Hypersensitivity to IL-27, and not IL-6, drives increased CD8⁺ T-cell proliferation in the absence of SOCS3. (A) Purified wt LN T cells were stimulated for the indicated times with anti-CD3 (10 μg/mL) plus anti-CD28 (5 μg/mL), and the expression of IL-6, IL-27p28, and IL-2 was measured by RT-PCR. CFSE-labeled wt (+/+) or Socs3^ΔLck/ΔLck (Δ/Δ) LN CD8⁺ T cells were stimulated for 3 days with anti-CD3 (0.6 μg/mL) with or without IL-6 (10 ng/mL) (B) or IL-27 (100 ng/mL) (C) and analyzed by flow cytometry. (D) The capacity of the WSX-1Fc reagent to inhibit IL-27 signaling was assessed by intracellular flow cytometry. STAT1 activation was measured in splenocytes from wt mice stimulated for 15 minutes with IL-6 (10 ng/mL) or IL-27 (5 ng/mL), with or without 5 μg/mL WSX-1Fc chimeric receptor. (E) CFSE-labeled wt or Socs3^ΔLck/ΔLck LN CD8⁺ T cells were stimulated for 3 days with anti-CD3 with or without 5 μg/mL WSX-1Fc chimeric receptor. (F) Purified, CFSE-labeled wt (+/+, shaded), Socs3^ΔLck/ΔLck (SOCS3, –), or IL-6–deficient Socs3^ΔLck/ΔLck (IL-6/SOCS3, ---) CD8⁺ LN T cells were stimulated for 3 days with anti-CD3 (10 μg/mL). The division index shows the average number of cell divisions in each culture. Data are representative of 2 to 3 independent experiments.