The ectodomain of Toll-like receptor 9 is cleaved to generate a functional receptor

Abstract

Mammalian Toll-like receptors (TLRs) 3, 7, 8 and 9 initiate immune responses to infection by recognizing microbial nucleic acids1, 2; however, these responses come at the cost of potential autoimmunity due to inappropriate recognition of self nucleic acid3. The localization of TLR9 and TLR7 to intracellular compartments appears to play a role in facilitating responses to viral nucleic acids while maintaining tolerance to self nucleic acid, yet the cell biology regulating the trafficking and localization of these receptors remains poorly understood4-6. Here, we define the route by which TLR9 and TLR7 exit the endoplasmic reticulum (ER) and traffic to endolysosomes. Surprisingly, the ectodomains of TLR9 and TLR7 are cleaved in the endolysosome, such that no full-length protein is detectable in the compartment where ligand is recognized. Remarkably, though both the full-length and cleaved forms of TLR9 are capable of binding ligand, only the processed form recruits MyD88 upon activation, arguing that this truncated receptor, rather than the full-length form, is functional. Furthermore, conditions that prevent receptor proteolysis, including forced TLR9 surface localization, render the receptor non-functional. We propose that ectodomain cleavage represents a strategy to restrict receptor activation to endolysosomal compartments and prevent TLRs from responding to self nucleic acid.

Ribonuclease H2 mutations induce a cGAS/STING‐dependent innate immune response

“…However, as IL‐1β and IL‐18 were both undetectable in culture supernatants from Rnaseh2b −/− cells (data not shown), an inflammasome‐mediated response was unlikely. While the observed ISG induction would be consistent with TLR9 activation, signalling is impaired by deficient proteolytic processing of the receptor in MEFs (Ewald et al , 2008), and we therefore prioritised assessment of the cGAS‐STING pathway. We performed siRNA depletion of cGAS in Rnaseh2b −/− MEFs and found that cGAS depletion significantly abrogated the ISG response and CCL5 production (Fig 4A).…”

Diverse regulatory roles for lysosomal proteases in the immune response

“…Asagiri et al, reported that chemical or genetic ablation of cathepsin K attenuated inflammatory autoimmune disease apparently via a marked reduction in TLR9-induced signalling leading to the production of . Three recent studies confirmed a requirement for endosomal proteolysis in TLR9 signalling and went further by showing that a key protease target is TLR9 itself [1,2,94]. Apparently, cleavage in the extracellular domain of TLR9 increases binding of CpG containing oligonucleotides and crucially, cleaved TLR9 recruited the signalling adaptor MyD88 much more efficiently than intact TLR9 [1].…”

“…Whether proteolysis of the other nucleic acid-sensing TLR is required is not yet clear. In the studies mentioned above, one demonstrated that TLR7 proteolysis occurred [1], but no cleavage of TLR7 was detected in another study [2].…”

“…Both have emerged as having many important regulatory functions in addition to their role in protein breakdown for amino acid recycling. For example, limited proteolysis within the endocytic pathway can induce activating conformational changes [1,2], release functional proteins from chaperones [3], and cleave soluble bioactive molecules from membrane-anchored precursors [4]. The concept of ''regulation through limited destruction'' is evident in many processes in innate and adaptive immunity.…”

“…Three recent studies confirmed a requirement for endosomal proteolysis in TLR9 signalling and went further by showing that a key protease target is TLR9 itself [1,2,94]. Apparently, cleavage in the extracellular domain of TLR9 increases binding of CpG containing oligonucleotides and crucially, cleaved TLR9 recruited the signalling adaptor MyD88 much more efficiently than intact TLR9 [1]. However, none of these studies confirmed a role for cathepsin K as the sole TLR9 convertase.…”