[Multilevel maturation of Toll-like receptor 9]

Toll-like receptors (TLRs) are essential elements of the innate immune response. TLRs induce expression of inflammatory cytokines or interferons after recognition of microbial or viral structures called pathogen-associated molecular patterns (PAMPs). Two different groups of TLRs can be distinguished: TLRs residing in the plasma membrane or in the endosomal compartment. TLRs localized in endosomes act as sensors for nucleic acids. TLR9, which recognizes unmethylated CpG, belongs to endosomal TLRs. The proper ligand detection by TLR9 depends on its specific subcellular localization and maturation. TLR9 delivery to the endosomes is mediated by two distinct proteins, UNC93B1 and AP-2, and post-early endosome distribution is determined by AP-3. TLR9 localized in the endosome is cleaved by at least two classes of proteases, AEP and cathepsins, which generate the mature form of receptor. Functional C-terminal form of TLR9 is capable of recognition of CpG and activation of signal pathways. Ligand binding to TLR9 causes conformational changes in the structure of this receptor which facilitates recruitment of MyD88 adaptor protein and activation of two distinct cytokine-inducing pathways: IRF-7- and NF-κB-dependent. The specific structure of the synthetic ligand (CpG-A or CpG-B) determines activation of certain transcription factors. Recognition of multimeric CpG-A results in IRF-7-dependent induction of type I interferon production. Monomeric CpG-B activates NF-κB-dependent induction of proinflammatory cytokines, in particular TNFα and IL-6.