Regulation of Legionella phagosome maturation and infection through flagellin and host Ipaf

Area of Science:

• Immunology
• Microbiology
• Cell Biology

Background:

• Legionella pneumophila causes Legionnaires' disease by replicating within human macrophages.
• The Legionella-containing phagosome (LCP) normally avoids lysosomal fusion, but this is restricted in most mouse macrophages.
• The role of NLR protein Ipaf in restricting intracellular Legionella replication is unclear.

Purpose of the Study:

• To elucidate the mechanism by which Ipaf restricts Legionella pneumophila replication in macrophages.
• To investigate the roles of bacterial flagellin and host Ipaf in activating caspase-1 during Legionella infection.

Main Methods:

• Utilized mouse macrophages lacking Ipaf or caspase-1 to assess Legionella replication.
• Investigated the requirement of flagellin and type IV secretion system for caspase-1 activation.
• Examined phagosome maturation and acquisition of endoplasmic reticulum vesicles.
• Conducted in vivo studies to confirm findings.

Main Results:

• Legionella flagellin and a functional type IV secretion system are essential for activating caspase-1 in macrophages.
• Ipaf and caspase-1 activation are required to prevent the LCP from acquiring ER-derived vesicles and to promote lysosomal fusion.
• A flagellin-deficient Legionella mutant replicated in wild-type macrophages, indicating flagellin's role in caspase-1 activation and bacterial control.
• Ipaf-mediated regulation of phagosome maturation occurred early post-infection and was independent of cell death.

Conclusions:

• Ipaf restricts Legionella replication by regulating phagosome maturation, a novel function for NLR proteins in host defense.
• Flagellin and Ipaf are critical for controlling Legionella clearance in vivo.
• This study reveals a new mechanism of innate immunity against intracellular bacterial pathogens.