We may age at different rates, but none of us escapes aging. A study in mice and in human cells by Stanford Medicine researchers has linked organ aging to the increased inability—with advancing age—of tissue resident macrophage (TRM) immune cells to clear aged neutrophils, another type of immune cell.
The study found that these TRMs appear to be central coordinators of age-related organ decline. Blocking a single receptor, EP2, on these cells preserved the youthfulness of multiple organs in mice, including the brain, heart, skeletal and heart muscle, liver, spleen, bone marrow, kidney, and colon. The receptor binds specifically to a hormone, prostaglandin E2, which is known to cause inflammation and pain in humans as well as in mice.
The researchers found that in mice, selectively disabling this receptor exclusively on tissue-resident macrophages genetically, or using an experimental selective EP2 antagonist drug, prevented chronic-inflammation-driven disorders of age—including frailty, excessive fat accumulation, and heart trouble—and also substantially slowed cognitive decline.
Research lead Katrin Andreasson, MD, the Edward F. and Irene Thiel Pimley Professor in Neurology and Neurological Sciences, said, “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 … We’ve been trying to figure out why we age. Now we know at least one big reason for it.”
The discoveries help to clarify systemic inflammation’s significant contribution to aging and the debilities that accompany it. The findings also point to a pharmaceutical approach that could restrain our organs’ unavoidable march toward senescence and so extend overall health span.
Senior author Andreasson, together with first author Jessy Tan, PhD, an instructor in neurology, and colleagues reported on their findings in Science, in a paper titled “Restored clearance of senescent neutrophils by tissue-resident macrophages limits organ aging.” In their research article summary, the team stated, “This work identifies EP2 signaling in TRMs as a central regulator of organ-wide aging through its control of senescent neutrophil clearance, reframing aging as a failure of active cellular clearance rather than passive degeneration.”
Aging is accompanied by parallel functional decline across organs, but the cellular drivers remain unclear, the authors wrote. “Although molecular hallmarks of aging have been identified, the cellular events that initiate and propel tissue decline remain poorly defined.”
The most abundant white blood cells in our immune system are neutrophils, which act as the body’s main first responders. Produced in bone marrow, new neutrophils are transferred to the bloodstream, where they circulate and attack bacterial, viral, or fungal pathogens that they encounter. Neutrophils are also extremely short-lived, surviving just 12–24 hours.
Some 90% of circulating neutrophils end up in the liver, spleen, and bone marrow, awaiting execution clearance by another type of immune cell. “Neutrophils are among the shortest-lived immune cells, aging within hours in the circulation and requiring continuous clearance,” the team noted.
This neutrophil clearance is critical. In aged animals, the vast bulk of neutrophils that never see combat undergo a fast transition to senescence, a zombie-like state in which they may injure, age, and inflame neighboring cells. And as we age, the neutrophil count rises, with senescent neutrophils constituting an ever higher percentage. “Senescent neutrophils are killing our tissues,” Andreasson said. “Clearance of these cells is essential for preventing chronic inflammation.”
That’s a job for macrophages. These cells comb the tissues for pathogens, signal other cells to lend a hand in the fight, and pump out growth factors that help repair damaged tissue. But first and foremost, Andreasson said, “They’re the body’s garbage collection crew. A lot of that garbage is defunct cells.” And a lot of those cells are neutrophils—to the tune of 100 billion a day.
Macrophages come in several subtypes. Tissue-resident macrophages are long-lived and ubiquitous. They take up residence in each of the body’s organs during fetal development and remain for their lifetimes in whatever organ they’ve inhabited, adapting their roles to fit that organ.
One of tissue-resident macrophages’ prime responsibilities is to swallow senescent cells. “A core TRM function is efferocytosis, the clearance of apoptotic, senescent, and damaged cells that is essential for preventing chronic inflammation,” they explained. Especially important targets for this operation, the study showed, are the potentially 100 billion neutrophils produced daily, which start showing signs of senescence within 8 to 12 hours after entering the bloodstream. (Neutrophils that haven’t arrived at senescence yet but have lived long enough and seen enough to put out “kill me now” flags of surrender on their cell surfaces are fair game.)
“Among primary TRM targets are neutrophils, the most abundantly produced immune cell, with more than 10 billion and 100 billion generated daily in mice and humans, respectively,” the investigator noted. “Uncleared aged neutrophils release proteases and extracellular traps that damage tissues, propagate inflammation, and promote aging, and are normally removed efficiently by TRMs in the liver, spleen, and bone marrow.”
But tissue-resident macrophages also grow old. As Andreasson and associates showed in a prior study, over the advancing years these long-lived cells become ever more prone to succumb to aging-associated inflammation and to propagate it. In their newly reported paper, they noted, “TRMs comprise 60–90% of macrophages in the brain, liver, lungs, heart, and kidneys, and their long lifespan makes them particularly vulnerable to aging, as they accumulate metabolic, oxidative, and inflammatory injury over years to decades.”
Immune cells produce hormones called prostaglandins. One of the five varieties of prostaglandin, called PGE2, can exert diverse effects on a cell, depending on which type of surface receptor is expressed on that cell’s surface. Of the various subtypes of receptors for PGE2, the EP2 receptor is highly pro-inflammatory. Tissue-resident macrophages are loaded with EP2.
