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Advantages of peripheral blood stem cells from unrelated donors versus bone marrow transplants in outcomes of adult acute myeloid leukemia patients

Open AccessPublished:June 18, 2022DOI:https://doi.org/10.1016/j.jcyt.2022.05.009

      Abstract

      Background aims

      In allogeneic stem cell transplantation, unrelated donors are chosen in cases where appropriate related donors are not available. Peripheral blood stem cells (PBSCs) are more often selected as a graft source than bone marrow (BM). However, the prognostic benefits of PBSCs versus BM transplants from unrelated donors have not been carefully examined in patients with acute myeloid leukemia (AML). This study compared outcomes of adult AML patients who underwent unrelated PBSC and BM transplantation, evaluating post-transplant complications, including engraftment, graft-versus-host disease (GVHD) and infections, and determined subgroups of patients who are most likely to benefit from unrelated PBSCs compared with BM transplants.

      Methods

      The authors analyzed 2962 adult AML patients who underwent unrelated PBSC or BM transplants between 2011 and 2018 (221 PBSC and 2741 BM) using the Japanese nationwide registry database, in which graft source selection is not skewed toward PBSCs.

      Results

      In 49.7% of patients, disease status at transplantation was first complete remission (CR1). In 57.1% of cases, HLA-matched donors were selected. Myeloablative conditioning was performed in 75.1% of cases, and anti-thymocyte globulin (ATG) was added to conditioning in 10.5%. Multivariate analyses showed a trend toward favorable non-relapse mortality (NRM) in PBSC recipients compared with BM recipients (hazard ratio [HR], 0.731, P = 0.096), whereas overall survival (OS) (HR, 0.959, P = 0.230) and disease-free survival (DFS) (HR, 0.868, P = 0.221) were comparable between PBSC and BM recipients. Although the rate of chronic GVHD (cGVHD) was significantly higher in PBSC patients (HR, 1.367, P = 0.016), NRM was not increased, mainly as a result of significantly reduced risk of bacterial infections (HR, 0.618, P = 0.010), reflecting more prompt engraftments in PBSC recipients. Subgroup analyses revealed that PBSC transplantation was advantageous in patients transplanted at CR1 and in those without ATG use. PBSC recipients experienced significantly better OS and/or DFS compared with BM recipients in this patient group.

      Conclusions

      The authors' results confirmed the overall safety of unrelated PBSC transplantation for adult AML patients and suggested an advantage of PBSCs, especially for those in CR1. Further optimization of the prophylactic strategy for cGVHD is required to improve the overall outcome in transplantation from unrelated PBSC donors.

      Keywords

      Introduction

      Allogeneic hematopoietic stem cell transplantation (allo-HSCT) can be a curative treatment for patients with hematological malignancies, including acute leukemia and malignant lymphoma [
      • Gale RP
      • Champlin RE.
      How does bone-marrow transplantation cure leukaemia?.
      ]. Unrelated donors are chosen when appropriate related donors are not available [
      • Arai Y
      • Konuma T
      • Yanada M.
      Hematopoietic cell transplantation in adults with acute myeloid leukemia: A review of the results from various nationwide registry studies in Japan.
      ]. Currently, according to reports from the Center for International Blood and Marrow Transplant Research and the European Society of Blood and Marrow Transplantation, peripheral blood stem cells (PBSCs) are more often selected than bone marrow (BM) as a graft source in such cases [
      • Phelan R
      • Arora M
      • Chen M.
      Current use and outcome of hematopoietic stem cell transplantation: CIBMTR US summary slides, 2020.
      ,
      • Anasetti C
      • Logan BR
      • Lee SJ
      • et al.
      Peripheral-blood stem cells versus bone marrow from unrelated donors.
      ,
      • Passweg JR
      • Baldomero H
      • Chabannon C
      • et al.
      Hematopoietic cell transplantation and cellular therapy survey of the EBMT: monitoring of activities and trends over 30 years.
      ]. Such a trend toward PBSCs in graft source selection is mainly due to less invasive procedures and more flexible harvest schedules for donors of PBSCs compared with BM [
      • Rowley SD
      • Donaldson G
      • Lilleby K
      • Bensinger WI
      • Appelbaum FR.
      Experiences of donors enrolled in a randomized study of allogeneic bone marrow or peripheral blood stem cell transplantation.
      ,
      • Karlsson L
      • Quinlan D
      • Guo D
      • et al.
      Mobilized blood cells vs bone marrow harvest: experience compared in 171 donors with particular reference to pain and fatigue.
      ].
      Thus, PBSCs are more often selected as a “donor-friendly” graft source in cases of unrelated allo-HSCT, but this selection preference should be revisited from the viewpoint of post-transplant outcomes in recipients [
      • Eapen M
      • Logan BR
      • Confer DL
      • et al.
      Peripheral blood grafts from unrelated donors are associated with increased acute and chronic graft-versus-host disease without improved survival.
      ]. Although the multicenter randomized trial from the Blood and Marrow Transplant Clinical Trials Network indicated non-significant differences in survival, non-relapse mortality (NRM) and relapse between PBSCs and BM in the whole cohort of patients with various hematological malignancies [
      • Anasetti C
      • Logan BR
      • Lee SJ
      • et al.
      Peripheral-blood stem cells versus bone marrow from unrelated donors.
      ], such prognostic similarities between the two sources of graft could differ depending on the underlying disease [
      • Eapen M
      • Logan BR
      • Appelbaum FR
      • et al.
      Long-term survival after transplantation of unrelated donor peripheral blood or bone marrow hematopoietic cells for hematologic malignancy.
      ], and further information is required on the superiority or inferiority of PBSCs over BM based on the underlying disease. Acute myeloid leukemia (AML) should be analyzed with priority because this is the most prevalent disease worldwide with an indication for unrelated HSCT [
      • Passweg JR
      • Baldomero H
      • Chabannon C
      • et al.
      Hematopoietic cell transplantation and cellular therapy survey of the EBMT: monitoring of activities and trends over 30 years.
      ,
      • D'Souza A
      • Fretham C
      • Lee SJ
      • et al.
      Current Use of and Trends in Hematopoietic Cell Transplantation in the United States.
      ]. As previous observations have suggested a higher risk of chronic graft-versus-host disease (cGVHD) in unrelated PBSC versus BM recipients, we are now taking more intensive measures to prevent cGVHD as well as acute GVHD (aGVHD) and to treat it earlier than before. In addition, the advent of novel targeted therapeutics for GVHD [
      • Zeiser R
      • Burchert A
      • Lengerke C
      • et al.
      Ruxolitinib in corticosteroid-refractory graft-versus-host disease after allogeneic stem cell transplantation: a multicenter survey.
      ,
      • Miklos D
      • Cutler CS
      • Arora M
      • et al.
      Ibrutinib for chronic graft-versus-host disease after failure of prior therapy.
      ] as well as novel antimicrobials [
      • Nava T
      • Ansari M
      • Dalle JH
      • et al.
      Supportive care during pediatric hematopoietic stem cell transplantation: beyond infectious diseases. A report from workshops on supportive care of the Pediatric Diseases Working Party (PDWP) of the European Society for Blood and Marrow Transplantation (EBMT).
      ] can further reduce the incidence of GVHD and serious infections. Such changes in clinical practice can potentially modify transplantation outcomes.
      Thus, the impact of using unrelated PBSCs versus BM as a graft source on patient outcomes should be updated. Moreover, the severity of adverse events can vary widely depending on patient-specific characteristics, such as disease status, conditioning regimens and HLA type. It is time to evaluate whether there are specific subgroups of patients who can benefit from PBSC or BM transplants in clinical practice.
      Therefore, the authors performed a retrospective cohort study to (i) compare outcomes of adult patients with AML who underwent unrelated PBSC and BM transplantation; (ii) evaluate post-transplant complications, including engraftment, GVHD and infections; and (iii) identify subgroups of patients who are most likely to benefit from unrelated PBSC compared with BM transplantation. In this study, the authors used the Japanese nationwide transplant registry. In Japan, the number of unrelated donor PBSC transplants is still low [

