In this report, using the same Hep-OVA Tg mice that have been employed in understanding the role of cellular autoimmunity in AIH (15
), we studied the role of the humoral autoimmunity in the pathogenesis of this liver disease. We found that although mouse hepatocytes express all the intrinsic cell surface complement regulators, complement activation initiated by a hepatocyte specific antibody overwhelms the protection of these complement regulators, resulting in liver injury. Furthermore, depleting complement by CVF or inhibiting complement activation by recombinant DAF protein attenuates the liver specific antibody induced hepatitis.
Knowledge of expression patterns of intrinsic cell surface complement regulators on hepatocytes is important for understanding the role of complement in AIH and in transplanted liver/hepatocyte rejection. The current known intrinsic cell surface complement regulators in humans are DAF, CD59 and CD46. DAF inhibits complement activation on cell surfaces by accelerating the decay of C3/C5 convertases (8
), while CD46 serves as a cofactor for factor I to inactivate C3b/C4b therefore preventing the assembly of C3/C5 convertases (9
). CD59 protects cells from complement mediated injury by inhibiting the formation of the membrane attack complexes (10
). Not all cells express all the three intrinsic complement regulators on their surface. For examples, human erythrocytes do not express CD46 (22
), and natural killer cells do not express DAF (23
). Earlier work on human liver tissues indicated that only CD46 is expressed on normal hepatocytes (24
), but later studies using human hepatoma cell lines showed that CD46, DAF and CD59 are all present on these tumorized hepatocytes (25
). Recently, a detailed study using isolated primary hepatocytes found that human hepatocytes express all the intrinsic cell surface complement regulators (26
). Although mice are commonly used to study human hepatitis and liver transplantation, surprisingly, expression patterns of the intrinsic cell surface complement regulators on mouse hepatocytes have not been systematically studied. Different from that in humans, CD46 expression in rodents is limited only in testis (27
). But there is another rodent specific cell surface complement regulator termed complement receptor-related gene y (Crry) (28
), which is widely distributed and considered the counterpart of human CD46 in mouse tissues. Using isolated primary murine hepatocytes, we found that similar to that in humans, murine hepatocytes constitutively express all intrinsic cell surface complement regulators DAF, CD59 and Crry, which could help to protect hepatocytes from autologous complement mediated injury.
It is the current understanding that autoreactive T cells induce liver injury in AIH. Despite the long time observations that autoantibodies are developed in most of the AIH patients (30
), the precise role of autoantibodies in AIH remains elusive. While many of the autoantibodies are not liver specific and are against soluble antigens (31
), several autoantibodies against hepatocyte surface antigens have been identified from patients (3
). There has been one study showing that an IgM isolated from AIH patients reacts to a ~190 kDa unknown hepatocytes surface antigen, activates complement, and induces acute liver injury in mice (12
). However, the precise role of complement in this intriguing model of AIH was not elucidated.
The lack of a good animal model for AIH contributes to limited understanding of the disease pathogenesis and inadequate development of effective therapeutics. In addition to liver transplantation, current therapeutic interventions for AIH are mainly immunosuppressive drugs such as Prednisone and Azathioprine, which are not always effective and possess many side effects (31
). It is important to understand the pathogenesis underlying AIH to develop better therapeutics for disease management. OVA is a model antigen which has been widely employed to study the pathogenesis of many diseases in their respective models. The reagents and assays required for studies on OVA specific T cell and antibody responses have been well developed. The generation of the unique OVA transgenic mice which selectively express OVA antigen on hepatocytes surface provides a simple model to better understand the role of both the cellular and humoral autoimmunity in the pathogenesis of AIH, and to develop novel therapies against this disease. Previous studies (15
) using the same mouse have shown that hepatitis can be induced by adoptive transfer of OVA specific CD8+
T cells from OT-I mice, providing a powerful tool to study the role of hepatocyte specific autoreactive T cell responses in AIH. In current report, we have demonstrated that by adoptive transfer of an anti-OVA antibody into the same Hep-OVA Tg mouse, the liver specific antibody-initiated, complement mediated hepatocytes injury can be reliably induced. And this autoantibody induced hepatitis depends on the selective expression of OVA antigen on hepatocytes surface because WT C57BL/6 mice which do not express OVA protein exhibited little, if any, liver injury after the same antibody treatment. We propose that this model could be used to study the pathogenesis of AIH from the humoral side of the autoimmunity, which should be helpful in understanding the role of autoantibodies in AIH.
In many autoantibody induced autoimmune diseases and their animal models, excessive complement activation initiated by autoantibodies that overwhelms self complement inhibitors’ protection play a pivotal role in the pathology, indicating that exogenous complement inhibitors can be used to treat these diseases. One of the examples is myasthenia gravis, in which anti-acetylcholine receptor (AchR) autoantibodies bind to AChR at neuromuscular junctions, initiates complement activation leading to endplates damage and eventually muscle paralysis (32
). In addition to conventional immunosuppressive drugs and thymectomy, a complement inhibitor (anti-C5 mAb) has shown promise in attenuating muscle weakness in an animal model of MG (34
) and the humanized anti C5 mAb (Eculizumab) is being evaluated in Phase II clinical trials as a new therapy for MG patients. Using the Hep-OVA Tg mice, we found that autoantibodies activate complement on surface of hepatocytes which overwhelms the protection of intrinsic cell surface complement regulators, resulting in C5b-9 deposition and neutrophils infiltration which lead to liver injury. Depletion of complement by CVF or administration of recombinant soluble DAF protein, a potent native complement inhibitor, effectively suppresses complement activation and thereby protecting Hep-OVA Tg mice from anti-OVA IgG induced liver injury. These results suggest that like MG, complement activation could be the primary mechanism underlying autoantibody mediated liver injury in AIH, and complement inhibitors, in principle, could be developed for AIH management.