At least 20% donor myeloid chimerism is necessary to reverse the sickle phenotype after allogeneic HSCT

Cell replacement in human lung bioengineering

BACKGROUND

As the number of patients with end-stage lung disease continues to rise, there is a growing need to increase the limited number of lungs available for transplantation. Unfortunately, attempts at engineering functional lung de novo have been unsuccessful, and artificial mechanical devices have limited utility as a bridge to transplant. This difficulty is largely due to the size and inherent complexity of the lung; however, recent advances in cell-based therapeutics offer a unique opportunity to enhance traditional tissue-engineering approaches with targeted site- and cell-specific strategies.

METHODS

Human lungs considered unsuitable for transplantation were procured and supported using novel cannulation techniques and modified ex-vivo lung perfusion. Targeted lung regions were treated using intratracheal delivery of decellularization solution. Labeled mesenchymal stem cells or airway epithelial cells were then delivered into the lung and incubated for up to 6 hours.

RESULTS

Tissue samples were collected at regular time intervals and detailed histologic and immunohistochemical analyses were performed to evaluate the effectiveness of native cell removal and exogenous cell replacement. Regional decellularization resulted in the removal of airway epithelium with preservation of vascular endothelium and extracellular matrix proteins. After incubation, delivered cells were retained in the lung and showed homogeneous topographic distribution and flattened cellular morphology.

CONCLUSIONS

Our findings suggest that targeted cell replacement in extracorporeal organs is feasible and may ultimately lead to chimeric organs suitable for transplantation or the development of in-situ interventions to treat or reverse disease, ultimately negating the need for transplantation.

The Hematopoietic Cell Transplant Comorbidity Index predicts survival after allogeneic transplant for nonmalignant diseases

Despite improvements, mortality after allogeneic hematopoietic cell transplantation (HCT) for nonmalignant diseases remains a significant problem. We evaluated whether pre-HCT conditions defined by the HCT Comorbidity Index (HCT-CI) predict probability of posttransplant survival. Using the Center for International Blood and Marrow Transplant Research database, we identified 4083 patients with nonmalignant diseases transplanted between 2007 and 2014. Primary outcome was overall survival (OS) using the Kaplan-Meier method. Hazard ratios (HRs) were estimated by multivariable Cox regression models. Increasing HCT-CI scores translated to decreased 2-year OS of 82.7%, 80.3%, 74%, and 55.8% for patients with HCT-CI scores of 0, 1 to 2, 3 to 4, and ≥5, respectively, regardless of conditioning intensity. HCT-CI scores of 1 to 2 did not differ relative to scores of 0 (HR, 1.12 [95% CI, 0.93-1.34]), but HCT-CI of 3 to 4 and ≥5 posed significantly greater risks of mortality (HR, 1.33 [95% CI, 1.09-1.63]; and HR, 2.31 [95% CI, 1.79-2.96], respectively). The effect of HCT-CI differed by disease indication. Patients with acquired aplastic anemia, primary immune deficiencies, and congenital bone marrow failure syndromes with scores ≥3 had increased risk of death after HCT. However, higher HCT-CI scores among hemoglobinopathy patients did not increase mortality risk. In conclusion, this is the largest study to date reporting on patients with nonmalignant diseases demonstrating HCT-CI scores ≥3 that had inferior survival after HCT, except for patients with hemoglobinopathies. Our findings suggest that using the HCT-CI score, in addition to disease-specific factors, could be useful when developing treatment plans for nonmalignant diseases.

Improving Gene Editing Outcomes in Human Hematopoietic Stem and Progenitor Cells by Temporal Control of DNA Repair

Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR)/CRISPR-associated system (Cas9)-mediated gene editing of human hematopoietic stem cells (hHSCs) is a promising strategy for the treatment of genetic blood diseases through site-specific correction of identified causal mutations. However, clinical translation is hindered by low ratio of precise gene modification using the corrective donor template (homology-directed repair, HDR) to gene disruption (nonhomologous end joining, NHEJ) in hHSCs. By using a modified version of Cas9 with reduced nuclease activity in G1 phase of cell cycle when HDR cannot occur, and transiently increasing the proportion of cells in HDR-preferred phases (S/G2), we achieved a four-fold improvement in HDR/NHEJ ratio over the control condition in vitro, and a significant improvement after xenotransplantation of edited hHSCs into immunodeficient mice. This strategy for improving gene editing outcomes in hHSCs has important implications for the field of gene therapy, and can be applied to diseases where increased HDR/NHEJ ratio is critical for therapeutic success. Stem Cells 2019;37:284-294.

