Lysosomal thiol reductase that can reduce protein disulfide bonds. May facilitate the complete unfolding of proteins destined for lysosomal degradation. Plays an important role in antigen processing. Facilitates the generation of MHC class II-restricted epitodes from disulfide bond-containing antigen by the endocytic reduction of disulfide bonds. Facilitates also MHC class I-restricted recognition of exogenous antigens containing disulfide bonds by CD8+ T-cells or crosspresentation (By similarity). Dimer; disulfide-linked. Expressed in many tissues, including spleen, liver, lung, heart, intestine, blood and kidney. By LPS in spleen and blood. N-glycosylated. Sugar chains contain mannose-6-phosphate (By similarity). Synthesized as a 35 kDa precursor which is then processed into the mature 30 kDa form via cleavage of N-terminal and C-terminal propeptides. Processing of the precursor is mediated by multiple lysosomal proteases (By similarity). Both precursor form and mature form have thiol reductase activity. Belongs to the GILT family.