New concepts in assessing sickle cell disease severity

Whole blood viscosity and red blood cell adhesion: Potential biomarkers for targeted and curative therapies in sickle cell disease

Sickle cell disease (SCD) is a recessive genetic blood disorder exhibiting abnormal blood rheology. Polymerization of sickle hemoglobin, due to a point mutation in the β‐globin gene of hemoglobin, results in aberrantly adhesive and stiff red blood cells (RBCs). Hemolysis, abnormal RBC adhesion, and abnormal blood rheology together impair endothelial health in people with SCD, which leads to cumulative systemic complications. Here, we describe a microfluidic assay combined with a micro particle image velocimetry technique for the integrated in vitro assessment of whole blood viscosity (WBV) and RBC adhesion. We examined WBV and RBC adhesion to laminin (LN) in microscale flow in whole blood samples from 53 individuals with no hemoglobinopathies (HbAA, N = 10), hemoglobin SC disease (HbSC, N = 14), or homozygous SCD (HbSS, N = 29) with mean WBV of 4.50 cP, 4.08 cP, and 3.73 cP, respectively. We found that WBV correlated with RBC count and hematocrit in subjects with HbSC or HbSS. There was a significant inverse association between WBV and RBC adhesion under both normoxic and physiologically hypoxic (SpO₂ of 83%) tests, in which lower WBV associated with higher RBC adhesion to LN in subjects with HbSS. Low WBV has been found by others to associate with endothelial activation. Altered WBV and abnormal RBC adhesion may synergistically contribute to the endothelial damage and cumulative pathophysiology of SCD. These findings suggest that WBV and RBC adhesion may serve as clinically relevant biomarkers and endpoints in assessing emerging targeted and curative therapies in SCD.

Influence of β-Globin Haplotypes on Oxidative Stress, Antioxidant Capacity and Inflammation in Sickle Cell Patients of Chhattisgarh

The present study was conducted to investigate the frequency of β-globin gene haplotype and their influence on oxidative stress, antioxidant level and inflammation of sickle cell patient of Chhattisgarh. The serum levels of total antioxidant capacity, oxidative stress and inflammation were measured in hundred SCD patients. The mean serum CRP and MDA levels were significantly (p < 0.0001) lower in Arab-Indian haplotype when compared with Bantu and other haplotype. However, the antioxidant capacity and HbF levels were significantly higher in Arab-Indian haplotype. These result indicate that β-globin gene haplotype have a role in modulating disease severity in SCD patients of Chhattisgarh.

Vascular complications of sickle cell disease

Sickle cell disease (SCD) is a monogenetic disorder caused by a mutation in the β-globin gene HBB leading to polymerization of red blood cells causing damage to cell membranes, increasing its rigidity and intravascular hemolysis. Multiple lines of evidence suggest that SCD can be viewed as pan-vasculopathy associated with multiple mechanisms but driven by hemoglobin S polymerization. Here we review the pathophysiology, clinical manifestations and management strategies for cerebrovascular disease, pulmonary hypertension and renal disease associated with SCD. These "vascular phenotypes" reflect the systemic nature of the complications of SCD and are a major threat to the well-being of patients with the disorder.

Oxidative stress in sickle cell disease; pathophysiology and potential implications for disease management

Sickle cell disease (SCD) is a hemoglobinopathy characterized by hemolytic anemia, increased susceptibility to infections and vaso-occlusion leading to a reduced quality of life and life expectancy. Oxidative stress is an important feature of SCD and plays a significant role in the pathophysiology of hemolysis, vaso-occlusion and ensuing organ damage in sickle cell patients. Reactive oxygen species (ROS) and the (end-)products of their oxidative reactions are potential markers of disease severity and could be targets for antioxidant therapies. This review will summarize the role of ROS in SCD and their potential implication for SCD management.

