01 December 2002
The Lancet Neurology
Volume 1, Number 8
Gianvito Martino, Luciano Adorini, Peter Rieckmann, Jan Hillert, Boris Kallmann, Giancarlo Comi, and Massimo Filippi
Department of Neuroscience, San Raffaele Scientific Institute, Milan, Italy.
BioXell, Milan, Italy.
Department of Neurology, Julius-Maximilians-University of Würzburg, Würzburg, Germany.
Department of Neurology, Huddinge University Hospital, Huddinge, Sweden.
Inflammation has always been thought of as detrimental in the pathophysiology of multiple sclerosis (MS). However, emerging genetic data, magnetic-resonance-imaging studies, and immunopathological evidence challenge this simplistic view. The evidence leads to the conclusion that inflammation is tightly regulated, and that its net effect may be beneficial in MS, thus explaining some of the results from recent trials of anti-inflammatory agents. We argue that the use of anti-inflammatory drugs to treat MS may not be appropriate in all cases. Precise identification of the inflammatory pathways to be targeted in the different phases of the disease and the timing of such interventions are therefore crucial.
Demyelinated areas in the CNS of patients with multiple sclerosis (MS) are characterised by inflammatory infiltrates that contain blood-derived myelin-specific T cells, B cells that secrete antibodies to myelin components, and a multitude of non-specific, effector mononuclear cells. This pathogenetic description has lead to the conclusion that MS is a chronic, inflammatory, autoimmune, demyelinating disease of the CNS.1–3 From an immunological point of view, chronic inflammation in MS can be thought of as an inflammatory process with a disordered resolution phase. We still do not know why inflammation in MS does not resolve in all cases, but there are several possible explanations. The persistence of inflammatory CNS infiltrates could be caused by long-lasting “danger signals”. Although many viruses have been implicated as possible danger signals in MS, there is no conclusive evidence that any pathogens have such a role.4
Chronic inflammation in MS results from in-situ production of proretention and prosurvival factors that prevent the clearance of blood-borne inflammatory cells that have invaded the CNS (figure 1 ). The clearance of inflammatory cells normally occurs via emigration or in-situ apoptosis. Primary inflammatory cytokines (eg, tumour necrosis factor [TNF] a and interleukins 1b and 6) and chemokines with a proretention activity (eg, macrophage inflammatory protein 3a, secondary lymphoid tissue chemokine, and B-lymphocyte chemoattractant) have been found in active MS plaques.5–11 The integrated activity of the prosurvival and proretention proteins causes blood-borne mononuclear cells to accumulate in and resist clearance from the CNS. In a typical patient with MS, lymphocytes and monocytes predominate in perivascular infiltrates within demyelinating plaques, but they can also be found in normal white and grey matter and meningeal spaces.2,12
Figure 1. Persistence of the inflammatory infiltrate due to
the persistence of the danger signal (1) is a key event. In-situ production
of prosurvival cytokines (eg, interleukin 6, TNF-a,
and interleukin 1b; 2) and proretention
chemokines (eg, macrophage inflammatory protein, secondary lymphoid tissue
cytokine, and B-lymphocyte chemoattractant; 3) by stromal cells (eg, glial
cells) is the subsequent step in the induction of persistent accumulation
of blood-derived mononuclear cells in the CNS. Myelin-antigen-specific
T cells, B cells, and macrophages (4) are the most common mononuclear cells
involved. In-situ inflammatory-cell division occurs (5). Ectopic lymphoid
aggregates of glial cells as well as dendritic cells develop (6). CNS-infiltrating
mononunclear cells are unable either to emigrate from (7) or to undergo
apoptosis in (8) the inflamed CNS.
Whether there is biased selection of effector cells to enter the brain is not known. Such selectivity could be mostly due to cerebral endothelial cells in the blood–brain barrier although glial cells either producing the factors that regulate the permeability of the barrier or acting as antigen-presenting cells, could also contribute. Cerebral endothelial cells, as an integrated part of the blood–brain barrier, are thought to regulate traffic of immune cells between the blood and the CSF. The cerebral endothelial cells and the tight junctions between them are the most important components of the blood–brain barrier (figure 2 ).13,14 In patients with MS, abnormal endothelial tight junctions are found not only in active lesions but also in microscopically inflamed microvessels in white matter that otherwise looks normal. Endothelial dysfunction in MS has been further supported by the detection of microparticles released from endothelial cells during disease relapses.15 The extent to which disruption of the blood–brain barrier influences demyelination and axonal injury during MS pathogenesis is unclear.16
Figure 2. In active inflammatory MS lesions, endothelial cells
of cerebral microvessels express chemokines (1) and adhesion molecules
(AM; 2) on their cellular surface to regulate traffic of mononuclear inflammatory
cells across the blood–brain barrier (BBB).13
The cell-surface expression of AM is followed by the release of soluble
AM (3). Matrix metalloproteinases (MMPs; 4) are implicated in extracellular-matrix
degeneration, opening of the BBB, myelin degradation, and shedding of cell-surface
molecules, and are probably involved in most pathogenetic steps in MS.14
Soluble adhesion molecules released from endothelial cells at the BBB have
further effects by downregulation of the disease process and may thus represent
a natural inflammation-limiting process.
