Demonstration of peptide-specific and cross-reactive epitopes in proteins reacting with antimitochondrial antibodies of primary biliary cirrhosis

Exploring the role of mitochondrial proteins SIRT5 and MRPL33 through Mendelian randomization in primary biliary cholangitis

BACKGROUND

Primary biliary cholangitis (PBC) is an autoimmune liver disease characterized by elevated serum antimitochondrial antibody levels in 90-95 % of cases. However, the exact causal relationship between mitochondrial proteins and PBC remains unclear. This study aims to investigate and clarify this relationship.

METHODS

Genome-wide association data for mitochondrial proteins and PBC were obtained from public databases. The assessment of causal relationships between exposures and outcomes employed the Inverse Variance Weighted (IVW) method, MR Egger regression, and Weighted Median. Sensitivity analyses were systematically carried out to appraise the robustness of the Mendelian Randomization (MR) findings.

RESULTS

The analysis revealed two mitochondrial proteins exhibiting a causal relationship with PBC. Elevated SIRT5 levels demonstrated a positive correlation with an augmented susceptibility to PBC in the IVW approach (odds ratio, OR: 1.2907, 95 % CI: 1.062-1.568, p = 0.0102). Conversely, increased MRPL33 levels were associated with a decreased risk of PBC (OR: 0.8957, 95 % CI: 0.807-0.993, p = 0.0376). Sensitivity analysis corroborated these findings consistently.

CONCLUSION

This investigation advances the notion of a potential causal association between elevated SIRT5 levels and an increased risk of PBC, alongside a decreased risk of PBC linked to elevated MRPL33 levels. The identified mitochondrial proteins may serve as viable biomarkers, offering pertinent insights for the understanding and addressing of PBC.

Is there a serological difference between men and women with primary biliary cirrhosis?

OBJECTIVE

Primary biliary cirrhosis (PBC) is an autoimmune disease affecting small intrahepatic bile ducts of the liver, causing destruction of the epithelium that results in eventual fibrosis and scarring. We still lack a complete epidemiological description of this disease, although interesting geographic differences in prevalence have been described. One consistent feature has been the relative scarcity of men with PBC. In fact, published ratios of women to men range from 3:1 to as high as 22:1. Thus far, the only clinical difference reported between men and women with PBC is a putative higher risk of hepatocarcinoma in men. Previous serological studies have shown that about 95% of all patients possess antimitochondrial antibodies to members of the highly conserved 2-oxo-acid dehydrogenase family of proteins, namely pyruvate dehydrogenase complex E2 (PDC-E2), branched-chain 2-oxo-acid dehydrogenase complex E2 (BCOADC-E2), and 2-oxo glutarate dehydrogenase complex E2 (OGDC-E2). However, there has been no information as to whether there is a difference in serological response between men and women. Using the serological hallmark of antimitochondrial antibodies (AMAs) and taking advantage of the availability of recombinant mitochondrial autoantigens, investigations were performed to determine if there were any serological differences between men and women with PBC.

METHODS

Sera were collected from 88 patients with PBC, of whom 46 were men and 42 were women. Using a combination of immunoblotting and enzyme-linked immunoabsorbent assay (ELISA) against beef heart mitochondria (BHM), recombinant PDC-E2, BCOADC-E2, and OGDC-E2, we determined the relative autoantibody reactivities of our study population.

RESULTS

Both men and women with PBC produced high titer antimitochondrial antibodies. The frequency of reactivity was similar in both groups and included, in descending order, PDC-E2, E3BP (Protein X), BCOADC-E2, and finally OGDC-E2. More importantly, antigenic specificity was nearly identical regardless of gender.

CONCLUSIONS

AMAs are the serological hallmark of PBC in both men and women, and there is no significant difference in reactivity between the two groups of patients.

Monoclonal Antibodies to Mitochondrial E2 Components Define Autoepitopes in Primary Biliary Cirrhosis

Primary biliary cirrhosis (PBC) is an autoimmune liver disease characterized by the presence of antimitochondrial Abs (AMA). The autoantigens recognized by AMA are the E2 components of the pyruvate dehydrogenase complex (PDC-E2), the branched chain 2-oxoacid dehydrogenase complex E (BCOADC-E2), and the 2-oxoglutarate dehydrogenase complex E (OGDC-E2). Previous studies using murine monoclonal and human combinatorial Abs to PDC-E2 have demonstrated an intense linear staining pattern in the apical region of biliary epithelial cells (BEC) in PBC but not control liver. We therefore examined whether mAbs to the other mitochondrial autoantigens BCOADC-E2 and OGDC-E2 demonstrated disease-specific patterns of reactivity. Using an expressed recombinant “trihybrid” protein containing the lipoyl domains of PDC-E2, OGDC-E2, and BCOADC-E2, we immunized BALB/c mice to produce 35 mAbs specific for one or more of the above mitochondrial autoantigens. Seven of these mAbs uniquely stained the apical region of BEC in PBC. Of these seven, one was reactive to PDC-E2, two recognized BCOADC-E2, three were reactive to OGDC-E2, and one recognized all three Ags. Our current data demonstrate that, similar to our previous studies regarding PDC-E2, mAbs to BCOADC-E2 and OGDC-E2, or a molecule that cross-reacts with the inner lipoyl domain of all three enzymes, also show a uniquely intense staining pattern in the apical region of BEC in patients with PBC when compared with diseased controls. The abundance of such disease-specific determinants in the target cells of PBC raises interesting possibilities regarding the role of these autoantigens in the pathogenesis of this disease.

