IRAP⁺ endosomes restrict TLR9 activation and signaling

Area of Science:

• Immunology
• Cell Biology
• Molecular Biology

Background:

• Intracellular Toll-like receptors (TLRs) are crucial for innate immunity.
• TLR9 activation requires sensing ligands within specific endosomal compartments.
• Dysregulated TLR signaling can lead to autoimmune diseases.

Purpose of the Study:

• To identify cellular compartments involved in early Toll-like receptor 9 (TLR9) activation.
• To elucidate the role of aminopeptidase IRAP in regulating TLR9 trafficking and signaling.
• To understand the mechanism preventing TLR9 hyperactivation in dendritic cells (DCs).

Main Methods:

• Immunofluorescence microscopy to track TLR9 and CpG localization.
• Biochemical assays to analyze protein interactions.
• Flow cytometry to assess TLR9 signaling.
• In vivo studies using mouse models of bacterial infection.

Main Results:

• IRAP-positive (IRAP+) endosomes are key compartments for early TLR9 activation.
• Both TLR9 and its ligand CpG are found within IRAP+ endosomes.
• IRAP deficiency enhances CpG and TLR9 trafficking to lysosomes and subsequent TLR9 signaling.
• IRAP interacts with FHOD4, stabilizing endosomes and slowing TLR9 lysosomal transport.

Conclusions:

• Endosomal retention of TLR9 by IRAP is a critical mechanism for preventing TLR9 hyperactivation in dendritic cells.
• IRAP acts as a regulator of TLR9 signaling by controlling its trafficking dynamics.
• Targeting the IRAP-mediated pathway could offer therapeutic strategies for TLR9-related inflammatory conditions.