      Hematopoietic Cell Transplantation in Japan. Annual Report of Nationwide Survey 2020: The Japanese Data Center for Hematopoietic Cell Transplantation/The Japan Society for Hematopoietic. Cell Transplantation. http://www.jdchct.or.jp/data/report/2020/

      ] but has gradually increased since 2010, when the Japan Marrow Donor Program started facilitating PBSC transplantation from unrelated donors [
      • Goto T
      • Tanaka T
      • Sawa M
      • et al.
      Prospective observational study on the first 51 cases of peripheral blood stem cell transplantation from unrelated donors in Japan.
      ]; thus, the authors were able to make legitimate comparisons between PBSC and BM recipients in real-world cohorts. Such comparisons are almost impossible outside the framework of clinical trials in US and European cohorts because the standard sources of unrelated donors are PBSC grafts [
      • Phelan R
      • Arora M
      • Chen M.
      Current use and outcome of hematopoietic stem cell transplantation: CIBMTR US summary slides, 2020.
      ,
      • Passweg JR
      • Baldomero H
      • Chabannon C
      • et al.
      Hematopoietic cell transplantation and cellular therapy survey of the EBMT: monitoring of activities and trends over 30 years.
      ], and BM is selected in only a minor proportion of cases. The authors’ findings provide valuable insights into the donor selection algorithm (PBSCs versus BM) in unrelated allo-HSCT for adult AML patients and should contribute to improvements in transplantation outcomes.

      Methods

      Patients

      Data on adult patients (aged ≥16 years) with AML who had undergone their first allogeneic PBSC or BM transplant from unrelated donors between 2011 and 2018 were identified through the Transplant Registry Unified Management Program sponsored by the Japanese Society for Transplantation and Cellular Therapy [
      • Atsuta Y.
      Introduction of Transplant Registry Unified Management Program 2 (TRUMP2): scripts for TRUMP data analyses, part I (variables other than HLA-related data).
      ,
      • Kanda J.
      Scripts for TRUMP data analyses. Part II (HLA-related data): statistical analyses specific for hematopoietic stem cell transplantation.
      ]. Patients without survival data or with HLA mismatches at three or more loci were excluded. The study was planned by the Adult AML Working Group of the Japanese Society for Transplantation and Cellular Therapy, approved by the data management committees of the Transplant Registry Unified Management Program and the institutional review board of Kyoto University Hospital and conducted in accordance with the Declaration of Helsinki.

      Study endpoints and definitions

      The primary endpoint was overall survival (OS) after transplantation. Death, regardless of cause, was considered an event. Secondary endpoints were disease-free survival (DFS); cumulative incidence of relapse; NRM; neutrophil and platelet engraftment; aGVHD; cGVHD; GVHD relapse-free survival (GRFS); viral, bacterial and fungal infection; and infection-related mortality. DFS was defined as survival without disease progression or relapse. aGVHD and cGVHD were assessed according to standard criteria [
      • Sullivan KM
      • Agura E
      • Anasetti C
      • et al.
      Chronic graft-versus-host disease and other late complications of bone marrow transplantation.
      ,
      • Przepiorka D
      • Weisdorf D
      • Martin P
      • et al.
      1994 Consensus Conference on Acute GVHD Grading.
      ]. GRFS was defined as survival without death, relapse, development of grade III–IV aGVHD or development of cGVHD that required systemic treatment [
      • Holtan SG
      • DeFor TE
      • Lazaryan A
      • et al.
      Composite end point of graft-versus-host disease-free, relapse-free survival after allogeneic hematopoietic cell transplantation.
      ]. Neutrophil and platelet engraftment was defined as the first three consecutive measures with a neutrophil count ≥0.5 × 109/L and a platelet count ≥50 × 109/L without platelet transfusion after transplantation. Viral infection included infections with cytomegalovirus; Epstein–Barr virus, including Epstein–Barr virus post-transplant lymphoproliferative disorder; and human herpesvirus 6. Bacterial infection included any bacterial infection, excluding febrile neutropenia without proven infection. Fungal infection included candidemia; proven, probable or possible aspergillosis with previously reported criteria [
      • De Pauw B
      • Walsh TJ
      • Donnelly JP
      • et al.
      Revised definitions of invasive fungal disease from the European Organization for Research and Treatment of Cancer/Invasive Fungal Infections Cooperative Group and the National Institute of Allergy and Infectious Diseases Mycoses Study Group (EORTC/MSG) Consensus Group.
      ]; and other proven fungal infections. Infection-related mortality was defined as death from infection as the primary cause of death. Cytogenetic risk was classified in accordance with criteria specified by the National Comprehensive Cancer Network guidelines, which have been described in detail elsewhere [
      • Yanada M
      • Mori J
      • Aoki J
      • et al.
      Effect of cytogenetic risk status on outcomes for patients with acute myeloid leukemia undergoing various types of allogeneic hematopoietic cell transplantation: an analysis of 7812 patients.
      ]. Conditioning intensity was defined according to operational definitions of the National Marrow Donor Program/Center for International Blood and Marrow Transplant Research [
      • Giralt S
      • Ballen K
      • Rizzo D
      • et al.
      Reduced-intensity conditioning regimen workshop: defining the dose spectrum. Report of a workshop convened by the center for international blood and marrow transplant research.
      ]. HLA matching was assessed using allele data for the HLA-A, -B, -C and -DRB1 loci [
      • Morishima Y
      • Sasazuki T
      • Inoko H
      • et al.
      The clinical significance of human leukocyte antigen (HLA) allele compatibility in patients receiving a marrow transplant from serologically HLA-A, HLA-B, and HLA-DR matched unrelated donors.
      ]. HLA mismatch was defined in the GVHD vector when recipient alleles were not shared by the donor and was defined in the host-versus-graft direction when donor alleles were not shared by the recipient.

