SUMMARY:It has been suggested that the behavior and function of Paneth cells in metaplasia are different from those found in normal intestinal mucosa. In this study, we investigated whether calnexin, a protein involved in secretory pathways, might be associated with differentiation and function of Paneth cells in normal small intestine, in complete intestinal metaplasia of the stomach, and in Paneth cell-rich adenomas. Differentiation and function of Paneth cells was monitored by Ki67, lysozyme, and morphologic features. Using a newly established monoclonal antibody, we found that calnexin is regularly synthesized by Paneth cells of normal small intestine. In these cells, the staining intensity of calnexin was inversely correlated with their content of secretory granules (lysozyme). In contrast, Paneth cells of intestinal metaplasia and Paneth cell-rich adenomas showed a reduced immunostaining of both calnexin and lysozyme. Moreover, these Paneth cells synthesized the proliferation marker Ki67, a phenomenon that was never observed in Paneth cells of normal small intestine. In vitro experiments using CaCo2 cells showed that the expression of calnexin is not directly affected by the induction of mitosis. In conclusion, calnexin probably reflects the status of Paneth cell differentiation and function. The results do not necessarily indicate that calnexin has a function in Paneth cell proliferation. (Lab Invest 2002, 82:1647-1659.T he four principal epithelial cell types of the human small intestinal mucosa (absorptive, goblet, Paneth, and endocrine cells) derive from a multipotent stem cell located near the base of crypts of Lieberkühn. In contrast to absorptive and goblet cells, which migrate in approximately 5 days from the crypt depths to the surface, endocrine cells and Paneth cells differentiate during a downward migration to the base of crypts. Paneth cells reside there for approximately 20 days (Bry et al, 1994). Paneth cells are generally recognized in hematoxylin and eosinstained sections by their large eosinophilic apical secretory granules and their characteristic ultrastructural pattern (Otto, 1973). Functions that have been attributed to Paneth cells include modulation of the intestinal microflora and maintenance of the mucosal defense barrier through production and secretion of several antimicrobial peptides, such as lysozyme, secretory phospholipase A 2 , and ␣-defensins (Nevalainen et al, 1998;Ouellette, 1999;Ouellette and Bevins, 2001). In addition, Paneth cells can also produce further regulatory molecules including growth factors, suggesting that Paneth cells may regulate the proliferation and differentiation programs of epithelial cell lineages by short-circuit paracrine loops (Bry et al, 1994;Ganz, 2000).In humans, Paneth cell metaplasia is preferentially found in atrophic gastritis (Wong et al, 2000). Other nonmalignant pathologic conditions associated with Paneth cell metaplasia are diversion colitis, ulcerative colitis, and collagenous colitis (Goff et al, 1997;Shah et al, 1998;Stamp et al, ...