In people past 110, one rare T cell keeps growing instead of fading

A team at the University of Osaka reported in Cell Reports that a rare hybrid immune cell, the CD4 cytotoxic T lymphocyte, takes up a progressively larger share of circulating T cells in the very old, reaching a median of 17.6 percent in people aged 110 and over ^[1][2]. That cuts against the default model of immune aging as subtraction, in which the system loses its capacity to clear pathogens and abnormal cells such as precancerous or senescent ones ^[17].

CD4 CTLs sit awkwardly in textbook immunology. T cells are conventionally split into helpers that coordinate a response and killers that destroy infected or malignant cells; CD4 CTLs do both, and they are uncommon in most people ^[21][12]. The same group flagged them as a hallmark of supercentenarians in 2019 ^[11]. The new work tries to quantify the trajectory.

The cohort is 28 people: eight aged 70 to 99, ten centenarians, and ten supercentenarians ^[3]. Median CD4 CTL share of total T cells was 4 percent, 9.6 percent, and 17.6 percent across those brackets ^[2], a 4.4-fold spread between the youngest and oldest groups ^[20]. The team ran integrated single-cell profiling combining transcriptome, surface protein, and T cell receptor sequence data ^[5] on more than 40,000 T cells ^[4], then applied a machine-learning model to a larger dataset of 1,500 people of all ages, which showed a similar trend ^[6].

Two details do more work than the percentages. First, participants with higher CD4 CTL levels carried many cellular clones, which the authors read as evidence the cells are responding to something rather than merely accumulating ^[7]. Second, the cells showed no signs of T cell exhaustion, the loss of killing capacity that usually accompanies chronic stimulation ^[8]. The authors conclude the population expands and diversifies as an adaptation to persistent antigens, potentially contributing to longevity through cancer suppression ^[9]. Hashimoto's framing is that immune aging is not simply decline, and that selective expansion of certain T cells suggests continued adaptation even in extreme old age ^[10].

The obvious limit is n. Ten supercentenarians is thin by any normal standard, though Hashimoto notes Japan's national census counts only about 150 in the country, making ten a substantial fraction ^[13] - roughly 7 percent of the national total ^[22]. The other limit is direction of causation. Rahul Sharma, an immunologist at the University of Virginia who was not involved, called the study commendable but said more work is needed, and raised the possibility that the cells are simply adapting to survive in older people without benefiting the host ^[14]. Alon Monsonego at Ben-Gurion University of the Negev, also uninvolved, said the work is not a breakthrough but could serve as a starting point ^[15]. Manel Esteller at the Sant Pau Research Institute, who studied María Branyas Morera before her death in 2024 at 117 years and 168 days, said the findings show supercentenarians are enriched in cells that attack threats and probably cancer cells ^[16].

What to watch: whether anyone identifies what these cells actually target. Hashimoto says the next questions are the specific antigens and the role of CD4 CTLs in younger people, some of whom also carry large numbers ^[19]. Until a target is named, expansion plus clonality plus absent exhaustion is a strong correlation and nothing more.