Mitochondrial antigens, molecular mimicry and autoimmune disease

The immune system is normally tolerant to mitochondrial self-antigens, but responsive against bacteria. Low-titre anti-mitochondrial antibodies (AMA) might be involved in this discrimination. Tolerance is broken in diseases characterised by high titre AMA. Some of these AMA, against cardiolipin, cross-react with DNA. The best studied AMA are those characterising primary biliary cirrhosis (PBC). These are directed against E2 subunits of the oxo-acid dehydrogenase complexes, and also against subunits E1 alpha, E1 beta and X of the pyruvate dehydrogenase complex. AMA of PBC patients also react with bacterial E2s. Reactivities are primarily peptide-specific but with cross-reactivity between mitochondrial and microbial antigens and between E2s of respective complexes. Immunodominant epitopes, for anti E2 AMA, include the conserved sequence flanking the site of lipoyl attachment. It is proposed that the initial stimulus for antibody production is chronic urinary tract infection. AMA themselves are not pathogenic, but CD4+ T-cells would be primed, recognising the lipoyl domain epitope in association with class II HLA. Inappropriate expression of class II antigens on bile duct epithelia, (as found in PBC), might lead to presentation of a particular fragment of HLA-DR alpha, known to be a major MHC presented self-peptide in the mouse. That sequence strongly mimics the lipoyl domain and might be recognised by primed T-cells, initiating the autoimmune cascade. In the mouse, a peptide of ND1 of Complex I is presented in association with class I MHC. Cells exhibiting somatic mutation of such a peptide might thus be subject to attack by CD8+ T-cells. If such peptides were presented by class II HLA, autoimmune diseases might arise, related to mimicry between such peptides and microbial sequences and/or self-antigens. These considerations might apply in Leber's disease and in age-related pathology.

Liver cytosolic 1 antigen-antibody system in type 2 autoimmune hepatitis and hepatitis C virus infection

Within the multiform liver/kidney microsomal (LKM) family, a subgroup of sera that reacts with a liver cytosolic (LC) protein has been isolated and the new antigen-antibody system is called LC1. Unlike LKM antibody type 1 (anti-LKM1), anti-LC1 is said to be unrelated to hepatitis C virus (HCV) infection and has therefore been proposed as a marker of 'true' autoimmune hepatitis type 2. Altogether 100 LKM1 positive sera were tested by immunodiffusion (ID). Twenty five gave a precipitation line with human liver cytosol; 17 of the 25 also reacted with rat liver cytosol. Thirteen of the 25 sera were anti-HCV positive by second generation ELISA: anti-HCV positive patients were significantly older (p < 0.001) and tended to have less active disease. No difference in anti-LC1 titre or ID immunoreactivity was found between anti-LC1/anti-HCV positive and anti-LC1/anti-HCV negative cases. In Western blotting experiments, 14 of 24 ID positive sera recognised a 58 kD protein of the human cytosolic fraction and 11 gave a similar reactivity when tested with human microsomes, suggesting the presence of the LC1 target antigen also in the microsomal preparation. Western blotting reactivity was similar for both anti-HCV positive and negative sera. These data confirm the existence of the LC1 antigen-antibody system that partially overlaps with LKM1, and that it is an additional marker of juvenile autoimmune hepatitis type 2. It does not, however, discriminate between patients with and without HCV infection.

Is primary biliary cirrhosis an autoimmune disease?

This review summarizes the experimental and clinical support for an autoimmune origin of primary biliary cirrhosis (PBC). Direct proof is lacking, but indications in favour of an immunologic destructive mechanism include the demonstration of antibodies and T cell clones with specificity for mitochondrial autoantigens, and the lymphocytic infiltration/destruction of small bile ducts similar to that of graft-vs-host disease and rejection. There is a weak association with other autoimmune diseases, but no clear HLA linkage. Spontaneous animal models for PBC are lacking, and immunization of animals with purified autoantigen does not result in typical disease. Anti-mitochondrial antibodies (AMAs) of M2 type are diagnostic of PBC, and are mainly directed against a functional, restricted epitope on the E2 subunit of the pyruvate dehydrogenase complex (PDC). PDC-E2 shows several similarities to other classical autoantigens. The pathogenic role of AMA remains elusive. Recent studies have shown that AMAs detect an antigenic epitope expressed on the luminal surface of biliary epithelium in PBC liver. The initial triggering event might represent a microbial infection (molecular mimicry), or an aberrant surface expression of a true autoepitope.