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Proteolytic cleavage in an endolysosomal compartment is required for activation of Toll-like receptor 9

Boyoun Park1, Melanie M Brinkmann, Eric Spooner

  • 1Whitehead Institute for Biomedical Research, Massachusetts Institute of Technology, Cambridge, Massachusetts 02115, USA.

Nature Immunology
|October 22, 2008
PubMed

Insights

Toll-like receptor 9 (TLR9) requires proteolytic cleavage to activate the innate immune system. This process, involving lysosomal proteases, generates a functional fragment essential for responding to pathogens like CpG DNA.

Area of Science:

  • Immunology
  • Molecular Biology
  • Cell Biology

Background:

  • Toll-like receptors (TLRs) are crucial components of the innate immune system.
  • TLR9 recognizes specific microbial DNA motifs, such as CpG sequences, initiating immune responses.
  • The precise mechanisms regulating TLR9 activation remain incompletely understood.

Purpose of the Study:

  • To investigate the role of proteolytic cleavage in Toll-like receptor 9 (TLR9) signaling.
  • To identify the specific proteases involved in TLR9 processing and activation.
  • To elucidate how TLR9 cleavage contributes to innate immunity.

Main Methods:

  • Inhibition of lysosomal proteolysis in cellular systems.
  • Analysis of TLR9 cleavage products using biochemical techniques.
  • Functional assays in Tlr9-deficient dendritic cells to assess restored signaling.

Main Results:

  • Proteolytic cleavage of TLR9 was found to be essential for its signaling function.
  • Inhibition of lysosomal proteolysis abolished TLR9 activity.
  • A carboxy-terminal fragment of TLR9, containing ectodomain, transmembrane, and cytoplasmic regions, was generated.
  • This fragment successfully bound CpG DNA and restored cytokine production in Tlr9-deficient cells.
  • While cathepsin L was involved in vitro, multiple proteases acted on TLR9 in intact cells.

Conclusions:

  • Lysosomal proteolysis is a prerequisite for TLR9 activation and subsequent innate immune responses.
  • Specific cleavage of TLR9 by proteases facilitates its function in pathogen recognition.
  • This mechanism highlights the importance of protease activity in regulating immune signaling pathways.

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