Rogue immune cells, not just cleanup crews, turn out to run spinal cord repair in zebrafish

Researchers in Germany and Scotland have identified a small population of immune cells that appears to determine whether a severed spinal cord heals or scars over, a finding drawn from zebrafish that can regrow their own spinal cords within weeks of injury. The study, led by scientists at the Center for Regenerative Therapies Dresden (CRTD) at TU Dresden and the University of Edinburgh, singles out a subset of neutrophils, the short-lived white blood cells usually associated with the earliest, most generic response to injury, as active managers of the inflammation that follows spinal trauma rather than mere first responders that show up and disperse.

The work, published in the Journal of Neuroinflammation, found that this neutrophil subgroup secretes a signaling protein called interleukin-4, or Il-4, at the site of a spinal cord lesion. When the researchers selectively eliminated these cells in larval zebrafish, other immune cells, chiefly macrophages and microglia, ramped up production of inflammatory proteins and the injury site became locked in a prolonged, damaging inflammatory state. Nerve fibers failed to regrow properly across the lesion, and the fish struggled to recover normal swimming movements.

The more striking result came when the team bypassed the neutrophils entirely. According to the university's account of the findings, supplying Il-4 directly to the wound site restored the animals' regenerative capacity even in the continued absence of the reparative neutrophils. Inflammation subsided, and the spinal cords healed and reconnected as they would in an unmanipulated fish. That result indicates Il-4 itself, rather than the neutrophils that normally deliver it, is the operative signal that tips the injury site from a destructive inflammatory loop toward repair.

What the numbers show

The study used larval zebrafish, a standard model organism in regeneration research because, unlike mammals, they can regrow severed spinal cord tissue and largely restore function within roughly two to three weeks of a lesion. The research team, led by Thomas Becker's group at the CRTD, tracked immune cell behavior at the injury site using genetic and imaging tools that let them identify, and selectively remove, the Il-4-producing neutrophil subpopulation while leaving other neutrophils intact.

• Removing the Il-4-producing neutrophils led to excess production of inflammatory cytokines by macrophages and microglia at the lesion.
• Fish lacking these neutrophils showed impaired nerve fiber regrowth across the injury and reduced recovery of swimming behavior.
• Direct application of Il-4 to the wound restored full anatomical and functional regeneration even without the neutrophils present.
• The authors describe Il-4 as necessary and sufficient for the neutrophils' reparative effect, rather than one factor among several.

The paper's senior authors are Thomas Becker and Catherina G. Becker, who jointly run research groups spanning Dresden and Edinburgh; the study's other authors include Xiaobo Tian, Alberto Docampo-Seara, Kim Heilemann, Friederike Kessel, Daniela Zöller and Anja Bretschneider.

Why inflammation cuts both ways

Spinal cord injury in humans and other mammals sets off an inflammatory response that is necessary in its early stages, to clear debris and fight infection, but that frequently persists too long or too intensely, contributing to the scar tissue and secondary cell death that block any chance of nerve regrowth. Decades of work on why mammals fail to regenerate spinal tissue has focused heavily on this problem: the same immune system that initiates healing after a cut also appears to entrench the damage after a severe spinal injury, and clinical attempts to simply suppress inflammation broadly have had limited success in restoring function.

Zebrafish and other fish species do not have this problem to the same degree, and researchers at the CRTD, working with the Edinburgh group on this and earlier projects, have spent years mapping which immune signals help and which hinder repair in a species that manages to do what mammals cannot, including a 2018 study on how macrophages regulate pro- and anti-regenerative cytokines after a spinal lesion and, this year, the identification of a separate microglial protein, Sema4ab, that suppresses new neuron formation after injury. The new findings add neutrophils, previously seen mainly as blunt first responders, to that list of fine-tuned regulators, and pin the effect specifically on Il-4 rather than on the presence of the cells themselves.

Who this matters to

The immediate audience for the finding is the basic-research community working on spinal cord injury and neuroinflammation, along with clinicians and patients for whom no therapy currently reverses paralysis after a severe spinal injury. An estimated 300,000 people are living with spinal cord injuries in the United States, according to figures compiled by the National Institute of Child Health and Human Development, and existing treatment options are limited to stabilizing the injury and managing complications rather than restoring lost nerve connections.

Whether the zebrafish finding has any direct bearing on those patients is, at this stage, an open question rather than a promise. Mammals possess Il-4 and the immune pathways it acts on, but whether the same neutrophil-to-macrophage signaling relay operates at a human injury site, and whether boosting Il-4 there would help rather than cause unintended side effects, has not been tested.

"For the first time, we have shown that neutrophils play a massive, active role in successfully repairing a spinal cord. 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," said Thomas Becker, who led the study at the CRTD.

Xiaobo Tian, the study's first author, was more measured about extending the result to people. "Of course, the question is to what extent our results apply to humans," Tian said, according to the university's release. "It remains to be seen if Il-4 plays a similar role in humans and whether it can finely balance the inflammation, allowing for better healing at the injury site. It is definitely a very promising avenue for future studies in humans."

Where the research goes from here

The Dresden and Edinburgh groups say their next step is to test whether the same neutrophil-Il-4 relay, or something like it, is present at injury sites in mammalian models, where inflammation is known to behave differently and where scar tissue, rather than regenerated nerve fiber, is the typical outcome. Comparing how immune signaling diverges between a species that regenerates and one that does not is a strategy other labs have used successfully in related zebrafish work, including the broader program of neuroinflammation research at the CRTD, and the authors frame Il-4 delivery as one of several immune-signaling targets now worth testing in mammalian injury models rather than a therapy ready for trial.

Any path toward a human application would still require showing that adding or boosting Il-4 at an injury site is safe, that it reaches the right cells at the right concentration, and that it produces a comparable shift from destructive to reparative inflammation in mammalian tissue, none of which has yet been demonstrated. The researchers describe the result as a mechanistic clue about how a regenerating vertebrate keeps its own immune response in check, rather than a demonstration that the same trick would work in a person with a spinal cord injury.