May have a role in immune function. Probably involved in the processing of antigenic peptides during MHC class II-mediated antigen presentation. May play a role in activation-induced lymphocyte depletion in the thymus, and in neuronal degeneration and glial cell activation in the brain. Similar to cathepsin D, but slightly broader specificity. Homodimer; disulfide-linked. The proenzyme is localized to the endoplasmic reticulum and Golgi apparatus, while the mature enzyme is localized to the endosome. Expressed abundantly in the stomach, Clara cells and alveolar macrophages of the lung, brain microglia, spleen and activated B-lymphocytes. Not expressed in resting B-lymphocytes. Glycosylated. The nature of the carbohydrate chain varies between cell types. In fibroblasts, the proenzyme contains a high mannose-type oligosaccharide, while the mature enzyme contains a complex-type oligosaccharide. Belongs to the peptidase A1 family.