor.

Two other HIV positive leukemia patients have received transplants from donors without the mutation, and while it originally looked like they had been cured, the virus eventually came back. This suggests that while the treatment was not effective, the transplants probably did help to reduce the amount of HIV in their bodies.

While this research did not provide a definitive answer, it has helped eliminate one possibility. The researchers say they will endeavour to solve the mystery and have further work planned.

[Via PLOS Pathogens, Science and Live Science]