The continuous influx of mononuclear cells into the brain through a damaged blood–brain barrier and the breakdown in clearance of these cells causes atypical ectopic lymphoid aggregates to develop. As a consequence, local immune cells (mainly astrocytes and microglial cells) are activated, which, along with secondary inflammatory mediators, set the stage for a CNS specific immune reaction. Activated microglia act as phagocytes, by endocytosing myelin debris in the early stage of demyelination,17 and as antigen-presenting cells.18 Ectopic lymphoid aggregates contain glia and dendritic cells,19 and they probably have a role in the continuous recruitment of mononuclear cells from the periphery. Accumulation of leucocytes within the CNS ultimately results in tissue hyperplasia, demyelination, axonal loss, and astrocytic scarring. Several final effector mechanisms in MS pathogenesis may be involved in the process of myelin damage; these include direct macrophage-mediated myelin phagocytosis and the secretion of antibodies against myelin, myelin-toxic cytokines, and nitric oxide derivatives.1–3
Although inflammation seems to have a crucial role in demyelination and axonal loss in the early phase of MS, recent evidence suggests that inflammation is not exclusively detrimental. The morbidity rate in mice with experimental autoimmune encephalomyelitis (EAE), an animal model of MS, increased from 20% to 80% when the animals were treated with monoclonal antibodies against interferon g, a proinflammatory cytokine.20 CNS-specific production of interferon g (before or after EAE onset) can protect mice from disease progression by improving the clearance of encephalitogenic T cells via an apoptotic pathway associated with upregulation of TNF receptor 1, the so-called death receptor.21 CD4-positive T cells can induce microglia to secrete interleukin-12-inhibiting agents, such as prostaglandin E2, thus self-limiting the inflammatory process.18 T cells from the brains of patients with MS and mice with EAE can secrete factors that protect myelin, such as brain-derived neurotrophic factor.22 Myelin-basic-protein-specific encephalitogenic T cells from EAE mice may prevent secondary demyelination by helping to clear toxic substances released by the initial damage.23 Macrophages promote remyelination by the removal of myelin debris—which, renders the local environment inconducive to remyelination—from the local environment.24,25 Macrophages can also promote remyelination by secreting proinflammatory cytokines, such as TNF-a that, in turn, can directly promote proliferation of oligodendrocyte progenitors via TNF-receptor-II signalling.26 Finally, antibodies against myelin antigen, which have been thought of as final effectors in EAE and MS,27 can also promote remyelination after lysolecithin-induced demyelination in mice by inducing oligodendrocyte proliferation via specific binding to cell-surface molecules.28
The role of inflammation in MS is more complex than previously thought and includes detrimental and protective components (figure 3 ).26,29 However, we still do not know the exact molecular mechanisms or the local factors—both of which are possibly related to the nature and the persistence of the danger signal as well as to the genetic background of the host—that regulate the different features of inflammation during autoimmune demyelination in the CNS.
Figure 3. Protective versus detrimental inflammation in MS.
The exact molecular mechanisms, as well as the local factors that tightly
regulate the different features of inflammation are still unclear (eg,
TNF-a ) may have a dual role in autoimmune demyelination.
TNF-a signalling through the p55 TNF receptor
I exerts a proinflammatory effect 29
whereas TNF-a signalling through the p75
TNF receptor II induces proliferation of oligodendrocyte progenitors.26
Proinflammatory and anti-inflammatory genes in MS
Analysis of recurrence rates of MS in families indicates the importance of genes in determining the risk of MS. This link is confirmed by the identification of the strongest of such factors within the HLA gene complex. Whereas most evidence implicates HLA class II genes, in particular DR and DQ alleles or haplotypes, 30,31 identification of the key genes 32 has not been possible, and the influences of independent factors in the HLA class I region further complicate the situation.33 The mere association of MS with the most typical immune-response genes is in itself evidence that inflammation is important. In comparison, identification of genes that also affect clinical manifestations of MS is even more difficult, but data are now being gathered to support genetic determination of severity34,35 and symptoms at onset.36
A proinflammatory genotype in MS
If proinflammatory alleles of key genes influence the risk of MS, people with the disorder must have an inherited risk for it and other autoimmune disorders. This link has been difficult to document, since co-occurrence of MS and other autoimmune disorders is rare. However, in a large survey, Broadley and co-workers 37 found a high frequency of autoimmune conditions in first-degree relatives of patients with MS. Further support for this notion stems from the observations that linkage peaks in various autoimmune disorders tend to cluster38,39 and that homologous chromosomal loci seem to be important for human autoimmune diseases and their animal models.38–40 Thus, genes that predispose to inflammation seem to affect the risk of MS. But what are these genes? MS is most obviously associated with the HLA haplotype DR15,DQ6, commonly referred to as DR2 or Dw2 and more correctly identified as DRB1*1501–B5*0101,DQA1*0102–B1*0602. In Europeans, 60% of patients with MS carry this haplotype compared with 25–30% of the background population, indicating a relative risk of 3–4 for MS.30 This haplotype also raises the risk of narcolepsy, but few other autoimmune disorders share this association. Dw2 is strongly protective against type 1 diabetes and accordingly does not appear to be an inflammation-inducing factor. However, the haplotype DR17–DQ2 is associated with several autoimmune disorders (including diabetes, lupus, various thyroid disorders, and myasthenia gravis) and has traditionally been thought to increase the risk of autoimmunity in general. In fact, a weak association with this haplotype in MS has occasionally been observed, and was confirmed when a sample of appropriate size was studied.41 DR17 appears to confer a relative risk of MS of about 1·5, which indicates that proinflammatory genetic factors increase susceptibility to MS.
