Neutrophils are the most abundant white blood cells in the immune system and serve as some of the body’s most important first responders. They are produced in bone marrow and then enter the bloodstream, where they patrol for bacterial, viral, and fungal threats.
The researchers found that tissue-resident macrophages, a type of immune cell that lives permanently within organs, become less able with age to dispose of another class of immune cells. That decline appears to contribute to ageing throughout the body.
When the scientists blocked a single receptor on these macrophages, multiple organs in mice retained more youthful characteristics. The effects were observed in the brain, heart, skeletal and heart muscle, liver, spleen, bone marrow, kidney, and colon. The receptor normally responds to a hormone involved in inflammation and pain in both mice and humans.
Disabling the receptor specifically in tissue-resident macrophages also protected mice from several problems associated with chronic inflammation and ageing, including frailty, excess fat accumulation and heart trouble. Cognitive decline was substantially reduced as well, according to Katrin Andreasson, MD, the Edward F. and Irene Thiel Pimley Professor in Neurology and Neurological Sciences.
“We’ve been trying to figure out why we age,” Andreasson said. “Now we know at least one big reason for it.”
The findings appear in a paper published in Science. Andreasson is the senior author, and Jessy Tan, PhD, an instructor in neurology, is the lead author.
The results provide new insight into the important role that chronic, body-wide inflammation plays in ageing and its associated health problems. They also point toward a potential drug strategy that could slow age-related deterioration in organs and possibly extend the number of years people remain healthy.
How the Immune System Clears Ageing Cells
When neutrophils encounter pathogens, they can release toxic substances and even destroy themselves, spilling long strands of biological material that form web-like traps around invading microbes.
These cells do not live for long. A neutrophil may survive for as much as 24 hours, although 12 hours is more typical. Roughly 90% of circulating neutrophils eventually arrive in the liver, spleen, and bone marrow, where other immune cells remove them.
That disposal process is especially important as the body grows older. In ageing animals, most neutrophils that never encounter a pathogen quickly enter senescence, a dysfunctional state in which they can release harmful chemicals that damage nearby cells and promote inflammation.
Neutrophil numbers increase with age, and a growing proportion of them become senescent.
“Senescent neutrophils are killing our tissues,” Andreasson said. “Clearance of these cells is essential for preventing chronic inflammation.”
The Body’s Cellular Garbage Collectors
Macrophages are responsible for much of that cleanup. These versatile immune cells fight pathogens, coordinate responses from other cells and release growth factors that help damaged tissues repair themselves.
They also remove dead and dysfunctional cells.
“They’re the body’s garbage collection crew. A lot of that garbage are defunct cells,” Andreasson said.
A large share of that cellular waste consists of neutrophils, with roughly 100 billion of them needing to be cleared every day.
There are several kinds of macrophages. Tissue-resident macrophages are unusually long-lived cells that settle in organs during foetal development. Once established, they remain in those organs throughout life and adapt to perform specialised roles in each organ.
One of their most important responsibilities is swallowing senescent cells. The new findings show that neutrophils are particularly significant targets. About 100 billion neutrophils are produced each day, and they begin showing signs of senescence only 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.)
The problem is that tissue-resident macrophages themselves deteriorate with age. Andreasson and her colleagues reported in a 2021 Nature study that these long-lived immune cells become increasingly vulnerable to inflammation as animals grow older. They can then contribute to that inflammation themselves.
An Inflammatory Signal Grows Stronger With Age
One important part of this process involves prostaglandins, hormones produced by immune cells. One of the five types, called PGE2, can affect cells in different ways depending on which receptors are present on their surfaces.
One receptor for PGE2, known as EP2, strongly promotes inflammation. Tissue-resident macrophages contain large amounts of EP2.
PGE2 production rises in response to infection, injury and toxic substances, including compounds produced as the body ages. The researchers’ earlier work showed that PGE2 levels increase substantially over time. At the same time, tissue-resident macrophages develop higher concentrations of EP2.
Together, those changes create a harmful feedback process. Increasing PGE2 activity repeatedly stimulates EP2 receptors on tissue-resident macrophages. The new study found that this stimulation weakens the macrophages’ ability to engulf neutrophils.
As a result, senescent neutrophils begin accumulating in the bloodstream and tissues.
