Where are we in clinical applications of stem cells in ischaemic stroke?

Posted in Clinical Review Article,Online First,Stroke Series on 28th Nov 2016


dheeraj_kalladka-for-web Dheeraj Kalladka is a Stroke Fellow and Neurology Registrar at the Queen Elizabeth University Hospital, Glasgow and an Affiliate Research Fellow at the University of Glasgow, United Kingdom.
keith_muir-for-web Keith Muir is a SINAPSE Professor of Clinical Imaging and Consultant Neurologist at the Queen Elizabeth University Hospital, University of Glasgow, Glasgow.

Conflict of interest statement: Dheeraj Kalladka received the Jim Gatheral Travel Scholarship, the Mac Robertson Travel Scholarship and the Guarantors of Brain Travel Award during the conduct of the study. Dr. Muir received grants and personal fees from ReNeuron Ltd for PISCES and for the ongoing Phase 2 stroke stem cell trial, and grants from European Union during the conduct of the study.
Provenance and peer review: Submitted and externally reviewed.
To cite: ACNR 2016; 16(3).
Date submitted: 13/4/16
Date resubmitted after peer review: 28/7/16
Acceptance date: 12/9/16
Published online first: 20th October 2016
Correspondence to: Dr Dheeraj Kalladka, Institute of Neuroscience and Psychology, University of Glasgow, Queen Elizabeth University Hospital, Glasgow, United Kingdom. Email: Dheeraj.Kalladka@glasgow.ac.uk


Safety and feasibility of novel stem cell therapy for ischaemic stroke is emerging from limited numbers of carefully selected patients. Exogenous cell therapy as a means of augmenting brain repair processes is promising supported by favourable outcomes in animal stroke models. Mesenchymal stem cell based trials outnumber neural stem cell trials due to ease of sourcing and administration. Further efficacy evidence from larger numbers of patients remains to be seen.


Since the discovery of pluripotency and the ability to guide cell differentiation both in-vitro and in-vivo,1 our understanding of the spectrum of stem cells and their properties has promised therapeutic applications in several neurological diseases, supported in many cases by favourable preclinical studies. Several reviews have discussed the potential indications in stroke, covering the various cell types, time and routes of administration, immunology, preclinical evidence, trial design issues and challenges in the development of clinical applications.2-4 To date, 19 completed human studies have been reported (Table 1), including a total of 275 stroke patients (range between 5 and 65 individuals per study), and in only 6 of these studies – 142 control subjects. The majority of these studies have been early phase 1 trials with their main objectives being to address safety and feasibility. Seven studies5-10 have adopted intra-cerebral implantation (IC), three11-13 have used the intra-arterial (IA) route and nine studies9, 10, 14-20 have used intravenous (IV) routes for cell delivery. The average minimum timing of cell delivery was 88 days post stroke, with very wide inclusion criteria ranging from 1 day to 6 years post stroke. Mean (range) follow-up has been 15.2 months (4 to 60 months). The majority of these studies (13/19 studies) have used mesenchymal stem cells (MSCs) of bone marrow origin or bone marrow mononuclear cells (BMMC) (IV,10, 14-20 IA,11-13 IC8, 10 delivery), two studies used cultured neuronal cells (IC implantation),6, 7 and one study each used neural stem cells (NSC) (IC delivery),10 neural stem / progenitor (NPC) (IV+IC delivery),9 foetal porcine (IC) and a cell suspension of neuronal and haematopoietic cells (intra-thecal (IT) delivery).5

Neural Stem Cells

Small trials began in the late 1990s based upon the concept of tissue replacement, something now considered to be a minor and possibly unachievable mechanism of action. In two studies, Kondziolka et al6, 7 used cultured neuronal cells of teratocarcinoma origin. In the first, uncontrolled safety study,6 non-significant improvement in various neurological scales at 6 months post implantation and increased relative uptake of fluorodeoxyglucose (FDG) on FDG-PET at the implant site or in ipsilateral adjacent brain was reported. In a further study7 from the same group improvements in some aspects of neurological function were noted at 6 months post-implantation compared with an untreated control group although these were not consistent across all neurological assessments. A study of IC implantation of porcine origin foetal cells was terminated due to adverse effects (seizures and cerebral vein thrombosis) not definitely related to cells.5 Neither of these cell lines has been developed further.

