Stem Cell Treatment for Stroke and Traumatic Brain Injury

Damage and Disability caused by Stroke
At present, ischemic stroke is the third leading cause of death in industrialised countries. With an annual incidence of 250–400 in 100 000 inhabitants, around 1 million people suffer from a stroke each year in the United States and in the European Union(1). Approximately a third of cases are left with some form of permanent impairment, making stroke the single largest cause of severe disability in the developed world.

Stroke is caused by the interruption of blood flow in a brain-supplying artery; commonly an embolus causes an occlusion (blockage) in the blood vessel. Ischemic stroke (cerebral infarction) and the even more devastating intracerebral haemorrhage, cause a disturbance of neuronal circuitry and disruption of the blood-brain-barrier that can lead to functional disabilities. At this time, therapy is primarily based on the prevention of recurrent (secondary) strokes. Rehabilitation therapy is important for maximizing functional recovery in the early phase after stroke, but once recovery has plateaued there is no known treatment.

Stem cell treatment could be the major breakthrough in effecting repair of some of the damage caused by stroke.

Cell transplantation in experimental models of stroke
Research: 2001-2008
Recent studies have highlighted the enormous potential of cell transplantation therapy for stroke. A variety of cell types derived from humans have been tested in experimental/rodent stroke models. Human cells that have been used in these studies belong in three categories: (i) neural stem cells cultured from foetal tissue; (ii) immortalised neural cell lines; and (iii) haematopoietic/endothelial progenitors and stromal cells isolated from bone marrow, umbilical cord blood or peripheral blood(3).

While human embryonic stem cells offer a virtually unlimited source of neural cells for structural repair in neurological disorders such as stroke, there are the ethical and safety concerns.Adult neural progenitor cells can be obtained from different tissues, can be safely expanded in vitro, and have shown promising therapeutic effects in several neurological disorders without causing serious side effects(2).

The purpose of this review is to focus specifically on the prospects of umbilical cord blood cells as stroke therapy.

Review of human umbilical cord blood cell (HUCBC) treatments for stroke:

As early as 2001, a study was conducted to assess whether an intravenous infusion of human umbilical cord blood cells in a rodent model, could enter the brain, survive, differentiate, and improve neurological functional recovery at 24 hours and 7 days after stroke. The study objectives were all achieved to a certain extent(4).

In 2005 a research team at the University of South Florida investigated strategies to effectively treat stroke patients other than by re-canalisation of the occluded vessels in the cerebral infarcted area. This group also investigated strategies to extend the narrow anticoagulant treatment window to which only a minority of patients have timely access. The following results were published: rats receiving human cord blood cells 24 h after stroke demonstrated improvements in behavioural defects; the 3 hour therapeutic window for anticoagulant treatment of stroke victims may be extended 24-72 hours post stroke with the use of umbilical cord blood cell therapy(5).

Paradoxically, a Finnish study (2006) reported that human cord blood cells, administered intravenously 24 h after stroke in rats, did not improve functional sensorimotor and cognitive recovery because of limited migration of cells(6), but that an infusion of pure CD34+ cells following focal cerebral ischemia demonstrated some improvement in functional outcome(7).

Recently, Kim et al(8) showed that human mesenchymal (CD34+) stem cells transplanted intravenously (ipsi- and contralateral) into a rat after ischaemic stroke, possessed the capacity to migrate extensively to the infarcted area. Promising data were also recently cited for treatment of intracerebral haemorrhage (ICH): intravenous delivery of cord blood cells might well enhance endogenous repair mechanisms and functional recovery after ICH(9, 10).

Current knowledge supports HUCBC as cell transplant candidate for stroke: It goes without saying that the ideal cell for transplantation should meet all the criteria of safety for the receiver as well as offer the highest therapeutic potential. Therapeutic preparations for stroke require an adequate cell number, which raises the need to expand the precursor cell source in vitro (cell culture).

• Cord blood is composed of many cell types including haematopoietic and endothelial stem/progenitor cells (CD34-), mesenchymal cells (CD34+), immature lymphocytes and monocytes. It is not clear which of these cells are important for functional recovery after stroke.
• Umbilical cord blood cells, whether delivered intracerebrally or intravenously, target the ischaemic border. Chemokines - induced by injury - are thought to mediate this migration process.
• Few transplanted cells are found in the brain, even when delivered intracerebrally. Given the controversy of whether these cells can really become neurons, it is unlikely that they act to replace the damaged tissue; it is more feasible that they secrete factors that enhance inherent brain repair mechanisms(11).

Evidence(review.3) suggests that transplanted cells may work in the following ways:

• increase vascularisation: Increased blood flow in the ischaemic area within a few days after stroke is associated with neurological recovery. The induction of new blood vessel formation (angiogenesis) has been reported with transplantation of several stem cells including those from human cord blood.
• enhance endogenous (inherent) repair mechanisms. Human cord blood cells in the ischemic cortex increased sprouting of nerve fibres.
• reduce death of host cells. Several cell types elicit a neuroprotective effect whereby, presumably by the secretion of trophic factors, there is often reduction in lesion size and inhibition of cell death.
• reduce inflammation. It has been reported that stem cells can directly inhibit T-cell activation, thus inhibiting the immune response. Intravenous injection of human umbilical cord blood cells reduced leukocyte infiltration into the brain thereby reducing the stroke-induced inflammatory/immune response.

