Immunopathogenesis and immunotherapy of multiple sclerosis - Nature

REVIEW www.nature.com/clinicalpractice/neuro

Immunopathogenesis and immunotherapy of multiple sclerosis Bernhard Hemmer*, Stefan Nessler, Dun Zhou, Bernd Kieseier and Hans-Peter Hartung

INTRODUCTION

S U M M A RY Multiple sclerosis (MS) is a chronic disease of the CNS that is characterized by inflammation, demyelination and axonal injury. Although the etiology of MS is still unknown, many findings point toward a central role for the immune system in the pathogenesis of the disease. This hypothesis is strongly supported by the beneficial effects of immunomodulatory and immunosuppressive therapy on disease activity. Over the past few years, substantial progress has been made in deciphering the immune response in MS. Although animal models have advanced our knowledge of basic mechanisms of immune responses in the CNS, recent studies have also highlighted the differences between MS and its animal equivalent, experimental autoimmune encephalomyelitis. New immunotherapeutic agents have been developed and evaluated in clinical trials. Here, we review current knowledge of the immunopathogenesis of MS and corresponding animal models of disease, and discuss new immunointerventional treatment strategies based on changing pathogenetic concepts. KEYWORDS B cells, immune system, immunotherapy, multiple sclerosis, T cells

REVIEW CRITERIA PubMed was searched using Entrez for articles published up to October 2005, including electronic early release publications. Search terms included “multiple sclerosis” or “CNS inflammation”, as well as “immune cells”, “T cells” or “B cells”. The abstracts of retrieved citations were reviewed and prioritized by relative content. Full articles were obtained and references were checked for additional material when appropriate.

Multiple sclerosis (MS) is a chronic disease of the CNS. This disease starts during early adulthood and, despite important advances in treatment in recent years, it remains a leading cause of disability in the white population.1 The etiology of MS is unknown, but many findings indicate a central role for the immune system in the disease pathogenesis, and both genes and environmental factors influence the risk of developing disease. The important role of genes in determining susceptibility to MS is exemplified by a concordance in the occurrence of disease of 30% in monozygotic twins and 3% in siblings of patients with MS.2 Family and genetic studies indicate a highly polygenetic mode of inheritance, with the human leukocyte antigen (HLA) region as the only major gene locus associated with disease. The HLA-DR1501 and HLA-DQ0601 alleles, which encode restriction elements of T cells, are associated with a 2–4-fold increased risk of developing MS in white populations.3 No other MS-associated genes have yet been unequivocally identified. Migration studies, a few apparent MS epidemics, and the association between relapses and viral infections, strongly support an additional role for environmental factors—most probably infectious agents—in the pathogenesis of MS.4–6 BASIC IMMUNOLOGY OF THE CNS

B Hemmer is a professor of neurology, S Nessler is a trainee neurologist, D Zhou is a research fellow, B Kieseier is a professor of neurology, and H-P Hartung is Professor and Chair of the Department of Neurology, at the Department of Neurology, Heinrich Heine University, Düsseldorf, Germany. Correspondence *Neuroimmunology Group, Department of Neurology, Heinrich Heine-University, Moorenstrasse 5, 40225 Düsseldorf, Germany [email protected] Received 11 October 2005 Accepted 8 February 2006 www.nature.com/clinicalpractice doi:10.1038/ncpneuro0154

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The brain was originally considered to be a highly immunoprivileged organ, a view that has been challenged by many recent studies.7 Any damage to CNS tissue leads to the activation of CNS resident immune cells—in particular, microglial cells, which upregulate major histocompatibility complex (mhc, also known as HLA in humans) and costimulatory molecules. These cells start to release cytokines and chemokines, thereby paving the way for the entry of monocytes, lymphocytes and cells with a phenotype similar to dendritic cells into the lesion. Microglial cells are important for generating and maintaining the inflammatory milieu, whereas

