Murine Toll-like receptor 8 is a functional sensor of nucleic acid fragments

Abstract

Toll-like receptors 7 and 8 (TLR7 and TLR8) detect nucleic acid degradation products to initiate innate immune responses, although their ligand-sensing functions differ between species. Whereas human TLR8 is a well-established nucleic acid sensor, murine TLR8 (mTLR8) has long been considered unable to detect nucleic acids. Here, we show that mTLR8 is an active receptor that responds to the synthetic agonist TL8-506 and the endogenous cyclic nucleotide 2',3'-cGMP, with signalling potentiated by cooperative binding of nucleic acid fragments at the secondary ligand-binding site. Using Tlr7-deficient mice expressing mTlr8 under control of the Tlr7 promoter, we demonstrate that mTLR8 partially restores TLR7-dependent tonic transcriptional programs in splenocytes, indicating responsiveness to physiological ligands in vivo. Comparison with a chimeric human-mouse TLR8 knock-in model expressing human TLR8 from the endogenous murine Tlr7 locus demonstrated that the mTLR8 ectodomain is intrinsically less sensitive, consistent with sequence divergence in the LRR11-12 region that links agonist recognition to antagonist regulation. Collectively, these findings establish mTLR8 as a functional nucleic acid sensor specialized for combinatorial detection of nucleic acid degradation products, extending the murine endosomal nucleic acid-sensing repertoire beyond TLR3, TLR7, TLR9, and TLR13.