Elevated fetal haemoglobin levels are associated with decreased incidence of retinopathy in adults with sickle cell disease

Among the many vascular complications of sickle cell disease (SCD), retinopathy is the most prevalent and represents a leading cause of blindness. Hydroxycarbamide therapy ameliorates many symptoms of SCD, and high fetal haemoglobin (HbF) levels have been shown to protect against the development of retinopathy in children with HbSS. Its effect on adults with SCD, who are at a much higher risk of developing retinopathy, has not been studied. We aimed to investigate the effect of hydroxycarbamide use and HbF level on sickle cell retinopathy development in adults. We performed a retrospective cross-sectional study and collected demographics, comorbidities, and ocular and haematological data from 300 adult sickle cell subjects examined at the Henkind Eye Institute at Montefiore Medical Center during a 5-year period, from October 2012 to November 2017. The cohort was comprised mainly of Black and Hispanic subjects with all SCD genotypes, aged 18-71 years. Results show that in HbSS patients treated with hydroxycarbamide, those with retinopathy had significantly lower HbF levels compared to patients without retinopathy (P = 0·018). Our study identified the optimal HbF cut-off point for retinopathy protection to be 14·87%. Thus, a HbF level of 15% appears to be the threshold above which the odds for developing retinopathy in SS patients are reduced by 50%.

Emerging Genetic Therapy for Sickle Cell Disease

The genetic basis of sickle cell disease (SCD) was elucidated >60 years ago, yet current therapy does not rely on this knowledge. Recent advances raise prospects for improved, and perhaps curative, treatment. First, transcription factors, BCL11A and LRF/ZBTB7A, that mediate silencing of the β-like fetal (γ-) globin gene after birth have been identified and demonstrated to act at the γ-globin promoters, precisely at recognition sequences disrupted in rare individuals with hereditary persistence of fetal hemoglobin. Second, transformative advances in gene editing and progress in lentiviral gene therapy provide diverse opportunities for genetic strategies to cure SCD. Approaches include hematopoietic gene therapy by globin gene addition, gene editing to correct the SCD mutation, and genetic manipulations to enhance fetal hemoglobin production, a potent modifier of the clinical phenotype. Clinical trials may soon identify efficacious and safe genetic approaches to the ultimate goal of cure for SCD.

How I treat the older adult with sickle cell disease

With increasing survival, cumulative complications of sickle cell disease (SCD), which develop insidiously over time, are becoming more apparent and common in older patients, particularly those in their fifth decade and beyond. The older patient is also more likely to develop other age-related nonsickle conditions that interact and add to the disease morbidity. A common misconception is that any symptom in a SCD patient is attributable to their SCD and this may lead to delays in diagnosis and appropriate intervention. We recommend regular comprehensive reviews and monitoring for early signs of organ damage and a low threshold for the use of hydroxyurea and blood transfusions as preventative measures for end-organ disease. Treatable comorbidities and acute deterioration should be managed aggressively. Although the primary goal in management of the older adult with SCD is improving anemia and minimizing organ damage, the time has come for us to be more proactive in considering curative therapies previously offered to the younger patient. Curative or experimental interventions should be discussed early, before complications render the patients ineligible for these treatments.

Hematopoietic stem cell transplantation for sickle cell disease: Progress and challenges

Sickle cell disease (SCD) presents challenges to hematopoietic stem cell transplantation (HSCT), including donor availability and morbidity with age/disease severity. However, severe SCD causes irreversible organ damage that HSCT can mitigate. This benefit must be balanced against preparative regimen toxicity, graft-versus-host disease, and mortality risk. We review efforts to balance HSCT complications with the promise of cure, and knowledge gaps that warrant further investigation. We highlight the burden of SCD, HSCT risks and benefits, and SCD families' approach to this balance. We emphasize the necessity for information exchange to ensure a joint decision-making process between providers and patients.

Gene therapy for sickle cell disease: An update

Sickle cell disease (SCD) is one of the most common life-threatening monogenic diseases affecting millions of people worldwide. Allogenic hematopietic stem cell transplantation is the only known cure for the disease with high success rates, but the limited availability of matched sibling donors and the high risk of transplantation-related side effects force the scientific community to envision additional therapies. Ex vivo gene therapy through globin gene addition has been investigated extensively and is currently being tested in clinical trials that have begun reporting encouraging data. Recent improvements in our understanding of the molecular pathways controlling mammalian erythropoiesis and globin switching offer new and exciting therapeutic options. Rapid and substantial advances in genome engineering tools, particularly CRISPR/Cas9, have raised the possibility of genetic correction in induced pluripotent stem cells as well as patient-derived hematopoietic stem and progenitor cells. However, these techniques are still in their infancy, and safety/efficacy issues remain that must be addressed before translating these promising techniques into clinical practice.