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GLOSSARY MAJOR HISTOCOMPATIBILITY COMPLEX (MHC) A set of molecules expressed on the cell surface that bind and present antigen for recognition by T cells COSTIMULATORY MOLECULES Molecules expressed on immune cells that enhance or inhibit the activation of an immune response CYTOKINES Intercellular soluble proteins that activate and regulate inflammatory and immune responses through interactions with specific receptors CHEMOKINES Soluble proteins that function to recruit other cells by chemoattraction MONOCYTES Phagocytic white blood cells that can develop into macrophages following migration into tissue DENDRITIC CELLS A subset of antigenpresenting cells found throughout the body and lymphoid tissues, which engulf antigen and present it to immune cells CLONOTYPIC EXPANSION Extensive proliferation of single T and B cells within the lymph nodes

dendritic cells seem to play a central role in antigen presentation to invading T cells.8,9 In parallel with the entry of immune cells into the lesion, antigens from the lesion gain access to the periphery, a phenomenon that is best demonstrated in CNS infection. Nonself antigens introduced into the CNS are rapidly detected in cervical or paraspinal lymph nodes, although it is unclear whether the antigens are passively drained or are actively carried by phagocytic cells to these sites. In the lymphnode environment, dendritic cells initiate an acquired immune response by processing the introduced proteins and presenting the resulting peptide antigens, bound to MHC class I and II molecules on their surface, to incoming T cells. CD8+ T cells recognize short peptides in the context of MHC class I, whereas CD4+ T cells recognize these peptides in the context of MHC class II molecules. High-affinity binding of T-cell receptors (TCRs) to the MHC–peptide complex results in the arrest of T-cell migration. In the presence of costimulatory molecules expressed on antigen-presenting cells, activation and clonotypic expansion of T cells occurs. These antigen-specific T cells, guided by chemoattractants, cross the blood– brain barrier and infiltrate the lesion. Their distribution throughout the lesion is reflected in the expression pattern of MHC molecules. MHC class II is sufficiently displayed only on professional antigen-presenting cells (e.g. dendritic cells, B cells, macrophages), whereas MHC class I can be expressed by all cells in the inflammatory milieu of the CNS.10,11 Accordingly, CD4+ T cells are predominantly found in perivascular cuffs and the meninges, whereas CD8+ T cells also seem to invade the parenchyma of the inflamed lesion. On contact with their cognate antigen, the T cells arrest and locally mediate their effector functions.12 CD4+ T cells recruit macrophages, which release proinflammatory cytokines and toxic molecules—such as nitric oxide, interleukin (IL)-1, IL-6, tumor necrosis factor-α (TNF-α) and matrix metalloproteinases—and CD8+ T cells might also directly attack MHC class I-expressing cells such as oligodendrocytes and neurons. B-cell responses are also initiated in the lymph nodes by dendritic cells. In contrast to T cells, B cells recognize, by means of their B-cell receptor, conformational or linear determinants of proteins displayed on dendritic cells. The activation and clonotypic expansion of a B cell is stimulated by the high affinity binding of a

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B-cell receptor to its specific antigen in the presence of antigen-specific T-cell help. Like T cells, activated B cells can pass through the blood– brain barrier and infiltrate the perivascular space and meninges. Locally, these cells release soluble immunoglobulins corresponding to the clonal specificity of their B-cell receptor that can bind membrane-bound or soluble antigens.13 The question of whether terminal differentiation of T and B cells takes place in the brain or the lymph nodes is still a matter for debate.14 ANIMAL MODELS OF NEUROINFLAMMATORY DISEASE

Animal models of autoimmune, infectious and neurodegenerative conditions have been developed to study the role of the immune system in CNS diseases. The most extensively studied animal model of neuroinflammatory disease is experimental autoimmune encephalomyelitis (EAE). EAE is a T-cell-mediated inflammatory disease of the CNS with variable degrees of demyelination and axonal damage. In susceptible animals, disease is elicited by immunization with myelin antigens—such as myelin-oligodendrocyte glycoprotein (MOG) and myelin basic protein (MBP)—and adjuvant, resulting in a CD4+ T helper-1 (Th1)-cell response that attacks the myelinated areas of the CNS.15 These T cells, supported by recruited monocytes and activated microglial cells, mediate inflammation and demyelination. B cells and antibodies are not essential for the induction of EAE, but antibodies that bind structural epitopes of MOG enhance demyelination in some models.16 CD8+ T cells play only a minor role in commonly used EAE models, although in particular circumstances they might also be encephalitogenic.17 Several concepts have been developed to explain the induction of autoimmunity to myelin antigens. Crossreactivity between nonself proteins— such as those from bacteria or viruses—and self proteins, is termed molecular mimicry, and has been proposed to be a possible mechanism for the onset of autoreactive T-cell responses.18–20 Once self-tolerance is broken, the repeated release of self antigens from the brain might promote additional autoreactive T cells that respond to additional myelin epitopes. This process, termed epitope spreading, is crucial in chronic EAE models.21,22 CNS pathology comparable to the pathology of MS is also seen in infectious disease models. For example, CNS infection with neurotropic viruses

