PBC is an autoimmune liver disease characterized by a targeted immune response to small biliary epithelial cells (BEC) and the presence of antimitochondrial autoantibodies (AMAs), which react specifically with the lipoyl domain of the pyruvate dehydrogenase complex E2 subunit (PDC-E2). Although considerable insights have been gained on both the AMAs and the CD4+ and CD8+ T cell responses, a detailed understanding of the mechanisms of the specific targeted destruction of BEC remains elusive. A key issue to be resolved is how loss of tolerance to a ubiquitous autoantigen (PDC-E2) results in an immune response to a highly specific cell type (BEC). In our most recent work we have established that BEC are unique in that during apoptosis, PDC-E2 remains intact and immunologically accessible within apoptotic blebs. In addition, the triad of AMA, BEC blebs and PBC monocyte-derived macrophages (MoMF) leads to a release of proinflammatory cytokines, with each component of the triad required for this response, suggesting an important role of the innate immune system in tissue specific targeting. Further, we have shown that antigen-specific CD8+ T cells are enriched within PBC liver and, in our murine models of autoimmune cholangitis, CD8+ T cells can adoptively transfer disease. Therefore, we will focus on the role of innate immune responses to BEC blebs, the phenotypic and functional attributes of PDC-E2 specific CD8+ T cells, and the interplay between these two immune responses. We will investigate the structural and specificity requirements for the bleb and AMA and use a proteomic approach to identify additional autoantigens unique to BEC. In addition, we will determine if other mononuclear cells, e.g. dendritic cells can elicit a similar response as MoMF. The unique ability of PBC MoMF to secrete proinflammatory cytokines in response to BEC blebs will be probed at the level of RNA transcription by next-generation sequencing and at the level of cell signaling. We will further test our hypothesis that this innate immune response is responsible for activation of PDC-E2 specific CD8+ T cells and we will use PDC-E2 epitope loaded tetramers to fully explore the relationship between antigen specific CD8+ T cells and disease progression. We submit that destruction of bile ducts requires an orchestrated interplay of AMA, intact PDC-E2 in BEC blebs and PBC antigen presenting cells, along with the recruitment/activation of cytotoxic CD8+ T cells which perpetuate BEC destruction. We submit that this rigorous dissection of PBC will define pathways that will direct us to potential therapeutic venues.
Primary biliary cirrhosis is an autoimmune disease of the liver that targets destruction of small bile duct cells. The etiology of this disease is unknown but there is mounting data that helps explain how the immune system attacks the bile ducts. In this application, we will take advantage of that data and define in more precise terms the mechanisms that lead to liver damage in the hopes of developing new therapeutic approaches.
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