Background

Stroke is still a primary cause of mortality and morbidity in developed countries [1, 2]. Despite the advent of thrombolytic therapy as a stroke treatment, the limited therapeutic window only benefits a small number of stroke patients. Of note, tissue plasminogen activator (tPA), which is the only approved drug by US Food & Drug Administration, needs to be administered to ischemic stroke patients within three hours [3]. Even those who receive such thrombolytic treatment display significant disability following 3 months of stroke onset. In accordance with the ECASS 3 trial and the American Stroke Association advisory panel, the window for treatment has been extended to 4.5-hours. However, despite the expansion of the tPA treatment time window, many stroke patients are still left with significant disability [4, 5]. Endovascular thrombectomy (ET) is also effective in acute ischemic stroke; however it must be started within 6 h of stroke onset, less than 5–10 % of stroke patients are eligible for treatment, and more than 50 % remain with significant disability after ET [6]. It is, therefore, essential to develop a neurorestorative and neuroreparative treatment for ischemic stroke that could be administered at a later time window since the current time window of 3–6 h limits the number of stroke patients whom can benefit [1, 2].

Cell therapy represents a treatment option for a wider therapeutic time window in stroke patients by targeting multiple reparative, anti-inflammatory, and cytoprotective processes [6, 7] The cells can be administered through intracerebral, intrathecal, intra-arterial, or intravenous (IV) settings. Since the IV route is available in the community hospital setting, it is the most feasible method of administration and the most likely to be disseminated throughout the healthcare system. Despite the belief that the IV route may not provide sufficient cells to the brain due to the distribution to peripheral organs, cells that get lodged into the lung and spleen appear to contribute to the overall benefit from cell therapy. For example, after IV delivery of MSCs, there is an improvement in cardiac performance and reduction of myocardial damage as the lungs trap MSCs and secrete the anti-inflammatory factor, alpha inducible protein (TNFA1P6, or TSG-6) [7, 8]. Similarly, if hematopoietic stem cells and human umbilical cord blood stem cells are delivered intravenously after stroke [8–10], there is reduction in the transmigration of splenocytes, counteracting both the acute inflammatory injury to the brain and the later stroke-induced shrinking and “exhaustion” of the spleen that makes the patient more vulnerable to infection [9–13]. Therefore, there may be advantages to IV delivery versus the intracerebral or intra-arterial methods.

The optimal time window for IV administration is inconclusive, but it may extend out as far as 30 days after the ischemic stroke [12, 14]. Specifically, the early, ‘tail end’ of the neuroprotective period in the time period of about 24 h, presents a rich array of therapeutic targets and an active period of brain remodeling that could be exploited to salvage penumbral tissue and alter inflammatory and immune responses, still evolving days to weeks after the initial stroke [1, 2, 7].

Finding an effective and safe stem cell therapy for stroke

Progenitor stem cells derived from bone marrow or placental tissue are advantageous in cell therapy because they pose no ethical problems or concerns and few safety concerns. Specifically, MSCs serve as a promising form of cell therapy due to their anti-inflammatory and immunomodulatory properties that permit the cells to be transplanted in genetically dissimilar tissue [13–16]. This could lead to cell manufacture of allogeneic cells from healthy unrelated donors (as opposed to autologous cells), allowing a scalable and ‘off the shelf’ stem cell product, that requires no tissue matching.

MAPCs, originally discovered by Verfaillie et al. [15–19] are a distinct adherent bone marrow population that are able to differentiate into cells from all three varying germ layers. After blastocyst injection, their multipotent nature was confirmed by the presence of the cells in all tissues. MAPCs can be isolated from either human or rodent bone marrow, and the cells’ highly expandable nature allows for the cells to be cryopreserved over extended periods and subsequently thawed for clinical use [15, 19]. Early studies showed the cells were effective in a rodent stroke model; intracerebral transplantation of MAPC one-week after cortical stroke resulted in the cells displaying a trophic effect on the host brain that improved sensorimotor ability [18, 20].

