CCL21 and chronic obstructive pulmonary disease: This study also shows that although activated dendritic cells conditioned with CSE have diminished migratory CCR7 expression - an effect induced by oxidative constituents in CSE - migration of CSE-conditioned activated dendritic cells towards the CCR7 ligand CCL21 is not impaired, implying that retention of activated dendritic cells due to impaired CCR7-dependent migration is unlikely to be an important mechanism for the increased dendritic cell numbers in COPD.