Fig. 7. T_RH help to CD8⁺ T cells is IL-21 dependent.

(A) Representative dot plot of IL-21^Hi or IL-21^Low CD4⁺ T cells from the IL-21-VFP reporter mice infected with PR8 (28 d.p.i.). (B to E) WT mice were infected PR8 with or without IL-21R blockade through intraperitoneal (i.p.) route starting at 14 d.p.i. in the presence of FTY720 administration (13 to 34 d.p.i.). (B) Experimental scheme. Cell numbers of lung parenchymal (C) B_GC, (D) HA-B_RM, and (E) CD8⁺CD69⁺ NP_366–374, or CD8⁺CD69⁺ PA_224–233 T_RM cells. (F to J) WT mice were infected with PR8 with or without IL-21R blockade through intranasal (i.n.) route at 14 d.p.i. (F) Experimental scheme. (G) Frequencies or (H) cell numbers of lung tissue CD8⁺CD69⁺ NP_366–374 or CD8⁺CD69⁺ PA_224–233 T_RM. (I) Percentage of apoptotic cells were identified by active caspase 3/7-FLICA staining within lung CD8⁺ NP_366–374 T_RM or splenic CD8⁺ NP_366–374 T_MEM at 28 d.p.i. (J) Representative histogram of BATF expression in lung CD8⁺ NP_366–374 or CD8⁺ PA_224–233 T_RM of mice received with Ctl IgG or α-IL21R at 35 d.p.i. FMO, fluorescence minus one control. (K to M) WT mice were infected with PR8 with or without IL-21R blockade in the presence of FTY720. Mice were challenged with X31 at 40 d.p.i. (K) Experimental scheme. (L) Lung infectious virus titer or (M) viral gene expression were measured at day 2 after rechallenge. In (A) and (I) to (M), representative data were from at least two independent experiments (four to five mice per group). In (C), (E), and (G) to (H), data were pooled from two independent experiments (three to five mice per group). P values of all experiments were calculated by unpaired two-tailed Student’s t test. Data are means ± SEM. *P < 0.05, **P < 0.01, and ***P < 0.001.