Cleavage and activation of a Toll-like receptor by microbial proteases

Abstract

Toll-like receptors (TLRs) are innate receptors that show high conservation throughout the animal kingdom. Most TLRs can be clustered into phylogenetic groups that respond to similar types of ligands. One exception is avian TLR15. This receptor does not categorize into one of the existing groups of TLRs and its ligand is still unknown. Here we report that TLR15 is a sensor for secreted virulence-associated fungal and bacterial proteases. Activation of TLR15 involves proteolytic cleavage of the receptor ectodomain and stimulation of NF-κB-dependent gene transcription. Receptor activation can be mimicked by the expression of a truncated TLR15 of which the entire ectodomain is removed, suggesting that receptor cleavage alleviates receptor inhibition by the leucine-rich repeat domain. Our results indicate TLR15 as a unique type of innate immune receptor that combines TLR characteristics with an activation mechanism typical for the evolutionary distinct protease-activated receptors.chicken | Pseudomonas | Toll-like receptor 9 | leucin-rich repeat

TLR15 Is Unique to Avian and Reptilian Lineages and Recognizes a Yeast-Derived Agonist

“…Nonetheless, the finding that TLR15 responds to yeast-derived agonists is functionally significant. The TLR15 stimulatory activity in the yeast lysates was heat labile and inhibited by PMSF, supporting the recent findings of de Zoete et al (42) with culture supernatants.…”

“…Because chitin is present in zymosan preparations, it is unlikely that chitin is an agonist for TLR15. Intriguingly, a recent report implicates proteolytic cleavage by bacterial and yeast-secreted proteases as a mechanism of activation for TLR15 (42). Our data support the hypothesis that whole-cell lysates from yeast contain a potent enzymatically active TLR15 agonist because the effect is abrogated by PMSF or heat treatment.…”

“…6B). During the preparation of this article, de Zoete et al (42) proposed that TLR15 was activated by secreted microbial proteases. To investigate the nature of the TLR agonist activity in whole-cell yeast lysates, we examined the effect of heat inactivation or pretreat- ment of the lysate with PMSF.…”

Proteolytic Processing Regulates Toll-like Receptor 3 Stability and Endosomal Localization

“…In addition to the processing between leucine-rich repeat (LRR) 14 and LRR15 to generate the CTF, TLR9 was also found to be cleaved at the C terminus of the ectodomain to generate a soluble fragment that negatively regulates TLR9 function (17). Avian TLR15 also requires proteolytic cleavage the ectodomain to be activated (18).…”

Effects of Virus-associated Molecular Patterns on the Expression of Cathelicidins in the Hen Vagina

“…The single-stranded RNA (ssRNA) of viruses is recognized by TLR7 (Diebold, 2008;Yue et al, 2014). Fungal and bacterial proteases are recognized by the chicken specific TLR15 (de Zoete et al, 2011;Keestra et al, 2013). The oligo-DNA with unmethylated CpG motifs of bacteria and virus is recognized by TLR21, a homologous of mammalian TLR9 (Brownlie et al, 2009;Keestra et al, 2010).…”