Stem cell transplantation in sickle cell disease: therapeutic potential and challenges faced

INTRODUCTION

Sickle cell disease (SCD) is the most common inherited hemoglobinopathy worldwide, and is a life-limiting disease with limited therapeutic options to reduce disease severity. Despite being a monogenic disorder, the clinical phenotypes of SCD are variable, with few reliable predictors of disease severity easily identifying patients where the benefits of curative therapy outweigh the risks. Allogeneic hematopoietic stem cell transplantation (HSCT) is the only curative option, though significant advances in gene therapy raise the promise for additional curative methods. Areas covered: Allogeneic transplantation in SCD has evolved and improved over the last two decades, now offering a standard of care curative option using a human leukocyte antigen (HLA)-matched sibling donor. Many of the seminal transplantation studies are reviewed here, demonstrating how initial failures and successes have influenced and led to current HSCT strategies. Such strategies aim to overcome setbacks and limitations, and focus on conditioning regimens, immune suppression methods, the use alternative donor sources, and gene therapy approaches. Expert commentary: SCD is a curable disease. Each dedicated effort to refine transplantation methods, expand the donor pool, and bring gene therapy models to fruition will make enormous impacts reducing disease burden and improving outcomes and quality of life for patients with SCD.

Hemoglobin disorders: lentiviral gene therapy in the starting blocks to enter clinical practice

The β-hemoglobinopathies, transfusion-dependent β-thalassemia and sickle cell disease, are the most prevalent inherited disorders worldwide and affect millions of people. Many of these patients have a shortened life expectancy and suffer from severe morbidity despite supportive therapies, which impose an enormous financial burden to societies. The only available curative therapy is allogeneic hematopoietic stem cell transplantation, although most patients do not have an HLA-matched sibling donor, and those who do still risk life-threatening complications. Therefore, gene therapy by one-time ex vivo modification of hematopoietic stem cells followed by autologous engraftment is an attractive new therapeutic modality. The first proof-of-principle of conversion to transfusion independence by means of a lentiviral vector expressing a marked and anti-sickling β^T87Q-globin gene variant was reported a decade ago in a patient with transfusion-dependent β-thalassemia. In follow-up multicenter Phase II trials with an essentially identical vector (termed LentiGlobin BB305) and protocol, 12 of the 13 patients with a non-β⁰/β⁰ genotype, representing more than half of all transfusion-dependent β-thalassemia cases worldwide, stopped red blood cell transfusions with total hemoglobin levels in blood approaching normal values. Correction of biological markers of dyserythropoiesis was achieved in evaluated patients. In nine patients with β⁰/β⁰ transfusion-dependent β-thalassemia or equivalent severity (β^IVS1-110), median annualized transfusion volume decreased by 73% and red blood cell transfusions were stopped in three patients. Proof-of-principle of therapeutic efficacy in the first patient with sickle cell disease was also reported with LentiGlobin BB305. Encouraging results were presented in children with transfusion-dependent β-thalassemia in another trial with the GLOBE lentiviral vector and several other gene therapy trials are currently open for both transfusion-dependent β-thalassemia and sickle cell disease. Phase III trials are now under way and should help to determine benefit/risk/cost ratios to move gene therapy toward clinical practice.

Genetic therapies for sickle cell disease

After decades with few novel therapeutic options for sickle cell disease (SCD), autologous hematopoietic stem cell (HSC) based genetic therapies including lentiviral gene therapy (GT), and genome editing (GE) now appear imminent. Lentiviral GT has advanced considerably in the past decade with promising clinical trial results in multiple disorders. For β-hemoglobinopathies, GT strategies of gene addition and fetal hemoglobin induction through BCL11A regulation are both being evaluated in open clinical trials. GE techniques offer the possibility of a nonviral curative approach, either through sickle hemoglobin mutation repair or fetal hemoglobin elevation. Although GE currently remains at the preclinical stage, multiple clinical trials will likely open soon. In addition to reviewing current strategies for GT and GE, this review highlights important next steps toward optimization of these therapies. All autologous cell-based genetic therapies rely on safely obtaining an adequate yield of autologous HSCs for genetic modification and transplantation. HSC collection is uniquely challenging in SCD. Peripheral mobilization with plerixafor has recently emerged as a promising approach. The acute and long-term toxicities associated with myeloablative conditioning are risks that may not be acceptable to a significant number of SCD patients, highlighting the need for novel conditioning regimens. Finally, increasing availability of autologous genetic therapies will require comprehensive and collaborative discussions regarding cost and access for SCD patients, at individual centers and worldwide.

