UCLA Scientists Eliminate 'Zombie' Immune Cells and Reverse Liver Damage in Mice
Rogue immune cells quietly destroy liver tissue from within. UCLA researchers found a way to remove them — and the results were stunning.
The Hidden Threat Quietly Destroying Your Liver
Deep within aging tissues, a rogue population of immune cells is silently fueling chronic inflammation and driving liver disease. UCLA scientists have identified these dysfunctional cells, developed a way to track them, and — in a breakthrough finding — demonstrated that eliminating them can dramatically reverse liver damage, even when dietary habits remain unchanged.
The study, published in Nature Aging, sheds new light on one of the most pressing public health challenges of our time: the rapid rise of fatty liver disease and the complex role that biological aging plays in its progression.
What Are 'Zombie' Cells — And Why Are They So Dangerous?
At the heart of this research is a process called cellular senescence. When cells experience significant stress, they stop dividing but refuse to die. Instead, they linger within tissues in a kind of suspended state, continuously releasing inflammatory signals that irritate and damage the healthy cells around them.
Scientists have long referred to these as "zombie cells" — neither fully alive in function nor properly cleared from the body. While they were already known to accumulate with age in various tissues, their specific role within the immune system remained hotly debated.
"Senescent cells are fairly rare, but think of them like a broken-down car on the 405," said Anthony Covarrubias, senior author of the study and a member of the Eli and Edythe Broad Center of Regenerative Medicine and Stem Cell Research at UCLA. "Just one stalled car can back up traffic for miles. Now imagine five or ten of them slowly accumulating. That's what these cells do to a tissue: even a small number causes enormous disruption."
A Long-Standing Scientific Debate — Now Resolved
For years, researchers disagreed over whether macrophages — the immune cells responsible for patrolling tissues and clearing cellular debris — could even become senescent. Many believed they could not, partly because healthy macrophages already display certain molecular features that overlap with those found in senescent cells, making accurate identification extremely difficult.
The UCLA team cut through this confusion by pinpointing a precise molecular signature. They discovered that the simultaneous presence of two proteins, p21 and TREM2, reliably identifies macrophages that have become truly senescent — cells that are no longer performing their protective duties but are actively inflaming surrounding tissue.
Armed with this marker, the researchers made a striking observation: in young mice, only around 5% of liver macrophages carried this senescent signature. In older mice, that figure surged to between 60 and 80%, directly mirroring the escalating chronic inflammation associated with aging livers.
Cholesterol: An Unexpected Trigger for Cellular Aging
Aging isn't the only pathway into senescence. The research team also found that elevated cholesterol levels can independently push macrophages into this dysfunctional state.
In laboratory conditions, exposing healthy macrophages to high concentrations of LDL cholesterol caused the cells to stop dividing, triggered the release of inflammatory proteins, and produced the same p21-TREM2 signature identified in aging tissue.
"Physiologically, macrophages can handle cholesterol metabolism," explained Ivan Salladay-Perez, first author of the study and a graduate student in the Covarrubias lab. "But in a chronic state, it's pathological. And when you look at fatty liver disease, which is driven by overnutrition and too much cholesterol in the blood, that excess cholesterol appears to be a major driver of the senescent macrophage population."
This discovery opens a broader and more troubling possibility: diets consistently high in fat and cholesterol may be accelerating biological aging itself — not just in the liver, but potentially in the brain, heart, and fatty tissues throughout the body.
Removing the Cells Reverses the Damage
To determine whether clearing senescent macrophages could produce measurable health benefits, the team administered ABT-263, a drug engineered to selectively destroy senescent cells, to mice fed a high-fat, high-cholesterol diet.
The outcomes were remarkable:
- Liver size dropped from approximately 7% of body weight down to a healthier 4–5%
- Body weight fell by roughly 25%, declining from around 40 grams to 30 grams
- Treated livers appeared visually healthier — smaller and a normal red color — compared to the enlarged, pale yellow livers observed in untreated animals
Critically, none of the mice changed their diet during the trial. The improvements came purely from eliminating the senescent cells.
"That's what wowed me," said Salladay-Perez. "Eliminating senescent cells doesn't just slow the fatty liver — it actually reverses it."
Do These Findings Apply to Humans?
To bridge the gap between mouse models and human medicine, the researchers analyzed an existing genomic dataset drawn from human liver biopsies. The same senescent macrophage signature was found to be significantly elevated in diseased human livers compared to healthy ones — suggesting that the same mechanism may be driving chronic liver disease in people.
This is particularly relevant in Los Angeles, where an estimated 30–40% of residents are affected by fatty liver disease, with even higher rates reported among Latino communities. Effective treatment options remain scarce, and early diagnostic tools are still underdeveloped.
"This is a huge public health crisis in the making," said Covarrubias, who also serves as an assistant professor of microbiology, immunology, and molecular genetics. "We're seeing fatty liver disease in younger and younger people. So we're really happy to make some inroads into understanding what's driving it and identifying cell types we might be able to target."
The Road Toward Human Treatment
Despite its effectiveness in mice, ABT-263 carries a toxicity profile that makes it unsuitable for broad human use. The research team is now actively screening for safer compounds that can selectively eliminate senescent macrophages without producing harmful side effects.
Beyond liver disease, the scientists are investigating whether comparable processes unfold in other age-related conditions. In the brain, for instance, microglia — the macrophages of the central nervous system — may undergo senescence when overwhelmed by the cellular debris associated with Alzheimer's disease.
These findings lend strong support to the geroscience hypothesis, which proposes that a single, shared aging mechanism underlies a wide spectrum of chronic diseases. In this framework, the accumulation of senescent macrophages may serve as a common thread linking fatty liver disease, atherosclerosis, Alzheimer's, and cancer.
"If you really understand the basic mechanisms driving inflammation with aging, you can target those same mechanisms to treat not just fatty liver disease, but atherosclerosis, Alzheimer's and cancer," said Salladay-Perez. "It all goes back to understanding how these cells arise in the first place."
Funding and Research Support
The study received funding from the National Institutes of Health, the Glenn Foundation for Medical Research, the American Federation for Aging Research, and the UCLA-UCSD Diabetes Research Center.
