CD4 and chronic obstructive pulmonary disease: In our study, we observed a negative correlation between the number of pack-years of smoking and the expression of PD-1 on CD4+T cells (r = −0.33, p = 0.04; Fig. 4I), Tregs (r = −0.37, p = 0.02; Fig. 4J), and percentages of G-MDSCs cells (r = −0.35, p = 0.03; Fig. 4K) in COPD patients, but not the percentages of M-MDSCs, PD-L1 expression on MDSCs subsets and CTLA-4 expression on Treg.