      Propensity score matching

      Propensity score was calculated to evaluate the intention to use PBSCs. Propensity score matching analysis accounted for patient age at the time of transplantation (<50 or ≥50 years), sex (male or female), Eastern Cooperative Oncology Group performance status (ECOG PS) (0–1 or 2–4), cytogenetic risk (favorable, intermediate, poor or unevaluable), disease status at transplantation (first complete remission [CR1], second complete remission [CR2], ≥3 complete remission [CR3] or non-complete remission [non-CR]), donor age (<40 or ≥40), donor–sex mismatch (matched, female to male or male to female), HLA mismatches (0, 1 or 2), intensity of the conditioning (myeloablative conditioning or reduced intensity conditioning), use of total body irradiation (TBI) (no or yes), GVHD prophylaxis (cyclosporin A-based or tacrolimus-based), addition of anti-thymocyte globulin (ATG) to conditioning (no or yes) and year of transplantation (2011–2014 or 2015–2018). Matching (1:1) was performed using the nearest neighbor matching method with a caliper width fixed at 0.2 standard deviations of the propensity score.

      Statistical analysis

      Categorical variables and continuous variables were compared between groups with Fisher exact test and two-tailed unpaired Student's t-test, respectively. The probabilities of OS and DFS were estimated according to the Kaplan–Meier method and compared among groups with the Cox proportional hazards model. Probabilities of NRM; relapse; engraftment; aGVHD; cGVHD; viral, bacterial or fungal infection; and infection-related mortality were estimated on the basis of cumulative incidence methods and compared among groups with the Fine–Gray proportional hazards model, considering death without relapse as a competing event for relapse, relapse as a competing event for NRM, death without engraftment as a competing event for neutrophil and platelet engraftment, death or relapse without GVHD as a competing event for aGVHD and cGVHD and death without infection as a competing event for infection and infection-related mortality. The following variables were considered in multivariate analyses: patient age at the time of transplantation, sex, ECOG PS, cytogenetic risk, disease status at the time of transplantation, donor age, donor–sex mismatch, HLA mismatches, intensity of conditioning, use of TBI, GVHD prophylaxis, addition of ATG to conditioning regimen and year of transplantation. All tests were two-sided, and P < 0.05 was considered statistically significant. All analyses were performed with Stata 17 software (StataCorp LLC, College Station, TX, USA).

      Results

      Patient characteristics

      A total of 2962 patients were eligible for analysis. Of these, 221 underwent unrelated PBSC transplantation (PBSC group), and 2741 received unrelated BM transplantation (BM group). Patient characteristics, including transplant procedures, are shown in Table 1. Median patient age was 53 years (range, 16–69) for the PBSC group and 53 years (range, 16–76) for the BM group. ECOG PS at time of HSCT and cytogenetic risk at initial diagnosis were equivalent between the groups. Disease status at time of HSCT was CR1 in 49.7% of the whole cohort, and no significant differences were observed according to graft source (P = 0.248). With regard to donors, distribution of donor age and recipient–donor–sex disparities were comparable between the PBSC and BM groups. By contrast, HLA matching showed significant differences, with HLA-matched donors selected in 70.1% of the PBSC group but in only 56.1% of the BM group (P < 0.001). Conditioning regimens were composed of myeloablative conditioning in 75.1% of the whole cohort (no statistical difference between PBSC and BM), and TBI was more frequently used in the BM group (45.2% versus 61.8%, P < 0.001). Regarding GVHD prophylaxis, PBSC patients were more likely to receive tacrolimus-based prophylaxis (95.0% versus 87.2%, P < 0.001) and ATG (25.3% versus 9.3%, P < 0.001) than were BM recipients. Median follow-up of survivors in the PBSC and BM groups was 1.6 years and 3.4 years, respectively.
      Table 1Patient characteristics.
      Total (N = 2962)PBSC (N = 221)BM (N = 2741)P value
      Patient age, years, median (range)53 (16–76)53 (16–69)53 (16–76)0.050
      Patient age, years, n (%)
      <501252 (42.3)83 (37.6)1169 (42.6)0.157
      ≥501710 (57.7)138 (62.4)1572 (57.4)
      Patient sex, n (%)0.046*
      Male1752 (59.1)145 (65.6)1607 (58.6)
      Female1209 (40.8)76 (34.4)1133 (41.3)
      ECOG PS, n (%)0.168
      0–12753 (92.9)211 (95.5)2542 (92.7)
      2–4205 (6.9)10 (4.5)195 (7.1)
      Cytogenetic risk, n (%)0.610
      Favorable351 (11.9)20 (9.0)331 (12.1)
      Intermediate1833 (61.9)143 (64.7)1690 (61.7)
      Poor613 (20.7)46 (20.8)567 (20.7)
      Unevaluable165 (5.6)12 (5.4)153 (5.6)
      Disease status, n (%)0.248
      CR11472 (49.7)112 (50.7)1360 (49.6)
      CR2495 (16.7)34 (15.4)461 (16.8)
      ≥CR327 (0.9)5 (2.3)22 (0.8)
      Non-CR967 (32.6)70 (31.7)897 (32.7)
      Donor age, years, median (range)39 (19–55)39 (19–55)39 (19–55)0.516
      Donor age, years, n (%)
      <401567 (52.9)113 (51.1)1454 (53.0)0.484
      ≥401367 (46.2)108 (48.9)1259 (45.9)
      Sex mismatch, n (%)0.813
      Matched1729 (58.4)134 (60.6)1595 (58.2)
      Female to male425 (14.3)30 (13.6)395 (14.4)
      Male to female792 (26.7)56 (25.3)736 (26.9)
      HLA mismatch, ABCDE genotype, total, n (%)< 0.001
      01692 (57.1)155 (70.1)1537 (56.1)
      11046 (35.3)57 (25.8)989 (36.1)
      2224 (7.6)9 (4.1)215 (7.8)
      Conditioning, n (%)0.936
      Myeloablative2223 (75.1)167 (75.6)2056 (75.0)
      Reduced intensity738 (24.9)54 (24.4)684 (25.0)
      TBI, n (%)< 0.001
      No1166 (39.4)121 (54.8)1045 (38.1)
      Yes1795 (60.6)100 (45.2)1695 (61.8)
      GVHD prophylaxis, n (%)< 0.001
      Tac-based2600 (87.8)210 (95.0)2390 (87.2)
      CyA-based309 (10.4)8 (3.6)301 (11.0)
      Addition of ATG to conditioning regimen, n (%)311 (10.5)56 (25.3)255 (9.3)< 0.001
      Years of transplant, n (%)< 0.001
      2011–20141555 (52.5)36 (16.3)1519 (55.4)
      2015–20181407 (47.5)185 (83.7)1222 (44.6)
      CyA, cyclosporin A; Tac, tacrolimus.

