Fig. 3.

IL-21 acts directly to sustain virus-specific CD8⁺ T cells during an ongoing infection. Cohorts of control Il21r^+/+/IL21r^+/+ (CD45.1/CD45.2) and experimental IL21r^+/+/IL21r^−/− (CD45.1/CD45.2) mixed bone-marrow chimeras were infected with LCMV-Cl 13 and CD8⁺ T cell responses evaluated over time. (A) PBMCs were evaluated by flow cytometry to check reconstitution of CD8⁺ T cells in Il21r^+/+/Il21r^+/+ or Il21r^+/+/Il21r^−/− mixed bone-marrow chimeras prior to infection. Gated CD8⁺ T cells are shown. (B) Flow cytometric analysis of GP₃₃- and GP₂₇₆-specific CD8⁺ T cell responses in the circulation at days eight and 16 after infection. Gated tetramer⁺ CD8⁺ T cells are shown. (C) Flow cytometric analysis of splenic CD8⁺ T cells and GP₃₃- and GP₂₇₆-specific responses at three weeks following infection. Gated CD8⁺ (left panel) or CD8⁺ tetramer⁺ (right panels) cells are shown. (D) Absolute numbers of GP₃₃- and GP₂₇₆-specific CD8⁺ T cells in mixed bone-marrow chimeras three weeks following infection. Graphs represent average + SD of Il21r^+/+ CD45.1 CD8⁺ T cells (black), Il21r^+/+ CD45.2 CD8⁺ T cells (gray) and Il21r^−/− CD45.2 CD8 T cells (white). **P < 0.01, ***P < 0.001. Representative results are shown from one of two similar experiments (n= 7 and 8 for the Il21r^+/+/IL21r^+/+ and IL21r^+/+/IL21r^−/− cohorts, respectively)