Neutrophil activation in sickle cell disease

Placenta growth factor activates monocytes and correlates with sickle cell disease severity

Sickle cell disease (SCD) results in chronic hypoxia and secondarily increased erythropoietin concentrations. Leukocytosis and activated monocytes are also observed in SCD in absence of infection or vaso-occlusion (steady state), the reasons for which are unknown. We found that erythroid cells produced placenta growth factor (PlGF), an angiogenic growth factor belonging to the vascular endothelial growth factor (VEGF) family, and its expression was induced in bone marrow CD34+ progenitor cells in the presence of erythropoietin. Furthermore, the steady state circulating PlGF levels in subjects with severe SCD (at least 3 vaso-occlusive crises [VOCs] per year) were 18.5 +/- 1.2 pg/mL (n = 9) compared with 15.5 +/- 1.2 pg/mL (n = 13) in those with mild SCD (fewer than 3 VOCs per year) and 11.3 +/- 0.7 pg/mL (n = 9) in healthy controls (P <.05), suggesting a correlation between PlGF levels and SCD severity. In addition, PlGF significantly increased mRNA levels of the proinflammatory cytochemokines interleukin-1beta, interleukin-8, monocyte chemoattractant protein-1, and VEGF in peripheral blood mononuclear cells (MNCs) of healthy subjects (n = 4; P <.05). Expression of these same cytochemokines was significantly increased in MNCs from subjects with SCD at steady state (n = 14), compared with healthy controls. Of the leukocyte subfractions, PlGF stimulated monocyte chemotaxis (P <.05, n = 3). Taken together, these data show for the first time that erythroid cells intrinsically release a factor that can directly activate monocytes to increase inflammation. The baseline inflammation seen in SCD has always been attributed to sequelae secondary to the sickling phenomenon. We show that PlGF contributes to the inflammation observed in SCD and increases the incidence of vaso-occlusive events.

Pathophysiological-based approaches to treatment of sickle cell disease

Sickle hemoglobin (HbS), as a result of its polymer-related and oxidant effects, damages the sickle erythrocyte, provokes inflammation, and causes endothelial injury. All these elements cause the phenotype of sickle cell disease. Novel treatments inhibit HbS polymerization by inducing fetal hemoglobin expression, prevent or repair erythrocyte dehydration by slowing cellular potassium and water loss, and replace HbS-producing erythroid progenitors by stem cell transplantation. Future treatment prospects include gene therapy, interruption of the interaction of sickle cells with the endothelium, inhibition of oxidative damage, and protection of an injured endothelium.

Transgenic sickle mice have vascular inflammation

Inflammation may play an essential role in vaso-occlusion in sickle cell disease. Sickle patients have high white counts and elevated levels of serum C-reactive protein (CRP), cytokines, and adhesion molecules. In addition, circulating endothelial cells, leukocytes, and platelets are activated. We examined 4 transgenic mouse models expressing human alpha- and sickle beta-globin genes to determine if they mimic the inflammatory response seen in patients. These mouse models are designated NY-S, Berk-S(Antilles), NY-S/S(Antilles) (NY-S x Berk-S(Antilles)), and Berk-S. The mean white counts were elevated 1.4- to 2.1-fold (P </=.01) in the Berk-S(Antilles), NY-S/S(Antilles), and Berk-S mice, but not in the NY-S mice compared with controls. Serum amyloid P-component (SAP), an acute-phase response protein with 60% to 70% sequence homology to CRP, was elevated 8.5- to 12.1-fold (P </=.001) in transgenic sickle mice. Similarly, serum interleukin-6 (IL-6) was elevated 1.6- to 1.9-fold (P </=.05). Western blots, confirming immunohistochemical staining, showed vascular cell adhesion molecule (VCAM), intercellular adhesion molecule (ICAM), and platelet-endothelial cell adhesion molecule (PECAM) were up-regulated 3- to 5-fold (P </=.05) in the lungs of sickle mice. Ribonuclease protection assays (RPAs) demonstrated VCAM mRNA also was elevated in sickle mice 1.2- to 1.4-fold (P </=.01). Nuclear factor kappaB (NF-kappaB), a transcription factor critical for the inflammatory response, was elevated 1.9-fold (P </=.006) in NY-S sickle mouse lungs. We conclude that transgenic sickle mice are good models to study vascular inflammation and the potential benefit of anti-inflammatory therapies to prevent vaso-occlusion in sickle cell disease.