Infection, injury, and toxic chemicals, including those produced by our aging bodies, increase PGE2 output. As the team’s prior work showed, that output grows substantially as we grow older. So does the concentration of EP2 on tissue-resident macrophages. “TRMs express the prostaglandin E2 (PGE2) receptor EP2, which suppresses macrophage metabolism and phagocytosis in aging,” the investigators noted.
This effectively creates a one-two punch. PGE2’s pro-inflammatory influence increases with age. The resulting unrelenting inflammatory PGE2 stimulation on tissue-resident macrophages, the new study showed, downshifts these cells’ ability to clear neutrophils. Senescent neutrophils then accumulate in tissues and blood.
Andreasson and her colleagues had previously shown that with aging, tissue-resident macrophages undergo a slow decay in their energy metabolism. “Once that starts, there’s a steady decline in a macrophage’s performance,” she said.
For their newly reported study, Andreasson’s lab bioengineered a mouse in which, at a time of the scientists’ choosing, the EP2 gene gets deleted—but only in tissue-resident macrophages. The results of their experiments showed that disappearance of EP2 from these cells reinvigorated the neutrophil-clearance process that PGE2 undermines.
For their experiments, the Stanford Medicine researchers studied younger normal mice, aged 6–8 months, which corresponds to late adolescence or early adulthood in humans, and they also studied older normal mice, at 23 to 25 months of age, whose human counterparts would be in their 60s or 70s. They also looked at older mice whose EP2-encoding gene had been deleted at 4 to 6 months of age (equivalent to their “teenage” years).
The team’s analyses identified 71 proteins, found in blood, whose levels were significantly altered in older normal mice. Of those proteins, 59 stayed at youthful levels in older mice whose tissue-resident macrophages lacked EP2. Many of these proteins originated in the liver. “The liver is one of the body’s most tissue-resident-macrophage-enriched organs and a major contributor to aging-related changes in blood chemistry,” Andreasson said. “It’s the central organ determining the body’s metabolic rate.”
The study showed that in normal old mice, smoldering senescent neutrophils accumulated in the liver, spleen, and bone marrow and, to a lesser extent, in many other organs the researchers looked at.
But the organs of older mice lacking EP2 on their tissue-resident macrophages retained the lower neutrophil numbers of youth. These mice looked younger, leaner, and more physically fit compared with control littermates. They evidenced less visceral fat and greater muscle mass. Their performance on tests of multiple organs’ function equaled that of young mice.
EP2 deletion in addition reduced inflammation in the blood, liver, colon, heart, kidney, and hippocampus (a brain region tightly tied to memory and navigation ability) in the older mice. Their speed, balance, and forelimb grip strength resembled that of young animals.
Reducing EP2 activity in older mice also preserved their memory capabilities. These animals could thread their way through a maze or recall previously encountered objects almost as well as younger mice—and far better than similarly old mice with tissue-resident macrophages expressing functional EP2. “Reducing TRM EP2 signaling in aged mice preserved youthful mitochondrial fitness and prevented cognitive decline, frailty, sarcopenia, adiposity, cardiac impairment, and systemic inflammation,” they wrote in summary.
There are, today, no approved drugs that selectively shut down EP2 activity, although there are several that target PGE2. Non-steroidal anti-inflammatory painkillers work by blocking PGE2 production, Andreasson said. That’s how aspirin and similar drugs reduce pain, fever, swelling, and redness. But to greater or lesser degrees these drugs all block other vital prostaglandins. Even PGE2 has beneficial properties when it binds to receptors other than EP2, rather than the detrimental inflammatory one examined in this study.
As part of their study, the investigators treated otherwise normal 22-month-old mice for two months with an EP2-inhibiting experimental drug. The results showed that the treatment reduced total and senescent neutrophil counts in old mice toward youthful levels. In culture dishes, old age diminished—but the EP2-blocking drug likewise significantly restored—the mice’s tissue-resident macrophages’ ability to engulf and digest burnt-out neutrophils. “Together, these results demonstrate that pharmacologic EP2 inhibition partially reverses age-associated TRM dysfunction and senescent neutrophil accumulation, with strongest rescue in the liver,” they stated.
Finally, the team turned to a large human database characterizing different cell types in young, old, and diseased human livers. This database revealed the same age-related neutrophil buildup, increased neutrophil senescence, tissue-resident-macrophage decline, and heightened EP2 activity in older—and even more so, diseased—livers that the Stanford Medicine researchers had seen in mice. This was a first-time observation in human cells, according to Andreasson. “These human findings, while correlative, position the TRM EP2-efferocytosis axis as a candidate mechanism in human aging that warrants further functional testing,” they noted. “Specifically, future studies should assess whether the impaired clearance of senescent neutrophils also occurs in human TRMs and whether pharmacological EP2 blockade can restore this defect.”
Andreasson suggested that targeting neutrophil clearance may yield big therapeutic benefits. “We need to develop a safe drug that incapacitates EP2 without disrupting upstream events such as PGE2 production.”
