Objective: MicroRNA have recently been identified as regulators that modulate target gene expression and are involved in shaping the immune response. This study was undertaken to investigate the contribution of microRNA‐146a (miR‐146a), which was identified in the pilot expression profiling step, to the pathogenesis of systemic lupus erythematosus (SLE).Methods: TaqMan microRNA assays of peripheral blood leukocytes were used for comparison of expression levels of microRNA between SLE patients and controls. Transfection and stimulation of cultured cells were conducted to determine the biologic function of miR‐146a. Bioinformatics prediction and validation by reporter gene assay and Western blotting were performed to identify miR‐146a targets.Results: Profiling of 156 miRNA in SLE patients revealed the differential expression of multiple microRNA, including miR‐146a, a negative regulator of innate immunity. Further analysis showed that underexpression of miR‐146a negatively correlated with clinical disease activity and with interferon (IFN) scores in patients with SLE. Of note, overexpression of miR‐146a reduced, while inhibition of endogenous miR‐146a increased, the induction of type I IFNs in peripheral blood mononuclear cells (PBMCs). Furthermore, miR‐146a directly repressed the transactivation downstream of type I IFN. At the molecular level, miR‐146a could target IFN regulatory factor 5 and STAT‐1. More importantly, introduction of miR‐146a into the patients' PBMCs alleviated the coordinate activation of the type I IFN pathway.Conclusion: The microRNA miR‐146a is a negative regulator of the IFN pathway. Underexpression of miR‐146a contributes to alterations in the type I IFN pathway in lupus patients by targeting the key signaling proteins. The findings provide potential novel strategies for therapeutic intervention.

Although circumstantial evidence supports enhanced Toll-like receptor 7 (TLR7) signalling as a mechanism of human systemic autoimmune disease^1–7, evidence of lupus-causing TLR7 gene variants is lacking. Here we describe human systemic lupus erythematosus caused by a TLR7 gain-of-function variant. TLR7 is a sensor of viral RNA⁸,⁹ and binds to guanosine¹⁰–¹². We identified a de novo, previously undescribed missense TLR7^Y264H variant in a child with severe lupus and additional variants in other patients with lupus. The TLR7^Y264H variant selectively increased sensing of guanosine and 2',3'-cGMP^10–12, and was sufficient to cause lupus when introduced into mice. We show that enhanced TLR7 signalling drives aberrant survival of B cell receptor (BCR)-activated B cells, and in a cell-intrinsic manner, accumulation of CD11c⁺ age-associated B cells and germinal centre B cells. Follicular and extrafollicular helper T cells were also increased but these phenotypes were cell-extrinsic. Deficiency of MyD88 (an adaptor protein downstream of TLR7) rescued autoimmunity, aberrant B cell survival, and all cellular and serological phenotypes. Despite prominent spontaneous germinal-centre formation in Tlr7^Y264H mice, autoimmunity was not ameliorated by germinal-centre deficiency, suggesting an extrafollicular origin of pathogenic B cells. We establish the importance of TLR7 and guanosine-containing self-ligands for human lupus pathogenesis, which paves the way for therapeutic TLR7 or MyD88 inhibition.

Systemic lupus erythematosus is a complex autoimmune disease caused by genetic and epigenetic alterations. DNA methylation abnormalities play an important role in systemic lupus erythematosus disease processes. MicroRNAs (miRNAs) have been implicated as fine-tuning regulators controlling diverse biological processes at the level of posttranscriptional repression. Dysregulation of miRNAs has been described in various disease states, including human lupus. Whereas previous studies have shown miRNAs can regulate DNA methylation by targeting the DNA methylation machinery, the role of miRNAs in aberrant CD4⁺ T cell DNA hypomethylation of lupus is unclear. In this study, by using high-throughput microRNA profiling, we identified that two miRNAs (miR-21 and miR-148a) overexpressed in CD4⁺ T cells from both patients with lupus and lupus-prone MRL/lpr mice, which promote cell hypomethylation by repressing DNA methyltransferase 1 (DNMT1) expression. This in turn leads to the overexpression of autoimmune-associated methylation-sensitive genes, such as CD70 and LFA-1, via promoter demethylation. Further experiments revealed that miR-21 indirectly downregulated DNMT1 expression by targeting an important autoimmune gene, RASGRP1, which mediated the Ras–MAPK pathway upstream of DNMT1; miR-148a directly downregulated DNMT1 expression by targeting the protein coding region of its transcript. Additionally, inhibition of miR-21 and miR-148a expression in CD4⁺ T cells from patients with lupus could increase DNMT1 expression and attenuate DNA hypomethylation. Together, our data demonstrated a critical functional link between miRNAs and the aberrant DNA hypomethylation in lupus CD4⁺ T cells and could help to develop new therapeutic approaches.