MHC class II-restricted Ag processing requires protein degradation in the endocytic pathway for the activation of CD4+ T cells. Gamma-interferon-inducible lysosomal thiol reductase (GILT) facilitates Ag processing by reducing protein disulfide bonds in this compartment. Lysosomal cysteine protease cathepsin S (CatS) contains disulfide bonds and mediates essential steps in MHC class II-restricted processing, including proteolysis of large polypeptides and cleavage of the invariant chain. We sought to determine whether GILT’s reductase activity regulates CatS expression and function. Confocal microscopy confirmed that GILT and CatS colocalized within lysosomes of B cells. GILT expression posttranscriptionally decreased the steady-state protein expression of CatS in primary B cells and B-cell lines. GILT did not substantially alter the expression of other lysosomal proteins, including H2-M, H2-O, or CatL. GILT’s reductase active site was necessary for diminished CatS protein levels, and GILT expression decreased the half-life of CatS, suggesting that GILT-mediated reduction of protein disulfide bonds enhances CatS degradation. GILT expression decreased the proteolysis of a CatS selective substrate. This study illustrates a physiologic mechanism that regulates CatS and has implications for fine tuning MHC class II-restricted Ag processing and for the development of CatS inhibitors, which are under investigation for the treatment of autoimmune disease.

MHC class II-restricted processing requires protein degradation in the endosomal pathway for activation of CD4+ T cells. Gamma-interferon-inducible lysosomal thiol reductase (GILT) facilitates the processing of class II-restricted antigens by reducing protein disulfide bonds. Our previous work demonstrates that reductase active site mutants of GILT have diminished processing of precursor GILT to mature GILT, suggesting that GILT’s reductase active site may decrease the expression or activity of lysosomal proteases responsible for cleavage of GILT’s N- and C-terminal pro-peptides. Cathepsin S (CatS) is capable of processing precursor GILT to its mature form and mediates essential steps in class II-restricted processing, including proteolysis of large polypeptides and invariant chain cleavage. Therefore, we sought to determine whether GILT’s reductase activity regulates CatS expression and function. Our studies show GILT and CatS colocalize within lysosomes of murine B cells. GILT expression acts post-transcriptionally to decrease CatS steady state protein expression. GILT’s reductase active site is necessary for diminished CatS protein levels. However, GILT does not substantially alter the steady state protein expression of other lysosomal proteins, H2-M, H2-O or CatL. In addition, GILT expression decreases proteolysis of a CatS-selective substrate. Thus, GILT’s reductase activity is likely to fine tune class II-restricted antigen processing through decreasing CatS activity.