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Published on: August 26, 2016
Bacterial ligands generated in a phagosome are targets of the cytosolic innate immune system
Anat A Herskovits1, Victoria Auerbuch, Daniel A Portnoy
1Department of Molecular and Cell Biology, University of California, Berkeley, California, United States of America.
Abstract:
Macrophages are permissive hosts to intracellular pathogens, but upon activation become microbiocidal effectors of innate and cell-mediated immunity. How the fate of internalized microorganisms is monitored by macrophages, and how that information is integrated to stimulate specific immune responses is not understood. Activation of macrophages with interferon (IFN)-gamma leads to rapid killing and degradation of Listeria monocytogenes in a phagosome, thus preventing escape of bacteria to the cytosol. Here, we show that activated macrophages induce a specific gene expression program to L. monocytogenes degraded in the phago-lysosome. In addition to activation of Toll-like receptor (TLR) signaling pathways, degraded bacteria also activated a TLR-independent transcriptional response that was similar to the response induced by cytosolic L. monocytogenes. More specifically, degraded bacteria induced a TLR-independent IFN-beta response that was previously shown to be specific to cytosolic bacteria and not to intact bacteria localized to the phagosome. This response required the generation of bacterial ligands in the phago-lysosome and was largely dependent on nucleotide-binding oligomerization domain 2 (NOD2), a cytosolic receptor known to respond to bacterial peptidoglycan fragments. The NOD2-dependent response to degraded bacteria required the phagosomal membrane potential and the activity of lysosomal proteases. The NOD2-dependent IFN-beta production resulted from synergism with other cytosolic microbial sensors. This study supports the hypothesis that in activated macrophages, cytosolic innate immune receptors are activated by bacterial ligands generated in the phagosome and transported to the cytosol.
Insights
Activated macrophages detect degraded Listeria monocytogenes in the phagosome. This triggers a specific gene expression program, including interferon-beta (IFN-beta) production, via nucleotide-binding oligomerization domain 2 (NOD2) signaling.
Area of Science:
- Immunology
- Microbiology
- Cell Biology
Background:
- Macrophages are key immune cells that eliminate intracellular pathogens.
- The mechanisms by which macrophages sense and respond to degraded pathogens within phagosomes are not fully understood.
- Interferon-gamma (IFN-gamma) activates macrophages to kill Listeria monocytogenes (L. monocytogenes) within phagosomes.
Purpose of the Study:
- To investigate how activated macrophages monitor the degradation of intracellular pathogens.
- To elucidate the signaling pathways involved in the immune response to phagosome-localized, degraded bacteria.
- To determine if degraded bacteria can activate cytosolic immune sensing pathways.
Main Methods:
- Macrophage activation with IFN-gamma.
- Stimulation of macrophages with L. monocytogenes.
- Analysis of gene expression programs.
- Toll-like receptor (TLR) and nucleotide-binding oligomerization domain 2 (NOD2) signaling pathway investigation.
- Assessment of phagosomal function and lysosomal protease activity.
Main Results:
- Activated macrophages induce a specific gene expression program in response to degraded L. monocytogenes.
- Degraded bacteria activate both TLR-dependent and TLR-independent pathways.
- A TLR-independent interferon-beta (IFN-beta) response is induced by degraded bacteria, similar to cytosolic bacteria.
- This IFN-beta response depends on NOD2, phagosomal membrane potential, and lysosomal proteases.
- NOD2-dependent IFN-beta production involves synergism with other cytosolic microbial sensors.
Conclusions:
- Activated macrophages can sense bacterial degradation within the phagosome.
- Bacterial ligands generated in the phagosome can activate cytosolic innate immune receptors like NOD2.
- This process leads to IFN-beta production, contributing to innate immunity against intracellular pathogens.
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