Long-term survival of ex-thalassemic patients with persistent mixed chimerism after bone marrow transplantation

Hematopoietic stem cell transplantation for B-thalassemia major with alemtuzumab

While matched related donor (MRD) allogeneic hematopoietic stem cell transplantation (HSCT) is a curative option for transfusion-dependent beta-thalassemia (TDT), the use of alternative sources has increased, resulting in the exploration of novel transplant-conditioning regimens to reduce the contribution of graft-versus-host disease (GVHD) and graft failure (GF) to transplant-related morbidity and mortality. Alemtuzumab is a CD52 monoclonal antibody that has been successfully incorporated into myeloablative conditioning regimens for other hematologic conditions, yet there have been limited studies regarding the use of alemtuzumab in HSCT for TDT. The purpose of this study was to evaluate engraftment, incidence of GVHD, and transplant related morbidity and mortality in patients with TDT who received alemtuzumab in addition to standard busulfan-based conditioning. The primary endpoint was severe GVHD-free, event-free survival (GEFS). Our cohort included 24 patients with a median age of 6.8 years (range 1.5-14.9). Eleven patients received a 10/10 MRD HSCT, eleven 10/10 unrelated donor (UD), and two mismatched UD. All patients achieved primary engraftment. For all patients, 5-year GEFS was 77.4% and 5-year overall survival (OS) was 91%. The 5-year cumulative incidence of GF (attributed to poor graft function) without loss of donor chimerism was 13.8% (95% CI: 4.5, 35.3). We report low rates of significant acute GVHD grade II-IV (12.5%) and chronic GVHD (4.4%). Younger age and MRD were associated with significantly improved GEFS, OS and EFS. Our results show that the use of alemtuzumab promotes stable engraftment, may reduce rates of severe GVHD, and results in acceptable GEFS, OS, and EFS.

Hemoglobinopathies (Sickle Cell Disease and Thalassemia)

Hematopoietic cell transplantation (HCT) using an HLA-matched sibling donor is a well-established curative therapy for pediatric patients with sickle cell disease (SCD) and transfusion-dependent thalassemias (TDT). In order to expand the donor pool, new approaches such as related haploidentical donor HCT have been used with encouraging results. These approaches aim for a higher overall survival, an effective reduction of acute and chronic GvHD and a reduced toxicity. Due to these alternative approaches and adult patients being increasingly transplanted, the number of HCT has dramatically increased in the last decade. Furthermore, different gene therapy and gene editing strategies are being developed in clinical trials, showing promising results.

Hematopoietic Stem Cell Transplantation in Thalassemia

Allogeneic hematopoietic stem cell transplantation (allo-HSCT) is the only consolidated, potentially curative treatment for patients with transfusion-dependent thalassemia major. In the past few decades, several new approaches have reduced the toxicity of conditioning regimens and decreased the incidence of graft-versus-host disease, improving patients' outcomes and quality of life. In addition, the progressive availability of alternative stem cell sources from unrelated or haploidentical donors or umbilical cord blood has made HSCT a feasible option for an increasing number of subjects lacking an human leukocyte antigen (HLA)-identical sibling. This review provides an overview of allogeneic hematopoietic stem cell transplantation in thalassemia, reassesses current clinical results, and discusses future perspectives.

Immunosuppression Boost With Mycophenolate Mofetil for Mixed Chimerism in Thalassemia Transplants

Declining mixed chimerism (MC) portending impending graft failure is an undesirable outcome. However, for hemoglobinopathies in a stable state of MC, residual host cells persist without rejection in 30% to 40% of patients after hematopoietic stem cell transplantation (HSCT). Early detection and level of MC have been attributed to be significant in predicting the outcome of MC. Common clinical approach on MC is removal of immunosuppression. We retrospectively evaluated MC in transfusion dependent thalassemia patients who underwent HSCT in our institution between September 2013 and January 2022 to determine the outcome of MC on the basis of our approach of immunosuppression boost in comparison to conventional approach of immunosuppression tapering. Among 90 patients, 22 (24.4 %) had MC at some time point after transplantation with a median follow-up of 496 (67-1492) days. Immunosuppression withdrawal was done in 12 (54.5%) patients, whereas immunosuppression boost was given in 8 (36.3%) patients. In the immunosuppression withdrawal group, 2 (16.6%) patients evolved to complete chimerism, 5(41.6%) patients had persistent MC (PMC), whereas 5 (41.6%) patients had secondary rejection. All these rejections were at median of 186 (89-251) days after transplantation. In the immunosuppression boost group, all patients (n = 8) had PMC with no secondary rejection until median follow-up of 255(97-812) days after transplantation. We acknowledge that we need more experience with our unconventional approach of immunosuppression boost to obtain statistical significance in comparison to the conventional approach of tapering of immunosuppression.

High-level correction of the sickle mutation is amplified in vivo during erythroid differentiation

Background

A point mutation in sickle cell disease (SCD) alters one amino acid in the β-globin subunit of hemoglobin, with resultant anemia and multiorgan damage that typically shortens lifespan by decades. Because SCD is caused by a single mutation, and hematopoietic stem cells (HSCs) can be harvested, manipulated, and returned to an individual, it is an attractive target for gene correction.

Results

An optimized Cas9 ribonucleoprotein (RNP) with an ssDNA oligonucleotide donor together generated correction of at least one β-globin allele in more than 30% of long-term engrafting human HSCs. After adopting a high-fidelity Cas9 variant, efficient correction with minimal off-target events also was observed. In vivo erythroid differentiation markedly enriches for corrected β-globin alleles, indicating that erythroblasts carrying one or more corrected alleles have a survival advantage.

Significance

These findings indicate that the sickle mutation can be corrected in autologous HSCs with an optimized protocol suitable for clinical translation.