      PBSC recipients demonstrated a lower incidence of NRM than BM recipients

      Respective 3-year OS and DFS rates were 57.5% and 51.6% in the PBSC group and 52.4% and 48.3% in the BM group (Figure 1A,B). Multivariate analyses revealed that OS (hazard ratio [HR], 0.859, P = 0.230) and DFS (HR, 0.868, P = 0.221) in the PBSC group were not inferior to those in the BM group (Table 2, Figure 1A,B; also see supplementary Table 1). Respective 3-year cumulative incidences of NRM and relapse were 17.9% and 30.6% in the PBSC group and 22.6% and 29.1% in the BM group (Figure 1C,D). Multivariate analyses revealed a trend toward a lower incidence of NRM in the PBSC group (HR, 0.731, P = 0.096), whereas the incidence of relapse was comparable between the PBSC and BM groups (HR, 0.978, P = 0.872) (Table 2, Figure 1C,D; also see supplementary Table 1). These results indicated that unrelated PBSC transplantation is at least safe for adult AML patients, with suggestions of a more favorable trend regarding NRM.
      Fig 1
      Fig. 1Comparison of outcomes between unrelated PBSC and BM transplants in the whole cohort. (A) OS. (B) DFS. (C) Cumulative incidence of NRM. (D) Cumulative incidence of relapse. HRs and P values were calculated using the Cox proportional hazards model (A,B) and Fine–Gray tests (C,D) after being adjusted for confounding factors.
      Table 2Multivariate analysis for OS, DFS, NRM and relapse.
      OSDFSNRMRelapse
      HR(95% CI)P valueHR(95% CI)P valueHR(95% CI)P valueHR(95% CI)P value
      Graft source
      PBSC versus BM0.859(0.670–1.101)0.2300.868(0.691–1.089)0.2210.731(0.505–1.058)0.0960.978(0.745–1.284)0.872
      Patient age
      ≥50 versus <501.538(1.357–1.743)< 0.0011.336(1.185–1.506)< 0.0011.823(1.511–2.199)< 0.0010.933(0.794–1.097)0.402
      Patient sex
      Female versus male0.809(0.681–0.960)0.0160.846(0.718–0.998)0.0470.681(0.520–0.893)0.0051.118(0.899–1.389)0.316
      Performance status
      2–4 versus 0–12.382(1.995–2.843)< 0.0011.943(1.631–2.314)< 0.0011.802(1.338–2.427)< 0.0011.223(0.958–1.561)0.105
      Cytogenetic risk
      Intermediate versus favorable1.372(1.111–1.694)0.0031.329(1.085–1.628)0.0061.279(0.958–1.708)0.0951.257(0.955–1.653)0.102
      Poor versus favorable2.295(1.828–2.882)< 0.0012.259(1.814–2.812)< 0.0011.130(0.810–1.576)0.4732.414(1.799–3.239)< 0.001
      Unevaluable versus favorable1.539(1.128–2.100)0.0071.552(1.153–2.090)0.0041.114(0.706–1.757)0.6431.705(1.147–2.533)0.008
      Disease status
      CR2 versus CR10.997(0.828–1.200)0.9740.991(0.829–1.184)0.9180.906(0.713–1.152)0.4201.055(0.822–1.357)0.680
      ≥CR3 versus CR12.539(1.507–4.278)< 0.0012.227(1.345–3.689)0.0022.003(1.015–3.955)0.0451.897(0.896–4.015)0.094
      Non-CR versus CR12.354(2.084–2.659)< 0.0012.525(2.246–2.838)< 0.0010.970(0.801–1.174)0.7543.308(2.826–3.872)< 0.001
      Donor age
      ≥40 versus <401.190(1.065–1.330)0.0021.192(1.071–1.326)0.0011.317(1.117–1.552)0.0010.996(0.862–1.151)0.955
      Sex mismatch
      Female to male versus matched1.031(0.881–1.207)0.7011.023(0.879–1.192)0.7651.008(0.800–1.271)0.9461.060(0.866–1.298)0.571
      Male to female versus matched1.086(0.899–1.312)0.3911.044(0.871–1.251)0.6421.359(1.014–1.820)0.0400.815(0.640–1.037)0.096
      HLA mismatch
      1 versus 01.128(1.001–1.272)0.0491.113(0.992–1.248)0.0681.414(1.189–1.680)< 0.0010.855(0.730–1.002)0.052
      2 versus 01.380(1.133–1.681)0.0011.404(1.162–1.696)< 0.0011.733(1.288–2.330)< 0.0010.985(0.757–1.282)0.912
      Conditioning
      RIC versus MAC0.997(0.872–1.139)0.9651.083(0.953–1.230)0.2211.032(0.848–1.257)0.7531.112(0.931–1.328)0.241
      TBI
      Yes versus no1.083(0.964–1.217)0.1801.063(0.951–1.189)0.2811.012(0.852–1.201)0.8961.082(0.930–1.260)0.307
      GVHD prophylaxis
      Tac-based versus CyA-based0.723(0.610–0.857)< 0.0010.803(0.680–0.947)0.0090.742(0.578–0.952)0.0191.010(0.794–1.285)0.934
      ATG
      Yes versus no0.837(0.692–1.012)0.0660.902(0.752–1.081)0.2640.655(0.484–0.886)0.0061.212(0.961–1.528)0.104
      Years of transplant
      2015–2018 versus 2011–20141.042(0.926–1.172)0.4991.048(0.937–1.173)0.4130.997(0.843–1.179)0.9690.978(0.840–1.137)0.769
      CI, confidence interval; CyA, cyclosporin A; MAC, myeloablative conditioning; RIC, reduced intensity conditioning; Tac, tacrolimus.

      PBSC recipients demonstrated fewer infection-related complications but higher incidence of cGVHD than BM recipients

      Next, the authors performed detailed analyses focusing on post-transplant engraftment and complications. The cumulative incidences of neutrophil engraftment at day 30 and platelet engraftment at day 60 were both significantly higher in the PBSC group than in the BM group (95.9% versus 93.1%, HR, 2.109, P < 0.001, and 84.6% versus 73.9%, HR, 1.920, P < 0.001), and the time between HSCT and engraftment was significantly shorter in the PBSC group (Figure 2A,B). Among patients who achieved engraftment, secondary graft failure was significantly less frequent in the PBSC group than in the BM group (0.9% versus 3.4%, P = 0.044), suggesting robust engraftment in the PBSC group.
      Fig 2
      Fig. 2Comparison of engraftment and post-transplant complications between unrelated PBSC and BM transplants in the whole cohort. (A) Cumulative incidence of neutrophil engraftment. (B) Cumulative incidence of platelet engraftment. (C) Cumulative incidence of grade II–IV aGVHD. (D) Cumulative incidence of grade III–IV aGVHD. (E) Cumulative incidence of cGVHD. (F) Cumulative incidence of extensive cGVHD. (G) Cumulative incidence of bacterial infection. (H) Cumulative incidence of infection-related mortality. HRs and P values were calculated using Fine–Gray tests (A–H) and adjusted for confounding factors (C–F). *P < 0.05.
      Fig 2
      Fig. 2Comparison of engraftment and post-transplant complications between unrelated PBSC and BM transplants in the whole cohort. (A) Cumulative incidence of neutrophil engraftment. (B) Cumulative incidence of platelet engraftment. (C) Cumulative incidence of grade II–IV aGVHD. (D) Cumulative incidence of grade III–IV aGVHD. (E) Cumulative incidence of cGVHD. (F) Cumulative incidence of extensive cGVHD. (G) Cumulative incidence of bacterial infection. (H) Cumulative incidence of infection-related mortality. HRs and P values were calculated using Fine–Gray tests (A–H) and adjusted for confounding factors (C–F). *P < 0.05.
      With regard to GVHD, the respective cumulative incidence of grade II–IV and III–IV aGVHD at 100 days in the PBSC group was comparable to that in the BM group (31.5% versus 37.4%, HR, 0.890, P = 0.369, 7.4% versus 10.8%, HR, 0.875, P = 0.586) (Figure 2C,D; also see supplementary Table 2). By contrast, the respective cumulative incidence of cGVHD and extensive cGVHD at 2 years was significantly higher in the PBSC group than in the BM group (39.8% versus 31.4%, HR, 1.367, P = 0.016, 24.9% versus 18.5%, HR, 1.450, P = 0.025) (Figure 2E,F; also see supplementary Table 2). Three-year GRFS was comparable between the PBSC and BM groups (32.0% versus 31.3%, adjusted HR, 0.975, 95% confidence interval, 0.809–1.174, P = 0.789), suggesting that the higher incidence of cGVHD did not lead to decreased GRFS.
      As regards infection-related complications, the cumulative incidence of bacterial infection at day 100 was significantly lower in the PBSC group than in the BM group (13.4% versus 20.4%, HR, 0.618, P = 0.010) (Figure 2G). The lower incidence of bacterial infection in the PBSC group accounted for the significantly lower cumulative incidence of infection-related mortality in this group (2.5% versus 7.5%, HR, 0.307, P = 0.019) (Table 3, Figure 2H). There was no significant difference in the cumulative incidence of viral or fungal infections between the PBSC and BM groups (see supplementary Figure 1A,B). These results suggested that the reduced risk of bacterial infection, possibly reflecting the more prompt and stable engraftment in the PBSC group, resulted in the lower NRM observed in this group, whereas the higher incidence of cGVHD did not cause increased mortality after transplantation.
      Table 3Comparison of causes of mortality in the PBSC and BM groups.
      TotalCR1≥CR2/non-CR
      PBSC

