ARTICLE IN PRESS
Transplantation and Cellular Therapy 000 (2022) 16

Transplantation andCellular Therapy
journal homepage: www.tctjournal.org

Allogeneic  Adult

Effect of Cryopreservation in Unrelated Bone Marrow and Peripheral
Blood Stem Cell Transplantation in the Era of the COVID-19 Pandemic:
An Update from the Japan Marrow Donor Program
Yoshinobu Kanda1,19,*, Noriko Doki2, Minoru Kojima3, Shinichi Kako4, Masami Inoue5,19,
Naoyuki Uchida6,19, Yasushi Onishi7,19, Reiko Kamata8,19, Mika Kotaki2,19, Ryoji Kobayashi9,19,
Junji Tanaka10,19, Takahiro Fukuda3,19, Nobuharu Fujii11,19, Koichi Miyamura12,19,
Shin-Ichiro Mori13,19, Yasuo Mori14,19, Yasuo Morishima15,19, Hiromasa Yabe16,19,
Yoshiko Atsuta17,19, Yoshihisa Kodera18
1

Division of Hematology, Department of Medicine, Jichi Medical University, Tochigi, Japan
Hematology Division, Tokyo Metropolitan Cancer and Infectious Diseases Center, Komagome Hospital, Tokyo, Japan3
Department of Hematopoietic Stem Cell Transplantation, National Cancer Center Hospital, Tokyo, Japan
4
Division of Hematology, Department of Medicine, Jichi Medical University, Tochigi, Japan
5
Department of Hematology/Oncology, Osaka Women’s and Children’s Hospital, Osaka, Japan
6
Department of Hematology, Toranomon Hospital, Tokyo, Japan
7
Department of Hematology, Tohoku University Hospital, Sendai, Japan
8
Nonproﬁt Organization Blood Disorder Information and Support Service “Tsubasa”, Tokyo, Japan
9
Department of Hematology/Oncology for Children and Adolescents, Sapporo Hokuyu Hospital, Sapporo, Japan10
Department of Hematology, Tokyo Women's Medical University, Tokyo, Japan
11
Division of Blood Transfusion, Okayama University Hospital, Okayama, Japan
12
Department of Hematology, Japanese Red Cross Nagoya First Hospital, Nagoya, Japan
13
Hematology Department, St. Luke's International Hospital, Tokyo, Japan
14
Medicine and Biosystemic Science, Kyushu University Graduate School of Medical Sciences, Fukuoka, Japan
15
Department of Promotion for Blood and Marrow Transplantation, Aichi Medical University School of Medicine, Nagakute, Japan
16
Department of Innovative Medical Science, Tokai University School of Medicine, Isehara, Japan
17
Japanese Data Center for Hematopoietic Cell Transplantation, Aichi, Japan
18
Aichi Medical University School of Medicine, Aichi, Japan
19
Medical Committee of the Japan Marrow Donor Program, Tokyo, Japan
2

Article history:
Received 15 April 2022
Accepted 23 June 2022
Key Words:
Cryopreservation
COVID-19
Bone marrow transplantation
Peripheral blood stem celltransplantation
Japan Marrow Donor ProgramA B S T R A C T
During the COVID-19 pandemic, donor grafts are frequently cryopreserved to ensure that a graft is available
before starting a conditioning regimen. However, there have been conﬂicting reports on the effect of cryopreservation on transplantation outcomes. Also, the impact of cryopreservation may differ in bone marrow (BM) transplantation (BMT) and peripheral blood stem cell (PBSC) transplantation (PBSCT). In this retrospective study, we
analyzed the clinical data of both cryopreserved unrelated BMTs (n = 235) and PBSCTs (n = 118) and comparedthese with data from a large control cohort without cryopreservation including 4133 BMTs and 720 PBSCTs.
Among the patients with cryopreserved grafts, 10 BMT recipients (4.3%) and 3 PBSCT recipients (2.5%) did notachieve neutrophil engraftment after transplantation, including 4 of the former and all 3 of the latter who diedearly before engraftment. In a multivariate analysis, cryopreservation was not associated with neutrophil engraftment in BMT but signiﬁcantly delayed neutrophil engraftment in PBSCT (hazard ratio [HR], .82; 95% conﬁdence
interval [CI], .69 to .97; P = .023). There was an interaction with borderline signiﬁcance between cryopreservationand the stem cell source (P = .067). Platelet engraftment was delayed by cryopreservation after both BMT and
PBSCT. Only 2 cryopreserved grafts (<1%) were unused during the study period. The cryopreservation of unrelateddonor BM and PBSC grafts is associated with a slight delay in neutrophil and platelet engraftment but an acceptable rate of graft failure. PBSC grafts may be more sensitive to cryopreservation than BM grafts. Cryopreservation

