T-cell autoimmunity in primary biliary cirrhosis

Glomerulonephritis in primary biliary cholangitis in China: an important complication

BACKGROUND

Extrahepatic manifestations in patients with primary biliary cholangitis (PBC) are frequently observed recently. We aimed in this study to explore the clinicopathological characteristics and prognosis of glomerulonephritis in patients with PBC.

METHODS

Consecutive PBC patients admitted to Peking Union Medical College Hospital from January 2002 to May 2019 were retrospectively enrolled. PBC patients with other autoimmune diseases which may have nephritis were excluded. Structured interview, systemic rheumatologic examination, and laboratory tests were conducted for each patient. Literature about patients with PBC and glomerulonephritis was reviewed and summarized.

RESULTS

Among the 330 PBC patients enrolled, glomerulonephritis were identified in 10 patients (3.0%). Eight (80.0%) were females and 2 (20.0%) were males. The mean age was 58.4 ± 9.5 years old. Membranous nephropathy (MN) was revealed in 4 patients, IgA nephropathy (IgA N) in 2 patients, minimal change disease (MCD) in 2 patients, mesangial proliferative glomerulonephritis in 1 patient, and renal amyloidosis in 1 patient. Compared to the literature reviewed, 10 cases of MN, 1 case of MCD, 1 case of IgA N, and 1 case of acute poststreptococcal glomerulonephritis (APSGN) were observed.

CONCLUSIONS

Glomerulonephritis may not be a well-recognized feature of PBC and is not a rare complication and deserved to be routinely screened in clinical practice. As MN is the most common form of glomerulonephritis in PBC patients and PBC can be asymptomatic at an early stage, patients presented with MN should be screened for PBC, so to avoid cirrhosis. Key Points • Patients with primary biliary cholangitis (PBC) can be complicated with glomerulonephritis, and membranous nephropathy (MN) is the most common form.

Integrative computational approach identifies drug targets in CD4+ T-cell-mediated immune disorders

CD4⁺ T cells provide adaptive immunity against pathogens and abnormal cells, and they are also associated with various immune-related diseases. CD4⁺ T cells’ metabolism is dysregulated in these pathologies and represents an opportunity for drug discovery and development. Genome-scale metabolic modeling offers an opportunity to accelerate drug discovery by providing high-quality information about possible target space in the context of a modeled disease. Here, we develop genome-scale models of naïve, Th1, Th2, and Th17 CD4⁺ T-cell subtypes to map metabolic perturbations in rheumatoid arthritis, multiple sclerosis, and primary biliary cholangitis. We subjected these models to in silico simulations for drug response analysis of existing FDA-approved drugs and compounds. Integration of disease-specific differentially expressed genes with altered reactions in response to metabolic perturbations identified 68 drug targets for the three autoimmune diseases. In vitro experimental validation, together with literature-based evidence, showed that modulation of fifty percent of identified drug targets suppressed CD4⁺ T cells, further increasing their potential impact as therapeutic interventions. Our approach can be generalized in the context of other diseases, and the metabolic models can be further used to dissect CD4⁺ T-cell metabolism.

Variation in the organization and subunit composition of the mammalian pyruvate dehydrogenase complex E2/E3BP core assembly

Crucial to glucose homoeostasis in humans, the hPDC (human pyruvate dehydrogenase complex) is a massive molecular machine comprising multiple copies of three distinct enzymes (E1–E3) and an accessory subunit, E3BP (E3-binding protein). Its icosahedral E2/E3BP 60-meric ‘core’ provides the central structural and mechanistic framework ensuring favourable E1 and E3 positioning and enzyme co-operativity. Current core models indicate either a 48E2+12E3BP or a 40E2+20E3BP subunit composition. In the present study, we demonstrate clear differences in subunit content and organization between the recombinant hPDC core (rhPDC; 40E2+20E3BP), generated under defined conditions where E3BP is produced in excess, and its native bovine (48E2+12E3BP) counterpart. The results of the present study provide a rational basis for resolving apparent differences between previous models, both obtained using rhE2/E3BP core assemblies where no account was taken of relative E2 and E3BP expression levels. Mathematical modelling predicts that an ‘average’ 48E2+12E3BP core arrangement allows maximum flexibility in assembly, while providing the appropriate balance of bound E1 and E3 enzymes for optimal catalytic efficiency and regulatory fine-tuning. We also show that the rhE2/E3BP and bovine E2/E3BP cores bind E3s with a 2:1 stoichiometry, and propose that mammalian PDC comprises a heterogeneous population of assemblies incorporating a network of E3 (and possibly E1) cross-bridges above the core surface.

