01 May 2002
The Lancet Neurology, Volume 1, Number 1
Christopher Halfpenny , Tracey Benn , and Neil Scolding
Institute of Clinical Neurosciences, Frenchay Hospital, Bristol, UK.
A decade ago, therapeutic strategies to remyelinate the CNS in diseases such as multiple sclerosis had much experimental appeal, but translation of laboratory success into clinical treatments appeared to be a long way off. Within the past 12 months, however, the first patients with multiple sclerosis have received intracerebral implants of autologous myelinating cells. Here we review the clinical and biological problems presented by multiple sclerosis disease processes, and the background to the development of myelin-repair strategies. We attempt to highlight those areas where difficulties have yet to be resolved, and draw on various experimental findings to speculate on how remyelinating therapies are likely to develop in the foreseeable future.
Dramatic scientific advances in brain repair are never far from the headlines. The transplantation of fetal neurons into the brains of patients with Parkinson's or Huntington's disease, whether appearing to go well or ill, rightly attracts attention. In addition, the highly publicised debate over stem cells and human cloning has alerted the public (raising its expectations the while) to the possibilities of transferring cell-repair strategies, particularly for neurological diseases, from laboratory to clinic.
Embryonic stem cells “could prove the Holy Grail in finding treatments for cancer, Parkinson's disease, diabetes, osteoporosis, spinal cord injuries, Alzheimer's disease, leukaemia and multiple sclerosis…transform[ing] the lives of hundreds of thousands of people” (authors’ italics; Yvette Cooper, UK Public Health minister, quoted in The Times, Dec 16, 2000). Multiple sclerosis has claimed further newsprint more recently with reports that the first remyelinating-cell implantation into the CNS of a patient with multiple sclerosis has been done (http://www.myelin.org/schwann_cells.html ).
How does this extensive lay coverage relate to the true clinical scientific picture? Here we review progress in glial-cell biology, in experimental myelin repair, and in our understanding of the pathology, pathogenesis, and treatment of multiple sclerosis. These advances have established remyelination therapy as a realistic prospect for the current generation of patients with multiple sclerosis.
Spontaneous myelin repair in multiple sclerosis
In 1965, ultrastructural studies provided the first clear evidence of spontaneous myelin repair in multiple sclerosis ( figure 1 ), 1 only a few years after the first demonstration of experimental spontaneous CNS remyelination. 2 It has been much studied by contemporary pathologists, 3–6 whose studies offered several interesting historical perspectives. The first was that the classically described “shadow plaque” (Markschattenherde) in fact contains large numbers of uniformly thin myelin sheaths, indicating successful repair of myelin across whole plaques. These shadow plaques are present in acute cases, comprising areas of pale-staining myelin either on the edge of a plaque, or in normal unaffected white matter, and originally were thought to represent a young and active, or incompletely demyelinated, lesion. The second important observation was that Joseph Babinski's often reproduced illustration of myelin phagocytosis in multiple sclerosis also clearly illustrates concurrent myelin repair.
Figure 1. The first demonstration of spontaneous myelin repair in multiple sclerosis—a substrate upon which all therapeutic remyelinating strategies aim to build. Reproduced with permission of Oxford University Press. 1
The occurrence of spontaneous remyelination has profound conceptual implications for myelin-repair therapies: no longer is it necessary to create a repair phenomenon de novo; rather, the therapy should stimulate or supplement a spontaneous process. How this might be done depends on understanding of the clinical biology of the disease. The reasons why endogenous repair is not more successful, and whether the limitations can be overcome, can only emerge from a fuller appreciation of the biology of myelinating glial cells.