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results in acute or chronic CNS inflammation.23 Depending on the features of the pathogen, the immune response might be beneficial or detrimental to the body. Beneficial responses of the immune system might prevent death of the animal by clearing the brain of highly neurotoxic viruses. By contrast, viruses with low neurotoxicity (e.g. Borna virus) might elicit little pathology in the brain per se, but the immune response that fights the virus could cause severe CNS damage.24 Accordingly, immunosuppression can prevent CNS damage in such infections. Chronic CNS infection might also lead to secondary autoreactive T-cell responses that accelerate CNS damage.22 In most infectious disease models, CD8+ and CD4+ T cells are essential for virus control, but also significantly contribute to tissue destruction.23 B cells appear to have a role primarily in virus control during subacute or chronic CNS infection.25 More recently, the role of inflammation in primary neurodegeneration was addressed in animal models. The impact of the immune system in these models is heterogeneous and depends on the genetic background of the strain, and also the timing and quality of the immune response. Although it has been well established that the immune response is detrimental in most models, immune cells are also capable of producing neurotrophic factors (e.g. brain-derived neurotrophic factor), which might be important for neuroregeneration.26 Whether an autoimmune response that generates neurotrophic factors has beneficial effects on neurodegeneration in vivo remains controversial.27,28 Although animal models have been extremely valuable for studying basic mechanisms of neuroinflammation and neurodegeneration, thereby providing a basis for the development of new therapies, these models do not thoroughly reflect every aspect of MS. In particular, there are diverging responses to immunomodulatory therapies between MS patients and experimental animal models; these differential responses have redirected research towards understanding the nature of the human disease.29 IMMUNOLOGY OF MULTIPLE SCLEROSIS Immunological changes in multiple sclerosis lesions

Few studies have addressed the difficult issue of lesion evolution in MS.30 Activation of microglia and macrophages, which express HLA class II molecules and complement receptor

C3d–immunoglobulin complexes on their surface, seems to represent the earliest change that occurs during the first days of lesion development. At this time, the blood–brain barrier seems to be intact, and few cell infiltrates are seen in the brain. Demyelination and astrogliosis are also largely absent. By contrast, between 6 and 20 weeks from the initiation of disease, lesions contain cell infiltrates, evidence of demyelination and blood–brain-barrier leakage, and reactive astrocytes, along with proliferating oligodendroglial cells at the lesion border. These lesions represent the most active phase of disease. Beyond week 20, the number of inflammatory cells decreases in the center of the lesion, and later at the lesion border. Many cytokines and chemokines are released within the lesion, including Th1 and Th2 cytokines.31,32 Axonal damage and demyelination is seen in all phases of disease,33 but appears to be most pronounced early during the disease course, correlating with the extent of cellular infiltration.34 Proliferation of oligodendrocytes and remyelination of axons is detectable in many lesions, but the remyelination is usually incomplete. Overall, the extent of inflammation, neurodegeneration and remyelination is heterogeneous between patients. This observation has prompted studies to stratify MS patients into subgroups according to their lesion pathology.35 Besides neuromyelitis optica,36 which is characterized by massive eosinophilic granulocyte infiltrates and perivascular immunoglobulin deposits, four subgroups were described on the basis of relative quantity and quality of inflammation, antibody deposition, and oligodendrocyte dystrophy.35 Two of these four subgroups were characterized by minor inflammatory changes, despite prominent oligodendrocyte pathology. This observation is further supported by a recent study that investigated the histopathology of very early MS lesions. In some patients, no inflammatory infiltrates were observed, although extensive oligodendrocyte apoptosis and microglia activation were present.37 Both studies indicate that in a subgroup of patients, or at certain disease stages, the immune system might play a less dominant role in the pathogenesis of MS. At present, it is unclear how oligodendrocyte death is triggered in these patients. T cells in multiple sclerosis

