CD4 and chronic obstructive pulmonary disease: Significant correlations were also found in BAL CD4+ and CD8+ T-cell subsets in the COPD smoker group with serotonin: CD4+ FOXP3+ (p = 0.02, r = -0.53), CD8+ CD103+ CD69− CD27+ (p = 0.03, r = 0.53), and CD8+ CD103+ CD69− CD27− (p = 0.04, r = 0.50) (Figure 6D–F).