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