Just as cancer cells were once healthy cells, the body also produces cells that naturally seek out and destroy cancer cells — aptly named natural killer (NK) cells. As far as immune cells go, NK cells are special. They are members of the innate immune system, so they patrol the body in search of cells that don’t belong — microbe and cancer cell alike (1). But NK cells can also act a little like T cells, their cousins in the adaptive immune system, by releasing perforin and granzymes to kill target cells (2). With these unique abilities, NK cells have translational potential as cancer cell therapies.
“This is so unique because this is the only cell in our entire body that has this capacity, so harnessing this capacity for cancer therapy is really exciting,” said Veronika Bachanova, a hematologist-oncologist at the University of Minnesota.
While NK cells and T cells attack cancer in a similar way, NK cells have some significant advantages compared to popular T cell-based cell therapies such as CAR T cell therapy. For one, NK cells don’t require a donor match, meaning that NK cell therapy could be an “off-the-shelf” cancer treatment.
“We can manufacture hundreds of doses from one donor, really reducing cost of therapy, being able to treat many more patients with them,” said Katayoun Rezvani, an NK cell biologist at the University of Texas MD Anderson Cancer Center. NK cells are also safer than CAR T cell therapy because they don’t cause cytokine release syndrome (CRS) or immune effector cell-associated neurotoxicity syndrome (ICANS).
With these traits, why haven’t NK cells overtaken other cancer immunotherapies yet? Unlike T cells, which can survive for a person’s entire life, NK cells only have a one- to two-week lifespan in the body. They also tire quickly and lose their cancer-killing ability, allowing the tumor to relapse. Culturing NK cells or engineering them to express the cytokine IL-15 drives NK cell proliferation and persistence, and studies have shown that adding IL-15 increases NK cells’ effectiveness as a cancer cell therapy (3,4). But even with the addition of IL-15, subsets of patients still don’t respond to the treatment.
“This is the field’s challenge,” Bachanova said, “how to harness the NK cells for therapy.”
Now, researchers, including Rezvani and Bachanova, are developing new strategies to give NK cells the boost of energy they need to keep fighting cancers. With that, they hope to present NK cell therapies as a safer cell therapy option to more patients than ever before.
A killer combination
NK cells are proficient cancer assassins by themselves. They use endogenous receptors on their surfaces to recognize cancers as “nonself” and to target them for killing. But not all cancers are equally susceptible to NK cells. To increase NK cell specificity for more cancer types, Rezvani reasoned, why not have NK cells express a chimeric antigen receptor (CAR)?
“Your NK cells will continue expressing their endogenous receptors, so potentially you have multiple mechanisms of killing: through the CAR but also through those endogenous receptors,” she said.
Rezvani and her team demonstrated that NK cells expressing anti-CD19 CAR along with IL-15 were both safe and led to a 73 percent response rate in patients with non-Hodgkin lymphoma or chronic lymphocytic leukemia in a first-in-human clinical trial (5). Even with that strong response rate and many of those patients having complete cancer remissions, Rezvani wanted to better understand why the treatment response rate was not higher.
“The question was, what determines that failure? Is it that the natural killer cells don't persist? Is it that they become exhausted?” Rezvani wondered. “And what happens to the tumor? Why is it that the tumor comes back?”
To answer those questions, she and her team turned to the most aggressive lymphoma mouse model that they could find: mice with Raji lymphoma. The researchers reasoned that this noncurable lymphoma model would give them the best simulation of what might happen in the patients that didn’t respond to the CAR-NK cell therapy.
To begin solving the puzzle, the researchers made a few different sets of NK cells: NK cells with no modifications, CAR-NK cells with no IL-15 expression (CAR19-NK), NK cells without a CAR but with IL-15 (IL-15-NK), and CAR-NK cells with IL-15 expression (CAR19/IL-15-NK). After administering the cell therapies to the mice, the team isolated the NK cells from mice at specific time points and performed single cell transcriptomics and proteomics on them (6).
As they expected, the NK cells by themselves disappeared from the mice within two weeks. The CAR19 cells only stuck around one week longer, and while the CAR19/IL-15-NK cells persisted the longest (for at least 35 days), the mice eventually succumbed to cancer. Once Rezvani and her team began to analyze the NK cells in these different populations, they realized exactly what was going on.
