TCF7 and neoplasm: ,19,25,26 The exhausted progenitor population is required for the proliferative burst observed after ICB, which produces significant expansion in the number of transitory effector cells.16, 17, 18 Thus, the progenitor population of exhausted CD8+ T cells has received significant attention in tumor immunology studies, many of which have quantified tumor-infiltrating TCF-1+ cells.21