What ten people over 110 have in their blood

Reaching age 110 means surviving roughly a century of infections, inflammation, and cellular wear that kills almost everyone else long before they get there. A study published today in Cell Reports, led by researchers at the University of Osaka and RIKEN Center for Integrative Medical Science, offers a specific clue about what separates supercentenarians from the rest of us: their blood carries an unusually large population of a rare immune cell type called cytotoxic CD4 T lymphocytes, or CD4 CTLs, and those cells appear to still be actively working rather than worn out.

The finding builds on earlier work from the same research group. Back in 2019, immunologist Kosuke Hashimoto and colleagues first reported that supercentenarians carry elevated levels of CD4 CTLs. What this new study adds is a clearer picture of when, exactly, that shift happens across a human lifespan, and evidence that these cells keep multiplying and adapting well into extreme old age rather than simply accumulating passively.

The numbers behind the pattern

The researchers drew blood from 28 people split into three groups: eight adults aged 70 to 99, ten centenarians aged 100 to 109, and ten supercentenarians aged 110 or older. In the youngest group, CD4 CTLs made up a median of 4% of circulating T cells, consistent with how rare these cells typically are in most people at most ages, since they usually account for less than 5% of the total T cell population throughout adulthood. Among the centenarians, that share jumped to a median of 9.6%. In the supercentenarians, it reached 17.6%, nearly a fifth of all T cells in the blood.

That's not a gradual, steady climb across a lifetime. For decades the proportion barely moves, then it surges after age 100. The researchers checked whether this pattern held beyond their small sample by comparing it against public data covering more than five million immune cells from donors ranging from newborns to people over 110, and found the same trend. One notable exception appeared in the study: a single participant under 100 had the highest CD4 CTL share of anyone in the entire dataset, and researchers confirmed the sample was healthy and passed every quality check, a reminder that individual variation exists even within a pattern this consistent.

Why these cells are unusual to begin with

CD4 T cells are typically known as "helper" cells, coordinating the immune system's response to threats rather than directly killing anything themselves. That job normally belongs to a different cell type, CD8 T cells and natural killer cells. CD4 CTLs blur that distinction: they retain features of helper cells but have also picked up direct killing ability, making them a hybrid that Hashimoto's team calls "atypical and relatively rare" under normal physiological conditions. These cells have previously been documented killing tumor cells in certain cancers, including lung cancer and melanoma, and they're commonly detected during viral infections, suggesting they play a genuine surveillance role against both infected and abnormal cells.

The current study went further than counting cells, using T cell receptor profiling to show that the expanded CD4 CTL populations in older participants were built through clonal expansion, meaning specific cell lineages multiplied repeatedly rather than many different, unrelated CD4 CTLs simply appearing independently. That distinction matters for interpretation: clonal expansion is what happens when the immune system repeatedly responds to the same persistent threat, whether a chronic infection or recurring abnormal cells, rather than a random accumulation with age. It's the kind of targeted, adaptive immune signature that echoes patterns seen elsewhere in aging research, including recent findings on how the brain's resident immune cells get progressively swapped out for a different population starting around age 50, suggesting immune remodeling across the body continues well beyond what researchers once assumed was a fixed adult baseline.

What the researchers still don't know

Hashimoto was careful about the limits of what this study actually shows. "Our study suggests that even at extreme old age, the immune system may still selectively adapt," he said, but the researchers stopped short of claiming these cells cause longevity. The data is cross-sectional, meaning it captures a single snapshot of very old people's blood rather than tracking anyone's immune system as they aged into their hundreds. That leaves open an important alternative explanation: supercentenarians may simply be an unusually selected group of survivors whose immune systems happened to already work this way, rather than people whose CD4 CTL expansion actively helped them reach 110.

The team also doesn't yet know precisely what these expanded cell populations are targeting. Hashimoto said the next step is identifying what these cells actually recognize in human tissue and testing directly whether they can eliminate those targets, work that would move the finding from correlation toward a mechanistic explanation. His team is also interested in studying the much rarer CD4 CTL populations in younger people to map how this response develops across a full lifespan rather than only appearing in its most extreme, elderly form.

Why this line of research matters beyond supercentenarians

Supercentenarians are genuinely difficult to study simply because there are so few of them; Japan's 2015 census counted only 146 people over 110 nationwide, against more than 61,000 centenarians. That scarcity is precisely why researchers keep returning to this population: supercentenarians don't just live long, they largely avoid the diseases that kill most people in old age, including heart disease, dementia, and cancer, making their immune profile a potential map toward healthier aging more broadly. If researchers can eventually identify what CD4 CTLs are targeting and confirm they play a protective role, a similar dynamic to how supercharged immune cells have been engineered to target tumors that manipulate the immune system to grow elsewhere in cancer research, it could eventually point toward therapies designed to encourage this kind of adaptive immune response earlier in life, long before anyone reaches age 110 to develop it naturally.