Estimation of fetal hemoglobin levels in individual red cells via fluorescence image cytometry

Differential sensitivity to hypoxia enables shape-based classification of sickle cell disease and trait blood samples at point of care

Red blood cells (RBCs) become sickle-shaped and stiff under hypoxia as a consequence of hemoglobin (Hb) polymerization in sickle cell anemia. Distinguishing between sickle cell disease and trait is crucial during the diagnosis of sickle cell disease. While genetic analysis or high-performance liquid chromatography (HPLC) can accurately differentiate between these two genotypes, these tests are unsuitable for field use. Here, we report a novel microscopy-based diagnostic test called ShapeDx™ to distinguish between disease and trait blood in less than 1 h. This is achieved by mixing an unknown blood sample with low and high concentrations of a chemical oxygen scavenger and thereby subjecting the blood to slow and fast hypoxia, respectively. The different rates of Hb polymerization resulting from slow and fast hypoxia lead to two distinct RBC shape distributions in the same blood sample, which allows us to identify it as healthy, trait, or disease. The controlled hypoxic environment necessary for differential Hb polymerization is generated using an imaging microchamber, which also reduces the sickling time of trait blood from several hours to just 30 min. In a single-blinded proof-of-concept study conducted on a small cohort of clinical samples, the results of the ShapeDx™ test were 100% concordant with HPLC results. Additionally, our field studies have demonstrated that ShapeDx™ is the first reported microscopy test capable of distinguishing between sickle cell disease and trait samples in resource-limited settings with the same accuracy as a gold standard test.

Targeting fetal hemoglobin expression to treat β hemoglobinopathies

INTRODUCTION

Sickle cell disease and β thalassemia are the principal β hemoglobinopathies. The complex pathophysiology of sickle cell disease is initiated by sickle hemoglobin polymerization. In β thalassemia, insufficient β-globin synthesis results in excessive free α globin, ineffective erythropoiesis and severe anemia. Fetal hemoglobin (HbF) prevents sickle hemoglobin polymerization; in β thalassemia HbF compensates for the deficit of normal hemoglobin. When HbF constitutes about a third of total cell hemoglobin, the complications of sickle cell disease are nearly totally prevented. Similarly, sufficient HbF in β thalassemia diminishes or prevents ineffective erythropoiesis and hemolysis.

AREAS COVERED

This article examines the pathophysiology of β hemoglobinopathies, the physiology of HbF, intracellular distribution and the regulation of HbF expression. Inducing high levels of HbF by targeting its regulatory pathways pharmacologically or with cell-based therapeutics provides major clinical benefit and perhaps a "cure."

EXPERT OPINION

Erythrocytes must contain about 10 pg of HbF to "cure" sickle cell disease. If HbF is the only hemoglobin present, much higher levels are needed to "cure" β thalassemia. These levels of HbF can be obtained by different iterations of gene therapy. Small molecule drugs that can achieve even modest pancellular HbF concentrations are a major unmet need.

Heterogeneity of fetal hemoglobin production in adult red blood cells

PURPOSE OF REVIEW

Small amounts of fetal hemoglobin can be expressed in a subset of adult red blood cells called F-cells. This review examines the potential mechanisms and clinical implications of the heterogeneity of fetal hemoglobin expression.

RECENT FINDINGS

Although the heterocellular nature of fetal hemoglobin expression in adult red blood cells has been noted for over 70 years, the molecular basis of this phenomenon has been unclear. Recent discoveries of novel regulators of fetal hemoglobin as well as technological advances have shed new light on these cells.

SUMMARY

Fetal hemoglobin reactivation in adult red blood cells through genetic or pharmacological approaches can involve both increasing the number of F-cells and cellular fetal hemoglobin content. New technologies enable the study and eventually the improvement of these parameters in patients with sickle cell disease and β-thalassemia.

