text: Distinct kinetics of T-bet and Eomes expression during CTL differentiation
  The T-box transcription factors T-bet and Eomes have been linked to the regulation of genes encoding effector cytokines (e.g., Ifng) and genes important for cytolytic function (e.g., Prf1 and GzmB) (20). We investigated the kinetics of expression of these transcription factors in our in vitro cultures (Fig. 1, A-D). T-bet mRNA and protein were not detectable in naive CD8+ T cells, but were strongly induced upon TCR priming (day 2) and remained expressed through day 6 of differentiation (Fig. 1, A and C; quantified in Fig. 1, B and D). In contrast, Eomes expression was low or undetectable at both the mRNA and protein levels in naive CD8+ T cells, and TCR priming in culture had only a modest effect on its expression at day 2 (Fig. 1, A and C). Strong induction of Eomes mRNA and protein was only observed at day 4 and later (Fig. 1, A and C). T-bet mRNA expression slightly preceded the expression of GzmB mRNA; similarly, Eomes mRNA and protein were expressed approximately1 d ahead of the reexpression of perforin mRNA and protein, respectively (Fig. 1, B and D). 
  This detailed kinetic analysis suggested that, under our culture conditions, T-bet and Eomes contribute to distinct aspects of gene transcription during CTL differentiation. T-bet is required early for IFN-gamma production, and our data suggested that Eomes might not function during this early period but rather might contribute later to the control of perforin expression. Our data seemed most consistent with a model in which TCR signals induce T-bet, which in turn induces IFN-gamma (17) and possibly granzyme B; subsequently, Eomes is induced during the period of clonal expansion in IL-2 and activates perforin expression. 
  