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The gene editing protocol corrects the sickle mutation in ∼30% of engrafting cells

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Random assortment of engrafting stem cell clones without clonal dominance was shown

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Corrected erythroid cells are preferentially enriched compared with unedited cells

Long-Term Mixed Chimerism After Ex Vivo/In Vivo T Cell-Depleted Allogeneic Hematopoietic Cell Transplantation in Patients With Myeloid Neoplasms

In patients who have undergone allogeneic hematopoietic cell transplantation (HCT), myeloid mixed donor chimerism (MC) is a risk factor for disease relapse. In contrast, several studies found favorable outcome in patients with lymphoid MC. Thus far, most studies evaluating MC focused on a short-term follow-up period. Here, we report the first case series of long-term survivors with MC. We screened 1,346 patients having undergone HCT for myeloid neoplasms at our center from 1996 to 2016; 443 patients with data on total peripheral blood mononuclear cells (PBMC)/CD4⁺/CD34⁺ short tandem repeat (STR) donor chimerism (DC) and follow-up ≥24 months post-HCT were included. We identified 10 patients with long-term MC (PBMC DC <95% at ≥12 months post-HCT). Median follow-up was 11 years. All patients had received combined ex vivo/in vivo T cell-depleted (TCD) peripheral blood stem cells; none experienced ≥grade 2 acute graft-versus-host disease (GVHD). The mean total PBMC, CD4⁺, and CD34⁺ DC of all patients were 95.88%, 85.84%, and 90.15%, respectively. Reduced-intensity conditioning (RIC) was associated with a trend to lower mean total DC. Of note, two patients who experienced relapse had lower CD34⁺ DC but higher CD4⁺ DC as compared with patients in continuous remission. Bone marrow evaluation revealed increased CD4⁺/FOXP3⁺ cells in patients with MC, which might indicate expansion of regulatory T cells (T_regs). Our results support known predictive factors associated with MC such as RIC and TCD, promote the value of CD34⁺ MC as a potential predictor of relapse, highlight the potential association of CD4⁺ MC with reduced risk of GVHD, and indicate a possible role of T_regs in the maintenance of immune tolerance post-HCT.

Editing outside the body: Ex vivo gene-modification for β-hemoglobinopathy cellular therapy

Genome editing produces genetic modifications in somatic cells, offering novel curative possibilities for sickle cell disease and β-thalassemia. These opportunities leverage clinical knowledge of hematopoietic stem cell transplant and gene transfer. Advantages to this mode of ex vivo therapy include locus-specific alteration of patient hematopoietic stem cell genomes, lack of allogeneic immune response, and avoidance of insertional mutagenesis. Despite exciting progress, many aspects of this approach remain to be optimized for ideal clinical implementation, including the efficiency and specificity of gene modification, delivery to hematopoietic stem cells, and robust and nontoxic engraftment of gene-modified cells. This review highlights genome editing as compared to other genetic therapies, the differences between editing strategies, and the clinical prospects and challenges of implementing genome editing as a novel treatment. As the world's most common monogenic disorders, the β-hemoglobinopathies are at the forefront of bringing genome editing to the clinic and hold promise for molecular medicine to address human disease at its root.

Sequential paternal haploidentical donor liver and HSCT in EPP allow discontinuation of immunosuppression post-organ transplant

BACKGROUND

EPP is characterized by photosensitivity and by liver disease. When LT is performed in EPP, recurrence often occurs in the allograft due to ongoing protoporphyrin production in bone marrow. Therefore, curative treatment requires allogeneic HSCT after LT. Long-term immunosuppression could be spared by using the same donor for both transplants.

METHODS

A 2-year-old girl with EPP in liver failure underwent liver transplant from her father. Transfusion and apheresis therapy were used to lower protoporphyrin levels before and after liver transplant. Ten weeks after liver transplant, she underwent HSCT, using the same donor. Conditioning was with treosulfan, fludarabine, cyclophosphamide, and ATG. GVHD prophylaxis was with abatacept, methotrexate, MMF, and tacrolimus. We followed the patient's erythrocyte protoporphyrin and liver and skin health for 2 years after transplant.

RESULTS

After hematopoietic stem cell engraftment, a decline in protoporphyrin levels was observed, with clinical resolution of photosensitivity. Liver biopsies showed no evidence of EPP. Mild ACR occurred and responded to steroid pulse. Two years post-HSCT, the patient has been weaned off all immunosuppression and remains GVHD and liver rejection free.

CONCLUSIONS

Sequential liver and HSCT from the same haploidentical donor are feasible in EPP. This strategy can allow for discontinuation of immune suppression.

Donor-Host Lineage-Specific Chimerism Monitoring and Analysis in Pediatric Patients Following Allogeneic Stem Cell Transplantation: Influence of Pretransplantation Variables and Correlation with Post-Transplantation Outcomes

The impact of donor-host chimerism in post-hematopoietic stem cell transplantation (HSCT) outcomes is poorly understood. We were interested in studying whether pre-HSCT variables influenced lineage-specific donor-host chimerism and how lineage-specific chimerism impacts post-HSCT outcomes. Our main objective was to study pre-HSCT variables as predictors of lineage-specific donor-host chimerism patterns and to better characterize the relationship between post-HSCT lineage-specific chimerism and adverse outcomes, including graft failure and disease relapse. We conducted a retrospective data analysis of all patients who underwent allogeneic HSCT at the Pediatric Transplantation and Cellular Therapy service at Memorial Sloan Kettering Cancer Center between January 2010 and June 2015 and had at least 2 measurements of split-lineage chimerism. The trend of lineage-specific donor-host chimerism post-HSCT and the impact of age, disease, graft type, and pretransplantation conditioning regimen on chimerism at 3 months and 12 months post-HSCT were studied. The Wilcoxon signed-rank test, Mann-Whitney-Wilcoxon test, and Cox proportional hazard models were used for statistical analyses. A total of 137 patients were included (median age, 11.3 years). Most patients had a hematologic malignancy (n = 95), and fewer had a nonmalignant disorder (n = 27) or primary immune deficiency (n = 15). Myeloablative conditioning regimens (n = 126) followed by T cell-depleted (TCD) peripheral blood stem cell or bone marrow grafts (n = 101) were most commonly used. Mixed chimerism (MC) of total peripheral blood leukocytes (PBLs) did not predict loss of donor chimerism in all lineages and when stable was not associated with graft failure or rejection in this analyses. Split chimerism with complete donor chimerism (CC) of myeloid, B, and natural killer cells, but not T cells, occurred early post-HSCT, but full donor T cell chimerism was achieved at 12 months post-HSCT by most patients. MC within the T cell lineage was the major contributor to PBL MC, with lower median donor T cell chimerism at 3 months than at 12 months (91%) post-HSCT (51% versus 91%; P < .0001). Predictors of MC at 3 and 12 months were (1) age <3 years (P = .01 for PBLs and P = .003 for myeloid lineage); (2) nonmalignant disorder (P = .007 for PBLs); and (3) the use of reduced-intensity conditioning regimens. TCD grafts produced lower donor T cell chimerism at 3 months post-HSCT compared with unmodified grafts (P < .0001), where T cell lineage CC was achieved early post-HSCT. The donor T cell chimerism was similar at 12 months in the 2 types of grafts. Umbilical cord blood grafts had CC in all lineages at all time points post-HSCT. Loss of donor B cell chimerism was associated with increased risk of relapse in hematologic malignancies (hazard ratio, 1.33; P = .05). Age, underlying disease, conditioning regimen, and graft manipulation can impact post-HSCT donor-host chimerism and be predictors for early MC. MC in total PBLs and T cells was not related to graft failure or disease relapse. Whole-blood PBL chimerism analysis is not sufficient to assess the significance of post-HSCT donor-host status; rather, lineage-specific chimerism, particularly for myeloid, T, and B cells, should be analyzed to guide interventions and inform outcomes.

