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Cell transplantation for stroke.

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NEUROLOGICAL PROGRESS
Cell Transplantation for Stroke
Sean I. Savitz, MD,1 Daniel M. Rosenbaum, MD,2 Jonathan H. Dinsmore, PhD,3
Lawrence R. Wechsler, MD,4 and Louis R. Caplan, MD1
Cell transplantation has emerged as an experimental approach to restore brain function after stroke. Various cell types
including porcine fetal cells, stem cells, immortalized cell lines, and marrow stromal cells are under investigation in
experimental and clinical stroke trials. This review discusses the unique advantages and limitations of the different graft
sources and emphasizes the current, limited knowledge about their biology. The survival, integration, and efficacy of
neural transplants in stroke patients will depend on the type, severity, chronicity, adequacy of circulation, and location
of the stroke lesion.
Ann Neurol 2002;52:266 –275
During the past decade, the goals of stroke management have evolved from mere localization and prevention to an emphasis on treatment. The success of the
National Institute of Neurological Disorders and
Stroke tissue plasminogen activator trial, for example,
has created an entirely new attitude toward stroke. In
addition to therapies aimed at improving cerebral
blood flow, there has been increasing emphasis on neuroprotective strategies. Once damage from a stroke has
maximized, however, little can be done to recover premorbid function. Despite immediate medical attention,
many patients still have permanent deficits. Recent attention has focused on restoring brain function
through cell transplantation. Emerging animal studies
have shown that cells transplanted to the brain not
only survive but also lead to functional improvement
in different models of neurodegenerative diseases. Clinical trials have supported the efficacy of intrastriatal fetal grafts in patients with early Parkinson’s disease. A
few preclinical studies also have shown the potential
efficacy of neural transplantation in models of focal
and global cerebral ischemia, and clinical trials already
are under way for patients with stable and persistent
stroke deficits. This review discusses the various types
of donor cells in experimental and clinical stroke trials
and emphasizes current limited knowledge about the
basic behavioral mechanisms of graft sources. Various
factors affecting the survival, integration, and efficacy
of neural transplants in stroke patients will be reviewed
including the type, severity, chronicity, and location of
the stroke.
From the 1Department of Neurology, Beth Israel Deaconess Medical Center and Harvard Medical School, Boston, MA; 2Departments of Neurology, Neuroscience, and Ophthamology, Albert Einstein College of Medicine, Bronx, NY; 3Diacrin, Inc., Department
of Cell Transplantation, Charlestown, MA; and 4Department of
Neurology, University of Pittsburgh Medical Center, Pittsburgh,
PA.
Received Jan 24, 2002, and in revised form May 15. Accepted for
publication May 16, 2002.
266
© 2002 Wiley-Liss, Inc.
Donor Cells for Intracerebral Grafting
What kinds of cells are appropriate for cell transplant in
stroke? An ideal cell would be proliferative, allowing for
ex vivo production of high numbers of cells from minimal donor material. Cell transplants also should remain
immature and phenotypically plastic so that they could
differentiate into appropriate neural and glial cell types
depending on the ectopic site. In contrast with Parkinson’s disease, which destroys a relatively homogenous
population of neurons, strokes affect multiple, different
neuronal phenotypes. If proliferation and differentiation
were controllable, it then would be important to show
functional reconstitution of neural circuits. Various cell
types ranging from porcine fetal cells to bone marrow
stromal cells (BMSCs) currently are under investigation.
Porcine Fetal Cells
Early transplantation experiments established that
grafted neurons survive and remain viable only if they
are immature before they have elaborated extensive axonal connections. Transplanted fetal neurons survive,
integrate, and ameliorate functional deficits in animal
models of neurodegenerative diseases.1,2 Fetal human
transplants have benefited some patients with Parkinson’s disease.3 Given the limited availability of human
tissue, some investigators have turned to fetal xenotransplants, specifically from pigs, which are considered
relatively safe as a donor cell source.
