Sjögren’s syndrome patients manifest impaired serum DNase1 activity and increased serum levels of cell-free genomic DNA (cfDNA). Such circulating cfDNA may accumulate in tissues and may act as danger signal able to activate the intracellular inflammasomes. Herein, we sought to investigate the occurrence of defective nuclease activity, as well as of inflammasome activation in various tissues of SS patients. The mRNA levels of inflammasome-related and nuclease-encoding genes were evaluated in PBMC by q-PCR. Serum and salivary gland (SG) tissue expression of inflammasome-related proteins were assessed by ELISA and confocal microscopy. DNase-II activity in PBMC was measured by the SRED-assay. Indicative of inflammasome activation, SS patients exhibited elevated serum levels of ASC (n=56; p=0.002), IL-1β (n=37; p=0.003) and IL18 (n=33; p<0.0001), compared to healthy controls (HC; n=20). The serum ASC protein levels were higher in SS patients at high risk for lymphoma development (SStype I; n=17) and in SS patients with lymphoma (n=28), compared to SS at low risk for lymphoma development (SS-type II; n=11, for p=0.030 and p=0.004, respectively). ASC and IL-18 serum levels correlated positively with cumulative ESSDAI score (p=0.005 and p=0.006, respectively), whereas ASC correlated with C4 hypocomplementemia, RF positivity and purpura. Compared to HC-PBMC (n=16), SS-PBMC expressed significantly higher levels of ASC, NLRP3, IL-1β, IL-18 and pro-caspase-1 transcripts (n=39, p<0.05). In CD68+macrophages of SS-SG, ASC protein localised in perinuclear aggregates (specks), also suggesting inflammasome activation. SS patients (but not controls) manifested extranuclear DNA accumulations in PBMC and SG macrophages, as well as extracellular dsDNA strands in the SG tissues. cfDNA from SS patients sera was able to serve as a priming signal for inflammasome activation in healthy monocytes. Among various nucleases tested, DNase-II transcriptional levels and enzymatic activity were diminished in SS-PBMC. DNase-II mRNA silencing in healthy monocytes led to upregulation of inflammasome-related genes and fueled caspase-1 activation. Overall, SS patients manifest inflammasome activation in peripheral blood and SG, which associates with deficient nuclease activity and the presence of extranuclear DNA accumulations. Several indices of inflammasome activation were found to correlate with the occurrence of severe SS manifestations, including lymphoma development, and may constitute valuable clinical biomarkers.
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