Oligodendrocyte progenitor cells
Quiescent oligodendrocyte progenitor cells (OPC), rather than surviving mature oligodendrocytes, are generally thought to bring about the majority of spontaneous remyelination. 7–10 Large populations of immature oligodendrocytes can be found in fresh lesions, 11,12 including significant numbers of cells with the phenotypic markers of oligodendrocyte precursors. 13–16 Additionally, Schwann cells make a small contribution to endogenous myelin repair, especially in the spinal cord. 6,17,18
Recent studies have investigated the origin, nature, and limitations of spontaneous myelin repair. That it starts while demyelination is still occurring, as unwittingly shown by Babinski, has been confirmed. 5,19 In the early inflammatory phase of multiple sclerosis some 40% of plaques may contain areas of remyelination occupying 10% or more of plaque volume. 19 This proportion seems to fall as the disease progresses, 20 with the majority of chronic plaques having either no new myelin or a thin peripheral rim.
Several possible explanations for the ultimate failure of repair have been proposed. First, repeated demyelination appears to impede subsequent repair, 19,21 and the depletion of remyelinating cells is commonly accepted to have a major role. 8,10,22–24 Indeed, oligodendrocyte progenitors may be directly targeted by disease processes. 25 Second, the limited migratory capacity of endogenous progenitors is likely to contribute. 26 Third, molecular changes in chronically demyelinated axons may make them no longer amenable to remyelination. 27 Finally, the progression of the astrocytic scar, obstructing the dispersal of myelin-forming glia within areas of demyelination, also might impede remyelination. 28
The influence of astrocytes is complex, however. In experimental studies, extensive myelination by implanted cells of oligodendroglial lineage or by Schwann cells occurred unimpeded by host or purified transplanted astrocytes. 29–31 Others have described cohabiting astrocytes and Schwann cells in lesions that show “peripheral” myelin repair. 17,18 Paradoxical roles for astrocytes are increasingly appreciated in oligodendrocyte remyelination: chronic astrocytic scars may obstruct myelin repair by OPC, whereas acutely reactive astrocytes synthesise and release promigratory and proliferative growth factors for OPC.
Supplementation of remyelination by glial-cell transplantation
Cells of the oligodendrocyte lineage
Oligodendrocytes were first discovered only 80 years ago by del Rio Hortega; he also described their principal function, the synthesis of myelin in the CNS. We now know that they develop from a progenitor cell, the properties of which have been much studied since culture techniques offered new opportunities for cell biological investigations. 32,33 Few cell types have been so intensively studied; the growth factors that lead to the proliferation, survival, differentiation, and maturation of oligodendrocyte-lineage cells have been minutely defined.
Much experimental work on remyelination has focused on the oligodendrocyte lineage. There is an inherent logic in concentrating on these cells. They are the cells lost in multiple sclerosis; it is their normal function to myelinate the CNS, and spontaneous oligodendrocyte remyelination in multiple sclerosis bears witness to their substantial inherent capacity for remyelinating damaged areas of the brain. In addition, there is a wealth of experimental evidence to demonstrate the production of new myelin in animals after transplantation of purified oligodendrocyte-lineage cells 24,34–39 or cell lines. 36,40,41 There is also evidence to suggest that this process is accompanied by both improved conduction 42 and demonstrable functional recovery. 43
The stage within the oligodendrocyte lineage most suitable for transplantation is important. Although some studies have suggested that mature differentiated oligodendrocytes are useful myelinating cells, 34 others suggest they have only limited capacity. 29,44 Comparative experiments have shown better myelin formation by implanted immature progenitors than mature oligodendrocytes. 37 The majority view is that mitotic potential is an important prerequisite for successful myelin formation, 45 and that postmitotic oligodendrocytes do not readily recapitulate their development to form mature myelin sheaths again.
A further significant advantage of the progenitor phenotype is a demonstrable migratory potential. 37,46,47 These cells seem to migrate better through demyelinated lesions than do their mature counterparts, which manifest a more complex morphology both in vitro and in vivo. However, endogenous progenitors migrate only 1–2 mm to repopulate demyelinated areas, 26 reflecting a very limited migratory potential through normal brain parenchyma. Astrocytes significantly impede progenitor migration in vitro, 48 as do mature oligodendrocytes, 49 suggesting significant inhibition by surface molecules.
