Functional Epitopes for Anti–Aquaporin 5 Antibodies in Sjögren Syndrome

• Published in: Journal of dental research Vol. 96; no. 12; pp. 1414 - 1421
• Main Authors: Alam, J., Koh, J.H., Kwok, S.-K., Park, S.-H., Park, K., Choi, Y.
• Format: Journal Article
• Language: English
• Published: Los Angeles, CA SAGE Publications 01.11.2017; SAGE PUBLICATIONS, INC
• Subjects: Amino Acid Sequence; Aquaporin 5; Aquaporin 5 - antagonists & inhibitors; Aquaporins; Autoantibodies; Autoantibodies - immunology; Data analysis; Epitopes; Epitopes - immunology; Fluorescent Antibody Technique, Indirect; Humans; Immunofluorescence; Immunoglobulin G; Immunoglobulin G - immunology; Immunoglobulins; Mimicry; Peptides; Peptides - immunology; Permeability; Sjogren's syndrome; Sjogren's Syndrome - immunology; Therapeutic applications; rheumatology; oral medicine; autoimmunity; MDCK cells; indirect immunofluorescence assay; salivary physiology
• DOI: 10.1177/0022034517717965
• ISSN: 0022-0345; 1544-0591

We recently reported the presence of anti–aquaporin 5 (AQP5) immunoglobulin G (IgG) in patients with primary Sjögren syndrome (SS) with a sensitivity of 0.73 and a specificity of 0.68. The aim of this study was to identify functional epitopes for the anti-AQP5 autoantibodies detected in control subjects and patients with SS. Recognition of epitopes by anti-AQP5 autoantibodies in sera (n = 13 for control and n = 24 for SS) or purified IgG (n = 1 for control and n = 3 for SS) was evaluated by indirect immunofluorescence (IIF) assay performed in the presence or absence of peptides corresponding to the second transmembrane helix and extracellular loops A, C, and E of AQP5. Functional epitopes were determined by measuring the effects of purified IgG and neutralizing peptides on transepithelial osmotic permeability (PfT) of MDCK cells expressing AQP5. In the IIF assay, 89% of SS samples were inhibited by at least 1 peptide, while only half of control samples were inhibited by any peptide. Overall, SS samples were inhibited by peptides corresponding to extracellular loops A, C, and E by 40% to 50%, whereas control samples were inhibited only by peptides corresponding to loop E by <20%. A cyclized peptide (E1) mimicking loop E was most frequently recognized and best differentiated between the SS and control samples. Incubation of MDCK-AQP5 cells with SS but not with control IgG, significantly decreased PfT, which was reversed by neutralization of IgG binding to any of the extracellular loops. In conclusion, the anti-AQP5 autoantibodies detected in control and SS groups showed differences in fine specificity to the functional epitopes of AQP5. The prevalent recognition of functional epitopes by anti-AQP5 autoantibodies from SS patients suggests that anti-AQP5 autoantibodies act as mediators of glandular hypofunction and are a potential therapeutic target in SS.