Gene replacement of α-globin with β-globin restores hemoglobin balance in β-thalassemia-derived hematopoietic stem and progenitor cells

β-Thalassemia pathology is due not only to loss of β-globin (HBB), but also to erythrotoxic accumulation and aggregation of the β-globin-binding partner, α-globin (HBA1/2). Here we describe a Cas9/AAV6-mediated genome editing strategy that can replace the entire HBA1 gene with a full-length HBB transgene in β-thalassemia-derived hematopoietic stem and progenitor cells (HSPCs), which is sufficient to normalize β-globin:α-globin messenger RNA and protein ratios and restore functional adult hemoglobin tetramers in patient-derived red blood cells. Edited HSPCs were capable of long-term and bilineage hematopoietic reconstitution in mice, establishing proof of concept for replacement of HBA1 with HBB as a novel therapeutic strategy for curing β-thalassemia.

Does Mixed Chimerism After Allogeneic Hematopoietic Cell Transplantation in Pediatric Patients With Fanconi Anemia Impact on Outcome?

Fanconi anemia (FA) cells are characterized by genomic instability, which places FA patients at risk for malignancies such as leukemia and oropharyngeal/urogenital cancers. The risk of development of leukemia is theoretically eliminated after hematopoietic cell transplantation (HCT). Mixed chimerism (MC) in FA patients might have a unique implication because the persistent existence of FA cells might give rise to a malignant clone. We have studied a large population of FA patients who underwent allogeneic HCT at our institution and report here the outcome according to chimerism status. Patients with FA who had evidence of progressive bone marrow failure and were blood products-transfusion dependent (packed red blood cells, platelets, or both) were included in the study. Those who had myelodysplasia (MDS) or an abnormal clone or evidence of leukemia were excluded. All but 3 patients had normal renal and cardiac function at the time of transplantation. In total, 160 patients with FA underwent allogeneic HCT at our center from January 1995 to December 2017; mean age at HCT was 8.4. Chimerism data at last follow-up visit were available on 97 patients who are the subjects of this analysis (no day +100 chimerism data on one of them). On day +100, 46 patients (47.9%) had full chimerism (FC) and 50 (52.1%) had MC, whereas at last follow-up 50 (51.5%) exhibited FC and the remaining 47 (48.5%) had MC. Cumulative incidence of all grades acute graft-versus-host disease (GVHD) was 13.4% and that of grade III to IV GVHD was 4.1%. Chronic GVHD was seen in eight (8.0%) patients. Incidence of severe acute GVHD (grade ≥ III) and that of chronic GVHD were not significantly associated with FC or MC measured at day +100 (P values = .347 and .254, respectively), nor at the last follow-up. Graft failure occurred in 2 patients; both from the MC at day +100 group. No graft failures occurred in the FC at day +100 group (P value = 1.00). At a median follow-up of 83.8 months (95% confidence interval, 51.0-116.6; range, 19.3-181.1 months) the cumulative probability of overall survival (OS) at 5 years was 95.7% ± 2.1%. Mean follow-up time in our cohort was 90.7 months. Five-year overall survival was not significantly associated with FC or MC evaluated at day +100 (95.7% ± 3.0% versus 95.6% ± 3.1%, P value = .908) nor at the last follow-up (96.0% ± 2.8% versus 95.4% ± 3.2%, P value = .925). No patient in either group developed MDS/leukemia during the follow-up period. We conclude that mixed chimerism in patients with FA appears to have no adverse effect on outcome in our follow-up period. A longer follow-up period is needed, however, to confirm the validity of this statement.

Peripheral host T cells survive hematopoietic stem cell transplantation and promote graft-versus-host disease

Graft-versus-host disease (GVHD) is a major cause of morbidity and mortality in hematopoietic stem cell transplantation (HSCT). Donor T cells are key mediators in pathogenesis, but a contribution from host T cells has not been explored, as conditioning regimens are believed to deplete host T cells. To evaluate a potential role for host T cells in GVHD, the origin of skin and blood T cells was assessed prospectively in patients after HSCT in the absence of GVHD. While blood contained primarily donor-derived T cells, most T cells in the skin were host derived. We next examined patient skin, colon, and blood during acute GVHD. Host T cells were present in all skin and colon acute GVHD specimens studied, yet were largely absent in blood. We observed acute skin GVHD in the presence of 100% host T cells. Analysis demonstrated that a subset of host T cells in peripheral tissues were proliferating (Ki67+) and producing the proinflammatory cytokines IFN-γ and IL-17 in situ. Comparatively, the majority of antigen-presenting cells (APCs) in tissue in acute GVHD were donor derived, and donor-derived APCs were observed directly adjacent to host T cells. A humanized mouse model demonstrated that host skin-resident T cells could be activated by donor monocytes to generate a GVHD-like dermatitis. Thus, host tissue-resident T cells may play a previously unappreciated pathogenic role in acute GVHD.

In utero Therapy for the Treatment of Sickle Cell Disease: Taking Advantage of the Fetal Immune System

Sickle Cell Disease (SCD) is an autosomal recessive disorder resulting from a β-globin gene missense mutation and is among the most prevalent severe monogenic disorders worldwide. Haematopoietic stem cell transplantation remains the only curative option for the disease, as most management options focus solely on symptom control. Progress in prenatal diagnosis and fetal therapeutic intervention raises the possibility of in utero treatment. SCD can be diagnosed prenatally in high-risk patients using chorionic villus sampling. Among the possible prenatal treatments, in utero stem cell transplantation (IUSCT) shows the most promise. IUSCT is a non-myeloablative, non-immunosuppressive alternative conferring various unique advantages and may also offer safer postnatal management. Fetal immunologic immaturity could allow engraftment of allogeneic cells before fetal immune system maturation, donor-specific tolerance and lifelong chimerism. In this review, we will discuss SCD, screening and current treatments. We will present the therapeutic rationale for IUSCT, examine the early experimental work and initial human experience, as well as consider primary barriers of clinically implementing IUSCT and the promising approaches to address them.

