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INTRODUCTION
Allogeneic hematopoietic stem cell transplantation (HSCT) is a potentially curative treatment for children and adolescents with various malignant and non-malignant diseases. Recent progress in HSCT contributed to the improvement of outcomes for patients with diseases curable by HSCT. While human leukocyte antigen (HLA)-identical sibling donor is the preferred choice, HLA-matched unrelated volunteer donor is also a realistic option for successful HSCT. However, it is not always possible to find a HLA-matched donor for patients requiring HSCT, leading to a considerable number of deaths of patients without undergoing transplantation. The need for alternative donors has driven the development of new transplantation approaches such as transplants from HLA-haploidentical family members or umbilical cord blood.
Recent advances in the effective ex vivo depletion of T cells or unmanipulated in vivo regulation of T cells, better supportive care, and optimal conditioning regimens have significantly improved the outcomes of haploidentical HSCT [1234567]. The ex vivo techniques to remove T cells have evolved from the selection of CD34+ hematopoietic stem cell progenitors to the depletion of CD3+ cells, and more recently, to the depletion of αβ+ T cells [89]. Currently, allogeneic HSCT using an HLA-haploidentical family donor (HFD) is considered an accepted treatment option for patients who cannot find an optimal related or unrelated donor.
Here, we review the major advances in haploidentical HSCT, focusing on the ex vivo depletion of T cells. We will also introduce our experiences with transplantation using this novel approach.
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THE HISTORY OF HSCT FROM A HAPLOIDENTICAL FAMILY DONOR
HSCT from HFD has several advantages (Table 1): 1) virtually all patients who need HSCT can find a donor; 2) transplantation could be performed without delay, which is critical to patients with high-risk malignant disease or very severe aplastic anemia requiring urgent treatment; 3) further access to the donor for cellular therapy to treat relapse or infection or for additional transplantations is easy. In addition, HFD could rescue the patients who experienced early graft failure (GF) which is a life-threatening complication requiring prompt intervention after allogeneic HSCT [10111213].
Even though haploidentical HSCT seemed to be an attractive procedure with the added benefit of readily available donors, the early attempts at haploidentical HSCT from genetically haploidentical family members were disappointing due to the development of refractory graft-versus-host disease (GVHD) and excessively high transplant-related mortality (TRM) [14]. A high rate of graft rejection (GR) and refractory GVHD were major drawbacks to the use of haploidentical HSCT for patients who required transplantation but lacked a suitable donor. In addition, delayed immune recovery and a high prevalence of infections were significant obstacles. Several initial trials revealed that haploidentical HSCTs had a considerably high incidence of GF and GVHD, resulting in high rates of morbidity and mortality [15161718].
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RECENT ADVANCES IN HAPLOIDENTICAL HSCT
T cell depletion of donor grafts to prevent fatal GVHD is crucial for successful haploidentical HSCT. The methods for T cell depletion (TCD) could be in vivo (T cell-replete transplant) or ex vivo (T cell-depleted transplant). Various approaches have been developed, including the ex vivo selection of CD34+ cells with or without a megadose of purified stem cells, ex vivo depletion of T cells, in vivo T cell depletion using T-cell antibodies such as anti-thymocyte globulin (ATG), or post-transplant cyclophosphamide (Table 2). The ex vivo techniques to remove T cells have evolved from the selection of CD34+ hematopoietic stem cell progenitors to the depletion of CD3+ cells, and more recently, to the depletion of αβ+ T cells (Fig. 1). Compared to the positive selection of CD34+ cells, the direct depletion of CD3+ cells has the advantage of increasing the number of natural killer (NK) cells, monocytes, and other immunomodulating cells [19]. The depletion of CD3+ cells is superior to selecting for CD34+ cells in terms of rapid engraftment and immune reconstitution [20212223]. Moreover, the preliminary report on the new method by the depletion of αβ+ T cells showed further improvements in the outcome of T cell-depleted haploidentical transplants. Depletion of αβ+ T cells produced grafts containing many γδ+ lymphocytes as well as other effector cells. While αβ+ T cells are known to be associated with the initiation of GVHD, γδ+ T cells can enhance immune reconstitution and are not implicated in GVHD [2425].
