SPP1 and glioblastoma: TAMs are attracted to the GBM milieu through GBM-derived cytokines such as C–C motif chemokine ligand (CCL)-2, colony stimulating factor (CSF)-1, granulocyte macrophage-stimulating factor (GM-CSF), C–X3–C motif chemokine ligand (CX3CL1; also known as fractalkine), VEGF, MMP2, TGF-β, SDF-1, and osteopontin (OPN) [62,63,64,65,66].