Transplantation of fetal cells from the porcine, primordial striatum, also called the lateral ganglionic emPublished online Aug 20, 2002, in Wiley InterScience
(www.interscience.wiley.com). DOI: 10.1002/ana.10303
Address correspondence to Dr Caplan, Department of Neurology,
Beth Israel Deaconess Medical Center, Dana 779, Boston, MA
02215. E-mail: lcaplan@caregroup.harvard.edu
inence (LGE), was first shown to promote graft integration and to improve deficits in an animal model of
Huntington’s disease.2,4 The use of LGE cells has been
extended to other striatal injury models including focal
ischemia. In one study, a total of 350,000 LGE cells or
saline was injected into the rat striatum and parietal
cortex 3, 7, 14, or 28 days after 90 minutes of middle
cerebral artery occlusion (MCAo). Greater than 80%
of the LGE grafts survived, based on detection of pig
repetitive DNA element, at 12 weeks after transplanta-
Fig 1. Lateral ganglionic eminence (LGE) grafts at 12 weeks after transplantation detected by in situ hybridization for porcine repetitive DNA element. The nuclei of the engrafted pig cells stain dark blue. The sections were counterstained with hematoxylin.
(A, B, and C) Twelve-week-old grafts transplanted in rats at 14, 7, and 3 days after focal ischemia, respectively. (D, E, and F)
Twelve-week-old grafts in spontaneously hypertensive rats transplanted 14, 14, and 3 days after focal ischemia, respectively.
CC ⫽ corpus collosum; CTX ⫽ cortex; LV ⫽ lateral ventricle; STR ⫽ striatum; § ⫽ ischemic infarct.
Savitz et al: Cell Transplant for Stroke
267
tion (Fig 1). Grafts differentiated into glia and neurons, some of which expressed markers for GABA and
a striatal phenotype. LGE cells formed solid grafts in
infarct cavity (Fig 2), and some pig neurons were
found to project extensive processes to host structures.
There was also evidence for synaptogenesis both within
the graft and within the host, as shown by pig-specific
synaptobrevin. Animals transplanted 14 days after
MCAo showed statistically significant functional improvement compared with controls 4 weeks after implantation. However, no statistical differences were
found at later time points.5 A phase 1 clinical trial is
testing the safety of intrastriatal implantation of LGE
cells to patients who had a prior striatal stroke and
fixed deficits.
Despite in vivo evidence supporting the safety and
Fig 2. Lateral ganglionic eminence (LGE) cells to the infarcted rat brain 12 weeks after transplant. The graft was
stained with an antibody specific to pig neurofilament protein
and another antibody to choline acetyl transferase. Note that
the graft has filled the entire infarct. Str ⫽ striatum.
viability of LGE transplants, several issues about the
biology of these cells remain unresolved. LGE neurons
are presumed to be undifferentiated striatal precursor
cells, but no studies have investigated the extent of
their development nor their potential for proliferation
or differentiation. It remains unknown, for example,
what percentage of LGE grafts differentiate into terminal neurons versus glial cells. The plasticity of LGE
neurons is also unknown. Do they retain broad lineage
potential? Do they differentiate into the phenotype of
the implanted ectopic site, or are they committed to
becoming mature striatal neurons? Studies suggest that
transplanted LGE neuroblasts differentiate into striatal
neurons after intrastriatal implantation. Even when injected into the hippocampus or spinal cord, LGE grafts
still retain a striatal phenotype.5,6 These observations
suggest that LGE cells at the time of donor harvest
appear to be committed to follow a specific lineage
pathway, and it therefore is questionable whether they
would be appropriate for treatment of strokes outside
of the striatum.
Other concerns include graft rejection and the potential infectious disease risks associated with pig-to-human
neural xenotransplantation. There has been increasing
speculation about the possibility of transmitting a porcine endogenous retrovirus (PERV). One study found
no evidence for in vivo transmission of PERV in the
blood of 24 patients with Huntington’s disease, Parkinson’s disease, or epilepsy who underwent transplantation
with fetal porcine neurons.7
Neural Progenitor Cells
Reconstitution of the complex and widespread neuronal and glial damage in stroke may require access to a
broader array of neural lineage species than more
committed neuronal phenotypes. During the past few
years, techniques have been developed that make it
possible to isolate and expand, from developing and
even adult central nervous system tissue, cells with
properties characteristic of early, multipotent, neural
progenitor cells (NPCs; Fig 3). Neural stem cells reside in periventricular regions and in the cerebral cortex during development and have been shown to persist into adulthood. NPCs are identified by their
ability to undergo continuous cellular proliferation, to
regenerate exact copies of themselves, to generate
many regional cellular progeny, and to elaborate new
cells in response to injury or disease. Various injuries
including focal ischemia8 –10 stimulate the proliferation and differentiation of NPCs outside of traditional periventricular generative zones in the adult
mammalian brain.