Plasma levels of pentraxin-3, an acute phase protein, are increased during sickle cell painful crisis

The painful crisis accounts for the majority of sickle cell disease (SCD) related hospital admissions. The prototypic long pentraxin 3 (PTX3), an acute phase protein, is elevated in patients with inflammatory and ischemic states. As the sickle cell painful crisis is associated with both inflammation and tissue ischemia, we questioned whether plasma PTX3 levels are increased during and associated with painful crisis severity. Furthermore, since PTX3 up-regulates endothelial expression of tissue factor we studied PTX levels in relation to markers of endothelial and coagulation activation. Plasma levels of PTX3, ultra-sensitive C-reactive protein (US-CRP), prothrombin fragment 1+2, thrombin-antithrombin (TAT) complexes, von Willebrand Factor antigen and soluble vascular adhesion molecule-1 were determined in 105 asymptomatic sickle cell patients, 33 patients during painful crisis and 30 race matched healthy controls. Plasma PTX3 levels were comparable between patients in asymptomatic state and healthy controls, but significantly higher during painful crisis (P<0.01). US-CRP levels were higher in asymptomatic patients compared to controls (P<0.0001) and increased further during painful crisis (P<0.0001). PTX3 levels at presentation with painful crisis correlated significantly with the duration of subsequent hospital admission (r(s) = 0.43; P = 0.013), whereas US-CRP levels did not. PTX3 levels did not correlate with markers of hypercoagulability. The increase of PTX3 levels during painful crisis and their relation to the duration of subsequent hospital stay suggest that PTX3 might serve both as a diagnostic and severity marker of the painful sickle cell crisis.

Vasculopathy in sickle cell disease: Biology, pathophysiology, genetics, translational medicine, and new research directions

Sickle cell disease has been very well characterized as a single amino acid molecular disorder of hemoglobin leading to its pathological polymerization, with resulting red cell rigidity that causes poor microvascular blood flow, with consequent tissue ischemia and infarction. More recently, an independent spectrum of pathophysiology of blood vessel function has been demonstrated, involving abnormal vascular tone and activated, adhesive endothelium. These vasculopathic abnormalities are attributable to pathways involving hemolysis-associated defects in nitric oxide bioavailability, oxidative stress, ischemia-reperfusion injury, hemostatic activation, leukocytes and platelets. Vasculopathy of sickle cell disease has been implicated in the development of pulmonary hypertension, stroke, leg ulceration and priapism, particularly associated with hemolytic severity, and reported also in other severe hemolytic disorders. This vasculopathy might also play a role in other chronic organ dysfunction in patients with sickle cell disease. These pathways present novel targets for pharmacologic intervention, and several clinical trials are already under way. The authors present their perspectives of a workshop held at the National Institutes of Health in August 2008 on vasculopathy in sickle cell disease, along with meritorious future scientific questions on the topic of vascular complications of sickle cell disease.

Cerebrovascular reserve capacity is impaired in patients with sickle cell disease

Sickle cell disease (SCD) is associated with a high incidence of ischemic stroke. SCD is characterized by hemolytic anemia, resulting in reduced nitric oxide-bioavailability, and by impaired cerebrovascular hemodynamics. Cerebrovascular CO2 responsiveness is nitric oxide dependent and has been related to an increased stroke risk in microvascular diseases. We questioned whether cerebrovascular CO2 responsiveness is impaired in SCD and related to hemolytic anemia. Transcranial Doppler-determined mean cerebral blood flow velocity (V(mean)), near-infrared spectroscopy-determined cerebral oxygenation, and end-tidal CO2 tension were monitored during normocapnia and hypercapnia in 23 patients and 16 control subjects. Cerebrovascular CO2 responsiveness was quantified as Delta% V(mean) and Deltamicromol/L cerebral oxyhemoglobin, deoxyhemoglobin, and total hemoglobin per mm Hg change in end-tidal CO2 tension. Both ways of measurements revealed lower cerebrovascular CO2 responsiveness in SCD patients versus controls (V(mean), 3.7, 3.1-4.7 vs 5.9, 4.6-6.7 Delta% V(mean) per mm Hg, P < .001; oxyhemoglobin, 0.36, 0.14-0.82 vs 0.78, 0.61-1.22 Deltamicromol/L per mm Hg, P = .025; deoxyhemoglobin, 0.35, 0.14-0.67 vs 0.58, 0.41-0.86 Deltamicromol/L per mm Hg, P = .033; total-hemoglobin, 0.13, 0.02-0.18 vs 0.23, 0.13-0.38 Deltamicromol/L per mm Hg, P = .038). Cerebrovascular CO2 responsiveness was not related to markers of hemolytic anemia. In SCD patients, impaired cerebrovascular CO2 responsiveness reflects reduced cerebrovascular reserve capacity, which may play a role in pathophysiology of stroke.

