Prospects and Advances in Adoptive Natural Killer Cell Therapy for Unmet Therapeutic Needs in Pediatric Bone Sarcomas

Abstract

Malignant bone tumors are aggressive tumors, with a high tendency to metastasize, that are observed most frequently in adolescents during rapid growth spurts. Pediatric patients with malignant bone sarcomas, Ewing sarcoma and osteosarcoma, who present with progressive disease have dire survival rates despite aggressive therapy. These therapies can have long-term effects on bone growth, such as decreased bone mineral density and reduced longitudinal growth. New therapeutic approaches are therefore urgently needed for targeting pediatric malignant bone tumors. Harnessing the power of the immune system against cancer has improved the survival rates dramatically in certain cancer types. Natural killer (NK) cells are a heterogeneous group of innate effector cells that possess numerous antitumor effects, such as cytolysis and cytokine production. Pediatric sarcoma cells have been shown to be especially susceptible to NK-cell-mediated killing. NK-cell adoptive therapy confers numerous advantages over T-cell adoptive therapy, including a good safety profile and a lack of major histocompatibility complex restriction. NK-cell immunotherapy has the potential to be a new therapy for pediatric malignant bone tumors. In this manuscript, we review the general characteristics of osteosarcoma and Ewing sarcoma, discuss the long-term effects of sarcoma treatment on bones, and the barriers to effective immunotherapy in bone sarcomas. We then present the laboratory and clinical studies on NK-cell immunotherapy for pediatric malignant bone tumors. We discuss the various donor sources and NK-cell types, the engineering of NK cells and combinatorial treatment approaches that are being studied to overcome the current challenges in adoptive NK-cell therapy, while suggesting approaches for future studies on NK-cell immunotherapy in pediatric bone tumors.

Keywords: Ewing sarcoma; NK cells; adoptive cell therapy; immune evasion; immunotherapy; osteosarcoma; pediatric bone sarcoma; tumor microenvironment.

Figure 1

The evolution of standard treatment in Ewing sarcoma and osteosarcoma over time. The associated change in survival is shown in the bottom panel. Purple [top line]: Ewing sarcoma; blue [bottom line]: osteosarcoma [5,20,38,44].

Figure 2

The effects of sarcoma therapy on bones. Both chemotherapy and radiotherapy have negative effects on bone growth and BMD. These therapies can obviate the stem cell niches required for BMD maintenance, as well as inhibit the differentiation of stem cells into osteoblasts (inhibition is shown by the red cross in the figure). The therapies can also increase bone resorption by inducing osteoclast activity and osteoblast apoptosis, leading to decreased BMD, increased fracture risk and decreased longitudinal bone growth.

Figure 3

Mechanisms that mediate the immune exclusion in osteosarcoma and Ewing sarcoma. Immune responses against primary bone malignant tumors are inhibited at multiple levels. MHC loss that is primarily mediated by reduced expression and shedding of MHC molecules decrease the T-cell recognition of the tumor cells. Immune recognition is also hampered by the low number of tumor-associated antigens (TAAs) and central and peripheral tolerance mechanisms that prevent immune cells from recognizing self-antigens on the tumor cells. Co-stimulation required for T-cell activation is also hampered by CTLA-4 upregulation. Dense, hypoxic and acidic TME conditions prevent immune effector infiltration and activation against the bone sarcoma cells. Furthermore, suppressive immune cell populations, which are either recruited to the tumor site or differentiate into a regulatory phenotype in TME, can cause anergy or death of anti-tumor immune effectors. Various immune checkpoint molecules expressed by the tumor cells also deliver inhibitory signals against immune cell activation (MHC: major histocompatibility complex; TAM: tumor-associated macrophages; Treg: regulatory T cells; MDSC: myeloid-derived suppressor cells).

Figure 4

Approaches for an effective adoptive NK cell therapy in pediatric bone sarcomas. NK cells can be activated and expanded ex vivo by cytokines or membrane-bound cytokines presented on the feeder cells. Ex vivo activation by a cocktail of cytokines can also be employed to induce memory-like NK cells. The in vivo persistence and activation of NK cells can also be promoted by cytokine coadministration. NK cells are also engineered to express either higher quantities of activating receptor or their higher affinity forms, and to knock out inhibitory molecules/receptors. Inhibition is shown by the red cross in the figure. CAR-NK cells that are specific to tumor-associated antigens on sarcoma cells are also generated. The multieffector function of NK cells also make them suitable for combinatorial therapies because the cytokine released by these cells can stimulate other cells (such as CAR T cells), ADCC can increase monoclonal antibody efficiency, stress signals generated by chemotherapy and radiotherapy can enhance NK-cell recognition, and NK cells can effectively lyse tumor cells infected by viral vaccines.