Inhibition of mTOR suppresses IFNα production and the STING pathway in monocytes from systemic lupus erythematosus patients

Objective. Increased IFN alpha is important in the pathogenesis of SLE. Plasmacytoid dendritic cells are considered the main producer of IFN alpha upon Toll-like receptor pathway activation. However, which cells produce IFN alpha following stimulation with cyclic GMP-AMP synthase (cGAS) and stimulator of IFN genes (STING) in SLE remains unknown. We investigated the IFN alpha producing capacity of myeloid cells under cGAS-STING pathway stimulation. Methods. IFN alpha levels in peripheral blood mononuclear cells from SLE patients and healthy controls stimulated with 2'3'c-GAMP, a stimulator of cGAS-STING, were measured by intracellular cytokine staining and flow cytometry. STING expression and its co-localization with TBK1 were examined by flow cytometry or confocal microscopy. The effects of in vitro exposure to IFN alpha on IFN alpha production and STING expression, and in vitro rapamycin treatment on IFN alpha production and STING, pTBK1 and IRF3 expression were examined. Results. IFN alpha was produced by monocytes, conventional dendritic cells and plasmacytoid dendritic cells upon cGAS-STING pathway activation. The frequency of IFN alpha-producing monocytes positively correlated with SLE disease activity. STING expression and its co-localization with TBK1 were increased in lupus monocytes. Prior exposure to IFN alpha enhanced the IFN alpha-producing capacity of monocytes. Inhibition of the mechanistic target of the rapamycin (mTOR) pathway suppressed IFN alpha production from monocytes and downregulated enhanced STING expression and its downstream molecules. Conclusion. Enhanced IFN alpha from lupus monocytes induced by augmented STING pathway activation is associated with SLE pathogenesis. Suppression of the mTOR pathway downregulated the enhanced STING expression and the subsequent IFN alpha production by monocytes.