By Budi Oetomo Narendra 
Aging, an inescapable biological process, manifests uniquely in each individual, yet its universal impact on health and vitality is profound. New research from Stanford Medicine, leveraging studies in both mice and human cells, has pinpointed a specific breakdown within the immune system that appears to be a major underlying cause of this differential and ultimately universal decline. This groundbreaking discovery offers unprecedented insight into the mechanisms of aging and opens new avenues for therapeutic intervention aimed at extending human healthspan.
The core finding of the Stanford team, led by Katrin Andreasson, MD, the Edward F. and Irene Thiel Pimley Professor in Neurology and Neurological Sciences, is that tissue-resident macrophages—a specialized type of immune cell permanently established within organs—lose their efficiency with age. Specifically, these critical cells become less capable of clearing out another class of immune cells, known as neutrophils, particularly those that have become senescent or dysfunctional. This age-related decline in cellular waste disposal, the researchers found, contributes significantly to systemic aging throughout the body, impacting a wide array of vital organs.
In a remarkable series of experiments, scientists demonstrated that by blocking a single receptor on these tissue-resident macrophages, multiple organs in mice retained more youthful characteristics. The positive effects were observed across diverse systems, including the brain, heart, skeletal and heart muscle, liver, spleen, bone marrow, kidney, and colon. The targeted receptor, known as EP2, normally responds to a hormone called prostaglandin E2 (PGE2), which plays a crucial role in inflammation and pain pathways in both mice and humans.
Disabling the EP2 receptor specifically in tissue-resident macrophages also conferred protection against several age-associated ailments, including chronic inflammation, frailty, excessive fat accumulation, and cardiac issues. Furthermore, cognitive decline was substantially mitigated in these mice. "We’ve been trying to figure out why we age," Dr. Andreasson stated, emphasizing the significance of the findings. "Now we know at least one big reason for it." The detailed results of this extensive study were published in the prestigious journal Science, with Dr. Andreasson as the senior author and Jessy Tan, PhD, an instructor in neurology, as the lead author. This research not only illuminates the critical role of chronic, body-wide inflammation in aging and its related health problems but also paves the way for a potential drug strategy to slow age-related deterioration and enhance the number of years individuals remain healthy and functional.
The Global Imperative of Understanding Aging
The demographic landscape of the 21st century is characterized by an unprecedented increase in the global elderly population. According to the World Health Organization (WHO), the number of people aged 60 years and older is projected to double by 2050, reaching 2.1 billion. This demographic shift presents immense societal, economic, and healthcare challenges. Age-related diseases—including neurodegenerative conditions like Alzheimer’s, cardiovascular diseases, metabolic disorders, certain cancers, and musculoskeletal frailty—impose a massive burden on healthcare systems and diminish the quality of life for millions. Consequently, understanding the fundamental biological processes of aging is not merely an academic pursuit but a global imperative to extend "healthspan"—the period of life spent in good health—rather than just lifespan.
Scientists have long sought to unravel the complex tapestry of aging, identifying various "hallmarks" of the process, such as genomic instability, telomere attrition, epigenetic alterations, loss of proteostasis, mitochondrial dysfunction, cellular senescence, altered intercellular communication, and stem cell exhaustion. The Stanford research adds a crucial piece to this puzzle, spotlighting the immune system’s role as a central orchestrator of systemic aging, particularly through the lens of chronic inflammation, often termed "inflammaging." This low-grade, persistent inflammatory state is now recognized as a significant driver of many age-related pathologies, accelerating the decline of organ function and contributing to overall frailty.
How the Immune System’s First Responders Turn Against the Body
Central to the new findings is the lifecycle and fate of neutrophils, the most abundant type of white blood cell in the immune system. These cells are the body’s rapid-response unit, tirelessly patrolling the bloodstream for bacterial, viral, and fungal threats. Produced in the bone marrow, they are deployed quickly to sites of infection or injury, where they can unleash toxic substances, phagocytose pathogens, and even self-destruct to form web-like traps (neutrophil extracellular traps, NETs) around invading microbes.
However, neutrophils are designed for short, intense bursts of activity. Their lifespan is remarkably brief, typically lasting only 12 to 24 hours. After their mission, or simply after reaching their natural lifespan, roughly 90% of circulating neutrophils make their way to the liver, spleen, and bone marrow, where other immune cells are tasked with their removal. This disposal process is not merely a cleanup operation; it’s a critical mechanism for maintaining immune homeostasis.
