AIM: To investigate a pathophysiological role of cathepsin W (CatW), a putative thiol-dependent cysteine protease,which is specifically expressed in cytotoxic lymphocytes,in different types of chronic inflammation of the gastric mucosa.METHODS: Gastric and duodenal biopsies of patients with Helicobacter pylori ( H pylori)-associated active gastritis ( Hp,n = 19), chemically induced reactive gastritis (CG, n = 17),autoimmune atrophic gastritis (AIG, n = 20), lymphocytic corpus gastritis (LG, n = 29), celiac disease (CD, n = 10),and corresponding controls (n = 24) were analyzed by immunohistochemistry for the expression of CatW and CD45. Furthermore, immunohistochemical double staining with anti-CD3 and anti-cathepsin was performed for the samples of AIG.RESULTS: Median values of CatW-expressing cells among CD45-positive immune cells were between 2% and 6% for normal gastric mucosa, CG, and LG, whereas the corresponding value was significantly increased for AIG (24.7%, P＜0.001) and significantly decreased for HP (0.7%, P＜0.05). Double staining with anti-CD3 and antiCatW antibodies revealed that ＞90% of CatW-expressing cells in gastric mucosa of AIG were T cells. Duodenal mucosa had significantly more CatW/CD45-positive cells than normal gastric mucosa (median: 17.8% vs 2%, P＜0.01).The corresponding proportion of CatW/CD45-positive cells was decreased in CD compared to duodenal mucosa (median: 2.1% vs 17.8%, P＜0.05).CONCLUSION: The opposite findings regarding the presence of CatW-positive cells in AIG (increase) and CD (decrease) reflects the different cellular composition of immune cells involved in the pathogenesis of these diseases.