PDCD1 and glioblastoma: Fourth, in addition to PD-L1, PD-1, and CTLA-4, TMEs of GBM may also contain other immunosuppressive factors, such as the A2aR high-affinity adenosine receptor (on lymphocytes and tumor-associated macrophages) or PD-L2 (on macrophages lacking PD-L1 expression).