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Blood, Vol. 94 No. 9 (November 1), 1999:
pp. 3251-3257
A Kinetic Model for the Homing and Migration of Prenatally
Transplanted Marrow
By
Aimen F. Shaaban,
Heung Bae Kim,
Ross Milner, and
Alan W. Flake
From The Center for Fetal Diagnosis and Treatment at the Children's
Hospital of Philadelphia, Philadelphia, PA.
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ABSTRACT |
Currently little is known about the mechanisms regulating the homing
and the early engraftment of prenatally transplanted hematopoietic
cells due to the lack of a relevant functional assay. In this study, we
have defined a reproducible kinetic profile of the homing and the early
engraftment events in a murine model of prenatal stem cell
transplantation. Light density mononuclear cells (LDMCs) from adult
C57Pep3b and SJL/J marrow were transplanted by intraperitoneal (IP)
injection into C57BL/6 fetuses (106 LDMCs/fetus) at 14 days
of gestation. The fetuses were sacrificed at early time points (1.5 to
96 hours) after transplantation. Recipient fetal liver and cord blood
were analyzed for donor cell frequency and donor cell phenotype by dual
color flow cytometry. Pertinent findings included the following: (1) a
triphasic kinetic profile exists after in utero hematopoietic stem cell
(HSC) transplantation (homing of circulating donor cells, rapid
reduction of donor cell frequency, and donor cell competitive
equilibration); (2) homing to the fetal liver is nonselective and
reflects the phenotypic profile of the donor population; and (3) the
kinetics after the prenatal transplantation of congenic or fully
allogeneic cells are identical. This model will facilitate a systematic
analysis of the mechanisms that regulate the homing of prenatally
transplanted hematopoietic cells.
© 1999 by The American Society of Hematology.
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INTRODUCTION |
THE MECHANISM of homing is most likely a
multistep process consisting of adhesion to endothelial cells of the
receptive microenvironment, followed by transmigration, and finally
tethering within the extravascular microenvironment where proliferation and differentiation occur.1-4 The mechanisms regulating the
homing and the early engraftment of hematopoietic stem cells (HSC)
after in utero transplantation are poorly understood due to the lack of
a relevant functional assay. In vitro adhesion assays do not adequately
model the complex network of integrin-ligand-cytokine interactions
present during development. Postnatal homing studies in irradiated
recipients reflect homing patterns to radiation altered postnatal
microenvironments and are not applicable to homing and engraftment
events in the nonirradiated fetal microenvironment.5-7 Similarly, studies in mutant or compromised fetuses or neonates fail to
recreate a setting of engraftment competition, but rather, offer a
competitive engraftment advantage to the normal donor cells.8-10 Therefore, in this study, we have developed a
novel murine model of in utero HSC transplantation to investigate the homing and the engraftment of transplanted bone marrow (BM) to the
fetal liver after in utero HSC transplantation into normal recipients.
Normal hematopoietic ontogeny is characterized by a sequential
migration of hematopoiesis from the yolk sac and/or periaortic splanchnopleure, to the fetal liver, and finally to the spleen and
BM.11-13 In the mouse, and in the human, the fetal liver is the predominant source of hematopoiesis until just before term. We took
advantage of this fact to develop an in vivo assay system for homing
and engraftment to the fetal liver. At 14 to 15 days gestation in the
mouse, hematopoiesis is derived exclusively from the fetal
liver.13 Hematopoietic cells are not observed in the spleen
until 15 days gestation and in the marrow until soon
thereafter.13 Therefore, analysis of the early homing and
engraftment events to the fetal liver can be performed in isolation of
other confounding hematopoietic environments at this stage of development.
