Proliferation and differentiation potential of human CD8+ memory T-cell subsets in response to antigen or homeostatic cytokines

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Abstract
Four human CD8+ T-cell subsets, naive (CCR7+CD45RA+), central memory (TCM, CCR7+CD45RA–), effector memory (TEM, CCR7–CD45RA–), and CD45RA+ effector memory cells (TEMRA, CCR7–CD45RA+) were compared for their capacity to proliferate and differentiate in response to antigen or homeostatic cytokines. Cytokine responsiveness and interleukin-15 receptor expression were low in naive T cells and progressively increased from TCM to TEM and TEMRA. In contrast, the capacity to accumulate in response to T-cell receptor (TCR) or cytokine stimulation showed a reciprocal pattern and was associated with resistance to cell death and Bcl-2 expression. Whereas all TCR-stimulated cells acquired a CD45RA–CCR7– phenotype, cytokine-stimulated cells maintained their phenotype with the exception of TCM cells, which expressed CCR7, CD45RA, and perforin in various combinations. Single CD8+ TCM cells, but not TEM cells, could be expanded with cytokines, and the obtained clones displayed several distinct phenotypes, suggesting that TCM cells are heterogeneous. Consistently, CCR4 expression in the CD8+ TCM pool discriminated CCR4+ type 2 polarized cells (Tc2) and CCR4–CTL precursors. Finally, ex vivo bromodeoxyuridine (BrdU) incorporation experiments revealed that memory subsets have different in vivo proliferation rates, with CCR4–TCM having the highest turnover and TEMRA the lowest. These results show that human CD8+ memory T-cell subsets have different proliferation and differentiation potentials in vitro and in vivo. Furthermore, they suggest that TEMRA cells are generated from a TCM subset upon homeostatic proliferation in the absence of antigen.