Background Systemic lupus erythematosus (SLE) is about 10 times more common among women than men. This sex bias was previously attributed to sex hormones. However, while sex hormones are abnormal in both men and women with SLE, the best evidence supports that sex hormone abnormalities are caused by SLE. Studies of four-genotype mice with a lupus-like illness suggest an effect of the X chromosome numbers. Our previous studies have shown that both men with 47XXY and women with 47XXX are enriched in SLE such that we calculate that SLE is present among 47XXY men at the same rate as 46XX women. Up to 15% of genes lying on the X chromosome within the X-linked region escape X inactivation. Two X chromosome genes, TLR7 and CXorf21, with SLE-associated risk alleles routinely escape X inactivation. Signaling through the TLR7 pathway is critical to SLE pathogenesis. The CXorf21 protein product, known as TASL, binds SLC15a4, another gene with SLE- risk alleles, on the surface of the endolysosome.

Methods We used CRIPR-Cas to knock down expression of CXorf21 (TASL) in primary monocytes, B cells and T cells. We studied lysomomal pH using LysoSensor Yellow/Blue DND-160 and pHrodo Red. We measured cytokines by ELISA of cell culture supernats. We used an assay for NADP/NADPH conversion to determine whether TASL is a short chain dehydrogenase. Finally, we typed SLE subjects and controls for the lupus-associated single nucleotide polymorphism within CXorf21.

Results Knock out of Slc15a4 abrogates TLR7 signaling. Our data demonstrate that CRISPR-cas knock down of TASL expression renders lysosomal pH more alkalotic, which should impair TLR7 signaling.

Furthermore, in immune cells expressing TASL (monocytes and B lymphocytes), lysosomal pH was lower in female cells compared to male cells. In female B lymphocytes or monocytes, CRISPR-cas knock down of TASL expression markedly reduced cytokine secretion in response to TLR7 ligand engagement. The TASL protein has several features suggesting it is a short chain dehydrogenase. Our functional studies indicate that TASL catalyzes conversion of NADP to NADPH. Genetic studies demonstrate synergistic interaction among CXorf21 and Slc15a4 for SLE risk.

Conclusions The CXorf21 gene is differentially expressed in female and male immune cells based on escape from X inactivation and is critical for lysosomal pH and TLR7 signaling. Risk of SLE may be enhanced among individuals with risk alleles at both CXorf21 and Slc15a4. Sex bias in SLE is attributable to an X chromosome dose effect, which may be mediated in part by CXorf21 and its protein product TASL.