An experimental treatment that transforms immune system cells into genetically engineered “serial killer” cells has produced dramatic results in three patients suffering from an advanced form of leukemia, researchers reported Wednesday.

In a pair of papers published in The New England Journal of Medicine and the journal Science Translational Medicine, researchers led by Carl June of the University of Pennsylvania described the results of a pilot study aimed at evaluating the new approach.

The therapy involves removing immune system cells known as a T cells from each patient and genetically engineering them in the laboratory to become “serial killer T cells” that hone in on specific cells—in this case leukemia cells—mostly sparing other types of cells.

For the pilot study, the researchers then infused the cells back into three patients suffering from advanced cases of chronic lymphocytic leukemia (CLL). Currently, the only way to treat such patients is with bone marrow transplants, which are dangerous, difficult and often fail.

All three study patients had strong responses, the researchers reported. Two of the three experienced complete remissions.

“Within three weeks, the tumors had been blown away in a way that was much more violent than we ever expected,” June said in a statement released with the findings. “It worked much better than we thought it would.”

Moreover, the cells persisted in the patients’ bodies long enough to work, the researchers said.

“It’s the first time we’ve shown that T cells are what’s called serial killers— that they can kill one tumor cell and then go and kill another,” June said in a video released with the research. “We found in all three of our patients that the T cells killed at least 1,000 tumor cells. That’s the first time it’s ever been shown anywhere near that kind of efficiency.”

The same approach potentially could be used to attack other types of cancer, including non-Hodgkin’s lymphoma, acute lymphocytic leukemia, ovarian cancer and pancreatic cancer, they said.

The treatment did have side effects, however. Patients experienced chills, nausea, fever and other symptoms. One also developed a condition known as hypogamma globulinemia, which inhibits the ability of the immune system to make antibodies.

In an editorial accompanying the paper in The New England Journal of Medicine, Walter Urba and Dan Longo of the Providence Cancer Center in Portland, Ore., caution that more research is needed to know whether the findings “reflect an authentic advance toward a clinically applicable and effective therapy or yet another promising lead that runs into a barrier that cannot be easily overcome.”