Erythropoiesis and serum sVCAM-1 levels in adults with sickle cell disease

Sickle cell patients are characterized by stress erythropoiesis involving cytokines, growth factors, and adhesion molecules. We set out to determine whether serum soluble vascular cell adhesion molecule-1 (sVCAM-1) levels, which are inversely related to red blood cell counts in sickle cell disease (SCD), reflect erythropoietic activity in adult HbSS patients. Serum levels of sVCAM-1 were compared to erythropoietin (EPO), granulocyte-macrophage colony-stimulating factor (GM-CSF), interleukin-3 (IL-3), and soluble transferrin receptor (sTfR) levels in 29 adults with HbSS, and their respective levels were also compared to 29 race- and age-matched HbAA controls. EPO and sTfR levels were increased as compared to healthy controls, whereas IL-3 and GM-CSF were not. No significant correlation of sVCAM-1 levels could be detected with any of the measured erythropoietic markers. Patients, but not controls, with detectable IL-3 levels had lower sTfR and GM-CSF levels as compared to patients with undetectable IL-3 levels. Even though a link of sVCAM-1 to erythropoiesis could not be established, it cannot be ruled out that sVCAM-1 levels reflect the release of young red blood cells into the circulation. IL-3 and GM-CSF levels suggest that different rates of erythropoiesis may be characterized by specific cytokine profiles in SCD. Further research should focus on the potential cytokines and adhesion molecules involved in sickle cell erythropoiesis, as this may increase our understanding of sickle cell complications and may provide us with potential markers for risk assessment in sickle cell disease as well.

Adhesion of normal erythrocytes at depressed venous shear rates to activated neutrophils, activated platelets, and fibrin polymerized from plasma

Deep vein thrombosis (DVT) is a low flow pathology often prevented by vascular compression to increase blood movement. We report new heterotypic adhesive interactions of normal erythrocytes operative at low wall shear rates (gamma(w)) below 100 s(-1). Adhesion at gamma(w) = 50 s(-1) of washed red blood cells (RBCs) to fibrinogen-adherent platelets was 4-fold less (P <.005) than to collagen-adherent platelets (279 +/- 105 RBC/mm(2)). This glycoprotein VI (GPVI)-triggered adhesion was antagonized (> 80% reduction) by soluble fibrinogen (3 mg/mL) and ethylenediaminetetraacetic acid (EDTA). RBC-platelet adhesion was reduced in half by antibodies against CD36 or GPIb, but not by antibodies against GPIIb/IIIa, von Willebrand factor (VWF), thrombospondin (TSP), P-selectin, beta(1), alpha(v), or CD47. Adhesion of washed RBCs to fibrinogen-adherent neutrophils was increased 6-fold in the presence of 20 microM N-formyl-Met-Leu-Phe to a level of 67 RBCs per 100 neutrophils after 5 minutes at 50 s(-1). RBC-neutrophil adhesion was diminished by anti-CD11b (76%), anti-RBC Landsteiner-Wiener (LW) (ICAM4; 40%), or by EDTA (> 80%), but not by soluble fibrinogen or antibodies against CD11a, CD11c, CD36, TSP, beta(1), alpha(v), or CD47. RBC adhesion to activated platelets and activated neutrophils was prevented by wall shear stress above 1 dyne/cm(2) (at 100 s(-1)). Whereas washed RBCs did not adhere to fibrin formed from purified fibrinogen, adhesion was marked when pure fibrin was precoated with TSP or when RBCs were perfused over fibrin formed from recalcified plasma. Endothelial activation and unusually low flow may be a setting prone to receptor-mediated RBC adhesion to adherent neutrophils (or platelets/fibrin), all of which may contribute to DVT.

