RADIATION RESEARCH 185, 69–76 (2016)0033-7587/16 $15.00
Ó2016 by Radiation Research Society.
All rights of reproduction in any form reserved.
DOI: 10.1667/RR14209.1

Circulating Hematopoietic Stem and Progenitor Cells in Aging Atomic
Bomb Survivors
Seishi Kyoizumi,a,1 Yoshiko Kubo,a Munechika Misumi,b Junko Kajimura,a Kengo Yoshida,a Tomonori Hayashi,aKazue Imai,a Waka Ohishi,c Kei Nakachi,a Lauren F. Young,d Jae-Hung Shieh,e Malcolm A. Moore,e Marcel R. M.
van den Brinkd and Yoichiro Kusunokia,1Departments of a Radiobiology/Molecular Epidemiology, b Statistics and c Clinical Studies, Radiation Effects Research Foundation, Hiroshima, Japan;
and d Departments of Medicine and Immunology and e Cell Biology Program, Memorial Sloan-Kettering Cancer Center, New York, New YorkINTRODUCTION
Kyoizumi, S., Kubo, Y., Misumi, M., Kajimura, J., Yoshida,K., Hayashi, T., Imai, K., Ohishi, W., Nakachi, K., Young, L.
F., Shieh, J-H., Moore, M. A., van den Brink, M. R. M and
Kusunoki, Y. Circulating Hematopoietic Stem and Progenitor
Cells in Aging Atomic Bomb Survivors. Radiat. Res. 185, 69–
76 (2016).

Hematopoietic functions in atomic bomb (A-bomb)
survivors are variably damaged according to the extent ofionizing radiation exposure (1). The initial damage is mostobvious in terms of pronounced blood cytopenias that result
from radiation-induced death of hematopoietic stem andprogenitor cells (HSPCs) (2, 3) and gene mutations in longlived hematopoietic stem cells (HSCs) (4–8). Several
months after irradiation, the hematopoietic system insurvivors nearly recovers from the damage (2, 9). However,even more than 60 years after irradiation the A-bomb
survivors’ proportion of naı̈ve T cells in peripheral blood
lymphocytes is decreased in association with age andradiation exposure dose (10–12), whereas the number ofwhite blood cells (WBCs), especially neutrophils, increases
with dose (13). With advanced age, lymphopoiesis tends todecline, whereas myelopoiesis generally increases (14, 15).
Thus, exposure to A-bomb radiation may accelerate the ageassociated shift toward myeloid-dominant hematopoiesis.
Furthermore, enhanced myelopoiesis may also be involvedin age- and radiation-associated increases in inflammatory
responses with a corresponding attenuation of adaptive
immunity in A-bomb survivors (12, 16). However, whether
aging, in combination with prior radiation exposure,significantly impacts the structure and function of vital
HSPC compartments within the hematolymphoid system is
unknown.
In the current study, we hypothesized that the effects of
A-bomb radiation induced premature aging of HSCs,
resulting in reduced numbers and impaired self-renewal
and lineage commitment that in turn accelerated loss oflymphoid potential and augmentation of myeloid potential.
To test this hypothesis, we performed various numerical andfunctional hematopoietic analyses of HSPCs circulating in
the peripheral blood obtained from A-bomb survivors.
These analyses included a cell sorter-based limiting-dilution
assay (LDA) using CD34-positive/lineage marker-negative(CD34 þLin) cells, which constitute the total HSPC
population.

It is not yet known whether hematopoietic stem andprogenitor cells (HSPCs) are compromised in the aging
population of atomic bomb (A-bomb) survivors after their
exposure nearly 70 years ago. To address this, we evaluatedage- and radiation-related changes in different subtypes ofcirculating HSPCs among the CD34-positive/lineage markernegative (CD34+Lin) cell population in 231 Hiroshima Abomb survivors. We enumerated functional HSPC subtypes,
including: cobblestone area-forming cells; long-term cultureinitiating cells; erythroid burst-forming units; granulocyte andmacrophage colony-forming units; and T-cell and naturalkiller cell progenitors using cell culture. We obtained the countof each HSPC subtype per unit volume of blood and the
proportion of each HSPC subtype in CD34+Lin cells torepresent the lineage commitment trend. Multivariate analyses, using sex, age and radiation dose as variables, showedsignificantly decreased counts with age in the total CD34+Lin
cell population and all HSPC subtypes. As for the proportion,
only T-cell progenitors decreased significantly with age,suggesting that the commitment to the T-cell lineage in HSPCs
continuously declines with age throughout the lifetime.
However, neither the CD34+Lin cell population, nor HSPC
subtypes showed significant radiation-induced dose-dependentchanges in counts or proportions. Moreover, the correlations of
the proportions among HSPC subtypes in the survivors
properly revealed the hierarchy of lineage commitments.
Taken together, our findings suggest that many years after
exposure to radiation and with advancing age, the number andfunction of HSPCs in living survivors as a whole may haverecovered to normal levels. Ó 2016 by Radiation Research Society

