Role of the IgG4-related cholangitis autoantigen annexin A11 in cholangiocyte protection.

BACKGROUND & AIMS: Annexin A11 was identified as autoantigen in IgG4-related cholangitis (IRC), a B-cell driven disease. Annexin A11 modulates calcium-dependent exocytosis, a crucial mechanism for insertion of proteins into their target membranes. Human cholangiocytes form an apical 'biliary bicarbonate umbrella' regarded as defense against harmful hydrophobic bile acid influx. The bicarbonate secretory machinery comprises the chloride/bicarbonate exchanger AE2 and the chloride channel ANO1. We aimed to investigate the expression and function of annexin A11 in human cholangiocytes and a potential role of IgG1/IgG4-mediated autoreactivity against annexin A11 in the pathogenesis of IRC.

METHODS: Expression of annexin A11 in human liver was studied by immunohistochemistry and immunofluorescence. In human control and ANXA11 knockdown H69 cholangiocytes, intracellular pH, AE2 and ANO1 surface expression, and bile acid influx were examined using ratio microspectrofluorometry, cell surface biotinylation, and 22,23-3 H-glycochenodeoxycholic acid permeation, respectively. Annexin A11-mEmerald and ANO1-mCherry localization in H69 cholangiocytes after incubation with IRC patient serum containing anti-annexin A11 IgG1/IgG4-autoantibodies or disease control serum was investigated by live-cell microscopy.

RESULTS: Annexin A11 was strongly expressed in human cholangiocytes, but not hepatocytes. Knockdown of ANXA11 led to reduced plasma membrane expression of ANO1, but not AE2, alkalization of intracellular pH and uncontrolled bile acid influx. High intracellular calcium conditions led to annexin A11 membrane shift and colocalization with ANO1. Incubation with IRC patient serum inhibited annexin A11 membrane shift and reduced ANO1 surface expression.

CONCLUSION: Cholangiocellular annexin A11 mediates apical membrane abundance of the chloride channel ANO1, thereby supporting biliary bicarbonate secretion. Insertion is inhibited by IRC patient serum containing anti-annexin A11 IgG1/IgG4-autoantibodies. Anti-annexin A11 autoantibodies might contribute to the pathogenesis of IRC by weakening the 'biliary bicarbonate umbrella'.

LAY SUMMARY: We previously identified annexin A11 as specific autoantigen in IgG4-related cholangitis (IRC), a B-cell driven disease targeting the bile ducts. Human cholangiocytes are protected against harmful hydrophobic bile acid influx by an apical defence mechanism referred to as the 'biliary bicarbonate umbrella'. We found that annexin A11 is required for the formation of a robust bicarbonate umbrella by modulating the plasma membrane expression of the anion channel ANO1. Binding of patient IgG4/IgG1 annexin A11 autoantibodies inhibits annexin A11 function possibly contributing to bile duct damage by weakening the biliary bicarbonate umbrella in patients with IRC.