Sickle cell vaso-occlusion: The dialectic between red cells and white cells.

The pathophysiology of sickle cell anemia, a hereditary hemoglobinopathy, has fascinated clinicians and scientists alike since its description over 100 years ago. A single gene mutation in the HBB gene results in the production of abnormal hemoglobin (Hb) S, whose polymerization when deoxygenated alters the physiochemical properties of red blood cells, in turn triggering pan-cellular activation and pathological mechanisms that include hemolysis, vaso-occlusion, and ischemia-reperfusion to result in the varied and severe complications of the disease. Now widely regarded as an inflammatory disease, in recent years attention has included the role of leukocytes in vaso-occlusive processes in view of the part that these cells play in innate immune processes, their inherent ability to adhere to the endothelium when activated, and their sheer physical and potentially obstructive size. Here, we consider the role of sickle red blood cell populations in elucidating the importance of adhesion vis-a-vis polymerization in vaso-occlusion, review the direct adhesion of sickle red cells to the endothelium in vaso-occlusive processes, and discuss how red cell- and leukocyte-centered mechanisms are not mutually exclusive. Given the initial clinical success of crizanlizumab, a specific anti-P selectin therapy, we suggest that it is appropriate to take a holistic approach to understanding and exploring the complexity of vaso-occlusive mechanisms and the adhesive roles of the varied cell types, including endothelial cells, platelets, leukocytes, and red blood cells.

Sickle hemoglobin disturbs normal coupling among erythrocyte O2 content, glycolysis, and antioxidant capacity.

Energy metabolism in RBCs is characterized by O2-responsive variations in flux through the Embden Meyerhof pathway (EMP) or the hexose monophosphate pathway (HMP). Therefore, the generation of ATP, NADH, and 2,3-DPG (EMP) or NADPH (HMP) shift with RBC O2 content because of competition between deoxyhemoglobin and key EMP enzymes for binding to the cytoplasmic domain of the Band 3 membrane protein (cdB3). Enzyme inactivation by cdB3 sequestration in oxygenated RBCs favors HMP flux and NADPH generation (maximizing glutathione-based antioxidant systems). We tested the hypothesis that sickle hemoglobin disrupts cdB3-based regulatory protein complex assembly, creating vulnerability to oxidative stress. In RBCs from patients with sickle cell anemia, we demonstrate in the present study constrained HMP flux, NADPH, and glutathione recycling and reduced resilience to oxidative stress manifested by membrane protein oxidation and membrane fragility. Using a novel, inverted membrane-on-bead model, we illustrate abnormal (O2-dependent) association of sickle hemoglobin to RBC membrane that interferes with sequestration/inactivation of the EMP enzyme GAPDH. This finding was confirmed by immunofluorescent imaging during RBC O2 loading/unloading. Moreover, selective inhibition of inappropriately dispersed GAPDH rescues antioxidant capacity. Such disturbance of cdB3-based linkage between O2 gradients and RBC metabolism suggests a novel mechanism by which hypoxia may influence the sickle cell anemia phenotype.

What dentists should know about sickle cell disease.

The medical history should be a communication between the patient and the dentist. A good history will reveal a patient's medical problems,concerns, ideas, and expectations. Understanding medical conditions on a patient's medical history is of up most importance in providing the patient with the best possible standard of care. Sickle cell disease is an inherited blood disorder that affects red blood cells. Normal red blood cells contain hemoglobin A. People with sickle cell disease have red blood cells containing mostly hemoglobin S, an abnormal type of hemoglobin. These mutated sickle cells do not have the smooth motion needed for oxygenation and deoxygenation. One of the main concerns in sickle cell disease is the reversible extreme pain episodes called "sickle cell crisis."Pain episodes occur when sickle cells clog small vessels, depriving the body of adequate blood and oxygen. Treatment of the sickle cell patient should be a team approach between dentist,patient, and physician. Dental treatments should be conservative and stress free for the patient.Prevention of dental disease and infections are of the up most importance to the sickle cell patient.If your patient has sickle cell disease, know about it and talk to your patient about the disease.Maintaining excellent oral health to decrease the possibility of oral infections will ensure the best care for these patients.Key words: communication, sickle cell disease (SCD), sickle cell anemia (SCA), blood inherited disorder, sickle cell trait, crisis, African Americans, deoxygenation, hemoglobin,supporting dentist, prophylactic antibiotics, and infection.

