PDCD1 and chronic obstructive pulmonary disease: As the inclusion of comorbidity assessment in the CODEX index was also a common prognostic indicator for COPD, the PD-1+CD4+T/ CD4+T% (r = 0.451) and PD-1+CD8+T/ CD8+T% (r = 0.383) at baseline were demonstrated the moderate positive correlations with patients’ CODEX indices, with statistically significant(p < 0.05).