In liver transplantation, the development of tolerance is associated with an increased rate of apoptosis of T lymphocytes in the portal inflammatory infiltrate and the presence of an intragraft Th2-like T cell population. Underlying mechanisms are poorly understood. Kupffer cells (KC), which reside in the hepatic sinosoids, can directly interact with circulating T lymphocytes and thus are uniquely positioned to play a role in immunomodulation. In this study, the immunoregulatory effects of KC were investigated. We show that KC can significantly suppress T cell proliferation in mixed leukocyte reaction (MLR).

Summary Kupffer cells (KC) act as APC in the liver and play a major role in the clearance of gut-derived antigens and pathogens entering the liver with portal venous blood. Antigen presentation by KC has been implicated in regulation of the local and systemic immune responses. In this study, modulation of KC antigen presentation by antioxidants and the role of reactive oxygen species (ROS) as essential mediators of antigen presentation in KC were investigated. Co-culture of KC with ovalbumin (OVA) antigens resulted in upstream intracellular endogenous ROS generation and increased expression of MHC class II and costimulator molecules, and consequent OVA-specific CD4 + T-cell proliferation in response to antigen presentation by KC. Scavenging of KC ROS by antioxidants, or blocking of KC ROS generation by specific inhibitors of NADPH oxidase and/or xanthine oxidase, or by specific inhibitors of the mitochondrial electron transport chain, significantly decreased OVA-specific T-cell proliferation in response to antigen presentation by KC. Increased expression of MHC class II and costimulatory molecules in KC pulsed with OVA antigens was blocked by inhibiting ROS generation enzymatically. Intracellular endogenous ROS generation during antigen processing may therefore provide essential secondary signalling for KC antigen presentation.

Fas-Fas ligand (FasL)-dependent pathways exert a suppressive effect on inflammatory responses in immune-privileged organs. FasL expression in hepatic Kupffer cells (KC) has been implicated in hepatic immunoregulation. In this study, modulation of FasL expression of KC by endogenous gut-derived bacterial LPS and the role of reactive oxygen species (ROS) as potential mediators of FasL expression in KC were investigated. LPS stimulation of KC resulted in upstream ROS generation and, subsequently, increased FasL expression and consequent Jurkat cell (Fas-positive) apoptosis. The NADPH oxidase and xanthine oxidase enzymatic pathways appear to be major sources of this upstream ROS generation. Increased FasL expression was blocked by antioxidants and by enzymatic blocking of ROS generation. Exogenous administration of H2O2stimulated KC FasL expression and subsequent Jurkat cell apoptosis. Intracellular endogenous ROS generation may therefore represent an important signal transduction pathway for FasL expression in KC.

Abstract. Synchronous or metachronous liver metastasis occurs in approximately 15% of colorectal cancer patients and is an important negative prognostic factor. We therefore need an effective therapy to prevent metastasis. It has become apparent that cyclooxygenase (COX)-2 plays an important role in cancer growth, invasion and metastasis and that there is potential for chemoprevention via inhibition of these processes. We injected colon 26, a colorectal cancer cell line, in CDF1 mouse spleen and, from the following day, two kinds of COX-2 inhibitor (etodolac and nimesulide) were administered orally. Two weeks later, the animals were sacrificed, the liver was excised, and we counted the number of metastatic nodules on the liver surface. In addition, COX-2 mRNA, matrix metalloproteinase (MMP)-9 mRNA, and tissue inhibitor of MMP (TIMP)-1 mRNA of cancer tissue were measured by means of real-time RT-PCR. The number of metastatic nodules on the liver surface was significantly lower in the etodolac-treated group than in controls (p=0.001), but no significant difference was noted in the nimesulide-treated group. The expression of COX-2 mRNA was also significantly lower in the etodolactreated group than in controls (p=0.04), but not in the nimesulide-treated group. In addition, the expression of MMP-9 mRNA was significantly lower in the etodolac group than in controls (p=0.02), but not in the nimesulide group. Among the groups, there were no significant differences in TIMP-1 mRNA. Expression of COX-2 mRNA and MMP-9 mRNA correlated significantly (r=0.78, p=0.001), but there was no correlation between either COX-2 mRNA and TIMP-1 mRNA expression or between MMP-9 mRNA and TIMP-1 mRNA expression. These findings indicate that the selective COX-2 inhibitor, etodolac, suppresses liver metastasis by reducing MMP-9 activity.