Study Finds People Who Live Past 100 Have Unusual Abundance of Cancer-Killing Immune Cells

People who live past age 100, and especially those who reach 110 or beyond, appear to carry a striking abundance of rare, cancer-fighting immune cells in their blood, according to new research that offers a possible clue into how so-called supercentenarians manage to avoid the illnesses that typically claim most people decades earlier.

The findings, published in the journal Cell Reports through Cell Press, focus on a rare category of T helper cells that appears to proliferate significantly as people age past roughly 100 years old, expanding and adapting over time in ways researchers say reflect an active, evolving immune response rather than simple biological decline.

The human immune system relies on T helper cells, also known as CD4 cells, as a core component of adaptive immunity. Among the many subtypes of CD4 cells, a particularly rare variant known as cytotoxic CD4 T cells, or CD4 CTLs, typically occurs only infrequently in the human body. Unlike most helper cells, which coordinate immune responses, CD4 CTLs function more like dedicated killers, patrolling the body to identify and destroy infected, damaged or cancerous cells before they can cause further harm.

According to the study, these normally scarce CD4 CTLs become increasingly common as people age into their 90s and beyond, with the effect growing even more pronounced among centenarians and supercentenarians specifically. Researchers analyzed blood samples from 28 Japanese participants across different age brackets, a group the study’s authors acknowledge represents a small sample size drawn from a single country and therefore may not necessarily be representative of the broader global population.

Among eight participants in their 70s, 80s and 90s, the median proportion of CD4 CTLs in their blood samples measured 4%. That figure rose sharply among the study’s 10 centenarian participants, where the median proportion reached 9.6%. Among supercentenarians specifically, individuals who have lived past 110, the median proportion climbed even further, to 17.6%. Notably, even within this small sample, researchers identified at least one outlier: a participant who had not yet turned 100 but nonetheless showed the highest proportion of these cells recorded anywhere in the study. Researchers also noted that the CD4 CTLs identified in older participants showed no signs of cellular exhaustion, remaining biologically active despite the advanced age of the individuals carrying them.

Kosuke Hashimoto, an immunologist at the University of Osaka in Japan and one of the study’s researchers, said the findings challenge a common assumption about how the immune system changes with age. “Immune aging is not simply a process of decline,” Hashimoto said. “The selective expansion of certain T cells suggests that, even in extreme old age, the immune system may continue to adapt to age-related challenges.”

To better understand how these cells develop, the research team examined specific proteins present on the surface of the cells. They found that CD4 CTLs appear to emerge through a process in which helper cells sequentially lose two particular surface proteins, first CD27 and later CD28, a transformation that effectively converts the cells from their original helper function into their rarer, cytotoxic killer form.

The researchers also observed a hallmark of active immune defense within the samples: clonal expansion, a process in which killer T cells replicate themselves in response to a perceived threat, effectively building a targeted immune “army” against a specific antigen. According to the study’s findings, roughly 33% of the CD4 CTLs identified in older participants’ blood were, on average, clones descended from a single original cell. In one particularly striking centenarian sample, researchers found that approximately half of that individual’s total CD4 CTL population consisted of copies derived from the very same original cell.

The study’s authors interpret this pattern as evidence that the human body faces a growing burden of immune threats as it ages, and that an expanding population of CD4 CTLs may be helping certain individuals, at least among the Japanese supercentenarians studied, successfully manage those threats well beyond the age at which most people succumb to illness. Describing the underlying immune activity captured in the research, the study’s authors wrote that “these cells exhibit stepwise differentiation and cytokine plasticity, suggesting adaptive responses to persistent antigens during healthy aging.”

Hashimoto connected the broader biological backdrop driving this immune adaptation to the natural accumulation of cellular abnormalities that occurs with advancing age. “As we age, abnormal cells, including senescent and cancerous cells, become more common,” Hashimoto said. “Our findings suggest that immune adaptation to these changes may contribute to exceptional longevity.”

The new findings add to a growing body of research examining the biological underpinnings of extreme human longevity, an area of study that has drawn increasing scientific attention as global populations of centenarians and supercentenarians, though still exceedingly rare, have continued to grow in recent decades. Related research has previously identified other biological markers associated with slower aging among centenarians, including a set of 37 distinct proteins found in the blood of long-lived individuals that researchers have linked to a slower overall pace of biological aging, according to prior coverage of separate longevity research.

Given the modest size and geographic limitation of the current study’s sample, the researchers have cautioned that further work involving larger and more diverse populations will likely be necessary to confirm whether the abundance of CD4 CTLs observed among Japanese supercentenarians represents a broader, universal biological feature of extreme human longevity, or whether it may instead reflect genetic, environmental or lifestyle factors more specific to the population studied. Even so, the findings offer researchers a promising new avenue for understanding how, at a cellular level, a small subset of humans manage to avoid succumbing to cancer and other age-related illnesses well beyond the point at which most people’s immune systems begin to falter.