Perez-Martinez et al. antibody-dependent NK cell cytotoxicity, counteracting the NK cell mechanisms of self-tolerance, and developing adoptive NK cell therapy including chimeric antigen receptor-expressing NK cells. Keywords:osteosarcoma, Ewing sarcoma, rhabdomyosarcoma, natural killer, immunotherapy == Introduction == Osteosarcoma, Ewing sarcoma (EWS), and rhabdomyosarcoma (RMS) are the most common sarcomas in children. Patients with metastatic, relapsed, or refractory pediatric sarcoma have a dismal prognosis with less than 30% maintaining long-term survival (1). Further, no significant improvement in patient outcome has been made over the last 3 decades with currently available therapies (surgery, radiation, and chemotherapy) (24). Hence, there is an urgent need for innovative therapeutic interventions, such as immunotherapy. Immunotherapy is not a new concept for sarcoma. The earliest explained sarcoma immunotherapy was in 1891, when William B. Coley observed tumor regression after locally injectingStreptococcusbacteria into patients sarcoma to generate an immune response (5). Now scientific and clinical evidence strongly supports the critical role for both early and late immune responses to control cancer growth. As an example, there is mounting evidence that hematopoietic stem cell transplantation (HSCT) has an allograft-versus-tumor effect in the treatment of leukemia and in a subgroup of solid tumors including sarcomas (6). Rapid immune reconstitution post HSCT is critical for the anti-tumor effect, particularly due to the recovery of natural killer (NK) cells (7). Nevertheless, studies investigating the function of NK cells in sarcoma in the clinical field are limited and further work aiming to unleash the full potential of NK cells to improve their anti-sarcoma activity are needed. This review highlights the current knowledge in the field and future perspectives of applying NK cell-based immunotherapies to treat pediatric sarcomas. Possible strategies to increase NK cells efficiency discussed here are the use SIS-17 of monoclonal antibodies (mAb) targeting tumor antigens or of pharmacological brokers to enhance antibody-dependent cell-mediated cytotoxicity (ADCC), the use of cytokines to enhance NK cell-mediated anti-tumor activity, strategies to counteract self-tolerance, and the development of adoptive therapy using NK cells, including chimeric antigen receptor (CAR)-expressing NK cells (Physique 1). == Physique 1. == Enhancing NK cells-versus-pediatric sarcoma effect.(A)Expanded and activated NK-cellsin vitroor(B)engineered NK cells expressing combined antigen-receptors (CAR) specific to the sarcoma cells can be infused to the patients.(C)Infused cytokines can stimulate the NK cell cytotoxic activity.(D)Specific anti-tumor antibodies activate NK cellsviatheir CD16 activating receptor and drive the antibody-dependent cell cytotoxicity (ADCC). Small molecules, such as EZH2 inhibitors and prochlorperazine, can increase antigen presentation to facilitate ADCC.(E)Self-tolerance can be counteracted if the NK cells are KIR-mismatched to the tumor, if the NK inhibitory receptors are targeted with specific inhibitory antibodies (anti-KIR, NKG2A or TIM-3) or if the NK activating receptors ligands, such SIS-17 as MIC-A/B, are overexpressed due to genotoxic stress such SIS-17 as radiation therapy. == NK Cell Activity in Pediatric Sarcoma == NK cells are cytotoxic innate immune cells that can eliminate infected or transformed cells without prior sensitization. NK cells express inhibitory surface receptors, such as killer-cell immunoglobulin receptors (KIR), which recognizes specific human leucocyte antigen (HLA) class I molecules HLA-A, B SIS-17 and C, and CD94/NK group 2 member A (NKG2A), which recognizes HLA-E (8). NK cells are educated to lyse target cells lacking expression of major histocompatibility complex (MHC) class I molecules expressed by the host cells (9). Their activating surface receptors include natural cytotoxic receptors (NCRs) and NK group 2 member D (NKG2D), which identify stress proteins on the surface of target cells such as MICA/B and ULBPs, and a Fc receptor CD16, that mediate ADCC through acknowledgement of the Fc portion of antibodies on opsonized cells (10). Coreceptors of NCRs and NKG2D, such as DNAM-1, are capable of amplifying the NK cell activation (11). The balance between activating and inhibitory signals received by the NK cells IgG2b/IgG2a Isotype control antibody (FITC/PE) determines their cytotoxic activity. This activity is usually mediated by the release of cytotoxic granules made up of granzymes and perforin, expression of death receptor ligands on their surface,.