Efforts are now under way to determine whether
NPCs from rat embryonic cortex or ventricular zones
can proliferate and differentiate into mature neurons
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September 2002
jor obstacle for all types of neural stem cells will be
expansion in vitro to the extent needed for transplantation. In vitro expanded NPCs lose the capacity to
differentiate, which limit their ability to form functional grafts.11
Immortalized Cell Lines
Because of the ethical dilemmas of embryonic stem cell
research and the limitations of xenotransplantation,
several laboratories have pursued alternative graft
sources including transformed cell lines in vitro. Two
immortalized cell lines are discussed here: NTerra-2
cells and the MHP clonal cell line. The NTerra-2
(NT2) cell line was derived from a human testicular
germ cell tumor more than 20 years ago.12 Unlike
other teratocarcinoma cell lines, the NT2 cells show an
exclusive commitment to a neural lineage when exposed to retinoic acid (RA). Several studies have shown
that NT2 cells resemble NPCs. They express cell sur-
Fig 4. Coronal sections of the postischemic rat brain 45 days
after transplantation of neural progenitor cells to the subventricular zone. (A) A rat brain exposed to 45 minutes of middle cerebral artery occlusion alone (n ⫽ 5 animals). (B) A rat
brain exposed to 45 minutes of middle cerebral artery occlusion followed 24 hrs later by progenitor cell transplant (n ⫽5
animals). Note that the progenitor cells repopulated the infarct
cavity.
Fig 3. Clonally derived cortical progenitor cells dissected from
the embryonic day 15 rat are multipotent. Single progenitor
neurospheres were generated in low-density cultures. (A)
Nestin-positive undifferentiated species. (B) Exposure to selective neurotrophins promotes the differentiation of specific lineages. O4 labels oligodendroglia (blue); ␤-tubulin labels neurons (green); glial fibrillary acidic protein labels astroglia
(red).
and glia when transplanted into the adult rat brain
subjected to MCAo. Preliminary data have shown that
NPCs survive, proliferate, and differentiate into all
brain cell types when transplanted into ischemic rats
(Figs 4 and 5). These preliminary observations suggest
that local environmental factors differentially regulate
the profile of progenitor cell regenerative responses to
central nervous system injury. Several issues remain including whether grafts survive for longer periods of
time, elaborate axonal processes, integrate into the host
brain, develop appropriate synaptic connections, and
promote functional recovery. Human neural stem cells
from fetal tissue are also being investigated for many
neurological disorders including focal ischemia. A ma-
Savitz et al: Cell Transplant for Stroke
269
Fig 5. Expression of various phenotypic markers of the transplanted progenitor cells in the
ischemic brain at 45 days after transplant.
Note that phenotypic markers for all three
major brain cell types are expressed. H and E
staining was performed on a sample taken
directly from the transplanted brain taken at
45 days after implant. CNPase ⫽ 2⬘,
3⬘-cyclic nucleotide 3⬘-phosphohydrolase;
GFAP ⫽ glial fibrillary acidic protein;
NeuN ⫽ neuronal N; MAP-2 ⫽
microtubule-associated protein-2; H and E ⫽
hematoxylin and eosin.