Financial disclosure: See Acknowledgments on page XXX.
*Correspondence and reprint requests: Yoshinobu Kanda, Division of Hematology, Department of Medicine, Jichi Medical University, 3311-1 Yakushiji, Shimotsukecity, Tochigi, 329-0498, Japan
E-mail address: ycanda-tky@umin.ac.jp (Y. Kanda).

https://doi.org/10.1016/j.jtct.2022.06.022
2666-6367/© 2022 The American Society for Transplantation and Cellular Therapy. Published by Elsevier Inc. All rights reserved.

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Y. Kanda et al. / Transplantation and Cellular Therapy 00 (2022) 16

is a reasonable option during COVID-19 pandemic, provided that the apheresis and transplantation centers are
adept at cryopreservation.
Ó 2022 American Society for Transplantation and Cellular Therapy. Published by Elsevier Inc.
© 2022 The American Society for Transplantation and Cellular Therapy. Published by Elsevier Inc. All rights
reserved.

INTRODUCTION
During the COVID-19 pandemic, the Japan Marrow Donor
Program (JMDP) is allowing the cryopreservation of donor
grafts as an exception to ensure that grafts are available before
starting a conditioning regimen. Similarly, the National Marrow Donor Program (NMDP) is temporarily requiring thattransplantation centers plan cryopreservation of unrelated
and related donor products facilitated by the NMDP [1].
Although previous studies have shown that the cryopreservation of allogeneic donor grafts is both safe and effective [26],others have raised concerns regarding the deleterious effectsof the cryopreservation of donor cells [79].
We previously reported that the cryopreservation of unrelated bone marrow (BM) grafts did not affect neutrophil
engraftment irrespective of the time from stem cell harvest to
cryopreservation [10]. However, the safety of cryopreservationof unrelated peripheral blood stem cell (PBSC) grafts was notanalyzed in detail, owing to the small number of patients.
Since then, we have accumulated 3 times more clinical data on
the cryopreservation of both unrelated BM and PBSC grafts
and reanalyzed the results of neutrophil and platelet engraftment, including a comparison with the data in a large control
cohort without cryopreservation.

METHODS
Patients
The policy for the cryopreservation of unrelated donor graft has been
described previously. In brief, the JMDP Central Ofﬁce reviewed all requests
for cryopreservation of stem cells. After approval, stem cells were harvested
and shipped from harvest centers to transplantation centers and then cryopreserved at the transplantation centers. Clinical data on unrelated cryopreserved BMTs and PBSCTs performed between April 2020 and October 2021
were collected by questionnaires sent to the transplantation centers. Control
data from a cohort of recipients of noncryopreserved BMTs and PBSCTs performed between January 2016 and December 2018 were provided by the Japanese Data Center for Hematopoietic Cell Transplantation. This study wasapproved by the Ethics Committee of the JMDP.

