Abstract

Systemic Lupus Erythematosus (SLE) is among the most sex-biased autoimmune diseases identified to date, affecting 9-times more women than men. Recognition of self-RNA by Toll-like receptor 7 (TLR7) is implicated as a central pathogenic process leading to the aberrant production of type-I interferon (IFN) in SLE, but the specific RNA molecules contributing to this process have not been defined. Given the role of self-RNA and biological sex in SLE pathogenesis, we investigated which sex-biased self-RNAs are potentially responsible for aberrant TLR7 activation in SLE. We used recent revelations about TLR7 sequence specificity and publicly available RNA sequencing data to identify sex-biased sources of self-RNA containing TLR7 ligands. We found X-inactive specific transcript (XIST) to be a particularly rich source of TLR7 ligands that is specifically expressed in women. We then investigated the capacity of XIST to act as a TLR7 ligand in vitro and found that multiple fragments of XIST induce IFNα production in a TLR7-dependent manner more than control RNA of equal length. Furthermore, we found that RNA isolated from XIST-knockout cells have significantly reduced capacity to stimulate TLR7 compared to RNA from wild-type cells. Finally, we used flow cytometry and publicly available RNA sequencing data to investigate the connection between XIST and SLE disease variables. We found that higher XIST expression correlated with SLE disease status, higher SLE disease activity index (SLEDAI) scores, and the interferon signature. Our data suggest that XIST is a source of TLR7 ligands that may underlie the sex bias in SLE. Supported by grants from National Institute of Dental and Craniofacial Research (NIDCR), NIH (R21 DE028391-02), the Jerome L. Greene Foundation and Scleroderma Research Foundation.