Cell Therapy for Multiple Sclerosis

Tamir Ben-Hur 0 ) Department of Neurology, The Agnes Ginges Center for Human Neurogenetics, Hadassah - Hebrew University Hospital , Jerusalem 91120, Israel The spontaneous recovery observed in the early stages of multiple sclerosis (MS) is substituted with a later progressive course and failure of endogenous processes of repair and remyelination. Although this is the basic rationale for cell therapy, it is not clear yet to what degree the MS brain is amenable for repair and whether cell therapy has an advantage in comparison to other strategies to enhance endogenous remyelination. Central to the promise of stem cell therapy is the therapeutic plasticity, by which neural precursors can replace damaged oligodendrocytes and myelin, and also effectively attenuate the autoimmune process in a local, nonsystemic manner to protect brain cells from further injury, as well as facilitate the intrinsic capacity of the brain for recovery. These fundamental immunomodulatory and neurotrophic properties are shared by stem cells of different sources. By using different routes of delivery, cells may target both affected white matter tracts and the perivascular niche where the trafficking of immune cells occur. It is unclear yet whether the therapeutic properties of transplanted cells are maintained with the duration of time. The application of neural stem cell therapy (derived from fetal brain or from human embryonic stem cells) will be realized once their purification, mass generation, and safety are guaranteed. However, previous clinical experience with bone marrow stromal (mesenchymal) stem cells and the relative easy expansion of autologous cells have opened the way to their experimental application in MS. An initial clinical trial has established the probable safety of their intravenous and intrathecal delivery. Short-term follow-up observed immunomodulatory effects and clinical benefit justifying further clinical trials. - Although current therapy in multiple sclerosis (MS) is directed at the underlying autoimmune pathogenic process, cell therapy has been advocated as a means of regenerative medicine. In this review, the complexity, advantages, and difficulties in cell therapy will be discussed. Is Cell Therapy a Valid Option for MS? Any discussion of cell therapy for MS needs to first take a look at the endogenous processes of brain repair and their failure to build a rationale for the feasibility and prospects of treatment by cell transplantation. Glial Progenitor Cells in the Adult Central Nervous System The identification of neural precursor cells and neurogenesis in the adult central nervous system (CNS) [13], including that of humans [46], and the identification of persistent neural stem cells (NSCs) as the parental cells from which new neurons are derived [714], has revolutionized our concepts of the adult brain as structurally immutable. There are several niches in the adult brain in which NSCs persist throughout life and can respond to injurious processes [1518]. New neurons are continuously generated in the anterior subventricular zone (SVZ) of adult rodents, from which they migrate via the rostral migratory stream to the olfactory bulb [1923], and in the subgranular zone of the hippocampal dentate gyrus of both adult rodents [2427] and humans [6, 28]. The subependymal cell layer of the ventricles [29] and spinal cord [30] contains stem cells that give rise to both neurons and glia. Multi-potential precursors are abundant in many regions of the adult brain parenchyma [3134]. In particular, oligodendrocyte progenitor cells (OPCs) were isolated from various regions of the adult rodent CNS [3537], and were identified also in the adult human brain [3842] and spinal cord [43]. OPCs are identified by expression of chondroitin-sulfate proteoglycan NG2+ and of platelet-derived growth factor receptor- are highly abundant in the adult CNS, comprising up to 5% of its cells [37]. The origin of endogenous remyelinating cells in the adult CNS has been subject to multiple studies (for more detail see Franklin and Ffrench-Constant [44]). Differentiated postmitotic oligodendrocytes are unable to rebuild myelin sheaths [4547] and remyelination is dependent on cycling cells [47, 48]. The notion that OPCs are the main remyelinating cells of the adult CNS emerged from studies showing remyelination after focal demyelination by resident progenitor cells [49, 50]. Tissue OPCs expressing NG2 and platelet-derived growth factor receptor- on their cell surface are mobilized in response to demyelination [49, 51 53]. Recent studies provided the definitive proof that these cells are indeed the main remyelinating cell in the CNS following a demyelinating injury [54, 55]. In addition, neural precursor cells (NPCs) of the adult SVZ expressing the embryonic polysialylated form of the neural cell adhesion molecule (PSA-NCAM) react to inflammation and demyelination with proliferation, and migration into the tissue and glial differentiation, generating both astrocytes and remyelinating oligodendrocytes [5659]. Myelin Regeneration Fails in MS In MS, the inflammatory process in the CNS leads to demyelination. The affected demyelinated regions can undergo partial remyelination, leading to structural repair and recovery of function [6063]. Attempts to regenerate myelin can be recognized pathologically in brains of MS patients by the existence of shadow plaques, which are partially remyelinated lesions. Analysis of brain tissue from MS patients suggests there are several different pathological patterns of demyelination [64]. In some patients, there was progressive loss of oligodendrocytes and myelin without reactive remyelination, whereas in others, who exhibited strong T-cell and macrophage activity, there was robust remyelination, indicating the important role of tissue support to the remyelinating response [65]. The sequential involvement of these processes underlies the clinical course, characterized by episodes of relapses, which after full remissions early in the course of disease, eventually leave persistent deficits, and finally deteriorate into a secondary chronic progressive phase. Moreover, remyelination is typically incomplete and ultimately fails in the setting of recurrent episodes contributing to the progressive demyelination, gliosis, axonal damage, and neurodegeneration typically noted in MS [66, 67]. Several studies have indicated that axonal pathology is the best correlate of chronic neurological impairment in MS and its animal model, experimental autoimmune encephalomyelitis (EAE) [6873]. It is unclear why remyelination fails in time with MS [44, 67]. Some studies showed a depletion of progenitor cells after focal demyelination in experimental animals [52, 74], whereas others showed that repeated episodes of demyelination did not slow down remyelination [75]. In pathological specimens of chronic MS lesions in human patients, neither decrease nor reactive increase was observed relative (...truncated)