(A and B) Analysis of RCDII IEL lines cultured with 5 ng/ml IL-15 in the presence of anti–IL-15Rα or anti–IL-15Rβ blocking mAbs or control isotype for 72 hours. (A) Flow cytometry analysis of apoptosis in 1 representative cell line, and mean fold increase in percent apoptotic (annexin V/PI+) cells in 4 RCDII IEL lines. Only anti–IL-15Rβ mAb significantly increased apoptosis. *P = 0.02 versus IgG, paired t test. (B) Analysis of Bcl-2, Bcl-xL, and Mcl-1 expression by Western blot and of Bcl-2 and Bcl-xL expression (black histograms and text) by flow cytometry. Blockade of IL-15Rβ, but not IL-15Rα, decreased Bcl-xL, and to a lesser degree Bcl-2, compared with cells cultured in the presence of control isotype (gray filled histograms and text); Mcl-1 expression was unchanged. Shown is 1 representative experiment of 4. (C and D) Analysis of IL-15Rβ–dependent signaling in RCDII IEL lines. (C) Comparison of cell lines cultured with or without IL-15 for 72 hours demonstrated IL-15–induced phosphorylation of Jak3, STAT5, STAT3, and AKT. (D) Flow cytometry showed that IL-15 starvation and IL-15Rβ blockade, but not IL-15Rα blockade, decreased phosphorylation of STAT5, STAT3, AKT, and ERK (black histograms and text) compared with control cells in IL-15 (gray filled histograms and text). Dashed lines in A, B, and D denote isotype controls.