Transplantation studies also show poor survival and migration when progenitors are implanted into normal white matter, although they are able to populate and remyelinate when injected into, or very close to, lesioned tissue. 50 By contrast, these cells survive well in X-irradiated tissue, which depletes endogenous progenitor numbers. 51 Part of this better survival may reflect competition between endogenous and implanted cells for survival factors, since progenitor numbers increase with increased availability of platelet-derived growth factor 52 or glial growth factor 2. 53 The possibility of improving graft survival and proliferation by the use of growth factors has been explored with some success in vivo. 54
Investigations of human CNS glia have consistently shown significant biological differences from rodent cells, so that data on rodent OPC cannot be directly extrapolated to human glia. Early studies identified glia similar to the rodent OPC in cultures derived from the fetal human CNS. 55 After transplantation, these cells can synthesise myelin in the dysmyelinated rodent CNS, even after cryopreservation. 56
More recently, a progenitor was identified in cultures of adult human brain ( figure 2 ) and shown to have similar immunophenotype and differentiation potential to its rodent counterpart. 57–59 But the response to growth factors seemed different—indeed no soluble mitogens for this stage of development have yet been defined. Furthermore, when mixed human oligodendrocyte-lineage cells containing small numbers of these cells were transplanted into demyelinated rat spinal cord, myelin membranes, but not compact multilamellar myelin sheaths, were observed. 60 Methods for selection of these cells (for experimental purposes) from samples of human white matter have since been perfected, 61 but as yet no reports of useful myelin formation after transplantation of adult human progenitors have been published.
Figure 2. Proliferating adult human oligodendrocyte progenitors, stained with the progenitor marker A2B5 (red), and counterstained for nuclear uptake of BrDU (yellow). Mature oligodendrocytes (stained for galactocerebroside) are stained green.
Further studies showed that progenitors are identifiable in situ by use of wet-mount tissue print preparations, 62 and more conventional immunohistological techniques have confirmed their presence in normal adult white matter and also in acute and chronic lesions from patients dying with multiple sclerosis ( figure 3 ). 13,15,16 Using the marker NG2, much larger numbers of these cells have recently been identified, 14 although we have not found NG2 to be a specific glial-cell marker. 63,64
Figure 3. Adult human progenitors grown in cell culture and stained with the monoclonal antibody O4 can exhibit a multi-processed morphology (left); a similar morphology is seen when progenitors in normal adult white matter are stained with antibodies against the platelet-derived growth factor receptor (right).
In the absence of defined proliferative signals, acquisition of adequate numbers of OPC for therapeutic purposes, whether autologous or heterologous, adult or fetal, remains a problem, and the elucidation of human glial progenitor mitogens (or, perhaps, factors that prevent the proliferation of human progenitors in response to rat OPC mitogens) is emphatically a research priority in assessing their potential for transplantation purposes.
As mentioned above, oligodendrocytes are not the only glial cells that can remyelinate CNS axons; inwardly migrating Schwann cells carry out a proportion of spontaneous myelin repair in multiple sclerosis. A reasonable presumption is that, by contrast with oligodendrocyte-established new myelin, Schwann cells and their myelin sheaths should be resistant to continuing disease-related immunological attack, because little or no peripheral nerve damage occurs in multiple sclerosis. Experimental methods for preparing cultures of adult human Schwann cells from peripheral nerve biopsy samples are now well established. 65,66 These cultures can be purified and expanded in vitro to generate large populations of human Schwann cells. 67
Perhaps surprisingly, the demonstration that implanted dissociated Schwann cells could remyelinate the rodent CNS preceded comparable oligodendrocyte studies, 39,68 and restoration of normal central conduction to the demyelinated rodent spinal cord by both endogenous 69 and exogenous (transplanted) Schwann cells has been demonstrated. 70 Human Schwann cells successfully lay down new myelin in the spinal cord of mice 71 and rats. The reparative capacity of purified, heregulin-expanded human Schwann cells is not compromised by long-term frozen storage. 67,72 Schwann cells were used in the few patients with multiple sclerosis so far transplanted in a phase I experimental clinical trial (http://www.myelin.org/schwann_cells.html ).