Solution structure and characterisation of the human pyruvate dehydrogenase complex core assembly

Mammalian pyruvate dehydrogenase complex (PDC) is a key multi-enzyme assembly that is responsible for glucose homeostasis maintenance and conversion of pyruvate into acetyl-CoA. It comprises a central pentagonal dodecahedral core consisting of two subunit types (E2 and E3BP) to which peripheral enzymes (E1 and E3) bind tightly but non-covalently. Currently, there are two conflicting models of PDC (E2+E3BP) core organisation: the 'addition' model (60+12) and the 'substitution' model (48+12). Here we present the first ever low-resolution structures of human recombinant full-length PDC core (rE2/E3BP), truncated PDC core (tE2/E3BP) and native bovine heart PDC core (bE2/E3BP) obtained by small-angle X-ray scattering and small-angle neutron scattering. These structures, corroborated by negative-stain and cryo electron microscopy data, clearly reveal open pentagonal core faces, favouring the 'substitution' model of core organisation. The native and recombinant core structures are all similar to the truncated bacterial E2 core crystal structure obtained previously. Cryo-electron microscopy reconstructions of rE2/E3BP and rE2/E3BP:E3 directly confirm that the core has open pentagonal faces, agree with scattering-derived models and show density extending outwards from their surfaces, which is much more structurally ordered in the presence of E3. Additionally, analytical ultracentrifugation characterisation of rE2/E3BP, rE2 (full-length recombinant E2-only) and tE2/E3BP supports the substitution model. Superimposition of the small-angle neutron scattering tE2/E3BP and truncated bacterial E2 crystal structures demonstrates conservation of the overall pentagonal dodecahedral morphology, despite evolutionary diversity. In addition, unfolding studies using circular dichroism and tryptophan fluorescence spectroscopy show that the rE2/E3BP is less stable than its rE2 counterpart, indicative of a role for E3BP in core destabilisation. The architectural complexity and lower stability of the E2/E3BP core may be of benefit to mammals, where sophisticated fine-tuning is required for cores with optimal catalytic and regulatory efficiencies.

A new level of architectural complexity in the human pyruvate dehydrogenase complex

Mammalian pyruvate dehydrogenase multienzyme complex (PDC) is a key metabolic assembly comprising a 60-meric pentagonal dodecahedral E2 (dihydrolipoamide acetyltransferase) core attached to which are 30 pyruvate decarboxylase E1 heterotetramers and 6 dihydrolipoamide dehydrogenase E3 homodimers at maximal occupancy. Stable E3 integration is mediated by an accessory E3-binding protein (E3BP) located on each of the 12 E2 icosahedral faces. Here, we present evidence for a novel subunit organization in which E3 and E3BP form subcomplexes with a 1:2 stoichiometry implying the existence of a network of E3 "cross-bridges" linking pairs of E3BPs across the surface of the E2 core assembly. We have also determined a low resolution structure for a truncated E3BP/E3 subcomplex using small angle x-ray scattering showing one of the E3BP lipoyl domains docked into the E3 active site. This new level of architectural complexity in mammalian PDC contrasts with the recently published crystal structure of human E3 complexed with its cognate subunit binding domain and provides important new insights into subunit organization, its catalytic mechanism and regulation by the intrinsic PDC kinase.

Hepatic manifestations of gastrointestinal diseases. Inflammatory bowel disease, celiac disease, and Whipple's disease

The gastrointestinal tract and the liver are closely related anatomically, physiologically, and pathologically. Some disease associations are well documented, such as PSC in association with IBD, whereas others are less well defined. A heightened clinical suspicion is required in these patients who do not present with the classical disease associations. The underlying causes of their diseases are the subject of much debate and research, and their diagnosis and management remain challenging.

Immunogenetic analysis reveals that epitope shifting occurs during B-cell affinity maturation in primary biliary cirrhosis

Primary biliary cirrhosis (PBC) is a liver disease characterized by serum autoantibodies against the pyruvate dehydrogenase complex (PDC) located in the inner mitochondrial membrane. The predominant target in PDC has previously been localized to the inner lipoyl domain (ILD) of the E2 subunit. The etiology of PBC is unknown, although molecular mimicry with bacterial PDC has been proposed. Here, we have investigated the etiology of PBC and nature of the autoimmune response by analyzing the structure of a human monoclonal antibody with ILD specificity. Mutants of the monoclonal antibody, which was originally isolated from a patient with PBC, were expressed as Fab by phage display, and tested for reactivity against recombinant domains of the E2 subunit. Fab in which the V(H)-encoded portions were reverted to germline lost reactivity against the ILD alone, but recognized a different epitope in a didomain construct encompassing the ILD, hinge region and E1/E3 binding domain. The complete V(H) and V(L )germline revertant was unreactive with the human ILD and didomain, the Escherichia coli didomain, and whole PDC. We hypothesize that the IgM on the surface of the naïve B-cell first recognizes an as yet unidentified antigen, and that accumulation of somatic mutations results in an intermolecular epitope shift directed towards an epitope involving the E1/E3 binding domain. Further mutations result in the specificity being redirected to the ILD. These findings also suggest that bacterial molecular mimicry is not involved in initiating disease.

