PDCD1 and chronic obstructive pulmonary disease: Because CXCR6high CD4 effector memory in CD4 positively correlated with preserved lung function in E-COPD, and immune checkpoint molecules (CTLA4, PDCD1, LAG3, and CD96) were among the signature genes expressed in this subset, we next performed interactome analyses to identify potential mechanisms for their function in E-COPD.