IMMU-55. THERAPEUTIC WINDOW ENABLING ERADICATION OF RESIDUAL GLIOMA STEM CELLS BY INTRACRANIAL NK CELL AND EXTRACELLULAR VESICLE-MEDIATED THERAPY FOLLOWING TEMOZOLOMIDE-INDUCED TUMOUR DEPOPULATION

Abstract

Abstract Glioblastoma (GBM) is an incurable brain cancer, where dismal outcomes result from disease recurrence driven by tumour-initiating glioma stem cells (GSCs). GSCs survive and expand in the brain after surgery, radiation and temozolomide (TMZ) amidst weak immune and natural killer (NK) cell surveillance. NK cell-mediated killing of GBM cells occurs at effector to target rations precluding effective eradication as enforced by clinical trials with autologous NK cells in an adjuvant setting. Here we explore in a human GSC xenograft model whether tumor depopulation using high dose Temozolomide (TMZ) could create a window of curative opportunity for endogenous or exogenous NK cells. We observed that while subcutaneous (sc) xenografts of patient derived GSCs are infiltrated by endogenous functional (NCR1+) NK cells in SCID mice, the corresponding intracranial (ic) tumours remained NK cell depleted. In NK-deficient NSG mice, neither site contained functional NK cells. Notably, while TMZ caused near complete regression of GSC xenografts in both strains of mice and both locations (sc and ic), this was followed by recurrence of drug resistant lesions in all cases except NK-infiltrated sc SCID xenografts. This effect was obliterated by anti-sialo GM1 treatment. To explore whether direct delivery of NK cells into depopulated intracranial tumours would change the lethal course of the disease, mice were injected with GSCs ic, and upon tumour formation were treated with a sequence of TMZ (systemically), and two days later with irradiated NK92MI. Remarkably, this combined therapy completely obliterated recurrent disease. To assess whether NK92MI cells could be replaced by their derived extracellular vesicles (NK-EVs), the latter were injected i.c. post TMZ in GSC xenograft bearing mice. A single injection of NK-EVs resulted in tumour eradication in some but not all mice. Thus, chemotherapy-dependent tumour depopulation may create a unique window of opportunity for curative NK-mediated immunotherapy in GBM.