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. Optimum pH is 4-5. Dimer; disulfide-linked. Strongly expressed in spleen and kidney. Also detected at lower levels in heart, liver, intestine and muscle. Up-regulated in spleen and kidney in response to bacterial lipopolysaccharide (LPS). Belongs to the GILT family.