Mechanisms of Granule Protein Mobiliation in Blood Eosinophils

Karawajczyk, M. 2000. Mechanisms of granule protein mobilization in blood eosinophils. Acta Universitatis Upsaliensis. Comprehensive Summaries of Uppsala Dissertations from the Faculty of Medicin 951 p.60 Uppsala. ISBN 91-554-4799-6. Serum levels of the eosinophil granule proteins ECP, EPO and EPX, which are stored in the matrix of specific granules, were shown to correlate with the course of disease in disorders involving eosinophils. The concentrations of eosinophil proteins in serum are the result of their release in vivo, and also ex vivo during the sampling procedure. Generally, eosinophils release the contents of their specific granules in three ways, namely by exocytosis, piecemeal degranulation (PMD), or cytolysis. The manner of eosinophil protein release in blood eosinophils has not been determined. The aim of this work was to study the mechanisms of granule protein release from blood eosinophils in respect to protein subcellular localization and cell ultrastructure. In patients with bacterial infections, the serum level of ECP but not of EPO was increased, while in patients with viral infections both proteins remained within the range of healthy controls. When G-CSF, a cytokine involved in the response mechanism to bacterial but not viral infections, was administered to healthy subjects, it induced an increase in eosinophil count and a preferential increase in serum EPX and ECP in comparison to EPO. The PMD model consists of stepwise transportation of specific granule contents from the granules towards the plasma membrane. We observed that administration of G-CSF in healthy subjects and allergen exposure of allergic subjects during the birch pollen season caused ultrastructural changes in the eosinophil specific granules, such as loosening of the matrix, granule matrix lucency, and ragged losses of their core. Similar morphological alterations had previously been observed in eosinophils undergoing PMD. ECP, EPX and EPO were localized not only in the specific granules but also in extragranular compartments, as shown both by immunoelectron microscopy and subcellular fractionations. An extragranular EPX compartment were present in healthy as well as in allergic and hypereosinophilic subjects, and there were no significant differences in its size between the groups. The extragranular compartments of ECP and EPO were increased in size in allergic subjects during the birch pollen season and were clearly separate from the extragranular compartment of EPX. Results of this investigation indicate differential mobilization of granule proteins in circulating eosinophils and point to piecemeal degranulation as its principal mechanism.