TGFB1 and central nervous system cancer: In addition, glioma-derived IL-6, together with other tumor-secreted factors, such as TGFβ, polarize glioma-infiltrating microglia toward the immunosuppressive M2 phenotype through STAT3 activation, which increases the expression of the immunosuppressive factors TGF-β2, IL-1 and the costimulatory molecules CD80/86 73-76 and confers high PD-L1 expression to promote tumor immune evasion 77.