Azusa Tanaka Ph.D. Product Manager Taconic Biosciences

Natural Killer Cells on the Loose

As the field of immunotherapy progresses, and more effective treatments reach patients, the use of live cells to attack tumors has become the next area of focus. While checkpoint inhibitors such as anti PD-1, PD-L1, and CTLA-4 have demonstrated they are capable of increasing T-cell activation—leading to enhanced antitumor responses—and FDA approvals have paved the way for chimeric antigen receptor T-cell (CAR-T) therapies, both approaches require sufficient T cells to stimulate or modify. That can prove challenging when a patient is immunosuppressed from a first-line therapy that affects T cells.

This challenge has placed the spotlight on the next frontier in immunotherapy, which will likely be natural killer (NK) cell therapy.    

The Power of NK Cells

Part of the innate immune system, NK cells are much like cytotoxic T cells, in that they can bind to and kill other cells. This capability has led investigators to explore whether NK cells can be used in immunotherapy much like T cells are used, without the side effects associated with T-cell–based treatments, such as cytokine storm or graft vs. host disease (GvHD).1,2,3

There are several ways in which investigators can harness the antitumor activity of NK cells. All of the approaches are already the subject of study by pharmaceutical and biotech companies; some companies are developing antibodies that leverage direct killing and antibody-dependent cell-mediated cytotoxicity (ADCC) or are utilizing cell and gene therapy approaches.   

One method that appears promising is the use of bispecific antibodies to engage NK cells and bring them closer to their cancer targets, causing direct killing. For example, Innate Pharma’s bispecific antibody therapy uses the NKp46 activating receptor found on all NK cells to cause direct killing. The antibody binds to an antigen on the surface of tumor cells on one end, and the NKp46 receptor on the surface of NK cells on the other end, to activate the NK cells and enable them to attack the tumor.4

The induction of ADCC is another mechanism by which NK cells can engage in antitumor activity. Fc receptors expressed on NK cells, such as CD16, allow NK cells to identify and kill antibody-coated target cells. One company exploring this mechanism, Affimed, has an AFM13 that binds to Hodgkin’s lymphoma through CD30 and recruits NK cells through CD16, inducing ADCC.5

A third approach to leveraging the cytotoxic properties of NK cells is to develop CAR-modified NK cells, similar to how T cells are modified for CAR-T cell therapies. Fate Therapeutics has obtained FDA approval for several investigational new drug applications for its FATE-NK100, which employs modified NK cells expressing the maturation marker CD57 and the activating receptor NKG2C. The company recently announced a collaboration with the University of California San Diego to study iPS-derived CAR-NK cell therapies.6,7

Regardless of the specific mechanism employed, NK cell–based cancer therapies offer crucial treatment advantages. Human leukocyte antigen (HLA) matching is less of a concern than with T cells; therefore, allogeneic transplantation may be possible, so that one donor/NK cell line could provide a cure to multiple patients.8  Therapy production also may be streamlined when therapies do not have to be personalized to each patient.

Advancing In Vivo NK Cell Study

Existing in vitro systems for the study of NK cell function appear to be sufficient, but improved in vivo systems are necessary to help improve the translation of NK cell–mediated therapies to the clinic. To study human NK cell–based therapies in vivo requires sufficient quantities of functional NK cells, and an environment that supports their survival.

It has been demonstrated that the presence of certain cytokines, including human interleukin 15 (hIL-15), is conducive to NK cell survival.9 This finding has spurred development of humanized immune system rodent models that express human IL-15 and support engraftment of NK cells. One such model is the hIL-15 NOG mouse, which is built on the Central Institute for Experimental Animals (CIEA) NOG mouse®, a super-immunodeficient platform used as a host for patient-derived xenograft (PDX) and other cell-based humanization, including hematopoietic cells.  

The hIL-15 NOG model transgenically expresses the human IL-15 cytokine and demonstrates expansion of NK cells following engraftment onto the model of CD56+ NK cells derived from peripheral blood mononuclear cells (PBMCs), without signs of xenogeneic GvHD. 9 The NK cells express various NK receptors and produce both granzyme A and perforin upon stimulation.

The utility of a model such as the hIL-15 NOG lies in helping to elucidate whether a drug candidate of interest can bind to human NK cells in vivo. Investigators have begun using this model to determine if the NK cells can traffic to the right place at the right time, and whether a drug candidate can recruit and/or stimulate those human NK cells to engage in antitumor activity.     

In one study, hydrodynamic injections were used to deliver human IL-15 to a hCD34+ human hematopoietic stem cell (HSC)–engrafted NOG mouse, resulting in significant uptake of NK cells in peripheral blood. The increase in NK cells was stable for a duration of time sufficient to engraft tumor cells and test drug efficacy.10

In their latest work, CIEA scientists describe the long-term maintenance of human NK cells in hIL-15 NOG mice, supporting the utility of this model for preclinical evaluation of NK cell–based cancer immunotherapies and longer-term in vivo NK cell studies.9 Human NK cells isolated from peripheral blood or expanded in vitro were maintained long term upon engraftment in hIL15-NOG mice, retaining key immunological features and reporting the ability to respond to monoclonal antibodies (mAbs) to mediate ADCC against an engrafted tumor.

Continued study of NK cell mechanisms within in vivo systems will enable investigators to determine how these natural killer cells can be employed to deliver immunotherapies with improved efficacy and reduced risk. Humanized immune system (HIS) rodent models will continue to serve as valuable tools for such study, providing the environment necessary to support survival of sufficient, functional NK cells.

Azusa Tanaka, Ph.D., is product manager at Taconic Biosciences. 

1. G. Pittari et al., “Revving up Natural Killer Cells and Cytokine-Induced Killer Cells Against Hematological Malignancies,” Front Immunol. 6: 230 (2015).
2. H. Klingemann “Are Natural Killer Cells Superior CAR Drivers?,” Oncoimmunology  e28147 (2014).
3. L.K. Schoch et al., “Checkpoint Inhibitor Therapy and Graft Versus Host Disease in Allogeneic Bone Marrow Transplant Recipients of Haploidentical and Matched Products with Post-Transplant Cyclophosphamide,” Blood 128:4571 (2016).
4. Innate Pharma, “Bispecific Antibodies Technology: NK Cells Engagers,” Accessed December 15, 2017.
5. R.E. Kontermann and U. Brinkmann, “Bispecific Antibodies,” Drug Discov. Today 20(7), 838–847 (July 2015).
6. Fate Therapeutics, “Fate Therapeutics Announces FDA Clearance of Investigational New Drug Application for FATE-NK100 Natural Killer Cell Cancer Immunotherapy,” (March 13 2017), accessed December 15, 2017.
7. Fate Therapeutics, “Fate Therapeutics and University of California San Diego Launch Research Collaboration to Develop iPSC-Derived CAR NK Cell Cancer Immunotherapies,” (December 6, 2017), accessed December 15, 2017.
8. Z.B. Davis et al., “Natural Killer Cell Adoptive Transfer Therapy: Exploiting the First Line of Defense Against Cancer,” Cancer J. 21(6), 486–491 (Nov.–Dec. 2015)
9. I. Katano et al., “Long-Term Maintenance of Peripheral Blood-Derived Human NK Cells in a Novel Human IL-15-Transgenic NOG Mouse,” Sci Rep. 7(1), 17230 (Dec 8 2017).
10. Taconic Biosciences, “Immuno-Oncology in Humanized Mice” (October 17, 2016), accessed December 15, 2017. 

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