In many studies of prenatal and postnatal homing of transplanted cells,
observations on homing have not been dissociated from observations on
engraftment.14-17 If homing is defined as a process by
which parenterally administered hematopoietic cells lodge and firmly
anchor themselves within the hematopoietic tissues, then the parameters
that influence this process need to be studied immediately after
transplantation, before cell proliferation ensues. The findings from a
number of postnatal studies suggest that the homing process is complete
within a few hours after transplantation, and that delayed analysis
must account for graft apoptosis and proliferation.18-20
Our goals in this study were to describe the kinetics of cellular
trafficking immediately after in utero HSC transplantation. We analyzed
multiple time points to assess the early events after transplantation
that reflect the homing of transplanted cells, as well as their
subsequent engraftment. We have also analyzed the kinetics of
circulating donor cells and their clearance from the circulation, which
is a necessary corollary for any analysis of homing. In addition, we
sought to better characterize the types of cells that home to the fetal
liver to determine if homing within the prenatal environment is limited
to a particular cell type. Lastly, we examined if there were any
differences in the kinetics of homing between congenic and fully
allogeneic donor cells.
Our findings included the following: (1) a triphasic kinetic profile
exists after in utero HSC transplantation (homing of circulating donor
cells, rapid reduction of donor cell frequency, and donor cell
competitive equilibration); (2) homing to the fetal liver is
nonselective and reflects the phenotypic profile of the donor
population; and (3) the kinetics after the prenatal transplantation of
congenic or fully allogeneic cells are identical.
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MATERIALS AND METHODS |
Animals.
Breeding stock for inbred strains of mice, C57BL/6 (H-2b,
CD45.2), C57Pep3b (H-2b, CD45.1), and SJL/J
(H-2s, CD45.1) were purchased from The Jackson Laboratory
(Bar Harbor, ME) and bred in our colony. Animals were
mated and the females were checked daily for introital
plugging. The day of plugging was defined as gestational day 0 for time
dating. All animals were housed in the Laboratory Animal Facility of
the Abramson Pediatric Research Center at the Children's Hospital of
Philadelphia. All experimental protocols were reviewed and approved by
the Institutional Animal Care and Use Committee at the Children's
Hospital of Philadelphia and followed guidelines set forth in the
National Institutes of Health Guide for the Care and Use of Laboratory Animals.
Donor BM harvest.
Donor BM was harvested from 4- to 8-month-old adult mice by flushing
the tibias and femurs with phosphate-buffered saline (PBS) (GIBCO-BRL,
Gaithersburg, MD) using a 26-gauge needle. A single
cell suspension was made by 3 gentle passes through the needle. Cell
suspensions were then filtered through a 70-µm nylon mesh and layered
over Ficoll (Histopaque 1077; Sigma, St Louis, MO).
After centrifugation at 600g for 15 minutes, the light density mononuclear cell (LDMC) layer was removed and washed twice with sterile
PBS. The cells were counted and greater than 95% viability was ensured
by trypan blue exclusion.
Marrow transplantation.
Recipient mice were injected on day 14 of gestation. Using
methoxyfluorane general anesthesia and sterile technique, a midline laparotomy was made and the uterine horns were delivered from the
wound. Each fetus was transplanted by intraperitoneal (IP) injection
with 1 × 106 cells in 5 µL of PBS under direct
visualization through the intact uterine wall using a 100-µm beveled
glass micropipette. After return of the uterus to the maternal
peritoneal cavity, abdominal closure was achieved using 2 layers of
absorbable 5-0 suture.
Kinetics of transplanted cells and assessment of long-term
chimerism.
