By Ed Cara

A fifth known person has likely been cured of HIV following a specialized stem cell transplant. According to the man’s doctors, he has lived essentially free of the virus for about a decade. Though the treatment isn’t practical for the general population living with HIV, the knowledge gained from these patients may help scientists figure out a more scalable cure down the road.

The 53-year-old German resident, known only as the Düsseldorf patient (after the city in Germany) underwent the procedure more than nine years ago. He needed the stem cell transplant to help treat a case of acute myeloid leukemia, a form of cancer that affects white blood cells. But his doctors had the opportunity to rebuild his immune system with compatible donor bone marrow from someone with a rare genetic mutation that provides natural resistance to HIV-1, the most common type of the virus.

Though the man did experience some health problems over the years (including a brief recurrence of his cancer a few months after the transplant), his HIV viral loads stayed consistently undetectable while he remained on antiretroviral therapy. At the same time, some tests suggested that his body still contained traces of HIV RNA and DNA, while others indicated that no surviving fragment would be able to replicate and restart the infection. Eventually, in 2018, his doctors made the choice to wean him completely off HIV treatment and monitor him closely. Thankfully, more than four years later, the infection has not returned and they feel confident enough to declare him cured of HIV.

“Four years after analytical treatment interruption, the absence of a viral rebound and the lack of immunological correlates of HIV-1 antigen persistence are strong evidence for HIV-1 cure,” they wrote in their paper, published Monday.

There have been four other reports of patients possibly being cured after receiving this kind of stem cell transplant, including two announced last year. Typically, the doctors involved are careful to state that their patient has only achieved long-term remission and that it would take more time to confirm a true cure. But this case now appears to be one of the longest gaps between the procedure and an ongoing HIV-free status.

There have been some improvements over the years in how viable these transplants can be for HIV patients. Last year, doctors reported on a woman who achieved sustained remission after being given umbilical cord blood, which meant that her donor only needed to be a partial match. But these procedures are still a high-risk intervention that come with many potential complications, which is why they’re typically used as a last resort option for other conditions like leukemia. And there’s been at least one recent case where this kind of transplant failed to completely clear a patient’s HIV.

While these limitations mean that donor stem cell transplants will never become a standard HIV cure, they could spark new avenues of research and eventually lead to a truly practical treatment, experts have said. Some researchers are already studying whether it’s possible to genetically engineer a recipient’s own immune cells to become HIV-proof in the lab and then transplant them back, for instance.

In the meantime, the Düsseldorf patient’s doctors say they’ve since treated several other patients with donor stem cells, with similarly positive initial results, though it’s still too early to declare them cured.