Today, the average human life expectancy is around 71 years. Only a few people live to be 100—earning the title of centenarian—and even fewer live past 110. We call such individuals supercentenarians.
A new study published in the journal Cell Press reveals a possible explanation for how people who reach advanced age manage to avoid the rising risk of cancer and other diseases as the years go by.
Around the age of 100, these fortunate individuals appear to develop a rapidly proliferating, rare form of helper T-cell that expands and adapts over time.
“These cells exhibit stepwise differentiation and cytokine plasticity, indicating adaptive responses to persistent antigens during healthy aging,” the study authors write.
We all possess helper T-cells (also known as CD4 cells), which are part of our adaptive immune system.
There are many different types of CD4 cells, some of which are rarer than others. For instance, cytotoxic CD4 T-cells (CD4 CTLs) are typically quite rare in the human body.
These T-cells act more like killers than helpers: their job is to patrol the body, identifying infected, damaged, or cancerous cells and destroying them before further harm can occur.
However, as people reach the age of 90 and beyond, these normally rare CD4 CTLs appear to become increasingly abundant. This statistically significant pattern was observed in blood samples from a group of 28 Japanese participants (admittedly a very small sample consisting solely of citizens from one country, and not necessarily representative of the global population).
In samples taken from eight individuals aged 70, 80, and 90, the median proportion of CD4 CTLs was 4 percent.
Among the 10 study participants who had reached the age of 100, the median figure was 9.6 percent, while among those over 100, it was 17.6 percent.
However, there were outliers even in this small sample: one participant who had not yet turned 100 actually had the highest proportion of these cells in their blood.
These cells were active and showed no signs of exhaustion.
“Immune system aging is not merely a process of decline,” says immunologist Kosuke Hashimoto of Osaka University in Japan. “The selective expansion of certain T cells suggests that, even in advanced age, the immune system can continue to adapt to age-related changes.”
By examining proteins on the cell surface, researchers discovered that these CD4 CTLs emerge when helper cells lose the CD27 protein, followed by CD28.
Killer T cells are known to clone themselves in response to an immune challenge, forming a microscopic army.
This is precisely what the researchers found in the blood samples of the elderly individuals.
On average, about 33 percent of the CD4 CTLs in their blood were clones of a single original cell.
In a sample from one centenarian, approximately half of their CD4 CTLs were copies of the same cell. This suggests that, beyond a certain age, the human body faces mounting threats involving the immune system. An abundance of CD4 CTLs may help centenarians—at least those living in Japan—survive in the face of such threats.
“The number of abnormal cells, including senescent and cancer cells, increases with age,” says Hashimoto. “Our findings indicate that the immune system’s adaptation to these changes may contribute to exceptional longevity.”
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