Fetal hemoglobin in sickle cell anemia

Fetal hemoglobin (HbF) can blunt the pathophysiology, temper the clinical course, and offer prospects for curative therapy of sickle cell disease. This review focuses on (1) HbF quantitative trait loci and the geography of β-globin gene haplotypes, especially those found in the Middle East; (2) how HbF might differentially impact the pathophysiology and many subphenotypes of sickle cell disease; (3) clinical implications of person-to-person variation in the distribution of HbF among HbF-containing erythrocytes; and (4) reactivation of HbF gene expression using both pharmacologic and cell-based therapeutic approaches. A confluence of detailed understanding of the molecular basis of HbF gene expression, coupled with the ability to precisely target by genomic editing most areas of the genome, is producing important preliminary therapeutic results that could provide new options for cell-based therapeutics with curative intent.

Individual red blood cell fetal hemoglobin quantification allows to determine protective thresholds in sickle cell disease

Polymerization of the sickle hemoglobin (HbS) is a key determinant of sickle cell disease (SCD), an inherited blood disorder. Fetal hemoglobin (HbF) is a major modulator of the disease severity by both decreasing HbS intracellular concentration and inhibiting its polymerization. However, heterocellular distribution of HbF is common in SCD. For HbS polymerization inhibition, the hypothesis of an "HbF per red blood cell (HbF/RBC) threshold" requires accurate measurement of HbF in individual RBC. To date, HbF detection methods are limited to a qualitative measurement of RBC populations containing HbF - the F cells, which are variable. We developed an accurate method for HbF quantification in individual RBC. A linear association between mean HbF content and mean RBC fluorescence by flow cytometry, using an anti-Human-HbF antibody, was obtained from non-SCD subjects presenting homogeneous HbF distribution. This correlation was then used to measure HbF/RBC. Hydroxyurea (HU) improves SCD clinical manifestations, mainly through its ability to induce HbF synthesis. The HbF distribution was analyzed in 14 SCD patients before and during HU treatment. A significant decrease in RBC population containing less than 2 pg of HbF/RBC was observed. Therefore, we tested associations for %RBC above different HbF/RBC thresholds and showed a decrease in the pathognomonic vaso-occlusive crisis incidence from the threshold of 4 pg. This quantity was also correlated with the level of sickle RBC after in vitro deoxygenation. This new method allows the comparison of HbF/RBC distributions and could be a useful tool to characterize baseline patients HbF distribution and therapeutic response to HbF inducers.

Perspective: A Novel Prognostic for Sickle Cell Disease

Sickle hemoglobin (α₂β^S ₂) polymerization drives disease pathophysiology in sickle cell anemia. Fetal hemoglobin (α₂γ₂) restricts disease severity by inhibiting the polymerization of sickle hemoglobin in a concentration-dependent manner. Clinical decision-making relies on diagnostic technologies evaluating fetal hemoglobin as mean percent or mean quantity in blood. Limitation of this approach is exemplified by patients with significant high fetal hemoglobin levels and severe disease, suggesting that fetal hemoglobin is unevenly distributed across F-cells. Therefore, determination of fetal hemoglobin/F-cell would provide a new paradigm for ascertaining prognosis and response to fetal hemoglobin-inducing agents. Measurement of fetal hemoglobin/F-cell, ultimately adapted to widespread standardized analytical use, is a promising fetal hemoglobin-related prognostic approach to monitor the severity of sickle cell disease and the best "phenotype" to follow when developing new candidate fetal hemoglobin inducers or titrating hydroxyurea in treated sickle cell patients.

Hydroxycarbamine: from an Old Drug Used in Malignant Hemopathies to a Current Standard in Sickle Cell Disease

While hydroxycarbamide (hydroxyurea, HU) has less and fewer indications in malignant hemopathies, it represents the only widely used drug which modifies sickle cell disease pathogenesis. Clinical experience with HU for patients with sickle cell disease has been accumulated over the past 25 years in Western countries. The review of the literature provides increasing support for safety and efficacy in both children and adults for reducing acute vaso-occlusive events including pain episodes and acute chest syndrome. No increased incidence of leukemia and teratogenicity was demonstrated. HU has become the standard-of-care for sickle cell anemia but remains underused. Barriers to its use should be identified and overcome.