Prediction, management, and prognosis of mixed chimerism after hematopoietic stem cell transplantation in transfusion-dependent pediatric thalassemia patients

BACKGROUND

Early-onset mixed chimerism (MC) with a high proportion of residual host cells is considered a signal of graft rejection in patients undergoing allogenic hematopoietic stem cell transplantation for transfusion-dependent thalassemia. In order to prevent graft rejection and minimize the risk of treatment-related graft-versus-host disease (GVHD), we established a hierarchical management system based on chimerism analysis.

METHOD

This retrospective study provides a comprehensive review of the characteristics, interventions, and outcomes of the 38 patients who developed MC after transplantation among the 144 pediatric thalassemia patients between July 2007 and January 2019 at our center.

RESULTS

A sibling donor, a blood type-matched donor, conditioning regimens without fludarabine, and transplants containing <10 × 10⁸ total nucleated cells/kg were identified to be associated with the development of MC. Among the 38 patients developing MC, only four patients rejected the grafts. The response rate to donor lymphocyte infusion (DLI, only for patients receiving sibling donor transplantation) and cytokine immunomodulation without DLI was 70.6% and 42.9%, respectively. Patients that developed GVHD after DLI or cytokine therapy had a more significant increase in donor cell chimerism (16%, range 0%-35%) than those without (8.5%, range -21% to 40%, P = .049). However, even when treatment-related GVHD was included, patients with MC had a lower cumulative incidence of total acute GVHD than patients with complete donor chimerism (29.2% vs 48.0%, P = .030).

CONCLUSIONS

Interventions based on chimerism analysis were effective in preventing graft rejection and did not increase treatment-related GVHD in thalassemia patients with MC.

Hematopoietic Stem Cell Transplantation in Patients with Hemoglobinopathies

Hemoglobinopathies are the most common single-gene diseases and are estimated to affect millions of people worldwide. Thalassemia and sickle cell disease are the most prevalent diseases of this group. Today, despite the decreasing number of newborns diagnosed with a hemoglobinopathy, it remains an important health problem for many countries. Although regular red blood cell (RBC) transfusions, advanced iron chelation, and supportive therapy alternatives have improved life expectancy, allogeneic hematopoietic stem cell transplantation (HSCT) is the only curative option for patients with hemoglobinopathies to prevent irreversible organ damage. Modern transplantation approaches and careful posttransplantation follow-up of patients have improved survival outcomes, and HSCT has now been performed in several patients with hemoglobinopathies worldwide. Considering current experiences, hematopoietic stem cell transplantation is recommended in cases of β-thalassemia (β-thal) in the presence of a matched family or unrelated donor, without secondary organ damage due to transfusion. In patients with sickle cell anemia, transplantation indications include transfusion dependence and cases of secondary organ damage. Recently, gene therapy as a possible treatment option has yielded promising results, though it is not in routine clinical use at its current stage.

Unmanipulated Stem Cell Boost for Mixed Chimerism in Transfusion Dependent Thalassemia

Early mixed chimerism (MC) can lead to secondary graft rejection post allogeneic hematopoietic stem cell transplantation in transfusion dependent thalassemia (TDT) patients. Reduction of immunosuppression and donor lymphocyte infusions is the mainstay for treating MC. We report our experience of administering unmanipulated stem cell boost (SCB) in reversing progressive early MC. There were 70 transplants done for 69 TDT patients at our center between September 2005 and January 2020. Mixed chimerism was defined by > 5% recipient cells and the severity was assigned according to the proportion of recipient cells as level 1 =  < 10%, level 2 = 10-25%, level 3 =  > 25%. For patients developing MC level 2 and 3, we administered unmanipulated SCB and analyzed its safety and efficacy. Out of 70 transplants 7 (10%) had MC level 2 (3/7) and 3 (4/7). These patients received unmanipulated SCB at a median CD34 cell dose of 4.5 × 10⁶/kg (range-3.5 × 10⁶/kg-5.5 × 10⁶/kg). Overall Response (stable MC and/or transfusion independency) to unmanipulated SCB was seen in 5 patients (71.4%). Five patients (71.4%) developed acute graft versus host disease (GVHD) of which 1 patient expired due to severe GVHD. SCB infusion was well tolerated by majority of our patients. The 3 year overall survival and thalassemia free survival was 85.7% (6/7) and 57.1% (4/7) respectively. Timely monitoring of chimerism is important for detecting early MC. Development of acute GVHD is common after administration of unmanipulated SCB and requires vigilance and prompt management. Unmanipulated SCB is a feasible modality for treating progressive MC and salvaging the graft especially in resource-constrained settings.

Non-myeloablative matched sibling stem cell transplantation with the optional reinforced stem cell infusion for patients with hemoglobinopathies

BACKGROUND

The NIH protocol for non-myeloablative (NMA) conditioning allogeneic stem cell transplantation (alloSCT) with alemtuzumab and low-dose total body irradiation corrected the abnormal sickle cell disease (SCD) phenotype without the risk of graft-versus-host disease. However, alloSCT using NMA conditioning had been rarely applied to β-thalassemia major (β-TM) patients.

METHODS

To avoid prolonged immunosuppression, we developed a two-stage strategy. Mixed donor chimerism was initially achieved using the protocol developed by the NIH protocol. Thereafter, we facilitated donor chimerism using the optional reinforced stem cell (SC) infusion in cases requiring protracted immunosuppression or experiencing impending graft failure.

RESULTS

In this study, β-TM (n = 9) and SCD (n = 4) patients were equally effectively treated with eradicating the abnormal hemoglobin phenotype. Five patients, including four β-TM, achieved stable mixed chimerism without receiving optional reinforced SC infusion. All patients that received optional reinforced infusion achieved complete (n = 4) or mixed chimerism (n = 1). The overall survival rate and event-free survival at 4 years were 91.7% (95% CI; 53.9-98.8) in both groups, with a thalassemia-free survival rate in β-TM patients of 87.5% (95% CI; 38.7-98.1).

CONCLUSION

This study is the first to report successful NMA conditioning alloSCT to achieve stable mixed chimerism correcting the abnormal hemoglobin phenotype in adult β-TM patients.

Donor cell engineering with GSK3 inhibitor-loaded nanoparticles enhances engraftment after in utero transplantation

Host cell competition is a major barrier to engraftment after in utero hematopoietic cell transplantation (IUHCT). Here we describe a cell-engineering strategy using glycogen synthase kinase-3 (GSK3) inhibitor-loaded nanoparticles conjugated to the surface of donor hematopoietic cells to enhance their proliferation kinetics and ability to compete against their fetal host equivalents. With this approach, we achieved remarkable levels of stable, long-term hematopoietic engraftment for up to 24 weeks post-IUHCT. We also show that the salutary effects of the nanoparticle-released GSK3 inhibitor are specific to donor progenitor/stem cells and achieved by a pseudoautocrine mechanism. These results establish that IUHCT of hematopoietic cells decorated with GSK3 inhibitor-loaded nanoparticles can produce therapeutic levels of long-term engraftment and could therefore allow single-step prenatal treatment of congenital hematological disorders.

