Hijacking of inflammasome responses by the complement system during Pseudomonas aeruginosa-Aspergillus fumigatus sur-infection

This study reveals that during *Pseudomonas aeruginosa* and *Aspergillus fumigatus* co-infection in cystic fibrosis patients, a complement-inflammasome signaling axis—where bacterial priming enhances macrophage C3 expression and fungal engagement of C3 receptors amplifies SYK/ERK-driven NLRP3 inflammasome activation—drives pathological IL-1$\beta$ release and lung inflammation.

The Gist

The Big Picture: A "Perfect Storm" in the Lungs

Imagine the lungs of a person with Cystic Fibrosis (CF) as a busy, slightly messy city street. Because of a genetic glitch, the "street cleaners" (mucus clearance) don't work well, so trash and invaders tend to pile up.

Usually, the city has a security force (the immune system) that handles one intruder at a time. But in this study, researchers discovered what happens when two different types of intruders show up one after the other:

1. The Bacteria: Pseudomonas aeruginosa (a tough, persistent bacterial gang).
2. The Fungus: Aspergillus fumigatus (a mold that loves to grow in damp, dirty places).

When these two invade together, they don't just cause a little trouble; they trigger a massive, destructive riot in the lungs that damages the city itself. This paper explains how they do it.

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The Story: How the Riot Starts

1. The Setup: The "Priming" Effect

Think of the immune cells (macrophages) as the city's police officers.

• Step 1: The bacteria (Pseudomonas) arrive first. The police fight them off and win. The bacteria are gone (cleared by antibiotics or the immune system).
• Step 2: The fungus (Aspergillus) arrives later.

The Surprise: Even though the bacteria are gone, the police officers are still acting like they are in a war zone. When the fungus shows up, the police don't just react normally; they go into overdrive. They start screaming (releasing massive amounts of a chemical called IL-1β) so loudly that it hurts the city buildings (lung tissue).

The researchers found that this overreaction only happens if the bacteria were alive during the first encounter. Dead bacteria or just their "trash" (parts of the bacteria) weren't enough to set this trap. The bacteria had to actually infect the cell to "reprogram" it.

2. The Secret Weapon: Hijacking the "Complement System"

This is the most exciting part of the discovery. The researchers found a secret backdoor the bacteria and fungus use to hijack the immune system.

• The Analogy: Imagine the immune system has a "Complement System" that acts like a high-tech tagging system. When a bad guy is spotted, the system sprays a glowing "TARGET" sticker (a protein called C3) all over them so the police can grab them easily.
• The Hijack:
  1. The bacteria (Pseudomonas) train the police officers to wear special gloves (a receptor called ITGAM or CD11b) that are designed to grab those "TARGET" stickers.
  2. When the fungus (Aspergillus) arrives, it gets covered in the "TARGET" stickers (C3) naturally.
  3. The police, wearing the special gloves, grab the fungus too hard.
  4. This super-tight grip sends a signal that says, "ALARM! ALARM! TOTAL ANNIHILATION!"

This signal triggers the Inflammasome (the immune system's "nuclear button"). Instead of just cleaning up the fungus, the cell explodes with inflammation, releasing toxic chemicals that damage the lungs.

3. The Culprits: Specific "Weapons"

The researchers identified exactly which parts of the bacteria and fungus were needed to pull this off:

• From the Bacteria: They needed specific "weapons" like flagella (whips), pili (grappling hooks), and a secretion system (a syringe).
• From the Fungus: They needed a sugary coating called GAG.
• The Result: If you remove any of these specific parts, the "Perfect Storm" doesn't happen. The police react normally, and the lungs stay safe.

4. The "Gas Switch" That Wasn't Needed

Usually, when immune cells release these toxic chemicals, they blow a hole in their own cell wall (like popping a balloon) to get the chemicals out. This is called "pyroptosis."

• The Twist: In this specific bacterial-fungal riot, the cells didn't need to pop the balloon. They found a different way to release the toxic chemicals without killing themselves immediately. This explains why the inflammation can keep going for a long time without the cells dying off first.