Correction of murine hemoglobinopathies by prenatal tolerance induction and postnatal nonmyeloablative allogeneic BM transplants

Sickle cell disease (SCD) and thalassemias (Thal) are common congenital disorders, which can be diagnosed early in gestation and result in significant morbidity and mortality. Hematopoietic stem cell transplantation, the only curative therapy for SCD and Thal, is limited by the absence of matched donors and treatment-related toxicities. In utero hematopoietic stem cell transplantation (IUHCT) is a novel nonmyeloablative transplant approach that takes advantage of the immunologic immaturity and normal developmental properties of the fetus to achieve mixed allogeneic chimerism and donor-specific tolerance (DST). We hypothesized that a combined strategy of IUHCT to induce DST, followed by postnatal nonmyeloablative same donor "booster" bone marrow (BM) transplants in murine models of SCD and Thal would result in high levels of allogeneic engraftment and donor hemoglobin (Hb) expression with subsequent phenotypic correction of SCD and Thal. Our results show that: (1) IUHCT is associated with DST and low levels of allogeneic engraftment in the murine SCD and Thal models; (2) low-level chimerism following IUHCT can be enhanced to high-level chimerism and near complete Hb replacement with normal donor Hb with this postnatal "boosting" strategy; and (3) high-level chimerism following IUHCT and postnatal "boosting" results in phenotypic correction in the murine Thal and SCD models. This study supports the potential of IUHCT, combined with a postnatal nonmyelablative "boosting" strategy, to cure Thal and SCD without the toxic conditioning currently required for postnatal transplant regimens while expanding the eligible transplant patient population due to the lack of a restricted donor pool.

Fetal hemoglobin in sickle cell anemia: a glass half full?

Fetal hemoglobin (HbF) modulates the phenotype of sickle cell anemia by inhibiting deoxy sickle hemoglobin (HbS) polymerization. The blood concentration of HbF, or the number of cells with detectable HbF (F-cells), does not measure the amount of HbF/F-cell. Even patients with high HbF can have severe disease because HbF is unevenly distributed among F-cells, and some cells might have insufficient concentrations to inhibit HbS polymerization. With mean HbF levels of 5%, 10%, 20%, and 30%, the distribution of HbF/F-cell can greatly vary, even if the mean is constant. For example, with 20% HbF, as few as 1% and as many as 24% of cells can have polymer-inhibiting, or protective, levels of HbF of ∼10 pg; with lower HbF, few or no protected cells can be present. Only when the total HbF concentration is near 30% is it possible for the number of protected cells to approach 70%. Rather than the total number of F-cells or the concentration of HbF in the hemolysate, HbF/F-cell and the proportion of F-cells that have enough HbF to thwart HbS polymerization is the most critical predictor of the likelihood of severe sickle cell disease.

No improvement in suboptimal vitamin A status with a randomized, double-blind, placebo-controlled trial of vitamin A supplementation in children with sickle cell disease

BACKGROUND

Suboptimal vitamin A status is prevalent in children with type SS sickle cell disease (SCD-SS) and is associated with hospitalizations and poor growth and hematologic status. The supplemental vitamin A dose that optimizes suboptimal vitamin A status in this population is unknown.

OBJECTIVE

The efficacy of Recommended Dietary Allowance (RDA) doses (based on age and sex) of vitamin A (300, 400, or 600 μg retinyl palmitate/d) or vitamin A + zinc (10 or 20 mg zinc sulfate/d) compared with placebo to optimize vitamin A status was assessed in children aged 2.0-12.9 y with SCD-SS and a suboptimal baseline serum retinol concentration (<30 μg/dL).

DESIGN

In this randomized, double-blind, placebo-controlled trial, vitamin A status (serum retinol, prealbumin, retinol-binding protein, and relative-dose-response test) and disease-related illness events were assessed.

RESULTS

Twelve months of vitamin A supplementation at the doses recommended for healthy US children (based on age and sex) failed to improve serum retinol values in either group (vitamin A: n = 23; vitamin A + zinc: n = 18) compared with placebo (n = 21). By 12 mo, the increase (±SD) in serum retinol (3.6 ± 2.8 μg/dL) in those taking 600 μg vitamin A/d was significantly different from the decrease (±SD; -2.8 ± 2.4 μg/dL) in those taking 300 μg/d, which possibly suggests a dose-response relation (P < 0.05) with RDA doses.