Because of the key role of CD4+ autoreactive T cells in EAE, MS has been considered primarily

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GLOSSARY AQUAPORIN A protein channel that selectively conducts water molecules through the cell membrane COMPLEMENT An inactive group of proteins, which, when activated, yields numerous peptides that have a wide variety of modulating and effector actions in inflammation and immunity

to be a T-cell-mediated disease. Many studies have investigated the importance of autoreactive, myelin-specific CD4+ T cells in MS,38 but their roles in the pathogenesis of the disease so far remain elusive. Myelin-protein-specific T cells can be retrieved from the blood or cerebrospinal fluid (CSF) of MS patients, but they are also detectable in healthy controls.39 Although human myelinspecific T cells were shown to be encephalitogenic in a transgenic animal model, no consistent difference has been observed with respect to the frequency and phenotype of these cells between MS patients and healthy controls.39–41 Both CD4+ and CD8+ T cells are present in MS lesions, with CD4+ T cells being found predominantly in the perivascular cuff, and CD8+ T cells being more prevalent in the center and border zone of the lesion.30 Early studies indicated that the T cells infiltrating lesions originate from the same precursor cell and therefore have the same antigenic specificity—a phenomenon known as clonotypic accumulation.42 Recent studies on MS lesions and in CSF from MS patients have confirmed this idea. Interestingly, this clonotypic accumulation was mostly present in the CD8+ T-cell compartment, and was only rarely observed in the CD4+ T-cell compartment.43,44 The persistence of these T cell clonotypes in lesions was verified in longitudinal studies, and the low abundance of these clonotypes in blood implied a specific enrichment of the clonotypes in the diseased organ.44,45 Although these studies imply antigen-driven infiltration of lesions by CD8+ T cells, the specificity and functional role of these cells remains to be determined. B cells and antibodies in multiple sclerosis

The local synthesis of antibody in CSF, as demonstrated by the presence of oligoclonal bands or intrathecal IgG production, is still the only diagnostic laboratory marker for MS. Paradoxically, however, B cells have been neglected in MS research for decades, owing to their dispensable role in EAE and a lack of suitable technology for investigating their functions;46 it is only recently that their impact in MS pathogenesis has been thoroughly studied. Dominant B-cell clonotypes, which contain replacement mutations in their B-cell-receptor genes, are found in CSF and lesions, compatible with an antigen-driven selection process.47–49 The same B-cell clonotypes are found in the CSF during the course of the disease, implying


that they are periodically recruited, or that they persist in the CNS compartment.50 This model is compatible with other CSF parameters. Immunoglobulin (predominantly IgG1) is released into the CSF and lesions of MS patients. B-cell cytokines, such as tumor-necrosis factor ligand superfamily member 13B (BAFF), are also detected in MS lesions.51 Plasma blasts and plasma cells—terminally differentiated B cells that are usually found only in acute infectious disease—can be identified in the CSF of MS patients.52,53 The numbers of these cells correlate not only with local IgG synthesis, but also with the extent of CNS inflammation.54 Although these findings strongly support a role for the humoral immune response in MS, the specificity and function of this response remains to be determined. In neuromyelitis optica, a diagnostic serum antibody that binds aquaporin has recently been described that might be present in approximately 50% of patients.55,56 This finding warrants confirmation, however, and it remains to be determined whether this antibody can serve as a marker of disease or prognosis. INFLAMMATION AND NEURODEGENERATION IN MULTIPLE SCLEROSIS

