Zebrafish Study Finds Immune Cells That Help Damaged Spinal Cords Regrow

Neutrophils, the immune cells that arrive earliest at an injury site, have long been seen mainly as a cleanup crew that removes debris. New research led by Prof. Thomas Becker's team found that a specific subgroup of neutrophils plays a far bigger role, helping steer the immune response away from damaging inflammation and toward conditions that support regeneration, using a signalling molecule called Il-4.

The researchers studied larval zebrafish, which are able to regenerate damaged spinal cords, and examined what happens when these neutrophils and Il-4 are active at an injury site. When they inactivated this group of neutrophils, the immune response became unbalanced: other immune cells produced excessive amounts of inflammatory proteins, and the zebrafish were unable to properly regrow nerve fibres or recover their movement. When the scientists instead added Il-4 directly to the injured area, inflammation subsided and the spinal cords regenerated fully, even though the neutrophils themselves were no longer present.

"For the first time, we have shown that neutrophils play a massive, active role in successfully repairing a spinal cord," said Becker. "They aren't just there to clear away debris; they act like conductors that tell other immune cells to return to a harmonious rhythm. Without them, the immune system locks into a destructive cycle and prevents healing. By using the Il-4 molecule, the neutrophils smooth out the inflammation, allowing the delicate nerve fibers to grow right through the injury zone."

Understanding why zebrafish can recover from spinal cord injuries while humans generally cannot is one of the major challenges in regenerative medicine; in people, the immune response after central nervous system damage can contribute to lasting injury rather than regeneration. The study, conducted by Xiaobo Tian with an international team at the Center for Regenerative Therapies Dresden at TU Dresden and the Centre for Discovery Brain Sciences at the University of Edinburgh, was funded by the Chinese Scholarship Council and the Alexander-von-Humboldt Foundation. Tian said it is not yet known whether Il-4 plays a similar role in humans or whether it could help balance inflammation for better healing, calling it a promising avenue for future human studies.