Natural killer cells in antitumour adoptive cell immunotherapy

Abstract

Natural killer (NK) cells comprise a unique population of innate lymphoid cells endowed with intrinsic abilities to identify and eliminate virally infected cells and tumour cells. Possessing multiple cytotoxicity mechanisms and the ability to modulate the immune response through cytokine production, NK cells play a pivotal role in anticancer immunity. This role was elucidated nearly two decades ago, when NK cells, used as immunotherapeutic agents, showed safety and efficacy in the treatment of patients with advanced-stage leukaemia. In recent years, following the paradigm-shifting successes of chimeric antigen receptor (CAR)-engineered adoptive T cell therapy and the advancement in technologies that can turn cells into powerful antitumour weapons, the interest in NK cells as a candidate for immunotherapy has grown exponentially. Strategies for the development of NK cell-based therapies focus on enhancing NK cell potency and persistence through co-stimulatory signalling, checkpoint inhibition and cytokine armouring, and aim to redirect NK cell specificity to the tumour through expression of CAR or the use of engager molecules. In the clinic, the first generation of NK cell therapies have delivered promising results, showing encouraging efficacy and remarkable safety, thus driving great enthusiasm for continued innovation. In this Review, we describe the various approaches to augment NK cell cytotoxicity and longevity, evaluate challenges and opportunities, and reflect on how lessons learned from the clinic will guide the design of next-generation NK cell products that will address the unique complexities of each cancer.

Fig. 1. Advantages and limitations arising from different sources of NK cells.

Natural killer (NK) cells can be derived from several different sources, each of which presents its own advantages and potential challenges. Chimeric antigen receptor (CAR) NK cells have successfully been engineered from different platforms including cord blood^,, peripheral blood^,,, NK-92 cells^,– and induced pluripotent stem cell (iPSC)-derived NK cells^,,. ADCC, antibody-dependent cellular cytotoxicity; HPSC, haematopoietic stem and progenitor cells; MDACC, University of Texas MD Anderson Cancer Center.

Fig. 2. Strategies to redirect NK cell specificity.

Natural killer (NK) cell specificity towards tumour cells can be redirected using different strategies. a,b | Chimeric antigen receptor (CAR) NK cell^,,,, (panel a) and T cell receptor (TCR) NK cell (panel b) approaches both build upon stable genetic engineering to endow NK cells with synthetic receptors that recognize extracellular and intracellular tumour antigens, respectively. c | Bi-specific and tri-specific engagers^,–,,,,, deploy two-directional or three-directional antibodies which crosslink NK cells with their respective tumour cell targets and circumvent the need for complex genetic editing. HLA, human leukocyte antigen; MHC, major histocompatibility complex; scFV, single-chain variable fragment.

Fig. 3. Principles and strategies for CAR design.

a,b | Chimeric antigen receptor (CAR) molecules have evolved dramatically over the past two decades, from simple first-generation designs (panel a), to second-generation^,, and third-generation^, CARs with added co-stimulatory molecules (panel b) and, finally, to current-generation CAR designs resembling a modular system that encompasses optimized extracellular domains for target recognition, intracellular co-stimulatory molecules for effective natural killer (NK) cell activation and added payloads which can enhance NK cell functionality. c–e | Current strategies leverage the core principles of CAR signalling and functionality, and provide innovative methods to improve tumour recognition and enhance cell activation, using sophisticated construct designs to allow targeting of multiple tumour antigens^, (panel c), provide cytokine support^, (panel e) and activate auxiliary cytotoxicity pathways. Integration of logic-gated circuits to guide selective killing of targeted malignant cells while sparing healthy tissues may lead to improved safety profiles^,, (panel d). aCAR, activating chimeric antigen receptor; AML, acute myeloid leukaemia; DAP12, DNAX-activation protein 12; EMCN, Endomucin; HLA, human leukocyte antigen; HSC, haematopoietic stem cell; iCAR, inhibitory chimeric antigen receptor; scFV, single-chain variable fragment.

Fig. 4. Genetic engineering strategies to overcome suppressors of NK cell function.

a–f | Immune cell function is severely compromised by the hostile tumour microenvironment (TME). Current strategies leverage engineering tools to disrupt suppressive signals in the TME (panel a) and improve immune cell homing into tumour beds by ectopic expression of chemokine receptors (panel d). A selection of natural killer (NK) cell-relevant pathways that have been targeted through genetic engineering is shown. Genetic engineering strategies that include targeted ablation of inhibitory checkpoints^,,, (panels b,c) as well as disruption of extracellular receptors which sense inhibitory stimuli including TGFβ^, and adenosine (panel a) have been shown preclinically to effectively target pathways to enhance metabolic fitness and persistence of NK cells, and efforts are ongoing to advance these findings into the clinic. Ablation of endogenous receptors allows for combinatorial therapeutic approaches, such as by rendering immune cells resistant to corticosteroid-induced immunosuppression (panel e), a principle previously established in severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2)-directed cytotoxic T lymphocytes (CTLs). Knockout of CD38 (panel f) renders NK cells resistant to CD38-mediated fratricide, which enables combination strategies of NK cells and anti-CD38-targeting monoclonal antibodies in the context of treating multiple myeloma. B_reg cell, regulatory B cell; MDSC, myeloid-derived suppressor cell; NKG2A, CD94/NK group 2 member A receptor; TIGIT, T cell immunoreceptor with immunoglobulin and ITIM domains; T_reg cell, regulatory T cell.