Cathepsin S is expressed in antigen-presenting cells and plays a role in invariant chain processing and major histocompatibility complex class II (MHCII) antigen presentation leading to CD4+ T-cell activation. An oral Cathepsin S inhibitor that blocks MHCII antigen presentation could result in a T-cell-selective immunosuppressant agent with improved safety over the current standard of care for the treatment of rheumatoid arthritis, psoriasis, multiple sclerosis and other autoimmune-based inflammatory diseases.