Cathepsin S is a cysteine protease of the papain family, found inside the lysosomal/endosomal compartments of antigen-presenting cells such as B cells, macrophages and dendritic cells. Cathepsin S participates in the degradation of antigenic proteins to peptides for presentation to the MHC class II. Cathepsin S has additional biological effects, acting as elastase on the surface of vascular endothelial cells, a process that contributes to microvascular complications in diabetes mellitus.