Abstract

Sickle cell disease (SCD) is caused by an abnormal hemoglobin (HbS), which results mainly in sickling and hemolysis of RBC. However, the platelets and the polymorphonuclear neutrophils (PMN) are also involved in the pathophysiology of the disease. Similar to the findings in thalassemia, some patients develop thromboembolic phenomena with hypercoagulable state, which is due in part to platelet activation. In addition, a growing body of evidence suggests that WBC, particularly PMN, are abnormal; their number is elevated during painful crises and the severity of the disease increases with their number. Several aspects of the changes in the three blood lineages are thought to result from oxidative stress, which represents the imbalance between enhanced generation of reactive oxygen species (ROS) and a low cellular content of antioxidants such as reduced glutathione (GSH), the major intracellular scavenger of ROS. In SCD, oxidative stress results primarily from the premature precipitation of the unstable HbS and the accumulation of excess iron, a catalyst in the formation of ROS. Accumulation of ROS may result in hemolysis of RBC, activation of platelets and a respiratory burst of PMN. We developed flow cytometry techniques for measuring oxidative-state markers, ROS generation and GSH content, simultaneously in RBC, platelets and PMN.

Peripheral blood samples obtained from 15 normal donors, 10 patients with SCD and 5 SCD/beta-thalassemia patients (two with the IVS2-1 mutation and 3 with the IVS1-6 mutation) were studied. Out of the 15 SCD patients, 5 were splenectomized, and all were treated with folic acid, except one who was treated with hydroxyurea. The blood was mixed with 3% gelatin, and following 30 min. incubation the upper layer, containing RBC, platelets and WBC, was collected. ROS and GSH were measured by FACS analysis following staining with 2′, 7′-dichlorofuoresceine (DCF) and mercury orange, respectively. Cells were gated on the basis of size and granularity to include platelets, RBC or PMN, exclusively. The cells in each gate were analyzed for green (DCF) or orange (mercury orange) fluorescence and the Mean Fluorescence Channel (MFC) was calculated. The results showed that ROS production increased by 10 to 30-fold in RBC, platelets and PMN from SCD patients compared with that of their normal counterparts. Concomitantly, the GSH content decreased by 20–50% in the SCD cells. It was possible to modulate the oxidative status of cells from both normal donors and SCD patients: Exposure of the cells to oxidants such as hydrogen peroxide (2 mM), hemin (0.1 mM) or iron (ferric ammonium citrate - 0.1 mM) increased the oxidative status in all cell types, while antioxidants such as N-acetyl cysteine, vitamin C (both at 1 mM) and vitamin E (0.2 mM) significantly decreased the oxidative stress. Our results indicate that similar to the findings in thalassemia, in SCD, RBC and platelets, as well as PMN, are in a state of oxidative stress, which could in part account for the clinical manifestations. Addition of antioxidants, which reduced the ROS and enhanced the GSH content of the cells, could protect against oxidative damage. The flow cytometry techniques we developed may prove useful for studying the effects of various antioxidants and for monitoring the patient’s oxidative status during therapy thereby providing an objective, quantitative evaluation of their efficacy.

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