1 Address for correspondence: Radiation Effects Research Foundation, Department of Radiobiology/Molecular Epidemiology, 5-2
Hijiyamakouen, Minami-ku, Hiroshima, Hiroshima 732-0815;
emails: kyoizumi@rerf.or.jp and ykusunok@rerf.or.jp.

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KYOIZUMI ET AL.

TABLE 1
Characteristics of Study Participants
Age at blood collection (years)b
Males (n ¼ 81)
DS02 dose (Gy),0.005a0.005–0.5
0.5–1.0
1.0þ
Total

Females (n ¼ 150)

66–73

74–80

81–91

66–73

74–80

81–91

Total

9
6
4
11
30

8
6
2
4
20

17
5
4
5
31

16
9
6
8
39

18
9
6
8
41

33
13
16
8
70

101
48
38
44
231

a

Study participants in this dose category were exposed at distances in excess of 3 km from the hypocenter and therefore received doses that arefunctionally equivalent to zero.
b
Age at the time of the examinations, which were conducted between July 2011 and December 2013 (mean age: 78 years).

For analyses of T cell and natural killer (NK) cellpotential, we performed previously established in vitro
functional and quantitative assays of circulating T-cell andNK-cell precursors among CD34þLin cells with LDA byco-culturing these cells with OP9-DL1 stromal cells
expressing the Notch 1 ligand, Delta-like 1, in a 384-wellplate (17, 18). The surface phenotype of the NK-cellprogeny generated in the culture represented CD56hi
CD127þCD16 thymus-derived (thymic) NK cells. The T
cell, but not NK cell, progenitor frequency in CD34þLincells significantly decreased with donor age in the analysis
of in-house volunteers. In the current study, we also usedcell sorter-based LDA to quantify cobblestone area-forming
cells (CAFCs) and long-term culture-initiating cells (LTCICs) generated from CD34þLin cells using co-culture with
MS5 stromal cells (19–21). These two HSPC subtypes are
believed to be in vitro surrogate parameters that reflect selfrenewal and the multilineage differentiation ability of HSCs.
Furthermore, we also quantified myeloid- or erythroidcommitted progenitors in peripheral blood HSPCs by
performing granulocyte-macrophage colony-forming unit
(CFU-GM) and erythroid burst-forming unit (BFU-E)
assays using conventional methylcellulose culture. We
evaluated age- and radiation-related changes in these
HSPCs in A-bomb survivors using multivariate analysis.
MATERIALS AND METHODS
Study Population
Study participants were selected from an A-bomb survivor cohort of
1,705 Hiroshima survivors participating in the Adult Health Study at
the Radiation Effects Research Foundation (RERF) in January 2011.
The selected study population consisted of two groups of Hiroshima
survivors. The first group was exposed to significant doses of 0.005
Gy or more because of their location within 2 km of the hypocenter;the second group was exposed at a distance in excess of 3 km from the
hypocenter and as a result would have received doses of less than
0.005 Gy. Of these 1,705 participants, 1,310 participants were selected
after exclusion of those with disease and/or treatment histories, as
follows. First, 223 participants were unexceptionally excluded becauseof histories that included infectious diseases, autoimmune diseases,
immunodeficiency, hematologic diseases, hematologic malignancies
and treatments with radiation, interferon, erythropoietin or a steroid.
Next, 172 participants were excluded because of solid cancers thatwere diagnosed less than 5 years prior to examination and solidcancers and other diseases that were treated with therapies that affectthe immune system, after close investigation with a medical doctor. Of
the remaining 1,310 individuals, 1,186 participants who were younger
than 90 years of age as of June 30, 2011 were selected. Of these 1,186
participants, a total of 245 individuals were selected using a stratified
random sampling method for sex, radiation dose and age category
strata. We used the bone marrow dose as the radiation dose, which isthe sum of the gamma-ray dose plus 10 times the neutron dose
calculated by the recently revised dosimetry system, DS02 (22–25). A
sufficient volume of blood was obtained from the 245 individuals andexamined for hematopoietic functions between July 2011 andDecember 2013. The results of 14 of these participants were excluded
from the data analysis due to an update of their disease data after bloodcollection. Finally, data for the proportion of CD34þLin cells in
PBMCs were obtained for 231 participants (Table 1).
Ethics Statement
This study was approved by the Human Investigation Committee of
the RERF and was conducted according to the principles expressed in
the Declaration of Helsinki. All participants provided written informed
consent before examination.
Cytokines and Antibodies
Recombinant human cKit ligand (KL), Flt3 ligand (FL), interleukin(IL)-3, IL-7 and granulocyte-colony stimulating factor (G-CSF) werepurchased from PeproTecht (Rocky Hill, NJ). Erythropoietin waspurchased from Invitrogene (Carlsbad, CA). Anti-CD3 (clone
number: SK7), CD5 (UCHT2), CD7 (M-T701), CD16 (3G8), CD19
(HIB19), CD20 (2H7) and glycophorin A (GA-R2) monoclonal
antibodies (mAbs) were purchased from BD Biosciences (San Jose,
CA). Anti- CD34 (581 and CD56 (N901) mAbs were purchased from
Beckman Coulter Inc. (Brea, CA). mAbs were used for cell surface
staining according to the manufacturer’s instructions.
Cell Preparation
Peripheral blood samples were collected from 231 adult healthstudy participants and six in-house volunteers. PBMCs were separated
from approximately 7.5 ml of peripheral blood using Ficoll density
gradient centrifugation (Lymphocyte Separation Medium 1077, Wako
Pure Chemical Industries Ltd., Osaka, Japan).
Stromal Cell Lines
Mouse stromal cells (MS-5) were kindly provided by Dr. Kazuhiro
J. Mori (Niigata University, Niigata, Japan) (26). Generation of themouse OP9-DL1 stromal cells, which were engineered to express the