Rabinovich et al21 reported significant improvement in Karnofsky functional performance status scores among a group of 10 patients who received a sub-arachnoid injection (via lumbar puncture) of cell suspension having immature nervous and haematopoietic (10:1) cells, compared to a control group (no lumbar puncture), at 6 months post therapy. Qiao et al9 compared IV MSC with a combination of NSPCs of unspecified foetal origin given IC and umbilical cord derived MSCs given IV in 6 subjects with stroke and reported improvement in neurological functions and disability levels in the combination therapy group.

The PISCES trial,10 the first fully regulated study of allogeneic genetically modified foetal NSCs in stroke, was a phase 1 safety and tolerability study in disabled patients 6 months to 5 years after stroke, using genetically modified foetal NSCs delivered by IC implantation into the putamen. No cell-related adverse effects were evident up to 24 months, and improvement in some neurological measures was observed. A phase 2 trial is recruiting in the UK at the present (PISCES 2, NCT02117635), investigating neurological effects of IC implantation on arm function change 6 months after treatment as the primary endpoint.

Mesenchymal Stem Cells and Bone marrow origin mononuclear cells

Given the more established technology of cell harvest for autologous transplantation and IV administration, bone marrow-derived mesenchymal stem cells (MSCs) and a less well characterised population of bone marrow mononuclear cells (only some of which are stem cells) have been the most frequently investigated in both preclinical and clinical studies to date. In animal studies, there is evidence of functional improvement, reduction in infarct volume, and systemic immunomodulatory effects (predominantly from acute administration within hours or days of induction of ischaemia), but IV administered cells neither engraft nor enter the brain in detectable numbers, indicating a paracrine or trophic effect. Intra-arterial (IA) administration delivers more cells to the brain but persistence is also limited,22 and IA delivery has been associated with more complications due to embolic stroke, presumably secondary to cell clumping, and necessitating careful modification of cell delivery protocols. Clinical studies are limited, but in humans, 2 months after stroke, IA administration of autologous bone marrow CD34+ cells11 labelled with Technetium-99 m showed transient distribution to brain at 2 hours post-delivery but persistence of signal in only 2/6 patients at 24 hours. Intravascular delivery is therefore unlikely to represent an engraftment strategy, and both animal and human studies have adopted a “neuroprotectant” paradigm for investigation.

Both bone marrow and other sources of MSCs (eg adipose tissue or umbilical cord blood) may also be used as allogeneic therapies, potentially circumventing one of the major drawbacks of autologous cell therapy, the delay incurred in laboratory characterisation of specific cell populations, and even greater delay involved in ex-vivo culture expansion – a particular issue when acute delivery within plausibly neuroprotectant time windows appears to be the likeliest relevant treatment paradigm. Average time to therapy from marrow aspiration was 6 days (range 0.37 to 9 days) among 17 myocardial infarction trials.23 Such autologous approaches also face the possible drawback of wide variations in dosing, since cell yield is unpredictable and varies among individuals, for example as seen in the study of Bang et al14 using ex-vivo culture-expanded autologous MSCs delivered IV in post-stroke patients. Trial design for autologous cells is further compromised by the ethical and logistical difficulties of undertaking blinded control studies, although this has been achieved in other disease areas such as cardiology. Autologous bone marrow derived MSCs have also been delivered by IC implantation in a single centre early phase study of 5 subjects.8 Autologous BMMC12 with early IA administration (5-9 days after stroke) showed no safety issues. From 2010 to 2015 seven further studies using IV delivery have reported no safety issues. Four15-17, 20 of these studies have relatively delayed (30 to 720 days) cell administration compared to three10, 18, 19 other studies which have administered cells within the first week post stroke. Follow-ups have ranged from 6-60 and 6-12 months respectively.

The great majority of studies report improvements in the treated group in a variety of functional measures including National Institutes of Health Stroke Scale, Barthel Index of activities of daily living, and the modified Rankin Scale between 3 and 6 months post therapy, but trial inclusion criteria are generally very broad, and control groups absent, so claims of efficacy are not yet supported by evidence. At best, it is possible to conclude that no major cell-related safety issues have been reported to date, although with the caveat that there have been a wide range of cell types used and follow-up reports are generally short term.

