TLR9 - toll like receptor 9 |Elisa - Clia - Antibody - 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.

Protein Structure

The structure of TLR9 is typical of the TLR family, featuring distinct domains that facilitate ligand recognition, membrane anchoring, and signal transduction:

Extracellular Domain:

• The extracellular domain of TLR9 is composed of leucine-rich repeats (LRRs) arranged in a horseshoe-like solenoid structure, a hallmark of TLRs, that mediates ligand recognition.
• This LRR region binds CpG DNA through two symmetrical binding sites on either side of the horseshoe, forming a TLR9 homodimer upon activation. The LRRs recognize CpG-rich DNA and facilitate receptor dimerization, which is essential for signal initiation.
• TLR9 is synthesized as an inactive precursor and undergoes proteolytic cleavage in the endosome, producing a functional receptor that can bind CpG motifs. This cleavage exposes CpG recognition sites, allowing TLR9 to activate efficiently in the presence of pathogenic DNA.

Transmembrane Domain:

• TLR9 has a single alpha-helical transmembrane domain that anchors it to the endosomal membrane. This anchorage is crucial for positioning TLR9 within endosomes, where it encounters nucleic acids from internalized pathogens.
• The transmembrane domain stabilizes the receptor within the membrane environment, ensuring proper spatial orientation for ligand binding and intracellular signaling.

Cytoplasmic Toll/Interleukin-1 Receptor (TIR) Domain:

• The cytoplasmic TIR domain is conserved among TLRs and serves as the signal-transducing domain of TLR9. This domain is responsible for recruiting adaptor molecules upon activation, particularly MyD88 (myeloid differentiation primary response gene 88).
• After CpG DNA binding and receptor dimerization, TLR9’s TIR domain associates with MyD88, initiating a downstream signaling cascade that activates NF-κB and IRF7 (interferon regulatory factor 7). These transcription factors then promote the expression of pro-inflammatory cytokines, such as TNF-α and IL-6, as well as type I interferons, which play critical roles in antiviral defense.

Classification and Subtypes

TLR9 is part of the TLR7/8/9 subfamily, which includes receptors that recognize nucleic acid-based PAMPs and are localized to endosomal compartments. Within this subfamily:

• TLR7 and TLR8 detect viral and bacterial single-stranded RNA (ssRNA).
• TLR9 is specifically tuned to recognize DNA with unmethylated CpG motifs.

TLR9 does not have distinct isoforms; however, its functional activity can vary based on cell type, post-translational modifications, and proteolytic processing. This variation allows TLR9 to respond to specific types of CpG DNA motifs and pathogens, with different subclasses of CpG motifs (e.g., CpG-A, CpG-B, CpG-C) eliciting distinct immune responses.

Function and Biological Significance

TLR9 has essential roles in pathogen recognition and the initiation of innate immune responses, as well as in shaping adaptive immunity:

Pathogen Recognition and Immune Activation:

• TLR9’s primary role is to recognize CpG DNA motifs present in the genomes of bacteria and viruses, making it a key sensor of infections. Upon binding to CpG DNA, TLR9 dimerizes and initiates a signaling cascade through MyD88, leading to NF-κB activation and production of inflammatory cytokines. These cytokines are crucial for the immediate immune response against pathogens.
• The activation of TLR9 results in the release of type I interferons (IFN-α and IFN-β), which have potent antiviral effects. This release is especially significant in plasmacytoid dendritic cells, which produce large amounts of IFN-α in response to viral infections.

Promotion of Adaptive Immunity:

• TLR9 activation in dendritic cells and B cells leads to antigen presentation and B-cell proliferation, bridging innate and adaptive immunity. TLR9 signaling in B cells enhances their ability to produce antibodies, a vital adaptive response that provides lasting immunity against pathogens.
• TLR9 also promotes the maturation of dendritic cells, upregulating co-stimulatory molecules necessary for T-cell activation. This activation helps guide T-helper cell responses and contributes to a more robust adaptive immune response.

