IL-4 inhibition of the caspase-3 pathway by a PARP-14–dependent mechanism. Splenocytes from littermate WT and PARP-14-null mice, along with age-matched Stat6-deficient counterparts, were cultured (20 hours) in medium with or without IL-4. (A) PARP-14 mediates repression of caspase activity by IL-4. Shown in top panels are FACS profiles of B220⁺ cells analyzed using a fluorophore-conjugated substrate of enzymatically active caspases. The bar graph in the bottom panel summarizes mean (± SEM) data on the protective effect of IL-4 in the experiments, calculated as in Figure 4. Additional statistical analyses confirmed that percentage of caspase-positive was higher in IL-4–treated PARP-14 KO samples than controls (40.3 ± 2.0 vs 31.3 ± 2.9; P < .05). (B) IL-4 acts on B cells with DNA damage to signal inhibition of caspase activity by a PARP-14–dependent mechanism. As in panel A, except that cells were γ-irradiated (2 Gy) before culture (20 hours) in the presence or absence of IL-4. (C) Suppression of caspase-3 activity in IL-4–treated B cells depends on PARP-14. As in panel A, except that the B cells were probed by intracellular staining with antibodies specific for the cleaved, activated form of caspase-3. (D) As in panel C, except that cells were irradiated (2 Gy) before culture with or without IL-4. All inset numbers represent the percentage of cells positive for the caspase signal (total activity or activated caspase-3); n = 4. *P < .05; **P < .01; ∼P = .07; #P = .14.