Effect of entinostat on NK cell-mediated cytotoxicity against osteosarcoma cells and osteosarcoma lung metastasis

There is a crucial need for a new therapeutic approach for osteosarcoma (OS) lung metastasis since this disease remains the main cause of mortality in OS. We previously demonstrated that natural killer (NK) cell therapy has minimal efficacy against OS metastasis. This study determined whether the histone deacetylase inhibitor entinostat could immunosensitize OS cells to NK cell lysis and increases the efficacy of NK cell therapy for OS lung metastasis. Entinostat upregulated ligands for NK cell-activating receptors (major histocompatibility complex [MHC] class I polypeptide-related chain A [MICA] and B [MICB]; UL16 binding proteins 1, 2, 5, and 6; and CD155) on OS cells both in vitro and in vivo and led to more susceptibility to NK cell-mediated cytotoxicity in vitro. Importantly, entinostat did not change NK cell viability, receptor expression, or function within the 24-h treatment. We also demonstrated two potential mechanisms by which entinostat enhanced expression of MICA and MICB on OS cells. Although entinostat upregulated ligands for the NK cell activating receptor on OS lung metastasis, it failed to augment the efficacy of NK cell therapy in our nude mouse human OS lung metastasis model. This can be partly explained by our finding that although the infused NK cells were active and functional and could penetrate into the lungs, they failed to infiltrate into the lung nodules. These challenges regarding cellular immunotherapy against solid tumors may be overcome by combination therapy, such as adding a NK cell-activating cytokine (IL-2 or IL-21).