Four of nine acute myeloid leukemia patients went into complete remission – and a fifth responded – after being transfused with donor natural killer cells in a phase I study from Washington University in St. Louis.

The natural killer (NK) cells had been differentiated into “memorylike” NK cells by brief exposure to interleukin (IL) 12, 15, and 18 prior to transfusion. Although NK cells have traditionally been considered part of the innate immune system, it’s become clear recently that they have some adaptive abilities. The cytokine exposure in the St. Louis study seemed, in a sense, to train NK cells to remember and attack acute myeloid leukemia (AML).

The treated cells had enhanced interferon gamma production and cytotoxicity against AML cells in vitro. Once in the patients, they “proliferated extensively” and demonstrated robust responses against leukemia targets. Preactivation of NK cells with IL-12, IL-15, and IL-18 promotes “potent antileukemia functionality in vitro and in vivo and thus represent[s] a promising immunotherapy strategy for AML,” said investigators led by Rizwan Romee, MD , of the oncology division and clinical director of the haploidentical transplant program at Washington University (Sci Transl Med. 2016 Sep. 21. doi: 10.1126/scitranslmed.aaf2341 ).

NK cell therapy is an emerging treatment for AML, but it’s been unclear, at least until now, how best to maximize the cells’ anti-AML effect before transfer.

Prior studies have tried IL-2 or IL-15 overnight, which does increase NK cell functional capacity, but the effect is rapidly lost after transfer into patients.

That didn’t seem to be much of a problem when NK cells were differentiated into memorylike cells. “The longer-lasting increase in functional capacity … combined with improved AML recognition, [enhanced] in vivo expansion and antileukemia responses, result[ed] in a several week ‘window of opportunity’ to attack AML blasts.” the authors said.

For safety, the initial cell dose was a tenth or twentieth of the typical adoptive NK cell dose. Even so, the memorylike cells consistently expanded to become greater than 90% of blood and most of bone marrow NK cells, which was “remarkable,” they said.

The National Cancer Institute and others funded the work. The authors had no disclosures.