CD4 and chronic obstructive pulmonary disease: It has been shown that dendritic cells in the epithelial and submucosal layers of the trachea mucosa activate CD4+ and CD8+ T cells upon stimulation and are involved in the development of COPD.[37,38] In a case-control study, an increased abundance of CD8+ T cells in the lungs of mild-to-moderate COPD associated with pre-severe lesions inflammation,[39] aligning with our finding of a causal outcome of BTN1A1 with early COPD.