Relationship between the clinical manifestations of sickle cell disease and the expression of adhesion molecules on white blood cells

BACKGROUND

The severity of sickle cell disease (SCD) increases with leukocyte count. The biological basis could be that leukocyte adherence to vascular endothelium mediated by adhesion molecules (AMs) facilitates vaso-occlusion, the basic pathological process in SCD.

OBJECTIVE

To find out if there is a relationship between expression of AMs by leukocytes and the clinical manifestations of SCD.

METHODS

Flow cytometry was used to study the relationship between leukocyte AM expression and disease manifestations in 100 patients with homozygous (HbSS) sickle cell disease and 34 genotype HbAA controls. The effect of hydroxyurea therapy on AM expression was also examined. We excluded HbSS patients with any other disease, pregnancy in the previous 3 months, or Haemogloben F (HbF) > or = 10%.

RESULTS

Patients with complications of SCD showed high expression of alphaMbeta integrin by the neutrophils; and l-selectin by lymphocytes and neutrophils (P < 0.03). CD18 was highly expressed by neutrophils in patients with sickle nephropathy (P = 0.018), and l-selectin by lymphocytes in those with stroke (P = 0.03). Monocyte l-selectin increased in sickle cell crisis relative to steady state (P = 0.04). Expression of alphaLbeta2 integrin by neutrophils, monocytes, and lymphocytes decreased within a month of hydroxyurea therapy (P < 0.05), with symptomatic improvement in the patients and no more than 3.3% rise in HbF level.

CONCLUSIONS

The findings suggest that in SCD (1): High steady-state expression of alphaMbeta2 integrin and l-selectin by leukocytes predisposes to severe manifestations. (2) Increased leukocyte AM expression above steady-state levels could be important in the genesis of crisis. (3) The early symptomatic improvement that follows hydroxyurea therapy is mediated via mechanisms independent of increased HbF, and may involve reduced AM expression in leukocytes. (4) Other treatment modalities that reduce leukocyte AM expression might also confer clinical benefit.

Neutrophils and leucocyte adhesion molecules in sickle cell retinopathy

AIMS

The expression of the adhesion molecules ICAM-1, VCAM-1, and P-selectin, and the distribution and number of polymorphonuclear leucocytes (PMNs) were investigated in sickle cell retinopathy (SCR) and compared to the normal retina.

METHODS

Postmortem ocular tissue was obtained from five subjects (16, 21, 28, 40, and 41 years of age) with sickle haemoglobinopathies and from one control subject. Tissue was cryopreserved, and streptavidin peroxidase immunohistochemistry was performed with antibodies against ICAM-1, VCAM-1, and P-selectin. Immunohistochemical reaction product was scored, and PMN numbers were counted in sections stained with non-specific esterase.

RESULTS

Increased ICAM-1, VCAM-1, and P-selectin immunoreactivities were observed in sickle cell subjects compared to the control subject. The highest ICAM and P-selectin immunoreactivity was associated with intraretinal vessels adjacent to the preretinal neovascular formation in subjects with proliferative retinopathy. This was not the case with VCAM-1 immunoreactivity, which was highest in intraretinal vessels adjacent to the sea fan when the sea fan was still "in statu nascendi." Fully formed, "older" sea fans had the highest levels of VCAM-1. The increase in adhesion molecule immunoreactivity was paralleled by an increase in intraretinal PMNs. The number of intraretinal PMNs increased with progression of the disease and the numbers surpassed those in control subjects by threefold. In the sea fan with the greatest VCAM-1 immunoreactivity, there were 20 times more PMNs were observed than in the rest of the retina in the same subject.

