Cystatin S-a candidate biomarker for severity of submandibular gland involvement in Sjogren's syndrome

Objectives. Salivary cystatin S is a defence protein mainly produced by submandibular glands and involved in innate oral immunity. This study aimed to verify whether cystatin S was diversely expressed in different disease subsets of primary Sjogren's syndrome (pSS) patients, defined on the basis of salivary flow [unstimulated salivary flow rate (USFR)], minor salivary gland (MSG) focus score and submandibular gland ultrasonography abnormalities. We also evaluated miR-126 and miR-335-5p expression in MSG biopsies to verify whether an aberrant regulation of cystatin S at the glandular level may influence its salivary expression. Methods. Forty pSS patients and 20 sex-and age-matched healthy volunteers were included. Salivary cystatin S levels were assessed by western blot analysis using a stain-free technology. The expression of miR-126, miR-335-5p and cystatin S was assessed by quantitative PCR in 15 MSG biopsies differing for USFR and MSG focus score. Results. We found that salivary cystatin S was significantly decreased in pSS patients vs healthy volunteers (P = 0.000), especially in those with hyposalivation. A positive correlation was observed between cystatin S and USFR (r = 0.75, P = 0.01). Salivary cystatin S was also significantly reduced in patients with a submandibular gland ultrasonography score 52. The expression levels of miR-126 and miR-335-5P increased in inverse proportion with USFR. The mRNA of cystatin S did not change significantly, suggesting post-transcriptional regulation. Conclusion. Cystatin S emerged as a promising biomarker for pSS, strongly correlated with glandular dysfunction. An upregulation of miR-126 and miR-335-5P might be implicated in its expression.