Future prospects for the management of multiple sclerosis

The neuroprotective effect of inflammation: implications for the therapy of multiple sclerosis

Autoreactive T cells are a component of the normal immune system. It has been proposed that some of these autoreactive T cells even have a protective function. Recent studies support this notion by demonstrating that (a) myelin basic-protein (MBP-) specific T cells show neuroprotective effects in vivo, and (b) activated antigen-specific human T cells and other immune cells produce bioactive brain-derived neurotrophic factor (BDNF) in vitro. Furthermore, BDNF is expressed in different types of inflammatory cells in brain lesions of patients with acute disseminated leukoencephalopathy or multiple sclerosis. We postulate that the neuroprotective effect of T cells and other immune cells observed in vivo is at least partially mediated by BDNF and other neurotrophic factors. The concept of neuroprotective autoimmunity has obvious implications for the therapy of multiple sclerosis and other neuroimmunological diseases.

Changes in neurotransmitters in multiple sclerosis

Patients with multiple sclerosis were found to have increased cerebrospinal fluid, noradrenaline, and excitatory amino acid (glutamate and aspartate) levels, with increased blood glutamine, asparagine, and glycine levels. An association was found between these biochemical parameters and the nature and severity of neurological symptoms, as well as with the course of the disease. Neurotransmitters are proposed to have a role in the pathogenesis of multiple sclerosis, particularly in the biochemical mechanisms of the relationship between the nervous and immune systems, as well as in the neurochemical mechanisms underlying the development of neurological deficit.

Repair of myelin disease: strategies and progress in animal models

Myelin disorders form an important group of human neurological diseases that are as yet incurable. Recent studies on experimental remyelination have suggested that it might be feasible to repair the CNS, either by transplanting normal myelinating cells or by enhancing endogenous repair. Progress in animal models, particularly in transplanting cells of the oligodendrocyte lineage, has resulted in significant focal remyelination and physiological evidence of restoration of function. These data suggest that focal lesions in multiple sclerosis could be repaired by the transplantation of myelin-forming cells. Future therapies could involve both transplantation and promotion of endogenous repair, and the two approaches could be combined with ex vivo manipulation of the donor tissue.

Endogenous progenitors remyelinate demyelinated axons in the adult CNS

Remyelination occurs in demyelinated CNS regions in diseases such as multiple sclerosis. Identification of the cell type(s) responsible for this remyelination, however, has been elusive. Here, we examine one potential source of remyelinating oligodendrocytes-immature, cycling cells endogenous to adult white matter-and demonstrate that this population responds to demyelination by differentiating into myelinating oligodendrocytes. Dividing cells in subcortical white matter of adult rats were labeled by stereotactic injection of a replication-deficient lacZ-encoding retrovirus (BAG). Following a focal demyelination induced with lysolecithin, many of the BAG-labeled cells differentiated into myelinating oligodendrocytes engaging in repair of the lesion. Identification of endogenous cells capable of remyelination provides a target for the study of CNS repair processes in demyelinating diseases.

Growth factors and myelin regeneration in multiple sclerosis

Insulin-like growth factor-I (IGF-I), platelet-derived growth factor (PDGF), fibroblast growth factor (FGF) and ciliary neurotrophic factor (CNTF) are multifunctional growth factors which are found in the CNS. Oligodendroglia are the cells that form and maintain myelin sheaths and many in vitro experiments have shown that these growth factors promote the proliferation, differentiation and survival of cells in the oligodendroglial lineage. Since myelin breakdown is often severe in multiple sclerosis (MS), the possibility of growth factor use in the treatment of MS has been considered and recently, IGF-I treatment has been shown to reduce lesion severity and promote myelin regeneration in experimental autoimmune encephalomyelitis (EAE), an animal model of MS. This review briefly summarizes the structural characteristics of these growth factors and the actions which might help reduce oligodendrocyte-myelin sheath injury in MS and promote myelin regeneration.

Myelination of the canine central nervous system by glial cell transplantation: a model for repair of human myelin disease

There is a lack of effective means of promoting remyelination of the central nervous system (CNS) in humans with chronic demyelinating disease. We have investigated the ability of transplanted glia to myelinate areas of the CNS equivalent to focal demyelinated lesions in multiple sclerosis (MS). In these studies we show that transplantation of oligodendrocytes or their progenitors into the CNS of a neonatal or adult canine myelin mutant results in repair of large areas similar in size to many MS plaques. Progenitor or pre-progenitor cells of the oligodendrocyte lineage have the greatest capacity for myelination following grafting, although cells of neonatal origin may also be used. Such an approach may therefore have therapeutic value in the repair of focal lesions in human myelin disease.

Investigational drug therapies of treatment of multiple sclerosis

The licensing of interferon beta-1b dramatically changed the treatment of multiple sclerosis (MS) in the United States. Although it was the first therapeutic agent shown to affect the natural course of the disease, interferon beta-1b is not appropriate for all patients and is far from being a cure. Several other promising therapies now under study include immunosuppressive and immunomodulatory drugs to limit inflammation; oral administration of myelin to induce tolerance; monoclonal antibodies designed to deliver targeted immunotherapy; potassium channel blockers to facilitate conduction along demyelinated axons; and glial growth factors to promote remyelination. Clinical trials of potential therapeutic agents have proliferated in the past decade in conjunction with rapid advances in our understanding of the immunologic basis of MS. Some investigational therapies are associated with problematic toxicities, others benefit only a minority of patients, and many are still in the early stages of development. Nevertheless, because current therapeutic options are limited, and because the history of MS therapy is one of disappointment and frustration, it is essential that legitimate, scientifically based advances be widely disseminated to the neurologic community. This article reviews some of the most promising current and investigational therapies for MS.

Glial cell transplantation and remyelination of the central nervous system

Glial cell transplantation has proved to be a powerful tool in the study of glial cell biology. The extent of myelination achieved by transplanting myelin-producing cells into the CNS of myelin mutants, or into focal demyelinating lesions has raised hope that such a strategy may have therapeutic applications. Oligodendrocytes or Schwann cells could be used for repair. It is likely that the immature stages of the oligodendrocyte lineage have the best phenotypic characteristics for remyelination when transplanted, either as primary cells or as immortalized cells or cell lines. Prior culturing and growth factor treatment provides opportunities to expand cell populations before transplantation as dissociated cell preparations. Cell lines are attractive candidates for transplantation, but the risk of transformation must be monitored. The application of this technique to human myelin disorders may require proof that migration, division and stable remyelination of axons by the transplanted cells can occur in the presence of gliosis and inflammation.