Hutch virologist and defeatHIV Co-Director Jerome said the goal of cure researchers is to go even gentler. “Our long-term goal is to develop a simple and inexpensive version of this cure, which can be offered to everyone living with HIV, regardless of where they are in the world,” he said.

Brown stopped taking antiviral drugs at the time of this first transplant; the London Patient stayed on his anti-HIV medications throughout the first 17 months of his transplant recovery. He stopped them only 18 months ago.

Both patients received blood stem cells from donors who carried HIV-resistant genes. The so-called CCR5 mutation deletes part of a protein on the surface of blood cells that HIV uses like a mechanical gate to pry its way in and infect the cell. About 1 percent of people of northern European ancestry carry that mutation from both their mothers and fathers. Another 10 percent of people from those northern climes carry at least one of those genes.

The donors for both Brown and the London Patient carried two copies of that mutation, one from the mother and one from the father, a “homozygous” trait that makes them virtually immune to HIV infection.

Transplant as an HIV cure: many attempts, few successes

As the Nature authors noted, the apparent cure of a second patient shows that Brown’s success “was not an anomaly.” Yet the news is tempered by the fact that it took doctors 12 years to repeat his favorable result.

One reason it has taken so long, Jerome said, is that it is difficult to find that 1 percent of donors who carry both copies of the rare gene. HIV-positive patients who require a transplant to survive cancer are also unfortunately rare, because they are often too sick to qualify for the difficult procedure.

Dr. Carl Dieffenbach, who heads AIDS research for the National Institutes of Health and who was in Seattle for CROI, said that experiments like these — which put patients at great risk — are still appropriate because the people involved are so desperately ill.

“These therapies are used in situations where the life of the patient is on the line,” he said. “It is important to remember there have been a significant number of attempts to achieve this, and many other cases resulted in the death of the patient from their cancers. That just points to the difficulty.”

Dr. Monique Nijhuis of the University Medical Center of Utrecht, the Netherlands, and a co-author of the Nature paper, described significant efforts in Europe to provide transplants to HIV-positive patients. Through a consortium, IciStem, doctors have amassed a registry of 22,000 potential transplant donors who carry HIV-resistant genes. The European group has also been tracking results from 39 HIV-positive blood stem cell transplant patients, 26 of whom are still living. The London Patient is one of those cases.

In the Nature paper, the authors said they know of only one other case of a patient who was transplanted and also interrupted his HIV treatment like Brown and the London Patient have done. That case involved a German known as the “Essen Patient,” and after stopping antivirals he experienced a rapid rebound of HIV.

In another closely watched set of cases, three HIV-positive patients from the Boston area received transplants of blood stem cells that did not carry the mutation. The idea was to see if the rigorous transplant procedure alone, rather than the mutant cell types, cleared HIV. In fact, the virus did not rebound quickly, but it eventually did, at 12, 32 and 41 weeks, respectively, for the three.