Raised Neutrophil Phospholipase A2 Activity and Defective Priming of NADPH Oxidase and Phospholipase A2 in Sickle Cell Disease

Abstract

Intermittent painful crises due to vasoocclusion are the major clinical manifestation of sickle cell disease (SCD), but subclinical episodes may also occur. There is sparse evidence for the involvement of neutrophils in the pathophysiology of SCD, but production of cytokines by the damaged endothelium might influence neutrophil function and modulate responses to subsequent cytokine exposure. In addition, the activation of neutrophils in the microcirculation could itself exacerbate vasoocclusion. To test whether neutrophil inflammatory responses were altered in SCD, neutrophil phospholipase A2 and NADPH oxidase activity in response to in vitro priming by granulocyte-macrophage colony-stimulating factor (GM-CSF) and tumor necrosis factor-α (TNF-α) were measured both during and between painful crises. Resting levels of neutrophil phospholipase A2 activity in steady-state SCD (4.0% ± 0.5% of total cell radioactivity) were raised relative to control values (2.0% ± 0.2%, n = 10, P = .008). There was no defect of agonist-stimulated phospholipase A2 or NADPH oxidase activity in steady-state SCD; however, the ability of phospholipase A2 to respond to priming with GM-CSF was attenuated to 63% ± 17% of control values (n = 10,P = .04). Similarly, neutrophil NADPH oxidase activity after priming with GM-CSF and TNF-α was, respectively, 65% ± 11% (n = 7, P = .03) and 57% ± 7% of control (n = 10, P = .007) in steady-state disease, and was further reduced during painful vasoocclusive crises to 34% ± 9% and 25% ± 3% of control for GM-CSF and TNF-α, respectively. These data were not explained by poor splenic function or any racial factor, as normal cytokine responses were seen in splenectomized patients in remission from Hodgkin's disease and in healthy Afro-Caribbean subjects. Abnormal neutrophil cytokine priming responses were not observed in either patients with rheumatoid arthritis or iron-deficiency anemia. Our findings are indicative of an ongoing inflammatory state in SCD between painful crises involving neutrophil activation and an abnormality of cytokine-regulated neutrophil function, which may compromise the host defenses against certain microorganisms.