Zoledronate sensitizes neuroblastoma-derived tumor-initiating cells to cytolysis mediated by human γδ T cells.

Neuroblastoma is the most common extracranial solid tumor in children that is refractory to intensive multimodal therapy. In particular, tumor-initiating cells (TICs) derived from neuroblastoma are believed responsible for tumor formation and resistance to the conventional therapy; an optimal strategy therefore should target this population. Technically, TICs can be enriched from neuroblastoma-derived spheres when the tumor cells are cultured in a serum-free medium supplemented with certain growth factors. Recently, a line of evidence has suggested antitumor potential of Vγ9Vδ2 T cells (γδ T cells), a T-cell population that recognizes and kills target cells independent of surface HLA expressions. Furthermore, a mevalonate pathway inhibitor, zoledronate, has been reported to enhance cytolytic activity of γδ T cells. On the basis of these findings, we hypothesized that zoledronate would sensitize neuroblastoma TICs to γδ T-cell-mediated cytolysis and promote therapeutic efficacy against neuroblastoma. In the current study, we show that zoledronate efficiently sensitizes both neuroblastoma-derived adherent cells and sphere-forming cells to γδ T-cell-mediated cytolysis. Subsequently, in vitro colony formation inhibition assay and in vivo animal studies reveal that the presence of γδ T cells decelerates outgrowth of neuroblastoma TICs. We finally show that addition of interleukin-15 and/or interleukin-18 in culture enhances the cytolytic activity of γδ T cells. On the basis of these data, we conclude that ex vivo expanded γδ T cells are a promising tool for antineuroblastoma immunotherapy with options for further improvement.