Inflammatory genes may influence the course of MS
HLA DR and DQ genes and DR,DQ haplotypes have been repeatedly studied for their possible importance in determining severity or clinical course in MS. Published results are equivocal, probably owing to the large number of studies and the limited number of patients in each study, which makes stratification difficult. Recently, large studies have provided a chance to settle these issues. In particular, Masterman and co-workers41 could not confirm previous reports of an effect on severity of DRB alleles and documented no difference between primary progressive MS and bout-onset MS. However, DR15 was found to be associated with an earlier age at onset, a finding that was later confirmed.42,43 Thus, this association seems to be the only striking effect of class II genes on the clinical features of MS. A less common genotype may also have a striking effect on the age at onset; homozygosity for a null allele of the ciliary neurotrophic factor was recently reported to occur in only a small number of patients with MS who had a very low mean age at onset (7 years).44 The apolipoprotein E allele APOE e4 is the most important genetic risk factor for Alzheimer's disease and has been reported to confer a poor outcome in head trauma and stroke; it therefore probably plays a part in neuronal recovery after stress or damage.45 Apolipoprotein E is also thought to be involved in the inflammatory cascade in Alzheimer's disease and may have similar effects in MS.46 Indeed, several studies have now reported that the APOE e4 is associated with a severe outcome in MS, although an effect on susceptibility seems less likely.47–49 In the largest of these studies, however, its effect was limited to less than 10% of the most severely affected patients.48 Thus, for the vast majority of MS patients, the e4 genotype seems to be of little importance.
Inflammation and progressive MS
Cytotoxic-T-lymphocyte-associated antigen 4 (also known as CD152) is a costimulatory molecule in T-cell activation generally thought to have a downregulatory role; mice deficient in this antigen develop a lethal inflammatory phenotype and die young. The CTLA4 gene is somewhat polymorphic and is associated with several autoimmune diseases such as diabetes and thyroid disorders. The basis for its importance seems to be that alleles conferring decreased expression of CTLA4 increase the risk of disease. Since these alleles are the most common ones, the reverse statement may be more useful: less common genotypes seem to protect from autoimmunity by conferring a high production of CD152.50 Both modest positive associations and negative associations of CTLA4 with susceptibility to MS have been shown.51–53 However, the association may now be explained by the findings of two parallel studies,54,55 one of which had more than 700 participants, that only those with primary-progressive MS differed from controls in showing increased frequency of low-production genotypes and decreased frequency of high-production genotypes. Similar indications are also discernible in previously published data.53,56 This counter-intuitive observation deserves some analysis, since primary-progressive MS is thought to be less inflammatory than relapsing-remitting MS. However, the roles of CD152 are complicated and include the generation of memory T cells, which could increase the likelihood of a chronic progressive variant of MS.
The presence of DR15 affects age at onset not only of bout-onset MS but also primary-progressive MS. In a group of 85 patients with primary-progressive MS, the mean age at onset was 44 years among the 36 DR15-negative patients but only 37 years among the 49 DR15-positive individuals. 57 This finding implies that the progressive phase in MS is also immunologically mediated.
Imaging of inflammation in MS
Gadolinium has strong paramagnetic properties and is routinely used as a contrast agent to enhance MRI (gadolinium enhancement; figure 4 ). The intensity and size of enhancement depends on the local concentration of gadolinium, which in turn depends on its intravascular concentration, the permeability of the blood–brain barrier, and the volume of the leakage space. 58 The pathophysiological mechanisms of gadolinium leakage are still not completely understood, though there is evidence to support an active, energy-dependent, vesicular or microvescicular transport rather than an opening of the tight junctions between adjacent endothelial cells.59–61
Figure 4. T2-weighted (A) and post-contrast T1-weighted (B) MR images. In A, many white-matter lesions are visible. Two of them are enhanced in B after the injection of 0·1 mmol/kg of gadolinium-DTPA. This is a sign of increased blood–bran barrier permeability and continuing inflammation.
