Memory-Like Natural Killer Cells
Authors: Berrien-Elliott Melissa M , Wagner Julia A , Cashen Amanda F , Fehniger Todd A 
Published: October 2019

Abstract
Natural killer (NK) cells, classically considered to be part of the innate immune system, are specialized for rapid responses that protect the host against pathogens and eliminate malignant cells. These functional responses are multi-faceted, and include not only direct target cell killing, but also production of cytokines and chemokines, proliferative expansion, and cross-talk with various immune cells to help orchestrate cellular immunity. NK cell recognition of a target cell is controlled by the integration of signals from germline-DNA encoded activating, inhibitory, and cytokine receptors. NK cell tolerance toward healthy host cells arises via education or licensing that requires self-inhibitory receptor engagement. Over the past decade, it has become clear that NK cells have the capacity to remember prior activation events, including hapten-exposure,1 viral infection,2 and combined cytokine stimulation.3 These studies have defined "memory", "adaptive", and "memory-like" responses by NK cells within both murine and human immune systems, which can result in long-lived NK cell populations with enhanced effector functionality.4-6 While certain types of NK cell "memory" can be specific to the original stimulation, other types of "memory-like" responses are non-specific, providing flexibility in the enhanced response to subsequent activating triggers.7 Cytokine-induced memory-like NK cells were originally discovered in mice, following a brief exposure to the potent activating combination of IL-12, IL-15 and IL-18.3 After this activation, murine memory-like NK cells differentiated in vivo, and demonstrated enhanced IFN-γ recall responses to IL-12 and IL-15 re-stimulation, even after extensive cell division. Subsequent studies identified human IL-12/15/18-induced memory-like NK cells that displayed enhanced function after re-stimulation via cytokine receptors, activating receptors, or tumor targets.8 Adoptive transfer of murine memory-like NK cells into syngeneic or immunodeficient mice resulted in an enhanced ability to control lymphomas and solid tumors in vivo,9 as well as the ability to persist in the recipient for months. In addition, memory-like NK cell differentiation restored the anti-tumor function of unlicensed NK cells, demonstrating that cytokine receptor signals can overcome a lack of NK cell education.10 Other studies showed that memory-like NK cells can ignore inhibitory receptor signals, and have enhanced anti-leukemia responses.11 Based on these pre-clinical findings, we translated allogeneic memory-like NK cells into the clinic in a first-in-human adoptive cell therapy trial for patients with relapsed/refractory (rel/ref) myeloid malignancies. This study demonstrated that rel/ref AML patients were able to safely receive IL-12/15/18-activated donor NK cells (up to 10x106/kg) without developing cytokine release syndrome, neurotoxicity, or graft-versus-host disease. Immune monitoring revealed that memory-like NK cells expanded, trafficked to the bone marrow, and exhibited enhanced anti-leukemia function ex vivo. Clinical responses (CR/CRi) were observed in >50% of patients with active rel/ref AML.11 Ongoing studies are exploring memory-like NK cell adoptive immunotherapy in a phase 2 trial for rel/ref AML, combined with same-donor allogeneic hematopoietic cell transplantation (HCT), and for relapse after allogeneic HCT. Active areas of investigation in memory-like NK cell biology and therapeutics include defining mechanisms that regulate memory-like differentiation and enhanced function, elucidating memory-like NK cell checkpoints, evaluating autologous memory-like NK cell responses against cancers, and developing strategies to enhance memory-like NK cell targeting of resistant malignancies.
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Disclosures
Fehniger: Altor BioScience: Research Funding; Cyto-Sen Therapeutics: Consultancy; Celgene: Research Funding; NIH/NCI: Other: R01 CA205239, P50CA171963; Affimed: Research Funding.