Sickle cell children traveling abroad: primary risk is infection

BACKGROUND

Pediatricians taking care of sickle cell children in France are concerned about giving travel advice. Very few articles are published and no study has been done about it. A lot of pediatricians are using their own experience to decide if sickle cell children can travel abroad. Studying the consequences of such travel for sickle cell children is important to discuss common recommendations.

METHODS

We conducted a prospective study from June 2006 to December 2007 on desires to travel expressed during our consultations with sickle cell children. We studied notable events that occurred during travel and at least 2 months after return.

RESULTS

Of 52 desires to travel, 10 were cancelled. All of the 42 trips were to Africa. Median duration of travel was 1.29 months (0.5-3). Median age at travel was 7.6 years (0.2-17.7). Events during travel were two hospitalizations (4.8%), a transfusion (2.4%), and four paramedical or medical examinations (9.6%). After return, four events occurred: two SS children had Plasmodium falciparum malaria (4.8%) and two had digestive bacteremia (4.8%) in SC and Sbeta+ children. No event occurred during plane travel. None of our patients died.

CONCLUSIONS

The primary risk for sickle cell children traveling to Africa is infection: malaria first and digestive septicemia second. These risks are increased by long travel and poor sanitary conditions. Each travel should be prepared a long time before departure, and each pediatrician should insist on malaria prophylaxis and sanitary conditions, especially for young children. Trips should be shorter than 1 month when possible. A longer prospective study will be done to confirm these results.

Elevated endothelial progenitor cells during painful sickle cell crisis

OBJECTIVE

Circulating endothelial progenitor cells (EPCs) counts were determined in patients with sickle cell disease (SCD) to elucidate their role in SCD-related ischemia-induced angiogenesis and reendothelialization.

MATERIALS AND METHODS

Circulating EPC counts (KDR(+)/CD34(+)/Cd45(dim) cells) and their relation to serum levels of EPC mobilizing growth factors erythropoietin, vascular endothelial growth factor, and interleukin-8 were investigated in SCD patients during asymptomatic state (n=66) and painful crisis (n=36) and compared to healthy controls (n=13).

RESULTS

EPC counts were comparable between controls (0; range, 0-1.1 cells/mL) and patients (0; range, 0-0 cells/mL) in asymptomatic state, but were significantly higher during painful crisis (41.7; range, 0-186 cells/mL; p<0.05). Also in a paired analysis of 12 patients who were included both during asymptomatic state and painful crisis, EPC counts increased significantly during painful crisis (from 0 [range, 0-0] to 26 [range, 0-149 cell/mL; p<0.05). EPC counts were not related to any of the measured growth factors.

CONCLUSION

The higher EPC counts during painful crisis might indicate a role for EPC mobilization in reendothelialization. As a relationship of EPCs with the established mobilizing growth factors, measured in this study was not observed, the mechanism of EPC mobilization in SCD remains to be elucidated.