Recognition of Self-DNA and Autoimmunity:

• TLR9 is also involved in detecting self-DNA under certain conditions, such as when host DNA complexes with endogenous nucleic acid-binding proteins or is released from apoptotic cells. Inappropriate recognition of self-DNA by TLR9 can lead to autoimmune responses and chronic inflammation, contributing to diseases such as systemic lupus erythematosus (SLE).
• The endosomal localization of TLR9 usually helps to limit its exposure to self-DNA; however, in certain pathological conditions, TLR9 can become overactive, driving unwanted inflammation.

Clinical Issues

Dysregulation of TLR9 signaling is implicated in various infectious diseases, autoimmune disorders, and cancers. Its roles in these conditions are associated with either excessive or insufficient immune responses:

Autoimmune Diseases:

• Overactivation of TLR9 is implicated in systemic lupus erythematosus (SLE), where self-DNA, complexed with other molecules, activates TLR9 and leads to chronic inflammation. This excessive signaling results in the production of inflammatory cytokines and type I interferons that contribute to tissue damage.
• TLR9 activity is also linked to rheumatoid arthritis (RA) and other autoimmune diseases, where inappropriate immune responses to self-DNA promote inflammation and joint damage.

Infectious Diseases:

• TLR9 plays a vital role in viral infections by recognizing viral DNA and inducing antiviral immune responses. However, some viruses have evolved mechanisms to evade TLR9 detection, which can impair the immune system’s ability to control infection.
• In bacterial infections, TLR9’s response to CpG DNA aids in pathogen clearance, but excessive activation can lead to septic shock, where overproduction of cytokines results in systemic inflammation and potentially fatal outcomes.

Cancer:

• The role of TLR9 in cancer is dual-faceted. In some cases, TLR9 activation may promote anti-tumor immunity by stimulating an inflammatory response and enhancing immune cell recruitment to tumor sites. TLR9 agonists are under investigation for their potential to boost immune responses in cancers.
• Conversely, chronic TLR9 activation can promote a pro-tumor environment in some cancers by increasing inflammation, which can aid in tumor progression and immune evasion. TLR9 activity in tumor-associated macrophages and myeloid-derived suppressor cells, for example, may create an immunosuppressive microenvironment that supports tumor growth.

Therapeutic Targeting:

• TLR9 agonists, such as CpG oligodeoxynucleotides (ODNs), are being developed as adjuvants to enhance immune responses in vaccines and as immunotherapies in cancer. These agonists can stimulate TLR9 in dendritic cells and B cells, boosting immune responses.
• TLR9 antagonists are explored as treatments for autoimmune diseases where excessive TLR9 activation contributes to chronic inflammation. By inhibiting TLR9, these therapies aim to reduce unwanted inflammation and prevent tissue damage in conditions like SLE.

Summary

TLR9 is a toll-like receptor crucial for detecting microbial DNA, especially CpG-rich DNA sequences, within endosomes. Its structure includes an extracellular domain with leucine-rich repeats that bind CpG DNA, a transmembrane domain that anchors it to the endosome, and a cytoplasmic TIR domain that transduces signals through MyD88. This signaling cascade leads to the production of pro-inflammatory cytokines and type I interferons, which are essential for immune defense against pathogens and contribute to the maturation and activation of adaptive immune cells.

While TLR9’s activity is indispensable in antiviral and antibacterial responses, its activation must be tightly regulated to avoid excessive inflammation or autoimmune responses. Dysregulated TLR9 activity is linked to autoimmune diseases like SLE and rheumatoid arthritis, where inappropriate recognition of self-DNA leads to chronic inflammation. In cancer, TLR9 plays a complex role, where its activation can either suppress or promote tumor progression depending on the context. TLR9 agonists and antagonists offer promising therapeutic potential, either enhancing immunity against infections and tumors or inhibiting excessive inflammation in autoimmune diseases. Understanding TLR9’s role and regulation continues to be an active area of research, with significant implications for treating infectious diseases, cancers, and autoimmune disorders.