Scientists have uncovered a hidden immune signal that might explain why zebrafish can regenerate their spinal cords so effectively. Researchers discovered that certain neutrophils — a type of immune cell — release a molecule called Il-4, which reduces harmful inflammation and allows damaged nerve fibers to regrow. In experiments, when these cells were missing, healing stopped, but adding Il-4 back restored the ability to regenerate. Scientists are now exploring whether this mechanism could one day be used to improve spinal cord repair in humans. The immune system uses many different types of cells to respond to injuries. Neutrophils are among the first to arrive at the site of damage, and they have traditionally been seen as cleanup cells that remove debris. However, new research led by Prof. Thomas Becker shows that a specific subgroup of neutrophils plays a more active role in the healing process. These cells appear to help coordinate the immune response, shifting it from a damaging inflammatory state to one that supports tissue repair. A key player in this process is the molecule Il-4. To study how neutrophils influence healing, researchers used larval zebrafish, which have the remarkable ability to regenerate damaged spinal cords. They observed what happened when these immune cells and the Il-4 molecule were active at an injury site. When the researchers inactivated this group of neutrophils, the immune response became unbalanced. Other immune cells began producing excessive amounts of inflammatory proteins, leading to an uncontrolled reaction. As a result, the zebrafish were unable to regrow nerve fibers and had trouble recovering movement. However, when scientists added Il-4 directly to the injured area, inflammation decreased, and the spinal cords regenerated fully, even without the presence of the neutrophils. "For the first time, we have shown that neutrophils play a major role in successfully repairing a spinal cord," says Prof. Thomas Becker, who led the study. "They are not just clearing away debris; they act like conductors, guiding other immune cells to restore balance. Without them, the immune system becomes destructive and prevents healing. By using the Il-4 molecule, the neutrophils reduce inflammation, allowing nerve fibers to grow through the injury site." One of the biggest challenges in regenerative medicine is understanding why zebrafish can recover from spinal cord injuries, while humans typically cannot. In people, the immune response after damage to the central nervous system often worsens the injury instead of supporting nerve regeneration. Zebrafish offer a valuable model for studying what makes regeneration possible. The findings suggest that controlling inflammation carefully is essential for healing and highlight how specific immune signals, given at the right time, can create conditions that allow nerve fibers to regrow. Researchers now want to determine if a similar process could work in humans. "Of course, the question is how much our results apply to humans," says Xiaobo Tian, who conducted the study. "It remains to be seen if Il-4 plays a similar role in humans and whether it can balance inflammation to allow better healing at the injury site." The study was led by Xiaobo Tian and an international team of scientists at institutions including the Center for Regenerative Therapies Dresden and the University of Edinburgh. It was supported by the Chinese Scholarship Council and the Alexander-von-Humboldt Foundation.