Background and objectives Inflammasomes are intracellular multiprotein complexes that sense pathogenic microorganisms, as well as DNA and other molecules released after tissue injury. Such activation of inflammasome leads to the upregulation of various inflammasome-related molecules and the release of pro-inflammatory cytokines, such as interleukin-1β (IL-1β) and IL-18. We have recently shown that patients with primary Sjögren’s syndrome (SS) manifest increased serum levels of circulating nucleosomes and cell-free genomic DNA (CF-DNA) owing to impaired DNA degradation. Herein, we assessed whether the inflammasome is activated in the peripheral blood and salivary glands (SG) of SS patients and whether serum CF-DNA represents a novel IL-1β stimulating agent.
Materials and methods The mRNA expression of inflammasome-related molecules ASC, NLRP3, AIM2 and IL-1β were evaluated by RT-PCR in PBMC (15 SS, 9 non-SS-controls). Sera were studied for the levels of circulating CF-DNA (by RT-PCR; 8 SS and 5 non-SS-controls) and ASC protein (by ELISA; 39 SS, 21 non-SS-controls). SG biopsies were examined by confocal microscopy for the presence of CF-DNA (anti-dsDNA/TO-PRO-3/membranous-WAG immunostaining; 3 SS, 2 non-SS-controls) and the expression of ASC protein (15 SS, 5 non-SS-controls). To evaluate the immunostimulatory capacity of CF-DNA, healthy PBMC were treated with 5 SS sera-derived CF-DNA and IL-1β produced was measured by ELISA.
Results Significantly high mRNA expression of ASC, IL-1β, NLRP-3 was detected in the PBMC of SS patients (all for p < 0.05, vs. controls), with ASC highly positively correlating with disease activity scores (ESSDAI; p = 0.007). SS patients manifested high serum levels of ASC protein (p < 0.0001, vs. controls) that were particularly high among SS patients who had developed lymphoma (p = 0.009). High serum levels of CF-DNA were also detected in SS patients (p < 0.05, vs. controls) that correlated positively with serum levels of ASC (r = 0.654, p = 0.015). The SG specimens of SS patients (but not non-SS-controls) manifested ample evidence of CF-DNA, as well as strong ASC protein expression by both epithelial and infiltrating cells, mainly CD68+ macrophages. The serum CF-DNA of SS patients induced high IL-1β secretion by healthy PBMC (8-fold induction over mock-treated cells).
Conclusions SS patients manifest evidence of chronic inflammasome activation in the peripheral blood and the salivary glands, likely due to the presence of lingering non-degraded CF-DNA. As supported by its correlation with disease activity and lymphoma development, inflammasome activation in SS patients likely participates in the pathogenesis of the adverse clinical outcomes of the disorder and may specify novel disease biomarkers.