Toll-like receptors 2, 4, 5, and 9 (TLR2/4/5/9)

Toll-like receptors (TLRs) 2, 4, 5, and 9 are critical components of the innate immune system that function as pattern recognition receptors (PRRs). They are responsible for detecting structurally conserved molecules from pathogens, known as pathogen-associated molecular patterns (PAMPs). Specifically, TLR2 recognizes lipoproteins and peptidoglycans, TLR4 detects lipopolysaccharide (LPS) from Gram-negative bacteria, TLR5 senses bacterial flagellin, and TLR9 identifies unmethylated CpG DNA motifs common in bacteria and viruses. Upon activation, these receptors initiate intracellular signaling pathways, primarily through the MyD88 and TRIF adapter proteins, leading to the activation of transcription factors like NF-κB and the subsequent production of pro-inflammatory cytokines and interferons. These receptors play dual roles in human health: while they are essential for defending against infections, their dysregulation is linked to severe conditions such as sepsis, chronic inflammatory diseases, and cancer progression. Consequently, they are major therapeutic targets, with agonists being developed as potent vaccine adjuvants and cancer immunotherapies, while antagonists are explored for treating hyper-inflammatory and autoimmune disorders.

Other names

CD282 (TLR2)CD284 (TLR4)CD285 (TLR5)CD289 (TLR9)TIL4 (TLR2)ARMD10 (TLR4)SLEB1 (TLR5)PRR (Pattern Recognition Receptors)

Drugs targeting these receptors act as either agonists or antagonists. Agonists (e.g., MPL, Entolimod, Lefitolimod) mimic pathogen-associated molecular patterns (PAMPs) to stimulate the innate immune system, often used as vaccine adjuvants or in cancer immunotherapy to enhance immune surveillance. Antagonists (e.g., Eritoran, Tomaralimab) block the binding of ligands like LPS or lipoproteins to prevent over-activation of the immune system, aiming to treat conditions like sepsis, chronic inflammation, and ischemia-reperfusion injury.