CXCL9 inhibits eosinophil responses by a CCR3- and Rac2-dependent mechanism.

Recently, inhibitory cytokine pathways for leukocyte chemoattraction and activation have been identified, but there is little insight into the operational mechanisms except for models that rely on simple receptor antagonism. We have previously identified the existence of a murine eosinophil inhibitory pathway mediated by the CXC chemokine ligand 9 (CXCL9, Mig [monokine induced by interferon-gamma]) that impressively blocks eosinophil chemoattraction and function, but the mechanism has remained elusive. We now demonstrate that Mig's inhibitory action extends beyond receptor antagonism alone. Notably, in addition to inhibiting eotaxin-induced filamentous actin (F-actin) formation and chemoattraction, Mig potently blocks platelet activating factor (PAF)- and leukotriene B4 (LTB4)-induced responses. Remarkably, Mig-treated eosinophils display an abnormal F-actin assembly in the absence of agonist stimulation. Additionally, Mig pretreatment inhibits eotaxin-induced activation of the Rho-guanosine triphosphatase (GTPase) Rac, and Rac2-deficient eosinophils demonstrate an impaired transmigration and actin polymerization response to eotaxin stimulation. Furthermore, Mig was unable to inhibit eotaxin-induced responses in Rac2-deficient eosinophils. Finally, using CCR3 gene-targeted cells, Mig's inhibitory activity is demonstrated to be mediated by CC chemokine receptor 3 (CCR3). Thus, by altering agonist-induced signaling and abrogating cytoskeletal reorganization by a Rac2-dependent mechanism, Mig markedly inhibits eosinophil responses to diverse stimuli. These results establish evidence that distinct chemokines can use CCR3 to induce opposing signals in eosinophils.