USP13 stabilizes prohibitin 1 to support CD14 turnover and restrain TLR4-NLRP3 inflammatory signaling in sepsis

When the body faces a severe infection, it must walk a tightrope. It needs to summon a powerful defense to fight the invading germs, but if that defense becomes too aggressive, it can turn on the body itself, causing organ failure and death. This dangerous overreaction is a hallmark of sepsis, a condition where the immune system loses its ability to calibrate its response. At the front lines of this battle are specialized sensors on the surface of immune cells. These sensors act like doorbells, ringing the alarm when they detect signs of bacteria. One such sensor, called CD14, works closely with another, TLR4, to recognize a common bacterial component and trigger the inflammatory response. The body has built-in mechanisms to turn these sensors off once the job is done, preventing the alarm from ringing forever. This process of turning off the sensors involves tagging them for removal, a bit like putting a "discard" sticker on a package so the cell's recycling center knows to break it down. If this recycling system fails, the sensors pile up, the alarm keeps ringing, and the body suffers from uncontrolled inflammation.

Researchers have long known that a protein called prohibitin 1, or PHB1, helps manage this recycling process for the CD14 sensor. However, a new study from scientists at Shanghai Jiao Tong University and Fudan University reveals that PHB1 itself is fragile and needs protection to do its job. The team discovered that another protein, USP13, acts as a guardian for PHB1. When the body is under the stress of sepsis, the levels of this guardian protein drop. Without enough USP13, PHB1 breaks down too quickly. This leaves the CD14 sensors without their manager, causing them to accumulate on the cell surface. The result is a runaway inflammatory signal that damages the lungs and increases the risk of death. The researchers found that restoring the levels of PHB1, even after the guardian protein was missing, could calm the storm and improve survival rates in animal models. This work identifies a specific chain of events that controls how the immune system turns off its own alarm, offering a new understanding of why sepsis spirals out of control.

The journey to this discovery began with a look at human data. The researchers analyzed genetic information from blood samples of fifty-one patients with sepsis and twenty-two healthy people. They found that the gene for USP13 was significantly quieter in the patients with sepsis compared to the healthy group. To ensure this wasn't just a side effect of having too many white blood cells circulating during an infection, they adjusted their analysis, and the drop in USP13 remained clear. They also looked at a small group of elderly patients admitted to a hospital in Shanghai, tracking them from the moment they were diagnosed with sepsis to a week later. In these patients, the levels of USP13 were low at the start but showed signs of recovering as they moved through the early stages of their illness. This pattern suggested that the loss of USP13 was a genuine feature of the disease, not just a random fluctuation.

To understand what happens when this protein is missing, the team turned to laboratory models. They grew human lung cells in dishes and reduced the amount of USP13 they produced. When these cells were exposed to a bacterial toxin known to trigger inflammation, those lacking USP13 suffered much more damage. They produced more harmful molecules called reactive oxygen species, which act like rust inside the cell, and they showed signs of dying much faster than normal cells. The cells also released higher levels of inflammatory chemicals, such as TNF-alpha and IL-6, which are the body's way of shouting for help but can become destructive in excess. These experiments confirmed that without USP13, the cells could not handle the stress of an infection as well as they should.

The scientists then investigated how USP13 protects the cell. They found that USP13 physically binds to PHB1. In the language of cell biology, proteins often carry tiny tags called ubiquitin that mark them for destruction. The team showed that when USP13 is present, it removes these tags from PHB1, effectively saving it from being broken down. Without USP13, PHB1 gets tagged and destroyed much faster. This relationship was proven by showing that a version of USP13 that had lost its ability to cut these tags could no longer protect PHB1. The result was a clear chain of command: USP13 keeps PHB1 stable, and PHB1 is needed to manage the CD14 sensors.

To see if this chain held up in a living animal, the researchers used mice. They injected a virus into the mice's veins to lower the levels of either USP13 or PHB1. When these mice were later given a dose of bacterial toxin to mimic sepsis, they died at much higher rates than normal mice. Their blood was filled with high levels of inflammatory chemicals, and their lungs showed signs of severe stress and cell death. This proved that losing either part of the pair made the animals much more vulnerable to the infection.

The most critical part of the study was testing whether the damage caused by losing USP13 could be fixed by restoring PHB1. The researchers created mice that lacked USP13 but then gave them a dose of PHB1 using a viral vector. These mice survived the sepsis challenge at much higher rates than the mice that lacked both proteins. Their levels of inflammatory chemicals dropped, and their lungs looked healthier. To make the findings even more relevant to real-world treatment, they also tested a version of PHB1 that could be injected directly into the body, rather than relying on genetic engineering. When they gave this protein to mice that had already been silenced for USP13, it still reduced inflammation and improved survival, even when given hours after the infection started. This suggested that the problem caused by the missing guardian could be solved by simply replacing the worker it was supposed to protect.

The team also explored how this mechanism worked in a more complex model of sepsis, one that involved injecting a mixture of human fecal bacteria into the mice's abdomen. This mimics a real-life infection where multiple types of bacteria are involved, rather than just a single toxin. The results were the same: mice lacking USP13 or PHB1 suffered more, and restoring PHB1 saved them. Furthermore, they showed that the relationship between these proteins was strictly one-way. Restoring PHB1 fixed the problems caused by a lack of USP13, but adding extra USP13 could not fix the problems caused by a lack of PHB1. This confirmed that PHB1 sits directly below USP13 in the hierarchy of control.

Finally, the researchers traced the effect all the way to the CD14 sensors. They found that when PHB1 was missing, the CD14 sensors were not being tagged for removal. Instead, they piled up on the surface of the cells. This accumulation meant the cells were hyper-sensitive to bacterial signals, leading to the excessive inflammation that killed the animals. By restoring PHB1, the tagging process returned to normal, the sensors were cleared away, and the inflammatory response was brought back under control.

This study does not offer an immediate cure for sepsis, nor does it claim that giving patients PHB1 is a ready-made therapy. The researchers are careful to note that they have not yet determined the best dose, the timing, or the long-term safety of such a treatment in humans. However, the work provides a clear map of a broken pathway. It shows that in sepsis, the body loses a protective protein that keeps another protein stable, which in turn allows the immune system's sensors to run wild. By identifying this specific link, the study offers a new target for future research, suggesting that therapies designed to stabilize this chain of proteins might one day help the body regain control over its own defenses.