Tcf1+ cells are required to maintain the inflationary T cell pool upon MCMV infection.

Cytomegalovirus-based vaccine vectors offer interesting opportunities for T cell-based vaccination purposes as CMV infection induces large numbers of functional effector-like cells that accumulate in peripheral tissues, a process termed memory inflation. Maintenance of high numbers of peripheral CD8 T cells requires continuous replenishment of the inflationary T cell pool. Here, we show that the inflationary T cell population contains a small subset of cells expressing the transcription factor Tcf1. These Tcf1+ cells resemble central memory T cells and are proliferation competent. Upon sensing viral reactivation events, Tcf1+ cells feed into the pool of peripheral Tcf1− cells and depletion of Tcf1+ cells hampers memory inflation. TCR repertoires of Tcf1+ and Tcf1− populations largely overlap, with the Tcf1+ population showing higher clonal diversity. These data show that Tcf1+ cells are necessary for sustaining the inflationary T cell response, and upholding this subset is likely critical for the success of CMV-based vaccination approaches. Upon infection with cytomegalovirus, CD8+ T cells undergo prolific expansion in a process known as memory inflation. Here the authors define a population of Tcf1 expressing cells within the inflationary pool that is critical in fuelling this process.