TLR8escapes X chromosome inactivation in human monocytes and CD4⁺T cells

AbstractHuman 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,TLR7andTLR8. We previously established thatTLR7evades X chromosome inactivation in female immune cells, and that mononuclear blood cells express more TLR7 protein in women than in men. Using RNA fluorescencein situhybridization, we now show thatTLR8likewise evades X chromosome inactivation in CD14+monocytes and CD4+T lymphocytes, and that cells harboringTLR7orTLR8transcript foci are more frequent in women than in men. In parallel, we foundTLR7andTLR8simultaneous 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, bothTLR7andTLR8escape 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 theTLR7andTLR8genes 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.HighlightsTLR8, likeTLR7, escapes X chromosome inactivation in immune cells from women and 47,XXY Klinefelter syndrome (KS) men.The frequency of cells double-positive forTLR7andTLR8primary transcripts is 7-fold higher in women than in men.TLR7andTLR8form a co-regulated gene cluster on the human X chromosome, with sex-specific, divergent transcriptional patterns observable in monocytes and CD4+T lymphocytes.Co-dependent transcription of theTLR7andTLR8genes on the active X was observed in women and KS men, contrasting with mutually exclusive transcription in euploid men.Blood mononuclear cells, including monocyte subsets, expressed higher levels of TLR8 protein in females than in males.