Lymphatic endothelial cells regulate neutrophil phenotypes and function in a microphysiological model of infection

This study utilizes a human 3D microphysiological model to demonstrate that lymphatic endothelial cells actively regulate neutrophil migration and function during skin infection, revealing distinct, bacteria-specific responses to *Escherichia coli* and *Staphylococcus aureus*.

The Gist

Imagine your skin is a bustling city. When a burglar (a bacteria) breaks in, the city's first responders are the neutrophils—the police officers of your immune system. Their job is to rush to the scene, catch the bad guys, and clean up the mess.

For a long time, scientists thought the blood vessels were the only highways these police officers used to get to the crime scene. But this new study asks a different question: What about the lymphatic system? Think of the lymphatic system as the city's "drainage and recycling network." It usually just carries away trash, but this study suggests it might actually be an active traffic controller that tells the police officers where to go and how to act.

Here is the story of what the researchers discovered, using a high-tech "mini-city" they built in a lab.

1. Building the Mini-City (The Model)

Instead of studying this in a petri dish (which is like looking at a flat map), the scientists built a 3D "microphysiological system."

• The Setup: Imagine a tiny, transparent block of jelly (collagen) representing your skin tissue.
• The Roads: Inside this jelly, they built a tiny, hollow tube lined with lymphatic cells. This is a "biomimetic" (fake but realistic) lymphatic vessel.
• The Invaders: They dropped in two types of "burglars":
  • E. coli: A common bacteria that acts like a straightforward criminal.
  • Staphylococcus aureus (S. aureus): A "super-villain" bacteria known for being tricky, resistant to antibiotics, and causing nasty skin infections.
• The Police: They added human neutrophils (white blood cells) to the system to see how they reacted.

2. The First Discovery: The Lymphatic "Traffic Light"

When the scientists introduced E. coli (the straightforward burglar), something amazing happened. The lymphatic vessel didn't just sit there; it acted like a magnet.

• The neutrophils saw the bacteria and the lymphatic vessel and said, "Let's go!"
• They swarmed toward the vessel, moving in a straight, organized line.
• The Metaphor: It's like the lymphatic vessel turned on a green light and a siren, directing the police officers straight to the trouble spot. Without the bacteria, the police just wandered aimlessly. The lymphatic system only "woke up" to help when there was a real infection.

3. The Second Discovery: The "Super-Villain" Trap

Then, they swapped the burglar for S. aureus. This is where things got weird.

• The Trap: Even though the lymphatic vessel was there, the neutrophils stopped moving. They didn't rush toward the vessel like they did with E. coli.
• The Paradox: The neutrophils were actually eating more bacteria (phagocytosis), but they were stuck in place. They were like police officers who grabbed the criminal but then froze, unable to run away with the prisoner.
• The Villain's Trick: S. aureus is known for being sneaky. It seems to confuse the police, making them stop and stare instead of clearing the area.

4. The "Web" Strategy (NETosis)

When the neutrophils got stuck with S. aureus, they tried a desperate move called NETosis.

• Suicidal vs. Vital: Usually, when a neutrophil traps a bacteria, it explodes (like a grenade) to kill the enemy. This is "suicidal NETosis."
• The Twist: With S. aureus, the neutrophils didn't explode. Instead, they stayed alive but released a sticky, web-like net of DNA to trap the bacteria. This is called "Vital NETosis" (vital meaning "alive").
• The Result: The bacteria got caught in the web, but the neutrophils were also trapped in the web. They were immobilized.
• The Fix: When the scientists added an enzyme (DNase) that cuts up DNA, the neutrophils were freed and could move again. This proved that the "web" was the reason they were stuck.

Why Does This Matter?

This study is a big deal because it's the first time we've seen lymphatic vessels directly controlling how immune cells behave during an infection in a realistic 3D environment.

• The Takeaway: The lymphatic system isn't just a passive drain; it's an active commander. It helps organize the immune response against normal bacteria (E. coli) but gets hijacked by tricky bacteria (S. aureus) that use "vital NETosis" to trap the immune cells and hide in plain sight.
• The Future: Understanding this "traffic jam" caused by S. aureus could help doctors develop new treatments. Instead of just using antibiotics (which the bacteria are learning to resist), we might be able to use drugs that cut the "DNA webs" or help the lymphatic system break the bacteria's control, allowing our own immune system to do its job.

In short: The lymphatic system is the traffic cop of your skin. It usually directs the police to the crime scene efficiently, but some super-villain bacteria can hack the traffic lights, trapping the police in a sticky web so they can't do their job. This study helps us figure out how to unplug that hack.

Technical Summary

1. Problem Statement

• Context: Skin inflammation and infection require coordinated immune responses, primarily driven by neutrophils as first-line defenders. While the role of blood vasculature in neutrophil recruitment is well-studied, the lymphatic system's specific role in regulating neutrophil behavior during active skin infection remains poorly understood.
• Pathogen Challenge: Staphylococcus aureus is a major driver of antibiotic-resistant skin infections. It employs immune evasion strategies, including the manipulation of neutrophil functions (phagocytosis, migration, and NETosis), yet the mechanisms by which it interacts with the lymphatic microenvironment are unclear.
• Limitation of Current Models: Traditional 2D cell cultures and animal models fail to recapitulate the complex 3D tissue architecture and dynamic crosstalk between immune cells, lymphatic vessels, and pathogens found in human skin.

