Furthermore, in GBM, the NK and CD8+ response and the adaptive immune system are largely suppressed by the recruitment of abundant immune modulators that make the TME “cold” and refractory to immunotherapy such as Treg, TAM, and immunosuppressive molecules, including IL-6, IL-10 and TGF-β, and enzymes such as indolamine 2,3-dioxygenase (IDO) [51,54]. This evidence concerns the gene IDO1 and glioblastoma.