 | Fig. 1Advances in ex vivo T cell-depleted haploidentical hematopoietic stem cell transplantation. The ex vivo techniques to remove T cells have evolved from the selection of CD34+ hematopoietic stem cell progenitors to the depletion of CD3+ cells, and more recently, the depletion of αβ+ T cells. Early attempts with haploidentical HSCT using CD34-selected stem cells, even with a megadose, were complicated by a high rate of infections likely related to delayed immune recovery. To overcome the limitation of CD34+ selection, the concept of direct depletion of T cells using anti-CD3 monoclonal antibody was introduced with the advantage of increasing the number of natural killer (NK) cells, and other immunomodulating cells. Depleting CD3+ cells was superior to selecting CD34+ cells in terms of engraftment speed and immune reconstitution. Although haploidentical HSCT using CD3-depleted grafts successfully reduce lethal infection rates, delayed immune recovery and the high rate of relapse were still problematic. The most recently developed approach using the negative depletion of αβ+ T cells improved the outcomes of T cell-depleted haploidentical transplant. Although recent advances in haploidentical HSCT, delayed immune reconstitution with subsequent infections and relapse for malignant disease are current major causes of treatment failure. New depletion technique to deplete naïve T cells or adoptive transfer of immune effector cells and cellular therapy based on γδ T cells or other immune cells could further improve the outcomes of haploidetical HSCT.
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CD3-depleted haploidentical HSCT for pediatric patients
The concept of direct depletion of T cells using an anti-CD3 monoclonal antibody with microbeads was introduced in early 2000 [2627]. Previous studies that used megadoses of CD34+ stem cells have found promising results with rapid engraftment as a possible alternative for children lacking suitable matched donors [282930]. However, haploidentical HSCTs using CD34-selected stem cells were complicated by a high rate of opportunistic infections likely related to delayed immune recovery. To overcome the limitation of CD34+ selection, a method for the negative depletion of T cells was developed. This provided T-cell-depleted grafts containing not only CD34+ stem cells, but also large numbers of NK cells and other effector cells, which were expected to reduce the risk of engraftment failure and facilitate immune reconstitution. There have been several reports on HHCT using CD3-depleted grafts in pediatric patients [3132333435].
An early experiment using CD3 antibody conjugated to magnetic microbeads showed that T cells were effectively depleted with a mean log depletion of 3.4 with 82% mean recovery of CD34+ stem cells [26]. This result suggested that a direct negative T-cell depletion method could effectively remove the CD3+ cells responsible for GVHD without negatively affecting the functions of the hematopoietic stem cells. The first published study for the clinical application of CD3-depleted grafts enrolled 22 pediatric patients with refractory hematological malignancies [36]. Reduced-intensity conditioning (RIC) regimen consisting of fludarabine, thiotepa, melphalan and OKT3 without total body irradiation (TBI) was employed to reduce TRM. Since T-cell depletion was one of the well-established risk factors for the development of post-transplant lymphoproliferative disorder (PTLD), in vivo B-cell depletion combined with T-cell depletion was performed using an anti-CD20 antibody. The study showed excellent engraftment (91%) with a low incidence of acute GVHD (9% grade III and no grade IV acute GVHD). The incidence of viremia was low, and no fatal infections were reported. A comparative analysis of the immune recovery profile between reduced-intensity and myeloablative conditioning regimen revealed that the RIC group had a faster recovery of T-cell populations and NK cells, and a much more rapid increase in T-cell receptor excision circles (TRECs). TREGs are small extrachromosomal fragments of DNA produced in T-cells during the rearrangement of T-cell receptor genes in the thymus and indicate the recovery of thymus dependent T-cell regeneration.
With the introduction of clinically available anti-CD19 antibody, a simultaneous in vitro T- and B-cell depletion method was performed in subsequent studies [27]. An early pilot study showed that CD3/CD19-depleted grafts might have benefits regarding engraftment and immune reconstitution compared with CD34-positive selection [19].
