CD80 and neoplasm: Tumor-initiating cancer stem cells (tCSC), which are critical for tumor formation and growth, are particularly difficult to eradicate and display intrinsic resistance to chemotherapy and ICB in part due to their slower growth rate and elevated expression of the ligands for PD-1 and CTLA-4 inhibitory receptors (PD-L1 and CD80, respectively) (10–12).