Daclizumab (Zenapax (R)) inhibits early interleukin-2 receptor signal transduction events

Daclizumab, a humanized antibody against the interleukin-2 (IL-2) receptor (R) alpha -chain, is a promising new immunosuppressant in transplantation. As its exact mechanism of action has remained unclear, we examined its shor t-term effects on primary human T lymphocytes expressing the high-affinity IL-2R. Daclizumab exposure for 20 min neither affected T cell viability nor their surface expression of the IL-2R alpha-, beta-, or gamma -chains. How ever, after IL-2 stimulation (200 U/ml, 20 min), immunoblots of cell lysate s demonstrated attenuation of the IL-2-induced tyrosine phosphorylation of 65-75 kDa proteins by Daclizumab, but not by isotype controls. Since this i s the molecular weight of the IL-2R beta- and gamma -chains, which are both tyrosine-phosphorylated by IL-2, we next examined the effect of Daclizumab on their IL-2-induced tyrosine phosphorylation. In immunoblots of IL-2R be ta- and gamma -chain-immunoprecipitates the tyrosine phosphorylation of bot h chains by IL-2, but not by IL-15, was attenuated in the presence of Dacli zumab. Furthermore, co-immunoprecipitation experiments showed that Daclizum ab inhibited the IL-2-induced association of these chains, a prerequisite f or their mutual tyrosine phosphorylation. Lastly, we demonstrated that Dacl izumab inhibits the receptor-downstream induction of the IL-2-activated DNA -binding protein STAT5 in gel shift assays. We conclude that Daclizumab dir ectly and specifically interferes with IL-2 signaling at the receptor level by inhibiting the association and subsequent phosphorylation of the IL-2R beta- and gamma -chains induced by ligand binding. Under our experimental c onditions, Daclizumab had no effects on cell viability, and it did not modu late the surface expression of the IL-2R alpha-, beta-, or gamma -chains. ( C) 2000 Elsevier Science B.V. All rights reserved.