Diagnostic value of the saliva immunoglobulin free light chains concentrations measured in primary Sjцgren's syndrome

Abstract

Proliferation and hyperactivation of B-lymphocytes in the salivary glands is a feature of primary Sjцgren's syndrome (pSS). Detection in saliva of proteins synthesized by B-lymphocytes may be important in the diagnosis of this disease.Objective: to evaluate the diagnostic value of measuring the concentration of immunoglobulin free light chains (FLC) in saliva in patients with pSS.Material and methods. The cross-sectional study included 24 patients with pSS over the age of 18 years. PSS was diagnosed according to the 2016 ACR/EULAR classification criteria. The control group consisted of 11 healthy volunteers. Blood-salivary glands histohematic barrier permeability ratio for albumin, FLC was measured. Quantitative determination of FLC and in blood and saliva was performed by enzyme immunoassay. An immunohistochemical study of biopsies of minor salivary glands (MSG) was carried out with a quantitative assessment of CD3+, CD4+, CD8+, CD20+, CD21+, CD68+, CD138+ cells. The Mann–Whitney U-test was used to compare quantitative traits. Identification of diagnostic thresholds for the concentration of FLC in saliva for the diagnosis of pSS was carried out using the ROC analysis method. An operating characteristic curve was plotted, the area under the curve, indicators of diagnostic specificity, diagnostic sensitivity, and diagnostic accuracy were calculated.Results and discussion. The obtained values corresponded to the low permeability of the histohematic barrier of the salivary glands for albumin and FLC in patients with pSS and healthy individuals. The median concentrations of FLC ê and ë in the saliva of patients with pSS and healthy volunteers were 1.08 [0.58; 1.91], 1.038 [0.55; 2.03] mg/l and 0.36 [0.32; 0.54], 0.35 [0.21; 0.52] mg/l, respectively. The concentration of FLC in the saliva of patients with pSS was statistically significantly higher than in the control group (p<0.01). The amount of FLC ê and ë in saliva correlated with the rate of unstimulated saliva flow: rs=-0.483 (p=0.02), rs=-0.491 (p=0.017), respectively.A relationship was found between the concentration of ê-chains in saliva and the specific number of CD138+ cells: rs=0.733 (p=0.025). Statistically significant correlations between the concentration of ë-chains and the number of mononuclear cells in the MSG have not been established.Based on the results of ROC analysis, diagnostic thresholds for FLC concentrations in the saliva of patients with pSS were determined. Concentrations of ê- and ë-type FLC in saliva of 0.56 and 0.68 mg/l correspond to area under the curve values of 0.84 (95% confidence interval, CI 0.69–0.98) and 0.83 (95% CI 0.71–0.97), sensitivity 79.2% (95% CI 59.5–90.8) and 75% (95% CI 55.1–88), specificity 81.8% (95% CI 52.3–96.8) and 90.9% (95% CI 62.3–99.5), respectively.Salivary FLC concentrations were compared in patients with pSS receiving and not receiving glucocorticoids (GC). The groups did not differ in a statistically significant way in terms of clinical and laboratory parameters. The median daily dose of GC was 10 [5; 10] mg in prednisolone equivalent. There were no significant differences between the concentrations of saliva FLC in patients of these groups.Conclusion. Salivary-fixed FLCs are most likely produced by cells localized in the stroma of the salivary glands. Determination of the concentration of FLC in saliva can be proposed as a diagnostic test for the pSS. The concentration of free ê-chains in saliva can be considered as a surrogate marker of benign B-cell proliferation in the MSG. Therapy with low and medium doses of GC in pSS does not affect the concentration of FLC in saliva.