Extracellular cathepsin K exerts antimicrobial activity and is protective against chronic intestinal inflammation in mice

Objective: Cathepsin K is a lysosomal cysteine protease that has pleiotropic roles in bone resorption, arthritis, atherosclerosis, blood pressure regulation, obesity and cancer. Recently, it was demonstrated that cathepsin K-deficient (Ctsk^-/-) mice are less susceptible to experimental autoimmune arthritis and encephalomyelitis, which implies a functional role for cathepsin K in chronic inflammatory responses. Here, the authors address the relevance of cathepsin K in the intestinal immune response during chronic intestinal inflammation. Design: Chronic colitis was induced by administration of 2% dextran sodium sulphate (DSS) in distilled water. Mice were assessed for disease severity, histopathology and endoscopic appearance. Furthermore, DSS-exposed Ctsk^-/- mice were treated by rectal administration of recombinant cathepsin K. Intestinal microflora was assessed by real-time PCR and 16srDNA molecular fingerprinting of ileal and colonic mucosal and faecal samples. Results: Using Ctsk^-/- mice, the authors demonstrate a protective role of cathepsin K against chronic DSS colitis. Dissecting the underlying mechanisms the authors found cathepsin K to be present in intestinal goblet cells and the mucin layer. Furthermore, a direct cathepsin K-mediated bactericidal activity against intestinal bacteria was demonstrated, which potentially explains the alteration of intestinal microbiota observed in Ctsk^-/- mice. Rectal administration of recombinant cathepsin K in DSS-treated Ctsk^-/- mice ameliorates the severity of intestinal inflammation. Conclusion: These data identify extracellular cathepsin K as an intestinal antibacterial factor with antiinflammatory potential and suggest that topical administration of cathepsin K might provide a therapeutic option for patients with inflammatory bowel disease.