IL-6R defines a CD8⁺ T cell subset capable of producing IL-21. (A) IL-6R expression in fresh CD4⁺ and CD8⁺ T cells. IL-6R KO T cells were used as negative controls (shaded). The gate defines the IL-6R^high (IL-6R^hi) population. (B and C) IL-6R MFI (B) and percentage of IL-6R^high (C) as in A (n = 3). (D) IL-6R expression on CD4/CD8 double positive (DP) and single CD4- or CD8-positive (CD4 SP or CD8 SP). Unstained DP thymocytes were used as negative control (shaded). (E) IL-6R expression on fresh CD8⁺ T cells (0 h) or after activation for the indicated periods of time. The unstained cells were used as negative controls (shaded). (F) Expression of CD25, KLRG1, CD127, and CD44 on IL-6R^high (IL-6R^hi) and IL-6R^low (IL-6R^lo) CD8⁺ T cells. The gate based on an unstained negative control is shown. (G) Percentage of CD44^high cells as gated in F (n = 3). (H) IL-6R MFI on CD44^high and CD44^low CD8⁺ T cells (n = 3). (I) FACS sorted CD44^low and CD44^high CD8⁺ T cells were activated in the absence or presence of IL-6. IL-21 production (72 h) was determined (n = 3). (J) Expression of IL-6R on OT-I CD8⁺ T cells (shaded histogram shows unstained cells). (K) IL-21 production from OT-I CD8⁺ T cells activated in the absence or presence of IL-6 for 72 h (n = 3). (L) FACS sorted IL-6R^hi and IL-6R^lo CD8⁺ T cells were activated in the absence or presence of IL-6. IL-21 production after 72 h was determined (n = 3). Error bars represent mean ± SD. *, P < 0.05, as determined by Student’s t test (B, C, G, H, and K) and two-way ANOVA (I and L). Results are representative of two to three experiments.