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Why This Matters for Patients

The Problem: People with Cystic Fibrosis often have both bacteria and fungus in their lungs. This leads to a vicious cycle of inflammation that destroys lung function faster than either infection would alone.

The Solution (Potential):
For years, doctors have tried to calm the immune system, but they were worried that if they turned down the volume, the body wouldn't be able to fight the infection anymore.

• This paper suggests a new strategy: Don't turn off the whole alarm system.
• Instead, cut the wire between the "TARGET stickers" (Complement C3) and the "special gloves" (ITGAM).
• If you block this specific connection, you stop the destructive overreaction (the riot) without stopping the police from doing their job of catching the bad guys.

Summary in One Sentence

This study discovered that when Pseudomonas bacteria and Aspergillus fungus team up in Cystic Fibrosis lungs, the bacteria "train" the immune cells to grab the fungus extra tightly using a specific receptor, causing a massive, damaging explosion of inflammation that can be stopped by blocking that specific handshake.

Technical Summary

1. Problem Statement

Patients with Cystic Fibrosis (CF) suffer from chronic polymicrobial lung infections, most notably co-colonization by the bacterium Pseudomonas aeruginosa and the fungus Aspergillus fumigatus. This co-infection is associated with accelerated lung function decline and excessive, dysregulated inflammation driven by Interleukin-1β (IL-1β). While both pathogens individually trigger immune responses, the molecular mechanisms underlying the synergistic hyperactivation of the inflammasome during sequential bacterial-fungal infection remain undefined. The study aims to identify the specific pathogen-associated molecular patterns (PAMPs), signaling pathways, and cellular interactions that drive this pathological amplification of IL-1β production in CF airways.

2. Methodology

The researchers employed a multi-faceted approach combining in vitro models, genetic mouse models, transcriptomics, and human clinical data analysis:

• Cellular Models:
  • Sequential Infection Model: Wild-type (WT) alveolar macrophages (AMs) and bone marrow-derived macrophages (BMDMs) were infected with live P. aeruginosa, treated with antibiotics to clear the bacteria, and then superinfected with A. fumigatus.
  • Priming Controls: Cells were primed with PAMPs (LPS, flagellin), cytokines (IFN-γ), or heat-killed/fixed bacteria to distinguish between classical priming and live infection effects.
  • Mutant Libraries: Screening of P. aeruginosa mutants (lacking flagellin, T3SS, Type IV pili) and A. fumigatus mutants (GAG-deficient $\Delta$gt4c, GAG-overproducing $\Delta$ugm1).
• Genetic Models:
  • Utilized knockout mice for key immune components: Nlrp3, Aim2, Asc, Casp1, Casp8, Gsdmd, Myd88, Ifnar1, Tnf, and C3.
  • Used pharmacological inhibitors (MCC950 for NLRP3, VX-765 for Caspase-1, z-IETD-fmk for Caspase-8) and neutralizing antibodies (anti-ITGAM/CD11b).
• Omics and Bioinformatics:
  • Bulk RNA-seq: Performed on BMDMs to analyze transcriptional changes during sequential infection.
  • Single-cell RNA-seq (scRNA-seq): Re-analyzed public datasets from CF patients (sputum and lung biopsies) stratified by infection status (P. aeruginosa and/or A. fumigatus).
  • CellChat Analysis: Used to infer cell-cell communication networks (ligand-receptor interactions) within the CF lung microenvironment.
• Assays: ELISA for cytokines (IL-1β, TNF, KC), Western Blot for signaling proteins (phospho-SYK, phospho-ERK, Caspase-1 cleavage), and live-cell imaging for fungal growth and cell death.