Statistical Analysis
The primary endpoint was neutrophil engraftment, deﬁned as the ﬁrst of
3 consecutive days with an absolute neutrophil count of at least .5 £ 103/mL,
and the secondary endpoint was platelet engraftment, deﬁned as the ﬁrst
day with a platelet count >20 £ 103/mL without platelet transfusion for atleast 7 days. Fisher’s exact test was used to compare categorical variables,
and the Mann-Whitney U test was used to compare continuous variables.
Time to engraftment data were analyzed while treating death without
engraftment as a competing risk and then compared between the groups
using Gray’s test. Multivariate analysis was performed using Fine-Gray proportional hazards modeling based on an available case analysis for missing
data. Information on the use of granulocyte colony-stimulating factor (G-CSF)
was not obtained, and thus background diseases were grouped into myeloid
malignancies and others; this was also included as an independent variable
as a substitute for the use of G-CSF, because G-CSF was not used for myeloid
malignancies in some centers.
All P values were 2-sided, and a P value <.05 was considered to indicate statistical signiﬁcance. All statistical analyses were performed with
EZR version 1.55 (Jichi Medical University Saitama Medical Center, Saitama, Japan) [11].

RESULTS
Patients and Stem Cell Grafts
During the study period, 242 of 1342 (18.0%) unrelated
BMTs and 118 of 435 (27.1%) of unrelated PBSCTs facilitated by the JMDP were performed using cryopreserved
grafts. Two other BM grafts were cryopreserved but notinfused owing to patient death and a freezer problem.
Questionnaires were sent to transplantation centers regarding the total 360 unrelated transplantations, and clinicaldata for 235 recipients of cryopreserved BM grafts and 118
recipients of cryopreserved PBSC grafts were collected. The
median age of the BMT recipients was 50.0 years (interquartile range [IQR], 36.0 to 61.0 years), and that of PBSCT
recipients was 53.0 years (IQR, 42.25 to 61.75 years)(Table 1). There was an HLA mismatch in approximately
40% of the transplantations.
In the noncryopreservation cohort, 4133 patients
received BM grafts and 720 received PBSC grafts, excluding
7 patients without engraftment data. The median age ofTable 1
Patient Characteristics
Characteristic

BMT

PBSCT

No. of patients
Age, yr, median (IQR)
Disease, n (%)

235
50.0 (36.0-61.0)
38 (16.2)
75 (31.9)
10 (4.3)
6 (2.6)58 (24.7)
17 (7.2)
10 (4.3)
21 (8.9)120 (51.1)
115 (48.9)
149 (63.4)
86 (36.6)
131 (55.7)
104 (44.3)

118
53.0 (42.25-61.75)
20 (16.9)
48 (40.7)
3 (2.5)6 (5.1)25 (21.2)
10 (8.5)
4 (3.4)2 (1.7)71 (60.2)
47 (39.8)
83 (70.3)
35 (29.7)
72 (61.0)
46 (39.0)

Disease status, n (%)
Myeloid malignancies, n (%)
HLA mismatch, n (%)

ALL
AML
ATL
CML
MDS
ML-CLL-MM
MPN
No malignancy
CR
NR
Myeloid
OthersNo
Yes

ALL indicates acute lymphoblastic leukemia; AML, acute myelogenous leukemia; ATL, adult T cell leukemia/lymphoma; CML, chronic myelogenous leukemia; MDS,
myelodysplastic syndrome; ML-CLL-MM, malignant lymphoma, chronic lymphocytic leukemia, multiple myeloma; MPN, myeloproliferative neoplasms; CR, complete
remission; NR, not in remission.