Transplantation kinetics and long-term chimerism levels were assessed
in recipients of Pep3b and SJL marrow by harvest of recipient cord
blood and fetal liver at time points of 1.5, 4, 24, 48, and 96 hours
after transplantation and of recipient peripheral blood and BM at 12 months of age. Cord blood and fetal liver were individually harvested
sequentially in petri dishes filled with cold, heparinized PBS. The
cord was occluded at harvest and the fetus transferred to the petri
dish. The fetus was allowed to exsanguinate into the dish and the media
processed for cord blood cells. The fetus was then transferred to
another petri dish and the fetal liver harvested by dissection. The
fetal livers were then individually passed through a 1-cc syringe to
form a single cell suspension. Fetal liver, BM, cord, and peripheral
blood suspensions were ficoll separated for enrichment of LDMC. LDMCs
were then stained with directly conjugated anti-CD45 phycoerythrin
(PE) (specific for either CD45 isoform) and anti-CD45.1
fluorescein isothiocyanate (FITC) (Pep3b and SJL) antibodies
(Pharmingen, San Diego, CA) then counted by 2-color
flow cytometry (FACScan; Becton Dickinson, Mansfield,
MA). Dead cells were excluded by propidium iodide. The
percent donor cells was defined as:
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This ratio provides an estimate of fractional donor
hematopoiesis without being confounded by the overwhelming
erythropoiesis of the fetal liver that is predominantly composed of
CD45 Ter119+ erythroid precursors.
Analysis of transplanted cell phenotype.
Lineage analysis of fetal liver LDMCs was performed at the same time
points. Purified antibodies to CD3, Mac-1, B220, Gr-1, and Ter119 were
individually combined with directly conjugated anti-CD45.1 PE in the
first step. After cold incubation and repeated washings, the cells were
then incubated with an isotype-specific FITC-conjugated secondary
antibody. By gating on the donor cell population, the frequency of
donor cells expressing various lineage differentiation antigens was
made. A total of 5,000 to 10,000 donor cells was counted in each group.
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RESULTS |
The early engraftment of transplanted marrow is specific and is
characterized by three phases: (1) homing, (2) graft reduction and
redistribution, and (3) equilibration.
Single-cell suspensions of the LDMCs from the recipient fetal liver and
cord blood were analyzed at serial time points from 90 minutes to 96 hours by dual-color flow cytometry using monoclonal antibodies that
were specific for donor cell surface antigens. Preliminary experiments
at earlier time points showed that deposition of donor cells within the
fetal liver occurred as early as 45 minutes after transplantation (data
not shown). Later time points showed only small changes in chimerism
beyond the 96-hour levels through 12 months (see Fig 2). Therefore,
these time points were chosen because they occur during the time of
maximal cellular trafficking. These methods provided a dynamic picture
of donor cell migration.
A total of 163 injections were performed and 160 fetuses survived for
analysis establishing an overall fetal loss rate of 1.8%.
Figure 1 illustrates the frequency of
CD45+ donor cells in the recipient fetal liver and cord
blood over time. The resulting donor hematopoietic cell fractions that
were determined by flow cytometry of the 160 viable recipients are plotted. As shown, a triphasic profile of donor cell frequency within
the recipient is evident. After an early peak at 90 minutes (52% to
78%), the donor cell fraction in the fetal cord blood fell
dramatically over the subsequent 24 hours (18% to 22%). Thereafter, the level within the cord blood remained essentially stable throughout the remainder of the study (15% to 18% at 96 hours). No differences were noted between the congenic and allogeneic groups. On the other
hand, the peak donor cell fraction within the recipient liver occurred
at 4 hours (17% to 19%), lagging slightly behind the peak in cord
blood levels. The subsequent drop in the levels within the fetal liver
was less dramatic and was essentially a linear decline over the ensuing
48 hours (2.2% to 3.2%). During this time, the total number of
recipient CD45+ cells expanded nearly 10-fold
(Table 1). Donor hematopoiesis in both
groups remained stable for the next 48 hours at 2.5% to 3.0%, then
exhibited a slow decay over the early weeks of postnatal life, reaching
a stable level of 0.5% to 1.3% in the marrow at 12 months as shown in
Fig 2. Again, no significant differences were noted between the groups.
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| Fig 1.
These graphs depict the triphasic kinetic profiles for
the frequency of donor cells circulating within the recipient fetal
peripheral blood (cord blood) or lodged within the fetal liver at
various time points (1.5 to 96 hours) after in utero transplantation.