3. Key Contributions & Results

A. Inflammasome Hyperactivation is Specific to Live Sequential Infection

• Sequential infection (P. aeruginosa $\to$ A. fumigatus) resulted in a massive increase in caspase-1 cleavage and IL-1β release compared to mono-infections.
• This hyperactivation was independent of exogenous priming (LPS/IFN-γ alone failed to mimic the effect) and required live bacterial infection (heat-killed or fixed bacteria were insufficient).
• The response was specific to IL-1β; other cytokines like TNF and KC showed different regulatory patterns, and cell death (pyroptosis) was not significantly increased, suggesting a decoupling of cytokine release from cell lysis.

B. Critical PAMPs and Pathogen Requirements

• Bacterial Factors: P. aeruginosa flagellin (FliC), Type III Secretion System (T3SS), and Type IV pili were all required for the hyperactivation.
• Fungal Factors: The fungal cell wall polysaccharide galactosaminogalactan (GAG) was essential. GAG-deficient A. fumigatus failed to trigger hyperactivation, while GAG-overproducing strains enhanced it.
• Clinical Relevance: The mechanism held true when using clinical CF isolates of both pathogens.

C. Molecular Mechanism: The NLRP3–ASC–Caspase-1/8 Axis

• The hyperactivation was strictly NLRP3-dependent (abolished in Nlrp3–/– and by MCC950) and required the adaptor ASC.
• It relied on both Caspase-1 and Caspase-8.
• Surprising Finding: The process was Gasdermin D (GSDMD) independent. IL-1β was released without GSDMD-mediated pyroptosis, suggesting an alternative secretion mechanism.

D. The MyD88–ITGAM–C3 Signaling Axis

• MyD88 Dependency: The hyperactivation was abolished in Myd88–/– macrophages, indicating a TLR-dependent (rather than IL-1R) mechanism.
• Transcriptional Reprogramming: P. aeruginosa pre-infection upregulated ITGAM (CD11b) and complement components (specifically C3) in macrophages.
• The Hijacking Mechanism:
  1. P. aeruginosa infection (via MyD88/TLR) upregulates ITGAM (CD11b) on macrophages.
  2. Macrophages secrete C3, which binds to the fungal surface (GAG).
  3. The fungal-bound C3 engages the ITGAM–ITGB2 (CR3) receptor on the macrophage.
  4. This engagement triggers SYK and ERK phosphorylation, amplifying NLRP3 inflammasome activation and IL-1β release.
• Blocking ITGAM or using C3–/– macrophages phenocopied the Myd88–/– defect, confirming the ITGAM–C3 axis as the central driver.

E. Translation to Human CF Disease

• scRNA-seq Analysis: In CF patient samples, macrophage subsets (specifically Alveolar_MQ and Mo_MQ) showed coordinated upregulation of both complement (C3, ITGAM) and inflammasome (NLRP3, IL1B) signatures during co-infection.
• CellChat Modeling: Predicted a feed-forward loop where epithelial/stromal cells secrete C3, which engages CR3 (ITGAM) on myeloid cells, driving IL-1β secretion that targets neutrophils and epithelium, perpetuating inflammation.

4. Significance

• Mechanistic Insight: The study defines a novel "hijacking" mechanism where the complement system, typically part of host defense, is co-opted to drive pathological inflammation during polymicrobial infection.
• Therapeutic Target: It identifies the ITGAM–C3 axis as a specific therapeutic target. Blocking this interaction could potentially dampen the destructive IL-1β-driven inflammation in CF patients without compromising the essential host defense mechanisms against the pathogens themselves.
• Paradigm Shift: It challenges the view that inflammasome activation in CF is solely driven by direct pathogen recognition, highlighting the critical role of inter-pathogen signaling (bacteria priming the host for fungal recognition via complement).
• GSDMD Independence: The finding that IL-1β release can occur without GSDMD in this context opens new avenues for understanding chronic inflammatory diseases where cell death is not the primary driver of cytokine release.

In summary, the paper elucidates how P. aeruginosa primes macrophages via MyD88 to upregulate ITGAM, which then senses complement-coated A. fumigatus via the C3 receptor, leading to SYK-mediated NLRP3 hyperactivation and excessive IL-1β production, a key driver of lung damage in Cystic Fibrosis.