The Killer T Cells of Supercentenarians: What They May Reveal About Healthy Aging

Edited and Approved by Stephen C. Rose, PhD, MS

Most of us learn a tidy division of labor in the immune system. CD4 T cells are the coordinators: they send instructions and help other immune cells respond. CD8 T cells are the assassins: they identify dangerous cells and kill them. Biology, as usual, is less interested in tidy categories. Some CD4 T cells also acquire the machinery to kill. These unusual cells are called CD4 cytotoxic T lymphocytes, or CD4 CTLs.

A new Cell Reports study suggests that these cells become increasingly common at the far edge of human life. Researchers studying Japanese centenarians and supercentenarians found that CD4 CTLs rose sharply around age 100, formed large families of genetically identical clones, and remained capable of diverse responses after laboratory stimulation [1]. The findings are intriguing, but they do not show that CD4 CTLs cause exceptional longevity or that increasing them would make anyone live longer.

Why study people who reach 110?

Supercentenarians are people who live to at least 110. They are rare, and they are not simply ordinary older adults with extra birthdays. As a group, they tend to delay or avoid several major age-related diseases. That makes them valuable natural experiments: their biology may reveal ways the body preserves function despite an enormous lifetime of infections, cellular damage, and inflammatory stress.

The same research team reported in 2019 that seven supercentenarians had far more CD4 CTLs than five younger controls. In that earlier study, CD4 CTLs averaged about one quarter of all T cells in the supercentenarians, and a few clones dominated the population [2]. The new work asks a more useful question: When does this expansion emerge, and what does it tell us about the cells' history?

A closer look at 43,584 T cells

The investigators collected blood from 28 Japanese adults: eight people in their 70s through 90s, 10 centenarians aged 100 to 109, and 10 supercentenarians aged 110 or older. They profiled 43,584 T cells with three complementary methods. One measured which genes were active, another measured proteins on each cell's surface, and a third read the T-cell receptor, the molecular sensor that recognizes a particular target.

The median proportion of CD4 CTLs increased from 4.0% in the 70-to-99 group to 9.6% in centenarians and 17.6% in supercentenarians. The age trend was statistically significant, although one healthy person younger than 100 had the highest individual proportion. To see whether this was merely a quirk of a tiny rare cohort, the researchers trained a machine-learning model to recognize CD4 CTLs and applied it to public single-cell datasets containing more than five million cells from 1,512 samples. That broader analysis also showed low levels in younger people and a marked rise in both the typical level and variability at extreme ages [1].

How a helper becomes a killer

The cells appeared to move through a recognizable transition. Conventional helper CD4 cells usually carry two surface proteins, CD27 and CD28, that support activation. The suspected intermediate cells had lost CD27 but retained CD28. Fully developed CD4 CTLs had lost both and increased genes for granzymes and perforin, molecules used to damage a target cell. This orderly sequence is consistent with repeated stimulation pushing helper cells toward a cytotoxic identity. Other human and animal work has also found that aging reorganizes the CD4 landscape toward more extreme regulatory and effector states [3].

Importantly, these highly differentiated cells showed low levels of several molecular markers associated with exhaustion. In plain language, they looked experienced without necessarily looking spent. That distinction matters because a chronically stimulated T cell can become ineffective. Here, the phenotype supports the possibility of sustained function, but it is still an indirect molecular inference rather than a demonstration of protection inside a living person.

The clue hidden in the clones

When a T cell recognizes its target, it multiplies. Its descendants carry the same receptor, creating a clone specialized for the same molecular pattern. In this study, CD4 CTLs were strongly clonally expanded in every age group. On average, the single largest clone made up 33.3% of a person's CD4 CTLs; in one centenarian, it accounted for more than half. The dominant receptor sequences differed from person to person, suggesting private immune histories rather than one universal longevity target.

Large private clones are consistent with repeated exposure to persistent antigens—molecular flags that remain or reappear over time. Latent viruses, emerging abnormal cells, and senescent cells are plausible sources. The clones alone cannot reveal which target triggered them, however. They tell us that the immune system has repeatedly invested in particular cells, not why it made that investment.

What might these cells be targeting?

One possibility is damaged or senescent tissue. Senescent cells have stopped dividing but remain metabolically active and can release inflammatory signals. A 2023 human skin study found that senescent fibroblasts displayed a cytomegalovirus protein and an immune-recognition molecule called HLA class II. Skin-resident CD4 CTLs recognized that combination and killed the senescent cells in laboratory experiments [4]. That provides a real mechanism by which antiviral memory could be repurposed for clearing damaged cells, but it was demonstrated in skin and cannot automatically be generalized to the blood cells in the new study.

Cancer surveillance is another possibility. Cytotoxic CD4 cells have been found inside human bladder tumors, where clonally expanded cells killed a patient's own tumor cells in an MHC class II-dependent manner [5]. Separate experiments across human cancers likewise showed direct, contact- and granzyme-dependent tumor killing by tumor-specific CD4 cells [6]. In the supercentenarian study, some receptor beta-chain sequences matched sequences previously found expanded in cancer samples, especially non-small cell lung cancer. But the match used only one half of the receptor and did not identify the recognized antigen. The authors correctly treat it as an exploratory clue, not evidence of hidden cancer or proven cancer prevention.