      (N = 221)
      BM (N = 2741)P valuePBSC

      (N = 112)
      BM (N = 1360)P valuePBSC

      (N = 109)
      BM (N = 1380)P value
      Infection, n (%)4 (1.8)180 (6.6)0.0022 (1.8)69 (5.1)0.1652 (1.8)111 (8.0)0.014
      Primary disease, n (%)42 (19.0)594 (21.7)0.39510 (8.9)188 (13.8)0.19332 (29.4)406 (29.4)1.000
      Graft failure, n (%)1 (0.5)13 (0.5)1.0000 (0.0)6 (0.4)1.0001 (0.9)7 (0.5)0.456
      GVHD, n (%)7 (3.2)91 (3.3)1.0001 (0.9)46 (3.4)0.2556 (5.5)45 (3.3)0.264
      Interstitial pneumonia/ARDS, n (%)5 (2.3)80 (2.9)1.0002 (1.8)43(3.2)0.5743 (2.8)37 (2.7)0.466
      Organ failure/toxicity, n (%)10 (4.5)220 (8.0)0.0675 (4.5)97 (7.1)0.3385 (4.6)123 (8.9)0.154
      Secondary malignancy, n (%)1 (0.5)12 (0.4)1.0001 (0.9)2 (0.1)0.2110 (0.0)10 (0.7)1.000
      Other, n (%)3 (1.4)78 (2.9)0.2801 (0.9)47 (3.5)0.2592 (1.8)31 (2.3)1.000
      Total73 (33.0)1268 (46.3)22 (19.6)498 (36.6)51 (46.8)770 (55.8)
      ARDS, acute respiratory distress syndrome.

      PBSC is clearly beneficial for patients with CR1 at transplantation and may improve DFS in patients on non-ATG regimens

      To identify the subgroup of patients who clearly benefit from PBSCs rather than BM transplantation, the authors performed subgroup analyses of OS (Figure 3). Significantly favorable effects of PBSCs over BM on OS were observed in the patient subgroup with CR1 at transplantation (HR, 0.624, P = 0.030) and conditioning regimens without ATG (HR, 0.743, P = 0.037).
      Fig 3
      Fig. 3Subgroup analyses of OS with respect to patient characteristics. OS is compared in each subgroup regarding patient characteristics. HRs of OS in the PBSC group are shown in comparison with the BM group. HR <1 indicates favorable OS in the PBSC group. Black dots = HRs. Black bars = 95% CI. *P < 0.05. CI, confidence interval.
      In patients with CR1 at transplantation, PBSCs were significantly associated with favorable 3-year OS and DFS (73.9% versus 61.7%, HR, 0.551, P = 0.011, 70.9% versus 57.3%, HR, 0.546, P = 0.005, respectively) (Figure 4A,B) as well as marginally reduced risk of NRM (HR, 0.570, P = 0.058) and relapse (HR, 0.612, P = 0.106) compared with BM patients (Figure 4C,D). By contrast, PBSC transplantation offered no advantage to patients with CR2 or more advanced-stage disease (non-CR or ≥CR3) compared with BM transplantation (see supplementary Figure 2A–D).
      Fig 4
      Fig. 4Beneficial effects of PBSCs in the CR1 and non-ATG regimen subgroups. (A) OS in the CR1 subgroup. (B) DFS in the CR1 subgroup. (C) Cumulative incidence of NRM in the CR1 subgroup. (D) Cumulative incidence of relapse in the CR1 subgroup. (E) OS in the non-ATG regimen subgroup. (F) DFS in the non-ATG regimen subgroup. (G) Cumulative incidence of NRM in the non-ATG regimen subgroup. (H) Cumulative incidence of relapse in the non-ATG regimen subgroup. HRs and P values were calculated using the Cox proportional hazards model (A,B,E,F) and Fine–Gray tests (C,D,G,H) after being adjusted for confounding factors. *P < 0.05.
      Fig 4
      Fig. 4Beneficial effects of PBSCs in the CR1 and non-ATG regimen subgroups. (A) OS in the CR1 subgroup. (B) DFS in the CR1 subgroup. (C) Cumulative incidence of NRM in the CR1 subgroup. (D) Cumulative incidence of relapse in the CR1 subgroup. (E) OS in the non-ATG regimen subgroup. (F) DFS in the non-ATG regimen subgroup. (G) Cumulative incidence of NRM in the non-ATG regimen subgroup. (H) Cumulative incidence of relapse in the non-ATG regimen subgroup. HRs and P values were calculated using the Cox proportional hazards model (A,B,E,F) and Fine–Gray tests (C,D,G,H) after being adjusted for confounding factors. *P < 0.05.
      The authors observed greater DFS (HR, 0.761, P = 0.043) in PBSC transplant recipients in comparison with BM in patients on non-ATG regimens as well as comparable NRM (HR, 0.706, P = 0.093), whereas the incidence of relapse was similar between the two groups (HR, 0.863, P = 0.383). These effects were not apparent in patients who received conditioning with ATG (Figure 4E–H; also see supplementary Figure 2E–H). This may be partially explained by the fact that adding ATG to the conditioning regimen had no beneficial effect on the risk of grade II–IV aGVHD (HR, 1.502, P = 0.222) and was associated with a tendency toward increased risk of relapse (HR, 1.691, P = 0.094) in PBSC transplantation. As a result, ATG was associated with worse OS (HR, 1.737, P = 0.084) and significantly worse DFS (HR, 1.773, P = 0.042) in PBSC recipients (see supplementary Table 3). In contrast to the PBSC group, in the BM group, ATG significantly reduced the cumulative risk of grade II–IV aGVHD (HR, 0.662, P = 0.001) and NRM (HR, 0.645, P = 0.007), which resulted in significantly better OS (HR, 0.774, P = 0.014) and comparable DFS (HR, 0.824, P = 0.054).
      These results suggested that PBSCs from unrelated donors should be considered for adult AML patients in CR1 or those transplanted without ATG instead of BM. There were no patient subgroups in which PBSCs were associated with significantly worse OS than BM. Of note, HLA mismatch and conditioning intensity did not obviously interact with the graft source with regard to survival as well as risk of GVHD, NRM and relapse (Figure 3; also see supplementary Figure 3A–H)

