Mechanical Ventilation Worsens Septic Lung Injury via HMGB1 Pathway: Study

How Mechanical Ventilation Turns a Septic Lung Against Itself: The HMGB1 Pathway Explained

Mechanical ventilation at moderate tidal volumes does not injure healthy lungs — but in septic patients, the same settings can dramatically worsen lung damage. A 2025 study in Shock from researchers at Renji Hospital, Shanghai Jiaotong University School of Medicine, identified a molecular culprit: the caspase-1/caspase-11–HMGB1–TLR4/RAGE signaling cascade, which converts routine ventilator support into a second inflammatory hit.

The Two-Hit Problem: Why Ventilation Harms Only Vulnerable Lungs

Intensivists have long observed a paradox. A patient with healthy lungs tolerates mechanical ventilation without tissue damage. A patient whose lungs are already primed by sepsis, infection, or trauma may deteriorate rapidly once ventilated — even at conservative settings.

The explanation is called the “two-hit” theory. The first hit — say, abdominal sepsis from a perforated bowel — sends inflammatory signals through the bloodstream and activates immune cells, leaving lung tissue fragile and hypersensitive. The second hit — the physical stretch and stress of ventilation — then triggers a disproportionate inflammatory response. A vicious cycle follows: inflammation damages the lung, damaged tissue releases more inflammatory signals, and the ventilator keeps stressing the wounded tissue.

Until recently, the molecular wiring behind this cycle was unclear. The Shanghai team, led by anesthesiologists across Renji Hospital, Shanghai Pulmonary Hospital, and the National Cancer Center Shenzhen Hospital, set out to trace it in mice.

HMGB1: The Danger Signal That Location Makes Deadly

The researchers induced sepsis in mice via cecal ligation and perforation — the standard model mimicking human abdominal sepsis — and after 12 hours ventilated them for 2 to 6 hours. Ventilation alone produced no HMGB1 release. In septic mice, ventilation sharply increased HMGB1 in both plasma and the cytoplasm of lung cells.

HMGB1 is a danger-associated molecular pattern: a protein normally tucked inside the cell nucleus doing DNA maintenance, but when released — by caspase-1 and caspase-11 enzymes during inflammatory cell death — it acts as an alarm signal. Once outside the cell, HMGB1 binds two receptors on other cells: TLR4 and RAGE. This binding activates MAP kinase signaling, which switches on production of cytokines and chemokines and pulls neutrophils into lung tissue, where they cause collateral damage.

The team tested this pathway systematically using genetically engineered mice lacking HMGB1 in specific cell types:

• Myeloid-cell deletion (LysM HMGB1−/−): cytokines, chemokines, MAPK activation, neutrophil recruitment, and lung injury all dropped substantially.
• Endothelial-cell deletion (EC-HMGB1−/−): inflammation stayed high, yet neutrophil recruitment and lung injury fell — showing vessel-lining HMGB1 specifically controls neutrophil trafficking into the lung.
• Whole-body deletion (iHMGB1−/−): the strongest protection across every measure.
• RAGE−/− and TLR4/RAGE−/− mice: removing both receptors protected more than removing RAGE alone.
• Caspase-11−/− and caspase-1/11−/− mice: both showed reduced inflammation, with no significant difference between them.

Their conclusion: location is the key to function. The same protein causes different damage depending on which cells release it and which receptors it reaches.

Why This Matters for Breathing and Immune Health

This study connects several threads covered elsewhere on this site. Cytokines and chemokines are the same inflammatory messengers implicated in asthma-related airway inflammation, and mitochondrial dysfunction plays its own role in the breath–immune connection. HMGB1 sits upstream of all of these signals, making it a master switch rather than just another messenger.

For intensive care, the findings point toward targeted therapies: drugs that block HMGB1 release (inhibiting caspase-1/11), antibodies that mop up circulating HMGB1, or receptor blockers targeting TLR4 and RAGE. Some experimental compounds already under investigation include HMGB1-neutralizing antibodies and small molecules like glycyrrhizin, a TLR4/RAGE signaling inhibitor studied in animal models. None are yet validated in large human trials — an honest limitation of this mouse research, though the two-hit model closely mirrors clinical ICU scenarios.

There is also a practical ventilation lesson: lung-protective ventilation strategies (low tidal volumes, careful pressure control) matter most in patients with any systemic inflammation, not just established lung disease.

Frequently Asked Questions

Does mechanical ventilation damage healthy lungs?

No — in this study, moderate tidal volume ventilation alone caused no HMGB1 release or injury in healthy mice. Damage occurred only when lungs were already primed by sepsis.

What is HMGB1 and why is it important?

HMGB1 is a nuclear protein that, when released by dying immune or endothelial cells, acts as a danger signal. It binds TLR4 and RAGE receptors, triggering cytokine release and neutrophil-driven lung injury.

Can I reduce HMGB1 through lifestyle or breathing exercises?

There is no direct human evidence yet. Chronic stress and inflammation can raise inflammatory signaling generally, and practices that reduce stress physiology may help indirectly, but HMGB1-specific interventions remain experimental.

Are these findings applicable to humans?

Possibly — the cecal ligation model closely mimics clinical sepsis, and HMGB1 biology is conserved between mice and humans. But confirmation in patient studies is still needed.

In summary, this work maps a precise molecular route — caspase-1/11, HMGB1, and the TLR4/RAGE receptors — by which lifesaving ventilation can backfire in inflamed lungs. Understanding the pathway gives clinicians and researchers concrete targets to break the two-hit cycle and protect fragile lungs during critical illness.

Sources:
https://pubmed.ncbi.nlm.nih.gov/39228020/
https://pubmed.ncbi.nlm.nih.gov/39089571/
https://pubmed.ncbi.nlm.nih.gov/38950078/
https://pubmed.ncbi.nlm.nih.gov/38902859/
https://pubmed.ncbi.nlm.nih.gov/38897669/