Harvesting of autologous Schwann cells from peripheral-nerve biopsy samples, expansion in vitro, and transplantation in patients with multiple sclerosis offers the attractions of ready availability and the avoidance of rejection. Firm evidence is required, however, that expanded human Schwann cells do not form tumours in vivo, a hazard described after transplantation of rodent Schwann cells immortalised by growth factor expansion; 73 unpurified preparations of human peripheral nerve cells result in substantial fibroblast overgrowth with axon destruction. 67 This process presents an imposing barrier to the clinical application of Schwann-cell transplants.
Another potential problem for the use of Schwann cells in remyelination therapy is the apparent inhibitory effect of astrocytes on Schwann-cell-mediated CNS remyelination, 74–76 though, as mentioned above, the interactions between astrocytes and myelinating glia are complex, suggesting that further research is necessary before the potential for use of Schwann cells in CNS remyelination therapy can be fully assessed.
Olfactory ensheathing cells (OEC) are found in the olfactory bulb, nerves, and epithelium. They ensheath the axons emanating from olfactory epithelial neurons that penetrate the olfactory bulb of the CNS. Normally, OEC are non-myelinating, but rodent OEC assume a myelinating phenotype similar to that of Schwann cells when transplanted into lesions containing demyelinated axons. 77,78 The ability of OEC to promote CNS axon regeneration and ensheath and myelinate demyelinated axons has led to much interest in the potential of olfactory glia in the field of CNS repair. 79
One of the potential advantages of OEC over Schwann cells relates to their relationship with astrocytes. In health, OEC coexist alongside astrocytes within the olfactory bulb, whereas in experimentally demyelinated lesions OEC can (by contrast with Schwann cells) ensheath and myelinate axons unimpeded by the astrocytic environment. 77 Rodent OEC migrate far more successfully over astrocytes than Schwann cells in vitro. 80 Human OEC ( figure 4 ), like rodent OEC, are capable of remyelination after transplantation into demyelinated lesions in the rodent spinal cord. 81,82
Figure 4. Adult human olfactory glia are readily grown in cell culture, and may be an important candidate for implantation in multiple sclerosis.
Immortalised cell lines
One possible solution to the problem of obtaining sufficient numbers of cells for transplantation is to use immortalised cell lines, which could yield large numbers of appropriate cells in homogeneity. These cells lines have already proved invaluable in probing the complexity of myelin formation. Much of the work on rodent progenitors has used spontaneously arising lines such as CG4, which retains many of the characteristics of rodent progenitors—including the capacity for spontaneous differentiation and myelination in vivo—but which proliferates indefinitely in vitro under the influence of mitogens. 83 Some human glial cell lines have also been described, mainly derived from tumours, but they have not proved entirely faithful to their primary cell.
However, the techniques of gene transfer have enabled cell lines to be engineered from primary cells, sometimes by methods in which oncogene activity can be controlled. Conditionally immortalised neonatal rodent progenitors generated in this way repair myelin after transplantation, 36,40,77 although only a proportion of transplanted cells appear to differentiate into terminal, myelinating oligodendrocytes, and some immortalised progenitors may continue to proliferate in vivo in an uncontrolled fashion. Attempts have been made to immortalise human OPC, but evidence of myelin formation in vivo is lacking.
Despite increasingly elegant and elaborate techniques to prevent malignant transformation, the axiomatic impairment of cellular growth regulation that accompanies the formation of cell lines still carries a significant risk of tumour formation, and this possibility will continue to represent a serious anxiety over the clinical use of implanted immortalised cell lines.
