A recombinant IL-4-Pseudomonas exotoxin inhibits protein synthesis and overcomes apoptosis resistance in human CLL B cells

We have determined that CLL B cells consistently express type 3 membrane receptors for the Th2-derived cytokine IL-4 (IL-4R). Furthermore, when added to CLL B cells, IL-4 induces increased apoptosis resistance, increased protein synthesis in CLL B cells and rapid onset activation of STAT1, STAT5 and STAT6. Since the IL-4-IL-4R pathway is intact in CLL B cells and is related to apoptosis resistance, we considered whether we could target this pathway. A recombinant IL-4 Pseudomonas exotoxin fusion protein (IL-4PE), known to bind to IL-4R, was incubated with CLL B cells. IL-4PE (10 ng/ml) cultured with CLL B cells resulted in an increase of apoptosis/death from mean levels of 46.6 ± 7.0 of non-exposed cells to 69 ± 8.6 (n = 6). By measuring in vitro protein synthesis, two predominant patterns of sensitivity were observed. In one, CLL B cell clones (n = 4) were found to be extremely sensitive to IL-4PE (IC50's range = 6-25 ng/ml). In the second, low concentrations of IL-4PE induced agonist activity while increasing concentrations induced cytotoxicity in 6 of 21 patient-derived cells. These studies suggest that the IL-4R, on B-CLL cells, can serve as a unique molecular target for directing cytotoxic agents in the therapy of B-CLL.