      Propensity score matching analyses confirmed the advantage of PBSCs over BM transplantation for adult AML patients

      To confirm the superiority of unrelated PBSC transplantation for adult AML patients indicated by the subgroup analyses, the authors performed a propensity score matching analysis. A total of 201 PBSC recipients were pair-matched with 201 BM recipients (see supplementary Table 4). The propensity score analysis (Figure 5A–D; also see supplementary Figure 4A–H) showed a trend toward better 3-year OS (58.4% versus 51.0%, HR, 0.764, P = 0.102) and 3-year DFS (55.3% versus 45.6%, HR, 0.763, P = 0.077) and a lower 3-year NRM rate (17.8% versus 24.8%, HR, 0.658, P = 0.080) in the PBSC group than in the BM group (Figure 5A–C). Three-year cumulative incidence of relapse was similar between the PBSC and BM groups (27.0% versus 29.6%, HR, 0.885, P = 0.533) (Figure 5D). In analysis of the propensity score-matched cohort, PBSCs in CR1 were associated with significantly better OS and DFS as well as significantly lower NRM and a marginally lower relapse rate, but not in those transplanted at more advanced stages (see supplementary Figure 4A–D). Trends for better OS and DFS as well as lower NRM and relapse rate in the PBSC group were also observed in patients transplanted without ATG, although the difference was not statistically significant (see supplementary Figure 4E–H). These results were consistent with those observed in the whole cohort, confirming the efficacy of unrelated PBSC transplantation in adult AML patients, especially those in CR1.
      Fig 5
      Fig. 5Propensity score matching analyses for transplant outcomes. (A) OS. (B) DFS. (C) Cumulative incidence of NRM. (D) Cumulative incidence of relapse. HRs and P values were calculated using the Cox proportional hazards model (A,B) and Fine–Gray tests (C,D).

      Discussion

      Using the Japanese nationwide registry database, the present retrospective cohort study analyzed outcome differences between PBSCs from unrelated donors and BM transplantation in adult AML patients. There were three major findings. First, there was a favorable NRM as well as non-inferior OS and DFS in PBSC recipients compared with BM recipients. Second, more rapid engraftment resulted in significantly lower mortality caused by bacterial infection despite a higher incidence of cGVHD with PBSCs. Third, PBSCs were associated with significantly better OS and DFS compared with BM in patients transplanted in CR1, suggesting an advantage of PBSCs over BM for these patient subgroups.
      Importantly, the authors found that OS, DFS, NRM and relapse in Japanese patients after unrelated transplantation with PBSCs were not inferior to those after unrelated transplantation with BM. These results are compatible with previous reports from US and European cohorts [
      • Anasetti C
      • Logan BR
      • Lee SJ
      • et al.
      Peripheral-blood stem cells versus bone marrow from unrelated donors.
      ,
      • Eapen M
      • Logan BR
      • Confer DL
      • et al.
      Peripheral blood grafts from unrelated donors are associated with increased acute and chronic graft-versus-host disease without improved survival.
      ,
      • Eapen M
      • Logan BR
      • Horowitz MM
      • et al.
      Bone marrow or peripheral blood for reduced-intensity conditioning unrelated donor transplantation.
      ], but fair real-world comparisons are guaranteed in this study using a Japanese cohort because the prevalence of PBSCs is still low [

      Hematopoietic Cell Transplantation in Japan. Annual Report of Nationwide Survey 2020: The Japanese Data Center for Hematopoietic Cell Transplantation/The Japan Society for Hematopoietic. Cell Transplantation. http://www.jdchct.or.jp/data/report/2020/