A shift in the paradigm of the mechanisms of MS has occurred on the basis of pathologic and MRI studies that collectively implicate axonal damage and loss as the most important determinants of permanent neurological disability.57 Axonal damage occurs even in the early stages of the disease.33 Several hypotheses that establish a link between an aberrant inflammatory response in the CNS and axonal damage have been developed. These hypotheses include activation of CD8+ T cells that directly target neurons, vigorous CD4+ T-cell responses that recruit macrophages, leading to release of inflammatory mediators and toxic molecules, and binding of antibodies to neuronal surface antigens, followed by complement activation or antibody-mediated phagocytosis of axons. Indirect mechanisms, such as loss of protective myelin, mitochondrial dysfunction, or release of glutamate or nitric oxide, might contribute to axonal damage. Although all of these mechanisms could be relevant, the molecular events that underlie the axonal damage in MS remain elusive. IMMUNOPATHOGENETIC CONCEPT

The evidence reviewed above highlights important differences between MS pathology in humans



Table 1 Outcome of immunotherapies in experimental autoimmune encephalomyelitis and multiple sclerosis. Substance

Initial therapeutic strategy

Efficacy in MS

Efficacy in EAE

Comments

Interferon-γ

Anti-viral

Exacerbates

Ameliorates or no effect

Opposite effects in MS and EAE

Interferon-β

Anti-viral

Reduces relapse rates and MRI activity

Ameliorates, but efficacy is variable

Efficacy was first demonstrated in MS

Glatiramer acetate

Immunomodulatory


Ameliorates

Efficacy was first demonstrated in EAE

Lenercept

TNF-α receptor blockade

Exacerbates

Ameliorates

Opposite effects in MS and EAE

Anti-CD4 antibody

Depletion of CD4+ T cells

No effect

Ameliorates

Different outcome in MS and EAE

Oral myelin

Immune tolerance

No effect

Ameliorates

Different outcome in MS and EAE

Mitoxantrone

Immunosuppression

Reduces relapse rates and disease progression

Ameliorates

Efficacy was first demonstrated in EAE

Natalizumab

Anti-migratory


Ameliorates

Efficacy was first demonstrated with an analog antibody in EAE

EAE, experimental autoimmune encephalomyelitis; MS, multiple sclerosis; TNF, tumor necrosis factor.

and the currently available animal models—in particular, EAE. CD8+ T cells are considered to play an important role in MS, but they are not relevant in most EAE models. Likewise, B cells seem to be critically involved in MS, but are dispensable in most EAE models (antibody responses are important only in MOG-induced EAE in rats). These findings indicate that the immunological pathways and target antigens differ between human disease and the EAE animal model. This evidence is also supported by the differential responses to treatment; although some treatment strategies work in both EAE and MS, others, such as TNF-α-receptor blockade, are effective in EAE but might even exacerbate MS (Table 1). The involvement of B cells and CD8+ T cells in MS strongly supports the primary inflammatory nature of this disease in the majority of cases (Figure 1). Although T cells are detected in the CNS tissue following neuronal damage, a local humoral immune response with activated B cells and clonal, persistent CD8+ T-cell infiltrates is apparently absent in primary neurodegenerative diseases (e.g. Alzheimer’s disease) in humans or their corresponding animal models. At present, it is not clear whether the invasion of the CNS by T cells and B cells is the initiating

event of MS, or whether it is secondary to the activation of microglia and macrophages, and the local release of self or foreign antigens. It is probable, however, that the highly focused and persistent acquired immune response in MS is driven by a small number of antigens—the identity of which is as yet unknown—that are presented in the CNS. The involvement of B cells with a dominant IgG1 antibody response would indicate that these target antigens are proteins either released or displayed on the surface of CNS cells. Among the possible candidates are myelin or neuronal antigens, and also antigens from infectious agents that have been epidemiologically associated with MS.58–62 Although we still do not know the target antigens of the immune response, most studies indicate a detrimental effect of CNS inflammation in MS. This observation is supported not only by many findings from experimental models, but also by the correlation between inflammation, demyelination and axonal damage in MS lesions. It is important to bear in mind, however, that such a detrimental effect is not proof of autoimmune disease, because these effects might also be observed in chronic CNS infection with microbes that have an inherently low pathogenicity but elicit detrimental immune responses.




