Research ArticleAUTOIMMUNE DISEASE

TLR7 escapes X chromosome inactivation in immune cells

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Science Immunology  26 Jan 2018:
Vol. 3, Issue 19, eaap8855
DOI: 10.1126/sciimmunol.aap8855

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The X chromosome link to lupus

Nine of 10 individuals who develop systemic lupus erythematosus (SLE) are women. Furthermore, individuals with Klinefelter syndrome (47,XXY) also have increased incidence of SLE, suggesting that X chromosome dosage could be an important risk factor in SLE. Using sensitive quantification methods, Souyris et al. demonstrate that Toll-like receptor 7 (TLR7) that is encoded from the X chromosome escapes X inactivation in B cells and myeloid cells in females and Klinefelter individuals. TLR7 binds single-stranded RNA and activates type I interferon signaling, a pathway that is also activated in SLE patients. On the basis of this, the authors propose that biallelic expression of TLR7 contributes to greater SLE risk in individuals with two X chromosomes.

Abstract

Toll-like receptor 7 (TLR7) is critical to the induction of antiviral immunity, but TLR7 dosage is also a key pathogenic factor in systemic lupus erythematosus (SLE), an autoimmune disease with strong female bias. SLE prevalence is also elevated in individuals with Klinefelter syndrome, who carry one or more supernumerary X chromosomes, suggesting that the X chromosome complement contributes to SLE susceptibility. TLR7 is encoded by an X chromosome locus, and we examined here whether the TLR7 gene evades silencing by X chromosome inactivation in immune cells from women and Klinefelter syndrome males. Single-cell analyses of TLR7 allelic expression demonstrated that substantial fractions of primary B lymphocytes, monocytes, and plasmacytoid dendritic cells not only in women but also in Klinefelter syndrome males express TLR7 on both X chromosomes. Biallelic B lymphocytes from women displayed greater TLR7 transcriptional expression than the monoallelic cells, correlated with higher TLR7 protein expression in female than in male leukocyte populations. Biallelic B cells were preferentially enriched during the TLR7-driven proliferation of CD27+ plasma cells. In addition, biallelic cells showed a greater than twofold increase over monoallelic cells in the propensity to immunoglobulin G class switch during the TLR7-driven, T cell–dependent differentiation of naive B lymphocytes into immunoglobulin-secreting cells. TLR7 escape from X inactivation endows the B cell compartment with added responsiveness to TLR7 ligands. This finding supports the hypothesis that enhanced TLR7 expression owing to biallelism contributes to the higher risk of developing SLE and other autoimmune disorders in women and in men with Klinefelter syndrome.

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