Systemic and local dexamethasone treatments prevent allergic eosinophilia in rats via distinct mechanisms

We have studied the effect of local and systemic treatment with dexamethaso ne for prevention of the pleural eosinophilia triggered by allergen in acti vely sensitised Wistar rats. Parallel changes in blood and marrow eosinophi l numbers were assessed for comparison. The intrapleural (i.pl.) injection of ovalbumin into ovalbumin-sensitised animals led to a long-lasting pleura l fluid eosinophilia which peaked from 24 to 72 h post-challenge. At these time points, there was a significant 2- to 3-fold increase in the blood eos inophil numbers, whereas the bone marrow number of mature eosinophils remai ned unaltered. Systemic treatment with dexamethasone (0.05-0.5 mg/kg, i.p.) abolished the pleural and blood eosinophilia observed 24 and 48 h post-cha llenge, also causing a significant reduction in marrow eosinophil numbers. Despite being unable to alter blood and bone marrow eosinophil numbers, the local i.pl, administration of dexamethasone (2.5-10 mu g/cavity) inhibited dose dependently the allergen-induced pleural eosinophil influx at 24 h bu t not at 48 h post-challenge. This treatment also shortened the time course of eosinophil accumulation in the pleural space from the 48 h time point o n. We conclude that the effect of systemic but not of local treatment with dexamethasone on allergen-induced eosinophil recruitment is well correlated with the inhibition of eosinophil production in bone marrow. In contrast, low amounts of dexamethasone injected into the pleural space seem to affect locally eosinophil recruitment and survival. (C) 1999 Elsevier Science B.V . All rights reserved.