Phenotypic and functional heterogeneity of naïve CD8⁺ T cells in human peripheral blood during aging

Naïve CD8⁺ T cells are key players of adaptive immunity, but their heterogeneity and age-related changes are not fully understood. This study aimed to compare naïve CD8⁺ T cell subsets defined by different combinations of markers, namely, N_CCR7 (CD45RA⁺CCR7⁺), N_CD28 (CD45RA⁺CD28⁺), N_CD27 (CD45RA⁺CD27⁺), and phenotypically most "true-naïve"-like, N_TN (CD45RA⁺CCR7⁺CD28⁺CD27⁺CD57^-). Peripheral blood was harvested from donors of various ages and the phenotype of the four subsets of naïve CD8⁺ T cells was analyzed. N_CD27 and N_TN cells showed similar phenotypes with low expression of differentiation markers, pro-inflammatory cytokines, and effector molecules. Furthermore, they exhibited optimal mitochondrial fitness, low senescence markers, reduced apoptosis, and high proliferation potential. Hierarchical clustering identified cluster one including N_CD27 and N_TN, with lower expression of differentiation markers and pro-inflammatory molecules, and cluster 2, including N_CCR7 and N_CD28 cells, in which these parameters were more expressed. Age-related changes were observed in all subsets, although they were less pronounced for the N_CD27 and N_TN subsets. Taken together, this study demonstrates significant heterogeneity among naïve CD8⁺ T cell subsets, with N_TN cells representing the most bona fide naïve phenotype and N_CD27 showing a partially similar phenotype. These findings significantly enhance our understanding of naïve CD8⁺ T cell biology and function.