Myeloperoxidase promotes fibrosis by inhibiting cathepsin K to bias the lung toward ECM accumulation

This study reveals that myeloperoxidase (MPO) drives pulmonary fibrosis by post-translationally inhibiting the collagen-degrading enzyme cathepsin K, thereby shifting the lung toward excessive extracellular matrix accumulation and correlating elevated MPO levels with poor patient outcomes.

The Gist

The Big Picture: A Construction Site That Won't Stop Building

Imagine your lungs are a busy construction site. In a healthy body, there is a perfect balance: construction crews (cells that build collagen, the "scaffolding" of your lungs) and demolition crews (cells that break down old or excess scaffolding). They work in a loop, keeping the lungs flexible and strong.

In Pulmonary Fibrosis (PF), this balance breaks. The construction crews go into overdrive, and the demolition crews stop working. The result? A massive pile-up of "scaffolding" (scar tissue) that makes the lungs stiff, like a brick wall, eventually causing respiratory failure.

For a long time, scientists thought the problem was just that the construction crews were too aggressive. But this paper reveals a different, sneaky culprit: The Demolition Crews aren't lazy; they are being handcuffed.

The Villain: Myeloperoxidase (MPO)

Meet MPO (Myeloperoxidase).

• What it is: MPO is a weapon released by neutrophils, which are the body's "first responder" white blood cells. When you get an injury or infection, neutrophils rush in to fight germs, and they release MPO to help kill them.
• The Problem: Usually, once the infection is gone, the neutrophils leave, and the MPO goes away. But in fibrosis, MPO sticks around long after the neutrophils have left the building. It's like a security guard who stays at the construction site weeks after the emergency is over, refusing to let anyone leave.

The Mechanic: Cathepsin K (CatK)

Meet CatK (Cathepsin K).

• What it is: CatK is the super-demolition expert. It is a powerful enzyme specifically designed to chew up collagen (the scar tissue).
• The Twist: In fibrosis patients, the body actually makes more CatK. It's trying to fix the problem! But despite having more demolition experts, the scar tissue keeps piling up. Why? Because the experts are being stopped before they can do their job.

The "Handcuff" Moment: How MPO Stops CatK

This is the main discovery of the paper. The researchers found that MPO directly handcuffs CatK.

Think of it this way:

1. The body sends in the neutrophils to fight an injury. They drop MPO on the ground.
2. The neutrophils leave, but MPO stays behind, stuck to the lung tissue.
3. The body sends in CatK (the demolition expert) to clean up the mess.
4. MPO grabs CatK and ties its hands. Even though CatK is present and ready to work, it cannot cut the collagen.
5. The result: The demolition crew is paralyzed, the scar tissue piles up, and the lungs get stiff.

The Evidence: What the Scientists Found

The team used mice and human data to prove this story:

1. The "No-MPO" Mice: They used mice that were genetically unable to make MPO. When these mice got lung injuries, they didn't get fibrosis. Their demolition crews (CatK) were free to work, and the lungs healed.
2. The "Late-Stage" Rescue: They gave mice a drug to block MPO after the initial injury had passed (when inflammation was already over). Surprisingly, this still stopped the fibrosis! This proves that MPO isn't just causing the initial injury; it's the reason the lungs stay scarred.
3. The Human Connection: They looked at patients with Pulmonary Fibrosis.
   • They found high levels of MPO in the blood and lung tissue of these patients.
   • The "Handcuff" Effect: In patients with high MPO, the CatK enzyme was inactive.
   • The Outcome: Patients with high MPO levels had much shorter survival times and worse lung function than those with low MPO levels.

The Takeaway: A New Hope for Treatment

For years, doctors have tried to stop the "construction crews" (the cells building scar tissue) to treat fibrosis. This paper suggests a smarter approach: Unlock the demolition crews.

If we can give patients a drug that blocks MPO (like taking the handcuffs off CatK), we might allow the body's natural cleanup crew to finally break down the scar tissue and restore flexibility to the lungs.

In short: The paper explains that in lung fibrosis, the body's own immune weapon (MPO) accidentally traps the cleanup crew (CatK), causing a permanent traffic jam of scar tissue. By removing that weapon, we might finally clear the road.

Technical Summary

1. Problem Statement

Pulmonary fibrosis (PF) is characterized by excessive accumulation of extracellular matrix (ECM), specifically collagen, leading to lung stiffening and respiratory failure. While the mechanisms driving increased collagen deposition are partially understood, the factors preventing normal collagen resorption (degradation) remain unclear.

• The Paradox: In PF, the expression of Cathepsin K (CatK), a potent collagenolytic enzyme, is often elevated, yet collagen degradation is impaired. This suggests a post-translational inhibition of CatK activity.
• The Gap: Neutrophils are known to infiltrate injured lungs and release reactive oxygen species (ROS), but their numbers typically return to baseline while fibrosis progresses. The mechanism linking transient inflammation to sustained fibrosis, and the specific factor inhibiting collagen degradation in the absence of active neutrophils, has not been fully elucidated.
• The Candidate: Myeloperoxidase (MPO), an enzyme released by neutrophils, persists in lung tissue long after neutrophils have cleared. High MPO levels correlate with poor survival in PF patients, but its direct mechanistic role in collagen turnover was untested.