As the body ages, this disposal mechanism becomes increasingly vital, yet paradoxically, less efficient. In aging animals and humans, a growing proportion of neutrophils that have not encountered a pathogen rapidly enter a state of senescence. Senescent cells are dysfunctional and undergo a profound change, developing a "senescence-associated secretory phenotype" (SASP), where they release a cocktail of harmful chemicals, including pro-inflammatory cytokines, chemokines, and proteases. These substances damage nearby healthy cells and tissues, creating a localized inflammatory microenvironment that contributes to systemic inflammation. Adding to the problem, neutrophil numbers generally increase with age, meaning there are more of these potentially harmful senescent cells circulating. "Senescent neutrophils are killing our tissues," Dr. Andreasson warned, underscoring the urgency of their efficient clearance to prevent chronic inflammation.
Macrophages: The Body’s Cellular Sanitation Department
The responsibility for clearing this cellular debris largely falls upon macrophages. These highly versatile immune cells are well-known for their diverse functions: fighting pathogens, coordinating immune responses by communicating with other cells, and releasing growth factors that facilitate tissue repair. Critically, they are also the body’s primary scavengers, responsible for removing dead, dying, and dysfunctional cells. "They’re the body’s garbage collection crew. A lot of that garbage is defunct cells," Andreasson explained.
A significant portion of this daily cellular waste consists of neutrophils. With approximately 100 billion neutrophils produced and circulating each day, and many becoming senescent within 8 to 12 hours of entering the bloodstream, the sheer scale of the macrophage’s cleanup task is immense. Even neutrophils that haven’t formally entered senescence but have lived long enough to display "kill me now" signals on their surface are targeted for removal.
Among the various types of macrophages, tissue-resident macrophages are particularly important in this context. These are unusually long-lived cells that colonize organs during fetal development and remain in place throughout an individual’s life. They adapt to perform specialized, tissue-specific functions in their respective locations, making them indispensable for maintaining organ homeostasis. One of their most crucial, and often overlooked, responsibilities is the engulfment and digestion of senescent cells, with the new findings highlighting senescent neutrophils as particularly significant targets.
The challenge, however, is that tissue-resident macrophages themselves are not immune to the ravages of time. Andreasson and her colleagues had previously reported in a 2021 study published in Nature that these long-lived immune cells become increasingly vulnerable to inflammation as animals age. This vulnerability ironically leads them to contribute to the very inflammation they are supposed to mitigate, creating a detrimental feedback loop.
The Inflammatory Signal: EP2 and the Cycle of Decline
A key player in this age-related macrophage deterioration involves prostaglandins, a group of lipid compounds with hormone-like effects, produced by various immune cells. Specifically, prostaglandin E2 (PGE2) can exert diverse effects depending on the specific receptors present on cell surfaces. One such receptor, known as EP2, is particularly pro-inflammatory. Tissue-resident macrophages, unfortunately, contain large quantities of EP2.
PGE2 production naturally increases in response to infection, injury, and exposure to toxic substances, including the metabolic byproducts that accumulate as the body ages. The researchers’ earlier work established that PGE2 levels substantially rise over time. Concurrently, tissue-resident macrophages develop higher concentrations of the EP2 receptor on their surfaces.
These two age-related changes converge to create a harmful feedback process. The escalating PGE2 activity repeatedly stimulates the EP2 receptors on tissue-resident macrophages. The new study conclusively demonstrated that this chronic stimulation progressively weakens the macrophages’ ability to efficiently engulf and dispose of neutrophils. As a direct consequence, senescent neutrophils begin to accumulate in the bloodstream and within various tissues, perpetuating and exacerbating chronic inflammation. Previous research from Andreasson’s team also indicated that the energy metabolism of tissue-resident macrophages gradually deteriorates with age, further impairing their function. "Once that starts, there’s a steady decline in a macrophage’s performance," Andreasson noted. The current work strongly implicates EP2 as a critical mediator of this decline. "We’ve shown that when tissue-resident macrophages don’t have EP2 on their surfaces anymore or when that receptor is plugged up by a drug, this decline doesn’t happen."
Experimental Validation: Blocking EP2 Restores Youthful Organ Function
To meticulously investigate the EP2 receptor’s role, Andreasson’s laboratory engineered mice where the EP2 gene could be precisely deleted at a chosen time, specifically within tissue-resident macrophages. This genetic manipulation allowed the researchers to observe the direct consequences of removing EP2. The results were striking: deleting EP2 effectively restored the macrophages’ ability to dispose of neutrophils, thereby reversing the functional disruption caused by chronic PGE2 stimulation.
The study compared three groups of mice: younger normal mice (aged 6 to 8 months, roughly equivalent to late adolescence or early adulthood in humans), older normal mice (aged 23 to 25 months, comparable to humans in their 60s or 70s), and nearly identical older mice whose EP2 gene had been deleted when they were 4 to 6 months old (their "teenage" years).