CONCLUSION

These data suggest that adhesion molecule mediated leucocyte adhesion might play an important part in the vaso-occlusive phase of sickle cell retinopathy and in autoinfarction of sea fan formations.

Hydroxyurea corrects the dysregulated L-selectin expression and increased H(2)O(2) production of polymorphonuclear neutrophils from patients with sickle cell anemia

Impaired polymorphonuclear neutrophil (PMN) functions during sickle cell anemia (SCA) may have a pathogenic role in the onset of vasoocclusive events. We used flow cytometry to study, in whole blood, the adhesion molecule expression and respiratory burst of PMNs from children with SCA. Three different clinical groups were studied: (1) patients with no history of vasoocclusive events (n = 15); (2) patients with a history of vasoocclusive events (n = 17); and (3) patients receiving hydroxyurea therapy for severe vasoocclusive events (n = 9). Unstimulated PMNs showed decreased L selectin expression and increased H(2)O(2) production whatever the severity of the disease, reflecting PMN activation. This could contribute to endothelial activation reflected by abnormal plasma levels of soluble adhesion molecules (soluble intercellular adhesion molecule-1, sE selectin, and sL selectin). After stimulation with bacterial N-formyl peptides (N-formyl-methionyl-leucyl-phenylalanine [fMLP]), PMNs from untreated patients with a history of vasoocclusive events showed dysregulated L selectin shedding and increased H(2)O(2) production. Furthermore, in these patients, tumor necrosis factor priming followed by fMLP stimulation induced an H(2)O(2) production significantly higher than in the other patient groups and controls. These impairments could immobilize PMNs on the endothelium, thereby inducing reduced blood flow and fostering microvascular occlusion and vascular damage. In contrast, children treated with hydroxyurea showed near-normal basal and poststimulation H(2)O(2) production as well as normal L selectin shedding after stimulation but no change in plasma levels of soluble adhesion molecules. To our knowledge, this is the first report showing major qualitative changes of PMN abnormalities upon hydroxyurea treatment in SCA patients. This strongly suggests that PMNs are a primary target of this drug.

Eicosanoids in sickle cell disease: potential relevance of neutrophil leukotriene B4 to disease pathophysiology

Neutrophil activation with the release of intracellular granule contents has been observed in sickle cell disease (SCD). Because leukotriene B(4) (LTB(4)), a 5-lipoxygenase metabolite of arachidonic acid in neutrophils, is a chemoattractant and enhances neutrophil adhesion to endothelium, we assessed plasma levels of this metabolite in controls (n = 9) and individuals with SCD, SS genotype, both in basal "steady state" (n = 37) and during episodes of vaso-occlusion (n = 10) and acute chest syndrome (n = 5). Thirteen patients with SCD, SC genotype, in steady state were also studied. Although no significant differences were noted between the control (136 +/- 32 fmol/mL) and SC genotype (177 +/- 83 fmol/mL, P >.15), LTB(4) levels were markedly increased in patients with SS genotype in basal steady state (207 +/- 64 fmol/mL, P <.003) compared with those in controls. Values were further increased during vaso-occlusion (264 +/- 94 fmol/mL) and acute chest syndrome (363 +/- 124 fmol/mL). These levels were significantly different from measurements taken during steady state (P <.04 and P <.0001, respectively). No correlation was noted between LTB(4) level and total white cell or neutrophil count. Additionally, the significant correlation noted in SCD between increased levels of plasma LTB(4) and soluble L-selectin (P <.03) reflects neutrophil activation. We also observed an effect of LTB(4) on red cell-endothelial adhesion at concentrations that appear clinically relevant (1-10 pmol/mL) with concomitant up-regulation of mRNA for the endothelial vitronectin receptor. These properties of LTB(4) are relevant to disease pathophysiology, providing further evidence of the contribution of the neutrophil to the proinflammatory and proadhesive phenotype in SCD.