When there is no match, the game is not over: Alternative donor options for hematopoietic stem cell transplantation in sickle cell disease

Many patients with sickle cell disease experience severe morbidity and early mortality. The only curative option remains hematopoietic stem cell transplantation. Although HLA-matched sibling transplantation has been very successful for adults and children, the vast majority of patients with sickle cell disease do not have an HLA-matched sibling. Alternative donor options include haploidentical, unrelated umbilical cord blood, and matched unrelated donor transplantation. This report summarizes major alternative donor transplantation studies reported to date and ongoing and upcoming clinical trials. We conclude that when there is no HLA-match, all these approaches should be systematically considered before ruling out the option of hematopoietic stem cell transplantation.

Curative therapies: Allogeneic hematopoietic cell transplantation from matched related donors using myeloablative, reduced intensity, and nonmyeloablative conditioning in sickle cell disease

Sickle cell disease (SCD) chronically damages multiple organs over the lifetime of affected individuals. Allogeneic hematopoietic cell transplantation (allo-HCT) is the most studied curative intervention. Fully matched related marrow, peripheral blood derived, or cord blood HCT have the best transplant outcome for symptomatic patients with SCD. For patients with asymptomatic or milder disease who have this donor option available, risks and benefits of HCT should be discussed among the patient, family, treating hematologist, and transplant physician, and decision to proceed to HCT should be individualized. Myeloablative conditioning with busulfan, cyclophosphamide, and ATG has been a commonly employed regimen for children and young adults. Recently, low intensity conditioning with low dose total body irradiation and alemtuzumab is emerging as an efficacious and safe regimen for adults, young adults, and possibly children. Mixed donor chimerism (minimum ≥20% myeloid cells), from myeloablative or nonmyeloablative conditioning regimen, produces robust normal donor erythropoiesis and is sufficient to provide a clinical cure. The proportion of patients remaining on immunosuppression beyond 2 years post-HCT is likely <10% with either myeloablative or low intensity regimens. Late effects from myeloablative or reduced intensity conditioning, or from several more months of immunosuppression in low intensity conditioning may be less common than those observed in HCT for malignant indications. Nonmyeloablative approaches with low toxicities should be the focus of future research efforts. Prevention of GVHD is a shared goal in all approaches of allo-HCT in SCD.

Sickle Cell Disease: Advances in Treatment

BACKGROUND

Sickle cell disease causes significant morbidity and mortality and affects the economic and healthcare status of many countries. Yet historically, the disease has not had commensurate outlays of funds that have been aimed at research and development of drugs and treatment procedures for other diseases.

METHODS

This review examines several treatment modalities and new drugs developed since the late 1990s that have been used to improve outcomes for patients with sickle cell disease.

RESULTS

Targeted therapies based upon the pathophysiologic mechanisms of sickle cell disease that result in organ dysfunction and painful episodes include hydroxyurea, L-glutamine, crizanlizumab, and other drugs that are currently on the market or are on the verge of becoming available. These agents have the potential to improve survival and quality of life for individuals with sickle cell disease. Also discussed is stem cell transplantation that, to date, is the only curative approach for this disease, as well as the current status of gene therapy.

CONCLUSION

These examples demonstrate how the current knowledge of sickle cell disease pathophysiology and treatment approaches intersect. Although interest in sickle cell research has blossomed, many more clinical trials need to be initiated and subjected to more strenuous examination and analysis than have been used in the past.

Relationship between Mixed Donor-Recipient Chimerism and Disease Recurrence after Hematopoietic Cell Transplantation for Sickle Cell Disease

Mixed donor chimerism after hematopoietic cell transplantation for sickle cell disease (SCD) can result in resolution of disease symptoms, but symptoms recur when donor chimerism is critically low. The relationship between chimerism, hemoglobin S (HbS) level, and symptomatic disease was correlated retrospectively in 95 patients who had chimerism reports available at day 100 and at 1 and 2 years after transplantation. Recurrent disease was defined as recurrence of vaso-occlusive crises, acute chest syndrome, stroke, and/or HbS levels > 50%. Thirty-five patients maintained full donor chimerism (myeloid or whole blood) through 2 years. Donor chimerism was less than 10% (defined as graft failure) in 13 patients during this period. Mixed chimerism was reported in the remaining 47 patients (range, 10% to 94%). The lowest documented donor chimerism without symptomatic disease was 26%. Of 12 surviving patients with recurrent disease, 2 had recurrence of symptoms before documented graft failure (donor chimerism of 11% and 17%, respectively). Three patients underwent second transplantation for graft failure. None received donor leukocyte infusion to maintain mixed chimerism or prevent graft failure. We conclude stable donor chimerism greater than 25% is associated with resolution of SCD-related symptoms, and HbS levels in transplant recipients should be interpreted in context of the sickle trait status of the donors.