Year: 2006
Type: article
Abstract: To the editor: The FAS/Fas ligand (FasL) system is one of the main apoptosis-inducing systems and is involved deeply in pathologic changes in many diseases including cancers and autoimmune diseases. Recently, FasL expression in macrophages has been reported, and macrophage is considered to be an important component in liver diseases. In primary biliary cirrhosis (PBC) and autoimmune hepatitis (AIH), FasL-positive macrophages were identified and it was considered that the expression was involved in apoptosis of hepatocytes and bile duct cells (1, 2). RCAS1 is also considered to be a mediator for another apoptosis-inducing system. RCAS1 was first identified as a tumor-associated antigen in gynecological cancer (3); however, the antigen was expressed in various kinds of carcinoma cells including hepatocellular carcinoma and cholangiocarcinoma (4, 5). RCAS1 exists both in membrane-bound and soluble forms. The receptor for RCAS1 is expressed on the surface of lymphocytes, especially in their activated state, and RCAS1 binding to the receptor results in apoptosis of lymphocytes (6). Therefore, RCAS1 is thought to function in the evasion of immune surveillance and in promoting tumor progression. We have reported that RCAS1, in addition to cancer cells, can also be produced by target cells in autoimmune liver diseases (5, 7). In PBC, biliary epithelial cells are RCAS1-positive, especially in active inflammatory areas, and may defend themselves from immune attack by inducing apoptosis of lymphocytes. Here, we report on another new and noteworthy finding in inflammatory liver diseases; namely that infiltrating macrophages and/or Kupffer cells can also be positive for RCAS1. In addition, based on previous in vitro studies and the results of immunohistochemical staining in this study, we infer the functional difference between RCAS1 and FasL. We immunohistochemically assessed expression of RCAS1, FasL, and a specific marker for macrophages/Kupffer cells, LN-5 (8), in liver biopsy samples from patients with chronic viral hepatitis (n=13, HBV/HCV=3/10), acute or acute on chronic hepatitis (n=12, HBV/drug-induced/unknown etiology=3/3/6), AIH (n=8), graft vs. host disease (GVHD, n=8), PBC (n=14), and primary sclerosing cholangitis (PSC, n=6). Informed consent was obtained from each patient prior to their inclusion in the study. In our samples of chronic viral hepatitis, Kupffer cells were reactive for LN-5 and, generally, LN-5 were positive with not very large number of infiltrated mononuclear cells in the portal/periportal areas regardless of serum alanine transaminase (ALT) levels in individuals. Serum ALT levels of these patients on the day of the biopsy were 27–365 IU/l (median 147 IU/l). Several FasL-positive cells were also seen mainly in the portal areas and many of them appeared to be positive for LN-5. On the other hand, we could not find RCAS1 positivity in mononuclear cells, hepatocytes, biliary epithelial cells, and other components. In case of acute or acute on chronic hepatitis, serum ALT levels of the patients on the day of the biopsy were 274–4660 IU/l (median 1131 IU/l). Generally, there were many LN-5-positive mononuclear cells in the hepatic lobules and portal areas. In the individual case, various percentages of LN-5-positive cells were also FasL-positive, and especially in the portal areas most of the infiltrated macrophages were FasL-positive. The positive cell number or percentage for LN-5 or FasL was not related to the serum ALT levels of the patients. By contrast, RCAS1-positive cells tended to be found in the patients with higher serum ALT levels (>647 IU/l), although the number was not many. RCAS1-positive cells were mainly shown at the areas of massive hepatocytic death, and most of the cells showed positivity for FasL and LN-5 (data not shown). Immunohistochemical staining for RCAS1, FasL, and LN-5 was tested on the tissue samples from patients with AIH (n=8, ALT levels: 26–609 IU/l, median 88 IU/l), GVHD (n=8, ALT levels: 48–998 IU/l, median 204.5 IU/l), PBC (n=14, ALT levels: 21–397 IU/l, median 48 IU/l), and PSC (n=6, ALT levels: 23–275 IU/l, median 58 IU/l). All the samples included LN-5- and FasL-positive cells in hepatic lobules and portal areas. Generally, the number of LN-5- and/or FasL-positive cells appeared to be related to the activity of inflammation. In contrast to FasL, fewer RCAS1-positive cells were observed in these samples. RCAS1-expressing cells were found in the samples from only eight patients in whom ALT levels were higher (73–998 IU/l, mean±SD 285.4±315.2 IU/l) than those in patients without RCAS1-positive mononuclear cells in their biopsy samples (15–166 IU/l, mean±SD 67.2±42.7 IU/l) (P<0.01). RCAS1-positive cells also appeared to express LN-5 and FasL (Fig. 1). In these autoimmune diseases, RCAS1-positive macrophages tended to be observed at the damaged lobular areas as well as portal/periportal areas. In some PBC cases, RCAS1 was occasionally positive in several bile ducts cells surrounded by inflammatory cells but was negative in other liver components. All the hepatocytes and biliary epitherial cells were negative for FasL expression. Serial sections of a liver biopsy sample from a patient with autoimmune hepatitis. LN-5 (A), Fas ligand (B), and RCAS1 (C) were immunohistochemically stained. Sections were counterstained with hematoxylin. Positive cells at the same position are indicated by white arrows (A–C). Original magnification: 200 ×. In vitro experiments indicate that RCAS1 and its receptor cannot be expressed simultaneously on a cell (6). It has been accepted that, in malignancy, RCAS1 plays a role in the ability of cancer cells to evade immune surveillance. In fact, apoptotic lymphocytes were found in liver tissue from patients (5). It is a reasonable assertion that RCAS1 produced by macrophages also induce apoptosis in the activated lymphocytes expressing the RCAS1-receptor. Our group has already reported studies in monocytes that showed that RCAS1 can be detected only when monocytes are activated by stimulators like LPS (9, 10). In addition, RCAS1 is expressed not on the cell surface but in the cytoplasm, and soluble RCAS1 molecules are detected in culture supernatants obtained from LPS-stimulated macrophages but not from unstimulated cells (9, 10). It is possible that macrophages in the liver also present the same expression pattern. Because FasL can also be expressed in macrophages, the RCAS1 and Fas/FasL systems may work cooperatively on apoptosis induction to immune cells. However, Fas and FasL can be expressed simultaneously on immune cells, target cells of autoimmune diseases, and cancer cells. Our results showed that FasL-expressing mononuclear cells including macrophages were found in any grade of inflammation. Therefore, we infer that the FasL function of inducing apoptosis is mainly directed toward virus-infected hepatocytes and target cells of autoimmune diseases. We speculate further that, in the earlier stage or the beginning of inflammation of hepatic diseases, macrophages show well-known immunological functions such as phagocytosis, antigen presentation, and production of cytokines/chemokines, and that, after the obvious infiltration of lymphocytes, RCAS1 expression becomes prominent in some macrophages. According to the progression of inflammation, macrophages may alter the expression pattern of proteins including RCAS1, resulting in, to some degree, switching their immunological effect from activation to suppression.
Source: Liver International
Institutions: Kyushu University, Fukuoka University
Language: English
Topic: Liver Diseases and Immunity
Subfield: Hepatology
Field: Medicine
Domain: Health Sciences
Open Access status: bronze
OpenAlex ID: W2004378757