It was the aim of the present study, therefore, to study major co-stimulatory molecules such as programmed death ligand 1 (PD-L1) or OX40 ligand (OX40L) and other function-associated surface molecules on circulating DCs of patients with COPD for the first time, and to compare the findings with control groups of asymptomatic smokers and never-smokers. Here, TNFSF4 is linked to chronic obstructive pulmonary disease.