Toll-like receptor 9 (TLR9) recognizes DNA containing CpG motifs derived from bacteria and viruses and activates the innate immune response to eliminate them. TLR9 is known to bind to CpG DNA, and here, we identified another DNA binding site in TLR9 that binds DNA containing cytosine at the second position from the 5′ end (5′-xCx DNA). 5′-xCx DNAs bound to TLR9 in the presence of CpG DNA and cooperatively promoted dimerization and activation of TLR9. Binding at both sites was important for efficient activation of TLR9. The 5′-xCx DNA bound the site corresponding to the nucleoside binding site in TLR7 and TLR8 as revealed by the structural analysis. This study revealed that TLR9 recognizes two types of DNA through its two binding sites for efficient activation. This information may contribute to the development of drugs that control the activity of TLR9.

Toll样受体9（TLR9）识别包含源自细菌和病毒的CpG模体的DNA，并激活先天免疫应答以消除它们。已知TLR9与CpG DNA结合，在这里，我们在TLR9中鉴定了另一个DNA结合位点，该结合位点在5'端第二个位置（5'-xCx DNA）结合含胞嘧啶的DNA。5'-xCx DNA在CpG DNA存在下与TLR9结合，并共同促进TLR9的二聚化和激活。两个位点的结合对于TLR9的有效激活很重要。结构分析表明，5'-xCx DNA结合了与TLR7和TLR8中核苷结合位点相对应的位点。这项研究表明，TLR9通过其两个结合位点识别两种类型的DNA以进行有效激活。