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Rationale and Objectives: Patients with severe asthma have a poor therapeutic response to corticosteroid therapy, and corticosteroid responsiveness cannot be easily measured in these patients. We hypothesized that this poor response is associated with a reduced effect of corticosteroids to inhibit cytokine release from activated peripheral blood mononuclear cells (PBMCs).Methods: Patients with severe asthma were defined by American Thoracic Society criteria. We compared the suppression of LPS-induced cytokine release (monocyte chemotactic protein-1 [MCP-1], macrophage inflammatory protein [MIP] 1alpha, RANTES, tumor necrosis factor alpha, interleukin 1beta (IL-1beta), IL-8, IFN-gamma, IL-6, IL-10, and granulocyte-macrophage colony-stimulating factor [GM-CSF]) by dexamethasone from PBMCs of patients with severe asthma (n = 16), patients with nonsevere asthma (n = 19), and normal volunteers (n = 10).Results: There was no difference in baseline spontaneous or stimulated release of these cytokines among groups. LPS-induced release of 10 cytokines was less suppressed by dexamethasone (10(-6) M) in patients with severe asthma compared with patients with nonsevere asthma, with statistical significance achieved for IL-1beta (p < 0.03), IL-8 (p < 0.03), and MIP-1alpha (p < 0.003), and borderline significance for IL-6 (p = 0.054). There was less difference between the two groups for dexamethasone at 10(-8) M. Nuclear histone deacetylase (HDAC) and histone acetyltransferase activities were reduced in patients with severe asthma compared with patients with nonsevere asthma (p < 0.01). HDAC activity reduction correlated directly to the degree of steroid insensitivity of GM-CSF (r = 0.57, p < 0.01) and IFN-gamma (r = 0.56, p < 0.05) release. Reduction in histone acetyltransferase activity related to corticosteroid use rather than asthma severity.Conclusions: Patients with severe asthma have diminished corticosteroid sensitivity of PBMCs when compared with patients with nonsevere asthma, associated with a reduction in HDAC activity that parallels the impaired corticosteroid sensitivity. [ABSTRACT FROM AUTHOR]