One possible explanation for the inverse correlation between plasma cathepsin S and severity of disease among the COPD patients is that cathepsin S is produced in lung tissue including activated macrophages [6], and as COPD severity increases there is progressive destruction of the alveolar walls with loss of source of cathepsin S. Other cellular sources of cathepsin S such as adipose tissue [21] could also be altered quantitatively and qualitatively between the early and late stages of COPD. Here, CTSS is linked to chronic obstructive pulmonary disease.