Cathepsin-k as a diagnostic marker in the identification of micro-granulomas in Crohn's disease

Crohn's disease is a chronic inflammatory bowel disease, whose aetiology and pathogenesis are still unknown. The occurrence of epithelioid granulomas is one characteristic feature of the disease since these lesions are found in the bowel wall in 50-87% of colectomy specimens. Although granulomas are not pathognomonic, their identification is considered a relevant element for diagnosis. Cathepsin-k, a papain-like cysteine protease, is involved in bone remodelling, and has been widely used as a immunohistochemical marker for the in situ detection of osteoclasts. Interestingly, the expression of this potent protease is also significantly increased in stimulated tissue macrophages, epithelioid cells and granulomas, but is not expressed in resident tissue macrophages. In the present study, we evaluated Cathepsin-k expression as a diagnostic tool in the identification of small granulomas in Crohn's disease. Formalin-fixed and paraffin-embedded samples of 10 cases of Crohn's disease were collected from surgical ileo-colic resections followed by comparison of Cathepsin-k and CD68 immunoreactivity. Granulomas were identified in 4 of 10 cases examined in haematoxylin & eosin preparations. Cathepsin-k enabled the identification of small granulomas (with a diameter between 100 and 200 μm) in 6 of 10 cases, mainly localized within the submucosa and muscular layers. When compared to CD68, Cathepsin-k immunoreactivity was generally absent or only weakly expressed in resting tissue macrophages, thus allowing better identification of activated epithelioid cells. Based on these results, Cathepsin-k appears to be a reliable tool for the precise and rapid identification of small epithelioid granulomas in Crohn's disease.