Dual Guardian: Decoding the Key Role of Toll-like Receptors in Immunity and Disease

Dual Guardian: Decoding the Key Role of Toll-like Receptors in Immunity and Disease

1. Introduction of the Toll-like receptor family

Toll-like receptors (TLRs) are a group of pattern recognition receptors that play a significant role in the development and maintenance of the immune system. These receptors can recognize foreign pathogen-associated molecular patterns (PAMPs) as well as endogenous by-products of cell damage, namely damage-associated molecular patterns (DAMPs). Signal transduction through TLR leads to the production of pro-inflammatory cytokines and other inflammatory response mediators. Therefore, TLR and its signaling pathway effectors are crucial for the functions of both the innate immune system and the adaptive immune system.

1.1 Structure of the TLR Family

TLRs are type I transmembrane proteins, containing ligand-recognizing leucine-rich repeat (LRR) domains, transmembrane domains, and Toll/ interleukin-1 receptor (TIR) homologous domains. The LRR domain is responsible for ligand recognition, while the cytoplasmic TIR domain initiates downstream signaling cascade reactions by interacting with its adaptor proteins. To date, ten TLRs (TLR1-10) have been identified in humans. Most TLRs function as homodimers after activation, except for TLR2, which functions as heterodimers with TLR1 or TLR6.

Fig.1 Schematic diagram of TLRs structure

(The figure is sourced from Protein Sci^[1])

1.2 Distribution of TLR Family

The 10 TLRs in humans are functionally divided into two subgroups. A group consists of TLR1, TLR2, TLR4, TLR5, TLR6 and TLR10, which are located on the cell membrane and mainly recognize bacterial components such as lipids, lipoproteins and proteins. Another group consists of TLR3, TLR7, TLR8 and TLR9, which are located in endosomes/lysosomes and mainly recognize microbial nucleic acids. TLRs are expressed in a variety of hematopoietic cells and non-hematopoietic cells, including effector immune cells (such as dendritic cells, macrophages, lymphocytes, granulocytes), hematopoietic stem cells and progenitor cells, as well as non-immune cells.

1.3 Signal Transduction pathways of TLR

The signal transduction of TLR involves the recruitment of intracellular adaptor proteins, ultimately leading to the activation of transcription factors and the production of pro-inflammatory cytokines. Except for TLR3, most activated TLRS recruit members of MyD88 and the IRAK family (including IRAK1, IRAK2 and IRAK4). These proteins work together to form "Myddosome". TRAF6 was subsequently recruited into the complex, stimulating TAK1, which then activated the NF-κB and MAPK pathways and produced pro-inflammatory cytokines. TLR3 recruits TRIF instead of MyD88, and TLR4 conducts signals through two pathways: MyD88 dependence and TRIF dependence. TRIF binds to TRAF3 and then recruits TBK1 and IKKε, thereby activating IRF3 and stimulating the production of type I interferons. In addition, TRIF interacts with TRAF6 to promote the activation of NF-κB and MAPK.

Fig.2 TLRs transmit signals through MyD88-dependent or TrIF-dependent pathways

(The figure is sourced from Cold Spring Harb Perspect Biol^[2])

2. Research on TLR signal transduction and cancer

The exogenous activation of the immune system through TLRs may, on the one hand, be an anti-cancer strategy, but on the other hand, it can exacerbate potential chronic inflammation, which in turn will further facilitate the progression of cancer. In K-ras mutant mice in the context of chronic obstructive pulmonary disease (COPD), knockout of TLR2, TLR4 or TLR9 can reduce tumor burden, decrease angiogenesis and tumor cell proliferation, accompanied by increased tumor cell apoptosis and reprogramming of the tumor microenvironment into an anti-tumor environment^[3]. TLR9 is highly expressed in prostate cancer (PCa) and is associated with regional lymph node involvement and the invasiveness of PCa^[4]. TLR4 is associated with PCa cell lines, the proliferation and invasion ability of hepatocellular carcinoma (HCC), and low survival rate, and is a feasible therapeutic target for non-small cell lung cancer (NSCLC) metastasis enhanced by Gram-negative pneumonia^[5-7]. Doxorubicin-induced HMGB1 release activates TLR2 signaling in breast cancer cells, thereby leading to a chemotherapy-resistant phenotype^[8]. In contrast, TLR2 inhibits the early progression of lung cancer by activating the intracellular cell cycle arrest pathway and pro-inflammatory senescent secretory phenotypes^[9]. TLR-mediated PI3K activation regulates the invasion and metastasis of ovarian cancer by generating galectin-1^[10]. Inhibiting TLR signaling can improve the production of pro-inflammatory cytokines and reduce the occurrence of diethylnitrosamine-induced liver cancer^[11]. These studies highlight the potential value of TLR signaling in cancer treatment.

