Abstract
Nucleic acid structures are highly conserved through evolution and when self nucleic acids are aberrantly detected by toll-like receptors (TLRs) they contribute to autoimmune disease. For this reason, multiple regulatory mechanisms exist to prevent immune responses to self nucleic acids. TLR9 is a nucleic acid-sensing TLR that is regulated at multiple levels including association with accessory proteins, intracellular localization and proteolytic processing. In the endolysosomal compartment TLR9 is proteolytically processed to an 80 kDa form (p80) and this processing is a prerequisite for activation. Here, we identified a soluble form of TLR9 (sTLR9) generated by a novel proteolytic event that cleaved TLR9 between amino acids 724-735. Similar to p80, sTLR9 was generated in endosomes. However, generation of sTLR9 was independent of the cysteine protease cathepsin B, active at acidic pH, but partially dependent on cathepsin S, a protease active at neutral pH. Most importantly, sTLR9 inhibited TLR9-dependent signaling. Altogether, these data support a model where an intrinsic proteolytic processing mechanism negatively regulates TLR9 signaling. A proper balance between the independent proteolytic events probabably contributes to regulation of TLR9-mediated innate immunity and prevention of autoimmune disease.