The PD-1/PD-1L interaction leads to downregulation of the PI3-K / m-TOR/ RAS/Ras/MAPK/Erk pathways and T cell exhaustion 

PD-1 binds to its ligand PD-1L and leads to T cell activation

PD-1 binds to its ligand CTLA4 and leads to T cell activation

PD-1 binds to its ligand PD-L1 and induces T cell division and expansion