TLR8 escapes X chromosome inactivation in human monocytes and CD4+ T cells

Human endosomal Toll-like receptors TLR7 and TLR8 recognize self and non-self RNA ligands, and are important mediators of innate immunity and autoimmune pathogenesis. TLR7 and TLR8 are encoded by the adjacent X-linked genes, TLR7 and TLR8. We previously established that TLR7 evades X chromosome inactivation in female immune cells, and that mononuclear blood cells express more TLR7 protein in women than in men. Using RNA fluorescence in situ hybridization, we now show that TLR8 likewise evades X chromosome inactivation in CD14+ monocytes and CD4+ T lymphocytes, and that cells harboring TLR7 or TLR8 transcript foci are more frequent in women than in men. In parallel, we found TLR7 and TLR8 simultaneous transcription to be disproportionally frequent in female monocytes and T cells, and disproportionally scarce in the male cells, resulting in a 7-fold difference in frequency. These transcriptional biases were again observable when comparing the single X of XY males with the active X of female cells. Among (47,XXY) Klinefelter syndrome males, both TLR7 and TLR8 escape X chromosome inactivation, and co-transcription frequencies on the active X of monocytes were intermediate overall between those for XY males and XX females, and encompassed both male- and female-like individual patterns. These findings indicate that the TLR7 and TLR8 genes form a co-regulated gene cluster, which we have called the X-linked Toll-like receptor locus, with different sex- and sexual karyotype-dependent modes of transcription. Interestingly, TLR8 protein expression was significantly higher in female mononuclear blood cells, including all monocyte subsets, than in the male cells. Thus, co-dependent transcription from the active X chromosome and escape from inactivation could both contribute to higher TLR8 protein abundance in female cells, which may have implications for the response to viruses and bacteria, and the risk of developing inflammatory and autoimmune diseases. HighlightsO_LITLR8, like TLR7, escapes X chromosome inactivation in immune cells from women and 47,XXY Klinefelter syndrome (KS) men. C_LIO_LIThe frequency of cells double-positive for TLR7 and TLR8 primary transcripts is 7-fold higher in women than in men. C_LIO_LITLR7 and TLR8 form a co-regulated gene cluster on the human X chromosome, with sex-specific, divergent transcriptional patterns observable in monocytes and CD4+ T lymphocytes. C_LIO_LICo-dependent transcription of the TLR7 and TLR8 genes on the active X was observed in women and KS men, contrasting with mutually exclusive transcription in euploid men. C_LIO_LIBlood mononuclear cells, including monocyte subsets, expressed higher levels of TLR8 protein in females than in males. C_LI