Hyperresponsive cytosolic DNA-sensing pathway in monocytes from primary Sjogren's syndrome

Objectives Cytosolic DNA-sensing pathway stimulation prompts type I IFN (IFN-I) production, but its role in systemic IFN-I pathway activation in primary SS (pSS) is poorly studied. Here we investigate the responsiveness of pSS monocytes and plasmacytoid dendritic cells (pDCs) to stimulator of interferon genes (STING) activation in relation to systemic IFN-I pathway activation and compare this with SLE. Methods Expression of DNA-sensing receptors cGAS, IFI16, ZBP-1 and DDX41, signalling molecules STING, TBK1 and IRF3, positive and negative STING regulators, and IFN-I-stimulated genes MxA, IFI44, IFI44L, IFIT1 and IFIT3 was analysed in whole blood, CD14(+) monocytes, pDCs, and salivary glands by RT-PCR, monocyte RNA sequencing data, flow cytometry and immunohistochemical staining. Peripheral blood mononuclear cells (PBMCs) from pSS, SLE and healthy controls (HCs) were stimulated with STING agonist 2 ' 3 '-cGAMP. STING phosphorylation (pSTING) and intracellular IFN alpha were evaluated using flow cytometry. Results STING activation induced a significantly higher proportion of IFN alpha-producing monocytes, but not pDCs, in both IFN-low and IFN-high pSS compared with HC PBMCs. Additionally, a trend towards more pSTING(+) monocytes was observed in pSS and SLE, most pronounced in IFN-high patients. Positive STING regulators TRIM38, TRIM56, USP18 and SENP7 were significantly higher expression in pSS than HC monocytes, while the dual-function STING regulator RNF26 was downregulated in pSS monocytes. STING was expressed in mononuclear infiltrates and ductal epithelium in pSS salivary glands. STING stimulation induced pSTING and IFN alpha in pSS and SLE pDCs. Conclusion pSS monocytes and pDCs are hyperresponsive to stimulation of the STING pathway, which was not restricted to patients with IFN-I pathway activation.