Activated polymorphonuclear cells increase sickle red blood cell retention in lung: role of phospholipids

This study investigates the role of the activated polymorphonuclear cell (APMN) products on sickle red blood cell (SRBC) retention/adherence in the pulmonary circulation. Isolated rat lungs were perfused with (51)Cr-labeled normal RBCs (NRBC) or SRBCs (10% hematocrit) suspensions +/- PMNs. Specific activities of lung and perfusate were measured and retention (the number of SRBC/g lung) was calculated. SRBC retention was 3.5 times greater than NRBC retention. PMN activation was required to increase SRBC retention. Supernatants from APMN increased SRBC retention, which suggested soluble products such as oxidants, PAF, and/or leukotriene (LTB(4)) are involved. Heat inactivation of PMN NADPH oxidase had no effect on retention. Whereas neither platelet-activating factor (PAF) nor LTB(4) (secreted by APMN) increased SRBC retention, PAF+LTB(4) did. The PAF antagonist, WEB-2170, attenuated SRBC retention mediated by PAF+LTB(4) and APMNs. Similarly, zileuton (5-lipoxygenase inhibitor) attenuated APMN-mediated SRBC retention. We conclude the concomitant release of PAF and LTB(4) from APMN is involved in the initiation of microvascular occlusion by SRBCs in the perfused rat lung.

Thrombophilia in sickle cell disease: the red cell connection

Complex pertubations of hemostasis occur in sickle cell disease (SCD). Although the procoagulant property of sickle erythrocytes in vitro is tied to exposure of phosphatidylserine (PS), no study has directly linked this PS positivity to in vivo thrombin generation. This study was designed to determine if thrombin generation in SCD correlates with erythrocyte PS, or whether platelets play a significant role. PS was quantified on erythrocytes and platelets from 40 patients with SCD (SS genotype = 25; SC genotype = 15) and 11 controls. Markers of thrombin generation (prothrombin fragment F1.2; thrombin-antithrombin or TAT complexes) and fibrin dissolution (D-dimer; plasmin-antiplasmin or PAP complexes) were also evaluated. Thrombin generation and activation of fibrinolysis occurred with elevations in F1.2, TAT, and D-dimer. Although numbers of both PS-positive erythrocytes and platelets were elevated, there was no correlation between PS-positive platelets and any hemostatic markers. In contrast, correlations were noted between PS-positive erythrocytes and F1.2 (P <.0002), D-dimer (P <.000002), and PAP (P <.01). Correlations between F1.2 and D-dimer (P <.0001) demonstrated that fibrinolysis was secondary to thrombin generation. In patients with the SC genotype, abnormalities in coagulation, although present, were of a lesser magnitude than in SS disease. This study suggests that the sickle erythrocyte is the cell responsible for the thrombophilic state in SCD because associations between erythrocyte PS and thrombin generation were observed. No such relationship with platelet PS was noted. The use of erythrocyte PS as a surrogate marker in trials testing new therapeutic modalities may provide insights into the vascular complications of SCD.

Pathophysiology of sickle cell disease: role of cellular and genetic modifiers

Sickle hemoglobin (HbS), caused by a point mutation in the beta-globin gene of hemoglobin, polymerizes when deoxygenated. The pathophysiology of sickle cell disease results from cellular defects caused directly by the hemoglobin mutation interacting with the environment and many other gene products--a few known, but most yet unidentified--a typical example of epistasis. How normal tissue perfusion is interrupted is complex and why the phenotype of sickle cell disease differs from patient to patient is poorly understood. We review the "classic" aspects of the pathophysiology of sickle cell disease and focus on known and potential modulators of the phenotype of this disorder.

Role of free radicals in the pathogenesis of acute chest syndrome in sickle cell disease

Acute chest syndrome (ACS) of sickle cell disease (SCD) is characterized pathologically by vaso-occlusive processes that result from abnormal interactions between sickle red blood cells (RBCs), white blood cells (WBCs) and/or platelets, and the vascular endothelium. One potential mechanism of vascular damage in ACS is by generation of oxygen-related molecules, such as superoxide (O2-), hydrogen peroxide (H2O2), peroxynitrite (ONOO-), and the hydroxyl (*OH) radical. The present review summarizes the evidence for alterations in oxidant stress during ACS of SCD, and the potential contributions of RBCs, WBCs and the vascular endothelium to this process.