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HEMATOPOIESIS IN A-BOMB SURVIVORS

green fluorescent protein and mouse delta-like 1, has been previously
described elsewhere (27). The MS-5 and OP9-DL1 stromal cells weremaintained by culturing in alpha Minimal Essential Medium (MEM;
Gibcot; Life Technologies, Grand Island, NY) supplemented with 4 3
106 M 2-mercaptoethanol, penicillin-streptomycin and 10 and 20%
fetal bovine serum (FBS; GE Healthcare Life Sciences/HyClonee
Laboratories, Logan, UT), respectively, at 378C in a humidified
atmosphere with 5% CO2.
LDA of CAFCs and LTC-ICs
For progenitor cell culture, MS-5 stromal cells were seeded in wells(50–80% confluent) of a 384-well flat-bottom black plate (Corning).
At least 4 h prior to progenitor cell sorting, culture media in each well
was replaced with 50 ll alpha MEM supplemented with 4 3 106 M 2mercaptoethanol, penicillin-streptomycin, 12.5% horse serum (Gibco)
and 12.5% FBS (GE Healthcare Life Sciences/HyClone). Forprogenitor cell sorting, PBMCs were stained with allophycocyanin(APC)-conjugated anti-CD34 Ab and phycoerythrin (PE)-conjugatedmAbs against lineage markers (anti-CD3, CD14, CD16, CD19, CD20
and CD56 mAbs) for 30 min on ice. One thousand CD34þLin cells
were sorted into 80 wells of a 384-well plate at 20, 15, 10 and 5 cells
per well (20 wells for each number of cells) with FACSAriae II (BD
Biosciences). After cell sorting, the proportion of CD34þLin cells in
PBMCs was determined for further statistical analysis, as described
below. LDA culture was maintained at 378C in a humidified
atmosphere with 5% CO2. Half of the culture media (25 ll) waschanged every week. After five weeks of culture, individual wells
were microscopically screened for the presence or absence ofcobblestone areas. A well was considered positive when at least six
cells in proximity to each other were growing underneath the stroma.
Within a day after scoring CAFCs, the culture media was removed andreplaced with methylcellulose media as described above. The presenceof CFU-GMs and/or BFU-Es in individual wells was determined forscoring LTC-ICs after 14 days.
Progenitor frequencies of CAFCs and LTC-ICs were calculatedwith online analysis using ELDA software (Linz, Austria) (28), whichis available on the Bioinformatics Division home page of the Walterand Eliza Hall Institute of Medical Research (Parkville, Australia). We
confirmed the reproducibility of CAFC and LTC-IC assays using
Spearman’s rank correlation analysis between the first and second
measurements among six volunteers CAFC: q ¼ 0.83, P , 0.05; LTCIC: q ¼ 1.00, P , 0.01).
LDA of T/NK Progenitors
Procedures and culture conditions for the LDA of T/NK progenitors
were described previously (17). Briefly, for progenitor cell culture,
OP9-DL1 stromal cells were seeded in wells (50–80% confluent) of a384-well flat-bottom black plate. Culture media in each well was
replaced with 50 ll of phenol red-free alpha MEM containing 20%
knock-out serum replacement (Gibco), 104 M monothioglycerol(Sigma-Aldricht LLC, St. Louis, MO), 50 lg/ml gentamicin (SigmaAldrich), 10 ng/ml KL, 10 ng/ml FL and 10 ng/ml IL-7. Half of theculture media (25 ll) was changed every week. After five weeks ofculture, all cells grown in wells were harvested by vigorous pipetting
