The second-ever leukemia patient to receive an infusion of CAR T cells—which had been harvested from his blood and modified to produce receptors that recognize cancer cells—is still cancer free more than 10 years later. Tumor-fighting immune cells are still circulating in his body, according to a study published yesterday (February 2) in Nature. The first patient to receive the treatment also went into remission and stayed cancer-free until the time of his death in January 2021, when he passed away from COVID-19.

“We can now conclude that CAR T cells can actually cure patients with leukemia,” study coauthor Carl June, a cancer immunologist at the University of Pennsylvania, told reporters at a press briefing, multiple outlets report.

The result is beyond anything June and his colleagues had imagined when they started the clinical trial in 2010, June said at the news briefing, describing how they were skeptical the injected T cells would last longer than “a month or two.” Even when the patients’ cancers quickly went into remission following their treatments, the team was cautious about assuming the disease would not come back. But these latest results, which June calls “the most mature and oldest results available reported in scientific literature,” are pretty convincing, he told reporters. “Ten years on, no leukemia cells, and we still have CAR-T cells that are on patrol and surveillance from leukemia. . . . We can say it was a cure.”

June’s coauthor David Porter, an oncologist at UPenn, agrees. The therapy has performed “beyond our wildest expectations,” he tells Science News.

See “Natural Killer Cell Therapies Catch Up to CAR T

The data presented in the new study, based on regular follow-up blood tests, show that the CAR T cells had originally differentiated into CD8+ killer T cells, then became CD4+ helper T cells, which can serve as a form of immune memory. The ratio of these two different types of T cells within the CAR T cell population was initially dynamic but they transformed into a stable population of helper T cells after nine years.

“The new paper really describes in great detail what happened to the first two patients,” Marcela Maus, an immunologist at Mass General Hospital Cancer Center who was not involved in the research, tells STAT. “As an immunologist, it’s really interesting to see the evolution of a very specific T cell response, when you know exactly the day they went in and had an exposure, then carefully followed over 10 years.”

Unfortunately, other patients who have received CAR T cell treatments have not been so lucky, especially those with solid tumors. But Joseph Melenhorst, an immunologist at the University of Pennsylvania and the lead author on the new study, tells STAT that the team’s results could help scientists figure out why CAR T therapy works only for some and develop a next generation of treatments that can be more widely helpful.

See “The Next Frontier of CAR T-Cell Therapy: Solid Tumors

“The potential impact of CAR T is tremendous,” National Cancer Institute pediatric hematologist Nirali Shah, who did not participate in the study, tells Nature. This study “gives you a proof of concept about the safety of having long-term persistence and integration of the T cells into your body.”