The nearly identical profiles for both the congenic (A) and the fully
allogeneic (B) strain combinations are shown. The donor cell frequency
is calculated as the percentage of cells expressing the donor CD45.1
isoform among all CD45+ cells within the host cord blood
or fetal liver.
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| Fig 2.
Analysis of long-term chimerism. (A) Sample dot plot and
histogram are shown from dual-color analysis of donor (CD45.1 FITC)
peripheral blood chimerism among cells expressing either CD45 isoform
(CD45 PE) within the recipients at 6 months of age. (B) Peripheral
blood and BM chimerism levels by FACS analysis at 12 months of age in
recipients of either congenic or allogeneic marrow.
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Table 2 displays the relative homing
efficiency for the congenic and allogeneic groups. The homing
efficiency is defined as the percentage of donor CD45+
cells, which were recovered from the fetal liver. As shown, the homing
efficiency to the fetal liver at 4 hours for either group was 4.4% to
4.9% without a significant difference.
Cells of all lineages are capable of homing to the fetal liver.
Phenotypic analysis of donor cells in the recipient fetal liver at 4 and 48 hours after transplantation shows that various lineages of cells
are capable of homing and surviving in the fetal hematopoietic
microenvironment. As shown in Fig 3, all
lineages of cells from either allogeneic or congenic marrow are capable of homing to the fetal liver. The early lineage analysis shows a
pattern similar to that of the donor cell population. After 48 hours,
the CD3 expressing cell population is essentially absent. At this
point, the pattern of donor hematopoiesis, which includes granulocyte,
macrophage, and B-lymphocyte lineages, continues in a manner mirroring
host fetal liver hematopoiesis. Analysis of long-term (12 months)
chimeras shows a similar pattern of multilineage donor hematopoiesis
within recipient BM (data not shown).
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| Fig 3.
These charts summarize the phenotypic profile of the
donor cell population at various stages: pretransplant and within the
recipient fetal liver at 4 and 48 hours posttransplant. The percentage
of donor CD45+ cells expressing each of the
differentiation antigens listed is shown for each time point in either
the congenic (A) or allogeneic (B) strain combination.
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DISCUSSION |
The process of homing and engraftment of hematopoietic cells to the
fetal liver has not been previously studied. Our study is unique in
several respects. First, we have studied the homing of transplanted
cells in a nonirradiated, competitive environment, which should reflect
normal events during hematopoietic ontogeny, or after in utero HSC
transplantation. Second, because of the developmental timing of these
studies, we have been able to examine homing events to the fetal liver
(completed 4 hours after transplantation) in isolation from other
competitive hematopoietic environments such as the spleen or marrow.
This feature is relevant because the hematopoietic environment at this
stage of development in the mouse is similar to that in humans when in
utero HSC transplantation is optimally performed (12 to 14 weeks
gestation).21,22 Finally, our model allows examination of
homing and engraftment of allogeneic cells before the development of
cell-mediated immunity.13,23,24 Thus, in our model, the
influence of histocompatibility on homing and engraftment can be
analyzed in the absence of T-cell-mediated immunologic response in an
otherwise intact hematopoietic environment.