A

CNS

Periphery Repair/reorganization

Susceptibility

Altered immune system

Infectious agent?

Inflammation

Peripheral activation

Blood–brain barrier passage B

Vessel

Perivascular space

Infiltration of immune cells

Lesion center

Activation of local immune cells Normal white matter

Resting microglia

Antibodies

Plasmablast CD4+ T cells

CD8+ T cells Macrophages

Myelin

Dendritic cell

Reactive astrocytes

Oligodendrocytes

Activated microglia

Neurons

Resting astrocytes

Figure 1 Immunopathogenesis of multiple sclerosis. (A) Pathogenetic concept for the development of multiple sclerosis (MS). Based on a genomic predisposition, MS patients inherit traits that lead to an altered immune response. When encountering infectious agents, immune responses crossreactive with self proteins are mounted in the peripheral lymphoid system. Activated antigen-specific T cells and B cells cross the blood–brain barrier and target self antigens expressed by oligodendrocytes and neurons. In concert with the innate immune response in the CNS, T cells and B cells cause inflammatory damage. Susceptibility of oligodendrocytes and neurons to inflammatory damage, and the capacity for CNS repair and reorganization, determines the extent and functional consequences of the inflammatory damage. (B) Schematic view of the immunopathology in the MS lesion. A number of immune and CNS cell types are involved in lesion development and repair. T cells, B cells and macrophages infiltrate the lesion. CD4+ T cells are located in the perivascular cuff. These cells become reactivated by antigens presented on dendritic cells and microglial cells, and locally release cytokines and other inflammatory mediators, thereby attracting macrophages to the lesions. CD8+ T cells infiltrate the parenchyma and, as well as secreting inflammatory mediators, they directly attack cells expressing human leukocyte antigen class I such as neurons and oligodendrocytes. B cells are predominantly found in the perivascular space and meninges, where they release IgG antibodies. These antibodies bind to proteins expressed on the surface of oligodendrocytes and neurons. Bound antibodies can fix complement, thereby initiating the complement cascade, or inducing antibody-mediated phagocytosis by macrophages. Activated macrophages also release inflammatory and toxic molecules (e.g. nitric oxide), which predominantly damage oligodendrocytes and neurons. Reactive astrocytes induce gliosis at the lesion border. Following the inflammatory damage, oligodendrocytes proliferate and remyelinate the demyelinated axons.




Altered immune system

Rematuration of immune system ■ Bone-marrow transplantation

Local activation

Peripheral activation

Inhibition of microglia

Infiltration

Immunosupression ■ Mitoxantrone Migration

Immunomodulation ■ Interferons, GA ■ Statins ■ Minocyclin

Selective intervention ■ B-cell, T-cell depletion ■ Anti-CD25 mAb

Anti-migration therapy ■ Natalizumab ■ FTY720

Inactivation of infiltrating cells ■ Systemic immunosuppression?

Figure 2 Immune intervention strategies currently used or in development for the treatment of multiple sclerosis. The genomic predisposition and the altered immune maturation process can be treated by allogeneic or autologous bone-marrow transplantation. Both methods have a high mortality that prohibits routine use. Therapies targeting the peripheral immune system have been widely used in multiple sclerosis. These treatments include global immunosuppression, immunomodulation and specific immunointervention. A novel approach aims to prevent migration of immune cells to the CNS. This approach has been shown to be effective, but was also associated with severe side effects. Inactivation of infiltrating immune cells or cells that mediate local innate immunity in the CNS are other promising strategies for the future. GA, glatiramer acetate; mAb, monoclonal antibody

IMMUNOTHERAPY FOR MULTIPLE SCLEROSIS

On the basis of the inflammatory nature of the disease, targeting of the immune response has so far been the most widely used—and only successful—treatment approach for MS. Strategies have been developed that range from nonselective immunosuppression to highly specific immune intervention (Figure 2). Global immunosuppression and immunomodulation

