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  • The first animal models have

    2019-07-08

    The first animal models have been generated in the early 1970s during a period of time when more and more of the basic mechanisms of the immune system have been identified and it has become clear that AIH has an immune mediated background. Meyer zum Buschenfelde and colleagues used two crude fractions of human liver homogenates, which contained one of the two liver-specific antigens LP-1 or LP-2 known at the time. They injected these fractions together with various adjuvants into rabbits and found that LP-1, that has been described as a water insoluble macromolecular low-density lipoprotein, was efficient in inducing “experimental immune hepatitis” [75]. In the meantime, LP-1 is known as LSP and has been identified as the asialoglycoprotein receptor (ASGPR), which is highly expressed at the surface of hepatocytes [31]. Anti-LSP Jasplakinolide are present in up to 88% of AIH-patients [31], and may be used as a general marker compatible with AIH, but not as a diagnostic tool, since such antibodies have also been detected in patients with chronic hepatitis B and C, alcoholic liver disease, and PBC [49]. Although, the approach was simple and the real identity of both the triggering antigen as well as the target liver autoantigen has not been known at that time, Meyer zum Buschenfelde et al. succeeded in inducing a state of autoimmune liver damage characterized by a diffuse portal and periportal infiltration with lymphoid cells and piecemeal necrosis [75]. Interestingly, the rabbits generated antibodies to LP-1 and LP-2, but these antibodies were not sufficient to induce AIH-like chronic hepatitis in transfer experiments, indicating the additional requirement of pathogenic T cells and/or a strong local inflammation in the liver [75]. A similar approach has been made by Kuriki et al. in the 1980s, who injected syngeneic liver homogenate or liver-specific lipoproteins with the polysaccharide of Krebsiella pneumoniae 03:K1 as an adjuvant into SMA mice, resulting in generating liver-specific lipoprotein antibodies and portal infiltrations of mononuclear cells in the liver [76]. In analogy to the then widely used experimental autoimmune encephalomyelitis (EAE) model for multiple sclerosis (MS), they termed their model experimental autoimmune hepatitis (EAH). Importantly, they were able to demonstrate the presence of pathogenic immune cells, since transfer of splenocytes from such EAH-mice into naïve recipients induced similar features as detected in the donor mice with established EAH within 14 days after the transfer. These features included portal infiltrations with mononuclear cells as well as necrosis of liver parenchymal cells [76]. Further, Lohse et al. generated an EAH model by injecting C57BL/6 mice with a crude 100,000 g supernatant of syngeneic liver homogenate (S-100) emulsified in complete Freund's adjuvant [77]. The important novel finding was that besides a transient liver damage (perivascular infiltrates and hepatocyte necrosis) the generation of S-100 protein-specific T-cells has been demonstrated [77]. A few years earlier, Watanabe et al. used a very similar approach by injecting inbred A/J mice subcutaneously eight times with the supernatant of liver homogenate centrifuged at 105,000g [78]. Although they did not use the term S-100, their liver extract must have been very similar to the one used by Lohse et al. They showed EAH characterized by histological features of hepatitis as well as the production of anti-LSP antibodies and a strongly pronounced delayed-type hypersensitivity to LSP after neonatal thymectomy. This finding suggests a possible presence of an immunosuppressive T cell component in such EAH mice, since neonatal thymectomy prevents the proper development of the CD4 + CD25+ population of regulatory T cells [79]. Since then variations of the S-100 model have been often used by several research groups worldwide. For example, it has been shown that vaccination with irradiated activated T cells isolated from the liver of EAH, but not naïve mice, evokes an anti-idiotypic regulatory response, directed against epitopes of the Vβ-chains of T cells used for vaccination, and protects recipient mice from S-100 induced EAH [80]. More recently, the S-100-induced EAH model was used to demonstrate that the mitogen-activated protein kinase (MAPK) p38 and the transcription factor nuclear factor kappa B (NF-κB) play an important role in the immunopathogenesis of AIH [81]. Blockade of p38, which normally reaches its peak of expression between days 14 and 21 after S-100 administration, resulted in a decreased NF-κB activation and subsequently a reduced expression of pro-inflammatory cytokines, such as IFNγ, TNFα, IL-1β, and IL-12. Due to such a reduced pro-inflammatory milieu in the liver, cellular infiltrates were diminished, and serum aminotransferase levels decreased [81]. The S-100 model has also been used to investigate the suppressive role of regulatory B cells expressing CD11b [82]. Thereby, Liu et al. showed that IL-10-dependent expression of CD11b on B cells rendered a regulatory state in such B cells. CD11b B cells mediated their suppressive/regulatory function through impairment of TcR signal transduction and promotion of TcR downregulation, resulting in CD4 T cell suppression and amelioration of S-100-induced EAH [82]. In summary, these models originated in the “pre-transgenic” area when many of the target autoantigens have not yet been identified and they were based on the injection of liver extracts with largely unknown compositions. Today, it is known that such a microsomal subcellular fraction, such as the S-100 extract, contains several liver autoantigens, such as CYP2D6 [26] and SLA/LP [33]. Nevertheless, these crude models gave valuable information on the presence of effector cells and regulatory phenomenon in liver-specific immune reactions. The advantage of the S-100 model lays in its simplicity that does not require transgenic animals or complicated ways of disease induction. However, these early models rarely induced a chronic relapsing AIH with similarities to the human disease and the lack of knowledge about the target antigens prevented the quantification and tracking of liver autoantigen-specific T cell responses.