As people age, their immune system does not function as well as it did when they were younger, probably due to natural aging processes. But new research has found that exposure to air pollution also takes a significant toll on the human immune system over decades. Reporting in Nature Medicine, researchers found that particles from environmental pollutants get inhaled and then stick around in immune cells in the lymph nodes, disrupting those crucial cells and interfering with their ability to stop lung infections.
These findings could explain why people are so much more vulnerable to respiratory infections when they're older. The COVID-19 death rate, for example, is 80 times higher in people older than 75 compared to young adults. Elderly people are also more susceptible to pneumonia and flu.
Over ten years ago, investigators at Columbia University started collecting tissue from multiple immune tissues of deceased organ donors, like mucosa and lymphatic tissues.
"When we looked at people's lymph nodes, we were struck by how many of the nodes in the lung appeared black in color, while those in the GI tract and other areas of the body were the typical beige color," said study leader Donna Farber, Ph.D., a professor at Columbia University Vagelos College of Physicians and Surgeons.
As they collected more samples, the researchers noticed that a pattern began to emerge in the tissue. Tissue samples that came from children and teenagers tended to be beige, while tissue samples from people over the age of 30 contained black spots, and as the donors got older, the tissue got darker.
Farber said that they imaged the darker lymph nodes in the lungs, and determined that they were full of particles from air pollution. Then the researchers began to wonder about how the darkening tissue was affecting people's ability to fight off a lung infection as they got older.
When tissue from 84 deceased volunteers, all of whom were nonsmokers, was examined, the scientists found that pollutant particles had lodged in the macrophages, crucial immune cells that can engulf and eliminate pathogenic invaders like bacteria, viruses, or debris.
Macrophages that contained particles were not as good at generating inflammatory signals known as cytokines, or ingesting other particles. Other macrophages in the same lymph node that did not carry pollution, however, were still able to perform those functions without disruption.
"These immune cells are simply choked with particulates and could not perform essential functions that help defend us against pathogens," Farber said.
"We do not know yet the full impact pollution has on the immune system in the lung, but pollution undoubtedly plays a role in creating more dangerous respiratory infections in elderly individuals and is another reason to continue the work in improving air quality," added Farber.
Sources: Columbia University, Nature Medicine