UCLA researchers have identified a class of malfunctioning immune cells that accumulate with age and promote fatty liver disease, then demonstrated they could reverse liver damage by selectively removing these cells in mice, even when the animals continued eating an unhealthy diet.
The cells in question are senescent macrophages, immune cells that have stopped dividing but remain metabolically active, continuously releasing inflammatory signals into surrounding tissue. The phenomenon is rooted in cellular senescence, a stress response in which cells enter a state of permanent arrest.
"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 published in Nature Aging. "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."
For years, scientists debated whether macrophages could actually become senescent. Healthy macrophages naturally display some of the same molecular markers seen in senescent cells, creating confusion about how to distinguish between normal function and pathological dysfunction.
The UCLA team solved the problem by pinpointing a specific molecular signature: the combination of two proteins called p21 and TREM2. When both proteins appear together on macrophages, they reliably identify cells that are truly senescent and no longer functioning properly while still driving inflammation.
In young mice, only about 5% of liver macrophages carried this senescent signature. In older mice, that figure jumped to between 60 and 80%, matching the increase in chronic liver inflammation observed with age.
The team discovered that excess cholesterol plays a major role in triggering this transformation. When healthy macrophages were exposed to high levels of LDL cholesterol in laboratory conditions, they stopped dividing, began releasing inflammatory proteins, and displayed the p21-TREM2 signature.
"Physiologically, macrophages can handle cholesterol metabolism," said Ivan Salladay-Perez, the study's first author. "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 finding raises the possibility that high-fat, high-cholesterol diets may accelerate biological aging by promoting macrophage senescence not only in the liver but also in other organs including the brain, heart, and fat tissue.
To test whether removing these cells could restore health, researchers treated mice with ABT-263, a drug designed to selectively kill senescent cells. The results were striking. In mice fed a high-fat, high-cholesterol diet, liver size dropped from approximately 7% of body weight to a healthier 4-5%. Body weight fell by about 25%, declining from roughly 40 grams to around 30 grams.
Treated livers appeared smaller and healthier with a normal red color, in sharp contrast to the enlarged, yellowish livers in untreated animals. The improvement occurred despite the mice continuing to consume the unhealthy diet.
"That's what wowed me," Salladay-Perez said. "Eliminating senescent cells doesn't just slow the fatty liver, it actually reverses it."
To determine if the findings translate to human disease, researchers analyzed genomic data from human liver biopsies. The same senescent macrophage signature appeared significantly higher in diseased livers compared to healthy ones, suggesting the mechanism operates in people as well.
The public health implications are substantial. In Los Angeles alone, an estimated 30-40% of residents are affected by fatty liver disease, with even higher rates among Latino communities. Treatment options remain limited, and early detection tools are inadequate.
"This is a huge public health crisis in the making," Covarrubias said. "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."
Although ABT-263 produced dramatic results in mice, the compound is too toxic for routine human use. The research team is now screening for safer alternatives that can selectively eliminate senescent macrophages without unacceptable side effects.
The findings support the geroscience hypothesis, which proposes that a single underlying aging mechanism can drive multiple chronic diseases. Senescent macrophage accumulation may contribute to conditions ranging from fatty liver disease to atherosclerosis, Alzheimer's disease, and cancer.
Scientists are also investigating whether similar processes occur in other age-related conditions. In the brain, for instance, microglia, the macrophages of the central nervous system, may become senescent in Alzheimer's disease as they encounter accumulating cellular debris.
The research was supported by the National Institutes of Health, the Glenn Foundation for Medical Research, the American Federation for Aging Research, and the UCLA-UCSD Diabetes Research Center.
Author Jessica Williams: "If this pans out in humans, it could reshape how we approach liver disease and aging itself, but the path from mouse livers to safe human treatment is long."
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