miR-31-5p Regulates Type I Interferon by Targeting SLC15A4 in Plasmacytoid Dendritic Cells of Systemic Lupus Erythematosus

Background: Plasmacytoid dendritic cells (pDCs) are the main producers of type I interferon (IFN-I), and the excessive production of IFN-I is a hallmark of systemic lupus erythematosus (SLE). Both SLC15A4 and miR-31-5p are SLE susceptibility-related genes, and SLC15A4 has been implicated an important role in endolysosomal toll-like receptor (TLR) activation in pDCs. However, whether miR-31-5p exerts a regulating effect on SLC15A4 expression in pDCs is unclear.Methods: The expression of SLC15A4 and miR-31-5p in peripheral blood mononuclear cells (PBMCs) of SLE patients was measured by RT-qPCR analyses. The quantitative analysis of IFN-alpha secretion in the patients' serum was performed by ELISA assay. Luciferase-reporter assay was applied to confirm the interaction between miR-31-5p and SLC15A4. The expression of miR-31-5p, SLC15A4 and IFN-stimulated genes (ISGs, such as MX1, OAS1 and IFIT3) was detected by Western blot and RT-qPCR assays and further IRF5 phosphorylation was evaluated by immunofluorescence after transfected with miR-31-5p mimics or inhibitor in THP-1 and CAL-1 cells.Results: The expression of miR-31-5p was downregulated and negatively correlated with the overexpression of SLC15A4 in PBMCs of SLE patients. In addition to this, the secretion of IFN-alpha was overexpressed in sera of SLE and positively correlated with SLC15A4 level. We found that miR-31-5p directly targeted SLC15A4 and negatively regulated the expression of SLC15A4 in THP-1 and CAL-1 cells. In vitro inhibition of miR-31-5p increased the phosphorylation of IRF5 and the induction of ISGs stimulated by R848, overexpression of miR-31-5p get the reverse results.Conclusion: miR-31-5p might involve in SLE pathogenesis through regulating IFN-I expression by negatively regulating SLC15A4 to increase the levels of IFN-alpha and ISGs in pDCs.