Lysosomal thiol reductase that catalyzes protein disulfide bonds reduction (PubMed:10639150). Plays an important role in antigen processing and presentation, of namely both major histocompatibility complex (MHC) class I- and class II-restricted antigen by reducing the disulfide bonds of endocytosed proteins and facilitating their unfolding and optimal degradation (PubMed:10639150). Shares with thioredoxin a reduction mechanism in which the N-terminal cysteine thiol group in the CXXC active site motif initiates a nucleophilic attack on the substrate disulfide bond,resulting in the formation of a mixed disulfide-linked enzyme-substrate intermediate (Probable). Subsequent intramolecular attack by the second cysteine thiol group enables the release of the reduced substrate and oxidized enzyme (Probable). Lysosomal cysteine is a physiological reducing agent that is capable of reducing IFI30, so that it can catalyzes the next reaction (PubMed:10852914). Dithiothreitol, cysteine, and cysteinyl glycine, but not glutathione, are capable of regenerating active precursor and mature IFI30 in vitro (PubMed:10639150, PubMed:10852914). {ECO:0000269|PubMed:10639150, ECO:0000269|PubMed:10852914, ECO:0000305|PubMed:10852914}. This is the function of ENSG00000216490 (IFI30, IFI30 lysosomal thiol reductase).