Evolution of novel small-molecule therapeutics targeting sickle cell vasculopathy.

A 34-year-old African American woman with sickle cell disease and history of relatively severe hemolysis, chronic leg ulcers, and mild pulmonary hypertension presented with a new ischemic stroke. Recent research has suggested a syndrome of hemolysis-associated vasculopathy in patients with sickle cell disease, which features severe hemolytic anemia and leads to scavenging of nitric oxide and its biochemical precursor l-arginine. This diminished bioavailability of nitric oxide promotes a hemolysis-vascular dysfunction syndrome, which includes pulmonary hypertension, cutaneous leg ulceration, priapism, and ischemic stroke. Additional correlates of this vasculopathy include activation of endothelial cell adhesion molecules, platelets, and the vascular protectant hemeoxygenase-1. Some known risk factors for atherosclerosis are also associated with sickle cell vasculopathy, including low levels of apolipoprotein AI and high levels of asymmetric dimethylarginine, an endogenous inhibitor of nitric oxide synthase. Identification of dysregulated vascular biology pathways in sickle vasculopathy has provided a focus for new clinical trials for therapeutic intervention, including inhaled nitric oxide, sodium nitrite, L-arginine, phosphodiesterase-5 inhibitors, niacin, inhaled carbon monoxide, and endothelin receptor antagonists. This article reviews the pathophysiology of sickle vasculopathy and the results of preliminary clinical trials of novel small-molecule therapeutics directed at abnormal vascular biology in patients with sickle cell disease.

Genetic epidemiology of susceptibility to malaria: not only academic exercises.

Descriptive genetic epidemiology represents the initial step of a logical procedure of linked and consequential phases spanning from the identification of genes involved in the resistance/susceptibility to diseases, to the determination of the underlying mechanisms and finally to the possible translation of the acquired knowledge in new control tools. In malaria, the rational development and potential of this pathway is based on complementary interactions of heterogeneus disciplines going from epidemiology (the transmission, the infection, the disease) to vaccinology passing through genetics, pathogenesis, and immunology. Several epidemiological approaches can be applied in the study of the genetic susceptibility to Plasmodium falciparum malaria: intra-ethnic case-control studies comparing genetic candidates of resistance/susceptibility between subjects with different presentation of malaria (from severe disease to asymptomatic infection) and the general healthy population is the classic approach; inter-ethnic comparative analyses among populations with different genetic backgrounds, exposed to the same epidemiological context and showing different susceptibility to the disease is a further, complementary, strategy.

Hemorheology and heart rate variability: is there a relationship?

The aim of the present study was to test the relationships between hemorheology and autonomic nervous system (ANS) activity in 23 sickle cell trait (SCT) carriers and 17 control subjects. Hemorheological parameters were assessed at rest. Nocturnal ANS activity was calculated from heart rate variability indices. Impairment was observed in both domains in SCT carriers as they showed higher blood viscosity, higher red blood cell rigidity and a lower index of oxygen supply to tissues, as well as lower ANS activity. The index of oxygen supply to tissues was positively correlated with some of the indices of parasympathetic activity and global ANS activity. Negative correlations were found between blood viscosity and the very low frequency of the spectrum (VLF) that contains partial parasympathetic activity and between red blood cell rigidity and the percentage of differences between adjacent normal RR intervals more than 50 ms (PNN50), which also describes parasympathetic activity. These results support the hypothesis that autonomic nervous system equilibrium is affected by perturbations in blood rheology. Mechanisms have yet to be elucidated.

The oxygen affinity of sickle hemoglobin.