An almost limitless source of glial cells for transplantation could potentially be available by xenogeneic transplantation. Many successful remyelination studies in various laboratories prove the feasibility of xenogeneic glial-cell transplantation. 84–86 One disadvantage, however, is the need for more potent immunosuppressive agents such as cyclophosphamide or ciclosporin to prevent tissue rejection; recent studies indicate that drugs such as cyclophosphamide may significantly inhibit remyelination, 87 but ciclosporin is toxic to oligodendrocytes in vitro, inducing apoptosis. 88 The development and availability of genetically engineered pigs designed to escape human rejection 89 could bypass this difficulty, but fears of retroviral or other host infection, much publicised in the UK, will properly delay clinical experimental trials—although patients (for example, with Parkinson's disease) have received porcine xenografts in the USA. 90
Rodent embryonic stem cells have substantial remyelinating potential, 91 but their equivalent human sources—aborted fetuses or embryos surplus to requirements from in vitro fertilisation—are not easy to access. Some researchers suggest that stem cells from embryos cloned for the purpose (by cell nuclear transfer) from each patient needing an implant, as recently uniquely legalised in the UK, would have the huge advantage of avoiding rejection. However, the proposal that every patient requiring a transplant would first have to be cloned seems quite unrealistic, and the serious ethical and practical difficulties pertaining to all sources of human embryonic stem cells have stimulated the largely successful search for other sources. 92 It is now clear that neural stem cells are present in the adult rodent brain; 93 large numbers of oligodendrocyte-lineage cells can be generated by neurosphere/oligosphere techniques, 94–96 and these, on transplantation, successfully remyelinate axons. 97 Neural stem cells are also present in the adult human brain ( figure 5 ). 98
Figure 5. A growing sphere of human neural cells cultured from a temporal lobectomy specimen in our laboratories. Cells are expanded under the influence of growth factors; both neuronal and glial progeny can be generated in this way (diameter of sphere ?50 mm).
Stem cells are also found in adult mammalian skin and in adipose tissue. Importantly, from a therapeutic perspective, adult bone-marrow-derived stem cells 99 also have clear neural potential ( figure 6 ). 100–103 Direct implantation of rodent bone-marrow cells can achieve successful remyelination in the rodent spinal cord. 104 Although it is a little early to predict how successfully the use of stem cells derived from adult human bone marrow will develop, the difficulties in relation to the primary glial cell types may well result in bone-marrow-derived cells becoming the main source of implantable remyelinating cells.
Figure 6. Adult human mesenchymal stem cells. The cells shown are second passage mesenchymal stem cells (MSC) cultured from normal adult human femoral shaft marrow. Reproduced with permission of Jill Hows, University of Bristol, UK.
Promotion of endogenous remyelination
The difficulties of obtaining usable cells and, given the disseminated nature of multiple sclerosis, of knowing where to implant them, increase the attraction of systemic therapies that might have a general effect of promoting endogenous repair.
In an extensive series of studies, the possibility that immunoglobulins might promote remyelination has been systematically explored. In animals with CNS demyelination caused by chronic Theiler's virus infection, myelin repair was increased by treatment with systemic whole antiserum or purified IgG directed against spinal-cord homogenate. 105 Polyclonal immunoglobulins against myelin basic protein also achieved this effect, as did a monoclonal antibody directed against an oligodendrocyte surface antigen. 106 The antibody belongs to the class of “natural autoantibodies”—naturally occurring polyreactive antibodies of uncertain function and significance. 107 Antibody binding to oligodendrocytes might directly stimulate myelinating function, but in vitro observations suggested that immunoglobulins have no direct effect on oligodendrocyte function. 108 A more likely possibility is that the immunomodulatory and anti-inflammatory consequences of immunoglobulin treatment encourage remyelination. 107,109–111 Furthermore, the first trials of intravenous immunoglobulin as a putative systemic remyelination therapy have now been completed, unfortunately with negative results. 112,113
Growth-factor treatment to promote remyelination has superficial attractions, but several factors mitigate against this approach: the complex requirements for multiple and different factors during the sequential phases of OPC proliferation, migration, differentiation, and myelination; 114–116 the clear but as yet incompletely explored differences in growth-factor requirements between human and rat OPC; and the fatal effects of the exposure of postmitotic oligodendrocytes to potentially mitogenic growth factors. 117–119 The first trial of systemically delivered insulin-like growth factor 1 in multiple sclerosis has recently been stopped early, and attention is increasingly turning towards glial-cell transplantation as a more promising therapeutic approach.