Imaging of inflammation
Studies in animals 61 and in people with MS62,63 show that gadolinium enhancement is consistent with histopathological findings of breakdown of the blood–brain barrier. In a study of chronic relapsing EAE the areas of gadolinium enhancement corresponded to areas of barrier breakdown labelled histochemically with horseradish peroxidase.61 The pattern of breakdown in inflammatory demyelination progresses from a diffuse short-lived disturbance in acute EAE to a more focal and longlasting breakdown in animals with the chronic relapsing form of the disease.64 This indicates that persistent breakdown of the barrier and inflammation may result in chronic demyelinating lesions.65 However, perivascular inflammation seems to be a necessary precondition for gadolinium enhancement, because non-inflammatory demyelination is not accompanied by changes in blood–brain-barrier permeability.2,12,66 Recent studies in animals have shown that gadolinium enhancement is related to the number of inflammatory cells within the lesions and mainly represents macrophage activation.67,68 These findings suggest that enhancement of MS lesions may reflect active inflammation and that an immunologically mediated inflammation is the cause of the initial vascular changes of acute lesions. This idea is supported by post-mortem data showing that gadolinium-enhanced lesions correspond to areas with intense inflammatory activity and mononuclear-cell infiltration.62
Duration of enhancement
Studies with weekly MRI have shown that nearly all new lesions show gadolinium enhancement during the earliest stages.69 Enhancement can, however, also reappear in chronic lesions, with or without a concomitant increase in size,70 which suggests that there is either incomplete repair of the blood–brain barrier or reactivation of abnormalities. The duration of enhancement, as measured by the administration of a standard dose of gadolinium, is an average of 4–8 weeks for most lesions,70–72 although a shorter period of enhancement (less than 1 month) was described in 44% of lesions seen in longitudinal studies with weekly scanning.69,73 The heterogeneity of MS inflammation is also reflected by the different morphological patterns of enhancement—ie, nodular, patchy or ring-like. Lesions can also change their enhancing pattern from ring (2–5 min after gadolinium injection) to homogeneous (15–20 min later), thus suggesting that the blood–brain-barrier permeability is lower at the centre than at the periphery of these lesions.74 A gradual increase in the area of enhancement over several hours after gadolinium injection has also been reported.74 This result could be explained by the extension of the oedema beyond the region of barrier breakdown and the subsequent diffusion of gadolinium into the extracellular space. Recent studies have shown that ring enhancement is not restricted to reactivation of older MS lesions, but that it may be the first manifestation of new activity or the progression of other enhancement patterns, especially in very large plaques.75,76
Heterogeneity of lesions
With conventional post-contrast MRI a substantial amount of inflammation is not detected. Lesions that enhance after a triple dose have higher initial magnetisation transfer ratio values than those responding to standard dose. The recovery of magnetisation transfer rate observed during a 3-month follow-up period is higher after a triple than after a standard dose.77 This finding indicates that MS lesions form a heterogeneous population and those that enhance only after a triple dose of gadolinium are characterised by a small and short-lived opening of the blood–brain barrier, which is most probably associated with less severe inflammatory changes than lesions that enhance after a single dose. A recent study showed that, after injection with gadolinium, a persistent increase of signal intensity occurs both in non-enhanced lesions and in the apparently normal white matter of patients with MS.78 These changes most likely result from leakage of gadolinium from the intravascular to the extracellular space via a dysfunctional blood–brain barrier.
The link between gadolinium enhancement and the development of permanent tissue damage in MS has still not been completely elucidated. Longitudinal MRI studies indicate that only a minority of MS lesions appear without prior enhancement.70,73,74,79 The development of tissue damage in MS can, however, occur in the absence of blood–brain-barrier breakdown; Lee and co-workers80 found that the spatial distribution of lesions showing gadolinium enhancement in the brains of patients with MS significantly differs from that of abnormalities seen with T2-weighted imaging.
Several studies have found that the number of lesions with gadolinium enhancement increases shortly before and during clinical relapses79,81,82 and relates to the imaging activity in the subsequent few weeks.81,83 In patients with clinically isolated syndromes suggestive of MS, the presence of one or more gadolinium-enhancing lesions at the onset is highly predictive of the subsequent conversion to clinically definite MS.84 In patients with relapsing–remitting MS, the presence of gadolinium-enhancing lesions predicts the number of relapses in the next 6 months.81 In a group of such patients, a moderate relation between the degree of clinical disability and the mean frequency of enhancing lesions over 3months was found.85 In patients with secondary progressive MS, the number of gadolinium-enhancing lesions detected by monthly MRI over a period of 6months was related to clinical progression 5years later.86 Nevertheless, the correlation between measures of brain-tissue loss and the frequency of gadolinium-enhancing lesions is poor in all the major clinical phenotypes of the disease.87–90 Although no firm conclusion can be drawn from the lack of such a correlation (indeed, both gadolinium enhancement and atrophy are rather crude measures of the underlying disease process and one cannot exclude the possibility that “MR-occult inflammation”, such as formation of cortical lesions, may contribute significantly to tissue loss), these observations suggest that neurodegeneration is independent of MRI-detectable inflammation in MS.
