PDCD1 and chronic obstructive pulmonary disease: Among individuals younger than 70 years old, the increased percentages of PD1+CD4+ and PD1+CD8+ cells at discharge in COPD patients were significantly associated with a higher risk of recurrent AECOPD within 1 year of rehospitalization, with risks elevated by 55.1% and 58.3%, respectively (P < 0.001).