Toll-like receptor 4 (TLR4)

toll-like receptor 4 (TLR4) is a pattern recognition receptor belonging to the toll-like receptor (TLR) family, encoded by the TLR4 gene. It is a transmembrane glycoprotein constitutively expressed on the surface of innate immune cells — including monocytes, macrophages, dendritic cells, neutrophils, and microglia — as well as on a variety of non-immune cell types such as hepatic stellate cells and neurons. TLR4 is best characterized as the primary sensor for lipopolysaccharide (LPS), a structural component of gram-negative bacterial cell walls, but it also recognizes endogenous danger signals (damage-associated molecular patterns, or DAMPs) such as high-mobility group box 1 protein (high mobility group box 1 (HMGB1)) and lumican. Upon ligand binding, TLR4 recruits the adapter proteins TIRAP/MAL and TRAM, which facilitate recruitment of myeloid differentiation primary response 88 (MyD88) and TRIF, initiating downstream signaling cascades that culminate in the activation of nuclear factor kappa B (NF-κB) and the production of proinflammatory cytokines including interleukin-1 beta (IL-1β) and Interleukin-6 (IL-6).

Beyond its canonical role in host defense against bacterial infection, TLR4 is increasingly recognized as a central mediator of sterile inflammation across a broad spectrum of pathological conditions. Its activation drives tissue-damaging neuroinflammation in neurodegenerative and neuropathic pain states, promotes fibrosis, facilitates tumor progression and metastasis, and modulates immunophenotypic changes in circulating leukocytes exposed to environmental or pharmacological stressors. This dual role — protective in acute infection yet injurious in chronic or dysregulated contexts — makes TLR4 a high-priority research target for therapeutic intervention in inflammation-driven diseases, ranging from metabolic dysfunction–associated steatotic liver disease (MAFLD/NAFLD) to Parkinson's disease and cancer.