An equilibrium binding model for CpG DNA-dependent dimerization of Toll-like receptor 9

Microbial nucleic acids in the extracellular milieu are recognized in vertebrates by Toll-like receptors (TLRs), one of the most important families of innate immune receptors. TLR9 recognizes single-stranded unmethylated CpG DNA in endosomes. DNA binding induces dimerization of TLR9 and activation of a potent inflammatory response. To provide insights on how DNA ligands induce TLR9 dimerization, we developed a detailed theoretical equilibrium ligand binding model. Light scattering and fluorescence polarization assays performed with a recombinant TLR9 ectodomain fragment and a panel of agonistic and antagonistic DNA ligands provide data that restrain the binding parameters in our binding model. This work brings us one step closer to establishing a rigorous biochemical understanding of how TLRs are activated by their ligands.