Flexible cells, not a single fixed program

The team stimulated cells from six participants outside the body. Activated CD4 CTLs increased granzyme B, perforin, interferon-gamma, and tumor necrosis factor. Yet cells belonging to the same clone did not all produce the same collection of signaling proteins. The researchers identified multiple cytokine patterns, including profiles resembling familiar helper-cell programs. A clone therefore appears able to keep one target-recognition system while adjusting how it communicates with the surrounding immune environment.

That flexibility may be useful during decades of repeated immune challenge. It also complicates any simple story that more CD4 CTLs must be better. Cytotoxic CD4 cells can participate in autoimmunity and excessive inflammation. In a human study of multiple sclerosis, older patients showed abnormal increases in activated and cytotoxic CD4 cells alongside reduced inhibitory control [7]. The same broad cell category can be protective in one setting and damaging in another; target, tissue, activation state, and restraint all matter.

What the study does—and does not—establish

The established finding is descriptive: CD4 CTLs became more abundant around extreme old age, showed a stepwise helper-to-killer transition, and were dominated by large clones. The persistent-antigen explanation is plausible and supported by the receptor patterns, but the antigens were not identified. The cancer- and senescent-cell-surveillance interpretation is preliminary because it draws on database overlaps and experiments from other tissues and diseases.

The cohort was small, cross-sectional, entirely Japanese, and limited to circulating cells. It cannot tell whether CD4 CTLs helped participants reach old age, emerged because they had already survived that long, or reflect another factor. The stimulation experiment used powerful chemicals outside the body, not a natural antigen. Even the public-data analysis depended on a model trained to classify these rare cells. No intervention was tested.

There is strong experimental evidence that immune aging can influence aging elsewhere in the body: in mice, selectively accelerating immune-cell senescence promoted damage in solid organs, while transfer of younger immune cells reduced some senescence markers [8]. That makes immune resilience a credible part of healthy-aging biology. It does not mean that copying one unusual immune-cell pattern from supercentenarians would be safe or beneficial.

The bottom line

The most compelling message is not that researchers have found a cellular fountain of youth. It is that the immune system can remain adaptive after a century of life. In these exceptionally old adults, some helper T cells appeared to retrain as killers, expand around personally relevant targets, and preserve functional flexibility. That is a remarkable sign of immune plasticity.

The next step is to identify what the dominant clones actually recognize and what they do in tissues. If they are clearing senescent, infected, or precancerous cells without causing damaging inflammation, they could reveal principles for healthier immune aging. Until those experiments are done, CD4 CTLs should be viewed as an intriguing biomarker and mechanistic lead—not a treatment target, a longevity test, or proof that killer T cells are the secret to reaching 110.

References

[1] Hashimoto K, Kojima-Ishiyama M, Inokuchi H, et al. CD4 CTLs in supercentenarians: Signs of adaptive expansion in healthy aging. Cell Reports. Published online August 19, 2026. doi:10.1016/j.celrep.2026.117728.

[2] Hashimoto K, Kouno T, Ikawa T, et al. Single-cell transcriptomics reveals expansion of cytotoxic CD4 T cells in supercentenarians. Proc Natl Acad Sci U S A. 2019;116(48):24242-24251. doi:10.1073/pnas.1907883116.

[3] Elyahu Y, Hekselman I, Eizenberg-Magar I, et al. Aging promotes reorganization of the CD4 T cell landscape toward extreme regulatory and effector phenotypes. Sci Adv. 2019;5(8):eaaw8330. doi:10.1126/sciadv.aaw8330.

[4] Hasegawa T, Oka T, Son HG, et al. Cytotoxic CD4+ T cells eliminate senescent cells by targeting cytomegalovirus antigen. Cell. 2023;186(7):1417-1431.e20. doi:10.1016/j.cell.2023.02.033.

[5] Oh DY, Kwek SS, Raju SS, et al. Intratumoral CD4+ T Cells Mediate Anti-tumor Cytotoxicity in Human Bladder Cancer. Cell. 2020;181(7):1612-1625.e13. doi:10.1016/j.cell.2020.05.017.

[6] Cachot A, Bilous M, Liu YC, et al. Tumor-specific cytolytic CD4 T cells mediate immunity against human cancer. Sci Adv. 2021;7(9):eabe3348. doi:10.1126/sciadv.abe3348.

[7] Zuroff L, Rezk A, Shinoda K, et al. Immune aging in multiple sclerosis is characterized by abnormal CD4 T cell activation and increased frequencies of cytotoxic CD4 T cells with advancing age. EBioMedicine. 2022;82:104179. doi:10.1016/j.ebiom.2022.104179.

[8] Yousefzadeh MJ, Flores RR, Zhu Y, et al. An aged immune system drives senescence and ageing of solid organs. Nature. 2021;594(7861):100-105. doi:10.1038/s41586-021-03547-7.