Clinical Trials

Results: As a consequence of the encouraging results from experimental studies, pre-clinical phase I and II trials, using different types of stem cells, were tested in patients suffering from stroke (see table). Although some of these trials could demonstrate neurological improvements and cell transplantations appeared to be a safe procedure, the precise mechanisms underlying the restorative effects of stem cells were poorly known at the time of trial(2).

NT2/D1 cells are from a human embryonic carcinoma–derived cell line and have the capacity to develop into diverse mature nerve-like cells (LBS neurons; Layton BioScience Inc.) When transplanted, these neuronal cells survived, extended processes, expressed neurotransmitters, formed functional synapses, and integrated with the host. Safety and feasibility of cellular repair were achieved in this setting. Although this small study was not powered to demonstrate efficacy, valuable data will help in the design of subsequent clinical trials(3).

Future clinical trials considerations: It has been widely proposed that further research should focus on the development of new cell lines; on refining clinical inclusion criteria; on evaluating the need for immunosuppression; and an evaluation of whether ischemic stroke may be more suited to cell therapy than haemorrhagic stroke.

• CTX0E03, a human neural stem cell line, has been developed for the treatment of stable ischemic stroke. The cell line has been tested in rodent stroke models and in normal nonhuman primates. An application for a Phase I clinical trial, running for 24 months, has been submitted to the US Food and Drug Administration and approved.(reported in 1).

• Human umbilical cord blood cells: The use of HUCBC for traumatic brain injury in children has just been approved (ClinicalTrials.gov Identifier: NCT00254722). This is the first clinical trial using these cells for a neurological disorder(reported in 3).

• Timing of transplantation: The brain environment changes dramatically over time after ischemia. The optimal time to transplantation after a stroke will depend on the cell type used and their mechanism of action. If a treatment strategy focuses on neuroprotective mechanisms, acute delivery of the cells will be critical; if the cells act to enhance repair mechanisms (e.g. angiogenesis) then early delivery would be pertinent because these events are most prevalent in the first 2 to 3 weeks after ischemia; if cell survival is important, then transplanting late, after inflammation has subsided, could be beneficial(3).

• Lesion location and size While experimental data suggest that recovery from cortical damage may be more complex than from striatal damage, a conclusive statement can not be made at this point. Precise anatomic location of the lesion and its functional implication, as well as lesion size, will be critical determinants to define the target patient populations for transplantation therapy clinical trials.

Conclusions
Stem cell therapy for stroke holds great promise. However, many fundamental questions related to the optimal candidate (including the patient age, anatomic location and size of the infarct, and medical history), the best cell type, the number and concentration of cells, the timing of surgery, the route and site of delivery, and the need for immunosuppression remain to be answered. Longer-term studies are required to determine whether the cell-enhanced recovery is sustained. Other challenges include ensuring appropriate manufacturing, and quality control of transplanted cells. Clearly, more research is needed to translate cell transplantation therapy to the clinic in a timely but safe and effective manner so that the remarkable potential already shown for cell transplantation to aid recovery from experimental stroke can become a reality for the patient(3).

REFERENCES
1. Stroke repair with cell transplantation: neuronal cells, neuroprogenitor cells, and stem cells Kondziolka D, Wechsler L. Neurosurg Focus. 2008; 24 (3-4):E13.

2 Neural stem cells for the treatment of ischemic stroke Bacigaluppi M, et al. Journal of the Neurological Sciences 265 (2008) 73–77

3. Cell Transplantation Therapy for Stroke Bliss T, Guzman R, Daadi M; Steinberg G. Stroke. 2007; 38[2]: 817-826.

4. Intravenous administration of human umbilical cord blood reduces behavioral deficits after stroke in rats. Chen J, et al. Stroke. 2001; 32 (11):2682-8

5. Stroke-induced migration of human umbilical cord blood cells Newman M, et al. Stem Cells Dev. 2005 Oct;14(5):576-86.

6. Human umbilical cord blood cells do not improve sensorimotor or cognitive outcome following cerebral artery occlusion in rats. Mäkinen S, Kekarainen T, Nystedt J, et al.. Brain Res. 2006 Dec 6; 1123 (1):207-15.

7. Human cord blood CD34+ cells and behavioral recovery following focal cerebral ischemia. Nystedt J, Mäkinen S, et al. Acta Neurobiol Exp. 2006; 66 (4):293-300

8. In vivo tracking of human mesenchymal stem cells in experimental stroke. Kim D, et al. Cell Transplant. 2008; 16(10):1007-12.

9. Intravascular cell replacement therapy for stroke Guzman R, Choi R, Steinberg G et al. Neurosurg Focus. 2008; 24 (3-4):E15.

10. Cell replacement therapy for intracerebral hemorrhage Andres R, Guzman R, et al Neurosurg Focus. 2008; 24 (3-4):E16.

11. Growth factors, stem cells, and stroke Kalluri H, Dempsey R. Neurosurg Focus. 2008; 24(3-4):E14.