`olsELSEVIER
`
`TRENDSin Pharmacological Sciences Vol.24 No.3 March 2003
`
`131
`
`Immunomodulation tn multiple
`sclerosis: from immunosuppression
`to neuroprotection
`Oliver Neuhaus’, Juan J. Archelos” and Hans-Peter Hartung’
`
`'Department of Neurology, Heinrich-Heine-Universitat, Moorenstrasse 5, 40225 Diisseldorf, Germany
`2Multiple Sclerosis Research Group, Department of Neurology, Karl-Franzens-Universitat, Auenbruggerplatz 22, 8036 Graz, Austria
`
`Multiple sclerosis (MS) is the most common disabling
`neurological disease of young adulthood. Following
`advances in the understanding of the immunological
`mechanisms that underlie the pathogenesis of MS, a
`growing arsenal of immunomodulatory agentsis avail-
`able. Two classes of immunomodulators are approved
`for long-term treatment of MS,the efficacy of several
`promising new conceptsis beingtestedin clinical trials
`and classical immunosuppressive agents used in MS
`treatment have been shownto exert specific, immuno-
`modulatory effects. Furthermore,
`two recent obser-
`vations have changed our basic understanding of the
`pathogenesis of MS. First, immunecells in MS lesions
`have neuroprotective activity, which indicates a bene-
`ficial role of neuroinflammation. Second, there is evi-
`dence that axonal
`loss,
`rather than demyelination,
`underlies the progression of MS and,hence, constitutes
`a therapeutic target.
`
`have been approved for the treatment of MS [8-10].
`Immunomodulators, which do not cause general suppres-
`sion of the immunesystem, shift immuneresponses from
`pro-inflammatory autoimmune conditions [mediated by
`T helper 1 (Th1) cytokines that are released by auto-
`reactive T cells] towards more beneficial anti-inflamma-
`tory circumstances (mediated through Th2 cytokines that
`are secreted by regulatory T cells). Both IFN-8 and GA
`have been provento be partially effective in clinical trials
`[1,11—14]. In the search for moreefficacious agents, many
`new drugs are under investigation in preclinical and
`clinical trials, but several promising approaches have
`failed [15]. In parallel there have been advances in
`understanding the underlying pathogenesis of the disease
`as well as modesof action of the different agents.
`Here, we summarize current concepts about the mech-
`anismsofaction of therapies already approved for MS and
`the most promising future candidates.
`
`‘The surest way to lose a reputation in neurology is to
`advocate a treatment for multiple sclerosis.’ (H. Houston
`Merritt)
`
`Multiple sclerosis (MS) is the most common inflamma-
`tory disorder of the CNS and the leading cause of
`neurological disability in young adults [1]. Many immune
`abnormalities have been described in MS, which indicates
`that the immune system plays a central role in its
`pathogenesis [2—4]. Although immuneresponsescontrib-
`ute to the formation and maintenance of MSlesions [5],
`neuroinflammation might have neuroprotective effects
`[6,7]. This crucial role of the immune system in disease
`pathogenesis has important therapeutic implications. For
`a long while corticosteroids were the only proven therapy
`for MS. However, these only shorten an acute attack and
`effective, long-term drug treatment was not available.
`Although several immunosuppressive agents (i.e. inhibi-
`tors of crucial components of the immune system that
`cause generalized immune dysfunction) were used off-
`label, the adverse systemic effects, such as increased risk
`of cancer and infection, limited the potential benefits in
`MS. More recently,
`two classes of immunomodulatory
`agents, interferon B (IFN-8) and glatiramer acetate (GA),
`
`Disease-relevant immune processes
`Sincethefirst description of MS in 1835 by J. Cruveilhier
`as ‘sclérose en taches, en iles par masses disséminées’[16],
`the concepts of its pathogenesis have been adapted
`continuously [1,2,4,17]. Although unproven, the current
`consensus is that MS pathogenesis comprises an initial
`inflammatory phase, which fulfils the criteria for an
`autoimmunedisease [18], followed by a phaseof selective
`demyelination andlast, a neurodegenerative phase [4,17].
`Subjects with genetically determined susceptibility to MS
`[19] harbor T cells that react with CNS autoantigens.
`Although these can remain dormant for decades, at some
`point they are activated in the periphery, probably by
`molecular mimicry (i.e. recognition ofepitopes that are com-
`mon to autoantigens and microbial antigens as exogenous
`triggers [20,21]). This enables them to migrate through the
`blood—brain barrier to the brain and spinal cord. Reacti-
`vated in the CNS,these T cells of either CD4* helper or
`CD8* cytotoxic phenotype [22] release pro-inflammatory
`Th1 cytokines and orchestrate the destruction of the
`myelin sheath by various types of immunecells. Destruc-
`tion follows the first two of four pathological patterns [5]:
`(1) T-cell- and macrophage-mediated demyelination; (2)
`antibody-mediated demyelination that involves comple-
`Corresponding author: Oliver Neuhaus(oliver.neuhaus@uni-duesseldorf.de).
