Cytochrome P450 2D6 as a model antigen

Abstract

Cytochrome P450 2D6 (CYP2D6) has been identified as the major autoantigen in type 2 autoimmune hepatitis (AIH). However, because of a lack of appropriate animal models, the etiology of AIH is still poorly understood. We generated a mouse model for AIH using the human CYP2D6 as a triggering molecule for autoimmunity. We infected wild-type FVB mice with an adenovirus expressing human CYP2D6 (Ad-2D6) to break self-tolerance to the mouse CYP2D6 homologues. Ad-2D6-infected mice showed persistent features of liver damage including hepatic fibrosis, cellular infiltrations, focal-to-confluent necrosis and generation of anti-CYP2D6 antibodies, which predominantly recognized the identical immunodominant epitope recognized by LKM-1 antibodies from AIH patients. Interestingly, Ad-2D6 infection of transgenic mice expressing the human CYP2D6 (CYP2D6 mice) resulted in delayed kinetics and reduced severity of liver damage. However, the quantity and quality of anti-CYP2D6 antibodies was only moderately reduced in CYP2D6 mice. In contrast, the frequency of CYP2D6-specific CD4 and CD8 T cells was dramatically decreased in CYP2D6 mice, indicating the presence of a strong T cell tolerance to human CYP2D6 established in CYP2D6 mice, but not in wild-type mice. CYP2D6-specific T cells reacted to human CYP2D6 peptides with intermediate homology to the mouse homologues, but not to those with high homology, indicating that molecular mimicry rather than molecular identity breaks tolerance and subsequently causes severe persistent autoimmune liver damage. The CYP2D6 model provides a platform to investigate mechanisms involved in the immunopathogenesis of autoimmune-mediated chronic hepatic injury and evaluate possible ways of therapeutic interference.

Fig. 1

Acute and chronic phases of Ad-2D6-induced AIH in the CYP2D6 mouse model. In the acute phase directly after Ad-2D6 infection, the liver still maintains a normal morphology (a), but small focal infiltrations are visible (H&E staining of a 5-μm liver section, b). Isolated hepatic stellate cells have a naïve phenotype (Oil red O staining of cultured hepatic stellate cells, c) and no significant subcapsular fibrosis is detectable (Sirius red staining of a 5-μm liver section, d). In the chronic phase of autoimmune liver damage, the morphology of the liver changes and individual lobes become fused (e). Large clusters of infiltrating cells become apparent predominantly in the perivascular region (H&E staining of a 5-μm liver section, f). Isolated hepatic stellate cells are activated and express α-smooth muscle actin [anti-α-smooth muscle actin antibody (red) and DAPI (blue) staining, g] and extensive subcapsular and perivascular fibrosis are found (Sirius red staining of a 5-μm liver section, h) ([36]; Hintermann and Christen, manuscript in preparation). Color in online version only.

Fig. 2

Schematic overview on the development of AIH in the CYP2D6 model. The antiviral phase includes acute inflammation, transient elevation of serum aminotransferases, cytokine and chemokine release, focal infiltration, and initial activation of hepatic stellate cells (HSC). The chronic progressive autoimmune phase is characterized by high titer anti-CYP2D6 antibodies, CYP2D6-specific CD4 and CD8 T cells, strong hepatic infiltration and extensive fibrosis. The degree and onset of liver damage is dependent on the degree of immunologic tolerance that is more efficiently broken in conditions of molecular mimicry than identity ([36]; Hintermann and Christen, manuscript in preparation; Holdener and Christen, manuscript in preparation).