and washed with phosphate buffered saline (PBS) containing 2 mM
EDTA, 0.01% NaN3 and 1% FBS (washing buffer). For detection of
T/NK and myeloid lineage progeny, cells were stained with APCconjugated CD5, PE-conjugated CD7 and PE-Cy7-conjugated CD56
mAbs in washing buffer, as described previously (17). Absolute
numbers of each progeny type per well were calculated from thenumber of events on a flow cytogram. A well with six or more positive
events of CD7þCD5þ and CD7þCD56þ cells was designated as apositive well for calculation of progenitor frequencies of T and NK
cells, respectively. High reproducibility of the T-cell and NK-cellassay was demonstrated in our previous report (17).

CFU-C Assay
CFU-GMs and BFU-Es were assayed with methylcellulose cultures
as previously described elsewhere (29). Briefly, PBMCs were plated
in 24-well plates at a concentration of 2.5 3 105 cells/ml in a 0.25 ml
culture containing 1.2% methylcellulose (STEMCELLe Technologies Inc., Vancouver, Canada) with erythropoietin (6 U/ml), KL (20ng/ml), G-CSF (20 ng/ml) and IL-3 (20 ng/ml). The methylcellulose
cultures were counted after 14 days to determine the number ofcolonies per well.
Blood Cell Counts
The number of WBCs per ll of blood and the percentages oflymphocytes and monocytes in WBCs for each participant were
measured with the automated hematology analyzer Sysmext XN1000e (Sysmex, Kobe, Japan). These values were used to calculate
the number of PBMCs per milliliter of blood and the actual numbersof CD34þLin cells and HSPC subtypes per milliliter of blood using
the following formula: Number of cells of HSPC subtypes per
milliliter of blood ¼ number of WBCs per microliter of blood[lymphocytes (%) þ monocytes (%)] 3 CD34þLin cells (%) in
PBMCs 3 HSPC subtypes (%) in CD34þLin cells 3 103.
Data Analysis
Statistical analyses were conducted on the actual calculated numberof HSPCs per unit volume of blood, designated as ‘‘count’’, as well as
the fractional component (percentage) of HSPCs within the
CD34þLin cell population, designated as ‘‘proportion’’. Associationsof the count and proportion with sex, age at the time of examination(age) and radiation dose (dose) were investigated using multivariate
linear regression. The possibility of nonlinear relationships of age anddose with each HSPC subtype was tested by including quadratic andcubic terms of age and radiation. As a result, only a linear term wasselected for both age and radiation with additional confirmation of
nonlinearity by comparing the parametric form of the regression curveto the nonparametric smoothing curves of residuals of the multivariate
regression models. When we did not find a significant association
between each HSPC subtype and radiation, we retained a linear term
for radiation in the model, and the association between each HSPC
subtype and other variables was evaluated. All tests were two-sided
and conducted using statistical package R (The R Foundation, http://cran.r-project.org). A partial correlation between the proportions of
two HSPC subtypes adjusted for sex, age and dose as controllingvariables was analyzed using SPSS 16.01 software (SPSSt, Chicago,IL).

