Syk-deficient eosinophils show normal interleukin-5-mediated differentiation, maturation, and survival but no longer respond to FcgammaR activation.

The tyrosine kinase Syk has been proposed to play a critical role in the antiapoptotic effect of interleukin (IL)-5 in human eosinophils. However, little is known about the involvement of Syk in other IL-5-mediated activation events. To further address these questions, the role of Syk in IL-5-induced eosinophil differentiation, activation, and survival was analyzed using cells obtained from Syk-deficient mice. We could demonstrate that Syk-deficient fetal liver cells differentiate into mature eosinophils in response to IL-5 at the same rate as wild-type fetal liver cells and generate the same total number of eosinophils. Moreover, no difference in IL-5-induced survival of mature eosinophils between Syk(-/-) and wild-type eosinophils could be demonstrated, suggesting that the antiapoptotic effect of IL-5 does not require Syk despite the activation of this tyrosine kinase upon IL-5 receptor ligation. In contrast, eosinophils derived from Syk-deficient but not wild-type mice were incapable of generating reactive oxygen intermediates in response to Fcgamma receptor (FcgammaR) engagement. Taken together, these data clearly demonstrate no critical role for Syk in IL-5-mediated eosinophil differentiation or survival but underline the importance of this tyrosine kinase in activation events induced by FcgammaR stimulation.