In a study of 46 pediatric patients with acute leukemia and MDS, primary engraftment was achieved in 88% of the patients, and engraftment after salvage transplantation was obtained in 100% of the patients [1]. Grade II acute GVDH and grade III-IV acute GVHD and chronic GVHD developed in 20% and 7%, and 21% of the patients, respectively. TRM was 8% at one year and 20% at 5 years. The 3-year event-free survival (EFS) was favorable (46%) for patients who were in complete remission (CR) when receiving the first haploidentical HSCT, whereas patients with leukemia and were not in CR at the time of transplantation or have received a subsequent HSCT had significantly higher risks of relapse (75% and 88%, respectively). This study showed that haploidentical HSCT using CD3/CD19-depleted allograft is a feasible treatment with low GVHD and low TRM, although the outcomes for patients with active diseases still need to be improved.
Haploidentical HSCT using αβ-depleted grafts in children and adolescents
Although donor T cells have anti-infectious and anti-tumor properties, they are responsible for GVHD in allogeneic HSCT. Gammadelta (γδ) T cells are a subset of T cells that account for 1–10% of the circulating peripheral blood T lymphocytes that express the γδ T cell receptors (TCRs) [3738]. The recently introduced method of negative depletion of αβ+ T cell is an effective strategy to dissect graft-versus-tumor effect and anti-infectious activities from GVHD. The γδ+ T cells are a small subset of T cells which can elicit both innate and adaptive immune responses to tumors and infections, while αβ+ T cells, a major subset of T cells, are the main inducers of GVHD [242539]. This manipulation removes αβ+ T cells and preserves NK cells and γδ+ T cells, which are expected to have activities against tumor and infections, thus improving the outcome of haploidentical HSCT. Although several studies have suggested beneficial roles of γδ T cells in the context of hematopoietic cell transplantation, reports of clinical experiences are still limited [2538404142434445].
A German group reported promising results of TCRαβ/CD19-depleted haploidentical HSCT [46]. In 41 patients with acute leukemia, MDS, solid tumors and nonmalignant disease, primary engraftment occurred in 88% of the patients. Acute GVHD grade II, III-IV, and extensive chronic GVHD were observed in 10% and 15%, and 9%, respectively. Compared with CD34+ selected haploidentical HSCT, recovery of CD3+, CD3+4+, and CD56+ cells were significantly faster with this method. Patients with leukemia and MDS who received a first haploidentical HSCT in CR1 showed a 1-year EFS of 100%, whereas no patient with active diseases survived. Owing to a short follow-up period, the clinical impact of this accelerated immune recovery remains to be clarified.
An Italian research group also reported rapid TCRγδ+ T cell reconstitution in 27 children with malignant and nonmalignant diseases after TCRαβ/CD19-depleted haploidentical HSCT [3]. Circulating γδ+ T cells are comprised of a major subset expressing the Vδ2 chain and a minor subset expressing the Vδ1 chain. They demonstrated prompt reconstitution of Vδ1 and Vδ2 T cells post-transplantation, and showed expansion of Vδ2 cells in vitro after exposure to zoledronic acid an activating antigen for TCRγδ+ T cell. These results suggest that αβ+ T-cell depleted haploidentical HSCT can be used as a platform for immunotherapy using zoledronic acid.
In a study of 22 children with nonmalignant disorders such as severe combined immunodeficiency (SCID), severe aplastic anemia (SAA), Fanconi anemia, other bone marrow failure syndrome, and immunodeficiencies, TCRαβ/CD19-depleted haploidentical HSCT showed promising outcome with favorable engraftment rates (80%), low incidence of GVHD (no visceral or chronic GVHD), and low TRM (9.3%) [44].
Recent studies demonstrated that αβ-depleted haploidentical HSCT is an attractive treatment option that can allow stable engraftment and has low toxicity profiles for children who lack suitable donors. Future studies should investigate whether rapid reconstitution of γδ+ T cells can translate into improved patient outcome by reducing both TRM and relapse.