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Y. Kanda et al. / Transplantation and Cellular Therapy 00 (2022) 16

the patients who underwent BMT was 50.0 years (IQR, 34.0
to 60.0 years) and that of PBSCT recipients was 53.0 years(IQR, 42.0 to 61.0 years).
Outcomes of Cryopreserved Graft RecipientsTen (4.3%) BMT recipients and 3 (2.5%) PBSCT recipients did
not achieve neutrophil engraftment. Of these, 4 of the former
and 3 of the latter died early before engraftment. The cause ofdeath was infection in 4 patients and progression of underlying malignancy, diffuse alveolar hemorrhage, and multiorgan
failure after early rescue transplantation on day 16 in 1 patienteach. The other 6 patients who did not achieve neutrophil
engraftment were considered to have graft failure, and none
were in remission before transplantation. In 4 of thesepatients, graft failure was attributed to persistent hematologic
malignancy. One patient showed hematologic recovery, but
chimerism analysis revealed autologous hematopoiesis. The
cause of graft failure in the remaining patient was considered
to be hemophagocytic lymphohistiocytosis.
The median time to neutrophil engraftment was 18 days
(95% conﬁdence interval [CI], 17 to 19 days) after cryopreserved BMT and 16 days (95% CI, 15 to 17 days) after cryopreserved PBSCT. The incidence of neutrophil engraftment at day+28 in the 2 groups was 93.6% and 94.9%, respectively. The
median days to platelet engraftment was 34 days (95% CI, 32
to 35 days) after cryopreserved BMT and 26 days (95% CI, 23 to
28 days) after cryopreserved PBSCT. The incidence of platelet
engraftment at day +28 was 32.3% and 61.9%, respectively.
Data on the time between graft harvest and graft freezing
were available in patients who received cryopreserved grafts.
We grouped patients into the longest one-third, shortest onethird, and remaining patients. After cryopreserved BMT, neutrophil engraftment was slightly delayed in the shortest group3(median of 19, 18, and 18 days, respectively, in the 3 groups),
but the median time to platelet engraftment was equivalent
among the 3 groups (32, 33, and 32 days). After cryopreserved
PBSCT, the times to neutrophil and platelet engraftment were
not different among the 3 groups (median of 15, 15, and16 days for neutrophil engraftment and 30, 30, and 28 days forplatelet engraftment).
Comparison of Cryopreserved and Noncryopreserved Grafts
Characteristics of the patients who received cryopreserved
and noncryopreserved grafts are summarized in Table 2. There
were signiﬁcant between-group differences in background diseases and disease status. The numbers of harvested nuclear
cells and CD34+ cells did not differ between BM and PBSC
grafts. Univariate analysis showed no difference in neutrophil
engraftment between recipients of cryopreserved grafts andrecipients of noncryopreserved grafts in both BMT and PBSCT,
although there was a slight tendency toward delayed engraftment in the cryopreservation group in PBSCT recipients(Figure 1A,B). After adjustment for age, myeloid disease, disease status, and HLA mismatch by a multivariate analysis,
cryopreservation was not associated with neutrophil engraftment in BMT (hazard ratio [HR], .98; 95% CI .87 to 1.10;
P = .74). However, cryopreservation signiﬁcantly delayed neutrophil engraftment in PBSCT (HR, .82; 95% CI, .69 to .97;
P = .023) (Table 3). There was an interaction with borderline
signiﬁcance between cryopreservation and the stem cellsource (HR, .78; 95% CI, .60 to 1.02; P = .067).
Platelet engraftment was delayed by cryopreservation forboth BMT and PBSCT (Figure 2A,B). This result was conﬁrmed
by multivariate analyses adjusted for age, myeloid disease,disease status, and HLA mismatch (HR, .75; 95% CI, .66 to .87;
P < .0001 after BMT and HR, .74; 95% CI, .61 to .90; P = .0029

Table 2
Patient Characteristics Grouped According to the Use of Cryopreservation in BMT and PBSCT
Characteristic

Cryopreservation

BMT
No. of patients
Age, yr, median (IQR)
Disease, n (%)

Leukemia
Lymphoma/myeloma
MDS/MPN
No malignancy
CR
NR
OthersMyeloidNo
Yes

Disease status, n (%)
Myeloid malignancies, n (%)
HLA mismatch, n (%)
10Harvested nuclear cells, £ 10 , median (IQR)
PBSCT
No. of patients
Age, yr, median (IQR)
Disease, n (%)

Disease status, n (%)
Myeloid malignancies, n (%)
HLA mismatch, n (%)+

8Harvested CD34 cells, £ 10 , median (IQR)
SMD indicates standardized mean difference.