The kinetic profile found in our study suggests that homing and
engraftment to fetal liver has a number of parallels to homing and
engraftment to postnatal BM. First, our results demonstrate that the
homing events after intraperitoneal transplantation are relatively
rapid. We found peak circulating levels of donor cells at 90 minutes
after transplantation and peak fetal liver frequencies of donor cells
at 4 hours after transplantation. We interpret these events as rapid
uptake from the peritoneal cavity with the peak circulating levels of
donor cells reflecting a "bolus" effect. Homing and
"lodgment" of donor cells in hematopoietic and other tissues then
follows resulting in a rapid decrease in the levels of circulating
donor cells. The second phase of our engraftment profile between 4 and
48 hours is marked by a rapid decrease in the frequency of donor cells
in the fetal liver and steady state circulating levels of donor cells
in the cord blood. We interpret this phase as engraftment and
dilutional reduction in donor cell frequency due to the exponential
increase in host fetal liver cell number at this stage of development
as shown in Table 1. In addition, donor cells do not begin cycling for
at least 12 to 24 hours after transplantation and the proliferative
rate of adult-derived hematopoietic cells is likely to be less than
that of endogenous fetal cells.25-27 Further loss of donor
cells from the fetal liver may be attributable to the initiation of
fetal marrow hematopoiesis on day 15 or 16 of
gestation11,13 and as early as 24 hours after
transplantation. The third phase of our profile is interpreted as
competitive equilibration of donor and host-derived hematopoiesis and
extends from 48 hours to the postnatal period. During this time, a slow
decrease in donor cell frequency occurs and eventually reaches a steady
state, which is presumably ultimately reflective of the fraction of
donor HSC to host HSC. Beyond 96 hours, any potential differences
between the groups were obscured by the error associated with
fluorescence-activated cell sorting (FACS) detection of a target cell
frequency below 1%.
Our current phenotypic analysis of the engrafting donor cells supports
a nonselective mechanism for lodgment and engraftment. The phenotypic
profile of engrafting donor cells is very similar to the phenotype of
the injected cells confirming that the vast majority of engrafting
cells are committed progenitors of the macrophage, granulocyte, and
B-lymphoid lineages. This phenotypic profile remains relatively
constant over 48 hours, although there is a trend toward a decrease in
donor lymphoid and erythroid cells (CD3+,
B220+, and TER119+ cells) with an increase in
donor myeloid cells (Mac-1+ and Gr-1+ cells).
Loss of T-cell lymphopoiesis might be expected because this is not
normally supported by the fetal liver environment and may also reflect
redistribution of T cells to a more receptive site such as the
thymus.23 Loss of donor erythroid progenitors may reflect
the accelerated differentiation of donor erythroblasts in the intensely
erythropoietic milieu of the fetal liver.11,13,28
Hematopoiesis during ontogeny is developmentally regulated and
presumably involves sequential "activation" and
"deactivation" of hematopoietic microenvironments.14
Activation requires the development of stromal infrastructure with
appropriate expression of chemokines, integrins/ligands, and cytokines
to attract and support hematopoiesis. The process of homing and
engraftment then becomes critical for the establishment and maintenance
of hematopoiesis in a given environment. Deactivation presumably occurs
by downregulation of these adhesion and supportive interactions or by
the development of a competitive environment with greater affinity for
HSC, resulting in the depopulation and quiescence of a once active
hematopoietic microenvironment.14 Hence, analysis of homing
and engraftment events during development must be taken in the context
of the specific stage of hematopoiesis being analyzed and may or may not be applicable to other stages or microenvironments. The molecular mechanisms of this process remain obscure, but their successful completion is required for normal hematopoietic events during development or for successful engraftment after in utero HSC
transplantation. In this study, we have developed a model that allows
us to focus on a single stage of development and a specific
microenvironment, ie, the fetal liver before development of BM hematopoiesis.
Most knowledge about hematopoietic homing and engraftment is based on
studies in the irradiated mouse model. Early studies tracked homing and
distribution of colony-forming units-spleen (CFU-S) and demonstrated
the transient presence of transplanted cells in all organs, as well as
the BM and spleen.19,20,29,30 Clearance from the peripheral
blood of all CFU-S occurred within 3 to 4 hours and from all, except
the hematopoietic organs, within 48 hours. Donor cell cycling and
proliferation then began within 24 hours after transplantation. Thus,
the initial process of homing, ie, lodgment, does not appear to be
specific to hematopoietic organs, but the subsequent survival and
engraftment of circulating cells depends on the hematopoietic
microenvironment. This supports the concept of a "niche" in the
hematopoietic microenvironment that is necessary for survival and
proliferation of HSC.
