Evaluation of the influence of immunoglobulin G4 (IgG4) on the innate immune response and tight junction-mediated barrier function of biliary epithelial cells in chronic inflammatory cholangiopathies

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There is growing evidence that inappropriate innate immune responses to bile- or portal venous blood-derived intestinal pathogen-associated molecular patterns (PAMPs) in biliary epithelial cells (BECs) lining the intra- and extra-hepatic bile ducts and consecutive biliary barrier dysfunction contribute to the pathogenesis of chronic inflammatory cholangiopathies, such as primary sclerosing cholangitis (PSC) and immunoglobulin G4 (IgG4)-associated cholangitis (IAC). Subsequent peribiliary leakage of bile containing intestinal PAMPs such as lipopolysaccharide (LPS) likely accelerates biliary damage. In contrast to PSC, IAC is a steroid-sensitive chronic inflammatory biliary disease that involves different parts of the pancreatobiliary system, but little is known about its mechanisms of pathogenesis.We isolated primary human BECs from explanted livers from patients with PSC. We were able to show that in the presence of the predominant T helper (Th) cell type 1 cytokine milieu in advanced PSC, activated BECs exhibit inappropriate pattern recognition receptor (PRR)-mediated innate immune responses to intestinal endotoxins and subsequent endotoxin intolerance because of enhanced PRR signaling, which accelerated chronic biliary inflammation. As TNF-alpha inhibition partly restored protective innate immune tolerance, endogenous TNF-alpha secretion likely contributed to inappropriate endotoxin responses in activated BECs. In contrast to PSC livers, bile samples from patients with IAC exhibited significant increases in levels of Th2 cytokines such as interleukin (IL)-4 and IL-5. IL-13 was not detected in bile samples, but brush cytology samples from the extra-hepatic bile ducts of these patients revealed enhanced levels of IL-13 mRNA, compared with controls. In vitro, IL-4 and IL-13 significantly reduced tight junction (TJ)-associated BEC barrier function by activating claudin-2-mediated paracellular pore pathways. These findings suggested that the specific Th2 cytokine milieu in IAC likely contributed to the pathogenesis of chonic biliary inflammation in these patients due to the induction of increased paracellular permeability of the lining biliary epithelium. Th2 cytokines also impaired wound closure in BEC monolayers. The role of IgG4 in the pathogenesis of IAC remains to be determined. Our preliminary work suggests that IgG4 selectively bind to BECs of the extra-hepatic bile ducts. Of note, autoreactive antibodies to BECs have also been described in patients with PSC. We hypothesize that autoreactive IgG4 promote aberrant innate immune responses and TJ-mediated barrier dysfunction in BECs by the induction of an enhanced secretion of pro-inflammatory respectively pro-fibriotic mediators such as TNF-alpha or TGF-ß and thus contribute to the pathogenesis of chronic biliary inflammation in these patients. This grant proposal aims to elucidate potential pathomechanisms underlying this hypothesis."