Lymphocyte recruitment to brain is diminished in IL-21 deficient mice. (a) Gating strategy for leukocytes isolated from brain after MCAO. (b) WT and IL-21 KO spleen cells 24 h after tMCAO or sham procedure (n = 3 mice per group). (c) Relative change in spleen weight of WT and IL-21 KO mice after tMCAO (n = 3–7 mice per group). (d) Percentage of blood and spleen CD4⁺ T cells expressing IL-21 after 5-h ex vivo stimulation with PMA (10 ng/ml) and Ionomycin (1 µg/ml) 4 d after MCAO or control treatment. (e and f) Leukocyte accumulation in the brain of WT mice compared with IL-21 KO mice 1, 4, and 7 d after tMCAO (n = 3–6 mice per group). (g–k) In vitro cytokine expression by WT and IL-21 KO CD4⁺ and CD8α⁺ T cells after 5-h stimulation under indicated conditions with or without recombinant mouse IL-21 (100 ng/ml). (l) TNF production by CD11b⁺ myeloid cells stimulated with LPS (500 ng/ml) for 5 h with or without recombinant mouse IL-21 (100 ng/ml). Cells isolated from n = 3 mice per group. Data are representative of two to four independent experiments. *, P < 0.05; **, P < 0.01; ***, P < 0.001; ****, P < 0.0001 by Student’s t test (single comparison) or one-way ANOVA (multiple comparisons). Error bars indicate SD (b–d and g–l) and SEM (e, f).