2. Methodology

The study employed a multi-modal approach combining in vivo mouse models, ex vivo tissue models, in vitro biochemical assays, and human clinical data.

• Animal Models:
  • Bleomycin-induced fibrosis: Wild-type (WT) and Myeloperoxidase knockout (MPOko) mice were treated with bleomycin.
  • Pharmacological Intervention: WT mice were treated with PF1355 (a selective MPO inhibitor) starting on Day 7 (post-peak inflammation) to test if MPO drives fibrosis during the remodeling phase.
  • Metrics: Survival, body weight, hydroxyproline content (total collagen), histology (H&E, Masson's Trichrome, Picrosirius Red), and flow cytometry for immune cell populations.
• Ex Vivo & In Vitro Models:
  • Precision-Cut Lung Slices (PCLS): Mouse lung slices were cultured with MPO, TGF-β, and/or MPO inhibitors (ABAH) to measure CatK activity and collagen turnover markers (PINP for synthesis, CTX-I for degradation).
  • Cell Culture: Human lung fibroblasts (IMR-90) were treated with TGF-β and MPO to assess CatK activity.
  • Biochemical Assays: Recombinant CatK was incubated with MPO and varying concentrations of hydrogen peroxide (H₂O₂) to determine direct inhibition mechanisms.
• Human Clinical Data:
  • Cohorts: Analysis of platelet-poor plasma and lung tissue from Idiopathic Pulmonary Fibrosis (IPF) patients and healthy controls.
  • Omics: Re-analysis of public single-cell RNA sequencing (scRNA-seq) datasets (GSE210341, GSE132771, LMEX0000004396) to map CTSK (CatK gene) expression.
  • Clinical Correlations: MPO levels were correlated with survival, Forced Vital Capacity (FVC), and Diffusing Capacity (DLCO).

3. Key Contributions

1. Mechanistic Discovery: Identified MPO as a direct, post-translational inhibitor of Cathepsin K activity, providing a molecular explanation for impaired collagen degradation in PF despite high CTSK expression.
2. Temporal Decoupling: Demonstrated that MPO persists in lung tissue long after neutrophil numbers return to baseline, acting as a "temporal bridge" connecting acute inflammation to chronic fibrosis.
3. Therapeutic Window: Showed that inhibiting MPO after the peak inflammatory phase (Day 7) still protects against fibrosis, suggesting that targeting MPO is viable even after the initial injury.
4. Patient Stratification: Established that a significant subset of IPF patients (approx. 62%) have elevated circulating MPO levels, which correlates strongly with reduced survival and lung function, identifying a specific patient population for targeted therapy.

4. Key Results

• MPO Drives Fibrosis Independently of Inflammation:
  • MPOko mice and WT mice treated with MPO inhibitors (starting Day 7) showed significantly reduced collagen accumulation (hydroxyproline) and fibrotic area compared to controls.
  • This protection occurred without altering the recruitment of immune cells (neutrophils, macrophages, etc.) during the inflammatory phase, indicating MPO acts directly on the fibrotic remodeling process.
• MPO Suppresses CatK Activity:
  • In WT mice with bleomycin-induced fibrosis, CatK activity was low despite high Ctsk mRNA expression.
  • In MPOko mice and inhibitor-treated mice, CatK activity was significantly restored.
  • Biochemical Proof: MPO directly inhibited recombinant CatK activity to 33% of control levels in the absence of H₂O₂. The presence of MPO also sensitized CatK to inhibition by lower concentrations of H₂O₂.
• Impact on Collagen Turnover:
  • In PCLS, MPO treatment reduced CatK activity and decreased the release of collagen degradation products (CTX-I) by ~20%, without altering collagen synthesis (PINP). This confirms MPO selectively impairs degradation.
• Human Relevance:
  • Cellular Context: scRNA-seq confirmed that CTSK is highly expressed in adventitial fibroblasts (the primary cells maintaining lung ECM) in both healthy and fibrotic lungs.
  • Clinical Correlation: IPF patients had significantly higher MPO levels in plasma and lung tissue compared to controls.
  • Prognostic Value: Patients with high plasma MPO (>80 ng/mL) had a median survival of 1.5 years vs. 4.8 years for low-MPO patients. High MPO levels were inversely correlated with FVC and DLCO.

5. Significance and Implications

• Novel Pathophysiology: The study redefines the role of neutrophil-derived factors in fibrosis. It suggests that the residual activity of MPO deposited in the tissue, rather than the presence of active neutrophils, drives the failure of collagen resolution.
• Therapeutic Potential: MPO inhibition represents a promising therapeutic strategy. Unlike broad anti-inflammatory approaches (which can be harmful in PF), MPO inhibition specifically targets the mechanism of impaired collagen degradation.
  • Existing MPO inhibitors (e.g., Mitiperstat, Verdiperstat) have known safety profiles, though the study notes the need for careful timing and patient stratification to avoid compromising innate immunity.
• Precision Medicine: The findings support the stratification of IPF patients based on MPO levels. Patients with elevated MPO are the most likely to benefit from MPO-targeted therapies, as their disease progression is driven by this specific mechanism.
• Paradigm Shift: The paper challenges the view that neutrophils are irrelevant to late-stage fibrosis, proposing instead that neutrophil-derived enzymes (like MPO) persist and actively maintain the fibrotic state by "braking" the body's natural collagen-degrading machinery.