Simple Approach to Increase Donor Hematopoietic Stem Cell Dose and Improve Engraftment in the Murine Model of Allogeneic In Utero Hematopoietic Cell Transplantation

The rationale for in utero hematopoietic cell transplantation (IUHCT) rests on exploitation of normal events during hematopoietic and immunologic ontogeny to allow allogeneic hematopoietic engraftment without myeloablative conditioning.  Host hematopoietic competition is among the primary barriers to engraftment in IUHCT. In the murine model this can be partially overcome by delivery of larger donor cell doses, but volume is limiting. Enrichment of donor hematopoietic stem cells (HSCs) would seem to offer a more efficient approach, but such enriched populations have engrafted poorly in existing models of IUHCT. To increase HSC dose while maintaining the presence of accessory cells, we used a less stringent enrichment protocol of single-step lineage depleted cells alone (lin-) or in combination with whole donor bone marrow mononuclear cells. Our results confirm that increasing doses of HSCs in combination with bone marrow accessory cells can dramatically improve engraftment after IUHCT. This represents a practical and clinically applicable strategy to maximize the engraftment potential of the donor graft without risk of treatment-associated toxicity.

G-CSF-Mobilized Blood and Bone Marrow Grafts as the Source of Stem Cells for HLA-Identical Sibling Transplantation in Patients with Thalassemia Major

As an inherited anemia, thalassemia major (TM) is currently only curable with allogeneic hematopoietic stem cell transplantation (allo-HSCT). Here we report an allo-HSCT protocol for patients with TM who received a combination of granulocyte colony-stimulating factor-primed bone marrow and peripheral blood stem cells (G-BM & PBSCs) from a matched sibling donor (MSD). The conditioning regimen consisted of i.v. busulfan, cyclophosphamide, fludarabine, and antithymocyte globulin. Chimerism analysis was performed for all patients. Immunosuppressive treatment was terminated if rejection was suspected, and donor lymphocyte infusion was administered once no response was observed. A total of 184 patients with TM were enrolled in the study between July 2007 and July 2018. The cumulative incidence of grade II-IV acute graft-versus-host disease (GVHD) was 13.1%, and that of moderate or severe chronic GVHD was 5.7%. The cumulative incidence of graft rejection was .6%. In the total cohort, the 3-year overall survival, thalassemia-free survival, and GVHD-free, relapse-free survival were 97.8%, 97.3%, and 89.5%, respectively. Collectively, our results indicate that G-BM & PBSCs from an MSD is be a good stem cell source for patients with TM undergoing allo-HSCT.

Beta Thalassemia: Monitoring and New Treatment Approaches

Beta thalassemias are a significant global health problem. Globin chain imbalance leads to a complex physiologic cascade of hemolytic anemia, ineffective erythropoiesis, and iron overload. Management of the broad spectrum of phenotypes requires the careful use of red blood transfusions, supportive care, monitoring, and management of iron overload. In this article, the authors discuss recommendations for monitoring of individuals with thalassemia, as well as ongoing preclinical and clinical trials of therapies targeting different aspects of thalassemia pathophysiology.

Pipeline for the generation of gene knockout mice using dual sgRNA CRISPR/Cas9-mediated gene editing

Animal models possess undeniable utility for progress on biomedical research projects and developmental and disease studies. Transgenic mouse models recreating specific disease phenotypes associated with β-hemoglobinopathies have been developed previously. However, traditional methods for gene targeting in mouse using embryonic stem cells (ESCs) are laborious and time consuming. Recently, CRISPR has been developed to facilitate and improve genomic modifications in mouse or isogenic cell lines. Applying CRISPR to gene modification eliminates the time consuming steps of traditional approach including selection of targeted ESC clones and production of chimeric mouse. This study shows that microinjection of a plasmid DNA encoding Cas9 protein along with dual sgRNAs specific to Hbb-bs gene (hemoglobin, beta adult s chain) enables breaking target sequences at exons 2 and 3 positions. The injections led to a knockout allele with efficiency around 10% for deletion of exons 2 and 3 and 20% for indel mutation.

Hematopoietic chimerism and donor-specific skin allograft tolerance after non-genotoxic CD117 antibody-drug-conjugate conditioning in MHC-mismatched allotransplantation

Hematopoietic chimerism after allogeneic bone marrow transplantation may establish a state of donor antigen-specific tolerance. However, current allotransplantation protocols involve genotoxic conditioning which has harmful side-effects and predisposes to infection and cancer. Here we describe a non-genotoxic conditioning protocol for fully MHC-mismatched bone marrow allotransplantation in mice involving transient immunosuppression and selective depletion of recipient hematopoietic stem cells with a CD117-antibody-drug-conjugate (ADC). This protocol resulted in multilineage, high level (up to 50%), durable, donor-derived hematopoietic chimerism after transplantation of 20 million total bone marrow cells, compared with ≤ 2.1% hematopoietic chimerism from 50 million total bone marrow cells without conditioning. Moreover, long-term survival of bone marrow donor-type but not third party skin allografts is achieved in CD117-ADC-conditioned chimeric mice without chronic immunosuppression. The only observed adverse event is transient elevation of liver enzymes in the first week after conditioning. These results provide proof-of-principle for CD117-ADC as a non-genotoxic, highly-targeted conditioning agent in allotransplantation and tolerance protocols.

Successful donor engraftment and repair of the blood-brain barrier in cerebral adrenoleukodystrophy

Adrenoleukodystrophy (ALD) is caused by mutations within the X-linked ABCD1 gene, resulting in the inability to transport acylated very long chain fatty acids (VLCFAs) into the peroxisome for degradation. VLCFAs subsequently accumulate in tissues, including the central nervous system. Up to 40% of boys develop a severe progressive demyelinating form of ALD, cerebral ALD, resulting in regions of demyelination observed on brain magnetic resonance imaging that are associated with a "garland ring" of gadolinium contrast enhancement. Gadolinium enhancement indicates blood-brain barrier (BBB) disruption and an active inflammatory disease process. Only hematopoietic cell transplant (HCT) has been shown to halt neurologic progression, although the mechanism of disease arrest is unknown. We evaluated imaging- and transplant-related biomarkers in 66 males who underwent HCT. In 77% of patients, gadolinium contrast resolved by 60 days post-HCT. We determined that time to neutrophil recovery and extent of donor chimerism correlated significantly with time to contrast resolution post-HCT. Graft failure was associated with a significantly slower rate of contrast resolution (P < .0001). Time to neutrophil recovery remained significant in multivariate analysis with other biomarkers (P = .03). Our data suggest that robust donor myeloid recovery is necessary for timely repair of the BBB.