Imaging of the cellular and metabolic features of MS inflammation
New and improved tools to image the cellular and metabolic features of MS inflammation are under development. Magnetic resonance spectroscopy and cellular imaging of lymphocytes labelled with with a superparamagnetic iron oxide contrast agent (ie, monocrystalline iron oxide nanoparticles) are the most promising.
Magnetic resonance spectroscopy studies of enhancing MS lesions have shown that, in some lesions during the first 6–10 weeks after the onset of enhancement, raised lactate concentrations are found; this could be due to the concomitant presence of inflammation, local ischaemia, and neuronal mitochondrial dysfunction. 91 In a recent study of magnetic resonance spectroscopy and pathological findings, high lactate concentrations were found in MS lesions with striking inflammation and mononuclear-cell infiltration. 92 Magnetic resonance spectra from enhancing lesions also show a choline peak returning to normal over 4–6 months and lipid peaks that suggest a high membrane turnover due to demyelination. 91 A longitudinal study with magnetic resonance spectroscopy reported that lipid peaks can be found both in non-enhanced T2 lesions and in the white matter of patients with relapsing–remitting MS.93 Apparently normal regions of white matter, where lipid concentrations subsequently increased, developed macroscopic T2 abnormalities. Data from several spectroscopy studies support the hypothesis that, in MS, perivenous inflammation and demyelination may represent two separate pathological processes.91–93 In gadolinium-enhancing lesions, lipid signal can persist for 4–8 months, whereas enhancement normally ceases within 2 months. Moreover, progressive brain atrophy was seen in patients with secondary progressive MS without evidence of continuing inflammation and the atrophy was associated with low concentrations of N-acetyl aspartate, which suggests axonal degeneration.87
A preliminary study with monocrystalline iron oxide nanoparticles has shown that there is a large group of “active” MS lesions that enhance only after nanoparticle or gadolinium injection (Dousset V, personal communication).
Anti-inflammatory therapies in MS: some successes, several failures
The improved understanding of the mechanisms of T-cell activation, inactivation, and modulation has been translated into different strategies for selective immunosuppression in MS ( table ).
The clinical development status is subject to modifications. Phase IV refers to drugs on the market, labelled for the indication MS. MBP=myelin basic protein, PLP=proteolipoprotein, MMP=matrix metalloproteinases, TGF=transforming growth factor, PDE=phosphodiesterase type IV, VLA=very late antigen
|Table. Current immunotherapeutic approaches in MS|
Antigen-based immunointervention is an appealing strategy, but so far the results have not matched the expectations. Oral administration of antigen can induce suppressor T cells that release inhibitory cytokines. The secretion of anti-inflammatory or regulatory Th2-type cytokines induced by a myelin antigen, such as myelin basic protein, could also inhibit pathogenetic responses to other myelin antigens, such as proteolipid protein, by a mechanism of bystander suppression. 94 This mechanism, which mirrors epitope spreading, could be instrumental in making antigen-based immunointervention feasible.
A double-blind pilot trial of oral tolerance of myelin antigens in MS patients showed a lower frequency of disease exacerbation in the myelin-fed group. 95 However, a pivotal double-blind, placebo-controlled, phase III multicentre trial of oral myelin in 515 patients with relapsing–remitting MS, which concluded in 1998, did not show efficacy over placebo. This result, coupled with the modest efficacy reported in phase II clinical trials of oral type-II collagen in rheumatoid arthritis and S-antigen in uveitis, has dampened the interest in this type of immunointervention.
An alternative antigen-based approach uses altered peptide ligands as T-cell-receptor antagonists.96 T-cell-receptor antagonists can inhibit EAE induced by different epitopes, and their activity seems to be determined by the capacity to modulate diverse autoreactive T cells.97 Thus, if the epitopes that induce pathogenetic T cells could be identified, T-cell-receptor antagonists could prevent, or possibly even treat, MS. However, the feasibility of selective immunosuppression or immunodeviation by altered peptide ligands acting as T-cell-receptor antagonists requires detailed knowledge of the antigenic epitopes involved and demands homogeneity of these epitopes in different individuals. Obviously, the antagonist-altered peptide ligand has to be designed in such a way that it will never, under any circumstance, become an agonist.
