CCL2 and chronic obstructive pulmonary disease: COPD lung cells exhibit a “senescence-associated secretory phenotype” (SASP), which is characterized by senescent lung cells secreting pro-inflammatory molecules such as IL-6, IL-8, monocyte chemoattractant protein-1, and plasminogen-activated inhibitor-1 [49,50], which further contribute to the pathogenesis of COPD.