Investigation Of The Role Of Anti-M3R Antibody And IL-18in Primary Sj(?)gren’s Syndrome

Objectives:Sjogren’s syndrome (SS) is an autoimmune disease mainly targeting the lacrimal and salivary glands and resulting a symptoms of xerophthalmia (dry eyes), xerostomia (dry mouth). The serum of the SS patients are characterized with many autoantibodies, such as SSA antibodies, SSB antibodies and anti-M3R antibodies as well as elevated proinflammatory cytokines including Interleukin-18(IL-18). Previouss tudies suggest that the anti-M3R antibody is a candidate of the pathogenic autoantibodies of SS and the second extracellular loop of M3R (M3R2ndEL) might contain a pathogenic epitope. Our study aimed to investigate the role of the anti-M3R2ndEL IgG and Interleukin-18in primary Sjogren’s syndrome.Methoths:In this study, three ELISA methods were used to investigate whether it is possible to detect the autoantibodies against the M3R2ndEL in the serum of pSS patients. To determine the pathogenicity of the autoantibodies against M3R2ndEL, BALB/c mice were immunized with the peptides from the M3R2ndEL. The autoantibody production from the immunized mice was detected by ELISA, and the binding of the autoantibody to the tissue was determineddetermined by the immunofluorescence staining. The function of the lacrimal and salivary glands was evaluated by determining the tear and salivary secretion respectively. The concentrations of the IL-18in the serum of patients and healthy controls were determined by ELISA, then the association between IL-18and pSS clinical phenotypes was evaluated.Results:The positive rates of autoantibodies against the peptides of the M3R2ndEL in pSS patients were less than10%in all three different ELISA methods, and there was no significant differences between patients and controls. Both sensitivity and the specificity of the ELISA methods detecting the M3R2ndEL were too low to be used as diagnostic methods for pSS. The mice immunized with the peptide of the M3R2ndEL produced autoantibodies against the peptide, and the autoantibodies were able to bind to the salivary glands of mice. However, the function of the salivary and lacrimal glands was not affected, with neither a decrease in the tear and salivary secretion nor lymphocytes infiltration in glands. Finally, levels of IL-18in pSS patients’ serum were significantly higher than those in healthy controls (65.03±45.70 pg/ml)(P<0.01). In addition, the levels of IL-18in patients with high disease activity were significantly higher than those in patients with low disease activity (166.12±120.20pg/ml vs80.66±57.86pg/ml, P<0.01).Conclusions:Our study shows that the ELISA method detecting the autoantibodies against the M3R2ndEL cannot be used as a diagnostic tool for pSS. The autoantibodies against the peptide of M3R2ndEL are not pathogenic in mice. The circulating levels of IL-18are positive correlated with the disease activity of pSS, suggesting that IL-18might be a biomarker indicating the disease activity of pSS.