Immunopathogenesis of cholestatic autoimmune liver diseases

Primary biliary cirrhosis and primary sclerosing cholangitis are well recognized chronic cholestatic liver diseases that are considered to have an autoimmune basis. Recent progress in the study of autoimmune liver diseases has improved the recognition and characterization of these conditions. An important component of this progress has been the identification of liver disease-associated autoantibodies and their respective target antigens, and the development of specific assays for these autoantibodies. In addition, some nonhumoral immunological findings imply an involvement of specific immunopathogenic mechanisms in the development of these conditions. Furthermore, immunogenetic factors associated with increased susceptibility to some of these diseases have been identified. This article reviews the most relevant information relating to the postulated autoimmune pathogenesis of these diseases, with special emphasis on their associated humoral and cellular immunological abnormalities and immunopathogenetic factors. Some of the remaining important unresolved issues relating to the pathogenesis of these diseases, that need to be addressed in further research, are highlighted.

CTLA-4 gene polymorphism confers susceptibility to primary biliary cirrhosis

BACKGROUND/AIM

Primary biliary cirrhosis (PBC) is an autoimmune cholestatic liver disease thought to develop through a complex interaction of genetic and environmental factors. It is characterised by T-cell-mediated non-suppurative destructive cholangitis. We have studied the polymorphic cytotoxic T lymphocyte-associated antigen-4 (CTLA-4) gene, which encodes a molecule that is a vital negative regulator of T-cell activation, as a candidate susceptibility locus for PBC. This gene on chromosome 2q33 (designated IDDM12) is associated with susceptibility to both type 1 diabetes and autoimmune thyroid disease.

METHODS

The CTLA-4 exon 1 polymorphism (A/G encoding for threonine or alanine, respectively) was genotyped via polymerase chain reaction in 200 Caucasoid PBC patients and 200 non-related geographically matched Caucasoid controls.

RESULTS

There was significant overrepresentation of the G/A and G/G genotypes in PBC patients compared to controls (G/A 53% vs 40%; G/G 18.5% vs 10.5%, Odds Ratio (OR)=2.45 [95% CI 1.6-3.7], p=0.00006, chi2=19.4). Likewise, there was a significant difference in allele frequencies (G encoding alanine at codon 17, PBC 0.45 vs controls 0.305: OR=1.9 [1.4-2.5], p<0.0002). This association remained significant (p=0.00027) when patients with autoimmune thyroid disease were excluded from the analysis.

CONCLUSIONS

The CTLA-4 exon 1 polymorphism is the first non-major histocompatibility complex gene to be identified as a susceptibility locus for PBC. Our data support the hypothesis that clinically distinct autoimmune disease may be controlled by a common set of susceptibility genes.

T cell responses to the putative dominant autoepitope in primary biliary cirrhosis (PBC)

PBC is characterized by T cell-mediated destruction of the biliary epithelial cells lining the small intrahepatic bile ducts. The E2 and E3 binding protein (E3BP (protein X)) components of pyruvate dehydrogenase complex (PDC) are disease-specific autoantigens in PBC. Attempts to localize the T cell autoepitopes within PDC-E2 have, however, generated contradictory results. One study has suggested the presence of T cell epitopes throughout PDC-E2, whilst another has identified a single dominant 14 amino acid T cell epitope (p163) spanning the lipoic acid binding lysine residue in the inner lipoyl domain (ILD) of PDC-E2. The aim of the current study was to determine the prevalence of T cell responses to p163 and PDC-E2 ILD, and the role played by lipoylation of these antigens in their immunogenicity, in a UK PBC population. We found that the majority of the PBC patients showing a 6-day peripheral blood T cell proliferative response to native human PDC also responded, in a MHC class II-restricted fashion, to biochemically purified PDC-E2 and E3BP (which co-purify) (9/10 positive (SI > 2.76), mean SI 5.74 +/- 5.04 (PDC-E2/E3BP) versus 6.67 +/- 3.84 (PDC), P = NS), implying that the important PBC-specific T cell epitopes are contained within the PDC-E2 or E3BP components of PDC. Only a minority of patients responsive to PDC, however, responded to either lipoylated recombinant PDC-E2 ILD (4/10 positive, mean SI 1.98 +/- 1.24, P < 0.005 versus PDC response) or lipoylated p163 (4/12 positive, mean SI 1.90 +/- 1.58, P < 0.001). The lipoylation state did not affect the T cell response to either ILD or p163. Our findings suggest that in some UK patients with PBC there are immunodominant T cell autoepitopes within PDC-E2/E3BP which are outside the ILD of PDC-E2.