IL-21 Promotes Apoptosis through Induction of the BH3 Only Protein Bim and Enhacnes Direct and Innate Immune-Promoted Death of Chronic Lymphocytic Leukemia Cells.

Chemoimmunotherapy with fludarabine and the anti-CD20 monoclonal antibody rituximab has demonstrated promising clinical activity in chronic lymphocytic leukemia (CLL). We hypothesized that the gamma chain receptor cytokine IL-21, which is currently in clinical trials for lymphoma, might enhance the efficacy of this regimen by augmenting both immune-mediated clearance of CLL cells and a direct apoptotic mechanism involved in the homeostasis of normal B cells. CLL cells expressed variable levels of the IL-21 receptor alpha subunit (<10–80% positive cells, n = 16). IL-21 induced direct apoptosis in CLL cells from a subset of patients (40% ± 15.9 apoptotic cells; n = 9; p <0.0001), which directly correlated with increased expression of surface IL-21R (p = 0.001). The in vitro apoptosis occurred at physiologically attainable concentrations (25 ng/ml) and was time- and dose-dependent. As a single agent, IL-21 did not activate CLL cells, as evidenced by lack of surface expression of CD86, HLADR, CD95, or CD40. However, IL-21 induced phosphorylation of STAT-1^Tyr-701 and STAT-3 ^Tyr-705 in CLL cells exhibiting > 20% apoptosis at 72 hours, whereas phosphorylation of these proteins was not seen in CLL cells failing to undergo apoptosis. Similar to normal murine B cells, IL-21-mediated death was associated with up-regulation of the pro-apoptotic BH3 only domain protein Bim, whereas Bim was not up-regulated in CLL cells insensitive to IL-21-induced apoptosis. Furthermore, silencing of Bim in primary CLL cells with siRNA antagonized IL-21-mediated death. Preliminary studies examining the mechanism of Bim up-regulation demonstrated that both total levels and phosphorylation of FOXO3A^Thr32 increases following IL-21 treatment. FOXO3a is involved in transcriptional regulation of Bim, and further mechanistic studies are ongoing and will be presented. Given the favorable modulation of Bim, we examined the ability of IL-21 to enhance apoptosis in response to rituximab, alemtuzumab, or fludarabine. Our studies confirm that CLL cells pre-treated with IL-21 are sensitized to rituximab and fludarabine, whereas IL-21 had no effect on fludarabine-mediated apoptosis of normal T cells. IL-21 also enhanced NK cell ADCC against rituximab-coated autologous CLL cells (42 ± 4.4% rituximab-specific lysis vs. 28 ± 3.1% at an E:T ratio of 25:1; p < 0.0001). These data provide evidence that IL-21 promotes direct apoptosis through induction of Bim and also enhances fludarabine- and rituximab-mediated apoptosis. Additionally, IL-21 enhances autologous innate immune activation of NK cells toward primary CLL cells coated with rituximab. Overall, these findings provide justification for combination studies of IL-21 with fludarabine and rituximab chemoimmunotherapy in CLL and point to Bim induction as a pharmacodynamic endpoint to predicting surrogate biologic activity of IL-21 in vivo as part of planned clinical trials with this agent.