Application of experimental remyelination biology to patients with multiple sclerosis
Site of implantation
Most experimental studies have explored the effects of transplantation into a single site, but in multiple sclerosis almost innumerable areas of demyelination are commonly disseminated through the CNS ( figure 7 ). Clearly, the prospect of many inoculations into widely disparate lesions is unrealistic. What should not be overlooked, however, is that many plaques are clinically silent, whereas a disproportionate degree of disability emanates from a few critical lesions in eloquent areas in many cases. Thus, implantation into a very small number of carefully selected lesions—for example, the optic nerves, the spinal cord, or the superior cerebellar peduncle—could yield a useful therapeutic dividend. 120 Early phase II clinical trials are likely to proceed on this basis.
Figure 7. The wide dissemination of lesions throughout the brain and spinal cord emphasises the enormity of the problem, and the need for a highly targeted or focused approach to any implantation strategy. Reproduced from Charcot JM. Lecons sur les Maladies du Systeme Nerveux faites a la Salpetriere, 1872.
Others continue to pursue a more global myelin-repair strategy, both for multiple sclerosis and for the significantly rarer group of patients with inherited disorders of myelin metabolism. The latter, characterised by diffuse dysmyelination, would require successful remyelination of large tracts to yield any functional improvement. However, in these patients, unlike those with multiple sclerosis, myelin formation is unlikely to be hampered by astrocytic scarring, and the absence of autoimmune myelin destruction could allow a more stable outcome, notwithstanding the suggested contribution of inflammatory processes to myelin damage in adrenoleucodystrophy. 121 Thus far, other strategies using bone-marrow transplantation, gene therapy, or both appear to offer more promise for certain inherited leucodystrophies. 121,122
An appealing solution to the problems of cell dispersal would be to encourage transplanted cells to migrate widely, as occurs during development. Migration through mature brain parenchyma is very limited, but promigratory agents have been identified, and supplemention of cellular transplantation with growth-factor infusions, 123 or even cotransplantation with growth-factor-secreting cells, 54 has been tried with limited success. Another approach would be to suppress the expression or function of molecules that inhibit migration. 28,124 Finally, studies in neonatal dysmyelinating rodents have shown substantial cell dissemination after intraventricular transplantation, 125 especially after two implantations separated by a few days. 126
The timing of implantation
The aim of a remyelinating therapy in patients with multiple sclerosis is to restore function—to reverse certain types of persistent disability, at least partially. The increasingly recognised importance of axonal loss ( figure 8 ) as a pathophysiological substrate of accumulating disability by no means excludes a significant or substantial contribution by persistent demyelination to chronic disability. 127,128 The relative preservation of axons in chronic lesions offers pathological reasons for believing remyelination to have a prospect of functional value, and the transient deterioration in acute and chronic symptoms with fever—Uthoff's phenomenon—provides strong pathophysiological evidence for demyelination as an important substrate for impaired function.
Figure 8. Axon damage in multiple sclerosis, illustrated here by Greenfield and King (Brain 1936; 59: 445-59). This shows why myelin-repair strategies are unlikely to succeed if initiated too late, while early, succesful repair may help prevent such axon loss. Reproduced with permission of Oxford University Press.
The timing of a remyelinating treatment in multiple sclerosis, therefore, remains difficult. Clinically, the unpredictable course increases the temptation to defer a potentially hazardous intervention until progressive disability is established and hope of spontaneous recovery extinguished—the first principle in any new therapeutic endeavour must always remain “first, do no harm”. From a biological perspective, however, early intervention may offer significant advantages: as mentioned above, this is when spontaneous remyelination occurs, suggesting the most propitious environment, and whatever the specific reasons for failed endogenous repair, most of the explanations offered relate to chronicity. Furthermore, the contribution of progressive axonal loss to secondary progressive multiple sclerosis 128,129 also mitigates against very late intervention; little can be expected of repair strategies when the axonal framework for remyelination has been lost.