face markers and cytoskeletal proteins unique to neural
stem cells. Exposure to RA induces the sequential expression of neural markers in subsequent dividing
progeny, recapitulating the maturational events during
neurogenesis in vivo. NT2-treated cells also yield a
complement of daughter cells that retain the original
phenotype.13 Treatment with RA and mitotic inhibitors for several weeks ultimately results in the production of postmitotic, neuron-like cells (NT2N); they assume a highly asymmetrical morphology, elaborate an
extended axon, and elongate dendrites.13,14 NT2N
cells express diverse neurotransmitters,15 functional
glutamate receptors,16 calcium channels,17 and proteins
indicative of secretory activity and synaptogenesis.14,18
However, they fail to acquire a fully mature, neuronal
phenotype in culture and have been shown to terminally differentiate only when transplanted to the rat
brain.19 Grafted NT2N cells to the brains of adult
nude mice show evidence for terminal differentiation
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by 6 to 8 weeks, elaborate processes within 3 weeks,
and survive up to 14 months without reverting to a
neoplastic state.19
Demonstrating the survival and partial integration of
grafted NT2N cells to the adult rat brain has led to
several studies on the efficacy of transplants in rodent
models of focal cerebral ischemia. Intrastriatal transplantation NT2N cells at 1 month after MCAo followed by immunosuppression with cyclosporin led to
significant functional improvement at 6 months compared with ischemic animals that received rat fetal cerebellar grafts or growth medium alone or cyclosporin
alone. Transplanted animals not treated with cyclosporin showed a trend of decreasing behavioral recovery starting at 2 months after implantation. Cryopreserved NT2N cells were as viable and effective in
promoting functional recovery as fresh NT2N cells.20
Both types of NT2N grafts, which received cyclosporin, stained for human neuronal markers in the host
striatum. As expected, grafts did not survive in nonimmunosuppressed animals. Based on enhanced recovery,
the authors speculated that NT2N cells may perform
the functions of lost striatal cells, but no studies have
addressed the phenotype of NT2N grafts in the transplanted ischemic brain, nor has it been shown whether
they extend processes and integrate into the host brain.
Aside from striatal injuries, the effects of NT2N cells
in other ischemic infarcts remain to be determined.
The ready, constant availability of cryopreserved,
pure neurons has made the NT2N cells an attractive
graft source. A phase 1 clinical trial is now assessing the
safety of intrastriatal transplantation in patients with
basal ganglia infarcts and stable motor deficits who had
strokes 6 months to 6 years before transplantation.21
Preliminary findings among 12 patients treated with
NT2N cell transplants and immunosuppressed using
cyclosporin for 9 weeks found no evidence of malignant transformation or other adverse reactions at 1 year
follow-up.21 The extent to which NT2N cells differentiate into postmitotic neurons substantially reduces the
chances of neoplastic reversion. Positron emission tomography scanning at 6 months in the phase 1 trial
showed greater than 15% relative uptake of 18Ffluorodeoxyglucose at the transplant site in six patients.
This may reflect surviving and functioning implanted
cells, enhanced host cell activity, or an inflammatory
response. A second study of 18 patients with controls is
now under way.
Unfortunately, purifying neuronal NT2N cultures
also may limit their clinical utility. Glia are likely to be
necessary for reconstruction of the infarcted cytoarchitecture, and oligodendrocytes will be needed to myelinate newly formed axonal connections. Neuronal graft
survival may depend on glial support. It has been
shown, for example, that coculture of NT2N cells with
astrocytes substantially prolongs survival and enhances
maturation and synaptogenesis in vitro compared with
NT2N cultures alone.18,22 Transplanting the undifferentiated NT2 cells to the rat brain, however, leads to
tumors and death within 10 weeks except if implanted
into the host caudoputamen where grafts cease proliferating and differentiate into neurons, some of which
stain for tyrosine hydroxylase and synaptophysin.23,24
Another approach to the generation of clonal precursor cells is to incorporate an immortalizing oncogene
into neural stem cells. The Mandsley hippocampal
stem cell lines clone 36 (MHP36) cells, an immortalized murine stem cell line, are derived from the E14
hippocampal proliferative zone of the tsA58 transgenic
mouse, which constitutively expresses the temperaturesensitive tsA58 oncogene. MHP36 cells proliferate at
low temperatures (33°C) in vitro and give rise to various neuronal and glial precursor phenotypes at least in
part by inductive signaling.25 They cease dividing and
develop into mature neurons and glia on implantation
into the higher temperature brain (37°C).