Athersys, Inc. has developed a clinical grade scale cell therapy product called MultiStem [21] based on Verfaillie et al. isolation procedure of MAPCs [15–19]. Multiple preclinical studies investigating MultiStem’s administration have provided insights into the cells’ mechanisms of action, including neurotrophic and/or neuroprotective improvements in animal models with neonatal hypoxic–ischemic encephalopathy or focal ischemia [20–25] as well as significant reduction in inflammatory cascades and immune modulation in rodents with traumatic brain injury and spinal cord injury [23, 24, 26–28].

There are many advantages of MultiStem for cell therapy compared with other stem cells. In accordance with Good Manufacturing Practice (GMP) conditions, MultiStem have the potential to be manufactured in large scale and in uniform clinical doses. Similarly, the nature in which the MultiStem cells can be cryopreserved and its subsequent use as an allogeneic product without matching tissues, as noted by the preclinical research completed by Athersys, would allow for easy distribution to patients at appropriate times and dosages [19, 21]. MultiStem cells suppress mixed lymphocyte reactions involving allogeneic T cells and peripheral blood cells and are non-immunogenic [25, 29]. Similarly, in a rat ischemic stroke model, MultiStem cells derived from either rats or humans, when transplanted allogeneic and xenogeneic cells, respectively, exhibited comparable level of functional recovery without the use of immunosuppression [22].

In focal cerebral ischemia, MultiStem cells’ mechanism of action is rooted in trophic and immunomodulatory actions [26, 27, 30, 31]. While human MAPCs cause a reduction in axonal dieback and the secretion of matrix metalloproteinase 9 (MMP-9), there is also a shift from the pro-inflammatory state of M1 to an anti-inflammatory, ‘alternatively activated’ state of M2 in an in vitro axonal dieback and an adult rat dorsal column crush model [23, 26]. MAPCs are also effective at improving ischemic limb function and preservation when delivered directly into the tissue as observed in a model of limb ischemia [24, 32]. Moreover, IV-administered MAPCs preserve splenic mass when they migrate to the spleen and prevent the increase in blood brain barrier permeability as shown in a rodent model of traumatic brain injury [28]. A distinct phenotypic feature of MAPCs over MSCs is their ability to differentiate into endothelial cells and smooth muscles [28, 29, 32, 33]. as well as their capability to secrete vascular endothelial growth factor (VEGF) and other angiogenic molecules in vitro, and to increase vascular density in vivo in a myocardial infarction model [30, 34].

Translation to the bedside

MultiStem cell-based therapy is a promising therapeutic avenue for ischemic stroke. In acute myocardial infarction and in prophylaxis of graft vs. host disease, MultiStem cells have completed early phase clinical trials. MultiStem cells that were administered in myocardium after coronary intervention were well tolerated in a phase I trial of patients with first ST-elevation [31, 35]. There was improvement in stroke volume 4 to 12 months after patients were treated with a dose of about 50 million cells [31]. The MultiStem in Acute Stroke Treatment to Enhance Recovery Study (MASTERS) is a multicenter, randomized, double-blind, placebo-controlled trial of MultiStem in acute ischemic stroke where MultiStem are administered IV 24 to 48 h in moderate to moderate-severe ischemic stroke patients [36]. ClinicalTrials.gov Identifier: NCT01436487. This trial demonstrated the feasibility of administering a very large dose of cells IV −1.2 billion per patient- a much higher dose than can be achieved with other types of cell therapy. The study just completed enrollment. Careful and rigorous preclinical studies and limited clinical trials of MultiStem cells will provide guidance into the safe and effective application of cell therapy for stroke.

Conclusion

MultiStem are a promising cell therapy for ischemic stroke. Their major mechanism of action is likely immunomodulatory and they target the spleen and immune system. Clinical trials to date show safety of MultStem. The major advantages of clinical grade MultiStem are scalability and “off the shelf” availability in a hospital pharmacy with no requirement for tissue matching. If shown effective in ongoing and future clinical trials in ischemic stroke, MultiStem would have a wider time window allowing more patients to benefit and would also be available not only in academic medical centers but also in community hospitals.