However, axonal loss may paradoxically provide another potent reason for early remyelinating intervention—namely, that progressive axonal damage might be a consequence of persistent myelin loss. 128,130 Axon loss is unlikely to have a single cause. Inflammatory processes almost certainly contribute, perhaps particularly to acute axonal fragmentation, 128,131,132 a feature of acute inflammatory demyelinating lesions. This feature is, however, rather less likely to have pathophysiological effects; the reversible nature of acute relapses is more easily explained by the resolution of oedema and inflammation and by spontaneous remyelination than by axon fragmentation. 128,130 More securely established is the clinical effect of accumulating axon loss in secondary progressive disease, the course of which seems not to be closely influenced by early inflammatory disease activity 133–135 or, sadly, by even the most profound immunosuppressant or anti-inflammatory treatments.
Such progressive axon fallout may be a late corollary of demyelination. Pathological studies have indicated that axon loss does not correlate with inflammatory cell infiltrate, expression of tumour necrosis factor or nitric oxide, or demyelinating activity, but it is related to the overall extent of established myelin loss. 133,135 Axon loss is seen in lesions that are demyelinated but show little or no inflammation, but it is rare in remyelinated lesions. 135 Demyelination-induced axon loss might occur by several possible mechanisms: directly, through the loss of oligodendrocyte-derived trophic support 136 or axon dependence on myelin sheaths; 137 as a consequence of sustained demyelination-induced conduction block and electrical silence; 138 or indirectly through increased vulnerability of the exposed axon to injurious agents. 139
Thus, the earlier the intervention, the greater the potential gain. But the significant obstacle to this approach is the risk of losing repaired areas, and carefully prepared and implanted remyelinating cells, to continuing disease activity. Although advances in immunotherapy feed a cautious optimism, no current therapies are able to stop myelin destruction. Concurrent use of potent immunosuppressive agents, perhaps required in any case to prevent graft rejection, might help, yet there is evidence that some of these agents themselves, 87 or the suppression of inflammation in general, 140 may impair myelin regeneration.
Attention needs to be focused on both clinical management and monitoring after transplantation. Three modes of treatment could be required: continually exogenous trophic support of grafted cells; immunosuppression to prevent graft rejection; and adequate control of disease activity to reduce graft loss. These issues cannot be fully addressed before the early clinical trials, not least because of the substantial differences between animal models and human beings.
Little information is available for the first issue—continuing exogenous trophic support of grafted cells—though some animal studies do illustrate a potential effect. 54 The adverse influence of antimitotic immunosuppressants on remyelination has been mentioned already. Ciclosporin has been successfully used in Parkinson's disease transplant trials. In relation to multiple sclerosis, antirejection prophylaxis might help inhibit disease progression. The antileucocyte humanised monoclonal antibody Campath-1H, currently under investigation for the treatment of both (solid-organ) transplant rejection and multiple sclerosis 141,142 is particularly promising in this respect. Nevertheless, if cell implantation is adopted, there may be a risk, despite immunosuppression, of inciting new antioligodendrocyte immune reactions that not only see off the graft, but also augment underlying disease processes. Autografting might avoid this outcome.
Sensitive and precise methods of monitoring graft survival, migration, efficacy of remyelination, and early detection of uncontrolled growth will be essential if these therapeutic protocols are to be explored responsibly in a clinical setting. Furthermore, myelin repair without clinical improvement will be a hollow victory, so robust and reproducible methods of clinical assessment would need to be applied from the start.
