Perforin and granzyme B expression are not appreciably regulated by T-bet
To test the model outlined in the previous paragraph directly, we compared the expression of IFN-γ, perforin, and granzyme B in CD8+ T cells from WT and Tbx21 (T-bet)-deficient mice. As expected (17, 21), naive Tbx21−/− CD8+ T cells produced IFN-γ poorly upon activation (Fig. 2 A). Notably, this deleterious effect of T-bet deficiency was only observed in differentiating CD8+ T cells until day 4 of culture but was almost completely mitigated by day 6 (Fig. 2 A). This most likely reflected compensation by Eomes, which was strongly induced between days 4 and 6 (Fig. 1). In contrast, T-bet–deficient T cells cultured for 6 d showed no defect in perforin mRNA expression (Fig. 2 B, compare lanes 1 and 4). We consistently observed a modest reduction in GzmB mRNA in T-bet–deficient T cells (Fig. 2 B, compare lanes 1 and 4), which did not translate into a decrease in expression of granzyme B protein (Fig. 2 C).
Figure 2. Regulation of perforin, granzyme B, and IFN-γ expression by T-bet and Eomes in differentiating CTLs. (A) IFN-γ expression by WT (Tbx21+/+) and T-bet–deficient (Tbx21−/−) T cells. Naive CD8+ T cells, or cells activated and cultured for 4 or 6 d, were restimulated with PMA and ionomycin for 6 h, and IFN-γ expression was assessed by intracellular staining. Numbers show the percentage of IFN-γ+ cells. (B) Northern blot analysis of Prf1 and GzmB mRNA expression in WT or T-bet–deficient CD8+ T cells activated and either left uninfected (uninf) or transduced with retroviruses expressing Eomes-VP16 (Eo-VP16) or an empty IRES-GFP cassette (GFP). Total cellular RNA was analyzed on day 6 of culture. The frequency of transduced cells in the cultures was equivalent for both constructs (∼65–70% GFP+ cells; not depicted). (C) Granzyme B and IFN-γ expression by Tbx21+/+ and Tbx21−/− T cells analyzed in restimulated cells that had been cultured for 5 d. (D) IFN-γ production by cells transduced with Eo-VP16 or control (GFP) retroviruses (RV) measured on day 4 after 6 h of restimulation with PMA and ionomycin. Numbers show the percentage of GFP+ IFN-γ+ cells. Results are representative of three (A and C) or two (B and D) independent experiments.To examine the role of Eomes, we transduced naive CD8+ T cells from WT and Tbx21−/− mice with retroviruses containing internal ribosome entry site (IRES)–GFP that were either empty or encoded a strongly transactivating version of Eomes (Eo-VP16) (8), and expanded them for 6 d under our culture conditions. Eo-VP16, but not the empty GFP retrovirus, increased perforin expression in both WT and T-bet–deficient CD8+ T cells (Fig. 2 B, lanes 2, 3, 5, and 6). As expected, Eo-VP16 also rescued the early defect in IFN-γ production observed in T-bet–deficient CD8+ T cells (Fig. 2 D). However, Eo-VP16 did not induce GzmB mRNA expression in either WT or T-bet–deficient cells; thus, the partial T-bet dependence of GzmB mRNA expression cannot be compensated for by Eo-VP16.