Global immunosuppression was the first approach that attempted to attenuate the immune response in relapsing–remitting MS. Small beneficial effects were demonstrated for therapeutics such as azathioprine and ciclosporin in early studies. The immunosuppressant mitoxantrone is widely used to treat worsening forms of MS, and has been shown to delay disability progression.63 Most patients with relapsing–remitting MS are currently treated with the immunomodulatory

agents interferon-β (IFN-β) or glatiramer acetate (GA). IFN-β has multiple immunomodulatory effects: it curtails T-cell trafficking, redresses a Th1–Th2 imbalance that is in favor of Th1 responses in MS patients, and exhibits antiviral properties.64 GA, a synthetic polypeptide composed of the most prevalent amino acids in MBP, is believed to modulate autoreactive T cells, inhibit monocyte activity and induce bystander immune suppression at lesion sites in the CNS. GA might theoretically also promote neuroregeneration, as GA-reactive T cells have been reported to release neurotrophic factors.64–66 Although the precise mode of action for these substances has not been fully clarified, they have been shown to reduce relapse rates, inflammatory activity and lesion load as measured by MRI.67–70 IFN-β might also ameliorate disease progression slightly, although this effect has not been demonstrated in all studies.71,72 The development of new treatment strategies that significantly delay long-term disease




progression is warranted. New immunomodulatory drugs that ameliorate EAE have shown some promise in small phase II trials. Statins, which exert a variety of immunomodulatory actions, are currently being tested in a large phase II trial.73,74 Minocycline, an immunomodulatory and possibly neuroprotective agent, is also being tested in a clinical trial.75 Hematopoietic stem-cell therapy to remove autoreactive T cells from the repertoire has also been explored in progressive MS, and some promising results have been obtained, but these need to be weighed against the high mortality associated with this approach.76 Selective immune intervention

With the introduction of humanized monoclonal antibodies and small specific molecules (e.g. receptor agonists or antagonists), specific ablation of distinct immune populations, or selective blockade or activation of immune molecules, has become possible. Antibodies that bind cell-specific surface molecules allow depletion of T cells, B cells and other immunecell subsets via antibody binding and complement-mediated cell lysis. Depletion of CD4+ T cells had no impact on MS,77 but depletion of B cells seems to be beneficial in a subgroup of patients with high humoral activity.78,79 Depletion of T cells using an anti-CD52 antibody (Campath®, Genzyme Corporation, Cambridge, MA) markedly diminished relapse rates and inflammatory MRI activity, and is now being investigated in a large phase II trial.80 An alternative approach is to use antibodies that interfere with cellular functions. One antibody that binds the receptor for the T-cell growth factor IL-2 showed promising results in a small phase II trial.81 On the basis of findings from EAE models, the potential therapeutic effects of selective blockade of proinflammatory cytokines or cytokine receptors, and administration of anti-inflammatory cytokines or soluble cytokine receptors, were evaluated.29 Although most of these therapies ameliorated EAE, none have been brought to phase III trials in MS, either because of significant toxicity or a lack of efficacy. The most discordant finding was obtained with antiTNF-α therapies; TNF-α-blocking antibodies and TNF-receptor antagonists showed strong ameliorative effects in the EAE model, but were associated with increased disease activity in MS.82


Antigen-based and T-cell-receptor-based therapies

With the decryption of encephalitogenic myelin protein epitopes in EAE and the identification of myelin-protein-reactive T cells in MS patients, antigen-based therapies were developed to specifically target T-cell responses against myelin proteins. Although the pathogenetic role of the T-cell response to myelin antigens has not been fully established in MS, peptide and protein therapies that ameliorate EAE were applied to the human disease. These strategies included the tolerization of autoreactive T cells by oral administration of myelin antigens or by the administration of an altered peptide ligand (APL) based on MBP. Both phase III clinical trials produced negative results,29 and the APL trial had to be stopped because of side effects.83 A small study even indicated a disease-exacerbating effect of the APL in some patients.84 Nevertheless, the APL is currently being re-evaluated at a lower dose in a new clinical trial. Suppression of T-cell responses by vaccination with DNA that encodes myelin antigens is also being explored in clinical trials.85,86 Primed by reports of a skewed TCR repertoire in some EAE models, and some early findings on biased TCR repertoires in MS patients, TCRspecific therapies (e.g. TCR–peptide vaccination) were also explored in small clinical trials. Although this approach seems to be able to deplete T cells bearing particular TCRs from the repertoire, no significant impact on the MS disease course was observed.87 In addition, a principal conceptual problem arises from broadening of the immune response with intramolecular and intermolecular spreading of T-cell epitopes, subsequent to the release of neoantigens as a consequence of repeated neural damage.21,22 Modulation of immune-cell migration