More recently, fluorescently labeled cells have been used to assess the
homing and engraftment of syngeneic BM progenitors in nonirradiated, as
well as irradiated recipients.17 These studies combined a
direct homing assay of enriched marrow with a CFU-S assessment of the
recovered cells. Relevant findings included an 8.06% homing efficiency
to the marrow of both recipient groups at the measured time points of
17 and 24 hours. In addition, the efficiency was significantly worse in
the irradiated recipients when compared with the nonirradiated
recipients. These studies once again support the existence of a
nonspecific lodgment mechanism. More importantly, however, they confirm
that engraftment is an active process requiring communication between
the stromal endothelium and the migrating hematopoietic cell and
further suggest that radiation preparation may disturb these important
mechanisms and impair the success of donor cell engraftment. These
findings along with those from other studies challenge the concept that
myeloablation is required to create "space" for donor cells.
To our knowledge, only 1 other study has attempted to analyze the
kinetics of engraftment after prenatal transplantation of hematopoietic
cells.14 In that study, ovine fetuses were transplanted at
40, 50, 60, 70, or 80 days gestation with hematopoietic cells from
ovine fetal liver, human fetal liver, and ovine or human adult BM. The
recipient fetuses were killed 10, 20, or 30 days later. The liver,
marrow, and blood were then analyzed for donor hemoglobin, donor
karyotype, and human leukocyte common antigen expression. The
investigators found that transplanted cells engrafted in the fetal
liver exclusively before formation of the BM. As gestation proceeded,
however, the transplanted cells engrafted preferentially to the
developing marrow. Over 90% of donor cells were detectable in the
fetal liver at 50 days gestation, equal numbers of donor cells were
present in fetal liver and BM at 70 days gestation, and by 80 days
gestation, nearly 90% of donor cells were detected in the marrow. This
shift toward marrow hematopoiesis was in spite of the observation that
the vast majority of total hematopoiesis was derived from the fetal
liver at all time points studied. These results support a model of
increasing microenvironmental affinity for circulating progenitors with
the fetal BM having higher affinity for circulating progenitors than
the fetal liver. Due to the lack of early time points, however, these
studies did not actually address homing, lodgment, or early engraftment
of donor cells, but rather, the end result of engraftment, which would
include subsequent donor cell redistribution, proliferation, and
apoptosis. Although only a semiquantitative analysis of redistribution to the developing fetal BM was possible, we were able to reliably quantify the early events that truly reflect donor cell homing and
fetal liver engraftment.
The importance of MHC compatibility in homing and engraftment events
has been difficult to evaluate independent of immunologic considerations. The use of genetic or irradiation-induced models of
immunodeficiency may or may not reflect events in the immunologically and hematologically intact recipient. In a compelling study, Hashimoto et al31 transplanted donor microenvironment (bone) versus
major or minor major histocompatibility complex (MHC)-mismatched
microenvironment into irradiated allogeneic recipients and followed
this with BM transplantation. Observations included: (1) increased
cellularity and number of progenitors in MHC-matched BM versus
recipient or third party BM; (2) the ability to engraft HSC when
combined with the matched, but not mismatched microenvironment, into
previously resistant strain combinations; (3) the dependence of this
advantage on MHC with no restriction demonstrated for minor antigen
disparity; (4) limitation of engraftment in non-MHC-matched
environments was not secondary to T or natural killer
(NK)-mediated mechanisms; and (5) MHC restriction was
abrogated by pretransplant irradiation of the donor microenvironment.
This study suggests that in the nonirradiated microenvironment,
MHC-matched cells have an advantage over non-MHC-matched cells, but
does not address whether the advantage is due to better homing and
engraftment or subsequent ability to maintain and expand hematopoiesis.