Hematopoietic Stem Cell Transplantation in Thalassemia

Although recent advances in gene therapy are expected to increase the chance of disease cure in thalassemia major, at present hematopoietic stem cell transplantation (HSCT) remains the only consolidated curative approach for this disorder. The widest experience has been obtained in the HLA-matched family donor (MFD) setting, with probabilities of overall and thalassemia-free survival exceeding 90% and 85%, respectively. As for most patients a suitable MFD is not available, alternative donors (HLA-matched unrelated donor, unrelated cord blood, HLA-haploidentical relative) have been increasingly explored, translating into the expansion of the number of patients treatable with HSCT.

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.

Fludarabine-based reduced toxicity yet myeloablative conditioning is effective and safe particularly in patients with high-risk thalassemia undergoing allogeneic transplantation

INTRODUCTION

Thalassemia major (TM) is an inherited disorder caused by ineffective erythropoiesis. At the present time, allogeneic stem cell transplantation (allo-SCT) is a curative option. Conventional busulfan and cyclophosphamide based myeloablative conditioning regimens are limited by increased toxicity, especially in high-risk patients. Replacement of cyclophosphamide with fludarabine has reduced toxicity and nonrelapse mortality (NRM), thus improving outcomes. We analyzed long-term data of our fludarabine-based myeloablative, reduced toxicity protocol, specifically in high-risk patients.

METHODS

We retrospectively analyzed a cohort of 47 consecutive patients with TM undergoing allo-SCT from matched donors, using the fludarabine-based regimen (reduced toxicity regimen). The median age of the cohort was 10 years. Thirty-eight patients (80%) were in the high-risk and nine patients (20%) were in the low-risk category. The primary aim of this analysis was thalassemia-free survival (TFS).

RESULTS

The rejection rate was 11% within high-risk patients with NRM of 2%. With a median follow-up period of 7 years (1-15 years), the 10-year TFS in the entire cohort was 87%, and the overall survival (OS) was 97%. The 10-year TFS and OS among the low-risk and high-risk groups were 90% versus 84%, respectively (P = 0.45) and 100% versus 96%, respectively (P = 0.5), and both subsets of patients did equally well.

CONCLUSION

In conclusion, replacement of high-dose cyclophosphamide with fludarabine is well tolerated with minimal regimen-related toxicity and acceptable rejection rates, especially in high-risk patients.

In utero nanoparticle delivery for site-specific genome editing

Genetic diseases can be diagnosed early during pregnancy, but many monogenic disorders continue to cause considerable neonatal and pediatric morbidity and mortality. Early intervention through intrauterine gene editing, however, could correct the genetic defect, potentially allowing for normal organ development, functional disease improvement, or cure. Here we demonstrate safe intravenous and intra-amniotic administration of polymeric nanoparticles to fetal mouse tissues at selected gestational ages with no effect on survival or postnatal growth. In utero introduction of nanoparticles containing peptide nucleic acids (PNAs) and donor DNAs corrects a disease-causing mutation in the β-globin gene in a mouse model of human β-thalassemia, yielding sustained postnatal elevation of blood hemoglobin levels into the normal range, reduced reticulocyte counts, reversal of splenomegaly, and improved survival, with no detected off-target mutations in partially homologous loci. This work may provide the basis for a safe and versatile method of fetal gene editing for human monogenic disorders.

A comparison of intrauterine hemopoietic cell transplantation and lentiviral gene transfer for the correction of severe β-thalassemia in a HbbTh3/+ murine model

Major hemoglobinopathies place tremendous strain on global resources. Intrauterine hemopoietic cell transplantation (IUHCT) and gene transfer (IUGT) can potentially reduce perinatal morbidities with greater efficacy than postnatal therapy alone. We performed both procedures in the thalassemic HbbTh3/+ mouse. Intraperitoneal delivery of co-isogenic cells at embryonic days13-14 produced dose-dependent chimerism. High-dose adult bone marrow (BM) cells maintained 0.2-3.1% chimerism over ~24 weeks and treated heterozygotes (HET) demonstrated higher chimerism than wild-type (WT) pups (1.6% vs. 0.7%). Fetalliver (FL) cells produced higher chimerism than BM when transplanted at thesame doses, maintaining 1.8-2.4% chimerism over ~32 weeks. We boosted transplanted mice postnatally with BM cells after busulfan conditioning. Engraftment was maintained at >1% only in chimeras. IUHCT-treated nonchimeras and non-IUHCT mice showed microchimerism or no chimerism. Improved engraftment was observed with a higher initial chimerism, in HET mice and with the addition of fludarabine. Chimeric HET mice expressed 2.2-15.1% engraftment with eventual decline at 24 weeks (vs. <1% in nonchimeras) and demonstrated improved hematological indices and smaller spleens compared with untreated HETmice. Intravenous delivery of GLOBE lentiviral-vector expressing human β-globin (HBB) resulted in a vector concentration of 0.001-0.6 copies/cell. Most hematological indices were higher in treated than untreated HET mice, including hemoglobin and mean corpuscular volume, but were still lower than in WT. Therefore, direct IUGT and IUHCT strategies can be used to achieve hematological improvement but require further dose optimization. IUHCT will be useful combined with postnatal transplantation to further enhance engraftment.

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.

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.

Transplantation in patients with iron overload: is there a place for magnetic resonance imaging? : Transplantation in iron overload

In iron overload diseases (thalassemia, sickle cell, and myelodysplastic syndrome), iron is deposited in all internal organs, leading to functional abnormalities. Hematopoietic stem cell transplantation (HSCT) is the only treatment offering a potential cure in these diseases. Our aim was to describe the experience in the field and the role of magnetic resonance imaging in the evaluation of iron overload before and after HSCT. Magnetic resonance imaging (MRI), using T2*, is the most commonly used tool to diagnose myocardial-liver iron overload and guide tailored treatment. Currently, HSCT offers complete cure in thalassemia major, after overcoming the immunologic barrier, and should be considered for all patients who have a suitable donor. The overall thalassemia-free survival of low-risk, HLA-matched sibling stem cell transplantation patients is 85-90%, with a 95% overall survival. The problems of rejection and engraftment are improving with the use of adequate immunosuppression. However, a detailed iron assessment of both heart and liver is necessary for pre- and post-transplant evaluation. In iron overload diseases, heart and liver iron evaluation is indispensable not only for the patients' survival, but also for evaluation before and after HSCT.