Zinc supplementation decreases oxidative stress, incidence of infection, and generation of inflammatory cytokines in sickle cell disease patients

Zinc deficiency is common in adult sickle-cell disease (SCD) patients. We previously demonstrated that zinc supplementation to adult SCD patients decreased the incidences of infections and hospital admissions. We hypothesize that zinc supplementation improves T-helper cell function and decreases vascular endothelial cell activation, oxidative stress, and nuclear factor-kappa B (NF-kappaB)-DNA binding in mononuclear cells (MNCs) in SCD patients. To test this hypothesis, 36 SCD patients were recruited and randomly divided into 2 groups. One group (n = 18) received 25-mg zinc orally thrice a day for 3 months. The other group (n = 18) received placebo. The results indicate that the zinc-supplemented group had decreased incidence of infections compared with the placebo group. After zinc supplementation, red blood cell, hemoglobin (Hb), hematocrit, (Hct), plasma zinc, and antioxidant power increased; plasma nitrite and nitrate (NOx), lipid peroxidation products, DNA oxidation products, and soluble vascular cell adhesion molecule-1 decreased in the zinc-supplemented group, compared with the placebo group. Zinc-supplemented patients exhibited significant decreases in lipopolysaccharide-induced tumor necrosis factor-alpha (TNF-alpha) and IL-1beta mRNAs, and TNF-induced nuclear factor of kappaB-DNA binding in MNCs, compared with the placebo group. Ex vivo addition of zinc to MNCs isolated from the placebo subjects decreased TNF-alpha and IL-1beta mRNAs. Zinc supplementation also increased relative levels of IL-2 and IL-2Ralpha mRNAs in phytohemagglutinin-p-stimulated MNCs. These results suggest that zinc supplementation may be beneficial to SCD patients.

ADAMTS13 activity in sickle cell disease

Sickle red blood cell (SRBC)-endothelial adhesion plays a central role in sickle cell disease (SCD)-related vaso-occlusion. As unusually large von Willebrand factor (ULVWF) multimers mediate SRBC-endothelial adhesion, we investigated the activity of ADAMTS13, the metalloprotease responsible for cleaving ULVWF multimers, in SCD. ADAMTS13 activity was determined using a quantitative immunoblotting assay. VWF:Ag and VWF:RCo were determined using commercial assays. The high-molecular-weight VWF multimer percentage was determined by employing gel electrophoresis. ADAMTS13 activity was similar among asymptomatic patients (n = 8), patients at presentation with a painful crisis (n = 23), and healthy controls. ADAMTS13/VWF:Ag ratios were lower in patients compared to healthy HbAA controls, with the lowest values at presentation with a painful crisis (P = 0.02). Division of samples in those with VWF:RCo/VWF:Ag ratios < 0.70 and those with ratios >or= 0.70 revealed significantly more samples with ratios >or= 0.70 (P = 0.01) collected during painful crises. ULVWF multimers were detected in 6 patient samples and in 1 control sample. ADAMTS13/VWF:Ag ratios were inversely related to the duration of symptoms at presentation with an acute vaso-occlusive event (r(s)-0.67, P = 0.002). Although SCD is characterized by elevated VWF:Ag levels, no severe ADAMTS13 deficiency was detected in our patients.

Impaired vasodilation by red blood cells in sickle cell disease

Red blood cells (RBCs) have been ascribed a unique role in dilating blood vessels, which requires O2-regulated binding and bioactivation of NO by Hb and transfer of NO equivalents to the RBC membrane. Vasoocclusion in hypoxic tissues is the hallmark of sickle cell anemia. Here we show that sickle cell Hb variant S (HbS) is deficient both in the intramolecular transfer of NO from heme iron (iron nitrosyl, FeNO) to cysteine thiol (S-nitrosothiol, SNO) that subserves bioactivation, and in transfer of the NO moiety from S-nitrosohemoglobin (SNO-HbS) to the RBC membrane. As a result, sickle RBCs are deficient in membrane SNO and impaired in their ability to mediate hypoxic vasodilation. Further, the magnitudes of these impairments correlate with the clinical severity of disease. Thus, our results suggest that abnormal RBC vasoactivity contributes to the vasoocclusive pathophysiology of sickle cell anemia, and that the phenotypic variation in expression of the sickle genotype may be explained, in part, by variable deficiency in RBC processing of NO. More generally, our findings raise the idea that defective NO processing may characterize a new class of hemoglobinopathy.

