PDCD1 and chronic obstructive pulmonary disease: (c) PD-1: programmed cell death-1; (d) PD-L1: programmed death ligand-1; (e) COPD: chronic obstructive lung disease; (f) ECOG: Eastern Cooperative Oncology Group; (g) Gy: Gray, a unit of ionizing radiation dose in the International System of Units (SI)