Fig.3 Schematic representation of the mechanism of TLR2-dependent chemoresistance

(The figure is sourced from Oncoimmunology^[8])

3. Research on TLR signal transduction and cardiovascular disease

The activation of TLR plays a significant role in the development, progression and outcome of cardiovascular diseases. TLR1, TLR2, TLR5, TLR6 and TLR7 promote the progression of atherosclerosis by enhancing macrophage accumulation and T-cell reactivity, regulating lesions and systemic inflammation^[12-15]. TLR2 signaling is crucial for protecting the heart from adverse remodeling and systolic dysfunction associated with aging in mice^[16]. ANG II-induced hypertension and cardiac hypertrophy are associated with differential activation of TLR4 and TLR3^[17,18]. TLR9 is a key mediator of perivascular adipose tissue (PVAT) dysfunction in hypertension, leading to inflammation, oxidative stress and vascular damage^[19]. Research shows that TLR3 is a key element in the conservative pathway of aortic valve calcification^[20]. The novel SNP interaction between TLR4 and MyD88 is associated with an increased risk of coronary artery disease^[21]. TLR6 promotes the progression of myocardial fibrosis through oxidative stress and inflammatory responses^[22]. These studies suggest that a better understanding of TLR signaling in cardiovascular diseases may contribute to the development of new therapies targeting TLR.

Fig.4 Schematic of proposed model of differential effects of ANG II on hypertension and cardiac hypertrophy.

(The figure is sourced from Am J Physiol Heart Circ Physiol^[17])

4. Research on TLR signal transduction and lung diseases

Due to the exposure of the lungs to multiple infectious sources, antigens and host origin danger signals, the stromal cells and myeloid cells in the lungs express TLR aggregates, which can sense DAMPs and PAMPs and trigger TLR-related signal transduction involved in host defense. Variations in the TLR1 and TLR10 genes increase the risk of asthma after bronchiolitis^[23], and TLR7 and TLR8 may confer susceptibility to asthma^[24]. SNPS in TLR2 and TLR4 are associated with the severity and disease progression of chronic obstructive pulmonary disease (COPD)^[25], and TLR7 mediates emphysema and COPD through mast cell activity^[26]. Inhibiting the NF-κB and MAPK pathways mediated by TLR2, TLR6, TLR3, and TLR4 alleviates transfusion-related lung injury and silica-induced pulmonary fibrosis mediated by Mycoplasma pneumoniae, LPS, and antibodies^[27-32]. TLR-4 expression induces the continuous activation of inflammation and the pathological metastasis of macrophages, which is one of the mechanisms of death caused by COVID-19^[33]. The inhibition of TLR5 eliminated the destructive inflammatory response produced by cystic fibrotic airway cells after exposure to Pseudomonas aeruginosa^[34]. The injured cells after lung contusion activate the acute inflammatory response through TLR-9^[35]. Therefore, inhibiting TLR may be a new treatment strategy for lung injury.

5. Research on TLR signal transduction and inflammatory bowel disease

TLR, as a sensor of the intestinal microbiota, plays a crucial role in maintaining intestinal homeostasis, controlling immune responses and shaping the microbiota. Non-synonymous variations in the TLR1, TLR2, TLR5 and TLR6 genes are associated with Crohn's disease (CD) and ulcerative colitis (UC)^[36,37]. ERS and TLR2 are upregulated in inflammatory bowel disease (IBD), and ERS may promote the inflammatory response mediated by the TLR2 pathway^[38]. Inhibiting the activation of the TLR2/NF-κB signaling pathway can effectively prevent IBD induced by dexglucan sulfate sodium (DSS)^[39]. TLR3 mediates the expression of CCL20 and CXCL10 in colonic epithelial cells and participates in active inflammation in IBD^[40,41]. Inhibit the TLR4/NF-κB/HIF-1α axis to enhance the therapeutic outcome of UC, thereby reducing the inflammatory response and improving colonic lesions^[42,43]. Activation of the TLR4/NF-κBs pathway aggravates inflammation and pyroptosis in DSS-induced IBD^[44]. TLR6 is an important driver of Th1 and Th17 responses in intestinal-associated lymphoid tissues^[45]. Contrary to the above studies, TLR3 and TLR7 recognize resident virus-mediated interferon β production to improve intestinal inflammation^[46], and the activation of the TLR9 signaling pathway improves the clinical symptoms of patients with active UC^[47]. These findings provide assistance for targeted TLR signal transduction therapy for IBD.

Fig.5 The molecular mechanism by which the TLR4/NF-κB/HIF-1α pathway promotes inflammation in UC

(The figure is sourced from Drug Des Devel Ther^[42])

6. Research on TLR signal transduction and neurodegenerative diseases

The activation of TLR can induce immune and inflammatory responses to central nervous system injury or infection. Targeted inhibition of neuroinflammation mediated by the TLR2 and TLR4 pathways improves motor and cognitive impairments in Parkinson's disease^[48,49]. Inhibiting the TLR4/NF-κB signaling pathway can improve Aβ -induced memory dysfunction by suppressing neuroinflammation and apoptosis^[50]. Enhanced TLR4 signaling in glial cells promotes disease progression in mice with amyotrophic lateral sclerosis (ALS)^[51]. microRNA and ssRNA can act as signaling molecules, activating TLR7 signaling in the central nervous system and promoting neurodegeneration and neuroinflammation^[52,53]. The activation of TLR9 signaling can exacerbate neurodegeneration by inducing oxidative stress and inflammation^[54]. On the contrary, other studies have shown that TLR2, as an endogenous receptor for bone marrow-derived immune cells, helps clear toxic Aβ. In the absence of TLR2, cognitive decline will accelerate significantly^[55]. Therefore, TLR can serve as a potential drug pathological target for the development of neuroprotective drugs.

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