Plasma levels of asymmetric dimethylarginine (ADMA), an endogenous nitric oxide synthase inhibitor, are elevated in sickle cell disease

In recent years an important role has been ascribed to a reduced nitric oxide (NO) availability in the pathophysiology of sickle cell disease (SCD). Endogenously produced inhibitors of NO synthase, in particular asymmetric dimethylarginine (ADMA), are currently considered of importance in various vascular disease states characterized by reduced NO availability. We determined ADMA levels in plasma of 12 adult sickle cell patients (eight HbSS and four HbSC), and compared these to plasma levels in race- and age-matched controls. Sickle cell patients were characterized by strongly elevated levels of ADMA [HbSS: median 0.63 micromol/l (interquartile range 0.54-0.85), HbSC: 0.43 micromol/l (0.40-0.46), HbAA: 0.33 micromol/l (0.32-0.35) p<0.001]. ADMA levels were highest in HbSS patients with lowest hemoglobin levels and highest leukocyte counts, and in HbSS patients ADMA levels were positively associated with serum levels of soluble vascular cell adhesion molecule-1. These results suggest an important role of ADMA in limiting NO availability in SCD, and its role in the pathophysiology of SCD should be further investigated.

Clinical relevance of vascular endothelial growth factor levels in sickle cell disease

In recent years, vascular inflammation, marked by activated monocytes and endothelium, has been proven to play a significant role in the pathogenesis of vaso-occlusive events (VOEs) in sickle cell disease (SCD). Vascular endothelial growth factor (VEGF), an endothelial cell mitogen, has been shown to contribute to the increased endothelial cell adhesivity by increasing the expression of cell adhesion molecules ICAM-1 (intercellular adhesion molecule-1) and VCAM-1 (vascular cell adhesion molecule-1) on the endothelium. We have investigated VEGF alterations in 37 patients with SCD during VOEs and/or steady state. VEGF levels (mean+/-SEM) were found to be significantly elevated during VOEs (703.1+/-119.0 pg/ml) when compared with those at steady state (258.0+/-57.8 pg/ml) and healthy controls (196.6+/-21.9 pg/ml) (p<0.001). However, we did not find a difference between VEGF concentrations in sickle patients at steady state and the normal subjects (p>0.05). We suggest that considering a possible stimulatory role of tissue hypoxia in VEGF production during VOEs, VEGF levels in sickle patients might be helpful in monitoring disease severity.

Protein C and S and inflammation in sickle cell disease

Reduced activity of naturally occurring anticoagulants (NOAC) protein C and protein S may contribute to vaso-occlusion in sickle cell disease (SCD). We studied whether protein C and S are related to clinical vaso-occlusion, hematological markers of disease severity (hemoglobin levels, leukocyte counts, and percentage of fetal hemoglobin), and inflammation in SCD. Protein C activity, protein S (free and total) antigen, endothelial activation markers (soluble vascular cell adhesion molecule-1 [sVCAM-1], von Willebrand antigen [vWF]), and high sensitive C-reactive protein (hsCRP) levels were measured in 30 HbSS and 20 HbSC patients and in race-matched HbAA controls. NOAC levels were reduced in patients, and endothelial activation markers and hsCRP were elevated (except vWF in HbSC patients). Protein C activity and vWF levels were lower in HbSC patients who experienced painful crises compared to HbSC patients who were clinically asymptomatic. No other differences were observed between patients who did and did not experience vaso-occlusive events (painful crises, stroke, acute chest syndromes) or leg ulcers. A significant positive correlation between total protein S with hemoglobin levels and a significant negative correlation between total and free protein S and sVCAM-1 were detected in HbSS patients. Except perhaps for protein C in relation to painful crises in HbSC patients, these markers were not associated with the occurrence of clinical events. The protein S, hemoglobin, and sVCAM-1 associations may suggest decreased endothelial protein S production due to the more severe endothelial perturbation in HbSS patients with lower hemoglobin levels.