      ]. In the US and Europe, PBSCs are the standard selection, and BM is selected only in special clinical situations, including donor health issues [
      • Worel N
      • Buser A
      • Greinix HT
      • et al.
      Suitability Criteria for Adult Related Donors: A Consensus Statement from the Worldwide Network for Blood and Marrow Transplantation Standing Committee on Donor Issues.
      ]; thus, such a comparison between PBSCs and BM is no longer possible.
      The favorable outcomes of PBSCs compared with BM in unrelated HSCT observed in this study contrast with significantly inferior outcomes for PBSCs relative to BM in related HSCT in a Japanese cohort [
      • Nagafuji K
      • Matsuo K
      • Teshima T
      • et al.
      Peripheral blood stem cell versus bone marrow transplantation from HLA-identical sibling donors in patients with leukemia: a propensity score-based comparison from the Japan Society for Hematopoietic Stem Cell Transplantation registry.
      ]. This discrepancy may be partly explained by the biased donor selection in the related HSCT setting, as PBSC grafts are more often used for patients with high-risk HSCT in donor coordination. Cryopreservation-available PBSC grafts from related donors are often selected for patients with chemorefractory disease or those with severe infection [
      • Nagafuji K
      • Matsuo K
      • Teshima T
      • et al.
      Peripheral blood stem cell versus bone marrow transplantation from HLA-identical sibling donors in patients with leukemia: a propensity score-based comparison from the Japan Society for Hematopoietic Stem Cell Transplantation registry.
      ,
      • Yanada M
      • Konuma T
      • Yamasaki S
      • et al.
      Time-Varying Effects of Graft Type on Outcomes for Patients with Acute Myeloid Leukemia Undergoing Allogeneic Hematopoietic Cell Transplantation.
      ].
      Regarding engraftment, the authors’ study demonstrates that there is an advantage provided by faster neutrophil recovery that results in reduced risk of bacterial infection in unrelated PBSC transplantation, which is consistent with previous reports [
      • Young JH
      • Logan BR
      • Wu J
      • et al.
      Infections after Transplantation of Bone Marrow or Peripheral Blood Stem Cells from Unrelated Donors.
      ]. Moreover, the authors found that the decrease in bacterial infections actually led to a significant reduction in infection-related death (Figure 2H); thus, an advantage in engraftment is one of the reasons for superior OS with lower NRM in PBSCs compared with BM. Indeed, the effect of PBSCs in lowering the odds ratio of NRM was more prominent in the early phase—where infection-related death accounts for the majority of NRM—than the later phase after transplantation (odds ratio, 0.368 at day 100 and 0.630 at 2 years). Furthermore, the authors speculate that bacterial infection in the early days after allo-HSCT has a significant impact on the post-transplant prognosis that goes beyond the infection itself, as severe infections along with concurrent cytokine storms can enhance various inflammation-related complications, including hemophagocytic syndrome, engraftment failure, thrombotic microangiopathy, sinusoidal obstruction syndrome and aGVHD [
      • Laskin BL
      • Goebel J
      • Davies SM
      • Jodele S.
      Small vessels, big trouble in the kidneys and beyond: hematopoietic stem cell transplantation-associated thrombotic microangiopathy.
      ,
      • Poutsiaka DD
      • Munson D
      • Price LL
      • Chan GW
      • Snydman DR.
      Blood stream infection (BSI) and acute GVHD after hematopoietic SCT (HSCT) are associated.
      ,
      • Dalle JH
      • Giralt SA.
      Hepatic Veno-Occlusive Disease after Hematopoietic Stem Cell Transplantation: Risk Factors and Stratification, Prophylaxis, and Treatment.
      ,
      • Noguchi M
      • Inagaki J.
      Hemophagocytic Lymphohistiocytosis and Graft Failure Following Unrelated Umbilical Cord Blood Transplantation in Children.
      ].
      With regard to adverse events with long-term follow-up, the authors’ study confirmed the significantly higher incidence of cGVHD in the PBSC cohort versus the BM cohort, and this result was compatible with previous reports from US and European cohorts [
      • Anasetti C
      • Logan BR
      • Lee SJ
      • et al.
      Peripheral-blood stem cells versus bone marrow from unrelated donors.
      ,
      • Eapen M
      • Logan BR
      • Appelbaum FR
      • et al.
      Long-term survival after transplantation of unrelated donor peripheral blood or bone marrow hematopoietic cells for hematologic malignancy.
      ,
      • Savani BN
      • Labopin M
      • Blaise D
      • et al.
      Peripheral blood stem cell graft compared to bone marrow after reduced intensity conditioning regimens for acute leukemia: a report from the ALWP of the EBMT.
      ]. In the Japanese cohort, the incidence of both aGVHD and cGVHD was lower compared with Western cohorts—probably due to less diverse ethnicity in Japan [
      • Kanda J
      • Brazauskas R
      • Hu ZH
      • et al.
      Graft-versus-Host Disease after HLA-Matched Sibling Bone Marrow or Peripheral Blood Stem Cell Transplantation: Comparison of North American Caucasian and Japanese Populations.
      ]—but the difference between PBSCs and BM with respect to cGVHD was recapitulated in the authors’ cohort. Although there was a higher incidence of cGVHD among PBSC recipients in this study, reduction in OS or DFS was not observed compared with BM recipients. The effects of cGVHD on quality of life should be further evaluated because long-term follow-up of a randomized clinical trial has shown that unrelated PBSCs result in a higher burden of cGVHD symptoms than BM [
      • Lee SJ
      • Logan B
      • Westervelt P
      • et al.
      Comparison of Patient-Reported Outcomes in 5-Year Survivors Who Received Bone Marrow vs Peripheral Blood Unrelated Donor Transplantation: Long-term Follow-up of a Randomized Clinical Trial.
      ].
      The third major finding of the authors’ study derived from the subgroup analyses was that PBSCs were associated with statistically better OS and DFS as well as reduced relapse compared with BM in patients transplanted at CR1, but not at more advanced stages (CR2 or later). The significantly lower risk of relapse in CR1 patients may reflect the presence of stronger graft-versus-leukemia (GVL) effects in PBSC versus BM transplants [
      • Byrne M
      • Savani BN
      • Mohty M
      • Nagler A.
      Peripheral blood stem cell versus bone marrow transplantation: A perspective from the Acute Leukemia Working Party of the European Society for Blood and Marrow Transplantation.
      ]. GVL effects are generally stronger with PBSCs than BM because of the relatively higher number of infused donor-derived cytotoxic T cells at transplantation. Such enhanced GVL effects can reduce the incidence of relapse most effectively in patient subgroups in which the incidence of post-transplant relapse is relatively low [
      • Kolb HJ.
      Graft-versus-leukemia effects of transplantation and donor lymphocytes.
      ,
      • Nakamura M
      • Arai Y
      • Hirabayashi S
      • et al.
      Residual disease is a strong prognostic marker in patients with acute lymphoblastic leukaemia with chemotherapy-refractory or relapsed disease prior to allogeneic stem cell transplantation.
      ]. By contrast, it is difficult to observe GVL effects when HSCT is performed in patients with advanced-stage disease without remission and/or with more unfavorable conditions after long-term, repetitive, intensive chemotherapy as well as infectious episodes [
      • Yeshurun M
      • Weisdorf D
      • Rowe JM
      • et al.
      The impact of the graft-versus-leukemia effect on survival in acute lymphoblastic leukemia.
      ]. However, the authors were still able to observe tendencies for improved outcomes with PBSCs in those transplanted at CR2, although not in those at ≥CR3 or non-CR.
      In addition, positive effects of PBSCs were observed only in subgroups without the use of ATG. Here the addition of ATG was associated with a reduced risk of aGVHD in BM recipients (see supplementary Table 3), and this observation was compatible with previous reports [
      • Bacigalupo A
      • Lamparelli T
      • Bruzzi P
      • et al.
      Antithymocyte globulin for graft-versus-host disease prophylaxis in transplants from unrelated donors: 2 randomized studies from Gruppo Italiano Trapianti Midollo Osseo (GITMO).
      ,
      • Bacigalupo A
      • Lamparelli T
      • Barisione G
      • et al.
      Thymoglobulin prevents chronic graft-versus-host disease, chronic lung dysfunction, and late transplant-related mortality: long-term follow-up of a randomized trial in patients undergoing unrelated donor transplantation.
      ]. Administration of ATG with PBSCs can reduce both aGVHD and cGVHD [
      • Finke J
      • Bethge WA
      • Schmoor C
      • et al.
      Standard graft-versus-host disease prophylaxis with or without anti-T-cell globulin in haematopoietic cell transplantation from matched unrelated donors: a randomised, open-label, multicentre phase 3 trial.
      ,
      • Wang Y
      • Fu HX
      • Liu DH
      • et al.
      Influence of two different doses of antithymocyte globulin in patients with standard-risk disease following haploidentical transplantation: a randomized trial.
      ,
      • Kroger N
      • Solano C
      • Wolschke C
      • et al.
      Antilymphocyte Globulin for Prevention of Chronic Graft-versus-Host Disease.
      ,
      • Walker I
      • Panzarella T
      • Couban S
      • et al.
      Addition of anti-thymocyte globulin to standard graft-versus-host disease prophylaxis versus standard treatment alone in patients with haematological malignancies undergoing transplantation from unrelated donors: final analysis of a randomised, open-label, multicentre, phase 3 trial.
      ], but at the same time, it can also induce severe infections and post-transplant lymphoproliferative disorders and increase the incidence of relapse by reducing GVL effects [
      • Finke J
      • Bethge WA
      • Schmoor C
      • et al.
      Standard graft-versus-host disease prophylaxis with or without anti-T-cell globulin in haematopoietic cell transplantation from matched unrelated donors: a randomised, open-label, multicentre phase 3 trial.
      ,
      • Wang Y
      • Fu HX
      • Liu DH
      • et al.
      Influence of two different doses of antithymocyte globulin in patients with standard-risk disease following haploidentical transplantation: a randomized trial.
      ,
      • Ofran Y
      • Beohou E
      • Labopin M
      • et al.
      Anti-thymocyte globulin for graft-versus-host disease prophylaxis in patients with intermediate- or high-risk acute myeloid leukaemia undergoing reduced-intensity conditioning allogeneic stem cell transplantation in first complete remission—a survey on behalf of the Acute Leukaemia Working Party of the European Society for Blood and Marrow Transplantation.
      ]. As a result, overall, ATG does not always improve NRM or survival [
      • Finke J
      • Bethge WA
      • Schmoor C
      • et al.
      Standard graft-versus-host disease prophylaxis with or without anti-T-cell globulin in haematopoietic cell transplantation from matched unrelated donors: a randomised, open-label, multicentre phase 3 trial.
      ,
      • Kroger N
      • Solano C
      • Wolschke C
      • et al.
      Antilymphocyte Globulin for Prevention of Chronic Graft-versus-Host Disease.
      ,
      • Socie G
      • Schmoor C
      • Bethge WA
      • et al.
      Chronic graft-versus-host disease: long-term results from a randomized trial on graft-versus-host disease prophylaxis with or without anti-T-cell globulin ATG-Fresenius.
      ,
      • Nagler A
      • Labopin M
      • Dholaria B
      • et al.
      Impact of antithymocyte globulin on outcomes of allogeneic hematopoietic cell transplantation with TBI.
      ] and can even reduce OS [
      • Soiffer RJ
      • Kim HT
      • McGuirk J
      • et al.
      Prospective, Randomized, Double-Blind, Phase III Clinical Trial of Anti-T-Lymphocyte Globulin to Assess Impact on Chronic Graft-Versus-Host Disease-Free Survival in Patients Undergoing HLA-Matched Unrelated Myeloablative Hematopoietic Cell Transplantation.
      ]. The authors’ study suggests that these negative impacts of ATG can be observed more in PBSC patients than in BM patients. Differences in the effects of ATG on cGVHD between previous randomized studies and this study were at least partly due to differences in the types of ATG preparations administered as well as the doses and ethnicities involved. In addition, this study included various transplantation protocols with different indications for and administration schedules of ATG in different centers. Although among PBSC recipients the proportion of patients who received ATG was higher in recipients transplanted from HLA-mismatched donors than in those transplanted from HLA-matched donors, ATG use did not improve OS in either the HLA-matched or -mismatched group (see supplementary Figure 5; see supplementary Table 1), suggesting that the negative effects of ATG observed in the PBSC group may not be attributable solely to the HLA disparity. Given that a larger proportion of PBSC recipients in the authors’ cohort underwent transplantation with ATG than BM recipients (Table 1; also see supplementary Table 5), wider use of ATG in PBSC recipients may offset the benefits of ATG. Indeed, a previous study reported that absolute lymphocyte count before admininistration of ATG can affect the incidence of cGVHD and relapse after unrelated HSCT from PBSC donors [
      • Shiratori S
      • Sugita J
      • Fuji S
      • et al.
      Low-dose antithymocyte globulin inhibits chronic graft-versus-host disease in peripheral blood stem cell transplantation from unrelated donors.
      ]. Therefore, more refined indications as well as optimal ATG administration protocols for GVHD prophylaxis in unrelated PBSC transplantation should be determined in future studies.
      None of the patient subgroups were related to the graft source in the authors’ study. It is especially notable that HLA status (matched versus mismatched) and conditioning regimen (myeloablative versus reduced intensity) did not interact with the graft source with respect to outcomes, which is in agreement with previous reports [
      • Nagler A
      • Labopin M
      • Shimoni A
      • et al.
      Mobilized peripheral blood stem cells compared with bone marrow as the stem cell source for unrelated donor allogeneic transplantation with reduced-intensity conditioning in patients with acute myeloid leukemia in complete remission: an analysis from the Acute Leukemia Working Party of the European Group for Blood and Marrow Transplantation.
      ,
      • Fuji S
      • Miyamura K
      • Kanda Y
      • et al.
      Short-term clinical outcomes after HLA 1-locus mismatched uPBSCT are similar to that after HLA-matched uPBSCT and uBMT.
      ]. Thus, the authors’ data support the use of unrelated PBSC grafts for adult AML patients irrespective of HLA status and conditioning regimen.
      Although the strengths of the study include its restriction to a single disease (AML) and detailed patient subgroup analyses using real-world data in which graft source selection is not skewed toward PBSCs, limitations of the study should be acknowledged. First, this was a retrospective multicenter registry study, and various protocols were used in different centers. Therefore, pre-transplant patient characteristics could not be completely adjusted between the PBSC and BM groups even though the authors utilized multivariate and subgroup analyses along with propensity score matching. Second, the effects of dose and branch of ATG used or of minimal residual disease on transplantation outcomes were not evaluated because of lack of information. Third, given the relatively short follow-up period of survivors in this study, further study of the long-term effects of PBSCs is required. Fourth, since ethnicity affects the incidence and severity of GVHD [
      • Sasazuki T
      • Juji T
      • Morishima Y
      • et al.
      Effect of matching of class I HLA alleles on clinical outcome after transplantation of hematopoietic stem cells from an unrelated donor. Japan Marrow Donor Program.
      ], the authors’ conclusions based on a Japanese cohort should be validated in other ethnic groups. Fifth, grading of cGVHD in this study was performed using conventional criteria [
      • Sullivan KM
      • Agura E
      • Anasetti C
      • et al.
      Chronic graft-versus-host disease and other late complications of bone marrow transplantation.
      ] because of frequent missing values with regard to National Institutes of Health criteria [
      • Jagasia MH
      • Greinix HT
      • Arora M
      • et al.
      National Institutes of Health Consensus Development Project on Criteria for Clinical Trials in Chronic Graft-versus-Host Disease: I. The 2014 Diagnosis and Staging Working Group report.
      ].

