Toll-like receptor 7 (TLR7), Toll-like receptor 8 (TLR8), and Toll-like receptor 9 (TLR9) (TLR7/8/9)

Toll-like receptor 7 (TLR7), Toll-like receptor 8 (TLR8), and Toll-like receptor 9 (TLR9) are critical components of the innate immune system located within the endosomal compartments of cells, particularly plasmacytoid dendritic cells and B cells. Their primary biological function is to sense foreign nucleic acids, such as viral RNA or bacterial DNA, to trigger an immune response (Schrezenmeier & Dörner, 2020). In autoimmune conditions like systemic lupus erythematosus (SLE), these receptors mistakenly recognize self-nucleic acids, driving the chronic production of interferons and other inflammatory mediators that lead to tissue damage (Kuznik et al., 2011). Hydroxychloroquine acts as a therapeutic modulator by accumulating in the endosomes and increasing the local pH, which effectively "blunts" the activation of these receptors. By preventing the proteolytic processing and ligand binding of TLR7, TLR8, and TLR9, hydroxychloroquine reduces the autoimmune flare-ups associated with SLE and rheumatoid arthritis (StatPearls, 2023). This mechanism of action also contributes to its antimalarial properties by interfering with the parasite's ability to digest hemoglobin in its acidic food vacuole.

Other names

TLR7TLR8TLR9Endosomal Toll-like receptorsCD287CD288CD289

Hydroxychloroquine is a weak base that partitions into acidic intracellular compartments, such as endosomes and lysosomes, leading to an increase in their internal pH (Schrezenmeier & Dörner, 2020). This pH elevation inhibits the activity of acid-dependent proteases, such as cathepsins, which are required to cleave and activate Toll-like receptors 7, 8, and 9 (Kuznik et al., 2011). Furthermore, the altered pH environment disrupts the binding affinity between these receptors and their respective nucleic acid ligands, such as single-stranded RNA and CpG DNA, thereby preventing the downstream activation of MyD88-dependent signaling pathways and the subsequent production of pro-inflammatory cytokines like Type I interferons (StatPearls, 2023).