Steady-state sVCAM-1 serum levels in adults with sickle cell disease

Cytokines and adhesion molecules play an important role in the pathophysiology of vaso-occlusion in sickle cell disease (SCD), and their in vivo profiles are potential tools for assessing SCD severity. We compared steady-state soluble vascular cell adhesion molecule-1 (sVCAM-1) serum levels to clinical (painful crisis frequency, occurrence of acute chest syndrome, leg ulcers, and cerebrovascular disease) and related hematological parameters of SCD severity (such as HbF%, hemoglobin levels, and leukocyte counts) in 29 HbSS adults. Serum sVCAM-1 levels were not related to clinical severity, but an inverse correlation was demonstrated between sVCAM-1 and hemoglobin levels (r=-0.71, p<0.001) with a positive correlation to serum lactate dehydrogenase levels (r=0.59, p=0.008). Based upon these results, steady-state serum sVCAM-1 levels do not seem to reflect clinical disease severity. However, as VCAM-1 is involved in hematopoiesis, sVCAM-1 levels might reflect bone marrow activity in SCD. This underlies the pleiotropic nature of adhesion molecules in vivo and the need for further research in this area, especially since therapies targeting (cellular) adhesive interactions involving the endothelium are emerging for SCD.

No effect of acenocoumarol therapy on levels of endothelial activation markers in sickle cell disease

Sickle cell patients are characterized by a chronic inflammatory and hypercoagulable state, depicted by elevated levels of pro-inflammatory cytokines, endothelial adhesion molecules, and elevated markers of thrombin generation. We set out to determine whether anticoagulation with a coumadin derivative reduces inflammation in sickle cell disease. Therefore, serum levels of NFkappaB-regulated endothelial adhesion molecule soluble vascular cell adhesion molecule-1 and serum levels of non-NFkappaB-dependent markers of endothelial activation (soluble cellular fibronectin and von Willebrand factor antigen) were compared during treatment with acenocoumarol (INR 1.6-2.0) and placebo. No effect on circulating levels of the measured parameters was observed during treatment with acenocoumarol as compared to placebo. In the targeted INR range, anticoagulation of sickle cell patients with acenocoumarol does not seem to reduce endothelial activation.

Silent brain infarcts are rare in Kuwaiti children with sickle cell disease and high Hb F

Overt stroke is rare among sickle cell disease (SCD) patients in Kuwait. However, there are no previous studies of silent cerebral infarcts, which have been described in up to 20% of American children with Hb SS. We have carried out a prospective brain MRI study among otherwise normal SCD patients, who were consecutive patients seen in a 1-year period to document the prevalence of silent cerebral infarcts in children with sickle cell disease in Kuwait. Any patient with a previous seizure or other neurological abnormality was excluded. MRI was done with a 1.5 Tesla unit with super-conducting magnet. T1- and T2-weighted sagittal and axial sections and proton density axial images were obtained in 5-mm thick sections. The study group consisted of 30 (23 SS and 7 Sbeta(0)Thal) patients-19 males and 11 females-whose ages ranged from 6 to 17 (mean of 9.8 +/- 3.5) years. Hb F ranged from 11% to 35% with a mean of 22.8% +/- 5.7%. Only one patient, a 10-and-a-half-year-old boy with Hb SS, showed hyperintense signals in the parietal white matter, consistent with small infarcts, thus giving a prevalence of 3.3%. Silent brain infarcts are uncommon in our patients, and the protective factors remain to be fully elucidated.