2. Methodology

The researchers developed a human-based 3D Microphysiological System (MPS) using the LumeNEXT platform to model an infected skin microenvironment.

• Device Fabrication:
  • Constructed using soft lithography with PDMS molds on glass-bottom dishes.
  • Features a central collagen matrix (Type I collagen) flanked by two lumens.
  • One lumen is seeded with primary human dermal lymphatic endothelial cells (HdLECs) to form a biomimetic lymphatic vessel. The opposing lumen serves as a reservoir for neutrophils.
• Bacterial Stimuli:
  • pHrodo-conjugated bioparticles of Escherichia coli and Staphylococcus aureus were embedded in the collagen matrix.
  • pHrodo fluorescence (green) activates only upon phagocytosis, allowing for real-time quantification of neutrophil uptake.
• Experimental Setup:
  • Neutrophils: Isolated from healthy human donors via negative selection.
  • Assays: Neutrophils were introduced into the empty lumen and allowed to migrate for 4–24 hours.
  • Conditions: Compared scenarios with/without lymphatic vessels and/or bacterial stimuli (E. coli vs. S. aureus).
• Analysis Techniques:
  • Imaging: Confocal and widefield fluorescence microscopy (Zeiss Axio Observer 7, Nikon spinning disk).
  • Quantification: StarDist (deep learning-based tool) was used for automated single-cell nuclear segmentation to measure migration distance, directionality, nuclear size, and phagocytic intensity.
  • NETosis Assessment: Immunostaining for Myeloperoxidase (MPO) and nuclear morphology analysis (Hoechst staining) to distinguish between suicidal and vital NETosis. DNase treatment was used to verify the role of extracellular DNA.

3. Key Contributions

• First 3D MPS of Lymphatic-Neutrophil-Pathogen Interaction: This is the first study to integrate a biomimetic lymphatic vessel, human neutrophils, and bacterial stimuli in a single 3D system to directly investigate lymphatic regulation of innate immunity.
• Differentiation of Pathogen-Specific Responses: The study demonstrates that the lymphatic endothelium does not act uniformly but modulates neutrophil behavior in a bacteria-dependent manner.
• Characterization of Vital NETosis: The model successfully identified a specific neutrophil phenotype induced by S. aureus consistent with vital NETosis (cells remain viable), distinct from the suicidal NETosis typically induced by PMA.

4. Key Results

A. Lymphatic Endothelium Amplifies Migration

• Synergy: Neutrophil migration increased significantly only when both the lymphatic vessel and bacterial stimuli were present.
• Directionality: In the presence of bacteria, neutrophils exhibited a strong preference for migrating toward the lymphatic vessel. This directional bias was lost in the absence of bacteria, indicating a coordinated signaling mechanism between the endothelium and neutrophils triggered by infection.

B. Bacterial Identity Dictates Neutrophil Phenotype

• E. coli (Commensal-like): Promoted strong, directional migration toward the lymphatic vessel.
• S. aureus (Pathogenic):
  • Suppressed Migration: Despite the presence of the lymphatic vessel, S. aureus significantly reduced overall neutrophil migration and directionality compared to E. coli.
  • Increased Phagocytosis: Paradoxically, neutrophils exposed to S. aureus showed higher phagocytic uptake (higher pHrodo fluorescence) than those exposed to E. coli.
  • Interpretation: S. aureus appears to impair neutrophil motility after uptake, potentially trapping them to facilitate infection persistence.

C. Induction of Vital NETosis by S. aureus

• Morphology: S. aureus exposure caused neutrophils to develop rounded or peanut-shaped nuclei with increased nuclear size, distinct from the diffuse, wispy nuclei seen in PMA-induced suicidal NETosis.
• MPO Positivity: S. aureus-exposed neutrophils were strongly positive for Myeloperoxidase (MPO), confirming NET-associated activation.
• DNase Rescue: Treatment with DNase reduced the enlarged nuclear size back to control levels, confirming that extracellular DNA release (NETosis) drives this phenotype.
• Conclusion: The data supports that S. aureus induces vital NETosis (where cells remain viable), which may serve as an immune evasion strategy to limit effective bacterial clearance.

5. Significance

• Mechanistic Insight: The study provides the first direct evidence that lymphatic endothelial cells actively drive neutrophil behavior during skin infection, challenging the view of lymphatics as passive drainage systems.
• Pathogen Evasion Strategy: It elucidates how S. aureus exploits the immune system by suppressing migration and inducing vital NETosis, potentially explaining its success in causing chronic, antibiotic-resistant infections.
• Therapeutic Implications:
  • The MPS model offers a high-fidelity platform for screening therapeutics that target immune-lymphatic crosstalk.
  • Understanding the induction of vital NETosis suggests that DNase-based therapies or strategies to modulate NETosis could be effective in treating S. aureus infections.
  • The system supports the development of microbiome-informed strategies to restore healing balance and reduce reliance on systemic antibiotics.