RESULTSHSPC Counts and Proportions

Statistical analyses were conducted on the actual
calculated numbers of HSPCs per unit volume of blood,
designated ‘‘counts’’, to assess the association of age anddose with the total capacity of HSPCs to self-renew anddifferentiate (Table 2). Similar analyses were performed forthe fractional component of HSPCs within the CD34þLincell population, designated ‘‘proportions’’, to evaluate theage- and dose-related changes in lineage commitment of
HSPCs (Table 2). For the CFC assays, we did not use sorted
CD34+Linã cells but used total PBMCs. However, we
confirmed that nearly 100% of both CFU-GMs and BFU-Es
in PBMCs resided in the CD34 þLin cell populationaccording to preliminary experiments using PBMCs72

KYOIZUMI ET AL.

TABLE 2
Summary of HSPC Data
HSPCsCount(per ml)
CD34þLin
CAFCs
LTC-ICs
BFU-Es
CFU-GMs
T cells
NK cells
Proportion (%)
CAFCs
LTC-ICs
BFU-Es
CFU-GMs
T cells
NK cells

No. ofparticipantsa

RangebMedian

Mean

230
227
227
230
230
221
221

78–3014
0.04–303
0.04–291
5.60–849
3.90–546
0.18–74
1.10–103

698
18.6
19.2
164
72.6
8.0
19.8

819
29.6
30.3
200
90.9
11.0
24.5

228
228
231
231
222
222

0.05–11.4
0.05–12.8
1.30–62.4
0.90–39.2
0.05–4.6
0.20–9.5

2.81
2.84
22.6
10.3
1.19
2.96

3.11
3.19
23.7
11.0
1.27
3.03

a
Number of individuals whose data were successfully obtained
among 231 participants in total. The numbers of participants whose
data were missing mainly due to insufficient blood volume orexperimental errors during measurements were: 1 for blood cell count,
3 for CAFCs and LTC-ICs and 9 for T and NK cells.
bData with a value of 0 were converted to half the value of thedetection limit for each HSPC assay.

obtained from in-house volunteers (median: 98.5%; range:
96.2–100%; n ¼ 3). Therefore, the proportions of CFU-GMs
and BFU-Es in CD34 þLin cells were obtained by
calculation from the numbers of CFU-GMs, BFU-Es andCD34þLin cells in PBMCs. Multivariate analyses withlinear regression using age, sex and dose as variables wereconducted to assess associations of log-transformed HSPC
counts and proportions with these variables.
Sex Differences

Both counts and proportions of T-cell progenitors infemales were significantly higher than those in males (P ¼
0.0067 and P ¼ 0.019, respectively) with multivariate
analyses (Tables 3 and 4). Similarly, females showed ahigher count (P ¼ 0.0004) and proportion (P ¼ 0.002) ofNK-cell progenitors. These results may be concordant with

our findings that the proportions of mature naı̈ve T cells andthymic NK cells (CD56hiCD16CD127 þ) in peripheral
blood lymphocytes were significantly higher in female thanmale survivors (data not shown). No other HSPC subtypes
showed a significant difference between males and females.
Furthermore, no significant sex differences were detected in
the age or dose trend for any HSPC subtypes withmultivariate analyses (data not shown).
Age-Related Changes

We observed a statistically significant, age-dependent
decline in counts of all HSPC subtypes (Table 3). These
trends are not surprising because the count of CD34þLincells, which give rise to all populations of HSPC subtypes,
decreased significantly with age (P ¼ 0.0022; Table 3, Fig.
1). Although the proportions of all HSPC subtypes showeda trend toward a decrease with age, only T-cell progenitors
showed statistical significance (P ¼ 0.010; Table 4, Fig. 2),
suggesting that age-associated attrition was most evident for
lineage commitment of HSPCs to T cells. Of note, both T
and NK lineage cells are generated from common lymphoidprogenitors. However, the proportion of NK-cell progenitors showed only a marginal trend with age.
Dose-Related Changes