CONCLUSIONS

Compared with placebo, 12 mo of vitamin A supplementation at the RDA for healthy children did not improve serum retinol values in children with SCD-SS, which possibly suggests that higher doses are needed. However, the existence of alternative conclusions emphasizes the need for future research.

Identification and quantification of fetal red blood cells in maternal blood by a dual-color flow cytometric method: evaluation of the Fetal Cell Count kit

BACKGROUND

As an alternative to the cumbersome Kleihauer-Betke test (KBT), flow cytometry represents a powerful method for the identification and quantification of fetal red blood cells (RBCs) in maternal circulation.

STUDY DESIGN AND METHODS

The aim of this study was to evaluate the Fetal Cell Count kit (IQ Products), an innovative flow cytometric method, based on the combination of antibodies directed, respectively, against fetal hemoglobin (HbF) and carbonic anhydrase (CA), a marker expressed after birth, to discriminate fetal RBCs from adult F cells containing HbF. The investigation was performed by two French laboratories that compared the data obtained by flow cytometry and KBT in 455 pregnant or just-delivered women as well as in 124 artificial mixtures containing from 0.01 to 5.00 percent cord cells.

RESULTS

The FL1/FL2 histogram allowed distinction between fetal RBCs (HbF+, CA-), F cells (HbF+, CA+), and adult RBCs (HbF-, CA+). The limits of detection and quantification were determined at 0.03 and 0.10 percent or 0.02 and 0.05 percent when analyzing 100,000 or 200,000 events, respectively. Linearity was demonstrated between 0.01 and 5.00 percent fetal cells in the mixtures (r = 0.95, p < 0.01). A good correlation between fluorescence-activated cell sorting (FACS) and KBT results was obtained with artificial mixtures (r = 0.94, p < 0.01). From the 405 Kleihauer-negative samples, none were identified as positive by FACS. Among the 50 Kleihauer-positive samples, 6 were shown not to contain fetal cells but F cells by FACS.

CONCLUSION

With this new dual-color flow cytometric method, accurate evaluation of fetomaternal hemorrhage was achieved even in the face of HbF of maternal origin.

Effect of fetal hemoglobin on microvascular regulation in sickle transgenic-knockout mice

In sickle cell disease, intravascular sickling and attendant flow abnormalities underlie the chronic inflammation and vascular endothelial abnormalities. However, the relationship between sickling and vascular tone is not well understood. We hypothesized that sickling-induced vaso-occlusive events and attendant oxidative stress will affect microvascular regulatory mechanisms. In the present studies, we have examined whether microvascular abnormalities expressed in sickle transgenic-knockout Berkeley (BERK) mice (which express exclusively human alpha- and beta(S)-globins with <1% gamma-globin levels) are amenable to correction with increased levels of antisickling fetal hemoglobin (HbF). In BERK mice, sickling, increased oxidative stress, and hemolytic anemia are accompanied by vasodilation, compensatory increases in eNOS and COX-2, and attenuated vascular responses to NO-mediated vasoactive stimuli and norepinephrine. The hypotension and vasodilation (required for adequate oxygen delivery in the face of chronic anemia) are mediated by non-NO vasodilators (i.e., prostacyclin) as evidenced by induction of COX-2. In BERK mice, the resistance to NO-mediated vasodilators is associated with increased oxidative stress and hemolytic rate, and in BERK + gamma mice (expressing 20% HbF), an improved response to these stimuli is associated with reduced oxidative stress and hemolytic rate. Furthermore, BERK + gamma mice show normalization of vessel diameters, and eNOS and COX-2 expression. These results demonstrate a strong relationship between sickling and microvascular function in sickle cell disease.

Combined use of nonmyelosuppressive nitrosourea analogues with hydroxyurea in the induction of F-cell production in a human erythroleukemic cell line

OBJECTIVE

Although hydroxyurea (HU) has been used clinically to treat patients with sickle cell disease (SCD), not all patients benefit from HU treatment due to its toxicity. The objective of this study was to investigate the effectiveness of the use of two new Hb F-inducing nitrosourea analogues, 2-[3-(2-methyl, 2-nitroso) ureido]-2-deoxy-D-glucopyranose (MNGU) and 2-[3-(2-chloroethyl) ureido]-2-deoxy-D-glucopyranose (CGU), in combination with HU in K562 cells or erythroid progenitors.

