Cytotoxic CD4⁺ T cells induce age-associated myelopoiesis through CCL5-CCR5 signaling

Age-associated hematopoietic skewing results in an increase in the neutrophil-to-lymphocyte ratio, which serves as a strong predictor of all-cause mortality in older adults, although its causes are incompletely understood. Here we show that cytotoxic CD4⁺ T lymphocytes accumulate in the bone marrow of mice during aging and induce myelopoiesis, increasing the neutrophil-to-lymphocyte ratio. T cell receptor-dependent induction of mitochondrial stress and activation of STING upregulates the chemokine CCL5 in CD4⁺ T lymphocytes. During aging, hematopoietic stem cells and downstream myeloid progenitors upregulate CCR5, the primary receptor for CCL5. Genetic ablation of Ccr5 in hematopoietic progenitors mitigates T cell-induced myeloid skewing and neutrophil expansion. Pharmacological blockade of CCR5 using the Food and Drug Administration-approved drug maraviroc normalizes myelopoiesis, reduces circulating and tissue-infiltrating neutrophils, and improves multiple aging-related biomarkers and functional outcomes in aged mice. Together, these findings demonstrate a T cell-bone marrow axis that exacerbates age-associated decline and highlight CCR5 inhibition as a potential geroprotective strategy.