No significant association with radiation dose wasobtained for the counts of CD34þLin cells or any subtypesof HSPCs after adjusting for sex and age effects (Table 3,
Fig. 1). As for the association of the proportion of HSPC
subtypes with dose, although CAFCs reflecting the selfrenewal ability showed a trend toward a decrease with dose,
the trend was not statistically significant (P ¼ 0.18; Table 4).
The proportion of CFU-GMs representing myeloid commitment showed a trend toward an increase with dose, but
the trend was not significant (P ¼ 0.46). Furthermore,
contrary to our expectations, both the count and proportionof T-cell progenitors showed a trend toward an increase
with dose, although these increases were not significant (P¼ 0.56 and P ¼ 0.23, respectively).

TABLE 3
Regression Coefficients for Variables Related to HSPC Countsa
Variablesb

HSPCs

Intercept a

Sex (b1)

Age (b2)

Dose (b3)CD34þLin cells
CAFCs
LTC-ICs
BFU-Es
CFU-GMs
T cells
NK cells

4.24
5.12
5.18
7.38
6.44
3.90
5.02

0.08; P ¼ 0.31
0.20; P ¼ 0.20
0.14; P ¼ 0.35
0.06; P ¼ 0.62
0.12; P ¼ 0.27
0.42; P ¼ 0.0067
0.38; P ¼ 0.0004

0.17; P ¼ 0.0022
0.31; P ¼ 0.0047
0.32; P ¼ 0.0047
0.27; P ¼ 0.0020
0.25; P ¼ 0.0024
0.44; P ¼ 0.0001
0.30; P ¼ 0.0001

0.045; P ¼ 0.37
0.135; P ¼ 0.18
0.134; P ¼ 0.19
0.127; P ¼ 0.12
0.003; P ¼ 0.97
0.060; P ¼ 0.56
0.049; P ¼ 0.49

aRegression coefficients of the counts of each progenitor for sex, age and dose were obtained using the following formula: Ln(HSPC count) ¼

a þ b1 3 sex þ b2 3 age þ b3 3 dose.
b

Sex ¼ 1 for male and 2 for female.

73

HEMATOPOIESIS IN A-BOMB SURVIVORSTABLE 4
Regression coefficients for variables related to HSPC proportions in CD34þLin cellsa
Variables
HSPCs

Intercept a

Sexb; b1

Age; b2

Dose; b3

CAFCs
LTC-ICs
BFU-Es
CFU-GMs
T cells
NK cells

0.90
0.95
3.14
2.20
0.31
0.81

0.11; P ¼ 0.300
0.05; P ¼ 0.610
0.13; P ¼ 0.170
0.05; P ¼ 0.580
0.30; P ¼ 0.019
0.25; P ¼ 0.002

0.14; P ¼ 0.070
0.14; P ¼ 0.066
0.10; P ¼ 0.084
0.09; P ¼ 0.150
0.24; P ¼ 0.010
0.10; P ¼ 0.092

0.092; P ¼ 0.18
0.091; P ¼ 0.19
0.077; P ¼ 0.16
0.039; P ¼ 0.46
0.101; P ¼ 0.23
0.006; P ¼ 0.91

Regression coefficients of the proportion (percentage) of each HSPC subtype in the total CD34þLin cell population for age, sex and dosewere obtained using the following formula: lnðHSPC proportionÞ ¼ a þ b1 3 sex þ b2 3 age þ b3 3 doseb
Sex ¼ 1 for male and 2 for female.
a

Correlation between Proportions of HSPC Subpopulations

To evaluate the link between lineage commitments of
HSPCs, we analyzed the partial correlation between the
proportions of different HSPC subtypes using sex, age and

FIG. 1. Effect of age (panel A) and radiation dose (panel B) on the
count of CD34þLin cells circulating in the peripheral blood of Abomb survivors. The regression lines for age and the radiation dosewere obtained by adjusting for dose or age, respectively according to
the formula described in Materials and Methods and were plotted witha blue line for males (blue closed circles) and red line for females (redclosed circles).