The right-shifted oxyhemoglobin dissociation curve of sickle cell disease (SCD) has been thought to result in abnormally low arterial oxygen saturation (S(o)(2)), even when oxygen partial pressure (P(o)(2)) is normal. However, without polymer formation (minimal under normoxic conditions), HbS oxygen affinity is normal. We hypothesized that in SCD, in vivo S(o)(2) is normal when P(o)(2) is normal. We retrospectively examined 50 blood gas and COoximetry samples from SCD patients and from controls matched for pH, P(o)(2), and carboxyhemoglobin. Control data fell close to the Severinghaus curve, as did non-hypoxemic ( [Formula: see text] ) SCD data. In contrast, hypoxemic (S(o)(2)) < 92.5% SCD data fell well below the standard curve. Thus, although SCD patients' oxygen affinity is low under hypoxic conditions, it is normal at normal arterial S(o)(2). Therefore, a finding of abnormally low saturation demonstrates that P(o)(2) is abnormally low. Given our previous finding that pulse oximetry faithfully reflects saturation in SCD, low pulse oximeter readings in SCD constitute reliable evidence of impaired gas exchange.

Haemoglobin C and S role in acquired immunity against Plasmodium falciparum malaria.

A recently proposed mechanism of protection for haemoglobin C (HbC; beta6Glu-->Lys) links an abnormal display of PfEMP1, an antigen involved in malaria pathogenesis, on the surface of HbC infected erythrocytes together with the observation of reduced cytoadhesion of parasitized erythrocytes and impaired rosetting in vitro. We investigated the impact of this hypothesis on the development of acquired immunity against Plasmodium falciparum variant surface antigens (VSA) encoding PfEMP1 in HbC in comparison with HbA and HbS carriers of Burkina Faso. We measured: i) total IgG against a single VSA, A4U, and against a panel of VSA from severe malaria cases in human sera from urban and rural areas of Burkina Faso of different haemoglobin genotypes (CC, AC, AS, SC, SS); ii) total IgG against recombinant proteins of P. falciparum asexual sporozoite, blood stage antigens, and parasite schizont extract; iii) total IgG against tetanus toxoid. Results showed that the reported abnormal cell-surface display of PfEMP1 on HbC infected erythrocytes observed in vitro is not associated to lower anti- PfEMP1 response in vivo. Higher immune response against the VSA panel and malaria antigens were observed in all adaptive genotypes containing at least one allelic variant HbC or HbS in the low transmission urban area whereas no differences were detected in the high transmission rural area. In both contexts the response against tetanus toxoid was not influenced by the beta-globin genotype. These findings suggest that both HbC and HbS affect the early development of naturally acquired immunity against malaria. The enhanced immune reactivity in both HbC and HbS carriers supports the hypothesis that the protection against malaria of these adaptive genotypes might be at least partially mediated by acquired immunity against malaria.

Haemoglobin C and S in natural selection against Plasmodium falciparum malaria: a plethora or a single shared adaptive mechanism?

Conclusive evidence exists on the protective role against clinical Plasmodium falciparum malaria of Haemoglobin S (beta 6Glu-->Val) and HbC (HbC; beta 6Glu-->Lys), both occurring in sub-Saharan Africa. However, the mechanism/s of the protection exerted remain/s debated for both haemoglobin variants, HbC and HbS. Recently, an abnormal display of PfEMP1, an antigen involved in malaria pathogenesis, was reported on HbAC and HbCC infected erythrocytes that showed reduced cytoadhesion and impaired rosetting in vitro. On this basis it has been proposed that HbC protection might be attributed to the reduced PfEMP1-mediated adherence of parasitized erythrocytes in the microvasculature. Furthermore, impaired cytoadherence was observed in HbS carriers suggesting for the first time a convergence in the protection mechanism of these two haemoglobin variants. We investigated the impact of this hypothesis on the development of acquired immunity against P. falciparum variant surface antigens (VSA) encoding PfEMP1 in HbC and HbS carriers in comparison with HbA of Burkina Faso. Higher immune response against a VSA panel and several malaria antigens were observed in all adaptive genotypes containing at least one allelic variant HbC or HbS in the low transmission urban area whereas no differences were detected in the high transmission rural area. In both contexts the response against tetanus toxoid was not influenced by the beta-globin genotype. Thus, these findings suggest that both HbC and HbS affect the early development of naturally acquired immunity against malaria. We reviewed the hypothesized mechanisms so far proposed in light of these recent results.

