ABOVE: A new way of engineering natural killer (NK) cells could make them better at hunting down cancer cells hiding out in the bone marrow. © iStock, PashaIgnatov

In cancer immunotherapy, T cells are often the stars of the show. Many immune checkpoint blockade drugs keep cancer cells from evading T cells, and cellular immunotherapies arm T cells with chimeric antigen receptors (CAR). But Mattias Carlsten, a hematologist at the Karolinska Institute, switched to studying natural killer (NK) cells when he realized that they offered a unique opportunity to treat blood cancers like acute myeloid leukemia (AML). 

Previous studies reported that NK cells are unique in that they target cancer cells without any prior training or specially engineered molecules like CAR.1 But NK cells did not efficiently penetrate the bone marrow, which as the source of AML cells, was a prime target for lasting treatment.

In a study published in the journal Leukemia, Carlsten’s team described a new way to engineer NK cells to head straight to the source for more potent anticancer immunity.2 By modifying molecules on the NK cells’ surfaces, they ensured that the immune cells homed to the bone marrow to hunt down cancerous cells. Carlsten thinks that this approach, which leverages molecules already found on NK cells, could make NK cell therapies a reality for blood cancers.

“As a researcher, I like to capitalize on normal biology and take it from one context to another,” Carlsten said.

In this study, the normal biology is two important cell surface molecules involved in NK cell function. First, Carlsten’s team focused on a receptor that binds the molecule E-selectin. In people with AML, bone marrow blood vessels increase E-selectin expression, which triggers signals that send the cancerous blood cells into a dormant state.3 The cells’ lack of activity hides them from chemotherapy, which targets dividing cells.   “If this is happening, why don't we utilize this cue to home the NK cells better to this particular subsite of the microenvironment where the AML cells are?” Carlsten said.

NK cells already express the E-selectin receptor, but it needs the sugar molecule fucose to become activated. To lend the NK cells a helping hand, Carlsten’s team introduced a piece of RNA encoding fucosyltransferase 7 (FUT7), an enzyme that adds fucose to the E-selectin receptors.

To make these NK cells even more effective at targeting bone marrow, Carlsten’s team introduced a second piece of RNA that encodes a mutated form of the CXC chemokine receptor type 4 (CXCR4) immune receptor. This mutation is associated with a congenital immune deficiency in which NK cells tend to accumulate in the bone marrow.4 However, in a person with AML, this mutation could provide a mechanism to attract NK cells to the cancer cells’ hideout, which Carlsten had already shown in previous studies where he used CXCR4-mutant NK cells to treat AML.5

The researchers injected their newly engineered NK cells into an immunodeficient mouse model transplanted with human AML cells. FUT7-engineered NK cells were better at traveling to the bone marrow than unmodified NK cells. Adding the CXCR4 mutation improved the NK cells’ bone marrow homing even more. 

Dean Lee, a pediatric oncologist at Nationwide Children’s Hospital who was not involved in this study, said that he was especially excited to see confirmation that fucosylation, or the attachment of a fucose to a molecule, is important for NK cell activity in AML. “One really important thing that this is going to drive is that we will now be forced to really look at the levels of fucosylation on our cells if getting them into the bone marrow is what leads to a cure,” Lee said. “It's an area we've ignored, and we don't have any data on this from [NK cell] products that are in clinics.”

Introducing RNA into NK cells may also speed up regulatory pathways for introducing cellular therapies to the clinic relative to other approaches using genome editing, Lee added. Carlsten is planning to test the engineered NK cells in other animal models and in human trials. He believes that people who have received an unsuccessful stem cell or bone marrow transplant to treat their cancer could be good candidates for trial treatment; researchers could engineer NK cells from transplant donors into cancer-fighting machines. 

In future work, Carlsten hopes to test the potential benefits of additional modifications to the system. For example, he plans to add a CAR or remove molecules that direct the NK cells to other organs. “Now we have a good basis to use the same model with similar questions,” he said.