Cathepsin L Inhibition Prevents Murine Autoimmune Diabetes via Suppression of CD8(+) T Cell Activity

BACKGROUND: Type 1 diabetes (T1D) is an autoimmune disease resulting from defects in central and peripheral tolerance and characterized by T cell-mediated destruction of islet β cells. To determine whether specific lysosomal proteases might influence the outcome of a T cell–mediated autoimmune response, we examined the functional significance of cathepsin inhibition on autoimmune T1D-prone non-obese diabetic (NOD) mice. METHODS AND FINDINGS: Here it was found that specific inhibition of cathepsin L affords strong protection from cyclophosphamide (CY)-induced insulitis and diabetes of NOD mice at the advanced stage of CD8(+) T cell infiltration via inhibiting granzyme activity. It was discovered that cathepsin L inhibition prevents cytotoxic activity of CD8(+) T cells in the pancreatic islets through controlling dipeptidyl peptidase I activity. Moreover, the gene targeting for cathepsin L with application of in vivo siRNA administration successfully prevented CY-induced diabetes of NOD mice. Finally, cathepsin L mRNA expression of peripheral CD8(+) T cells from NOD mice developing spontaneous T1D was significantly increased compared with that from control mice. CONCLUSIONS: Our results identified a novel function of cathepsin L as an enzyme whose activity is essential for the progression of CD8(+) T cell-mediated autoimmune diabetes, and inhibition of cathepsin L as a powerful therapeutic strategy for autoimmune diabetes.