Previous research from Andreasson’s group also showed that the energy metabolism of tissue-resident macrophages gradually deteriorates with age.
“Once that starts, there’s a steady decline in a macrophage’s performance,” she said.
The new work suggests that EP2 is critical to that 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.”
Blocking One Receptor Protects Multiple Organs
To investigate the receptor’s role more closely, Andreasson’s laboratory engineered mice whose EP2 gene could be deleted at a time chosen by the scientists, specifically in tissue-resident macrophages.
Removing EP2 restored the macrophages’ ability to dispose of neutrophils, reversing the disruption caused by PGE2.
The researchers compared younger normal mice, aged 6 to 8 months, which corresponds roughly to late adolescence or early adulthood in humans, with older normal mice, aged 23 to 25 months, which are roughly comparable to humans in their 60s or 70s. They also studied nearly identical older mice whose gene for EP2 had been deleted when they were 4 to 6 months old (their “teenage” years).
The team identified 71 blood proteins whose levels had changed significantly in normal older mice. Remarkably, 59 of those proteins remained at youthful levels in older mice whose tissue-resident macrophages lacked EP2. Many of the proteins came from the liver.
“The liver is one of the body’s most tissue-resident-macrophage-enriched organs and a major contributor to ageing-related changes in blood chemistry,” Andreasson said. “It’s the central organ determining the body’s metabolic rate.”
Normal old mice accumulated senescent neutrophils in the liver, spleen and bone marrow. The researchers also observed smaller increases in the many other organs they examined.
Older mice whose tissue-resident macrophages lacked EP2 were different. Their organs maintained the lower neutrophil levels normally seen in younger animals.
The mice also appeared younger, leaner and more physically fit than control animals of the same age. They had less visceral fat and more muscle, and their performance on tests measuring the function of several organs matched that of young mice.
Memory, Strength and Inflammation Also Improved
Removing EP2 from tissue-resident macrophages reduced inflammation in the blood, liver, colon, heart, kidney and hippocampus (a brain region tightly tied to memory and navigation ability).
Older mice without EP2 also performed like younger animals on tests of speed, balance and forelimb grip strength.
Their memory remained stronger as well. They navigated mazes and remembered previously encountered objects almost as effectively as younger mice, while outperforming similarly aged mice whose EP2 receptors continued to function normally.
Searching for a Drug That Can Target EP2
No approved drug currently exists that can selectively shut down EP2 activity, although several medications affect PGE2.
Nonsteroidal anti-inflammatory pain medications reduce PGE2 production. Andreasson noted that this is the mechanism through which aspirin and related drugs reduce pain, fever, swelling, and redness (that’s how aspirin and similar drugs reduce pain, fever, swelling, and redness, the “four horsemen” of inflammation.)
The difficulty is that these medications also interfere to varying degrees with other prostaglandins that perform important functions. PGE2 itself can also have beneficial effects when it interacts with receptors apart from EP2.
Instead of broadly suppressing PGE2, researchers would therefore like to target the specific EP2 receptor responsible for the harmful inflammatory response.
To test whether that approach might work, the scientists gave otherwise normal 22-month-old mice an experimental drug that inhibits EP2 for two months.
The treatment brought both total neutrophil levels and the number of senescent neutrophils in old mice closer to youthful levels. Experiments in cell cultures also indicated that ageing reduced the ability of tissue-resident macrophages to engulf and digest worn-out neutrophils, while the EP2-blocking drug significantly restored that ability.
Similar Changes Appear in Human Liver Cells
The researchers then examined a large database containing information about different cell types in young, old and diseased human livers.
They found patterns resembling those observed in the mice. Older human livers showed increased neutrophil accumulation, greater neutrophil senescence, declining tissue resident macrophage function and elevated EP2 activity. Those changes were even more pronounced in diseased livers.
According to Andreasson, this study was the first time these changes had been observed in human cells.
Strengthening the body’s ability to remove ageing neutrophils could eventually offer significant therapeutic benefits.
“We need to develop a safe drug” that blocks EP2 without interfering with earlier processes such as PGE2 production, Andreasson said.
A researcher from the University of Munster in Germany also contributed to the study.
The research was funded by 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 work was conducted at the Neurosciences Preclinical Imaging Community Laboratory at the Wu Tsai Neurosciences Institute.
