Approaches to Enhance Natural Killer Cell-Based Immunotherapy for Pediatric Solid Tumors

Abstract

Treatment of metastatic pediatric solid tumors remain a significant challenge, particularly in relapsed and refractory settings. Standard treatment has included surgical resection, radiation, chemotherapy, and, in the case of neuroblastoma, immunotherapy. Despite such intensive therapy, cancer recurrence is common, and most tumors become refractory to prior therapy, leaving patients with few conventional treatment options. Natural killer (NK) cells are non-major histocompatibility complex (MHC)-restricted lymphocytes that boast several complex killing mechanisms but at an added advantage of not causing graft-versus-host disease, making use of allogeneic NK cells a potential therapeutic option. On top of their killing capacity, NK cells also produce several cytokines and growth factors that act as key regulators of the adaptive immune system, positioning themselves as ideal effector cells for stimulating heavily pretreated immune systems. Despite this promise, clinical efficacy of adoptive NK cell therapy to date has been inconsistent, prompting a detailed understanding of the biological pathways within NK cells that can be leveraged to develop "next generation" NK cell therapies. Here, we review advances in current approaches to optimizing the NK cell antitumor response including combination with other immunotherapies, cytokines, checkpoint inhibition, and engineering NK cells with chimeric antigen receptors (CARs) for the treatment of pediatric solid tumors.

Keywords: CAR NK cells; Ewing sarcoma; NK cells; childhood cancer; chimeric antigen receptor; hepatoblastoma; neuroblastoma; osteosarcoma; rhabdomyosarcoma; solid tumor.

Figure 1

Activating and inhibitory NK cell receptors present in mice and humans. Abbreviations: PVR—polio virus receptor, HL—human leukocyte antigen, KIR—killer-cell immunoglobulin-like receptor, NKG2A—natural killer group 2A, NKG2D—natural killer group 2D, NKG2C—natural killer group 2C, DNAM-1—DNAX accessory molecule-1, LLT1—lectin-like transcript-1, HA—hemagglutinin, MICA/B—MHC class I chain-related protein A and B, ULBP-1—UL16 binding protein 1, AICL—activation-induced C-type lectin, PtdSer—phosphatidyl serine, Tim-3—T-cell immunoglobulin and mucin domain 3. Ceacam-1—carcinoembryonic antigen-related cell adhesion molecule 1, HMGB1—high-mobility group box 1, LSECtin—lymph node sinusoidal endothelial cell C-type lectin.

Figure 2

Approaches to clinical applications of adoptive NK cell therapy. Sources of NK cells may originate from an autologous donor or allogeneic donor, which will influence how they are collected. Peripheral blood mononuclear cells from autologous donors are collected using leukapheresis and may be subject to KIR genotyping, whereas peripheral blood mononuclear cells from allogeneic donors may be subject to HLA and KIR genotyping. Other allogeneic sources of NK cells include NK cells from cord blood, a cell line such as NK-92 cells, or NK cells derived from induced pluripotent stem cells (iPSC). These sources of NK cells offer the benefit of being an “off-the-shelf” adoptive cell therapy. NK cells require expansion and activation through cytokines, such as IL-2, IL-12, IL-15, IL-18, or IL-21. Membrane bound (mb) IL-15 or IL-21 with 41BB ligand have been genetically engineered into feeder cell lines, such as K562 cells, to generate robust NK expansions. NK cells can be genetically modified to express CARs that enable them to target a specific antigen. Recent developments in CAR NK technology have found that co-expression of IL-15 enhances CAR NK persistence, therefore obviating the need for systematic administration of IL-15. Abbreviations: HLA—human leukocyte antigen, KIR—killer immunoglobulin-like receptor, CAR—chimeric antigen receptor.