Production and Expression of Inflammatory Mediators in Leukocytes of Sickle Cell Anaemia Patients and Effects of Hydroxyurea Therapy on This Production

Leukocytosis is frequently observed in sickle cell disease (SCD) in the absence of bacterial infection. An elevated baseline leukocyte count is associated with an increased risk of early death and leukocytes play a significant role in the initiation of vaso-occlusive events. Inflammation, cell adhesion to vascular endothelium, and subsequent endothelial injury appear to contribute to sickle cell anemia (SCA) vaso-occlusion. Furthermore, blood levels of inflammatory and anti-inflammatory cytokines are reported to be elevated (TNF-α, IL-6, IL-10, GM-CSF), both in steady state and during crisis, but reports have been conflicting and a conclusive role for these molecules in the disease remains to be established. Furthermore, the effect of hydroxyurea therapy (HU) on the release of inflammatory mediators is not understood. The aim of this study was to determine plasma levels and leukocyte gene expressions of inflammatory mediators in healthy controls (n=30), steady-state SCA patients (n=45) and SCA patients on HU therapy (n=24). qRT-PCR analysis was use to examine gene expression and ELISA protein production. TNF-α, IL-8 and PGE₂ plasma levels were significantly higher in the plasma of steady-state SCA individuals, when compared to control individuals (2.95 ± 0.4 pg/ml; 16.5 ± 2.5 pg/ml; 5.7 ± 0.6 pg/ml; 128.3 ± 12.2 pg/ml vs 1.43 ± 0.2 pg/ml, 88.5 ± 5.9 pg/ml, P=0.006; P<0.0001; P=0.012, respectively). HU therapy significantly reversed augmented TNF-α (1.6 ± 0.2 pg/ml, P=0.006) and, interestingly, increased plasma anti-inflammatory IL-10 (P<0.05). IL-10, IFN-γ, COX-2 and iNOS gene expressions were unaltered in SCA mononuclear cells (MC), however gene expressions of TNF-α, IL-8 and the protective enzyme, heme oxygenase-1 (HO-1), were significantly higher compared to healthy controls (0.46 ± 0.01; 0.08 ± 0.02; 0.21 ± 0.05 vs 0.18 ± 0.04; 0.02 ± 0.005; 0.035 ± 0.008; respectively, P<0.02). HU therapy was not associated with significantly altered SCA MC inflammatory gene expression, although COX-2 mRNA expression was decreased (0.11 ± 0.05; 0.37 ± 0.12, SCAHU and SCA, respectively; P<0.05). In SCA neutrophils, gene expressions of IL-8, IFN-γ, iNOS and HO-1 were significantly higher compared to those of control subjects (0.32 ± 0.07; 0.69 ± 0.19; 0.19 ± 0.06; 0.33 ± 0.09, P=0.02, P=0.025, P<0.05; P=0.027, respectively). Patients on HU therapy demonstrated lower iNOS and higher IL-10 neutrophil gene expressions compared to SCA not on HU therapy (0.038 ± 0.03; 0.72 ± 0.13, P<0.05; P<0.05, respectively). Taken together, data suggest that alterations in the gene expressions and productions of a number of pro-and anti-inflammatory mediators are present in SCA and knowledge of these pathways may be important for identifying novel drug targets for the disease.