Leukemia
Lymphoma/myeloma
MDS/MPN
No malignancy
CR
NR
OthersMyeloid
No
Yes

No

Yes

4140
50.0 (34.0-60.0)2289 (55.3)
658 (15.9)
830 (20.0)
363 (8.8)1647 (39.8)
2493 (60.2)
1853 (44.8)
2287 (55.2)
2233 (54.0)
1900 (46.0)
13.9 (10.3-17.7)

235
50.0 (36.0-61.0)
119 (50.6)
27 (11.5)
68 (28.9)
21 (8.9)120 (51.1)
115 (48.9)
86 (36.6)
149 (63.4)
131 (55.7)
104 (44.3)
13.9 (10.0-17.7)

720
53.0 (42.0-61.0)426 (59.2)
113 (15.7)
176 (24.4)
5 (.7)
274 (38.1)
446 (61.9)
235 (32.6)
485 (67.4)
430 (59.9)
288 (40.1)
2.4 (1.5-3.7)

118
53.0 (42.3-61.8)
74 (62.7)
13 (11.0)
29 (24.6)
2 (1.7)71 (60.2)
47 (39.8)
35 (29.7)
83 (70.3)
72 (61.0)
46 (39.0)
2.7 (1.8-4.2)

P Value

SMD

.545
.009

.047
.226

.001

.228

.015

.167

.638

.034

.66

.017

.867
.326

.002
.164

<.001

.454

.595

.064

.84

.023

.11

.17

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Y. Kanda et al. / Transplantation and Cellular Therapy 00 (2022) 16

Figure 1. Time to neutrophil engraftment grouped according to the use of cryopreservation after BMT (A) and after PBSCT (B).

after PBSCT) (Table 3). There was no signiﬁcant interaction
between cryopreservation and the stem cell source (HR, .98;
95% CI, .74 to 1.30; P = .90).
DISCUSSION
The cryopreservation of BM and PBSC grafts ensures thatthey are available before the start of conditioning and has
been used frequently during the COVID-19 pandemic. There
has been some concern about stem cell damage from the procedure, but several studies have shown no signiﬁcant delay inhematopoietic recovery after transplantation of cryopreserved
stem cells [26]. On the other hand, a retrospective study
showed delayed neutrophil and platelet engraftment with theuse of cryopreserved PBSC grafts with no effect on overall survival [9]. Eapen et al. [7] reported that the use of cryopreserved
grafts was associated with increased graft failure and 1-year
mortality after transplantation for aplastic anemia, in which
two-thirds of the cases involved BM grafts and one-third
involved PBSC grafts. In addition, in a recent large-scale retrospective study by the Center for International Blood and Marrow Transplant Research (CIBMTR), the use of cryopreserved
graft signiﬁcantly delayed neutrophil and platelet engraftment
in PBSCT but not in BMT [8]. In unrelated PBSCT, overall survival was signiﬁcantly inferior with cryopreserved grafts;

however, more than one-half of the cases involving cryopreservation were due to the patient condition including changes
in disease status, reaction to conditioning regimen, and infection. Multivariate analysis revealed signiﬁcantly shorted survival in patients who received cryopreserved grafts because of
their condition; thus, the reasons for cryopreservation areimportant when comparing cryopreserved and noncryopreserved grafts.
In the era of the COVID-19 pandemic, donor grafts are
cryopreserved mainly to ensure their availability before thestart of the conditioning regimen. Devine et al. [12]recently compared the outcomes of patients who underwent cryopreserved BMT or PBSCT between March andAugust 2020 with those of recipients of noncryopreserved
BMT or PBSCT in 2019 using the CIBMTR database. Neutrophil and platelet engraftment were delayed after cryopreservation, but there were no signiﬁcant differences in theincidence of graft failure and overall mortality.
In the current study, cryopreservation was performed
exclusively to ensure graft availability. Similar to the recent
CIBMTR study, the time to engraftment was longer after cryopreservation, but the effect on neutrophil engraftment wasmore prominent in PBSCT than in BMT, although the incidenceof graft failure was not increased. In previous studies analyzing