Two clinical trials have recently tested these concepts in patients with MS. Kappos and colleagues 98 did a double-blind placebo-controlled study with 142 patients and three different doses of an altered peptide ligand of the immunodominant myelin-basic-protein peptide (amino-acid residues 83–99). There was no difference in the relapse rates between patients treated with ligand and those treated with placebo. However, the volume and number of gadolinium-enhancing lesions were reduced in patients that received the lowest ligand dose. The administration of altered peptide ligand induced a Th2 response to myelin basic protein in a subset of patients, which could explain the high frequency of immediate-type hypersensitivity reactions observed in the treated group, leading to the termination of the trial. Bielekova and co-workers99 tested only the highest ligand dose in eight patients. Three patients developed exacerbation of MS that could be linked to the ligand treatment by immunological studies in two patients; this finding suggests an encephalitogenic capacity for the peptide. Both trials serve as important lessons on the safety and efficacy of therapy with altered peptide ligands. They also highlight the difficulties in translation of basic concepts into clinical situations.
Disruption of costimulation
Induction of T-cell responses requires activation of the T-cell receptor and costimulatory interactions between dendritic cells and T cells; in the absence of costimulatory interactions T cells become anergic. The two major costimulatory pathways for T-cell activation depend on engagement of CD28 by CD80 or CD86 on T cells and of CD154 by CD40 on dendritic cells. Once activated, T cells also express CD152, which is a CD28 homologue that binds to CD80 and CD86 with higher affinity than CD28 itself and inhibits production of interleukin 2, expression of this cytokine's receptor, and cell-cycle progression in activated T cells. Disruption of these costimulatory pathways by biological agents such as CD152-immunoglobulin and monoclonal antibodies against CD154 has been beneficial in autoimmune diseases and allograft rejection. 100
CD40–CD154 interaction is crucial for the development of EAE, as indicated by the requirement for interleukin-12 secretion by microglia during antigen presentation to Th1 cells101 and by the suppression of relapses when the interaction is disrupted.102 Treatment with monoclonal antibody to CD154 at either the peak of acute disease or during remission blocks clinical disease progression and CNS inflammation. This treatment also impairs the expression of clinical disease in adoptive recipients of encephalitogenic T cells, which suggests that CD40–CD154 interactions are involved in the migration of these cells to the CNS or their ability to activate macrophages and microglia.103 On the basis of these results, blockade of the CD40–CD154 interaction is a promising immunotherapeutic strategy in MS. However, all the trials with monoclonal antibodies against CD154, including treatment of MS patients, had to be suspended because of severe thromboembolitic events, which were probably caused by the expression of CD154 by activated platelets.104
CD152 can also regulate relapsing EAE. Antibodies to this protein (or their F[ab] fragments) promoted proliferation of and proinflammatory cytokine production by lymph-node cells primed with phospholipoprotein 139–151 in vitro, and administration of antibodies to recipients of T cells specific for phospholipoprotein 139–151 resulted in more aggressive encephalitis.102 These results suggest that CD152 can downregulate autoimmune diseases. This idea has been confirmed in a phase I clinical trial that showed the immunosuppressive activity of CD152-immunoglobulin in psoriasis, a T-cell-mediated autoimmune disease of the skin.105 Although efficacy results from phase I trials should be interpreted with caution, the treatment appeared safe and at least as effective as conventional therapy. This work may pave the way for the clinical reassessment of costimulation blockade in MS. In light of the genetic studies reviewed above, targeting of CD152 should decrease the likelihood of progression of MS to the chronic progressive variant.
MS seems to be mediated by type 1 T cells positive for both CD4 and CD8, and most T-cell clones derived from peripheral blood or CSF of patients with MS show a proinflammatory Th1-like lymphokine profile.106 Expression of intereukin-12p40 mRNA has been detected in acute MS lesions, particularly from early stages of the disease.107 This finding suggests that interleukin-12 upregulation is an important event in disease initiation. In addition, T cells from MS patients induce CD40-ligand-dependent interleukin-12 secretion in the progressive but not the relapsing–remitting form of the disease, which suggests a link to disease pathogenesis.108
The reciprocal regulation between subsets of T cells predicts a role for Th2 cells in the inhibition of autoimmune diseases. Regulatory T cells that suppress the development of EAE produce Th2-type cytokines, and recovery is associated with increased concentrations of Th2 cytokines in the CNS. 109 This regulation could depend more on interleukin 10 than on interleukin 4, as indicated by the observation that interleukin-4 transgenic mice do develop encephalitis but interleukin-10 transgenic mice are completely protected. 110 These results indicate that activation of Th2 cells may prevent EAE. However, in immunodeficient hosts, myelin-basic-protein-specific Th2 cells cause encephalitis, which suggests that Th2 cells are not the final effectors of protection. 111 Moreover, induction of a myelin-oligodendrocyte-glycoprotein-specific Th2 response exacerbates EAE via an antibody-dependent mechanism in non-human primates, which suggests possible side-effects of immunodeviation.112
Although there is evidence for therapeutic manipulation of the Th1–Th2 balance in experimental models, its efficacy in people with MS is still unclear. The goal is to develop effective approaches that inhibit established pathogenetic Th1 responses by converting the response in the lesion site from aggressive to protective. This change should provide a suppressive environment for the response to any autoantigen and disrupt disease progression. Modification of the Th1–Th2 balance in primed individuals could be a therapeutic approach for other autoimmune diseases. In addition, treatment of MS patients with CAMPATH-1H, a humanised CD52-T-cell-depleting monoclonal antibody, has led to improvement of MS symptoms associated with the induction of antibodies against the thyrotropin receptor. This result seems to show a switch from a Th1 to a Th2-type response.113
In addition to interferon beta—now a mainstay treatment for MS—several cytokine-based immunointerventions have been tried, such as targeting of TNF-a and administration of transforming growth factor b.
