Abstract 2503: Multicellular Ewing sarcoma spheres are efficiently targeted by activated NK cells

The outcome of disseminated Ewing sarcoma remains poor despite intensive multimodal treatment regimens. The disease often responds well to chemotherapy, but systemic relapses occur in the majority of patients. Targeting of residual disease by cellular immunotherapy may sustain remission and improve outcome. Specifically, Ewing sarcoma cells have been shown to be exquisitely sensitive to targeting by activated NK cells. To further explore the value of cellular strategies, preclinical models are needed that mimic the anchorage-independent, multicellular growth of Ewing sarcoma micrometastases. Here, we generated Ewing sarcoma spheres from cell lines (VH-64, TC-32, TC-71, A4573) and from four low-passage cell cultures established from Ewing sarcoma biopsies at primary diagnosis or at relapse under serum-free growth conditions. Standard monolayer cultures were used for comparisons. Phenotypic analysis revealed considerable heterogeneity among individual Ewing sarcomas and between spheres and monolayers. While the Ewing sarcoma marker CD99 as well as CD133 were expressed at comparable densities, spheres had significantly higher expression of the neural crest marker CD57 (HNK-1) and of MHC class I than monolayers, whereas CD117 (c-kit) expression was lower. Side populations characterized by Hoechst dye exclusion and previously associated with cancer stem cell function were identified in one of two primary sphere cultures and in VH-64 spheres but were absent or reduced in monolayer cultures. However, cells resuspended from spheres did not form subcutaneous tumors in immunodeficient (NOD/scid) mice at higher efficiencies than monolayer cultures, arguing against higher tumorigenicity of sphere-cultured cells. VH64 spheres were significantly more resistant towards doxorubicine than monolayers, and resuspended cells from sphere cultures remained less susceptible to lysis than monolayer cultures, but among primary tumor cells, consistent differences in chemosensitivity were not observed between the two culture systems. In vitro activated allogeneic NK cells were uniformly capable to lyse single cells derived from both monolayer and sphere cultures from established cell lines and primary cell cultures. Moreover, NK cells efficiently eliminated intact Ewing sarcoma spheres. Thus, cultured Ewing sarcoma cells are highly heterogenous. Their phenotype, function and susceptibility to both chemo- and immunotherapy differs among individual cell lines and primary cultures and under variable in vitro growth conditions. Activated NK cells efficiently target Ewing sarcoma cells both as monolayers and as spheres. The sphere model may provide a useful tool to analyze the contribution of micrometastatic architecture and serum-free niches to immune evasion. Experiments with autologous NK cells and with NK cells engineered to express tumor antigen-specific chimeric receptors are ongoing. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 2503. doi:1538-7445.AM2012-2503