Our finding that the early homing and engraftment events for congenic
or allogeneic donor cells are identical would suggest that any
MHC-derived advantage occurs after lodgment and engraftment and is more
likely related to ongoing stromal/HSC interaction. It would also
support a model of homing and engraftment that is entirely independent
of MHC-associated interactions.
A criticism of our current study is that conclusions drawn from
transplantation of a heterogeneous BM population may or may not reflect
homing and engraftment of long-term repopulating cells. Nevertheless,
these findings do represent numerous mechanisms that regulate the
trafficking of progenitors and it is likely that there is overlap with
the proposed homing mechanisms of HSC. The persistence of detectable
chimerism through 12 months within the recipient BM supports the
engraftment of donor HSCs. Because the vast majority of engrafting
cells do not have long-term repopulating capacity, the donor cell
compartment is reduced to a very small fraction of total
hematopoiesis.32-34 Based on a phenotypic analysis, Morrison and Weissman33 calculated the frequency of stem
cells in the marrow of an adult mouse to be in the range of 0.005% to 0.01%. Slightly lower levels were suggested by the work of Osawa et
al,34 who concluded an HSC frequency of 0.004% ± 0.003% based on a more restricted phenotype. From these detailed
studies, we conclude that the number of HSCs in the transplanted
inoculum was in the range of 50 to 100 cells. Considering the
documented homing efficiency of around 5% and assuming an equal homing
efficiency for HSCs, transplantation of 1 × 106 BM
cells results in the homing of only 2 to 5 HSCs to the fetal liver.
This calculation is consistent with the low-level of engraftment that
is observed when using this dose of BM cells. The purpose of this study
was not to achieve high levels of long-term engraftment, but rather to
document the early events after transplantation of a standard dose of
adult BM. In separate studies using this model,35,36 we
have documented higher levels of multilineage long-term engraftment
using fetal liver-derived cells or higher doses of BM-derived cells.
This latter finding is consistent with our interpretation of the
low-level of long-term engraftment seen in this study.
Although these findings may be applicable to events during normal
hematopoietic ontogeny, our primary interest in this model is its
potential for the study of in utero HSC transplantation. Despite
clinical success with this approach in immunodeficiency disorders in
which there is a survival advantage for normal cells (X-linked severe
combined immunodeficiency),22,37 broader clinical application of in utero HSC transplantation has been limited by minimal
donor cell engraftment in disorders in which there is no competitive
advantage. Models of prenatal or neonatal transplantation into mutant
or compromised fetuses fail to recreate this setting of engraftment
competition, but rather, offer a competitive bias to the normal donor
cells.8-10 The receptive environment in the human fetus
during the preimmune "window of opportunity" consists of only
fetal liver-derived hematopoiesis. Therefore, a developmentally analogous model in which the early homing and engraftment events can be
systematically explored and manipulated should prove invaluable in
developing clinical strategies to improve donor HSC engraftment. Our
ability to reproducibly characterize the kinetics of peripheral blood
and fetal liver donor cell expression in this model will pave the way
for a systematic analysis to elucidate the adhesion interactions that
are critical for fetal liver homing, as well as to examine the homing
capability of specific donor cell populations. Further development of
the model should allow studies of the competitive capacity of different
donor cell populations after engraftment and the ability of fetal liver
engraftment to redistribute to other hematopoietic environments.
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FOOTNOTES |
Submitted January 6, 1999; accepted June 25, 1999.
Supported in part by a National Research Service Award HL09856 from the
National Institutes of Health, Public Health Service Grant No. HL53998,
and funds from the Ruth and Tristram C. Colket, Jr, Chair of Pediatric Surgery.
The publication costs of this
article were defrayed in part by
page charge payment. This article
must therefore be hereby marked
"advertisement"
in accordance with 18 U.S.C. section
1734 solely to indicate this fact.
Address reprint requests to Alan W. Flake, MD, The Center for Fetal
Diagnosis and Treatment, The Children's Hospital of Philadelphia, 34th
St and Civic Center Blvd, Philadelphia, PA 19104.
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