MATERIALS AND METHODS

After K562 cells were cultured with different concentrations of HU with CGU or MNGU, aliquots of the cells were obtained to determine the total (benzidine-positive) hemoglobin level, number of F cells, and Hb F level. Erythroid progenitor cells of SCD patients and healthy donors were cultured with the optimal drug concentrations, and the number of BFU-E and Hb F level were determined.

RESULTS

Our results showed that the combined use of HU with CGU or MNGU increased the number of both benzidine-positive normoblasts and F cells in a synergistic manner. Further, a lower concentration of HU was required to induce a significant level of Hb F synthesis when combined with either of the two compounds in comparison with treatment with HU alone. On day 4, the number of benzidine-positive cells was 4.5- to 6.5-fold and the number of F cells was 5.0- to 8.0-fold higher than the respective numbers in the untreated K562 cells. Similarly, a 3.2- to 14.3-fold induction of Hb F was obtained when human erythroid progenitors from SCD patients were treated with the same drug combinations.

CONCLUSION

Based on these results, the use of CGU or MNGU in combination with HU might offer substantial benefits to patients with SCD and other hemoglobinopathies.

Mixed chimerism following in utero hematopoietic stem cell transplantation in murine models of hemoglobinopathy

OBJECTIVE

Mixed hematopoietic chimerism after bone marrow transplantation can provide effective treatment for beta-thalassemia because of the selective advantage that exists for donor erythropoiesis. In utero hematopoietic stem cell transplantation (IUHSCTx) can achieve mixed hematopoietic chimerism, particularly when a selective advantage exists for donor cells. To investigate the biology of IUHSCTx in hemoglobinopathies, we performed fully allogeneic IUHSCTx in murine models of beta-thalassemia (Thal) and sickle cell disease (SCD).

MATERIALS AND METHODS

We serially assessed and compared levels of mononuclear cell (MNC) and erythroid chimerism after IUHSCTx of either adult bone marrow (BM)- or fetal liver (FL)-derived allogeneic donor cells in the two hemoglobinopathy models, which differ significantly in their degree of anemia (Thal>>SCD) and red cell half-life (Thal<<SCD).

RESULTS

The mean level of donor MNC chimerism was higher for SCD and Thal chimeras receiving FL- compared to adult BM-derived donor cells and tended to increase over time in the FL recipients. Donor hemoglobin (Hb) levels also were higher in all groups receiving FL compared to adult BM. Donor Hb levels in chimeric Thal mice were significantly higher than those in SCD or wild-type mice. Hematologic parameters such as Hb, hematocrit (Hct), mean cell volume (MCV), membrane-associated denatured Hb, and the oxygen equilibration curve were improved in chimeric hemoglobinopathy mice. However, the improvement in Hb, Hct, and MCV was not sustained despite stable levels of donor leukocyte engraftment.

CONCLUSION

The severity of the hemoglobinopathy being treated and the source of donor cells may be important determinants of success in the treatment of hemoglobinopathy by IUHSCTx.

Hemoglobinopathies

The outlook for patients with sickle cell disease has improved steadily during the last two decades. In spite of these improvements, curative therapies are currently available only to a small minority of patients. The main theme of this chapter is to describe new therapeutic options that are at different stages of development that might result in further improvements in the outlook for patients with these disorders.

Dr. Joseph DeSimone and his colleagues had previously made the important observation that the hypomethylating agent 5-azacytidine can reverse the switch from adult to fetal hemoglobin in adult baboons. Although similar activity was demonstrated in patients with sickle cell disease and β-thalassemia, concern about the toxicity of 5-azacytidine prevented its widespread use in these disorders. In Section I, Dr. DeSimone discusses the role of DNA methylation in globin gene regulation and describe recent clinical experience with decitabine (an analogue of 5-azacytidine) in patients with sickle cell disease. These encouraging studies demonstrate significant fetal hemoglobin inducing activity of decitabine in patients who fail to respond to hydroxyurea.

