The XIST lncRNA is a sex-specific reservoir of TLR7 ligands in SLE.

Systemic lupus erythematosus (SLE) is a systemic autoimmune disease with a dramatic sex bias, affecting 9-times more women than men. Activation of toll-like receptor 7 (TLR7) by self-RNA is a central pathogenic process leading to aberrant production of type-I interferon (IFN) in SLE, but the specific RNA molecules that serve as TLR7 ligands have not been defined. We therefore sought to identify female-specific endogenous RNAs containing canonical TLR7 stimulatory motifs. By leveraging gene expression data and the known sequence specificity of TLR7, we identified the female-specific X-inactive specific transcript (XIST) long non-coding RNA as a uniquely rich source of TLR7 ligands in SLE. XIST RNA stimulated IFNα production by plasmacytoid DCs in a TLR7-dependent manner, and deletion of XIST diminished the ability of whole cellular RNA to activate TLR7. XIST levels were elevated in blood leukocytes from female SLE patients compared to controls, correlated positively with disease activity and the IFN signature, and were enriched in extracellular vesicles released from dying cells in vitro. Importantly, XIST was not IFN-inducible, suggesting that XIST is a driver, rather than a consequence of IFN in SLE. Our work suggests a novel role for XIST RNA as a female-specific danger signal underlying the sex bias in SLE.