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HAPLOIDENTICAL HSCT WITH EX VIVO T CELL-DEPLETED GRAFTS AT ASAN MEDICAL CENTER CHILDREN'S HOSPITAL (AMCCH)
Since 2008, haploidentical HSCT using ex vivo depletion of T cells has been practiced at our center. The depletion of CD3+ cells was introduced initially, and the depletion of αβ+ T cells was subsequently applied for allogeneic transplantation from HFD with several modifications of the treatment protocol (Fig. 2). The summary of our experience with ex vivo T cell-depleted haploidentical HSCT is provided below.
 | Fig. 2Major progress in ex vivo T cell-depleted haploidentical HSCT at AMCCH. In 2008, allogenetic HSCT from haploidentical family donor was initiated at our center using CD3-depleted grafts after reduced-intensity conditioning (RIC) with cacineurin inhibitors (CI) and mycophenolate mofetil (MMF) for the prevention of graft versus host disease (GVHD). Our early experience with CD3-depleted haploidentical HSCT showed a high incidence of graft failure (GF); therefore, low-dose total body irradiation (LD-TBI) was added to the conditioning regimen in an attempt to decrease GF in early 2011. In addition, the infused cell dose was targeted after add-back of T cells from negative selection product. The targeted dose of CD3+ T cells was gradually reduced from 1-5×106/kg to 6-8×105/kg to decrease the risk of severe GVHD and ensure stable engraftment. At the end of 2012, the ex vivo αβ+ T cell depletion technique with targeted dose of αβ cells at 1-5×105/kg by add-back was introduced. The depletion efficacy improved with the use of anti-TCRαβ monoclonal antibody instead of anti-CD3 monoclonal antibody for depletion, leading to ≤5×104/kg of recipient body weight of the residual αβ+ T cells. At the end of 2015, immunosuppressive drugs to prevent GVHD were eliminated for αβ-depleted haploidentical HSCT.
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CD3-depleted haploidentical HSCT
Between July 2008 and January 2013, 28 children underwent haploidentical HSCT using in vitro CD3-depleted peripheral blood stem cells after RIC [2]. Of the 28 patients, 9 had hematologic malignancy (HM) and 18 had non-malignant diseases (NM), including 16 patients with acquired SAA and one with refractory neuroblastoma. Twenty-six patients achieved neutrophil engraftment at a median of 11 days (range, 9–15 d). Two patients failed to achieve primary engraftment and five experienced GR. All seven patients received a second haploidentical HSCT and achieved stable engraftment. The cumulative incidences (CIs) of ≥grade II and ≥grade III acute GVHD were 33.3% and 14.3%, respectively, and the 1-year CI of extensive chronic GVHD was 11.1%. TRMs at 100 days, 1 year, and 2 years were 0.0%, 10.7%, and 14.3%, respectively. At a median follow-up of 32.8 months (range, 17.0–72.5 mo), the 2-year OS was 82.1% (94% for NM and 60% for malignant diseases, P=0.019).
Our trials with CD3-depleted haploidentical HSCT showed a rather higher incidence of GF in the early period of the study; therefore low-dose TBI (LD-TBI) was added to the conditioning regimen in an attempt to decrease GF. In addition, we modified the targeted dose of T cells by add-back of T cells from negative selection product in various ranges to improve the outcomes. Initially, targeting the infused CD3+ cell dose at 1-6×106/kg with the use of post-transplant immunosuppressants seemed to be associated with a higher incidence of severe acute GVHD and extensive chronic GVHD. A reduction of T cell dose to around 6-8×105 CD3+ cells/kg decreased the incidence of severe GVHD without increasing the incidence of GF.
