Emergence and fate of stem cell-like Tcf7 + CD8 + T cells during a primary immune response to viral infection.

In response to infection, naïve CD8 + T (T N ) cells yield a large pool of short-lived terminal effector (T TE ) cells that eliminate infected host cells. In parallel, a minor population of stem cell-like central memory (T CM ) cells forms, which has the capacity to maintain immunity after pathogen clearance. It has remained uncertain whether stem-like T CM cells arise by dedifferentiation from a subset of cytolytic T TE cells or whether priming generates stem-like cells capable of seeding the T CM compartment and, if so, when cytolytic T TE cells branch off. Here, we show that CD8 + T cells with stem-like properties, which are identified by the expression of TCF1 (encoded by Tcf7 ), are present across the primary response to infection. Priming programs T N cells to undergo multiple cell divisions, over the course of which TCF1 expression is maintained. These TCF1 + cells further expand relatively independently of systemic inflammation, antigen dose, or affinity, and they quantitatively yield TCF1 + T CM cells after pathogen clearance. Inflammatory signals suppress TCF1 expression in early divided TCF1 + cells. TCF1 down-regulation is associated with the irreversible loss of self-renewal capacity and the silencing of stem/memory genes, which precedes the stable acquisition of a T TE state. TCF1 expression restrains cell cycling, explaining in part the limited expansion of TCF1 + relative to TCF1 - cells during the primary response. Thus, our data are consistent with terminal differentiation of effector cells being a step-wise process that is initiated by inflammation in primed stem-like cells, which would otherwise become central memory cells by default.