Infliximab, a chimeric antibody against TNF-a, and etanercept, a recombinant human TNF-receptor-(p75)–Fc fusion protein, are examples of a new class of disease-modifying anti-inflammatory drugs that interfere with the action of TNF-a. These agents are effective in the treatment of rheumatoid arthritis, Crohn's disease, and psoriasis, and hold promise for the treatment of several other inflammatory disorders, although the long-term risks and benefits of these biological molecules are not yet known. In any case, their clear-cut efficacy and the modest short-term toxicity clearly show the power of appropriate immunointervention in chronic inflammatory disorders.
The role of TNF-a in EAE or MS is complex. TNF-a has a demyelinating effect in vitro, 114 it exacerbates EAE, 115 and its neutralisation inhibits chronic relapsing EAE.116 However, TNF-a-deficient mice immunised with myelin-oligodendrocyte glycoprotein develop severe neurological impairment with extensive inflammation and demyelination leading to high mortality;117 hence, TNF-a may actually limit the extent and duration of severe CNS injury. This view is consistent with increased gadolinium-enhancing lesions and lack of efficacy of treatment with antibodies against TNF-a in patients with MS.118 In a study with lenercept (a recombinant TNF-receptor-p55-immunoglobulin fusion protein), there were no significant differences between groups in any MRI measure; however, the number of patients who experienced exacerbation of disease symptoms was significantly higher in those treated with lenercept than in those receiving placebo.119 Moreover, exacerbation occured earlier in patients given lenercept, thus the pathology resembled the aggressive EAE seen in TNF-a-deficient mice. The disappointing outcome of targeting TNF-a in the treatment of MS compared with the successful results obtained in other autoimmune disorders shows that immunotherapies effective in a given disorder will not necessarily work in another. In addition, these results raise important questions about the pathogenesis of MS and highlight the apparently positive influence of TNF-a, at least in some phases of the disease.
Transforming growth factor b, a molecule known for its pleiotropic activities, can promote or inhibit cell growth and function. Every leucocyte lineage, including lymphocytes, macrophages, and dendritic cells, produces this growth factor. The molecule can modulate expression of adhesion molecules, provide a chemotactic gradient for leucocytes and other cells participating in an inflammatory response, and inhibit them once they have been activated.120 The beneficial role of this growth factor in autoimmune diseases is shown, for example, by its inhibition of EAE. In addition, transforming growth factor b is thought to be a major mediator in oral tolerance.93 Although the disease-limiting properties of transforming growth factor b in autoimmune diseases seem attractive, disruption of the balance between its opposing activities can contribute to aberrant development, malignancy, or pathogenetic immune and inflammatory responses characterised by widespread tissue fibrosis and deposition of extracellular matrix.121 The safety of the b2 type was tested in an open-label trial of 11 patients with secondary progressive MS. There was no change in score on the expanded disability status scale or lesions seen on MRI during treatment, but five patients experienced a reversible decline in the glomerular filtration rate, possibly due to tissue fibrosis.122 Thus, the side-effects of systemic treatment with transforming growth factor b2 overshadow the possible beneficial effects on the inflammatory process, and further investigation of its therapeutic potential should be done with caution.
Induction of regulatory cells
Induction of regulatory T cells has potential as a treatment for MS. Several types of regulatory cells have been described, including those positive for CD4 and CD25, interleukin-10-secreting T regulatory cells, and transforming-growth-factor-b-secreting Th3 cells.123,124 T cells with regulatory properties can be induced by various strategies. In addition, the restricted T-cell-receptor V gene repertoire expressed by autoreactive pathogenetic T cells in models of autoimmune diseases, such as EAE, has raised the possibility of self-reactivity control at the network level. The aim is to boost specific, anti-idiotypic, T-cell responses able to inhibit the pathogenetic activity of T cells expressing the target T-cell-receptor V-region gene product.
