CTSS (Cathepsin S) is a lysosomal enzyme that belongs to the papain family of cysteine proteases. This protein is expressed by antigen presenting cells including macrophages, B-lymphocytes, dendritic cells and microglia. Moreover, CTSS is expressed in some epithelial cells. In contrast, cortical thymic epithelial cells do not express CTSS. Normal epidermal keratinocytes do not express CTSS. However, level of CTSS is elevated in psoriatic keratinocytes due to stimulation by proinflammatory factors. CTSS can function as an elastase over a broad pH range in alveolar macrophages.