Long-term outcome of mixed chimerism after stem cell transplantation for thalassemia major conditioned with busulfan and cyclophosphamide

Mixed chimerism (MC) occurs frequently after allogeneic hematopoietic stem cell transplantation (HSCT) for thalassemia major (TM) and may be associated with rejection. We report the outcome of MC in 132 TM patients conditioned with Busulphan/Cyclophosphamide, who had successful engraftment and had ⩾1 year follow-up. Chimerism was first assessed at day +28, then every 3-9 months or more frequently if there was MC. If rejection was suspected, immunosuppression was stopped and donor-lymphocyte infusion (DLI) was given if there was no response. Among 132 patients, aged 7 years (range: 2-24), 46/132 (34.8%) had MC in the first year, 32/46 (69.6%) at day +28 and another 14 (30%) between day +28 and 1 year post HSCT. MC was quantified at level 1 (residual host chimerism (RHC) <10%) in 20 (43.5%), level II (RHC 10-25%) in 14 (30.4%) and level III (RHC >25%) in 12 (26.1%). On tapering immunosuppression, 15 (32.6%) developed acute GvHD and 8 (17.4%) had chronic GvHD with reversal to complete chimerism (CC). DLI was administered to 5/46 (10.9%), 1 evolved to CC but 4 rejected the graft. At median follow-up of 60 months (range: 16-172), 20/46 (43.5%) had CC, 18/46 (39.1%) had persistent MC with hemoglobin of 11.5 g/dL (range: 8.4-13.6), whereas 8 (17.4%) rejected the graft. Close monitoring and early intervention is needed with increasing recipient chimerism. Novel strategies are required for preventing graft rejection.

Detection of Residual Donor Erythroid Progenitor Cells after Hematopoietic Stem Cell Transplantation for Patients with Hemoglobinopathies

The presence of incomplete chimerism is noted in a large proportion of patients following bone marrow transplant for thalassemia major or sickle cell disease. This observation has tremendous implications, as subsequent therapeutic immunomodulation strategies can improve clinical outcome. Conventionally, polymerase chain reaction-based analysis of short tandem repeats is used to identify chimerism in donor-derived blood cells. However, this method is restricted to nucleated cells and cannot distinguish between dissociated single-cell lineages. We applied the analysis of short tandem repeats to flow cytometric-sorted hematopoietic progenitor cells and compared this with the analysis of short tandem repeats obtained from selected burst-forming unit - erythroid colonies, both collected from the bone marrow. With this method we are able to demonstrate the different proliferation and differentiation of donor cells in the erythroid compartment. This technique is eligible to complete current monitoring of chimerism in the stem cell transplant setting and thus may be applied in future clinical studies, stem cell research and design of gene therapy trials.

A Multicenter Retrospective Analysis Stressing the Importance of Long-Term Follow-Up after Hematopoietic Cell Transplantation for β-Thalassemia

Allogeneic hematopoietic cell transplantation (HCT) is curative in patients with β-thalassemia major. However, most reports on HCT outcomes lack long-term follow-up data with the exception of single-center reports. An international multicenter retrospective data collection and analysis was conducted in 176 β-thalassemia patients who were 1 year or beyond after first HCT to evaluate follow-up methods and outcomes at 7 centers. Median age at HCT was 5.5 years (range, .6 to 18.5), and median follow-up was 7 years (range, 1 to 20). HCT was predominantly from HLA-matched related donors (91%) with bone marrow as stem cell source (91%) and myeloablative conditioning regimens (88%). Late mortality or persistent chronic graft-versus-host disease (GVHD) was rare (<2%). Graft rejection was reported in 23% (24% of these occurred beyond 1 year) post-HCT. Of 119 patients with donor chimerism results available for ≥4 years post-HCT, 50% had >95%, 22% had 50% to 95%, 7% had 20% to 50% and 25 (21%) had <20% donor chimerism. Organ dysfunction was identified in 10% pre-HCT and in 20% post-HCT even without complete clinical details on all patients. Hypogonadism and elevated creatinine for age were most commonly reported and significantly higher in recipients ≥ 7 years at the time of HCT (P = .007) and in those with pre-existing morbidity before HCT (P = .02). Outcomes were unaffected by pre-HCT ferritin or GVHD. Mean z scores for height and weight were low at baseline and remained low post-HCT (79%), confirming that growth impairment from disease lacked recovery post-HCT during this follow-up period. HCT for β-thalassemia has a high rate of cure and low mortality, especially in the young and from HLA-matched related donors. Half of the number of recipients live with mixed chimerism that requires continued follow-up because of a risk of late graft rejection (14%). Organ function after HCT when <7 years of age was generally preserved. Hypogonadism, renal dysfunction, and growth impairment that failed to correct were late complications identified most frequently in older transplant recipients. Systematic follow-up of individual organs such as lung and heart were inadequate but important. These data support the development of simple measures of uniformly tracking long-term HCT outcomes and organ functions in children and adolescents who undergo HCT for thalassemia, allowing for systematic identification and implementation of standardized surveillance strategies and interventions.

ATG vs thiotepa with busulfan and cyclophosphamide in matched-related bone marrow transplantation for thalassemia

Matched-related bone marrow transplantation (BMT) may cure >80% of low-risk children with severe thalassemia (ST). Very long-term follow-up studies have shown how the standard busulfan-cyclophosphamide (BuCy) regimen may be associated with normalization of health-related quality of life, no second malignancies in the absence of chronic graft-versus-host disease, and fertility preservation in many patients. However, because BuCy may be associated with high rejection rates, some centers incorporate thiotepa (Tt) in busulfan- or treosulfan-based regimens, a combination that may increase the risk of permanent infertility. This study retrospectively compares matched-related BMT outcomes in 2 groups of low-risk ST patients conditioned with either Tt or anti-thymocyte globulin (ATG) in addition to BuCy. A total of 81 consecutive first BMTs were performed in 5 collaborating startup BMT centers in the Indian subcontinent between January 2009 and January 2016; 30 patients were transplanted after conditioning with Tt-BuCy between January 2009 and July 2013, whereas between August 2013 and January 2016, 51 patients received ATG-BuCy. All patients were <15 years and had no hepatomegaly (liver ≤2 cm from costal margin). Actuarial overall survival in the Tt-BuCy and ATG-BuCy groups was 87% and 94% and thalassemia-free survival was 80% and 85% at a median follow-up of 37 and 17 months, respectively, with no significant differences by log-rank statistics. Substituting Tt with ATG in the standard BuCy context seems safe and effective and may decrease transplant-related mortality. Higher fertility rates are expected for patients who received ATG-BuCy.

