Mitochondrial mass governs the extent of human T cell senescence

The susceptibility of human CD4⁺ and CD8⁺ T cells to senesce differs, with CD8⁺ T cells acquiring an immunosenescent phenotype faster than the CD4⁺ T cell compartment. We show here that it is the inherent difference in mitochondrial content that drives this phenotype, with senescent human CD4⁺ T cells displaying a higher mitochondrial mass. The loss of mitochondria in the senescent human CD8⁺ T cells has knock-on consequences for nutrient usage, metabolism and function. Senescent CD4⁺ T cells uptake more lipid and glucose than their CD8⁺ counterparts, leading to a greater metabolic versatility engaging either an oxidative or a glycolytic metabolism. The enhanced metabolic advantage of senescent CD4⁺ T cells allows for more proliferation and migration than observed in the senescent CD8⁺ subset. Mitochondrial dysfunction has been linked to both cellular senescence and aging; however, it is still unclear whether mitochondria play a causal role in senescence. Our data show that reducing mitochondrial function in human CD4⁺ T cells, through the addition of low-dose rotenone, causes the generation of a CD4⁺ T cell with a CD8⁺-like phenotype. Therefore, we wish to propose that it is the inherent metabolic stability that governs the susceptibility to an immunosenescent phenotype.