Differential regulation of TNFalpha and GM-CSF induced activation of P38 MAPK in neutrophils and eosinophils.

P38 MAPK is a central mediator in cytokine signalling in human leukocytes. P38 MAPK is activated by phosphorylation of a conserved Thr180-X-Tyr182 motif by dual phosphorylation via the upstream kinases MKK3 and MKK6. Alternatively, P38 MAPK can be activated via autophosphorylation when associated with TAB1. In this study P38 MAPK phosphorylation and activation (measured via phosphorylation of P38 MAPK downstream target MK2) were investigated upon engagement of the GM-CSF- and TNFalpha-receptors expressed on both eosinophils and neutrophils. The MKK3/MKK6 pathway mediated neutrophil P38 MAPK activation after stimulation with TNFalpha (100U/ml) or GM-CSF (10(-10)M). Under these conditions the activation but not phosphorylation of P38 MAPK could be inhibited by SB203580 (10(-5)M or 10(-6)M). In eosinophils SB203580 (10(-6)M) inhibited both the phosphorylation and activation of P38 MAPK after stimulation with several doses of TNFalpha (10-10000 U/ml) or GM-CSF (10(-11) to 10(-9)M), indicating that P38 MAPK activation is mediated via autophosphorylation in eosinophils. This hypothesis was supported by the finding that in marked contrast to neutrophils, MKK3/MKK6 did not show enhanced phosphorylation in eosinophils after cytokine stimulation, but were constitutively phosphorylated. Therefore, the involvement of TAB1 was investigated with regard to this cytokine-induced autophosphorylation. Co-immunoprecipitation experiments showed that TAB1 was constitutively associated with P38 MAPK in eosinophils and neutrophils and that cytokine-induced autophosphorylated P38 MAPK was co-precipitated with TAB1. These findings are consistent with the hypothesis that cytokine-induced autophosphorylation of P38 MAPK in primary granulocytes depends on the interaction with TAB1.

Rapid selective priming of FcalphaR on eosinophils by corticosteroids.

Preactivation or priming of eosinophils by (proinflammatory) cytokines is important in the pathogenesis of allergic diseases. Several priming-dependent eosinophil responses, such as migration and adhesion, are reduced by treatment with corticosteroids. Many inhibitory effects of corticosteroids are mediated by the glucocorticoid receptor via genomic mechanisms, which are evident only after prolonged interaction (>30 min). However, also faster actions of corticosteroids have been identified, which occur in a rapid, nongenomic manner. In this study, fast effects of corticosteroids were investigated on the function of eosinophil opsonin receptors. Short term corticosteroid treatment of eosinophils for maximal 30 min with dexamethasone (Dex) did not influence eosinophil cell surface CD11b/CD18 expression, adhesion, and/or chemokinesis. In marked contrast, incubation with Dex resulted in a rapid increase in binding of IgA-coated beads to human eosinophils, showing that Dex can up-regulate the activation of FcalphaR (CD89). This priming response by Dex was dose dependent and optimal between 10(-8) and 10(-6) M and was mediated via the glucocorticoid receptor as its selective antagonist RU38486 (10(-6) M) blocked the priming effect. In contrast to FcalphaR, eosinophil FcgammaRII (CD32) was not affected by Dex. Further characterization of the Dex-induced inside-out regulation of FcalphaR revealed p38 MAPK as the central mediator. Dex dose dependently enhanced p38 MAPK phosphorylation and activation in situ as measured by phosphorylation of its downstream target mitogen-activated protein kinase-activated protein kinase 2. The dose responses of the Dex-induced activation of these kinases were similar as seen for the priming of FcalphaR. This work demonstrates that corticosteroids selectively activate the FcalphaR on eosinophils by activation of p38 MAPK.