CD274 and neoplasm: The role of CTCs extends beyond representing tumor burden and aggressiveness; they may directly inhibit T and natural killer (NK) cell activation via surface expression of immune checkpoints (PD-L1, CTLA-4, CD47) and indirectly suppress immunity by secreting factors that recruit immunosuppressive myeloid-derived suppressor cells (MDSCs) and regulatory T cells (Tregs) [69].