Induction of regulatory T cells by T-cell vaccination has been accomplished in several experimental models125 and has also been tested in a limited number of patients with MS. Inoculation of patients with irradiated myelin-basic-protein-reactive T cells promoted anticlonotypic class I restricted T cells, which are able to deplete circulating myelin-basic-protein-reactive T cells; thus, clonotypic interactions that regulate autoreactive T cells can be induced by T-cell vaccination.126 The anti-clonotypic response induced by the T-cell vaccine was characterised by class I restricted cytotoxic CD8-positive cells that recognise the hypervariable regions of the T-cell receptor expressed by clones used for immunisation but not of other myelin-basic-protein specific clones. Although T-cell vaccination is an interesting model of immune regulation, it can hardly be considered a feasible treatment for MS, although some potential clinical benefit related to a depletion of myelin-basic-protein-reactive T cells was seen in a recent open-label trial of T-cell vaccination.127
A simplified version of T-cell vaccination uses a specific sequence of the T-cell receptor expressed by pathogenetic T cells, instead of inactivated T cells. On the basis of the increased frequency of T cells specific for a T-cell-receptor peptide in patients vaccinated with CDR2 peptides from Vb5.2 and Vb6.1 T-cell receptors, a double-blind trial was carried out in MS patients.128 Patients who responded to the vaccine showed a lower frequency of T cells specific to myelin basic protein and remained clinically stable. Vaccine-specific T cells were Th2 type and inhibited vaccine-specific Th1 cells via interleukins 4 and 10, but not transforming growth factor b. Conversely, patients who did not respond to the vaccine had an increased anti-myelin-basic-protein T-cell response and progressed clinically. The trial was not powered to show efficacy. Thus, the extent to which immunity to receptor peptides can regulate myelin-basic-protein-specific responses and whether this immunity has any beneficial effect on the course of MS are unclear. Clinical efficacy could be expected only if common T-cell-receptor V genes are used by pathogenetic autoreactive T cells in different MS patients, as is the case in EAE. In any case, from a pharmacological point of view, the clinical applicability of this strategy appears to be limited.
We can conclude that inflammation has both good and bad features in the pathogenesis of MS. The good effect is that inflammation is a tightly regulated process able to trigger counter-reactive mechanisms that can physiologically downregulate itself. The bad effect is the detrimental role of inflammation in MS. The complexity of inflammation in MS pathogenesis has so far confounded attempts to treat this feature of the disease. Several open questions remain. What is the initiating inflammatory event in the CNS of patients with MS? Is the pathogenetic mechanism due to a secondary autoimmune phenomenon triggered by inflammation in the CNS? Are myelin antigens the primary target of the immune process or is the primary target the oligodendrocytes? Do distinct immunological mechanisms work in different disease courses and phases? Is axonal loss in the later phases of MS independent of the inflammatory process? Is there any immunological reason to explain the peculiar topography of MS lesions in the CNS? Is inflammation in MS always detrimental?
Available data support the notion that genetic factors mediating predisposition to inflammation influence not only the risk of MS but also clinical features such as age at onset, severity, and disease course. A predisposing versus a protective genetic background could thus limit or amplify the extent of the inflammatory reaction either in the CNS or in the periphery, regulate the number of myelin-specific T cells that cross the blood–brain barrier, and determine the net effect of inflammation on peripheral and CNS-confined immune cells. Genetics might also explain the apparent paradox of the dual role of detrimental versus protective inflammation in MS. Inflammation in MS is not purely detrimental because it counter-induces protective mechanisms that tend to restore the immune-privileged status of the CNS. From a teleological point of view, the CNS should be able to protect itself from any injury, more than any other organ in the body. Therefore, treatments in MS should inhibit inflammation as well as promote the integrated protective pathways that the brain and peripheral immune systems are already trying to implement in response to the initial danger signal.
The many failed or suspended trials of anti-inflammatory agents should not be forgotten. The inflammatory process sustaining the disease is more complex than previously thought, and to design more effective anti-inflammatory therapies for MS we need to understand in detail the different phases of the inflammatory process, in particular the timing and the kinetics of its detrimental and protective components. MS is a heterogeneous disease, and inflammation may have different roles and characteristics in different forms of the disease. The good, bad, and complex may ultimately segregate into different disease categories.
Search strategy and selection criteria
Data for this review were identified by searches of Medline, Current Contents, and references from relevant articles. Abstracts and reports from meetings were included only when they related directly to previously published work. Only papers published in English were reviewed.
GM, MF, GC wrote the sections on immunological and MRI features of inflammation in MS. LA wrote the sections on anti-inflammatory treatment for MS. PRand BK wrote about the role of the blood–brain barrier. JH wrote the section on proinflammatory genes. All authors reviewed and discussed the whole review.
Conflict of interest
There are no conflicts of interest
Role of the funding source
No funding bodies had any involvement in the preparation of the review or in the decision to submit the paper for publication.
We thank R Furlan and M Rovaris for helping us in the preparation of
the review. This work was partly supported by Ministero della Salute (progetti
finalizzati) and Associazione Italiana Sclerosi Multipla (AISM).
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