The adaptive immune system of an older person is a very different beast in comparison to that of the younger self. It has lost the supply of new T cells due to atrophy of the thymus, and the remaining population of T cells becomes ever more damaged, misconfigured, strange, and different. The immune system as a whole is complex enough to still be hiding many unexplored details, even in this era of biotechnology. Here, researchers outline a novel discovery in the immune function of supercentenarians. It seems that at very advanced ages, some T cells start to undertake radical shifts in function in order to compensate somewhat for the growing lack of capacity. It remains to be seen whether or not this only occurs to a significant degree in a minority of the population, and is thus a feature of supercentenarians because it increases the odds of survival.
Supercentenarians, people who have reached 110 years of age, are a great model of healthy aging. Their characteristics of delayed onset of age-related diseases and compression of morbidity imply that their immune system remains functional. Here we performed single-cell transcriptome analysis of 61,202 peripheral blood mononuclear cells (PBMCs), derived from 7 supercentenarians and 5 younger controls. We identified a marked increase of cytotoxic CD4 T cells as a signature of supercentenarians. This characteristic is very unique to supercentenarians, because generally CD4 T cells have helper, but not cytotoxic, functions under physiological conditions. Furthermore, single-cell T cell receptor sequencing of two supercentenarians revealed that cytotoxic CD4 T cells had accumulated through massive clonal expansion, with the most frequent clonotypes accounting for 15-35% of the entire CD4 T cell population.
The cytotoxic CD4 T cells exhibited substantial heterogeneity in their degree of cytotoxicity as well as a nearly identical transcriptome to that of cytotoxic CD8 T cells. This indicates that cytotoxic CD4 T cells utilize the transcriptional program of the CD8 lineage while retaining CD4 expression. Indeed, cytotoxic CD4 T cells extracted from supercentenarians produced IFN-γ and TNF-α upon ex vivo stimulation. Our study reveals that supercentenarians have unique characteristics in their circulating lymphocytes, which may represent an essential adaptation to achieve exceptional longevity by sustaining immune responses to infections and diseases.