Non-invasive imaging is clearly attractive, most obviously magnetic resonance imaging (MRI) since it is widely available, safe, and well tolerated. Resolution is high, but contrast and specificity are more problematic. Standard protocols cannot disclose remyelination, but advances continue to offer new techniques, of which magnetisation transfer contrast is the strongest candidate for imaging remyelination. 143 Measurement of N-acetyl aspartate levels by magnetic resonance spectroscopy offers the means of assessing any impact on local neuron/axon survival. 144,145
Use of paramagnetic particles to label cells before transplantation, enabling their dispersion to be tracked by MRI, 146–148 has promise, though from a safety perspective even the most trivial manipulation of cells before implantation would be better avoided. Furthermore, graft survival cannot be inferred from migration, since cells which subsequently die remain visible, 146 and this method not only fails to show new myelin formation but may also impair the ability of other magnetic resonance modalities to do so. Positron emission tomography, although expensive and of limited availability, can be both sensitive and specific, but no appropriate ligands are yet available for monitoring remyelination.
Serial neurophysiology may also prove to be valuable, and by monitoring conduction times may provide evidence of returning saltatory conduction in the targeted pathways. The optic nerve has particular advantages in this respect.
If the sites mentioned above—the optic nerve, spinal cord, or brainstem—are selected for the first experimental trials seeking clinical benefit, programmes for rigorous monitoring of both the biological and clinical effects of the intervention need to be established, including not only imaging and electrophysiological examination but also physical assessment of any clinically relevant effects. Specific clinical outcome measures of function, disability, and handicap must be adopted and tailored for each group of patients. Ultimately, success will need to be measured by properly designed clinical trials, in which clinical outcomes are likely to carry the greatest weight. Advances in clinical scale design have improved physical and functional measurement in multiple sclerosis, 149 so that the tools for assessment of clinical outcome, on which remyelination therapies must stand or fall, are becoming available.
Continued progress in our understanding of the clinical biology of remyelination and of myelinating glia have significantly improved the prospects for cell-implantation therapy to promote remyelination in patients with multiple sclerosis and other diseases characterised by myelin loss. Indeed, clinical experiments have started at Yale University, USA, exploring the effects of cerebral implantation of autologous Schwann cells in patients with multiple sclerosis (http://www.myelin.org/schwann_cells.html ). In these studies, patients who have received implants will undergo surgical biopsy to assess the effects of implantation after about 6 months.
The Yale group were also the first to show successful remyelination by bone-marrow-derived stem cells in rodent demyelinating models. Since clinical studies are now being reported of cardiac-muscle repair after direct autologous bone-marrow stem-cell injection in patients with myocardial infarction 150 (the capacity of these cells to differentiate into cardiac myocytes and repair defects having previously been clearly demonstrated in experimental models 151), human mesenchymal stem cells may also be used in patients with multiple sclerosis before long. Other centres are likely to adopt different implantation strategies and may use different sources of glial cells; some will pause until some of the complex hurdles outlined above have been addressed. 152 The next decade will show how successful CNS repair strategies will prove for patients with multiple sclerosis.
Search strategy and selection criteria
Data for this review were identified by searches of MEDLINE with the search terms “remyelination” and “myelin repair”. Many articles were also identified through searches of the extensive files of the authors. Abstracts and reports from meetings were included only when they related directly to previously published work. Only papers published in English were reviewed.
All authors contributed to all parts of the text: CH made the major contribution to the sections on oligodendrocytes and on clinical application; TB made the major contribution to the Schwann cell and olfactory cell sections; NJS planned, wrote and revised the manuscript.
Conflict of interest
None of the authors has a conflict of interest.
Role of the funding source
CH is supported by the Multiple Sclerosis Society, and TB by the Wellcome Trust. The Burden Chair Clinical Neurosciences (NS) is supported by the Burden Trust. None of these funding bodies had a role in the preparation of the manuscript.
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Correspondence: Professor Neil J Scolding, Burden Professor of Clinical Neurosciences, University of Bristol Institute of Clinical Neurosciences, Department of Neurology, Frenchay Hospital, Bristol BS16 1LE, UK. Tel +44 (0) 117 970 1212; fax +44 (0) 117 975 3824