Table 3
Multivariate Analyses for Neutrophil and Platelet Engraftment after Unrelated BMT and PBSCT
FactorBMT
Age
Not in remission
Myeloid malignancies
HLA mismatch
Cryopreservation
PBSCT
Age
Not in remission
Myeloid malignancies
HLA mismatch
Cryopreservation

Neutrophil Engraftment

Platelet Engraftment

HR (95% CI)

P ValueHR (95% CI)

P Value1.00 (1.00-1.00).83 (.78-.88).92 (.87-.98).88 (.84-.93).98 (.87-1.1).27<.0001.0092<.0001.74

1.00 (.99-1.00).76 (.72-.82).98 (.91-1.04).80 (.75-.86).75 (.66-.87)<.0001<.0001.48<.0001<.00011.00 (.99-1.00).90 (.79-1.02)
1.01 (.87-1.16)
1.02 (.9-1.16).82 (.69-.97)

.3
.11
.92
.75
.023

1.00 (.99-1.00).72 (.62-.83)
1.07 (.92-1.24).94 (.81-1.08).74 (.61- .9).11<.0001
.4
.38
.0029

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Y. Kanda et al. / Transplantation and Cellular Therapy 00 (2022) 16

5

Figure 2. Time to platelet engraftment grouped according to the use of cryopreservation after BMT (A) and after PBSCT (B).

the impact of cryopreservation separately in BMT and PBSCT, asimilar tendency toward a stronger effect in PBSCT has been
observed (Table 4) [8,13]. Therefore, PBSC grafts may be moresensitive to cryopreservation than BM grafts.
Another concern has been that the cryopreservation ofdonor grafts may increase the number of unused grafts. The
NMDP reported that 222 of 9294 products (2.4%) collectedfrom March 17, 2020, through June 30, 2021, were not infused
for a variety of reasons, including patient death, patient choice,poor product quality, clumps in the product, viability, apositive culture, and others [14]. However, the proportion ofnoninfused grafts decreased over time, likely because transplantation centers and apheresis centers became more adept
at cryopreservation during the study period. In fact, in theearlier report during the COVID-19 pandemic, transplantation
centers reported problems with 29% of the products, including
damage during transit, low cell dose, inadequate labeling,missing representative samples, and missing documentation,
which resulted in noninfused products in 22 of 191 (12%) collections [15]. On the other hand, only 2 cryopreserved grafts(<1%) were not infused in Japan during the study period. The
policy of the JMDP to strictly restrict cryopreservation of donor
grafts before the COVID-19 pandemic might have contributed
to the high awareness in transplantation centers of the importance of limiting the number of unused grafts.
A major limitation of this study is the lack of clinical outcomes other than engraftment, such as the incidences of
GVHD and nonrelapse mortality. In previous studies, the effect

Table 4
Summary of Studies Comparing Cryopreserved and Noncryopreserved Grafts with Regard to Clinical Outcomes [4-9,12,13,17-24]
Authors

Year

SourceCryopreserved Noncryopreserved Neutrophil
Engraftment

PlateletEngraftment

Acute GVHD

Chronic GVHD

Eckardt et al. [17]Stockschl€
ader et al. [4]Kim et al. [5]Lioznov et al. [13]Lioznov et al. [13]Medd et al. [9]Parody et al. [18]Dagdas et al. [19]Eapen et al. [7],*

1993
1997
2007
2008
2008
2013
2013
2020
2020

BM
BM
PB
PB
BM
PB
PB
PB
BM or PB

10
40
105
31
8
76
224
30
52

33
40
106
493
14
123
107
42
194

No difference
No difference
No difference
ND
ND
DelayedNo difference
No difference
No difference

