Functional autoantibodies against aquaporin-5 detected in Sjögren's syndrome patient's sera

[Speaker] Shinichi Muroi:1
[Co-author] Hiromu Honma:1, Ichiro Horie:1, Yoichiro Isohama:1
1:Lab. Appl. Pharmacol., Facul. of Pharm. Sci., Tokyo Univ. of Sci., Japan

Sjögren's syndrome (SjS), a systemic autoimmune disease targeting salivary and lacrimal glands, results in severe dry mouth and dry eyes. The pathogenesis of SjS, including autoantibodies, which cause these specific symptoms of SjS, have been unclear. Aquaporins (AQPs) are a family of membrane proteins that largely function as water channels. Among 13 isoforms of AQPs, AQP5 is expressed in the apical membrane of the salivary glands and lacrimal glands, and plays important roles for exocrine secretion. AQP5-deficient mice have shown the reduction of saliva or tear secretion. In addition, abnormal localization of AQP5 in salivary glands of some SjS patients have been reported. In the present sutdy, we hypothesized that SjS patients have anti-AQP5 autoantibodies, and the autoantibodies may affect AQP5 function and/or localization.
Sera from SjS patients (University of Tsukuba Hospital) or its IgG fractions (SjS-IgG) were treated to AQP5-expressing human salivary gland cells (HSG-AQP5) as primary antibody, following immunocytochemistry. To examine AQP5 function, membrane water permeability was measured by Stopped-flow light-scattering method. SjS-IgG were treated with MLE-12 cells, expressing endogenous AQP5, and AQP5 protein expression and localization were examined by western blot and immunocytochemistry.
Immunocytochemistry using HSG-AQP5 cells demonstrated that 70% of sera from SjS patients specifically reacted HSG-AQP5 cells, but not control cells as well as the anti-AQP5 antibody (from commercial source). Although some of the sera also reacted AQP2- (32%) or AQP4-expressing cells (33%), AQP5-reactive sera were majority. SjS-IgG also reacted HSG-AQP5 cells, suggesting that quite a few SjS patients have anti-AQP5 autoantibodies. Interestingly, several SjS-IgG significantly decreased water permeability in membrane of AQP5-expressing HEK293T cells as well as HgCl2, an AQP inhibitor. In addition, SjS-IgG also induced AQP5 internalization via lipid-raft-dependent endocytosis, resulting in the decrease of cell surface expression of AQP5.
These data indicated that anti-AQP5 autoantibodies exist in SjS patient's sera, and the autoantibodies down-regulated AQP5 function or membrane localization. The anti-AQP5 autoantibodies could be a new biomarker of SjS, and provide new insight into the pathogenesis of SjS.

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