Regulatory T cells (Tregs) are known to be defective in SLE (systemic lupus erythematosus) patients and animals with lupus-like autoimmunity. Treg repopulation or differentiation stimulation with intravenous IL2 administration stalls the progression of lupus-like autoimmunity in mice. However, continuous treatments are required in order to achieve the efficacy, which also bring in the risk of unnecessary complications. Cathepsin S (CatS) is a lysosomal protease that mediates mouse H-2 haplotype-dependent antigen presentation. CatS also activates toll-like receptor-7 (TLR7) that interferes with Treg differentiation and T-effector cell (Teff) immunosuppression. Our preliminary data indicated increased CatS expression in lupus-prone mice. Systemic CatS inhibition demonstrated H-2 haplotype-independent mitigation of lupus-like autoimmunity in mice. CatS-selective inhibition of Tregs enhanced cell lifespan, Teff immunosuppressive activity, and therapeutic efficacy in lupus-prone mice. This study is proposed to test the role of CatS in TLR7 activation, Treg immunobiology and regulation, and associated molecular and cellular mechanisms using CatS-deficient mice and a CatS-selective inhibitor in cultured cells and/or in lupus-prone mouse models of various genetic backgrounds. This study may open the prospect using CatS inhibitor-modified Tregs to treat human SLE without bring in adverse effects from a systemic CatS inhibition.