“The clusters that were infused, they either evolve or they disappear, and then there are certain subsets that became dominant, especially in the CAR19/IL-15-NK group,” said Rezvani. High levels of that dominant CAR19/IL-15-NK cell cluster correlated with the optimal control of the tumor. “But then the tumor came back (because that was the model that we picked), and the tumor coming back correlated with that highly active cluster disappearing,” said Rezvani. When they looked at the transcriptomic data, they saw that like a weightlifter after a long day in the gym, the CAR19/IL-15-NK cells became exhausted and lost their metabolic fitness.
We can manufacture hundreds of doses from one donor, really reducing cost of therapy, being able to treat many more patients with them.
- Katayoun Rezvani, University of Texas MD Anderson Cancer Center
“It's interesting because when we first started this study, we were not necessarily very much focused on the metabolism of the cell,” Rezvani said. They had been more interested in looking at the expression of checkpoint molecules and transcription factors. “During the study, though, a lot of papers came out in the T cell field with a lot of focus on metabolism, so that made us dig deeper into our study to look and see whether metabolic competition was also a feature of NK cells,” she added.
The researchers reasoned that the NK cells likely competed for nutrients and oxygen in the tumor microenvironment, and the tumor outcompeted them. In fact, from their transcriptomic and proteomic profiling of these NK cells, they noticed that the NK cells lost their fitness via multiple different metabolic pathways. Her team is now hard at work investigating how these metabolic pathways influence NK cell metabolism and how to boost the metabolism most optimally for cancer treatment.
The complexity of these metabolic pathways is not lost on Rezvani. She thought that she’d seen the last of them after she finished memorizing them in medical school: “I thought, ‘Okay, great, I never have to think about this again.’ And it's come back to haunt me!” she laughed. “Thankfully, I have some very smart people in my lab that are very interested, and they're basically digging deep into the mechanism of each single enzyme and pathway.”
Most of all, Rezvani is eager to continue developing better CAR19/IL-15-NK cell therapies for hematological cancers as well as solid tumors and to bring those to the clinic.
“There's so much we still need to learn about natural killer cells,” said Rezvani. “The signals of activity that we see in our patients is what keeps me going to try and make a better version of these cells.”
A nicotinamide energy boost
While adding a CAR is one way to help direct NK cells to cancer cells, another strategy is to combine NK cell therapy with an antibody that binds to a specific type of cancer. After the antibodies bind to the cancer cell, NK cells then bind to the antibodies, which triggers their cancer killing activity via a process called antibody-dependent cellular cytotoxicity (ADCC) (1).
Bachanova and her team used this approach in a recent clinical trial (7). They treated patients with refractory non-Hodgkin lymphoma with a combination of NK cell therapy and the antibody rituximab. Like in Rezvani’s NK cell therapy trials, Bachanova saw that only some patients responded to the therapy. There had to be a way to improve NK cell therapy for the other patients.
As luck would have it, a potential way to boost NK cell treatment came from the cell therapy company Gamida Cell. Over the years, the researchers at Gamida Cell had developed a proprietary formulation of the small molecule nicotinamide (NAM) to improve and expand stem cells for bone marrow transplants.
“While our work was going on to develop the stem cell product, we looked at the ability to use the NAM platform with other cells,” said Ronit Simantov, the chief medical and scientific officer at Gamida Cell. “NK cells seemed like a great candidate because they're a cell type that's made to professionally kill foreign cells.”
Nicotinamide plays an important role in cellular metabolism. For example, it regulates many different nicotinamide adenine dinucleotide dependent reactions in the cell, including mitochondrial reactions and protection from reactive oxygen species (8). The Gamida Cell researchers discovered that when they cultured stem or NK cells in the presence of NAM, it prevented the cells from becoming exhausted in culture.
“Usually when you culture cells, they either differentiate — become other cells — or they have this exhausted phenotype where they don't function normally anymore. And what NAM does is protect them from that,” said Simantov.
Using the NAM platform with IL-15, the Gamida Cell researchers developed a technique to expand NK cells in culture, and they discovered that these NAM-NK cells (now called GDA-201) did a great job killing tumors both in vitro and in animal models. They then linked up with Bachanova to test the safety and efficacy of GDA-201 in combination with monoclonal antibodies in a Phase 1 clinical trial treating people with relapsed or refractory non-Hodgkin lymphoma or multiple myeloma (9).
This is the field’s challenge: how to harness the NK cells for therapy.
