Malignant gliomas are heterogeneous, aggressive tumours. A small fraction of glioma cells is undifferentiated and defined as glioma initiating cells (GICs), whereas the majority of glioma cells are differentiated (GDCs). Malignant glioma patients have a dismal prognosis, even with standard-of-care treatment. The use of T-cell immunotherapy is a promising approach, but needs to be potentiated. The hypomethylating agent Decitabine (DAC) up-regulated expression of glioma surface molecules involved in cell-cell interactions and enhanced cytotoxic T-cell (CTL) killing of both GDCs and GICs. Thus, DAC sensitises glioma cells (with diverse differentiation levels) to T-cell effector responses. However, in vivo results showed no benefit of systemic DAC administration before CTL adoptive transfer, with limited hypomethylating effects on brain tumour. Overall, the promising in vitro immunosensitisation of glioma cells using DAC should encourage development of efficient drug delivery to the tumour site in vivo to explore the therapeutic potential of hypomethylating drugs and immunotherapy.