FIG. 2. Effect of age on the count (panel A) and the proportion(panel B) of T-cell progenitors in the peripheral blood of A-bomb
survivors. The regression lines were obtained by adjusting for dose
according to the formula described in Materials and Methods and wereplotted with a blue line for males (blue closed circles) and red line forfemales (red closed circles).

74

KYOIZUMI ET AL.

FIG. 3. Partial correlation coefficients between the proportions of
HSPC subtypes using sex, age and dose as controlling variables.
Positive and negative correlations are indicated by solid and dottedlines, respectively (n ¼ 215). Statistically significant correlation
coefficients are shown by red lines and numerical numbers with P
values. The line width is proportional to the value of the correlation
coefficient. LTC-IC data are shown in parentheses.

dose as controlling variables (Fig. 3). The partial correlation
coefficients of both the count and proportion between
CAFCs and LTC-ICs were approximately 0.94, because
LTC-ICs are believed to be directly derived from the CAFC
population. Thus, LTC-IC data are shown in parentheses
following CAFC data in Fig. 3. Interestingly, significant
correlations were observed between CAFCs (LTC-ICs) and
BFU-Es, CFU-GMs and T-cell progenitors, whereas no
significant correlations were obtained between any two of
the three HSPC subpopulations: BFU-Es, CFU-GMs and Tcell progenitors. Although the correlation coefficient
between CAFCs (LTC-ICs) and T-cell progenitors was
not sufficiently large to declare a correlation and mostcorrelations were weak, these findings may suggest that
CAFCs (LTC-ICs) are upstream of the three lineage
progenitors in the differentiation tree. The strongest
correlation was observed between T-cell and NK-cellprogenitors, because both lineage progenitors are thought
to be directly derived from common T/NK progenitors.
DISCUSSION

Hematopoiesis has been reported to recover autonomously to nearly normal levels within 10 weeks, even after
exposure to high-dose radiation as in the case of some Abomb survivors (2, 9). Nonetheless, radiation-associatedexcess risks of myeloid leukemia and myelodysplastic
syndromes persist in survivors more than 50 years after
exposure (30, 31). In our previous studies using humanized
mice, compromised human hematopoiesis continued formore than 10 weeks after irradiation (32, 33). However, nosystematic studies have been conducted to assess whether
the hematopoietic functions of HSPCs in survivors arecompromised about 70 years after radiation exposure. In thecurrent study, we found strong age effects on the numbersof functional HSPCs with advancing age of the survivors
whose ages ranged from 66–91 years. This finding
suggested that human HSPCs continuously senesce overthe entire life. Contrary to expectations, we found noevidence supporting the hypothesis that exposure to Abomb radiation induces premature aging of HSCs. This may
imply that many years after radiation exposure and withadvancing age, the number and function of HSCs in thesurvivors have returned to normal levels, although it is notclear whether circulating HSPCs correctly reflect thehematopoietic functions in bone marrow HSC.
All HSPC counts showed a highly significant decrease
with age, because the counts of CD34þLin cells, whichgive rise to all HSPCs, significantly decreased with age.
Although the data are not shown, the proportion of
CD34þLin cells in PBMCs also significantly decreased
with age. Furthermore, we found an age-dependent decreasein the CAFC count, which may indirectly reflect HSC
functional status. Therefore, we presume that the number offunctional HSCs may also decline with age, although we did
not enumerate primitive HSCs according to either thesurface phenotype (e.g., CD34þCD38CD45RACD90þ)with flow cytometry and repopulating capacity in vivo using
humanized mice. These findings may be concordant with aprevious study using humanized mice, in which it wasreported that the repopulation ability of human bonemarrow HSCs decreases with age (15), although the age
range of our study subjects was skewed to older ages (66–
91 years). However, this previous study also showed thatthe population size of bone marrow HSCs with specificsurface phenotypes increases with age (15). However,
because contradictory findings were also observed in humanbone marrow HSCs by other researchers (34, 35), the exact
features of age-dependent changes in human HSCs arecontroversial.
An age-dependent decrease in lymphoid potential in
HSCs is well recognized and commonly observed in bothmice (36–39) and humans (15, 34). We also found that boththe count and proportion of T-cell progenitors significantly
decreased with age. This suggests that the commitment of
HSCs to the T-cell lineage may continuously decline withadvanced age even after thymic involution. However, theproportion of NK-cell progenitors showed only a marginal
decrease with age in survivors, although both types oflineage progeny are generated from common T/NK
progenitors (40). These results are consistent with thefindings obtained from our previous study of in-housevolunteers whose ages (28–64 years) were younger thanthose of survivors (17). Clonal analyses of HSPCs from inhouse volunteers suggested that bifurcation of T/NK coprogenitors shifts from the T-cell lineage to the NK-cell

