Human Toll Like Receptor 9 (TLR9) Protein

Background

Toll-like receptor 9 (TLR9) is an integral component of the immune system and a member of the Toll-like receptor (TLR) family, primarily involved in the recognition of pathogen-associated molecular patterns (PAMPs) within microbial DNA. TLR9 is uniquely specialized to detect unmethylated CpG motifs in DNA, which are frequently found in bacterial and viral genomes but rare in vertebrate DNA due to methylation. This distinction allows TLR9 to effectively recognize pathogenic DNA and initiate immune responses against infections. By detecting microbial DNA, TLR9 plays a pivotal role in the innate immune response and contributes to the activation of adaptive immunity. TLR9 is expressed in plasmacytoid dendritic cells (pDCs), B cells, and monocytes/macrophages, with its expression localized to the endosomes of these cells. This intracellular positioning is essential for its function, as it enables the receptor to sample nucleic acids from pathogens engulfed in endosomal compartments. Upon recognizing CpG motifs, TLR9 initiates a signaling cascade that promotes cytokine production and type I interferon release, critical for mounting an immune defense against pathogens and influencing the maturation of dendritic cells and the activation of T and B cells.