Recent cancer advance CAR T therapy relies on the killing power of cytotoxic CD8+ T cells to destroy a patient’s cancer, but they are not the only cells that can kill. Natural killer cells also target cancerous cells and could be used instead of CD8+ T cells to address several CAR T therapy limitations. On the road to translating this vision into a therapeutic reality, a study published in Scientific Advances uncovers a critical insight into why natural killer cells with chimeric receptors can lose efficacy over time. They also propose a means to overcome this setback. This study and others venture into a new realm of chimeric receptor technology that may lead to more ideal cancer therapy.

Killer T Cells vs. Natural Killer Cells

Chimeric Antigen T cell therapy (CAR T therapy) involves extracting a patient’s cytotoxic CD8+ T cells—also known as “killer” T cells—and boosting their natural abilities to fight cancer with bioengineering. Chimeric antigen receptors are fitted to the cell surface, allowing T cells to precisely target and eliminate cancer cells. Then, the edited cells are returned to the body via infusion.

The chimeric receptors activate faster than the receptors T cells naturally equip. Typically, an antigen-presenting cell must first show the T cell the exact antigen, or a protein target, to search for. Then, a second costimulatory signal must follow to complete the activation. Now activated, the T cell can recognize and bind to specific protein complexes found on cancer cells; the complex comprises a target antigen and a molecule called major histocompatibility complex (MHC) class I.

In comparison, CAR T cells bind directly to the target antigen and activate in one fell swoop—no prior exposure needed—which allows them to act more swiftly in comparison.

Natural Killer Cells 

Conventional CAR T therapy can effectively treat patients with certain blood cancers, including lymphomas, leukemias and multiple myeloma. However, manufacturing this customized treatment is time-consuming, resource-intensive and costly. While some look to allogeneic or donor-derived CAR T cells to solve this issue, another potential solution is to use a different immune cell as the therapy’s base. Natural killer cells are a particularly prime alternative.

Natural killer cells can also cull infected and cancerous cells, but the process differs from cytotoxic T cells. These cells earned the title of “natural” killers because they do not rely on prior exposure to specific antigens to find their target. Instead, they only attack stressed cells or cells that downregulate the expression of major histocompatibility complex class I molecules, as cancer cells often do to avoid immune system detection. This generalized response is characteristic of innate immune cells, our body’s first-line responders, while cytotoxic T cells operate under a slower and more tailored adaptive immune response.

Once activated, natural killer cells wipe out their target in one of three main ways: they can release perforin and granzymes—the same molecules cytotoxic T cells release—to destroy cancer; they can release chemicals called cytokines to recruit other cells to attack; or they can bind to a specific molecule on the target cell to trigger its cell death pathways.

CAR NK Cells: A Promising Alternative 

Natural killer cells may be an apt candidate for chimeric antigen receptor therapy. CAR NK cells can eliminate a broader range of tumor cells than CAR T cells, as they can kill via their chimeric receptor or their antigen-independent mechanisms.

CAR NK cells also demonstrate a favorable safety profile. These cells express different cytokines than cytotoxic T cells, and therefore are not as prone to common adverse effects of CAR T therapy such as cytokine release syndrome (CRS) or neurotoxicity. In addition, natural killer cells do not convert into memory cells as T cells do. Although this translates to a shorter lifespan, it also lowers the risk of on-target/off-tumor toxicity to normal tissues.

Most importantly, making an allogeneic, “off-the-shelf” CAR therapy from NK cells may be more advantageous. Their natural ability to distinguish between healthy and cancerous cells lowers the risk of graft-vs-host-disease (GvHD), a complication where the immune system recognizes a donor transplant as harmful and rejects it.

CAR NK and Metabolic Fitness 

CAR NK therapy is a developing concept, and much is still unknown. To this end, University of Texas MD Anderson Cancer Center researchers closely monitored how NK cells behave in different settings. They tested NK cells that either express nothing (control), anti-CD19 antigen CAR, a cytokine that encourages NK cell proliferation and survival called interleukin-15 (IL-15), or a combination of CAR and IL-15. Analyzing how the cells express genes revealed a previously unknown mechanism behind why CAR NK therapy can weaken over time—loss of metabolic fitness—and a possible means to combat this resistance.

All cells require energy to carry out their functions. Metabolic fitness here refers to a natural killer cell’s ability to produce enough energy for optimal functioning—to proliferate, to kill cancer cells, and to sustain their activity over time.

CAR19/IL-15 NK cells show the most robust enrichment for metabolic pathways among the four products. This means that integrating IL-15 expression into the synthetic receptor may help NK cells fight cancer. The team confirmed this with mouse models of lymphoma. Mice treated with CAR19/IL-15 NK cells demonstrate significantly lower tumor burden and improved survival compared to controls or CAR19 NK cells, but these mice still eventually die.

With more experimentation, the researchers observe that tumor relapse occurs despite the higher metabolic activity and persistence of IL-15-expressing CAR NK cells. The tumor cells appear to activate the NK cells; the NK cells initially exhibit increased function and metabolism for a time; then, the NK cells become dysfunctional and antitumor responses decline.

Possible Mitigation Strategy: Double Dosing 

If IL-15 expressing CAR NK cells lose steam over time, could a second infusion improve overall performance? The team tested this hypothesis by giving mice with lymphoma two infusions of the CAR19/IL-15 NK cells two weeks apart. Mice with two infusions achieve a 60% survival rate—a much better response than mice dosed with two control infusions or a single infusion of IL-15-expressing CAR NK cells. While the first infusion begins to dysfunction, an additional infusion may increase the population of functional NK cells ready to combat the blood tumors.

CAR NK Cells in Humans 

How do IL-15-expressing CAR NK cells perform in humans? To understand this, CAR NK cells were extracted from two participants with relapsed/refractory lymphoma seven and 14 days after infusion. Each patient received a single infusion of CAR-NK cells. The CAR NK cells did not work for one patient, but cleared all signs of cancer for the second patient within 30 days of infusion.

The NK cells from the responder patient showed higher expression of genes associated with NK cell activation and cytokine activity. This gene expression pattern is even more pronounced in Day 14 CAR NK cells. The results suggest that the CAR NK cells in the responder patient likely possess better metabolic fitness and gene expression than those of the nonresponder patient.

Future Implications

Natural killer cells possess attractive qualities for chimeric receptor therapy, but more research is needed to understand potential roadblocks. This study demonstrates that gradual loss of metabolic fitness may limit this concept’s clinical feasibility. Future strategies may benefit from addressing this resistance mechanism, such as with IL-15 expression or repeat dosing.