In Section II, Dr. George Atweh continues the same theme by describing recent progress in the study of butyrate, another inducer of fetal hemoglobin, in patients with sickle cell disease and β-thalassemia. The main focus of his section is on the use of a combination of butyrate and hydroxyurea to achieve higher levels of fetal hemoglobin that might be necessary for complete amelioration of the clinical manifestations of these disorders. Dr. Atweh also describes novel laboratory studies that shed new light on the mechanisms of fetal hemoglobin induction by butyrate.

In Section III, Dr. Ronald Nagel discusses the different available transgenic sickle mice as experimental models for human sickle cell disease. These experimental models have already had a significant impact on our understanding of the pathophysiology of sickle cell disease. Dr. Nagel describes more recent studies in which transgenic sickle mice provide the first proof of principle that globin gene transfer into hematopoietic stem cells inhibits in vivo sickling and ameliorates the severity of the disease.

Although stroke in adult patients with sickle cell disease is not as common as in children, adult hematologists, like their pediatric colleagues, need to make management decisions in adult patients with a stroke or a history of stroke. Dr. Robert Adams has led several large clinical studies that investigated the role of transfusions in the prevention of stroke in children with sickle cell disease. Much less is known, however, about the prevention of first or subsequent strokes in adult patients with sickle cell disease. In Section IV, Dr. Adams provides some general guidelines for the management of adult patients with stroke while carefully distinguishing between recommendations that are evidence-based and those that are anecdotal in nature.

The survival characteristics of dense sickle cells

Sickle red blood cells (RBCs) become depleted of potassium, leading to dehydration and abnormally elevated cellular density. The increased sickling that results is important for both hemolysis and vasocclusion. In this study, sickle cells were subjected to high-speed centrifugation, and the bottom 15% were isolated. This procedure removed light cells and to a variable degree enriched cells that were denser than normal to produce a high-density–enriched (HDE) population of sickle cells. Autologous HDE cells from 3 subjects were labeled with biotin and re-infused. The following determinations were performed: (1) the survival and density changes of HDE cells; (2) the amount of fetal hemoglobin (HbF) in labeled cells after magnetic isolation; (3) the percentage of labeled F cells; (4) the percentage of labeled cells displaying external phosphatidylserine (PS). For patients with 3.5%, 4.5%, and 24% HbF in the HDE RBCs, the circulation half-time was 40, 80, and 180 hours, respectively. The percentage of HbF (measured in all 3 subjects) and of F cells (measured in 2 subjects) in labeled RBCs increased with time after re-infusion, indicating that HDE F cells have longer in vivo survival than HDE non-F cells. The percentage of PS+, biotin-labeled HDE cells showed no consistent increase or decrease with time after re-infusion. These data provide evidence that HDE sickle cells, especially those that do not contain HbF, have a very short in vivo survival, and that the percentage of PS+ cells in a re-infused HDE population does not change in a consistent manner as these cells age in the circulation.

The survival characteristics of dense sickle cells

Sickle red blood cells (RBCs) become depleted of potassium, leading to dehydration and abnormally elevated cellular density. The increased sickling that results is important for both hemolysis and vasocclusion. In this study, sickle cells were subjected to high-speed centrifugation, and the bottom 15% were isolated. This procedure removed light cells and to a variable degree enriched cells that were denser than normal to produce a high-density–enriched (HDE) population of sickle cells. Autologous HDE cells from 3 subjects were labeled with biotin and re-infused. The following determinations were performed: (1) the survival and density changes of HDE cells; (2) the amount of fetal hemoglobin (HbF) in labeled cells after magnetic isolation; (3) the percentage of labeled F cells; (4) the percentage of labeled cells displaying external phosphatidylserine (PS). For patients with 3.5%, 4.5%, and 24% HbF in the HDE RBCs, the circulation half-time was 40, 80, and 180 hours, respectively. The percentage of HbF (measured in all 3 subjects) and of F cells (measured in 2 subjects) in labeled RBCs increased with time after re-infusion, indicating that HDE F cells have longer in vivo survival than HDE non-F cells. The percentage of PS+, biotin-labeled HDE cells showed no consistent increase or decrease with time after re-infusion. These data provide evidence that HDE sickle cells, especially those that do not contain HbF, have a very short in vivo survival, and that the percentage of PS+ cells in a re-infused HDE population does not change in a consistent manner as these cells age in the circulation.

