Using a threshold of significance of p <0.05 and |r| >0.5, serum IDO positively correlated with blood CXC chemokine ligand 9 (CXCL9, p = 0.009, r = 0.59) and chemokine (C-C motif) ligand 4 (CCL4, p = 0.02, r = 0.56), as well as with CD4+ forkhead box P3 (FOXP3)+ broncheoalveolar lavage (BAL) cells (p = 0.01, r = 0.57) in COPD smokers (Table 2). Here, CD4 is linked to chronic obstructive pulmonary disease.