Current Results and Future Research Priorities in Late Effects after Hematopoietic Stem Cell Transplantation for Children with Sickle Cell Disease and Thalassemia: A Consensus Statement from the Second Pediatric Blood and Marrow Transplant Consortium International Conference on Late Effects after Pediatric Hematopoietic Stem Cell Transplantation

Sustained donor engraftment after allogeneic hematopoietic cell transplantation (HCT) converts to healthy donor hemoglobin synthesis and halts disease symptoms in patients with sickle cell disease and thalassemia major. A disease-free survival probability that exceeds 90% has been reported when HCT using an HLA-matched sibling donor is performed in young patients with low-risk disease or treatment-related risk factors. Alternate donor HCT and HCT in adults is performed infrequently because of a higher risk profile. Transplant-specific risks include conditioning regimen-related toxicity, graft-versus-host disease, graft rejection with marrow aplasia or disease recurrence, and infections associated with immunosuppression and delayed immune reconstitution. The magnitude of risk depends on patient age, clinical status of the underlying disease (eg, organ injury from vasculopathy and iron overload), donor source, and intensity of the conditioning regimen. These risks are commonly monitored and reported in the short term. Documenting very late outcomes is important, but these data are rarely reported because of challenges imposed by patient drop-out and insufficient resources. This report summarizes long-term follow-up results after HCT for hemoglobin disorders, identifies gaps in knowledge, and discusses opportunities for future investigations. This consensus summary will be followed by a second article detailing comprehensive long-term follow-up recommendations to aid in maintaining health in these individuals and identifying late complication risks that could facilitate interventions to improve outcomes.

Non-genotoxic conditioning for hematopoietic stem cell transplantation using a hematopoietic-cell-specific internalizing immunotoxin

Hematopoietic stem cell transplantation (HSCT) offers curative therapy for patients with hemoglobinopathies, congenital immunodeficiencies, and other conditions, possibly including AIDS. Autologous HSCT using genetically corrected cells would avoid the risk of graft-versus-host disease (GVHD), but the genotoxicity of conditioning remains a substantial barrier to the development of this approach. Here we report an internalizing immunotoxin targeting the hematopoietic-cell-restricted CD45 receptor that effectively conditions immunocompetent mice. A single dose of the immunotoxin, CD45-saporin (SAP), enabled efficient (>90%) engraftment of donor cells and full correction of a sickle-cell anemia model. In contrast to irradiation, CD45-SAP completely avoided neutropenia and anemia, spared bone marrow and thymic niches, enabling rapid recovery of T and B cells, preserved anti-fungal immunity, and had minimal overall toxicity. This non-genotoxic conditioning method may provide an attractive alternative to current conditioning regimens for HSCT in the treatment of non-malignant blood diseases.

Alternative donor allogeneic hematopoietic cell transplantation for hemoglobinopathies

Allogeneic hematopoietic stem cell transplantation (HSCT) offers a curative therapy for patients with hemoglobinopathies, mainly severe sickle cell disease (SCD) and thalassemia (TM). However, the applicability of HSCT has been limited mainly by donor availability, with a less than 25%-30% of eligible patients having human leukocyte antigen (HLA)-matched sibling donors. Previous outcomes using alternate donor options have been markedly inferior due to increased regimen-related toxicity, transplant-related mortality, graft failure, and graft-versus-host disease (GVHD). Advances in transplant technology, including high-resolution HLA typing, improved GVHD prophylactic approaches with tolerance induction, and better supportive care over the last decade, are addressing these historical challenges, resulting in increasing donor options. Herein, we review alternate donor HSCT approaches for severe SCD and TM using unrelated donors, umbilical cord blood units, or related haploidentical donors. Though this is an emerging field, early results are promising and in selected patients, this may be the preferred option to mitigate against the age-related morbidity and early mortality associated with these disorders.

Chimerism analysis in clinical practice and its relevance for the detection of graft rejection and malignant relapse in pediatric hematopoietic stem cell transplant patients

Chimerism and clinical outcome data from 244 hematopoietic stem cell transplants in 218 children were retrospectively analyzed to assess their relevance for the detection of graft rejection and malignant relapse. Patients transplanted for a non-malignant disease had significantly higher proportions of residual recipient T cells in peripheral blood at one, three, and six months compared with patients transplanted for malignant disease. Recipient T-cell levels were below 50% at one month after transplantation in most patients (129 of 152 transplants). Graft rejection occurred more frequently in the group of patients with high levels of recipient cells at one month (10 graft rejections in the 23 patients with recipient T cells >50% at one month as compared to seven graft rejections occurred in 129 patients with recipient T cells <50% (p < 0.001). Multilineage chimerism data in 87 children with leukemia at one, three, and six months after transplantation were not correlated with subsequent relapse of malignant disease. In conclusion, early analysis of lineage-specific chimerism in peripheral blood can be used to identify patients who are at high risk of graft rejection. However, the efficacy of early chimerism analysis for predicting leukemia relapse was limited.

Naturally acquired microchimerism: implications for transplantation outcome and novel methodologies for detection

Microchimerism represents a condition where one individual harbors genetically distinct cell populations, and the chimeric population constitutes <1% of the total number of cells. The most common natural source of microchimerism is pregnancy. The reciprocal cell exchange between a mother and her child often leads to the stable engraftment of hematopoietic and non-hematopoietic stem cells in both parties. Interaction between cells from the mother and those from the child may result in maternal immune cells becoming sensitized to inherited paternal alloantigens of the child, which are not expressed by the mother herself. Vice versa, immune cells of the child may become sensitized toward the non-inherited maternal alloantigens of the mother. The extent of microchimerism, its anatomical location, and the sensitivity of the techniques used for detecting its presence collectively determine whether microchimerism can be detected in an individual. In this review, we focus on the clinical consequences of microchimerism in solid organ and hematopoietic stem cell transplantation, and propose concepts derived from data of epidemiologic studies. Next, we elaborate on the latest molecular methodology, including digital PCR, for determining in a reliable and sensitive way the extent of microchimerism. For the first time, tools have become available to isolate viable chimeric cells from a host background, so that the challenges of establishing the biologic mechanisms and function of these cells may finally be tackled.