- Veronika Bachanova, University of Minnesota
“Some patients came to the trial with relapse even after a bone marrow transplant, so they were quite advanced,” said Bachanova.
Because NK cell therapy has a much lower risk of CRS or ICANS compared to CAR T cell therapy, the researchers tried to enroll patients who were weaker or older than others and who would be classified as a high risk group for CAR T cell therapy. After the researchers dosed the first half of the patients in their study and proved GDA-201’s safety, the FDA allowed them to remove the age limit on the trial. Thereafter, the team enrolled patients up to age 82. Over the course of the trial, they found that 74 percent of the patients with advanced non-Hodgkin lymphoma responded to the treatment.
One patient in particular had a noteworthy response to the treatment. The 57-year-old man entered the study with a history of chronic lymphocytic leukemia (CLL) and Richter’s transformation, a rare and much more aggressive form of CLL, and his cancer had still progressed even after a bone marrow transplant from a sibling.
“He was a man who was retired [and] was an avid motorcycle rider but wasn't able to do that when the disease came back,” Simantov said. “He got GDA-201, and the disease originally shrunk a little bit and then a few months later, disappeared. He took pictures of himself riding his motorcycle again.”
When the researchers looked at a biopsy of this patient’s lymph node tissue as well as a blood sample taken after GDA-201 treatment, they saw that not only were the levels of NAM-NK cells elevated, but that additional T cells had been recruited to fight the cancer. Simantov hypothesized that the initial antibody-mediated killing by NK cells exposed neoantigens on the tumors that helped the immune system find and kill them.
“It's really nice to have the whole picture where a patient came to us and said, ‘You know, I feel better,’ as well as having the scientific backing of ‘There's a reason.’ We see what the GDA-201 did here,” said Simantov. “It's something that we're really happy to have seen and hope to be able to demonstrate in further patients as we get biopsy samples.”
The one disappointment from the trial results, however, was that while the NAM-NK cells persisted in multiple myeloma patients, the treatment was not very effective in this cancer type. Bachanova thinks that the limited efficacy may have to do with the choice of antibody administered in combination with GDA-201. She plans to investigate that question further with a colleague who is an expert in multiple myeloma and NK cell biology to find a way to improve the response to NK cell therapy.
To move this treatment forward, the team at Gamida Cell developed a cryopreserved formulation of GDA-201, which makes it much easier for them to expand their clinical research into a multicenter trial. In fact, they have already initiated a larger Phase 1/2 study testing this new formulation of GDA-201 at six different sites around the United States, including one back at the University of Minnesota. Eventually, because of how safe NK cell therapy is, Bachanova hopes to move treatment to an outpatient setting, which would improve accessibility.
“This is an effective therapy. It is going to require more investigations of finding the best combinations and the ultimate disease for which it's going to harness the efficacy,” she said. “It has potential. We just have to be smart about how to harness it.”
- Vivier, E. et al. Functions of natural killer cells. Nat Immunol 9, 503–510 (2008).
- Narni-Mancinelli, E., Vivier, E., Kerdiles, Y.M. The ‘T-cell-ness' of NK cells: unexpected similarities between NK cells and T cells. International Immunology 23, 427–431 (2011).
- Rautela, J. and Huntington, N.D. IL-15 signaling in NK cell cancer immunotherapy. Current Opinion in Immunology 44, 1-6 (2017).
- Liu, E. et al. Cord Blood Derived Natural Killer Cells Engineered with a Chimeric Antigen Receptor Targeting CD19 and Expressing IL-15 Have Long Term Persistence and Exert Potent Anti-Leukemia Activity. Blood 126, 3091 (2015).
- Liu, E. et al. Use of CAR-Transduced Natural Killer Cells in CD19-Positive Lymphoid Tumors. N Engl J Med 382, 545-553 (2020).
- Li, L. et al. Loss of metabolic fitness drives tumor resistance after CAR-NK cell therapy and can be overcome by cytokine engineering. Sci Adv 9, eadd6997 (2023).
- Bachanova, V. et al. Haploidentical natural killer cells induce remissions in non-Hodgkin lymphoma patients with low levels of immune-suppressor cells. Cancer Immunol Immunother 67, 483–494 (2018).
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- Cichocki, F. et al. Nicotinamide enhances natural killer cell function and yields remissions in patients with non-Hodgkin lymphoma. Sci Transl Med 15, eade3341 (2023).