Optimization of folic acid, vitamin B(12), and vitamin B(6) supplements in pediatric patients with sickle cell disease

Using homocysteine as a functional marker, we determined optimal folic acid, vitamin B(12), and vitamin B(6) dosages in 21 pediatric sickle cell disease (SCD) patients (11 HbSS, 10 HbSC; 7-16 years). Daily supplements of folic acid (400, 700, or 1,000 microg), vitamin B(12) (1, 3, or 5 U.S. 1989 RDA), and vitamin B(6) (1 or 3 U.S. 1989 RDA) were gradually increased in an 82-week dose-escalation study. Blood was taken at 9 occasions for measurements of erythrocyte (RBC) and serum folate, plasma vitamin B(12), whole-blood vitamin B(6), and plasma homocysteine. Augmentation of folic acid from 700 to 1,000 microg and vitamin B(12) from 3 to 5 RDA did not further decrease homocysteine. Percentages of patients exhibiting significant individual homocysteine decreases amounted to 43% (folic acid from 0 to 400 microg, vitamins B(12) and B(6) from 0 to 1 RDA), 14% (folic acid from 400 to 700 microg), 24% (vitamin B(12) from 1 to 3 RDA), and 18% (vitamin B(6) from 1 to 3 RDA ). The lowest plasma homocysteine at 82 weeks was 5.9 +/- 2.2 micromol/L. Patients with HbSS had higher RBC folate than HbSC. The entire group exhibited an inverse relation between RBC folate and hemoglobin. We conclude that RBC folate is less valuable for folate status assessment in SCD patients. Optimal dosages are as follows: 700 microg folic acid (3.5-7 U.S. 1989 RDA), 3 U.S. 1989 RDA vitamin B(12) (4.2-6.0 microg), and 3 U.S. 1989 RDA vitamin B(6) (4.2-6.0 mg). A practical daily combination is 1 mg folic acid (4.3-8.5 U.S. 1998 RDA when taken with meals), 6 microg vitamin B(12) (2.5-5 U.S. 1998 RDA), and 6 mg vitamin B(6) (4.6-10 U.S. 1998 RDA). This combination may by simple and relatively inexpensive means reduce these patients' inherently high risk of endothelial damage.

Low adjusted-dose acenocoumarol therapy in sickle cell disease: a pilot study

Vasoocclusion is a continuous process in sickle cell disease (SCD) and accumulates to significant end organ damage, mostly irrespective of the occurrence of manifest acute vasoocclusive events. As there are indications that reversing the hypercoagulable state may be of clinical benefit in sickle cell patients, we performed a randomized, double blind, placebo-controlled, cross-over pilot study to assess the efficacy and safety of low-adjusted dose acenocoumarol therapy (International Normalized Ratio: 1.6-2.0) in SCD. Treatment consisted of either acenocoumarol or placebo for 14 weeks, after which treatment was discontinued for a period of five weeks. Then, patients initially on acenocoumarol received placebo (and vice versa) for 14 weeks. Therapy efficacy was assessed by comparing the frequency of vasoocclusive complications, the occurrence of bleeding, and clotting activation between acenocoumarol and placebo treatment of each individual patient. Twenty-two patients (14 homozygous [HbSS] and 8 double heterozygous sickle-C [HbSC]; aged 20-59 years) completed the entire study. Acenocoumarol treatment did not result in a significant reduction of acute vasoocclusive events (three painful crises during acenocoumarol, five painful crises during placebo). There was a marked reduction of the hypercoagulable state (depicted by a decrease in plasma levels of prothrombin F1.2 fragments [P = 0.002], thrombin-antithrombin complexes [P = 0.003], and D-dimer fragments [P = 0.001]) without the occurrence of major bleeding. Even though no clinical benefit (pertaining to the frequency of painful crises) was detected in this pilot study, the value of low adjusted-dose acenocoumarol for preventing specific events (such as strokes) and as a long-term treatment of sickle cell patients should be subject of further study.

