S100A9 aggravates hepatic ischemia–reperfusion injury associated with hepatocyte CCL2 expression and macrophage recruitment

When a surgeon temporarily stops blood flow to a part of the liver to remove a tumor or prepare for a transplant, the organ faces a paradoxical danger. The moment blood rushes back in, it can trigger a violent, self-inflicted inflammatory storm. This phenomenon, known as ischemia-reperfusion injury, occurs because the sudden return of oxygen and nutrients wakes up the immune system in a way that damages the very tissue it is meant to save. The liver cells, already stressed from the lack of blood, release signals that call in white blood cells. These arriving immune cells, intended to clean up the damage, often end up causing more harm, leading to swelling, cell death, and sometimes organ failure. Understanding exactly how this chain reaction starts and how it spreads is critical for improving outcomes in liver surgery and transplantation, yet the specific molecular triggers that turn a necessary medical procedure into a dangerous inflammatory event have remained somewhat hidden.

Researchers at Central South University in China have now traced a specific path through this chaos, identifying a key protein that acts as a dangerous messenger between injured liver cells and the immune system. By combining advanced computer analysis of genetic and protein data with experiments in mice and human cells, the team discovered that a protein called S100A9 plays a central role in worsening liver damage after blood flow is restored. They found that this protein does not just sit passively in the tissue; it actively instructs stressed liver cells to release a chemical beacon that recruits more immune cells, creating a cycle of injury that could potentially be stopped with targeted treatment.

The investigation began with a broad search for clues. The scientists took liver tissue from mice that had undergone a procedure mimicking human liver surgery, where 70 percent of the liver's blood supply was blocked for one hour before being restored. They compared this tissue to liver tissue from mice that had undergone the same surgery but without the blood blockage. Using powerful sequencing tools, they read the genetic instructions and measured the protein levels in both groups. This massive data comparison highlighted S100A9 as a standout suspect. In the injured livers, the levels of this protein were significantly higher than in the healthy ones, appearing consistently in both the genetic code and the actual protein structures. The researchers then confirmed this finding using standard laboratory techniques, showing that the protein was indeed abundant in the damaged tissue and was closely associated with neutrophils, a type of white blood cell that rushes to the site of injury early in the process.

To prove that S100A9 was actually causing the harm rather than just being a bystander, the team tested two different strategies to stop it. In the first approach, they injected mice with a specialized antibody designed to neutralize S100A9, effectively blocking its ability to function. In the second approach, they used mice that had been genetically engineered to lack the gene that produces S100A9 entirely. Both methods yielded the same reassuring result: the liver damage was significantly reduced. The mice that received the blocking antibody or lacked the protein showed less swelling, fewer dead cells, and lower levels of harmful inflammatory chemicals in their blood compared to the control mice. This provided strong evidence that S100A9 is a functional driver of the injury, not merely a symptom of it.

The study then moved to the cellular level to understand the mechanism behind this damage. The researchers exposed mouse liver cells to a condition that mimics the lack of oxygen and glucose seen during surgery, followed by a return to normal conditions. When they added S100A9 to these stressed cells, the cells died at a higher rate, confirming that the protein directly worsens cell death. More importantly, they discovered what the protein was telling the cells to do next. By analyzing the genetic activity of the cells after exposure to S100A9, they found that the cells began producing large amounts of a chemical signal called CCL2. This chemical acts as a homing beacon, specifically designed to attract immune cells known as macrophages.

The team verified this connection by showing that when liver cells were treated with S100A9, the fluid surrounding them became much more attractive to macrophages, drawing them in more aggressively. Conversely, in the mice that lacked S100A9, the liver tissue produced far less of this chemical beacon, and significantly fewer macrophages accumulated in the injured area. This revealed a clear chain of events: the initial injury triggers the release of S100A9, which then forces the surviving liver cells to produce CCL2, which in turn calls in more immune cells that cause further damage. The researchers also looked at human patients undergoing partial liver removal. They measured the levels of S100A9 in blood samples taken before and after surgery and found that the levels rose significantly after the procedure, mirroring the changes seen in the mice. This suggests that the same biological process observed in the lab is happening in humans.

While the study establishes a strong link between S100A9, the chemical beacon CCL2, and the arrival of damaging immune cells, the authors note that they have not yet proven that blocking this specific link is the only way to stop the injury. They suggest that S100A9 might work through other pathways as well, and that the exact role of neutrophils as the primary source of the protein remains to be fully clarified. However, the evidence is robust enough to suggest that S100A9 is a major player in the inflammatory storm that follows liver surgery. By identifying this protein and the pathway it activates, the researchers have highlighted a potential new target for drugs that could calm the immune response, protecting the liver during the critical moments after blood flow is restored. This work does not offer an immediate cure, but it provides a clear map of a dangerous roadblock in the recovery process, pointing the way toward future treatments that could make liver surgery safer for patients.