The cysteine endoprotease cathepsin S mediates degradation of the MHC class II invariant chain Ii in human and mouse antigen-presenting cells. Studies described here examine the functional significance of cathepsin S inhibition on autoantigen presentation and organ-specific autoimmune diseases in a murine model for Sjögren syndrome. Specific inhibitor of cathepsin S (Clik60) in vitro markedly impaired presentation of an organ-specific autoantigen, 120-kDa α-fodrin, by interfering with MHC class II-peptide binding. Autoantigen-specific T cell responses were significantly and dose-dependently inhibited by incubation with Clik60, but not with inhibitor s of cathepsin B or L. Clik60 treatment of mouse salivary gland cells selectively inhibited autopeptide-bound class II molecules. Moreover, the treatment with Clik60 in vivo profoundly blocked lymphocytic infiltration into the salivary and lacrimal glands, abrogated a rise in serum autoantibody production, and led to recovery from autoimmune manifestations. Thus, inhibition of cathepsin S in vivo alters autoantigen presentation and development of organ-specific autoimmunity. These data identify selective inhibition of cysteine protease cathepsin S as a potential therapeutic strategy for autoimmune disease processes.

The cysteine endoprotease cathepsin S mediates degradation of the MHC class II invariant chain Ii in human and mouse antigen-presenting cells. Studies described here examine the functional significance of cathepsin S inhibition on autoantigen presentation and organ-specific autoimmune diseases in a murine model for Sjögren syndrome. Specific inhibitor of cathepsin S (Clik60) in vitro markedly impaired presentation of an organ-specific autoantigen, 120-kDa α-fodrin, by interfering with MHC class II-peptide binding. Autoantigen-specific T cell responses were significantly and dose-dependently inhibited by incubation with Clik60, but not with inhibitor s of cathepsin B or L. Clik60 treatment of mouse salivary gland cells selectively inhibited autopeptide-bound class II molecules. Moreover, the treatment with Clik60 in vivo profoundly blocked lymphocytic infiltration into the salivary and lacrimal glands, abrogated a rise in serum autoantibody production, and led to recovery from autoimmune manifestations. Thus, inhibition of cathepsin S in vivo alters autoantigen presentation and development of organ-specific autoimmunity. These data identify selective inhibition of cysteine protease cathepsin S as a potential therapeutic strategy for autoimmune disease processes.

1Four 3-alkylxanthines (3-methylxanthine, 3-n-propylxanthine (enprofylline), 3-n-butylxanthine and 3-iso-butylxanthine) and four 1-methyl-3-alkylxanthines (1-methyl-3-methylxanthine (theophylline), 1-methyl-3-n-propylxanthine, 1-methyl-3-n-butylxanthine and 1-methyl-3-iso-butylxanthine (IBMX)), were compared in terms of cyclic AMP phosphodiesterase (PDE) inhibition and trachealis muscle relaxation. The relationship between xanthine structure and cyclic AMP PDE inhibition was also studied. 2 Xanthine induced relaxation of guinea-pig isolated trachealis muscle was measured against spontaneous tone.3 The four 1-methyl-3-alkylxanthines were each significantly more potent than the corresponding 3-alkylxanthines in relaxing the isolated trachealis muscle. The 1-methyl-3-alkylxanthines were similarly more potent than the corresponding 3-alkyl derivatives in inhibiting low Km cyclic AMP PDE. There was a strong positive correlation between low Km cyclic AMP PDE inhibition and the tracheal smooth muscle relaxation evoked by the xanthine derivatives. 4 Since methylation of the 1-position of each 3-alkylxanthine increased the potency of the derivative in inhibiting low Km cyclic AMP PDE and in relaxing trachealis muscle and since a strong positive correlation was observed between the relaxant EC50 and the Ki value of each xanthine derivative, it is suggested that low Km cyclic AMP PDE inhibition by xanthines plays an important role in their tracheal relaxant effect.

Activation-induced cell death (AICD) is a well-known mechanism of peripheral T cell tolerance that depends upon an interaction between Fas and Fas ligand (FasL). In this study, we demonstrate that the administration of a soluble form of anti-FasL Ab, FLIM58, results in severe destructive autoimmune exocrinopathy in the murine model of human Sjögren’s syndrome (SS), and we found that an organ-specific autoantigen may play an important role on down-modulation of AICD. A high titer of serum autoantibodies against 120-kDa α-fodrin autoantigen was detected in the FLIM58-treated mice, and splenic T cell culture supernatants contained high levels of IFN-γ. In vitro T cell apoptosis assay indicated that FasL-mediated AICD is down-regulated by autoantigen stimulation in spleen cells from the murine SS model, but not from Fas-deficient MRL/lpr mice and FasL-deficient MRL/gld mice. FasL undergo metalloproteinase-mediated proteolytic processing in their extracellular domains, resulting in the release of soluble trimeric ligands (soluble FasL). We showed that the processing of soluble FasL occurs in autoantigen-specific CD4+ T cells, and that a significant increase in expressions of metalloproteinase-9 mRNA was observed in spleen cells from SS model mice. These findings indicate that the increased generation of soluble FasL inhibits the normal AICD process, leading to the proliferation of effector CD4+ T cells in the murine SS model.

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