Supercentenarians harbor abundant cancer-killing CD4 cytotoxic T cells without exhaustion
核心洞察
A study in Cell Reports found that a rare hybrid immune cell, the CD4 cytotoxic T lymphocyte (搜索) (CD4 CTL), becomes markedly more abundant in people who live past 100 years.
CD4 CTLs made up about 4% of T cells in people aged 70–90, roughly 10% in centenarians, and about 18% in supercentenarians (aged 110 and older).
These expanded cells showed no signs of T-cell exhaustion and retained functional flexibility, producing diverse cytokine patterns even within a single clone.
People who live to 110 years and older carry unusually large populations of a rare type of immune cell that can kill cancer (搜索), according to a study published in Cell Reports. The cells, called CD4 cytotoxic T lymphocytes (CD4 CTLs), typically make up less than 5% of the body's T cells, yet they account for nearly one in five T cells in supercentenarians. The findings suggest that immune aging is not simply a process of decline, but may involve selective adaptation even at extreme old age.
The work was led by Kosuke Hashimoto, with appointments at the Institute for Protein Research at The University of Osaka (搜索) and at the RIKEN Center for Integrative Medical Sciences (搜索), joined by colleagues at Keio University School of Medicine. "Most of immune ageing research has focused on decline," Hashimoto said. "Our study suggests that even at extreme old age, the immune system may still selectively adapt."
A rare hybrid cell surges after age 100
T cells have two familiar roles: helper T cells spot a threat and call in the rest of the immune system, while killer T cells do the destroying. The CD4 CTL at the center of this study does both, carrying the helper cell's marker while packing the killer cell's weapons, placing it outside the textbook split.
In most people at most ages, fewer than 5% of T cells are these hybrids. The team read 43,584 T cells one at a time from 28 donors in all. Among the eight donors in their 70s to 90s, the hybrids made up 4% of T cells in the blood. That share rose to 9.6% in the 10 centenarians and 17.6% in the 10 people past 110. The count does not rise steadily across a lifetime; for decades it barely changes, then climbs several-fold after age 100.
The team confirmed this pattern against public measurements of more than five million cells from people ranging from newborns to those over 110, and the same pattern appeared. One donor under 100 broke the pattern, showing the highest share of hybrids of anyone in the study; that person was healthy, and the sample passed every quality check.
Helper T cells convert into killer cells
A helper T cell does not become a killer T cell overnight. It drops two surface proteins along the way, CD27 and CD28, a familiar sign of an aging immune system. The team caught cells mid-switch: about 10.5% of the CD4 T cells had lost CD27 but still carried CD28, while fewer than 0.3% had it the other way around. That lopsided count fixes the order of conversion, with CD27 lost first and CD28 second. Those halfway cells carried a partial load of the proteins a killer uses to punch holes in a target.
Notably, the hybrids in these donors had no PD-1 (搜索) at all, and the genes tied to shutdown stayed quiet. A T cell triggered repeatedly normally shuts itself down and flags that state with PD-1, but these cells showed no signs of T-cell exhaustion. "One notable finding was that many CD4 CTLs had expanded into very large clones without showing signs of T-cell exhaustion," Hashimoto said.
Large clones dominate the population
When one T cell keeps meeting the same target, it divides, and its descendants all carry the same receptor. In these donors, the largest clone averaged 33.3% of a person's hybrid cells, roughly one in three. In one centenarian, a single clone accounted for 126 of that person's 234 hybrid cells, or 53.8%. A family that large means the cell has been triggered again and again by something that keeps coming back.
Clones also appeared in the younger donors, so expansion on its own cannot explain the higher numbers past 100. What may help explain it is time, with more years for repeated stimulation and clonal expansion. Every one of the 28 people carried a receptor sequence nobody else in the study had.
Receptors match tumor cells
To understand what these cells recognize, the team searched a public collection of more than 690 million receptor sequences and found 617 exact matches to their donors' biggest clones. Of those 617 matches, only 36 corresponded to T cells that had expanded into large clones in the people they came from. Thirty-two of the 36 came from people with cancer (搜索), 17 of them lung cancer (搜索). None of the centenarians or supercentenarians had a recorded history of the cancers on that list.
Hashimoto kept the interpretation narrow. "These matches are suggestive of a possible response to cancer (搜索)-related targets, although the exact targets remain unknown. We therefore see this as an interesting clue rather than direct evidence that these cells were recognizing cancer," he said.
Functional flexibility despite expansion
Immune cells communicate with small proteins called cytokines, and which ones a cell makes is a strong clue to what it is trying to do. The team took cells from six donors, stimulated them in a dish, and read those messages. All of the hybrids made the same two inflammatory signals, but beyond that they split into eight groups, with one producing signals associated with allergy-type reactions and another producing an immune-suppressing signal.
When the receptors were matched back to the groups, the clones turned out to be scattered across them, and the single largest clone appeared in all eight. "In ex vivo experiments, these cells also responded to stimulation and produced diverse patterns of cytokines, even among cells belonging to the same clone. This suggests that they retain functional flexibility despite extensive clonal expansion," Hashimoto said.
Open questions remain
Researchers have not yet shown what these cells recognize in the lungs or skin, or whether they destroy those targets. The receptor search used only one of the two chains a receptor is built from, so two cells can match on that chain while recognizing different targets; Hashimoto's team calls those matches exploratory for that reason. The functional tests used cells in a dish because obtaining tissue samples from people past 110 is difficult, and 28 people is a small group, meaning individual differences could have a large effect on the results.
"To show whether these cells are actually protective, we need to identify what they recognize in human tissues and test whether they can eliminate those targets. We are also interested in studying the much rarer CD4 CTLs in younger people to understand how this response develops over the lifespan," Hashimoto said. For now, the researchers know these cells become more common with extreme age, but they still do not know what the cells are targeting.