Toll-like receptor 9 (TLR9) often known as CD289 (cluster of differentiation 289), is a member of the Toll-like receptor family that recognizes pathogen-associated molecular pattern. TLR9 was first cloned and identified as a receptor for unmethylated CpG-DNA as well as for bacterial DNA. It is essential not only for pro-inflammatory cytokine production and other inflammatory responses, but it also plays a role in the induction of T helper 1 (Th1) acquired immune response and in the proliferation of B cells. Like all other members of the TLR family, TLR9 is composed of an extracellular domain containing multiple leucine-rich repeats (LRRs), a transmembrane region, and a cytoplasmic tail containing the conserved TIR domain. The TLR9 sequence encodes a 1032 aa protein containing 27 N-terminal LRRs with a calculated molecular weight of 116 kDa . The gene for TLR9 has been mapped to human chromosome 3p21.3. TLR9 is most closely related to TLR7 and TLR8 with 36% and 35% overall amino acid sequence identity, respectively and thus along with TLR7 and TLR8 constitutes a new sub-family of the TLRs.

In vivo, TLR9 mRNA is expressed in spleen, lymph node, bone marrow, and PBLs. (1) Specifically, TLR9 mRNA is expressed at the highest levels in B cells and dendritic cells (DC). In vitro, TLR9 is moderately upregulated by autocrine IFN-γ, IL-1ß, IL-6, IL-10, and TNF-α in PMA-differentiated THP-1 cells. TLR9 mRNA expression in THP-1 cells is unaffected by exposure to both Gram-positive and Gram-negative bacteria. Ex vivo, TLR9 expression in monocytes and particularly in granulocytes is downregulated in response to Gram-negative bacteria. (2, 3) TLR9 also appears to be localized internally, perhaps in lysosomic or endocytic compartments where it would more likely encounter PAMPs including unmethylated DNA.

TLR9 is expressed primarily on antigen presenting cells such as B cells and DC. In human DC, TLR9 is restricted to a subset of DC, plasmacytoid DC, responsible for production of high levels of type I IFN (IFNalpha). TLR9 recognizes synthetic CpG oligonucleotides and unmethylated CpG motifs in bacterial and viral DNA. Phagocytes endocytose microorganisms and lyse them in phagolysosomes, where their DNA is released and presumably interacts with TLR9, initiating an inflammatory response resulting in rapid secretion of a large quantity of IFNα and the production of inflammatory cytokines.(4) Two signaling pathways of TLR9 are thought to induce inflammatory cytokine expression: the MyD88-TRAF6-IRF5 pathway and the MyD88-TRAF6-TAK1-MAPK/IKK-AP-1/NF-κB pathway. Further TLR9 induce IFNα expression by activating IRF7 via TNF receptor-associated factor 3. The cytosolic TIR domain of TLR9 recruits the adaptor molecule MyD88 and other signaling molecules such as IRAK-4, and TRAF6 that are required for the signaling complex. The transcription factors such as IRF-1, IRF-5, and IRF-7 are also recruited to the complex and activated. The complex in turn activates other signaling cascades that lead to the activation of NF-κB and AP-1. These activated transcription factors induce diverse immunity-related genes (5). 

TLR9 signaling is recently implicated in the pathogenesis of autoimmunity, especially in systemic lupus erythematosus (SLE). TLR9 can suppress the pathology of autoimmunity in certain cases, although it may also act as a trigger and a center for a feedback loop of autoimmunity.

Reference: 1. Chuang, T.H. & R.J. Ulevitch (2000) Eur. Cytokine Netw. 11:372.

2. Zarember, K.A. & P.J. Godowski (2002) J. Immunol. 168:554.

3. Hornung, V. et al. (2002) J. Immunol. 168:4531.

4.  Cynthia A. Leifer The Journal of Immunology, 2004, 173: 1179-1183.

5. Yutaro Kumagai doi:10.1016/j.addr.2007.12.004  Article in press 2008.