Toll-Like Receptor (TLR)

Toll-like receptors are the best-characterised pattern recognition receptors, with ten functional members in humans. They divide by location. Surface receptors, including TLR2, TLR4 and TLR5, detect microbial surface structures such as lipoproteins, lipopolysaccharide and flagellin. Endosomal receptors, TLR3, TLR7, TLR8 and TLR9, detect nucleic acids, safely treated as foreign only in that compartment. Signalling runs through MyD88, giving nuclear factor kappa B and inflammatory cytokines, or TRIF, giving type I interferon.

TLR4 dominates practical work. With MD-2 and CD14 it detects the lipid A portion of lipopolysaccharide at picogram concentrations, a sensitivity that sets the endotoxin limits applied to injectables. Agonism is also deliberate: monophosphoryl lipid A, a detoxified TLR4 agonist, is a component of the adjuvant systems in licensed HPV and zoster vaccines, a CpG TLR9 agonist is the adjuvant in a licensed hepatitis B vaccine, and imiquimod, a TLR7 agonist, is approved topically.

For anyone reading peptide literature this family is the main source of artefact. A synthetic peptide carrying trace endotoxin will activate TLR4 on macrophages and produce cytokine release and nitric oxide that look like an intrinsic immunomodulatory effect. Chromatographic purity does not exclude it, since endotoxin is not a peptide impurity and is invisible to peptide purity methods.

The controls that settle it are standard and frequently missing: measure endotoxin in the actual test article, repeat the experiment with polymyxin B to sequester lipid A, and confirm the response is absent in cells lacking TLR4. A paper reporting that a peptide activates innate immunity without any of these has not excluded the simplest explanation, and the effect often disappears when they are applied.