SIRPA and glioblastoma: In addition, CD47, an immune checkpoint protein highly expressed in GBM cells,686,687 can inhibit macrophage phagocytosis of GBM cells by binding to the signal-regulatory protein α (SIRPα) receptor of macrophages.688,689 Downregulation of CD47 promotes immune clearance of GBM cells by blocking the interaction between CD47 and the SIRPα receptor.