Haploidentical HSCT using αβ+ T cell-depleted graft (Unpublished data)
Based on our previous results with CD3-depleted grafts, our recent study used αβ+ T cell-depleted grafts with a targeted dose of αβ+ cells at 1-5×105/kg by add-back of αβ+ T cells from the negative selection product after a uniform RIC with fludarabine, cyclophosphamide, r-ATG, and LD-TBI. Forty-two children and adolescents (31 with HM, 8 with NM, and 3 with solid tumors) underwent transplantations using αβ+ T cell-depleted grafts with a target of 1-5×105 αβ+ cells/kg and post-transplant immunosuppressants of tacrolimus and mycophenolate mofetil (MMF). All 42 patients achieved neutrophil engraftment at a median of 10 days (range, 9–17 d). The CIs of ≥grade II and ≥grade III acute GVHD were 31% and 12%, respectively, and the 1-year CI of chronic GVHD was 15%. One patient died of cytomegalovirus pneumonia, resulting in a TRM of 2.6%. Sixteen patients relapsed, and 11 died of disease. At a median follow-up of 19 months (range, 5–43 mo), the estimated two-year EFS for NM and HM were 88% and 50%, respectively. Our study demonstrated that haploidentical HSCT after ex vivo depletion of αβ+ T cells with the targeted dose noticeably reduced the GF and TRM in pediatric patients and could be applied to patients who lack a suitable related or unrelated donor.
αβ+ T cell-depleted haploidentical HSCT without post-transplant immunosuppressants
Pharmacologic prevention using immune-suppressive drugs such as calcineurin inhibitors, methotrexate and MMF, commonly in combination, is routine practice after the infusion of stem cells. Although advances in immunosuppressants have effectively prevented the development of acute GVHD, there are many serious toxic side effects and drug interactions requiring serial blood level monitoring [47484950]. Our targeted and ranged T cell dose-strategy improved the outcomes of ex vivo T cell-depleted haploidentical HSCTs. In addition, the depletion efficacy using anti-TCRαβ monoclonal antibody resulted in an approximately 4-log reduction of αβ+ T cells in most of the depletion procedures. Given that the reduction of target cells is more effective with αβ-depletion methods and considering the adverse effects of post-transplant immunosuppressants, pharmacological prophylaxis to prevent GVHD could be safely eliminated. Recently, seven patients received αβ-depleted haploidentical HSCT without post-transplant immunosuppressants. The median infused doses of CD34+ cells and αβ+ T cells were 6.1×106/kg (range, 3.0–12.8) and 4.9×104/kg (range, 1.0–5.0), respectively. All seven patients achieved a sustained neutrophil engraftment at a median of 10 days (range, 10–12 d). Two patients developed grade II acute GVHD and none developed severe acute GVHD greater than grade III. Early post-transplant outcomes were promising. However, further observations are necessary to assess any negative effects of the lower dose of T cells on immune recovery, relapse rate, and overall survival.
Haploidentical HSCT for hematologic malignancy
Forty-six patients with HM received ex vivo T cell-depleted haploidentical HSCT (9 with CD3-depleted graft and 37 with αβ-depleted graft) between July 2008 and January 2016. Of the 46 patients, 11 had ALL, 21 had AML, 2 had MPAL, 7 had MDS, 2 had JMML, and 3 had NHL. At a median follow up of 24.6 months (range, 1.5–93 mo), TRM, relapse rate, EFS, and OS at 2 years were 6%, 39%, 55% and 65%, respectively. The phase of disease was a significant risk factor for EFS [68% for any CR (N=35) vs. 0% for active disease (N=10), P=0.000]. Subsequent transplantations for patients who relapsed after previous allogenetic HSCT showed poorer outcomes compared to the single transplantation [EFS, 31% for subsequent transplants (N=12) vs. 63% for single transplantations (N=34), P=0.008]. Haploidentical HSCT is a feasible treatment option for pediatric patients with HM who have no suitable donors. However, further innovative strategies for the patients with active diseases at the time of transplantation or experience relapse after the initial transplantation should be researches to improve patient outcomes.
