For over thirty years, a stubborn biological puzzle sat unsolved.
Scientists knew the immune system could turn on its own tissues. They just didn't understand how it stopped itself from doing so. Recently, that gap finally closed. Researchers uncovered a surprising twist in how our defenses work.
The same chemical signal that summons white blood cells to fight infection also shields healthy cells from collateral damage.
It is both the alarm and the armor. This revelation emerged from Memorial Sloan Kettering Cancer Center (MSK) in New York City.
The team there made a choice that many of their peers would have questioned. They skipped the mice. Instead, they turned to zebrafish. That single decision cracked open a mystery that had resisted investigation for decades.
Their findings appeared October 7 in the journal Nature. The implications reach far beyond basic biology.
Cancer research stands to benefit. So does work on inflammatory bowel disease and asthma. Most importantly, the discovery opens a door to therapies that boost the body's own protective mechanisms rather than simply suppressing them.To appreciate why this matters, you have to go back to 1992.
That year, scientists at McGill University were tracking a molecule called 5-oxoETE. Its full chemical name is a mouthful: 5-oxo-6,8,11,14-eicosatetraenoic acid.
Think of it as a fatty distress flare. When your body detects trouble, this molecule goes up.
It tells immune cells where to go. It says, essentially, "come here, there's a threat."
During those early studies, researchers noticed something strange. An enzyme seemed to control 5-oxoETE levels by both creating and destroying it. Enzymes are molecular machines that speed up chemical reactions. This one was clearly important.
But when the team tried to find the human gene responsible for building it, they hit a wall. The gene simply wasn't there in any database anyone could access. It was invisible.
Without the genetic blueprint, progress stalled completely. You can't tweak what you can't locate.
You can't test whether its failure causes disease if you don't know which gene to switch off in an experiment.
So scientists gave the enzyme a functional label based on what it did chemically. But its true identity remained hidden.
More than three decades passed. The mystery gathered dust while other fields raced ahead. Then came Yanan Ma, a postdoctoral researcher at MSK's Sloan Kettering Institute.
Ma built on groundwork laid by King Lam Hui, a former colleague in the same lab. Together, their efforts pinpointed the missing gene.
She works under Philipp Niethammer, a cell biologist who studies how cells handle injury, infection, and stress.
His group focuses on inflammation, tissue repair, and regeneration. These processes matter enormously in cancer and countless other conditions.
Here's the basic problem the immune system faces. When bacteria invade, chemical signals call specialized white blood cells to the scene. 5-oxoETE is one of those calling cards. It draws in cells armed to destroy microbes.
But those weapons aren't precision instruments. The toxic compounds immune cells unleash to kill pathogens also harm nearby healthy tissue.
"They flood the site with toxic substances to kill the bacteria," Dr. Niethammer explains. "But this can cause damage to healthy cells if it gets out of control."
His lab had spent years mapping the molecular players in 5-oxoETE signaling. Piece by piece, the picture came together.
Except for one glaring blank space. The unknown enzyme.
Most biomedical research relies on mice or rats. Their biology overlaps substantially with ours. But for this particular question, those familiar models had a fatal flaw.
"Mice and rats are the primary model used in biomedical research," Dr. Ma notes. She earned a prestigious Marie-Josée Kravis WISE Fellowship in 2023 for this pathway work. "It makes sense they're mammals, and there's about an 80% overlap between their genes and ours."
Yet mice lack a known receptor that recognizes 5-oxoETE. Without that receiver, the whole signaling conversation can't happen.
You literally cannot study the complete pathway in a mouse. The circuit is broken from the start.
Zebrafish kept the machinery intact. Every component still connects properly.
"This means that you can't study the pathway in mice," Dr. Ma says. "But the pathway is intact in fish. So picking the right alternative model was critical to advancing our understanding of how the human body responds."
Working with living fish let the team watch the signal operate in real time inside a functioning organism. Before moving to zebrafish, though, Ma and Hui started with cultured human cells. They needed to find the gene first.
They picked several candidate genes that might encode the mystery enzyme. Then they systematically silenced each one. One produced an unmistakable result. When DHRS7 was disabled, enzyme activity vanished entirely.