In a focal ischemia model, MHP36 cells were transplanted to the intact somatosensory cortex and striatum, contralateral to the lesion cavity, followed by immunosuppression with cyclosporin, 2 to 3 weeks after
60 minutes of MCAo.26 Most of the grafted cells remained in the intact hemisphere, but some (an estimated 30%) migrated to the lesioned cortex and striatum. Compared with sham-operated animals, MHP36engrafted animals had significantly reduced infarct
volumes and significantly better sensorimotor recovery
over the following 18 weeks. MHP36 grafts, however,
did not improve marked deficits in spatial learning and
memory resulting from MCAo. Determination of the
effects of ipsilateral transplants on functional outcome
in addition to graft survival and migration is crucial.
The plasticity of MHP36 cells in this model also remains unknown. Although double labeling was not
performed to assess phenotypes, MHP cells showed diverse patterns of different neuron and glial-like morphologies.26 The pattern of differentiation is likely influenced by the various growth factors these cells are
exposed to before transplantation, but information regarding in vitro preparation is lacking in these studies.
Bone Marrow Stromal Cells
Research into stem cell biology also has focused attention on BMSCs that, unlike hematopoietic stem cells,
adhere to plastic and cause a variety of tissues including
bone, cartilage, adipose, muscle, hepatocytes, glia, and
neurons.27–29 When exposed to epidermal growth factor or BDNF in vitro or cultured with rat fetal mesencephalic or striatal cells,27 human BMSCs differentiate into cells expressing markers of NPCs. Whole bone
marrow cells from male mice systemically infused into
irradiated female mice have been shown to contribute
to the microglial population. When transplanted into
the albino rat striatum, human BMSCs engraft and
migrate along paths similar to NPCs.30 In irradiated
rats subjected to focal ischemia, systemically infused
BMSCs migrate to the ischemic cortex and become astrocytes.31
Chopp and colleagues have shown that intrastriatal
transplantation of BMSCs in mice 4 days after MCAo
survive, migrate, and differentiate into cells that stain for
neuronal or glial markers. Only 1% of the labeled BMSCs expressed neuronal markers, although it is difficult
to estimate the percentage of BMSCs in the pretransplanted marrow cell culture. Although transplanted and
control animals had no difference in infarct volume, the
transplanted mice showed significant behavioral improvement 28 days later compared with controls subjected to focal ischemia alone.32 Both intravenous and
intracarotid administration of BMSCs similarly improve
Savitz et al: Cell Transplant for Stroke
271
behavioral recovery at 35 or 14 days after MCAo, respectively.33,34 Among 2 million BMSCs transplanted
through the carotid artery, only 0.02% stained for neural markers in the ischemic hemisphere.
These preliminary results raise intriguing therapeutic
possibilities because BMSCs can be obtained readily
from bone marrow under local anesthesia, expanded in
culture, and potentially could be delivered to injured
brain tissue without need for invasive stereotaxic operations. Using the patient’s own BMSCs should circumvent the problems of host immunity and graft-versushost disease. However, neuronal differentiation in bone
marrow transplants is poorly understood. The relatively
large functional effects assessed within a short period of
time from such a low percentage of cells raises doubts
that BMSCs engraft to the injured sites and differentiate into mature neurons and/or glia. It remains purely
speculative whether BMSCs can differentiate into mature neurons that elaborate processes with the ability to
reconstruct neural circuitry. The authors note that the
mechanisms of recovery may be more related to production of trophic factors released by BMSCs.34
Another concern will be obtaining adequate cell numbers for human transplantation. The limited supply of
BMSCs will require new techniques to isolate and expand these cells in vitro. It also will be difficult to preferentially target BMSC transplants to the brain. A parenteral injection distributes BMSCs to other organs
including muscle, spleen, kidney, lung, and liver.34
Many issues need further research including the longterm survival and safety of transplanted BMSCs, their
plasticity and behavior, and optimal in vitro conditions
before transplantation.
Umbilical Cord Blood Cells
The umbilical cord is another source of multipotential
stem cells that, when exposed to NGF and RA, create
progeny that stain for neuronal and glial cell markers.35
Even less is understood about the biology of human
umbilical cord blood cells (HUCBCs). Nevertheless, a
study already has shown that intravenous infusion of 3
million HUCBCs to rats 24 hours after MCAo improved behavioral recovery 14 days after implantation
compared with control animals subjected to focal ischemia alone or focal ischemia with saline injection.36 An
unknown percentage of these cells preferentially survived and migrated to the ischemic hemisphere where
an estimated 2% expressed neuronal markers. As expected, HUCBCs also spread to various other organs.36
Similar to the BMSCs, the functional effects of the
HUCBCs observed within such a short period of time
likely result from the release of trophic factors (Table).