Haploidentical HSCT for acquired severe aplastic anemia
Several notable reports in recent years have supported haploidentical transplant as a viable option for the treatment of acquired SAA [5152535455565758]. In our center, 25 pediatric patients with acquired SAA received haploidentical HSCT (16 with CD3-depleted graft and 9 with αβ-depleted graft) between July 2009 and January 2016. Of the 25 patients, one patient experienced primary GF and four experienced GR. All five of these patients received CD3-depleted graft and achieved sustained engraftment after salvage transplantation. Eight of the 25 patients developed acute GVHD ≥grade II (six grade II and two grade III), leading to a CI of 32%. Twenty-three of the patients survived and were transfusion-independent. At a median follow up of 40 months (range, 1–80 mo), estimated OS at 3 years was 91%. HSCT from HFD with ex vivo T cell depletion could be offered for children and adolescents with refractory SAA who lack suitable donors.
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FUTURE PERSPECTIVES OF HAPLOIDENTICAL HSCT
The recent emerging evidences for haploidentical HSCT has provided additional therapeutic options for pediatric patients with malignant and non-malignant diseases curable with HSCT but do not have a suitable related or unrelated donor. In spite of the promising results for haploidentical HSCT in pediatric patients, there are still several obstacles to overcome. Although our targeted and ranged T cell dose-strategy improved the outcomes of ex vivo T cell-depleted haploidentical HSCTs, our current protocol (Fig. 3) is only a step in the development of a suitable haploidentical transplant protocol for patients who lack a donor. Unresolved issues include optimizing conditioning regimens, donor T cell regulation method, stem cell source, donor selection, management for graft failure, novel strategies to enhance immune recovery, and the prevention of relapse. Delayed immune reconstitution and subsequent infections are not uncommon and are a major cause of death after haploidentical transplantation. New depletion techniques to deplete naïve T cells or the adoptive transfer of immune effector cells such as pathogen-specific T cells could enhance the recovery of immune function after haploidentical HSCT [59606162636465]. In addition, relapse is another major treatment failure in haploidentical HSCT for malignant diseases. Patients with active diseases or who have relapsed after previous transplantation showed poor outcomes, necessitating further treatment strategies such as cellular therapy based on γδ+ T cells or other immune cells [66676869707172].
 | Fig. 3Current haploidentical HSCT strategy for pediatric patients at AMCCH. The donor will receive G-CSF for a minimum of four consecutive days and peripheral blood mononuclear cells (PBMCs) will be collected on days -1 and 0. The αβ+ T cells will be depleted by negative depletion using the CliniMACS system (Miltenyi-BioTec, Bergisch-Gladbach, Germany). The final dose of αβ+ T cells is targeted ≤5×104/kg by adding back αβ+ T cells from the negative selection product. The patient will receive conditioning regimen consisting of fludarabine (FLU), cyclophosphamide (CY), rabbit ATG (r-ATG), and low-dose total body irradiation (LD-TBI). After that, stem cells will be infused on day 0 without any post-transplant immunosuppressants. The patient will also receive rituximab post-transplant to deplete B cells at approximately day +28 or earlier if EBV was detected with PCR. For cytomegalovirus (CMV) prophylaxis, the CMV-seropositive patient will receive ganciclovir prior to transplant and foscarnet after transplantation up until engraftment. After engraftment, ganciclovir or valganciclovir will be administered until 100 days post-transplantation with CD4+ cells at >100/µL.Abbreviations: HSC, hematopoietic stem cells; αβ, αβ+ T cells; γδ, γδ+ T cells; DC, dendritic cells; B, B cells; HR, high-risk.
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CONCLUSIONS
Haploidentical HSCT using ex vivo T cell depleted grafts is a promising therapeutic approach for the treatment of patients without an optimal related or unrelated donor. Currently, substantial progress in haploidentical HSCT has been achieved in pediatric patients, providing a chance to cure the patients in need of HSCT. Further improvements to decrease the rates of GF and GVHD, to enhance immune recovery to reduce serious infections and to develop effective prevention and management strategies of relapse will enable haploidentical HSCT to become an established therapy for pediatric patients lacking a suitable donor. In addition, future clinical trials with larger number of patients will help to establish the most effective conditioning regimen, the best donor source, and the optimal regulation of donor T cells, thus maximizing the outcome of this novel approach.
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