Experiments, which took place at London's Great Ormond Street Hospital, raise the possibility of off-the-shelf cellular therapy using inexpensive supplies of universal cells that could be dripped into patients' veins on a moment's notice.

The ready-made approach could pose a challenge to companies including Juno Therapeutics and Novartis, each of which has spent tens of millions of dollars pioneering treatments that require collecting a patient's own blood cells, engineering them, and then re-infusing them.

Both methods rely on engineering T cells—the hungry predator cells of the immune system—so they attack leukemic cells.

The British infants, ages 11 and 16 months, each had leukemia and had undergone previous treatments that failed, according to a description of their cases published Wednesday in Science Translational Medicine. Waseem Qasim, a physician and gene-therapy expert who led the tests, reported that both children remain in remission.