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 (By similarity). 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 lymphocytes and macrophages of the thymus and spleen, and in the M cells of the intestine. In the brain, expression is limited to reactive microglial cells, the large pyrimidial neurons in the cerebral cortex, the CA1 and CA3 pyrimidial neurons of the hippocampus, the large neurons of the neostriatum, and the Purkinje neurons of the cerebellum. Expression increases in all brain regions examined with age, and increases markedly in reactive microglial cells amd CA1 pyrimidial neurons following ischemic injury. In the thymus expression increased steadily up to 8 weeks of age before decreasing to a much lower level by 52 weeks. Expression levels in the spleen and stomach do not appear to vary with age. Glycosylated. The nature of the carbohydrate chain varies between cell types. In brain microglia, the proenzyme contains a high mannose-type oligosaccharide, while the mature enzyme contains a complex-type oligosaccharide. In stomach and spleen, the mature enzyme contains a high mannose-type oligosaccharide. In erythrocyte membranes, the mature enzyme contains a complex-type oligosaccharide. Administration of dexamethasone results in the conversion of the proenzyme to the mature form in thymocytes. Belongs to the peptidase A1 family.