Human red blood cell polymorphisms and malaria.

Genetic factors are a major determinant of child survival in malaria endemic countries. Identifying which genes are involved and how they affect the malaria disease risk potentially offers a powerful mechanism through which to learn more about the host-parasite relationship. The past few years have seen significant progress towards achieving this goal for some of the best-known malaria resistance genes that determine the structure or function of red blood cells: Gerbich blood group antigen negativity; polymorphisms of the complement receptor genes (most notably CR1); Southeast Asian ovalocytosis; pyruvate kinase deficiency; haemoglobin E; the sickle cell trait; and alpha-thalassaemia are all examples. The challenge for the future must be to translate such advances into fresh approaches to the prevention and treatment of malaria.

Hypoxaemia in sickle cell disease: biomarker modulation and relevance to pathophysiology.

BACKGROUND: Nocturnal oxyhaemoglobin desaturation might have a role in CNS complications related to sickle cell disease, and rates of painful crises. We attempted to examine the biological relations, and describe the haematological risk factors for oxyhaemoglobin desaturation. METHODS: The study population included children with sickle cell disease and controls. Cellular activation was assessed by measurement of soluble vascular cell adhesion molecule 1, P-selectin, L-selectin, and leukotriene B4. Erythrocyte-endothelial adhesion and routine haematological variables were assessed. Oxygen saturation (SaO2) was measured by pulse oximetry while children were awake and asleep. Children with a mean sleeping SaO2 of < or =93% were identified as hypoxaemic. Children were divided into four groups: controls (ten children), HbSC (nine, all normoxic), HbSS normoxic (13), and HbSS hypoxaemic (15). FINDINGS: Among haematological variables, sleeping SaO2 correlated only with packed-cell volume (r=0.7; p<0.0001). Inverse relations were noted between sleeping SaO2 and adhesion (-0.45; p<0.01), and markers of white-cell (-0.51; p<0.01), platelet (-0.61; p<0.001), and endothelial activation (-0.46; p<0.01). In the HbSS group who had sleeping hypoxaemia, waking SaO2 measurements showed continuing hypoxaemia, with similar correlation between SaO2 and cell activation markers. INTERPRETATION: Our adhesion-related findings suggest a potential mechanism for the increased occurrence of clinical vaso-occlusive crises in individuals with sickle cell disease who have oxyhaemoglobin desaturation. Release of cellular mediators in hypoxaemia, and the relation between anaemia and oxyhaemoglobin desaturation, suggest that risk factors for stroke, including anaemia, might have a role in CNS-vasculopathy through hypoxia-mediated pathways. Further more, hypoxaemia in the older child also occurs during the day; such mild untreated hypoxia could lead to an increased risk of vaso-occlusive episodes.

Prevalence of Plasmodium falciparum infection in pregnant women in Gabon.

BACKGROUND: In areas where malaria is endemic, pregnancy is associated with increased susceptibility to malaria. It is generally agreed that this risk ends with delivery and decreases with the number of pregnancies. Our study aimed to demonstrate relationships between malarial parasitaemia and age, gravidity and anaemia in pregnant women in Libreville, the capital city of Gabon. METHODS: Peripheral blood was collected from 311 primigravidae and women in their second pregnancy. Thick blood smears were checked, as were the results of haemoglobin electrophoresis. We also looked for the presence of anaemia, fever, and checked whether the volunteers had had chemoprophylaxis. The study was performed in Gabon where malaria transmission is intense and perennial. RESULTS: A total of 177 women (57%) had microscopic parasitaemia; 139 (64%)of them were primigravidae, 38 (40%) in their second pregnancy and 180 (64%) were teenagers. The parasites densities were also higher in primigravidae and teenagers. The prevalence of anaemia was 71% and was associated with microscopic Plasmodium falciparum parasitaemia: women with moderate or severe anaemia had higher parasite prevalences and densities. However, the sickle cell trait, fever and the use of chemoprophylaxis did not have a significant association with the presence of P. falciparum. CONCLUSIONS: These results suggest that the prevalence of malaria and the prevalence of anaemia, whether associated with malaria or not, are higher in pregnant women in Gabon. Primigravidae and young pregnant women are the most susceptible to infection. It is, therefore, urgent to design an effective regimen of malaria prophylaxis for this high risk population.