Much work lies ahead to determine whether umbilical
cord blood cells will serve as a potential graft source for
cell transplantation. It remains to be shown whether
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these cells can differentiate into functional neurons.
Techniques are needed to first isolate and expand
HUCBCs and then to delineate the appropriate neural
lineage pathways.
From Bench to Clinical
Although we have a limited understanding of the basic
biology of transplanted cells, clinical stroke trials already have commenced. It is a question of not only
which cells are more safe and effective but also which
stroke patients should undergo transplantation. The location, severity, and chronicity of the stroke and the
adequacy of blood supply will likely affect the safety
and efficacy of transplantation.
Location
What stroke lesions are amenable to cell transplantation? Most preclinical ischemia studies involve intrastriatal implantation. Studies of the MCA rodent
model have shown that the striatum is the primary site
of damage, and many believe that the resulting deficits
in memory, learning, and motor behavior are directly
associated with striatal injury. The striatum and the
rest of the basal ganglia are anatomically well defined
and surgically accessible by following a trajectory under
the sylvian fissure. Injection into a fluid-filled cavity in
the putamen facilitates migration of transplanted cells.
Without a definable, cavitated area, transplantation requires more direct pressure to inject cells, risking damage to normal tissue. The two ongoing clinical trials
therefore have restricted inclusion criteria to patients
with basal ganglia infarcts.
Cortical lesions also may be accessible to transplantation, but infarcts involving the white matter are more
problematic. A proliferation of transplanted cells in the
cortex might not repair underlying axonal damage.
There is even less rationale for neural transplants in
patients with pure white matter infarcts. For these lesions, one possible regenerative strategy would be to
direct stem or progenitor cells toward an oligodendroglial lineage and create appropriate cells to remyelinate
host axons.
Acute versus Chronic
The appropriate time to transplant after a stroke is unknown. Animal studies have injected cells anywhere
from 1 day to 1 month after MCAo. Few investigations have examined whether transplantation at different times after ischemic damage affects proliferation,
differentiation, integration, and functional outcome. In
the LGE studies, equal graft survival and volume were
obtained when porcine cells were transplanted 3, 7, 14,
or 28 days after ischemia.5
Transplantation to an acute infarct would be un-
Table. Various Graft Sources under Investigation in Experimental and Clinical Stroke Trials
Graft Source
Cell Type
LGE5
Porcine, striatal progenitor
NPC
NT2N21
MHP3626
BMSC32–34
Rat, embryonic neural progenitor
Cultured, postmitotic neurons
Cultured, transformed hippocampal stem cell line
Stromal cells from bone marrow
HUCBC36
Stem cells from umbilical cord
Comments
Grafts differentiate into neurons and glia, elaborate extensive
processes in focal model
Differentiate into all brain cell types in focal model
Safety established at 12 mo after transplant in human stroke trial
Transplants contralateral to lesions may improve recovery in focal
model
Functional benefit may be related to release of trophic factors in
focal model
Functional benefit may be related to release of trophic factors in
focal model
LGE ⫽ lateral ganglionic eminence; NPC ⫽ neural progenitor cell; BMSC ⫽ bone marrow stromal cell; HUCBC ⫽ human umbilical cord
blood cell.
likely to succeed if there were a severe arterial occlusion; inadequate blood flow would not support graft
viability. Grafted cells to the penumbra might be supported by collateral circulation. The release of excitotoxic neurotransmitters, free radicals, and proinflammatory mediators might threaten new tissue
introduced into the periinfarct region. Ischemic damage may be an ongoing process. Li and colleagues,37
for example, found evidence for apoptotic cells in the
penumbra persisting for 4 weeks after focal ischemia.
Injecting cells at sites distant from the infarct in an
acute stroke may improve deficits, as shown by
Veizovic and colleagues26 using the MHP36 cell line,
but transplantation in this study occurred 4 days after
focal ischemia.
