Central memory CD8+ T cells derive from stem-like Tcf7hi effector cells in the absence of cytotoxic differentiation.

Central memory CD8 <sup>+</sup> T cells (Tcm) control systemic secondary infections and can protect from chronic infection and cancer as a result of their stem-cell-like capacity to expand, differentiate, and self-renew. Central memory is generally thought to emerge following pathogen clearance and to form based on the de-differentiation of cytolytic effector cells. Here, we uncovered rare effector-phase CD8 <sup>+</sup> T cells expressing high amounts of the transcription factor Tcf7 (Tcf1) that showed no evidence of prior cytolytic differentiation and that displayed key hallmarks of Tcm cells. These effector-phase Tcf7 <sup>hi</sup> cells quantitatively yielded Tcm cells based on lineage tracing. Mechanistically, Tcf1 counteracted the differentiation of Tcf7 <sup>hi</sup> cells and sustained the expression of conserved adult stem-cell genes that were critical for CD8 <sup>+</sup> T cell stemness. The discovery of stem-cell-like CD8 <sup>+</sup> T cells during the effector response to acute infection provides an opportunity to optimize Tcm cell formation by vaccination