The findings were profound. The team identified 71 blood proteins whose levels had significantly altered in normal older mice, reflecting systemic age-related changes. Remarkably, in older mice whose tissue-resident macrophages lacked EP2, 59 of these proteins remained at youthful levels. Many of these proteins originated from the liver, an organ rich in tissue-resident macrophages and a major contributor to age-related changes in blood chemistry and metabolic rate, as Andreasson highlighted.
Normal old mice showed a noticeable accumulation of senescent neutrophils in the liver, spleen, and bone marrow, with smaller increases observed across numerous other organs. In stark contrast, older mice whose tissue-resident macrophages lacked EP2 maintained lower neutrophil levels, mirroring those typically seen in younger animals.
Beyond cellular and molecular markers, the physical manifestations were equally compelling. The EP2-deficient older mice appeared visibly younger, leaner, and more physically fit than their age-matched control counterparts. They exhibited less visceral fat accumulation, greater muscle mass, and their performance on tests measuring the function of various organs matched that of young mice.
Cognitive and Physical Rejuvenation
The benefits extended to cognitive and physical capabilities. Removing EP2 from tissue-resident macrophages significantly reduced inflammation not only in the blood but also in critical organs such as the liver, colon, heart, kidney, and hippocampus—a brain region intrinsically linked to memory and navigation.
Behavioral tests further underscored the remarkable rejuvenation. Older mice without EP2 performed comparably to younger animals on assessments of speed, balance, and forelimb grip strength, indicators of overall physical vitality. Their memory also remained significantly stronger; they navigated mazes and recognized previously encountered objects almost as effectively as young mice, substantially outperforming similarly aged mice whose EP2 receptors were functioning normally. These results suggest a broad protective effect against age-related cognitive decline, a major public health concern.
The Search for a Targeted Therapeutic
While the findings are exceptionally promising, a key challenge lies in developing a safe and selective therapeutic. Currently, no approved drug specifically targets and shuts down EP2 activity without interfering with other crucial biological pathways. Existing nonsteroidal anti-inflammatory drugs (NSAIDs) like aspirin reduce PGE2 production, which explains their effectiveness in reducing pain, fever, swelling, and redness—the classic signs of inflammation. However, these medications broadly inhibit various prostaglandins, many of which perform essential physiological functions. PGE2 itself can also have beneficial effects when it interacts with receptors other than EP2.
Therefore, the scientific community’s goal is to develop a drug that can precisely target the EP2 receptor responsible for the harmful inflammatory response, leaving other beneficial prostaglandin pathways untouched. To assess the viability of such an approach, the Stanford team administered an experimental EP2-inhibiting drug to otherwise normal 22-month-old mice for two months. The treatment yielded encouraging results, bringing both total neutrophil levels and the number of senescent neutrophils in old mice closer to youthful levels. In vitro experiments on cell cultures corroborated these findings, showing that while aging reduced the ability of tissue-resident macrophages to engulf worn-out neutrophils, the EP2-blocking drug significantly restored this critical function.
Translational Promise: Similar Patterns in Human Liver Cells
To bridge the gap between animal models and human biology, the researchers meticulously examined a vast database containing detailed information on different cell types within young, old, and diseased human livers. They discovered patterns remarkably similar to those observed in their mouse models. Older human livers exhibited increased neutrophil accumulation, a greater proportion of senescent neutrophils, evidence of declining tissue-resident macrophage function, and elevated EP2 activity. These detrimental changes were even more pronounced in diseased human livers, suggesting a direct link between this immune system dysfunction and human pathology. According to Dr. Andreasson, this marks the first time these specific age-related changes, involving the EP2 pathway and senescent neutrophil clearance, have been mechanistically observed in human cells, significantly bolstering the translational potential of their findings.
Improving the body’s intrinsic ability to remove aging, dysfunctional neutrophils could eventually offer substantial therapeutic benefits across a spectrum of age-related conditions. The immediate next step, as Andreasson emphasized, is the development of a safe and highly selective drug that specifically blocks EP2 without interfering with earlier processes like PGE2 production or other beneficial prostaglandin signaling. Such a drug could represent a paradigm shift in geroscience, moving beyond symptomatic treatment of age-related diseases to directly address a fundamental mechanism of aging itself.
This pioneering research, which also involved a researcher from the University of Munster in Germany, was supported by significant funding from the National Institutes of Health (grants 1RF1AG080742, 1RF1AG070839, and P30AG066515), the American Heart Association, the Phil and Penny Knight Initiative for Brain Resilience (at the Wu Tsai Neurosciences Institute), Stanford University, the Arc Institute, and the Chan-Zuckerberg Biohub. Part of the experimental work was conducted at the Neurosciences Preclinical Imaging Community Laboratory at the Wu Tsai Neurosciences Institute. The implications of this work are far-reaching, offering a tangible pathway to not just extending life, but more importantly, enriching the quality and health of those extended years.
