Human TLR9 confers responsiveness to bacterial DNA via species-specific CpG motif recognition

TLDR

The Toll‑like receptor family are conserved transmembrane proteins that mediate innate immunity by recognizing pathogen‑derived ligands and activating cells through the Toll/IL‑1R signaling pathway. Human TLR9 expression confers responsiveness to bacterial CpG‑DNA, and gain of function is achieved by transfecting TLR9 into nonresponder cells, with transfection of human or murine TLR9 activating the Toll/IL‑1R pathway via MyD88 in a CD14‑ and MD2‑independent manner but requiring species‑specific CpG motifs (GTCGTT for human, GACGTT for mouse), demonstrating that hTLR9 directly engages immunostimulatory CpG‑DNA to trigger innate immune activation.

Abstract

The Toll-like receptor (TLR) family consists of phylogenetically conserved transmembrane proteins, which function as mediators of innate immunity for recognition of pathogen-derived ligands and subsequent cell activation via the Toll/IL-1R signal pathway. Here, we show that human TLR9 (hTLR9) expression in human immune cells correlates with responsiveness to bacterial deoxycytidylate-phosphate-deoxyguanylate (CpG)-DNA. Notably “gain of function” to immunostimulatory CpG-DNA is achieved by expressing TLR9 in human nonresponder cells. Transfection of either human or murine TLR9 conferred responsiveness in a CD14- and MD2-independent manner, yet required species-specific CpG-DNA motifs for initiation of the Toll/IL-1R signal pathway via MyD88. The optimal CpG motif for hTLR9 was GTCGTT, whereas the optimal murine sequence was GACGTT. Overall, these data suggest that hTLR9 conveys CpG-DNA responsiveness to human cells by directly engaging immunostimulating CpG-DNA.