Autoantibodies and autoantigens in autoimmune hepatitis: important tools in clinical practice and to study pathogenesis of the disease

Abstract

Autoimmune hepatitis (AIH) is a chronic necroinflammatory disease of the liver characterized by hypergammaglobulinemia, characteristic autoantibodies, association with HLA DR3 or DR4 and a favorable response to immunosuppressive treatment. The etiology is unknown. The detection of non-organ and liver-related autoantibodies remains the hallmark for the diagnosis of the disease in the absence of viral, metabolic, genetic, and toxic etiology of chronic hepatitis or hepatic injury. The current classification of AIH and the several autoantibodies/target-autoantigens found in this disease are reported. Current aspects on the significance of these markers in the differential diagnosis and the study of pathogenesis of AIH are also stated. AIH is subdivided into two major types; AIH type 1 (AIH-1) and type 2 (AIH-2). AIH-1 is characterized by the detection of smooth muscle autoantibodies (SMA) and/or antinuclear antibodies (ANA). Determination of antineutrophil cytoplasmic autoantibodies (ANCA), antibodies against the asialoglycoprotein receptor (anti-ASGP-R) and antibodies against to soluble liver antigens or liver-pancreas (anti-SLA/LP) may be useful for the identification of patients who are seronegative for ANA/SMA. AIH-2 is characterized by the presence of specific autoantibodies against liver and kidney microsomal antigens (anti-LKM type 1 or infrequently anti-LKM type 3) and/or autoantibodies against liver cytosol 1 antigen (anti-LC1). Anti-LKM-1 and anti-LKM-3 autoantibodies are also detected in some patients with chronic hepatitis C (HCV) and chronic hepatitis D (HDV). Cytochrome P450 2D6 (CYP2D6) has been documented as the major target-autoantigen of anti-LKM-1 autoantibodies in both AIH-2 and HCV infection. Recent convincing data demonstrated the expression of CYP2D6 on the surface of hepatocytes suggesting a pathogenetic role of anti-LKM-1 autoantibodies for the liver damage. Family 1 of UDP-glycuronosyltransferases has been identified as the target-autoantigen of anti-LKM-3. For these reasons the distinction between AIH and chronic viral hepatitis (especially of HCV) is of particular importance. Recently, the molecular target of anti-SLA/LP and anti-LC1 autoantibodies were identified as a 50 kDa UGA-suppressor tRNA-associated protein and a liver specific enzyme, the formiminotransferase cyclodeaminase, respectively. Anti-ASGP-R and anti-LC1 autoantibodies appear to correlate closely with disease severity and response to treatment suggesting a pathogenetic role of these autoantibodies for the hepatocellular injury. In general however, autoantibodies should not be used to monitor treatment, predict AIH activity or outcome. Finally, the current aspects on a specific form of AIH that may develop in some patients with a rare genetic syndrome, the autoimmune polyendocrinopathy-candidiasis-ectodermal dystrophy syndrome (APECED) are also given. Autoantibodies against liver microsomes (anti-LM) are the specific autoantibodies detected in AIH as a disease component of APECED but also in cases of dihydralazine-induced hepatitis. Cytochrome P450 1A2 has been identified as the target-autoantigen of anti-LM autoantibodies in both APECED-related AIH and dihydralazine-induced hepatitis. The latter may indicate that similar autoimmune pathogenetic mechanisms can lead to liver injury in susceptible individuals irrespective of the primary defect. Characterization of the autoantigen-autoantibody repertoire continues to be an attractive and important tool to get access to the correct diagnosis and to gain insight into the as yet unresolved mystery of how hepatic tolerance is given up and AIH ensues.

Figure 1

High titer antinuclear antibodies (ANA) of the homogeneous pattern by indirect immunofluoerescence on immobilized HEp-2 cells in a female patient with autoimmune hepatitis type 1 (AIH-1). Homogeneous ANA are frequently found in AIH-1 (original magnification 40×).

Figure 2

Typical staining of antinuclear antibodies in the serum of a patient with autoimmune hepatitis type 1 visualized by indirect immunofluoerescence on cryostat sections of rat liver (original magnification 40×).

Figure 3

Smooth muscle antibodies by indirect immunofluoerescence on rat kidney (from a female patient with autoimmune hepatitis type 1). The immunofluorescence involves smooth muscle fibers within blood vessels (original magnification 40×).

Figure 4

Smooth muscle autoantibodies by indirect immunofluoerescence on rat stomach (serum from a female patient with autoimmune hepatitis type 1). The immunofluorescence involves smooth muscle fibers within muscularis mucosa (original magnification 40×).

Figure 5

Perinuclear staining of anti-neutrophil cytoplasmic autoantibodies (p-ANCA) by indirect immunofluoerescence on ethanol fixed human granulocytes (serum from an ANA negative patient with autoimmune hepatitis type 1). Original magnification 40×).

Figure 6

Antibodies against liver-kidney microsomes type 1 (anti-LKM-1) react to the proximal tubules of the rat kidney. The absence of reactivity against thedistal tubules of the rat kidney (see also Fig. 6B) and parietal cells of the rat stomach distinguishes anti-LKM-1 autoantibodies from antimitochondrial antibodies (original magnification 40×).

Figure 7

Antimitochondrial antibodies react to the proximal and distal tubules of the rat kidney (original magnification 40 ×). In these cases there is also reactivity to the parietal cells of the rat stomach.

Figure 8

Linear B-cell epitopes on cytochrome P450 2D6 in autoimmune hepatitis type 2. The immunodominant epitope 257–269 aa shares sequence homology with the immediate early protein IE 175, a transcription factor of herpes simplex virus type 1 (now known as infected cell protein 4 or ICP4). Although this is an attractive model for the hypothesis of molecular mimicry, overall evidence for mimicry as a driving force of autoimmune hepatitis is not convincing.