Endosomes are specialized platforms for bacterial sensing and NOD2 signalling

Area of Science:

• Immunology
• Cell Biology
• Molecular Biology

Background:

• Host detection of microbial pathogens relies on Toll-like receptors (TLRs) and NOD-like receptors (NLRs).
• NLRs, like NOD2, sense cytosolic microbial products, but mechanisms of ligand escape from endosomes are unclear.
• NOD2 variants are linked to inflammatory bowel disease and other chronic inflammatory conditions.

Purpose of the Study:

• To investigate the mechanism by which NOD2 ligands exit endosomes.
• To identify host factors involved in sensing endosomally derived bacterial components.
• To elucidate the role of dendritic cell endosomes in pathogen sensing.

Main Methods:

• Investigated the expression and function of endo-lysosomal peptide transporters SLC15A3 and SLC15A4.
• Utilized dendritic cells and TLR stimulation models.
• Assessed the requirement of transporters for NOD2 activation by muramyl dipeptide (MDP).

Main Results:

• SLC15A3 and SLC15A4 are preferentially expressed by dendritic cells, particularly after TLR stimulation.
• These transporters mediate the egress of bacterially derived components, including the NOD2 ligand MDP, from endosomes.
• Enhanced transporter expression leads to the formation of endosomal membrane tubules, boosting NOD2-dependent responses.

Conclusions:

• SLC15A3 and SLC15A4 are essential for NOD2 sensing of endosomally derived bacterial components.
• Dendritic cell endosomes act as specialized platforms for both lumenal and cytosolic pathogen sensing.
• This highlights a novel pathway for innate immune recognition and its implications in inflammatory diseases.