Differential regulation of human eosinophil IL-3, IL-5, and GM-CSF receptor alpha-chain expression by cytokines: IL-3, IL-5, and GM-CSF down-regulate IL-5 receptor alpha expression with loss of IL-5 responsiveness, but up-regulate IL-3 receptor alpha expression.

Our recent data suggested that tissue eosinophils may be relatively insensitive to anti-IL-5 treatment. We examined cross-regulation and functional consequences of modulation of eosinophil cytokine receptor expression by IL-3, IL-5 GM-CSF, and eotaxin. Incubation of eosinophils with IL-3, IL-5, or GM-CSF led to reduced expression of IL-5R alpha, which was sustained for up to 5 days. Eosinophils incubated with IL-5 or IL-3 showed diminished respiratory burst and mitogen-activated protein kinase kinase phosphorylation in response to further IL-5 stimulation. In contrast to these findings, eosinophil expression of IL-3R alpha was increased by IL-3, IL-5, and GM-CSF, whereas GM-CSF receptor alpha was down-regulated by GM-CSF, but was not affected by IL-3 or IL-5. CCR3 expression was down-regulated by IL-3 and was transiently reduced by IL-5 and GM-CSF, but rapidly returned toward baseline. Eotaxin had no effect on receptor expression for IL-3, IL-5, or GM-CSF. Up-regulation of IL-3R alpha by cytokines was prevented by a phosphoinositol 3-kinase inhibitor, whereas this and other signaling inhibitors had no effect on IL-5R alpha down-regulation. These data suggest dynamic and differential regulation of eosinophil receptors for IL-3, IL-5, and GM-CSF by the cytokine ligands. Since these cytokines are thought to be involved in eosinophil development and mobilization from the bone marrow and are present at sites of allergic inflammation, tissue eosinophils may have reduced IL-5R expression and responsiveness, and this may explain the disappointing effect of anti-IL-5 therapy in reducing airway eosinophilia in asthma.

Proteoglycans are potent modulators of the biological responses of eosinophils to chemokines.

Chemokines play critical roles in governing the recruitment and activation of eosinophils at sites of allergic inflammation, particularly the asthmatic lung. However, we know little of how chemokine function is regulated post-translationally. Proteoglycans, consisting of a core protein and glycosaminoglycan (GAG) side chains, are cell surface molecules and components of the extracellular matrix that are able to bind chemokines, whilst heparin is a GAG with therapeutic value in asthma. We examined whether soluble GAG could alter the actions of chemokines in assays of eosinophil activation. Heparin inhibited intracellular calcium flux, respiratory burst and chemotactic responses of eosinophils to CCL11, but not to the chemoattractant C5a, and inhibited binding of CCL11 to CCR3. Heparin also inhibited eosinophil stimulation by CCL11, CCL24, CCL7, CCL13 and CCL5 to differing degrees, which broadly correlated with their relative affinities for heparin. Heparan sulfate and dermatan sulfate, but not chondroitin sulfate, also inhibited the actions of CCL11 and CCL13 in assays of cellular shape change and chemotaxis. Following treatment with the sulfation inhibitor chlorate or proteoglycanases, no inhibition of CCL11-induced activity was observed using either eosinophils or a CCR3-expressing transfectant cell line. This suggests that cell surface proteoglycans are not necessary for signaling via CCR3. However, the GAG context in which chemokines are expressed is likely to represent an important level of regulation of allergic inflammation.