GDF15 and glioblastoma: Some studies demonstrated that CSCs deriving from different solid tumors, such as glioblastoma multiforme (GBM) and melanoma, release a variety of immunosuppressive cytokines, such as IL-13 and IL-10, transforming growth factor β1 (TGF-β), growth/differentiation factor 15 (GDF-15), prostaglandin E2 (PGE2), and galectin-3, that can render the TME protected from effector immune cells [84].