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lineage with aging. Thus, this biased lineage commitment
seems to be imprinted in long-lived HSCs throughout theentire life.
Our previous study demonstrated that the proportion of
mature naı̈ve T cells in peripheral blood lymphocytes isdecreased in association with radiation dose (11). A
reduction in the naı̈ve T-cell pool in the periphery may
result from a decline in T-cell production in association withthymic dysfunction. The main causes of thymic dysfunction
are thought to involve impairments in both HSPCs and thethymic microenvironment (41). The results of the current
study are not consistent with a radiation-induced reductionin the T-cell potential of HSPCs, as mentioned above. Thus,an impaired T-cell potential in HSCs may recover to normal
levels over a long period. Regarding thymic function, we
have reported that T-cell receptor excision circle (TREC)
numbers in both CD4 and CD8 T cells of survivors decrease
with age, suggesting an age-dependent reduction in T-cell
production levels in the thymus of survivors (42). The
TREC numbers showed a significant correlation with theproportion of naı̈ve cells in both the CD4 and CD8 T-cell
populations. However, no significant radiation effects on
TREC numbers were found in either CD4 or CD8 T cells.
Again, this may be because the radiation effect on
peripheral TRECs may have been diluted over a long
period of time after radiation exposure. Therefore, analysisof archived biological samples including thymus, bonemarrow and blood cells shortly after radiation exposure may
be required to detect radiation effects on HSCs and thymus
function. We are now conducting analyses of the architecture and function of archived thymus specimens embedded
in paraffin blocks that were obtained from autopsied
survivors.
In the current study, numerical data regarding thedifferent subtypes of HSPCs were obtained with threedifferent methods: CAFC/LTC-IC, T/NK-cell and CFUGM/BFU-E assays. Interestingly, although these assays
used different culture conditions, the correlations between
the proportions of HSPC subtypes showed reasonable links
among the types of lineage commitment. CAFC (LTC-IC)
commitment showed a significant correlation with commitment to BFU-Es, CFU-GMs and T-cell precursors, whereas
no significant link was observed between any two of thethree HSPC subtypes. These findings suggest that the threelineage progenitors, BFU-Es, CFU-GMs and T cells, aredownstream of CAFCs (LTC-ICs) in the differentiation tree,
and the commitments of the three lineages are considered to
be independent of each other. Furthermore, NK-cell
commitment was strongly linked to T-cell commitment.
This finding is in good agreement with a previously
reported study where at least a portion of thymic NK cells
were generated from T/NK bipotential early thymic
progenitors (40). Interestingly, no significant link wasfound between NK cell and CAFC (LTC-IC) commitments,
in contrast to the significant link between T cells and
CAFCs (LTC-ICs). This suggests that thymic NK-cell

commitment may be independent of T-cell commitment at
the stage of divergence from CAFCs (LTC-ICs) and thatthymic NK-cell commitment occurs downstream of T-cell
commitment in the thymus. These results of commitment
linkage analyses appear to be reasonable and may
demonstrate the validity of our in vitro progenitor assays
for the purpose of studying in vivo hematopoiesis.
ACKNOWLEDGMENTS
We thank M. Yamaoka and A. Nishikiori for their excellent assistancewith FACS analyses and E. Douple and R. Ullrich for their valuablesuggestions. The Radiation Effects Research Foundation (RERF),
Hiroshima and Nagasaki, Japan, is a private, nonprofit foundation
funded by the Japanese Ministry of Health, Labor and Welfare and the
U.S. Department of Energy, the latter in part through DOE award no. DEHS0000031 to the National Academy of Sciences. This study was basedon RERF research protocol no. RP 5-09 and was supported by the U.S.
National Institute of Allergy and Infectious Diseases (NIAID contract no.
HHSN272200900059C). The views of the authors do not necessarilyreflect those of the two governments.
Received: July 24, 2015; accepted: October 28, 2015; published online:
December 31, 2015

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