STAT3 and neoplasm: Furthermore, TGF-β establishes an immunosuppressive axis through: (1) Upregulating PD-L1 on CAFs via p38 MAPK/STAT3 signaling, enabling T-cell exhaustion through PD-1 ligation (29); (2) Inducing FAS ligand expression that triggers apoptosis of tumor-infiltrating CD8+ T cells (30); (3) Activating IDO1/kynurenine pathway in dendritic cells, which expands myeloid-derived suppressor cells (31).