<i>TLR7</i> escapes X chromosome inactivation in immune cells

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 <i>TLR7</i> gene evades silencing by X chromosome inactivation in immune cells from women and Klinefelter syndrome males. Single-cell analyses of <i>TLR7</i> 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 <i>TLR7</i> on both X chromosomes. Biallelic B lymphocytes from women displayed greater <i>TLR7</i> 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. <i>TLR7</i> 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.