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.

A human embryonic hemoglobin inhibits Hb S polymerization in vitro and restores a normal phenotype to mouse models of sickle cell disease.

The principle that developmentally silenced globin genes can be reactivated in adults with defects in beta-globin gene expression has been well established both in vitro and in vivo. In practice, levels of developmental stage-discordant fetal gamma globin that can be achieved by using currently approved therapies are generally insufficient to fully resolve typical clincopathological features of sickle cell disease. The therapeutic potential of another developmentally silenced globin--embryonic epsilon globin--has been difficult to evaluate in the absence of a convenient expression system or an appropriate experimental model. The current work analyzes the antisickling properties of an epsilon -globin-containing heterotetramer (Hb Gower-2) both in vitro as well as in vivo in a well-established mouse model of sickle cell anemia. These animals, expressing 100% human Hb S, display a chronic hemolytic anemia with compensatory marrow and extramedullary erythropoiesis, abundant circulating sickled erythrocytes, and chronic tissue damage evidenced by parallel histopathological and functional deficits. By comparison, related mice that coexpress Hb S as well as Hb Gower-2 exhibit normal physiological, morphological, histological, and functional attributes. Subsequent in vitro analyses substantiate results from whole-animal studies, indicating that the polymerization of deoxygenated Hb S can be significantly slowed by relatively small quantities of Hb Gower-2. Together, the in vivo and in vitro analyses suggest that reactivation of epsilon-globin gene expression would be therapeutically beneficial to adults with sickle phenotypes, and provide a rationale for detailed investigations into the molecular basis for its developmental silencing.

Primary role for adherent leukocytes in sickle cell vascular occlusion: a new paradigm.

Vascular occlusion is the major cause of morbidity and mortality in sickle cell disease but its mechanisms are poorly understood. We demonstrate by using intravital microscopy in mice expressing human sickle hemoglobin (SS) that SS red blood cells (RBCs) bind to adherent leukocytes in inflamed venules, producing vasoocclusion of cremasteric venules. SS mice deficient in P- and E-selectins, which display defective leukocyte recruitment to the vessel wall, are protected from vasoocclusion. These data uncover a previously unsuspected paradigm for the pathogenesis of sickle cell vasoocclusion in which adherent leukocytes play a direct role and suggest that drugs targeting SS RBC-leukocyte or leukocyte-endothelial interactions may prevent or treat the vascular complications of this debilitating disease.

The lung in sickle cell disease.

Sickle cell disease is the most common inherited disorder in African-Americans. Although the primary defect is hematological, the changes in the erythrocytes lead to a vasculopathy with multiorgan injury. The pulmonary complications, i.e., acute chest syndrome and chronic sickle cell lung disease, are significant causes of morbidity and mortality. The pulmonary manifestations result from a unique constellation of factors which come into play in sickle cell disease. Based on the growing understanding of the molecular and cellular biology of sickle cell disease, new therapies are being developed that are likely to ameliorate the natural history of this disease and its complications.

[Pathophysiology of acute thoracic syndrome]. / Physiopathologie du syndrome thoracique aigu.

Pulmonary complications are the leading cause of morbidity and mortality in sickle cell disease patients. Acute chest syndrome (ACS), in which chest pain and dyspnea, occurs in combination with a recent chest radiograph abnormality, raises both diagnostic and therapeutic challenges. The pathogenesis of ACS involves alterations in blood rheology, increased coagulability, and, above all, increased adhesion of sickle cells to the vascular endothelium and nitric oxide-mediated dysregulation of vascular reactivity. Sickle cell disease thus impacts all the cells in the vascular environment. Recently gained insights into pathophysiology offer hope that new treatments for preventing and treating acute and chronic pulmonary complications will soon become available.