Green tea extract and aged garlic extract inhibit anion transport and sickle cell dehydration in vitro

Both green tea extract (GTE or tea polyphenols) and aged garlic extract (AGE) effectively inhibited in vitro dehydration of sickle red blood cells induced by K-Cl cotransport or red cell storage. For K-Cl cotransport induced by 500 mM urea, 0.3 mg/ml EGCg (epigallocatechin gallate; a major component in GTE) almost completely inhibited dehydration, and 6 mg/ml AGE inhibited dehydration to 30% of the control level. Both vitamins E and C had no effect at the level of 2 mM. Different tea extracts had different degrees of inhibition, but the inhibitory activity increased when the number of hydroxyl groups in the compounds increased. With storage of sickle cells at 4 degrees C for 6 days, the cells started to undergo spontaneous dehydration when incubated at 37 degrees C. Neither inhibitors for Ca-induced K efflux nor K-Cl cotransport could inhibit cell dehydration of stored sickle cells, but both GTE and AGE effectively inhibited it. Chloride efflux measurements using a chloride electrode demonstrated that both GTE and AGE inhibited anion transport in red blood cells. The inhibitory mechanism of these compounds may be related to anion transport inhibition, although involvement of their antioxidant activities can not yet be ruled out.

Platelet and leucocyte activation in childhood sickle cell disease: association with nocturnal hypoxaemia

We hypothesized that vaso-occlusive events in childhood sickle cell disease (SCD) may relate to inflammatory cell activation as well as interactions between sickle erythrocytes and vascular endothelium. Peripheral blood was examined from 24 children with SCD, of whom 12 had neurological sequelae and seven had frequent painful crises, and 10 control subjects. Platelet (CD62P and CD40L expression) and granulocyte (CD11b expression) activation and levels of platelet-erythrocyte and platelet-granulocyte complexes were determined by flow cytometry. Platelets (P = 0.019), neutrophils (P = 0.02) and monocytes (P = 0.001) were more activated and there were increased platelet-erythrocyte complexes (P = 0.026) in SCD patients compared with controls. Platelet-granulocyte complexes were not raised. There were no differences between the different groups of SCD. As hypoxia activates monocytes, platelets and endothelial cells and causes sickling of SCD erythrocytes, we also investigated 20 SCD patients with overnight pulse oximetry. Minimum overnight saturation correlated with the level of platelet-erythrocyte complexes (Spearman's rho -0.668, P < 0.02), neutrophil CD11b (Spearman's rho -0.466, P = 0.038) and monocyte CD11b (Spearman's rho -0.652, P = 0. 002). These findings provide important clues about the mechanism by which SCD patients may become predisposed to vaso-occlusive events.

Duffy phenotype does not influence the clinical severity of sickle cell disease

The red blood cell Duffy antigen receptor for chemokines serves as a sink for the clearance of chemokines such as interleukin-8 (IL-8) from the circulation. We analyzed the impact of the Duffy phenotype on sickle cell disease (SCD) severity and serum IL-8 levels in 15 Duffy-positive and 36 Duffy-negative sickle cell patients. There was no difference in clinical severity between Duffy-positive and Duffy-negative sickle cell patients. In asymptomatic sickle cell patients the upward deviation of mean serum IL-8 levels was significantly greater in Duffy-negatives (n = 20) than in Duffy-positives (n = 8) (P = 0.011). However, during a vasoocclusive episode, serum IL-8 levels were similar between Duffy-negatives (n = 11) and Duffy-positives (n = 3). Although the Duffy phenotype seems to influence steady-state serum IL-8 levels, it does not seem to have an effect on SCD severity.