Lyn tyrosine kinase is important for IL-5-stimulated eosinophil differentiation

IL-5 plays a pivotal role in growth and differentiation of eosinophils. The signal transduction mechanism of IL-5Rα is largely unknown. We have demonstrated that IL-5 induces tyrosine phosphorylation of IL-5Rα in eosinophils. To identify IL-5Rα-associated tyrosine kinases, we have examined the expression of Src family tyrosine kinases in eosinophils. Among the Src family members, Lyn, Hck, Fgr, and Lck are present in eosinophils, and, among these four kinases, only Lyn is associated with the IL-5Rα under basal conditions. We also confirm the association of Janus kinase (Jak)2 with IL-5Rα. Lyn kinase phosphorylates both IL-5Rα and βcR in vitro. The importance of Lyn kinase for eosinophil differentiation was studied using antisense oligodeoxynucleotides. Lyn antisense oligodeoxynucleotide blocks eosinophil differentiation from stem cells in a dose-dependent manner. The Jak2 inhibitor tyrphostin AG490 also inhibits eosinophil differentiation. The importance of Lyn for eosinophil differentiation was further studied using Lyn knockout mice. The IL-5-stimulated eosinophil differentiation from bone marrow cells is significantly inhibited in Lyn^-/- mice as compared with that in control mice. We conclude that both Lyn and Jak2 play an essential role in IL-5Rα signaling, leading to eosinophil differentiation. The effect of Lyn appears to be relatively specific for the eosinophilic lineage.