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Evaluation of Caspase Activation to Assess Innate Immune Cell Death
Published on: January 20, 2023
Host-dependent trigger of caspases and apoptosis by Legionella pneumophila
Marina Santic1, Rexford Asare, Miljenko Doric
1Department of Microbiology and Immunology, University of Louisville College of Medicine, 319 Abraham Flexner Way 55A, Louisville, KY 40202, USA.
Abstract:
The Dot/Icm system of Legionella pneumophila triggers activation of caspase-3 during early stages of infection of human macrophages, but apoptosis is delayed until late stages of infection. During early stages of infection of mouse macrophages, the organism triggers rapid caspase-1-mediated cytotoxicity, which is mediated by bacterial flagellin. However, it is not known whether caspase-1 is triggered by L. pneumophila in human macrophages or whether caspase-3 is activated in permissive or nonpermissive mouse macrophages. Using single-cell analyses, we show that the wild-type strain of L. pneumophila does not trigger caspase-1 activation throughout the intracellular infection of human monocyte-derived macrophages (hMDMs), even when the flagellated bacteria escape into the cytoplasm during late stages. Using single-cell analyses, we show that the Dot/Icm system of L. pneumophila triggers caspase-3 but not caspase-1 within permissive A/J mouse bone marrow-derived primary macrophages by 2 to 8 h, but apoptosis is delayed until late stages of infection. While L. pneumophila triggers a Dot/Icm-dependent activation of caspase-1 in nonpermissive BALB/c mouse-derived macrophages, caspase-3 is not activated at any stage of infection. We show that robust intrapulmonary replication of the wild-type strain of L. pneumophila in susceptible A/J mice is associated with late-stage Dot/Icm-dependent pulmonary apoptosis and alveolar inflammation. In the lungs of nonpermissive BALB/c mice, L. pneumophila does not replicate and does not trigger pulmonary apoptosis or alveolar inflammation. Thus, similar to hMDMs, L. pneumophila does not trigger caspase-1 but triggers caspase-3 activation during early and exponential replication in permissive A/J mouse-derived macrophages, and apoptosis is delayed until late stages of infection. The Dot/Icm type IV secretion system is essential for pulmonary apoptosis in the genetically susceptible A/J mice.
Insights
Legionella pneumophila infection activates caspase-3 in human and permissive mouse macrophages but not caspase-1. Apoptosis is delayed, occurring late in infection, particularly in susceptible mice.
Area of Science:
- Microbiology
- Immunology
- Cell Biology
Background:
- Legionella pneumophila (L. pneumophila) infection dynamics involve host cell apoptosis.
- The Dot/Icm secretion system is crucial for L. pneumophila virulence.
- Caspase activation (caspase-1 and caspase-3) plays a role in host defense and pathogen survival.
Purpose of the Study:
- To investigate the differential activation of caspase-1 and caspase-3 by L. pneumophila in human and mouse macrophages.
- To determine the role of the Dot/Icm system in caspase activation and apoptosis during L. pneumophila infection in vivo.
- To elucidate the mechanisms underlying delayed apoptosis in L. pneumophila-infected macrophages.
Main Methods:
- Single-cell analyses of human monocyte-derived macrophages (hMDMs) and mouse bone marrow-derived macrophages (A/J and BALB/c strains).
- Infection with wild-type L. pneumophila.
- Assessment of caspase-1 and caspase-3 activation.
- In vivo studies in susceptible (A/J) and nonpermissive (BALB/c) mice to evaluate bacterial replication, pulmonary apoptosis, and alveolar inflammation.
Main Results:
- L. pneumophila did not trigger caspase-1 in hMDMs, but activated caspase-3, with delayed apoptosis.
- In permissive A/J mouse macrophages, L. pneumophila activated caspase-3 but not caspase-1, delaying apoptosis.
- In nonpermissive BALB/c mouse macrophages, L. pneumophila activated caspase-1 (Dot/Icm-dependent) but not caspase-3.
- In A/J mice, L. pneumophila replication correlated with late-stage Dot/Icm-dependent pulmonary apoptosis and inflammation, while BALB/c mice showed no replication or inflammation.
Conclusions:
- L. pneumophila differentially regulates caspase activation in host cells, favoring caspase-3 over caspase-1 in permissive environments.
- The Dot/Icm system is essential for triggering late-stage apoptosis and inflammation in susceptible hosts.
- Delayed apoptosis is a conserved feature of L. pneumophila infection in both human and permissive mouse macrophages, suggesting a mechanism for prolonged intracellular replication.
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