This recombinant protein represents a stable and soluble fragment (residues 26-431) of human Toll-like receptor 9 (TLR9/CD289), a key pattern recognition receptor in innate and adaptive immunity. This 406-amino acid fragment has been computationally optimized for structural and biochemical studies, providing researchers with a valuable tool for investigating nucleotide-sensing immune pathways.

Toll-like receptor 9 is a crucial component of both innate and adaptive immunity, functioning as a nucleotide-sensing receptor that recognizes unmethylated cytidine-phosphate-guanosine (CpG) dinucleotides. Upon activation, TLR9 initiates immune responses through MYD88 and TRAF6 signaling pathways, leading to NF-kappa-B activation, cytokine secretion, and inflammatory responses. The receptor plays a vital role in B-cell proliferation, activation, survival, and antibody production following CpG stimulation. TLR9 exhibits dynamic subcellular localization, primarily residing in the endoplasmic reticulum membrane as a single-pass type I membrane protein, and relocalizing to endosomes, lysosomes, phagosomes, and Golgi apparatus upon agonist stimulation. This intracellular trafficking, regulated by UNC93B1, is essential for proteolytic cleavage and receptor activation, while also serving to prevent inappropriate responses to self nucleic acids.