Killer T Cells That Increase Only After Age 100 Discovered — And They Weren't Exhausted

Explanation

It is generally understood that immune function declines with age. Indeed, older people become more susceptible to infectious diseases, and vaccines become less effective. However, when people over 100 years old are examined, phenomena appear that this explanation cannot account for.

On August 21, 2026, a joint research group consisting of the Institute for Protein Research at Osaka University, the Keio University School of Medicine Center for Supercentenarian Medicine led by Center Director Yasumichi Arai, and the RIKEN Center for Integrative Medical Sciences led by Team Director Piero Carninci announced results showing that, in extreme old age, immunity does not "decline" but instead "reorganizes."

The focus was on cells called CD4-positive killer T cells. There are several types of T cells, broadly divided into helper T cells (CD4-positive), which act as commanders, and killer T cells (usually CD8-positive), which directly kill infected cells. CD4-positive killer T cells are a minority group in between, carrying the marker of a commander cell while also having the ability to kill.

The research group found that these cells remained rare up through the 90s. However, they increased markedly after age 100. Moreover, these increased cells did not show signs of "exhaustion." It is known that immune cells that continue fighting the same opponent can become depleted and lose function, and this was thought to be common in the elderly. The cells found in this study were not in that state.

Furthermore, although these cells originated from clones—that is, they arose from the expansion of a small number of cells—the types of cytokines (intercellular signaling molecules) they released were diverse. In other words, despite expanding from the same parent, their roles had diverged.

The analysis used single-cell level data from centenarians (aged 100 and older) and supercentenarians (aged 110 and older), further integrated with large-scale public datasets covering ages from 0 to over 110, and AI-based modeling was performed.

The question of "what is different about the immune systems of long-lived people" offers clues for thinking about healthy aging. This result shows that immunity in extreme old age does not simply weaken, but shifts to a different configuration.

The paper was published in Cell Reports (DOI: 10.1016/j.celrep.2026.117728).

Source: RIKEN, "Revealing the reorganization of T cell immunity in extreme old age" (August 21, 2026)
https://www.riken.jp/press/2026/20260821_1/index.html