GDNF and glioblastoma: The infiltration of peripheral immune cells, in particular bone marrow-derived monocytes and macrophages, is favored by the disruption of the blood–brain barrier determined by GBM itself and the release of cytokines and chemokines by glioma cells such as CC-chemokine ligand 2 (CCL2), CCL7, glial cell line-derived neurotrophic factor (GDNF), colony-stimulating factor-1 (CSF-1), granulocyte-macrophage colony-stimulating factor (GM-CSF), hepatocyte growth factor (HGF), stroma cell-derived factor 1 (SDF-1).