Regulation of Legionella phagosome maturation and infection through flagellin and host Ipaf.
Legionella pneumophila is an intracellular bacterium that causes an acute form of pneumonia called Legionnaires' disease. After infection of human macrophages, the Legionella-containing phagosome (LCP) avoids fusion with the lysosome allowing intracellular replication of the bacterium. In macrophages derived from most mouse strains, the LCP is delivered to the lysosome resulting in Legionella degradation and restricted bacterial growth. Mouse macrophages lacking the NLR protein Ipaf or its downstream effector caspase-1 are permissive to intracellular Legionella replication. However, the mechanism by which Ipaf restricts Legionella replication is not well understood. Here we demonstrate that the presence of flagellin and a competent type IV secretion system are critical for Legionella to activate caspase-1 in macrophages. Activation of caspase-1 in response to Legionella infection also required host Ipaf, but not TLR5. In the absence of Ipaf or caspase-1 activation, the LCP acquired endoplasmic reticulum-derived vesicles, avoided fusion with the lysosome, and allowed Legionella replication. Accordingly a Legionella mutant lacking flagellin did not activate caspase-1, avoided degradation, and replicated in wild-type macrophages. The regulation of phagosome maturation by Ipaf occurred within 2 h after infection and was independent of macrophage cell death. In vivo studies confirmed that flagellin and Ipaf play an important role in the control of Legionella clearance. These results reveal that Ipaf restricts Legionella replication through the regulation of phagosome maturation, providing a novel function for NLR proteins in host defense against an intracellular bacterium.
Amer
Amal
A
Department of Pathology and Comprehensive Cancer Center, University of Michigan Medical School, Ann Arbor, Michigan 48109, USA.
Franchi
Luigi
L
Kanneganti
Thirumala-Devi
TD
Body-Malapel
Mathilde
M
Ozören
Nesrin
N
Brady
Graham
G
Meshinchi
Sasha
S
Jagirdar
Rajesh
R
Gewirtz
Andrew
A
Akira
Shizuo
S
Núñez
Gabriel
G
D048868:Adaptor Proteins, Signal Transducing / Q000235:genetics / Q000378:metabolism; D000818:Animals; D051017:Apoptosis Regulatory Proteins / Q000235:genetics / Q000378:metabolism*; D001854:Bone Marrow Cells; D002135:Calcium-Binding Proteins / Q000235:genetics / Q000378:metabolism*; D002352:Carrier Proteins / Q000235:genetics / Q000378:metabolism; D020170:Caspase 1 / Q000235:genetics / Q000378:metabolism; D004721:Endoplasmic Reticulum / Q000378:metabolism; D004789:Enzyme Activation; D005408:Flagellin / Q000378:metabolism*; D005786:Gene Expression Regulation; D016952:Legionella pneumophila / Q000378:metabolism*; D008264:Macrophages / Q000378:metabolism; D051379:Mice; D018345:Mice, Knockout; D000071199:NLR Family, Pyrin Domain-Containing 3 Protein; D053474:Nod1 Signaling Adaptor Protein / Q000235:genetics / Q000378:metabolism; D010588:Phagosomes / Q000378:metabolism*
D048868:Adaptor Proteins, Signal Transducing; D051017:Apoptosis Regulatory Proteins; D002135:Calcium-Binding Proteins; D002352:Carrier Proteins; C512299:Ipaf protein, mouse; D000071199:NLR Family, Pyrin Domain-Containing 3 Protein; C492716:Nlrp3 protein, mouse; D053474:Nod1 Signaling Adaptor Protein; C507873:Nod1 protein, mouse; D005408:Flagellin; D020170:Caspase 1