Decreased
No difference
No difference
ND
No difference
No difference
Increased
No difference
No difference

ND
No difference
No difference
ND
ND
Increased
No difference
No difference
No difference

Alotaibi et al. [20]Hamadani et al. [6]**Fernandez-Sojo et al. [21]Valentini et al. [22]Maurer et al. [23]Hsu et al. [8]Hsu et al. [8]Hsu et al. [8]Novitzky-Basso et al. [24]Devine et al. [12]Current study
Current study

2021
2020
2021
2021
2021
2021
2021
2021
2022
2021ASH
2022
2022

PB
BM or PB
PB
PB
PB
Related PB
Unrelated PB
BMPB
BM or PB
PB
BM

310
274
32
32
101
1051
678
154
135
959
118
235

648
1080
32
106
203
3030
2028
456
348
2499
720
4133

No difference
No difference
No difference
No difference
DelayedDelayedDelayedDelayedNo difference
DelayedDelayedDelayedNo difference
No difference
No difference
No difference
Increased
Increased
No difference
No difference
No difference
ND
ND
ND

Increased
Decreased
ND
No difference
ND
ND
ND
ND
Decreased
ND
ND
ND

No difference
No difference
No difference
Increased graft failure
No difference
DelayedFasterDelayedIncreased 1-yr
graft failure
No difference
No difference
No difference
No difference
No difference
No difference
DelayedNo difference
DelayedDelayedDelayedNo difference

ND indicates not described.
* Transplantation only for aplastic anemia.yAll used post-transplantation cyclophosphamide.

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Y. Kanda et al. / Transplantation and Cellular Therapy 00 (2022) 16of cryopreservation on the incidences of acute and chronic
GVHD has been inconsistent, but several recent studies of cryopreserved PBSCT have reported an increased incidence ofacute GVHD compared with noncryopreserved PBSCT (Table 4).
In addition, data on the viability of graft cells before and after
cryopreservation were not available in this study. In this study,
we observed a stronger effect of cryopreservation on PBSC
grafts compared with BM grafts. A possible explanation forthis difference might be the difference in time between graft
collection and cryopreservation. When apheresis for 2 days
was required to collect a sufﬁcient number of CD34+ cells, cells
collected on the ﬁrst day may be transferred together withthose collected on the second day, leading to a longer time
between collection and freezing for the graft collected on theﬁrst day. However, the time from collection to freezing was
not associated with time to engraftment in the current cohort.
The different effects of cryopreservation between BMT and
PBSCT were observed only for neutrophil engraftment and not
for platelet engraftment; therefore, the difference might bedue to the sensitivity of the mobilized myeloid progenitor
cells. Viability data for graft cells in each lineage before andafter cryopreservation are needed to further clarify the difference in sensitivity between BM and PBSC grafts. Another limitation of this study is the lack of information on infectious
events before neutrophil engraftment, which might haveaffected the time to neutrophil engraftment. However, previous studies including ours have shown an association between
high-risk malignant disease and a higher incidence of bloodstream infection before engraftment [16], and in the current
study, cryopreserved graft recipients were signiﬁcantly more
frequently in remission before transplantation. Thus, it isunlikely that an infectious event before engraftment was the
major reason for the delayed engraftment after transplantationof cryopreserved grafts.
In conclusion, the cryopreservation of unrelated donor BM
and PBSC grafts is associated with slight delays in neutrophil
and platelet engraftment but with an acceptable rate of graft
failure. Cryopreservation is a reasonable option in the era of
the COVID-19 pandemic, provided that the apheresis andtransplantation centers are adept at cryopreservation. Further
analyses are warranted when the data on clinical outcomes,
including the incidence of GVHD and nonrelapse mortality,
become available.
Conﬂict of interest statement: There are no conﬂicts of interest to report.

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