GDNF and glioblastoma: After activation by TLR-3 ligands such as Poly I:C, dsRNA, and other TLR agonists, microglial cells synthesize and secrete IL-12, TNF-α, IL-6, IL-3, IL-10, CXCL-10, IFN-β, nerve growth factor (NGF), neurotrophin (NT)-4/5, transforming growth factor β1 (TGF-β1), glial-derived neurotrophic factor (GDNF), and fibroblast growth factor 2 (FGF2), producing chronic neuroinflammation and eventually GBM progression [185,186].