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

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) expres sion in human immune cells correlates with responsiveness to bacterial deox y-cytidylate-phosphate-deoxyguanylate (CpG)-DNA. Notably "gain of function" to immunostimulatory CpG-DNA is achieved by expressing TLR9 in human nonre sponder cells. Transfection of either human or murine TLR9 conferred respon siveness in a CD14- and MD2-independent manner, yet required species-specif ic 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 se quence was GACGTT. Overall, these data suggest that hTLR9 conveys CpG-DNA r esponsiveness to human cells by directly engaging immunostimulating CpG-DNA .