Immune Cell Atlas Charts How People’s Aging Paths Diverge

Studies of the immune system show that two people born on the same day can have immune systems that age differently, leaving some older adults with resilient defense systems and others with dysfunctional ones that put them at increased risk for age-related diseases. Now a new study led by scientists at Washington University School of Medicine in St Louis, Nationwide Children’s Hospital in Ohio, and King’s College London provides a map of human immune aging that could help explain these different trajectories. Details are published in a new Immunity paper titled “Single-cell analyses of global cohorts outline determinants of immune aging and link the ratio of GZMK⁺ to GZMB⁺ Tem cells to health trajectories.”

According to the paper, a ratio between two immune cell types can reveal whether someone stays healthy or may face risk of future disease. To get to this insight, the scientists first analyzed approximately 12.4 million individual immune cells from the blood of over 2,600 mainly healthy adults ages 20 to 90-plus across eight cohorts in North America, the United Kingdom, Asia, and Australia. 

Their analysis showed that while the blood of younger people was uniformly enriched with naive immune cells that had not been exposed to specific pathogens and remained adaptable to new infections or vaccines, older immune systems were more diverse. Specifically, some older participants had a greater abundance of immune cells that reflect inflammation.

To map this shift, the researchers placed each participant on a spectrum based on the ratio between two types of white blood cells: specifically, granzyme B-producing cells and granzyme K-producing cells. Some white blood cells, known as effector memory CD8 T cells, use granzymes to destroy target cells or coordinate immune responses. While granzyme B-producing cells serve as direct destroyers of diseased cells, granzyme K-producing cells have been less studied and may play a role in boosting immune responses. 

Next, the researchers evaluated whether participants’ positions on the spectrum, which corresponds to a specific type of CD8 T cell, could dictate their risk of getting sick. In other words, they looked at whether having more of one type of CD8 T cell has an impact on the person’s health as they age. To answer that question, the scientists turned to the UK Biobank. 

While that dataset includes protein information from the participants, it lacks counts of CD8 T cells. To get around this hurdle, the scientists used a smaller dataset of seemingly healthy adults that included data on CD8 T cell counts and blood proteins to create a computer model to map out the protein tracks left behind when either granzyme B-producing or granzyme K-producing cells dominated. They then applied this model to the baseline blood protein samples of 50,000 participants from the UK Biobank, calculating their position on the immune cell spectrum.

Using this approach, the scientists were able to draw out some useful insights for their work. Specifically, they found that healthy adults whose immune system leaned heavily toward granzyme B cells at baseline tipped the scale toward a higher risk of death a decade later compared to those with more granzyme K cells. The granzyme B-dominant group also was more likely to develop chronic conditions, including Type 2 diabetes, hypertension, liver disease, and renal failure, among others, over the following decade.

“A higher level of granzyme B cells in a healthy state means the immune system might be already reacting to something,” said Marina Terekhova, MD, a professor in pathology & immunology at WashU Medicine and one of the co-authors on the paper. “It isn’t a formal disease diagnosis, but it may indicate the body is off track.”

A healthy immune system gradually shifts toward granzyme K cells over time, while an immune system that leans toward granzyme B cells signals unhealthy aging. Because individuals can reach this granzyme B state much earlier in life, their immune systems age on different trajectories, explaining why two people born in the exact same year can face different risks for chronic disease and death decades later.

These findings point to “clues in the blood that can tell us whether someone is on a healthy or unhealthy aging trajectory,” said Maxim Artyomov, PhD, the alumni endowed professor of pathology and immunology at WashU Medicine and one of the corresponding authors on the paper. Artyomov and his lab are now adapting their research into a simple blood test that can be processed using standard equipment. “Identifying specific biological red flags in the immune system could pave the way for early-warning tests that spot disease risks years before a clinical diagnosis.”

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