The largest multicentre study to date in stroke has been that of Hess and colleagues, using allogeneic cells from a donor bone-marrow derived cell line (“Multistem”) characterised as multipotent adult progenitor cells (MAPCs) that have been depleted of CD45 (+)/glycophorin-A (+) cells. The trial included 126 subjects (65 patients given MAPCs and 61 placebo control subjects) delivered IV within 24-48 hours of stroke onset. A trend towards better functional outcomes in the MAPC group has been presented, based on the subset of control subjects recruited within 36h. Consistent with MSC’s pre-dominant anti-inflammatory effects in general, in the treated group, 2 days post administration, significant lower level of circulating CD3+ T-cells were observed, suggesting a reduction in the inflammatory response post-stroke.

Mechanisms of Action:

Early embryonal stem cell (ESC) work focussed on cell engraftment and replacement as prime concept for neuro-restoration. Other mechanisms that are now widely investigated include concepts of stimulating endogenous brain remodelling in particular angiogenesis, neurogenesis, favourable gene expression and axonal restoration and paracrine effects of modulating post-stroke inflammation. The different routes of administration of stem cells dictate and/or limit certain actions. Detailed review is out of scope and has been published elsewhere.2, 3 The neural stem cells in the sub ventricular zone of the adult human brain proliferate and differentiate in response to focal ischaemia and can potentially be stimulated by injected NSCs. Angiogenesis is key to maintaining neural proliferation and both NSCs and MSCs have been known to stimulate angiogenesis to varying degrees. Although stroke limits axonal sprouting, NSCs and MSCs have shown to promote growth factors to improve sprouting, increase axonal density and downregulate inhibitory proteoglycans. Oligodendrocyte numbers are observed to increase which help remyelinate new or damaged axons. Uncontrolled inflammatory response can be deleterious but when controlled can help with repair and the ability of stem cells to modulate host inflammatory microenvironment has been observed resulting in favourable functional outcomes in animal models.

Next Steps:

A large European multicentre randomized, placebo-controlled, double blind trial to investigate the efficacy of IV allogenic adipose derived MSCs (RESSTORE) has been funded and will commence recruitment in the coming months.24 A UK multicentre open-label phase II study25 of intracranial administration neural stem cells (PISCES-2) in subacute stroke is currently recruiting with the primary aim to determine the possible relationship with functional recovery of a paretic arm, measured by the action research arm test. Further trials of MAPCs are planned, and a large number of small, predominantly single-centre studies of IV autologous cells are registered on international clinical trials sites.


Concepts of the potential mechanisms for cell therapy in stroke have moved substantially over the past 5 years, away from a paradigm that envisioned cell engraftment and replacement (albeit a mechanism still potentially relevant, if minor, for intracerebral implantation) and towards a view of cells as a stimulant for endogenous recovery processes and modulator of immunological and inflammatory changes after stroke. Intravascular delivery in particular has more in common with neuroprotectant approaches and this increasingly informs trial design. Investigation of stem cell therapy in stroke remains in early phase trials. The widely different populations of cells that are termed “stem cells” may have very different properties and should not be considered as homogeneous. Several phase II/III trials that are ongoing or planned will refine clinical trial paradigms and pave the way for definitive trials.

#conference proceedings; BI= Barthel Index; BM MNC= Bone Marrow derived Mononuclear cells; ESOC= European Stroke Organization Conference; ESS= European Stroke Scale; FDA= Food and Drug Administration (United States Federal Government Agency); FM= Fugl-Meyer scale; IA= Intra-arterial; IC= Intracerebral; IT= Intrathecal; IV= Intravenous; MRI= Magnetic resonance imaging; MHC= Major Histo-Compatibility; MSC= Mesenchymal stem cells; mRS= modified Rankin Scale; mBI= modified Barthel Index; NIHSS= National Institutes of Mental Health Stroke Scale; NSC= Neural stem cells; NSPC= Neural stem progenitor cells; PET= Positron emission tomography.


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