Structural and functional diversity of blood group antigens

Biochemical and molecular genetic studies have revealed that blood group antigens are present on cell surface molecules of wide structural diversity, including carbohydrate epitopes on glycoproteins and/or glycolipids, and peptide antigens on proteins inserted within the membrane via single or multi-pass transmembrane domains, or via glycosylphosphatidylinositol linkages. These studies have also shown that some blood group antigens are carried by complexes consisting of several membrane components which may be lacking or severely deficient in rare blood group 'null' phenotypes. In addition, although all blood group antigens are serologically detectable on red blood cells (RBCs), most of them are also expressed in non-erythroid tissues, raising further questions on their physiological function under normal and pathological conditions. In addition to their structural diversity, blood group antigens also possess wide functional diversity, and can be schematically subdivided into five classes: i) transporters and channels; ii) receptors for ligands, viruses, bacteria and parasites; iii) adhesion molecules; iv) enzymes; and v) structural proteins. The purpose of this review is to summarize recent findings on these molecules, and in particular to illustrate the existing structure-function relationships.

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.

Effect of zinc supplementation on incidence of infections and hospital admissions in sickle cell disease (SCD)

Zinc deficiency is a common nutritional problem in adult sickle-cell disease (SCD) patients. Hyperzincuria and increased requirement of zinc due to continued hemolysis in SCD are probable bases for zinc deficiency in these patients. Zinc deficiency affects adversely T-helper1 (TH1) functions and cell mediated immunity and interleukin (IL)-2 production is decreased in zinc deficient subjects. We hypothesized that zinc supplementation will improve T-helper1 function and decrease incidence of infections in patients with SCD. We tested this hypothesis in 32 SCD subjects who were divided in three groups (Grs A, B, and C). Grs A (n = 11) and B (n = 10) were zinc deficient based on cellular zinc criteria and Gr C (n = 11) were zinc sufficient. Gr A subjects were observed for 1 year (baseline), following which they received zinc acetate (50 to 75 mg of elemental zinc orally daily) for 3 years. Gr B subjects were observed for 1 year (baseline), following which they received placebo for 1 year and then switched to zinc supplementation (50 to 75 mg of elemental zinc orally daily) for 2 years. Gr C subjects did not receive any intervention inasmuch as they were zinc sufficient. Prolonged zinc supplementation resulted in an increase in lymphocyte and granulocyte zinc (P = 0.0001), and an increase in interleukin-2 production (P = 0.0001), decreased incidence of documented bacteriologically positive infections (P = 0.0026), decreased number of hospitalizations and decreased number of vaso-occlusive pain crisis (P = 0.0001). The predominant pathogens isolated were staphylococci and streptococci involving the respiratory tract and aerobic gram-negative bacteria, particularly Escherichia coli, involving the urinary tract. Further confirmation of our observations will require prospective studies of zinc supplementation in a larger number of SCD patients.

Elevated homocysteine levels indicate suboptimal folate status in pediatric sickle cell patients

We investigated whether pediatric patients with sickle cell disease (SCD) (9 +/- 4 years; 27 homozygous SCD [HbSS]; 19 sickle-C disease [HbSC]) have different folate status compared with age-, sex-, and race-matched normal hemoglobin (HbAA) controls (n = 20), and whether their folate status can be improved by folate supplementation. The patients were supplemented with vitamins B6 and B12 during one week and with folate during the following week. Circulating folate, homocysteine, vitamin B6 and vitamin B12 levels were measured at baseline (patients and controls), after one week and after two weeks (patients). The patients had similar folate, vitamin B6, and vitamin B12, but higher homocysteine levels compared with HbAA controls (12.7 +/- 4.5 vs. 10.9 +/- 3.5 micromol/l; P = 0.04). Vitamin B6 and B12 supplementation did not change their homocysteine levels, but folate supplementation caused a 53% reduction (to 5.7 +/- 1.6). We conclude that patients with SCD have adequate vitamin B6 and B12 status, but suboptimal folate status, leading to elevated plasma homocysteine levels. They may therefore benefit from folate supplementation to reduce their high risk for endothelial damage.