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Evaluation of Caspase Activation to Assess Innate Immune Cell Death
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Evaluation of Caspase Activation to Assess Innate Immune Cell Death

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Pseudomonas aeruginosa activates caspase 1 through Ipaf

Edward A Miao1, Robert K Ernst, Monica Dors

  • 1Institute for Systems Biology, Seattle, WA 98103, USA.

Proceedings of the National Academy of Sciences of the United States of America
|February 8, 2008
PubMed

Insights

Macrophages detect bacterial flagellin using the NLR Ipaf protein, activating caspase 1 and inflammatory cytokine secretion. This conserved innate immune pathway identifies pathogens employing type III secretion systems.

Area of Science:

  • Immunology
  • Microbiology
  • Cell Biology

Background:

  • The innate immune system utilizes cytosolic Nod-like receptors (NLRs) to detect pathogens.
  • Several NLRs, like Ipaf, trigger caspase 1 activation, leading to IL-1beta and IL-18 secretion.
  • Macrophages activate caspase 1 in response to Salmonella typhimurium via cytosolic flagellin and type III secretion systems (T3SS).

Purpose of the Study:

  • To investigate the role of Pseudomonas aeruginosa in activating caspase 1 and IL-1beta secretion in macrophages.
  • To determine if P. aeruginosa flagellin and T3SS are involved in this activation pathway.
  • To elucidate the contribution of Naip5 in P. aeruginosa-induced immune responses.

Main Methods:

  • Infection of macrophages with wild-type and mutant P. aeruginosa strains (T3SS and flagellin mutants).
  • Assessment of caspase 1 activation and IL-1beta secretion.
  • Delivery of P. aeruginosa flagellin directly into the macrophage cytosol.
  • Comparison of immune responses in Naip5-deficient (A/J) and Naip5-sufficient (C57BL/6, BALB/c) macrophages.

Main Results:

  • Live, virulent P. aeruginosa induced caspase 1-dependent IL-1beta secretion.
  • Mutations in P. aeruginosa T3SS or flagellin abolished IL-1beta secretion.
  • Reconstitution of caspase 1 activation by delivering flagellin to the cytosol confirmed its role.
  • Naip5-deficient macrophages showed slightly reduced, but still present, IL-1beta secretion against various pathogens and flagellin.

Conclusions:

  • Ipaf-dependent recognition of cytosolic flagellin is a conserved mechanism for macrophages to detect pathogens utilizing T3SS.
  • Flagellin delivered via T3SS is a key P. aeruginosa component triggering innate immune activation.
  • Naip5 plays a role, but other pathways also contribute to flagellin sensing in macrophages.

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