That was the answer. DHRS7 was the gene researchers had hunted since the early nineties.
Next, they tested the rodent version of DHRS7. Almost no enzyme activity showed up. This matched perfectly with the missing receptor in mice. Rodents apparently lost key pieces of this signaling system during evolution.
Finding DHRS7 solved the old puzzle. But it also linked Ma's work to a parallel project happening in the same lab. Miklos Lengyel, another postdoctoral researcher with an MD and PhD, was studying colitis in zebrafish.Colitis is inflammation of the colon. His approach involved knocking out selected genes to see how intestinal inflammation changed.
One knockout produced a baffling outcome. Lengyel removed hcar1-4, the zebrafish equivalent of the human OXER1 gene. This gene builds the receptor that detects 5-oxoETE. Logic dictated that removing the receiver should dampen the response. Less signal detection, less inflammation. Simple. "So if you remove the receptor that detects the signal, you'd expect less inflammation, not more," Dr. Lengyel says. The fish developed spontaneous intestinal inflammation instead. No infection present. No external trigger.
The team first suspected neutrophils, the white blood cells normally recruited by this pathway. Maybe they were acting independently.
Eliminating those cells didn't stop the inflammation either. Something else was going on. The pathway wasn't just activating immunity. It was doing something far more subtle.
The two projects converged once Ma confirmed DHRS7's identity. Lengyel then knocked out DHRS7 in zebrafish. Again, the expectation was reduced inflammation. Again, the result was the opposite. Inflammation spiked.
"It made absolutely no sense," Dr. Lengyel admits. "I had to prove using several different methods before the rest of the lab was convinced." The data pointed toward a previously unrecognized role. The pathway didn't merely kick-start immune responses. It actively prevented unnecessary harm to healthy cells. Two jobs, one system. Removing the pathway stripped away both functions simultaneously. Scientists accidentally deleted protection along with activation. Further experiments revealed the mechanism behind that protection. When immune cells attack microbes, they release highly reactive chemicals. Those chemicals injure neighboring healthy cells as collateral damage. The 5-oxoETE pathway helps tissues tolerate this chemical assault. It reduces the risk that your own defenses become self-destructive.
"This makes sense when you think about it," Dr. Niethammer reflects. "These immune cells unleash chemical warfare against a pathogen, and organisms have evolved an elegant way of protecting healthy cells at the same time."
Zebrafish intestinal cells became especially fragile without 5-oxoETE or its Hcar1-4 receptor. Even ordinary gut bacteria generated enough low-level chemical stress to cause real problems when the shield was gone.
The team traced the cellular defense to a family of enzymes called NUDIX hydrolases. These enzymes clean up damaged DNA building blocks before those fragments can disrupt normal cell function. Strip away the protective signaling, and intestinal cells couldn't withstand routine bacterial stress. Some died. Their death triggered inflammation without any pathogen involved. The body uses one molecular system to summon defenders and protect bystanders.
"As it turns out, this signal isn't just an alarm, it's also a shield," Dr. Niethammer summarizes. This insight carries weight for diseases driven by runaway or poorly regulated inflammation. The 5-oxoETE pathway is already known to malfunction in cancer, IBD, and asthma. Now researchers see how its loss might worsen tissue damage specifically. In human tumors, mutations hitting DHRS7 and OXER1 appear in gastrointestinal and uterine cancers.
Studies also connect dysfunctional OXER1 to worse outcomes in colorectal, pancreatic, and kidney cancer patients. Boosting the pathway might help healthy tissues endure hostile environments better. One promising application involves shielding the gut during cancer treatment.
"More research is needed, but there's a possibility that activating the pathway could increase the resilience of the gut to oxidative stress  which might help counteract damage caused by radiation therapy or chemotherapy," Dr. Lengyel says.
The discovery extends to mucosal tissues broadly. These protective linings coat the intestines, airways, and other surfaces exposed to the outside world. People with IBD show lower 5-oxoETE levels in intestinal tissue. Those lower levels correlate with poorer disease trajectories.
Asthma research offers another clue. A separate primate study found that blocking OXER1 weakened immune recruitment but also eliminated protective mucus-producing cells in airways.