Overexpression of cathepsin S induces chronic atopic dermatitis in mice

Atopic dermatitis (AD) is a chronically relapsing, noncontagious pruritic skin disease with two phases: acute and chronic. Previous studies have shown that the cysteine protease cathepsin S (CTSS) is linked to inflammatory processes, including atherosclerosis and asthma. The possibility that this or other cysteine proteases might cause itching or be part of a classical ligand-receptor signaling cascade has not been previously considered. Recently, CTSS was shown to be a ligand for proteinase-activated receptor-2 (PAR-2), which is associated with itching. In this study, we show that CTSS-overexpressing transgenic (TG) mice spontaneously develop a skin disorder similar to chronic AD. The results of this study suggest that CTSS overexpression triggers PAR-2 expression in dendritic cells (DCs), resulting in the promotion of CD4 ⁺ differentiation, which is involved in major histocompatibility complex (MHC) class II expression. In addition, we investigated mast cells and macrophages and found significantly higher mean levels of T helper type 1 (Th1) cell-associated cytokines than T helper type 2 (Th2) cell-associated cytokines in CTSS-overexpressing TG mice. These results suggest that increased PAR-2 expression in DCs as a result of CTSS overexpression induces scratching behavior and Th1 cell-associated cytokine expression, and can trigger chronic AD symptoms.