Enhancement of hemoglobin and F-cell production by targeting growth inhibition and differentiation of K562 cells with ribonucleotide reductase inhibitors (didox and trimidox) in combination with streptozotocin

Upon appropriate drug treatment, the human erythroleukemic K562 cells have been shown to produce hemoglobin and F-cells. Fetal hemoglobin (Hb F) inhibits the polymerization events of sickle hemoglobin (Hb S), thereby ameliorating the clinical symptoms of sickle cell disease. Ribonucleotide reductase inhibitors (RRIs) have been shown to inhibit the growth of myeloid leukemia cells leading to the production of Hb F upon differentiation. Of the RRIs currently in use, hydroxyurea is the most effective agent for Hb F induction. We have examined the capacity of two novel RRIs, didox (DI) and trimidox (TRI), in combination with streptozotocin (STZ), to induce hemoglobin and F-cell production. The K562 cells were cultured with different concentrations of didox-STZ or trimidox-STZ at a fixed molar ratio of 3:1 and 1:5 for 96 hr, respectively. At pre-determined time intervals, aliquots of cells were obtained and total hemoglobin (benzidine positive) levels, number of F-cells, and Hb F were determined by the differential staining technique, fetal hemoglobin assay kit, and fluorescence cytometry respectively. The effect of combined drug treatment on the growth of K562 cells was examined by isobologram analysis. Our results indicate that a synergistic growth-inhibitory differentiation effect occurred when didox or trimidox was used in combination with STZ on K562 cells. There was an increase in the number of both benzidine-positive normoblasts and F-cells, accompanied by morphologic appearances typical of erythroid maturation. On day 4, the number of benzidine-positive cells showed a 6-9-fold increase and the number of F-cells was between 2.5- and 5.7-fold higher than the respective controls. Based upon these results, treatment with a ribonucleotide reductase inhibitor, such as didox or trimidox, in combination with STZ, might offer an additional promising option in sickle cell disease therapy.

F reticulocytes assay: a method to evaluate fetal hemoglobin production

Production of fetal hemoglobin (Hb F) involves molecular as well as cellular aspects as, among red blood cells, it is restricted to a specific population referred as the F cells. Thus understanding the mechanisms involved in persistence or re-emergence of Hb F production in various inherited or acquired conditions requires the measurement of both Hb F and F cells. In addition, in disorders with a hemolytic component, including sickle cell disease (SCD), because of a probable preferential survival of F cells as compared to non-F cells, the true parameter of F cell production is the F reticulocyte count. The F cells/F reticulocytes ratio then selectively reflects this preferential survival. Here we describe an original immunofluorescence microscopy assay that permits the simultaneous measurement of F cells and F reticulocytes. For this assay to be widely usable, we chose to use commercially available monoclonal antibodies.

Cross evaluation of three flow cytometric F cell counting methods performed by different laboratories

Three flow cytometric methods of counting F cells were evaluated in the settings of an external laboratory assessment scheme. The laboratories to participate with a different method were located in Oxford (method O), Athens (method A) and Jerusalem, (method J). Two monoclonal anti-gamma chain antibodies were used: monoclonal antibody produced by P. Beverley (Oxford) (BEV) and an antibody provided by Bioatlantic S.A.R.L. (France) (BIO). The specimens tested were mixtures in five predefined ratios of a sample with homozygous deltabeta-thalassemia with 100% F cells with a sample with no F cells. A central independent laboratory prepared and distributed the aliquots (at room temperature) to the participating centers within 2 (O), 3 (A), and 6 (J) days. The performance of the three methods was evaluated by: 1) deviation indices, 2) relative accuracy, as percent difference of the counts from the target values, and 3) bias and linearity by linear regression of the counts versus the target values [parameters: slope (s), y-intercept (y), R squared (Rs), and F ratio]. The highest score of performance was obtained by method A with both monoclonal antibodies.