The timing of transplantation also must consider
the natural course of recovery from stroke. Many neurologists would delay transplantation until deficits
reached a plateau. Impairments, however, have different courses of improvement depending on the type,
for example, hemiparesis versus apraxia38 and severity.
The prognosis for fine finger movements of the hand,
for example, is harder to predict for a slight versus a
severe deficit.
For these reasons and many others, some investigators have preferred to transplant at least months after
a stroke. Indeed, the two clinical trials have chosen to
study disabled patients at least 6 months after a
stroke. However, delaying allows for the formation of
scar tissue, which might adversely affect implanted
grafts. Unfortunately, there are no corroborating animal models of chronic stroke to investigate transplantation several months after MCAo. Few outcome
measures exist for animals with chronic stroke infarcts. In general, however, functional recovery in animal models cannot be easily equated across studies or
related to humans.
Site of Implantation
Another important factor is the site of transplantation.
Does the region of the brain that receives the transplanted cells influence different responses of the donor
cells? Many studies directly inject cells into the infarct
where it remains unclear whether new tissue can remain viable. Specific neuropathological conditions may
alter the balance of regional environmental signals by
releasing, for example, proinflammatory and other
modulatory cytokines, which, in turn, may adversely
affect survival and differentiation of the grafted populations. It may be more appropriate to inject cells in
the salvageable periinfarct regions of the penumbra,
but grafts still might be exposed to detrimental effects
of spreading depression and excitatory neurotransmitters. Differences in graft behavior depending on the injection site were noted by Hadani and colleagues39
who found that fetal cortical grafts to ischemic rat
brain survived in the penumbra but not in the core
lesion. Among the various graft sources under investigation, it remains to be established whether the penumbra versus the infarct is the more ideal site for transplantation.
For some types of transplanted cells, there may be
limited regions in the brain to support their growth
as evidenced by greater proliferation of implanted
NPCs in the subventricular zone compared with the
ischemic cortex. Some posit that grafts could be more
effective if the poorly vascularized, inflammatory environment of the ischemic region is avoided altogether and suggest the plausibility of transplantation
to distant regions, even to the contralateral side26;
however, no evidence supports this notion. Conversely, certain areas of the brain may allow some
types of grafts, particularly the more multipotential
and proliferative types, to grow unchecked and form
tumors in contrast with regions that promote graft
differentiation.23
Savitz et al: Cell Transplant for Stroke
273
Other Considerations
The number of cells needed to enhance recovery in
stroke is unknown. This issue will be important in
planning delivery methods given the volumes needed
for adequate cell number. Another concern is graft rejection and immunosuppression, which carries its own
inherent risks.21 The use of immunosuppressive agents
such as cyclosporin also may complicate the interpretation of any therapeutic effect from transplanted cells
as this agent protects neurons from focal ischemia and
promotes nerve regeneration via immunophilins.40 Finally, the neoplastic potential of different graft sources
especially multipotential cells needs to be better addressed. Embryonic stem cells have been shown to
form fatal teratomas at the transplantation site in a
Parkinson’s animal model.41 Tumor formation would
be a serious drawback to transplantation in humans.
Conclusion
Cell transplantation is a novel therapy for stroke patients, but much work lies ahead to further characterize
the biology of the different graft sources under investigation. In the future, head-to-head comparisons of
different donor cells in animals may help establish
which types are most beneficial. The mechanism by
which cells might improve function may differ depending on the cell type. Rather than replace infarcted tissue and form functional connections, some cell types
may enrich the local neural environment through
region-specific synaptic connections and trophic factors. Alternatively, grafts may upregulate endogenous
recovery mechanisms and induce surviving cells to establish new circuits. The selection of stroke patients for
transplantation will depend on the location of the infarct, time after stroke, the mechanism of the stroke,
the adequacy of circulation, and the temporal course of
the patients’ deficits.
This work was supported by a NIH EY11253 Research to Prevent
Blindness grant (D.M.R.) and grant funding from Layton Bioscience (L.R.W). Diacrine Inc. is funding the porcine transplant
trial at the Beth Israel Medical Center.
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