Quizartinib (AC220) Besides an infection of animals
Besides an infection of animals with liver-tropic pathogens, an acute and/or chronic inflammatory milieu in the liver has also been generated by direct in situ expression of inflammatory factors, such as cytokines. Djilali-Saiah et al. used TTR-LCMV mice, which express the nucleoprotein (NP) of LCMV as a target antigen expressed under the control of the transthyretin (TTR) promoter in the liver. By DNA-vaccination with plasmids encoding for NP as well as the pro-inflammatory cytokine IL-12 they could cause liver damage characterized by elevated serum aminotransferase levels and minor cellular infiltrations. Further, a NP-specific CTL response was detectable after 2 months and persisted up to 5 months post-vaccination. . In a follow-up study they used the natural human AIH autoantigens formiminotransferase cyclodeaminase (FTCD) and CYP2D6 to replace the AIH-unrelated NP of LCMV as triggering antigen. DNA-vaccination with a plasmid encoding for the antigenic regions of FTCD and CYP2D6 as well as the N-terminal region of mouse CTLA-4 and IL-12 resulted in significant inflammation in the liver. A vaccination with a CTLA-4-CYP2D6-FTCD-plasmid without IL-12 had no significant impact, . Thus, IL-12 indeed seems to be an important driver of the inflammatory response in the liver. This conclusion has been further confirmed by Tamaki et al. who showed that vaccination of wild type C57BL/6 mice with dendritic cells loaded with well-differentiated hepatocellular carcinoma cells (DC/Hepa1–6) followed by intraperitoneal injection of recombinant human IL-12 caused a liver-specific inflammation and the generation of liver autoantigen (S-100)-specific proliferative and cytotoxic immune responses. . In adoptive transfer experiments DC/Hepa1–6 activated splenocytes proved to exhibit a pathogenic phenotype, but only if the non-vaccinated recipient mice had also been treated with IL-12 . In another model IL-12 has been administered with the help of an adeno-associated viral vector (AAV). Considering the abovementioned models, it is surprising that wild type mice that have been administered with AAV-IL-12 only developed AIH-like disease characterized by persistent cellular infiltrations of the liver, hepatic fibrosis, elevated serum aminotransferase levels, hypergammaglobulinemia, as well as generation of ANA and anti-SMA Quizartinib (AC220) . Mechanistically, they found that although AAV-mediated IL-12 was short lived, it induced the persistent expression of endogenous IL-12 and IFNγ throughout the observation time of 60 days. Further, they found that the observed hepatic damage was predominantly caused by CD4 and CD8 T cells rather than NK, NKT or B cells . The strength of this model is surely its simplicity, since neither a particular target and/or triggering autoantigen nor the presence of autoantigen-specific TcR transgenic T cells is required. However, the absence of a defined autoantigen as target and/or trigger comes also with the disadvantage of not being able to track and quantify the autoantigen-specific immune response.
Animal models: Models using naturally occurring autoantigens
Conclusions Many models for human AIH have been generated (Table 1). Some with the goal to develop a model system that closely mimics the human disease and thereby be able to evaluate possible treatment, and some that were intended to provide a tool to investigate basic aggressive or regulatory immune mechanisms. Many of which have been identified and extensively characterized in models, such as the Con A hepatitis model or models using TcR-transgenic T cells that allow for proper quantification and tracking of liver autoantigen-specific T cells. Hence, inflammatory factors that are critical for acute and/or chronic forms of hepatitis as well as many tolerogenic processes that maintain a healthy immune balance in the liver are known today. Unfortunately, many models that have helped identifying mechanisms of liver immunity and/or liver tolerance have not been pursued after one or two initial publications. Interestingly, most publications report about the failure or success of a specific model in inducing AIH-like disease and many even identified critical factors driving the disease but have not aimed at a blockade of the immunopathogenesis. Therefore, it may not come a s surprise that current therapy of AIH still largely relays on glucocorticoids and cytostatic drugs. Another reason might be that many of the models used antigens that are not target of the aggressive immune system in patients. Further, some models used rather artificial ways of breaking or circumventing liver tolerance by using disease unrelated target autoantigens, TcR-transgenic T cells, or thymectomy. However, often this was not sufficient to induce chronic hepatitis resembling human AIH and an additional local inflammation had to be induced by either using liver-tropic pathogens or by locally overexpressing critical inflammatory factors, like IL-12. Therefore, in the future it will be important to evaluate novel therapeutic strategies in models that are closer to the human situation. Models like the CYP2D6 or FTCD mouse model use natural occurring liver autoantigens as targets. In addition, an acute inflammatory insult of the liver is caused by infection of a liver-tropic pathogen that carries molecules that share a structural similarity or identity with the target autoantigen. Thus, the way for breaking tolerance is feasible and reflects a possible mechanism by which AIH might be initiated and/or promoted in patients. In future models it will be also important to incorporate humanized mice that for example carry critical human liver autoantigens in combination with defined HLA-risk alleles. Such a combination would allow a presentation of the same critical T cell epitopes than in patients and thereby an investigating of how tolerance to “real-life” autoantigen epitopes is broken and how the specific T cell response is perpetuated.