Original Article Cytokeratin subtypes in biliary atresia: Immunohistochemical study Hideyuki Sasaki, 1,2 Masaki Nio, 2 Daiji Iwami, 2 Nobuo Funaki, 2 Ryoji Ohi 2 and Hironobu Sasano 1 Departments of 1 Pathology and 2 Pediatric Surgery, Tohoku University School of Medicine, Sendai, Japan Biliary atresia (BA), the most common cause of obstructive jaundice in infancy, 1,2 has been considered to be the result of progressive destruction of the bile ducts through a necroin- flammatory process. 1,3 This process affects not only part or all of the extrahepatic bile ducts, but also the intrahepatic bile ducts, which subsequently results in paucity of the interlobu- lar bile ducts, or ductopenia. 1,2 The development of human intrahepatic bile ducts has been relatively well docu- mented, 1,4–9 but the etiology of BA remains unknown. Ductal- plate malformation, or insufficient ductal-plate remodeling, has been suggested to be involved. 1–3 Cytokeratins (CK) belong to the intermediate filament family. Moll et al. identified 19 different CK polypeptides using 2-D gel electrophoresis 10 and CK-20 was subsequently added to this family of proteins in 1990. 11 Different types of normal epithelia are characterized by their expression of specific CK. Reports indicate that this cell-type-specific CK expression is, in general, maintained during neoplastic trans- formation. 10,12,13 Therefore, CK are considered to serve as good ‘lineage markers’ in identifying epithelia and in tracing the cell-of-origin in tumors. Differences in the expression of CK subtypes between human hepatocytes and biliary cells have been well studied. 14–16 Hepatocytes express CK-8 and -18, and intrahepatic bile-duct cells also express these, in addition to CK-7 and -19. 14–16 In the rat liver, bile ducts have also been reported to express CK-20 in addition to CK-8, -18, -7 AND -19, 11,17 but no immunoreactivity for CK-20 has been detected in the human liver. 11 Abnormalities in the development and/or maturation of bile-duct cells have been suggested to play a role in the pathogenesis and development of BA. It is therefore impor- tant to examine the degree of maturation and differentiation in the bile ducts of BA. Immunolocalization of CK subtypes has been well studied in the normal human fetal and adult liver, 14–16,18–20 but not in BA. Therefore, in this study, we exam- ined the immunolocalization of CK subtypes in BA, and com- pared the findings with those in fetal and pediatric non-BA liver, in order to study possible abnormalities of bile-duct development and/or maturation in BA. Pathology International 2001; 51: 511–518 The etiology of biliary atresia (BA) remains unknown, but ductal-plate malformation and insufficient ductal-plate remodeling have been suggested to play important roles, so it is beneficial to examine the maturation and differentiation of bile ducts in BA. Different epithelial types are character- ized by the expression of specific cytokeratin (CK) sub- types. CK can therefore serve as a ‘lineage marker’ of epithelial cells. CK subtypes have not been previously examined in BA. In this study, we examined the maturation of bile-duct cells in BA (n = 45) using immunohistochem- istry of CK subtypes, with mouse monoclonal antibodies to CAM5.2, and CK subtypes 7, 8, 13, 14, 17, 19 and 20. We then compared these findings with pediatric non-BA (n = 11) and fetal (n = 21) liver. We semiquantitatively evaluated the find- ings using a H score method. In the fetal liver, immunoreac- tivity for CAM5.2, CK-7, CK-8 and CK-19 was detected in bile-duct cells, and CAM5.2 and CK-8 immunoreactivity was also detected in hepatocytes. The distribution of these CK subtypes was the same in fetal, pediatric non-BA and BA liver. However, CK-7 immunoreactivity was markedly weaker in bile ducts of fetal (H scores: ductal plate 0 ± 0; remodel- ing 9.5 ± 40.3; remodeled 37.3 ± 60.8) and BA (H score: 200.9 ± 55.3) liver compared to non-BA liver (H score: 251.1 ± 33.5). In addition, CK-20 was detected in the bile ducts of the fetal and BA liver, but not in non-BA liver. These findings suggest that the expression patterns of CK subtypes in bile-duct cells in BA are similar to that in devel- oping bile-duct cells, which is indicative of bile-duct cell immaturity. Key words: biliary atresia, cytokeratin subtype, development, immunohistochemistry, liver Correspondence: Hideyuki Sasaki, MD, Department of Pathology, Tohoku University School of Medicine, 1-1 Seiryou-machi, Aoba-ku, Sendai 980-8574 Japan. Email: h-sasaki@ped-surg.med.tohoku.ac.jp Received 3 October 2000. Accepted for publication 16 March 2001.