Human Stem Cell-like Memory T Cells Are Maintained in a State of Dynamic Flux

Adaptive immunity requires the generation of memory T cells from naive precursors selected in the thymus. The key intermediaries in this process are stem cell-like memory T (T(SCM)) cells, multipotent progenitors that can both self-renew and replenish more differentiated subsets of memory T cells. In theory, antigen specificity within the T(SCM) pool may be imprinted statically as a function of largely dormant cells and/or retained dynamically by more transitory subpopulations. To explore the origins of immunological memory, we measured the turnover of T(SCM) cells in vivo using stable isotope labeling with heavy water. The data indicate that T(SCM) cells in both young and elderly subjects are maintained by ongoing proliferation. In line with this finding, T(SCM) cells displayed limited telomere length erosion coupled with high expression levels of active telomerase and Ki67. Collectively, these observations show that T(SCM) cells exist in a state of perpetual flux throughout the human lifespan.