      Conclusions

      Outcomes after unrelated HSCT from PBSC donors for adult patients with AML indicate a trend toward favorable NRM as well as OS and DFS, which is comparable to that of BM recipients. Although the rate of cGVHD was significantly higher with PBSCs, NRM was not increased, primarily because of the reduced risk of bacterial infection following the more robust engraftment in PBSC recipients. These tendencies were prominent in patient subgroups of HSCT at CR1, suggesting that PBSCs are the preferred source in these situations. The authors did not find any subgroup of patients for which BM transplants are advantageous. Therefore, the authors’ study supports the recent preferred donor selection of PBSCs in unrelated allo-HSCT in AML. Further optimization of the prophylactic strategy for cGVHD is required to improve outcomes after allo-HSCT from PBSC donors.

      Funding

      This work was supported in part by Program for the Development of Next-Generation Leading Scientists with Global Insight (L-INSIGHT) sponsored by the Ministry of Education, Culture, Sports, Science and Technology to YA.

      Author Contributions

      Conception and design of the study: TJ, YArai, TK and MY. Acquisition of data: TJ, YArai, TK, MY, ND, TF, YO, YKatayama, YKanda, KF, KM, ST, MS, TA, MO, TI and YAtsuta. Analysis and interpretation of data: TJ, YA, TK, SM, SH, YI, JK and MY. Drafting or revising the manuscript: TJ and YArai. All authors have approved the final article.

      Declaration of Competing Interest

      The authors have no commercial, proprietary or financial interest in the products or companies described in this article.

      Acknowledgments

      The authors thank all physicians and data managers at the centers who contributed valuable data on transplantation to the Japanese Society for Transplantation and Cellular Therapy.

      Appendix. Supplementary materials

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