Cathepsin S is not crucial to TSHR processing and presentation in a murine model of Graves' disease

By regulating invariant (Ii) chain processing and MHC class II peptide loading, the endosomal protease cathepsin S (Cat S) has a potential role in autoimmune susceptibility. Indeed, Cat S null mice are resistant to I-Ab-restricted experimental myasthenia gravis due to inadequate peptide presentation. To explore the role of Cat S in a Graves' disease model, I-Ad-restricted wild-type (WT) and Cat S^-/- mice were immunized with adenovirus encoding the A subunit of thyroid stimulating hormone receptor (TSHR). TSHR adenovirus immunized mice develop Th1 T cells, TSHR antibodies, and a proportion become overtly hyperthyroid. Although TSHR presentation in vitro was initially impaired in Cat S^-/- mice, subsequent TSHR presentation in vitro and disease development were similar in both groups but with higher antibody responses in Cat S null mice. WT and Cat S^-/- mice recognized similar T cell epitopes from a panel of overlapping TSHR peptides. TSHR responses were found to be I-Ad-restricted and Cat S^-/- I-Ad B cells had marked defects in Ii processing. These data imply that loading of TSHR peptides critical to TSHR antibody responses becomes Ii-independent. Contrasting findings among organ-specific murine autoimmune models imply that potential uses of Cat S inhibitors to ameliorate autoimmunity must be determined empirically.