Recently, the first therapies that interfere with cell migration have entered the clinic.88 Blockade of adhesion molecules prevents leukocyte binding to the vessel wall, and eventually prevents leukocyte passage across the blood–brain barrier. On the basis of positive results in the EAE model, the humanized monoclonal antibody natalizumab was developed to block β4 integrin. Natalizumab was efficient in phase II and two phase III trials, producing a profound reduction in MRI activity, relapse rates and disease progression.89 After the occurrence of progressive multifocal



leukoencephalopathy in three patients, however, marketing of natalizumab was suspended, and the safety profile of the drug is being further reevaluated in clinical trials.90 The further development of similar approaches using orally active small-molecule inhibitors of adhesion molecules will depend on the re-introduction of this drug into the market. FTY720, a fungal metabolite with sphingosine1-phosphate-receptor agonist activity, induces homing of lymphocytes to the lymph nodes and traps them at this site, thereby preventing their migration to inflamed organ compartments.91 This orally administered drug is effective in transplantation and in autoimmune animal models. A phase II trial in patients with MS revealed positive results, prompting a phase III trial that is currently in progress. At present, it is unclear whether any immunointervention that effectively blocks access of T cells into the CNS, thereby abrogating immune surveillance, is associated with an increased risk of progressive multifocal leukoencephalopathy or other opportunistic infections. Altering the immune response in the CNS

Targeting the immune response in the periphery has led to a number of efficient therapies for MS, but little effort has been made to target CNS-infiltrating lymphocytes, perivascular macrophages, dendritic cells or microglia in the brain. To reduce systemic side effects, it is worth exploring therapies that focus on the affected immune compartment. In particular, the targeting of microglial cells, which seem to play a central role in maintaining inflammation, might be a promising avenue for future therapeutic exploration. Furthermore, it might be possible to influence the local inflammatory milieu by exogenous administration of neuronal stem cells that migrate into the lesion and mediate local anti-inflammatory activity.92 All the local strategies, however, are at the experimental stage, and it might be years before they enter the first clinical trials in patients with MS. CONCLUSIONS

MS is a chronic disabling disease, and its cause remains uncertain, despite considerable progress in research during the past decade. The immune system seems to play a central role in the disease pathogenesis, and new, more-selective immunotherapies have been developed with demonstrable impact on the disease course; however, the serious

side effects of these drugs have so far prevented their widespread clinical application. New therapies are in the development pipeline, some of them showing promising results. Continuous concerted efforts are necessary to fully decrypt the pathogenesis of MS and develop efficient and safe therapies for the future. KEY POINTS ■ The etiology of multiple sclerosis (MS) is unknown, but many findings indicate a central role for the immune system in the disease pathogenesis, and both genes and environmental factors influence the risk of developing disease ■ The most extensively studied animal model of MS is experimental autoimmune encephalomyelitis, a T-cell-mediated inflammatory disease of the CNS with variable degrees of demyelination and axonal damage ■ Traditionally, MS has been considered primarily to be a T-cell-mediated disease, but the importance of B lymphocytes in the pathogenesis of MS is beginning to be appreciated ■ There are important differences between the pathology of MS in humans and EAE in animals, particularly with regard to the involvement of B cells and CD8+ T cells ■ Targeting of the immune response is the most widely used treatment approach for MS; strategies range from nonselective immunosuppression to highly specific immune intervention

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Acknowledgments B Hemmer, S Nessler and D Zhou were supported by grants from the Deutsche Forschungs-gemeinschaft, the Gemeinnützige HertieStiftung and the German MS Society.

Competing interests B Hemmer, B Kieseier and H-P Hartung declared competing interests. The other authors declared they have no competing interests.
