How was the autoantigen and Primary Biliary Cholangitis (PBC) discovered?

Discovery of the Autoantigen and Primary Biliary Cholangitis (PBC)

The major breakthrough in understanding Primary Biliary Cholangitis came in 1987-1988 with the identification of the 74-kD mitochondrial autoantigen as dihydrolipoamide acetyltransferase (PDC-E2), which remains the principal autoantigen in PBC to this day. [1, @18@]

Key Discoveries in PBC Autoantigen Identification

Initial Identification of the Mitochondrial Autoantigen

The journey to understanding PBC began with a landmark discovery in 1988 when Van de Water and colleagues identified that the autoepitope of the 74-kD mitochondrial autoantigen in PBC corresponds to the functional site of dihydrolipoamide acetyltransferase (PDC-E2) [@18@]. This discovery was pivotal as it:

• Established that the primary target of antimitochondrial antibodies (AMAs) was a specific enzyme component
• Revealed that the autoantigen was located at the functional site of the enzyme
• Provided the first molecular basis for understanding the autoimmune response in PBC

Mapping of the Autoepitope

Following the initial identification, researchers made significant progress in characterizing the autoantigen:

• In 1990, Surh and colleagues determined the structural requirements for autoreactivity on human PDC-E2, confirming it as the major autoantigen of PBC [@21@]
• The epitopes were mapped to the highly conserved lipoyl domain of the E2 subunits 1
• Further research revealed that the epitopes recognized by CD4+ and CD8+ T cells are mapped to the same region of the inner lipoyl domain of PDC-E2 2

Expanded Understanding of PBC Autoantigens

Research continued to identify additional autoantigens related to the 2-oxo-acid dehydrogenase complexes (OADC):

• E2 subunits of the branched chain 2-oxo-acid dehydrogenase complex (BCOADC-E2) [@19@]
• 2-oxo-glutarate dehydrogenase complex (OGDC-E2) [@20@]
• Dihydrolipoamide dehydrogenase-binding protein (E3BP) [@17@]
• Protein X and E1 alpha components 1

Mechanisms of Autoimmunity in PBC

Unique Properties of Biliary Epithelial Cells

A critical discovery explaining the tissue specificity of PBC came when researchers found that:

• Biliary epithelial cells (BECs) uniquely express PDC-E2 on apoptotic bodies (apotopes) in an immunologically intact form during apoptosis 2
• Other epithelial cells do not maintain this immunologically intact form during apoptosis
• This phenomenon helps explain why the autoimmune attack is specifically directed at bile ducts

Role of Xenobiotics in Breaking Tolerance

Research has revealed how environmental factors may trigger PBC:

• Xenobiotics such as 2-octynamide or 2-nonyamide can modify the lipoylated PDC-E2 2
• These modified proteins can be endocytosed by antigen-presenting cells and presented to T cells
• Studies showed that immunization with xenobiotic 6-bromohexanoate bovine serum albumin conjugate induces anti-mitochondrial antibodies [@27@]
• Loss of tolerance to the autoantigen E2 subunit of pyruvate dehydrogenase can be induced by xenobiotics with subsequent biliary ductular disease [@29@]

Animal Models Contributing to PBC Understanding

Several animal models have been developed that have enhanced our understanding of PBC pathogenesis:

• NOD.c3c4 mice develop autoimmune biliary disease that serologically and pathogenetically models human PBC [@35@]
• TGF-beta receptor II dominant-negative mice develop anti-mitochondrial antibodies and PBC-like features [@31@]
• IL-2 receptor alpha-deficient mice develop features of PBC [@34@]
• ARE Del-/- mouse model presents disease modeling human PBC, including female dominance 2

Recent Advances in Autoantigen Discovery

Beyond the classic mitochondrial autoantigens, recent research using human proteome microarrays has identified additional PBC autoantigens:

• Hexokinase-1 (isoforms I and II)
• Kelch-like protein 7 and 12
• Zinc finger and BTB domain-containing protein 2
• Eukaryotic translation initiation factor 2C, subunit 1 3

These newer discoveries may provide additional diagnostic markers and therapeutic targets for PBC.

Clinical Significance of Autoantigen Discovery

The identification of PBC autoantigens has had profound implications:

• Enabled the development of highly specific diagnostic tests for PBC
• Provided insights into disease pathogenesis
• Opened avenues for potential therapeutic interventions
• Allowed for better understanding of the natural history of the disease

Pitfalls and Caveats

• While AMA testing is highly specific for PBC diagnosis, the titer does not directly correlate with disease progression 4
• Approximately 5-10% of PBC patients are AMA-negative despite having the disease
• The presence of AMAs alone does not necessarily predict disease development, as some AMA-positive individuals may not develop clinical PBC
• Multiple autoantigens are involved in PBC, and focusing solely on PDC-E2 may provide an incomplete picture of disease pathogenesis

The discovery of the PBC autoantigen represents one of the most significant breakthroughs in autoimmune liver disease research, transforming our understanding of this condition from a mysterious inflammatory disease to a well-characterized autoimmune disorder with specific molecular targets.