`mentactivation [3,23]; (3) distal oligodendrogliopathy and
`Merck 2018
`http://tips.trends.com 0165-6147/03/$ - see front matter © 2003 Elsevier ScienceLtd. All rights reserved. doi:10.1016/S0165-6147(03)00028-2
`TWiv Merck
`IPR2023-00050
`
`

`

`132
`
`TRENDSin Pharmacological Sciences Vol.24 No.3 March 2003
`
`Periphery
`Activation
`
`Macrophage
`
`Migration,
`adhesion,
`penetration
`
`Central nervous system
`Reactivation
`
`Inflammation
`
`Microglia
`
`Macrophage
`
`Demyelination
`patterns (1)—(4)
`Neuron Oligodendrocyte
`
`Axonal damage
`
`
`
`
`
`Antigen-
`
`presenting
`Adhesiesion
`cell
`TNF-o.,
`x
`
`MHC-II Jal molecules,|MHC-IIJal < Qo, NO
`
`
`
`Microbial Ag © proteases,|CNS Ag O Uf Activated S (1) T-cell and
`
`
`chemokines
`IFN-y ,
`T cell
`SS
`macrophage
`
`TCR
`TCR
`7
`attack
`1
`;
`’
`TNF=c.
`i
`vt
`
`
`Autoreactive oy(Oy)I Le >(1) ( Antibodyattack
`3
`\
`Complement
`4
`apoptosis
`2
`\
`~
`(4) Primary OG
`7
`Thi
`a
`IFN-y, ‘Activated
`Plasma
`F
`degeneration
`Axonalswelling,
`2
`4 Beell
`IL-2
`cell SQ
`lobulation,
`5
`<x!
`disconnection
`}
`©--Oz
`s
`= (2)
`a]
`8
`=f
`eS
`
`no Demyelinating_—Target cellsEffector cells
`antibodies
`
`--»>
`
`
`
`TRENDSin Pharmacological Sciences
`
`Fig. 1. Pro-inflammatoryTcells in the periphery are activated whenforeign antigens (Ags) andself-antigens that are presented on major histocompatibility complexclassII
`(MHC-Il) by Ag-presenting cells, such as macrophages,bind to T-cell receptors (TCRs). Activated T cells migrate to, adhere at and penetrate through the blood-brain bar-
`rier, steps that are mediated by adhesion molecules, proteases and chemokines. In the CNS,the T cells are reactivated by CNS Ags presented on MHC-II by other Ag-pre-
`senting cells, predominantly microglial cells. The reactivated T cells secrete pro-inflammatory cytokines, such as interferon y (IFN-y) or interleukin 2 (IL-2), which induce
`CNSinflammation by subsequent activation of macrophages,other T cells and B cells. MacrophagesandTcells attack the myelin sheath of oligodendrocytes (OGs) by
`cytotoxic mediators, mainly tumornecrosis factor « (TNF-«), Oz radicals and nitric oxide (NO). B cells differentiate into plasma cells. These secrete demyelinating antibodies
`that can guide and activate macrophages, and ignite the complement cascade, which causes assembly of the membrane attack complex and causes pore formation in
`myelin membranes. Demyelination occurs by four different pathological patterns (1-4), as described in the main text.
`
`oligodendrocyte apoptosis; and (4) primaryoligodendrocyte
`degeneration. The mechanismsofthe latter two patterns
`remain elusive.
`In addition to this autoaggressive inflammatory phase,
`axonal loss, which causes irreversible disability, occurs
`early in the course of the disease [24,25]. It is unclear
`whether axonal damageis the consequence of a primary
`active destructive process executed by,
`for example,
`macrophages and cytotoxic molecules derived from CD8
`cells [26], or a (patho)physiological response that occurs
`secondarily to demyelination and is based on increased
`vulnerability [24,27]. Axonal damage appears to be
`initiated by increased membrane permeability followed
`by enhanced Ca?" influx. Disruption of axonal transport
`alters the cytoskeleton and leads to axonal swelling,
`lobulation and, finally, disconnection [28].
`The different molecules involved in each phase of MS
`are summarized in Fig. 1 [1,17,29].
`
`MStherapeutics: immunomodulatory profile in vitro and
`in vivo
`The structural features of the therapeutic agents are
`shown in Fig. 2. Interferon Bla (IFN-Bla), IFN-B1b and
`glatirameracetate (GA) are approved for the treatment of
`relapsing-remitting (RR) MS,
`IFN-§1b for secondary-
`progressive (SP) MS, IFN-B1a for SP MS with super-
`imposedrelapses and mitoxantrone for worsening forms of
`RR and SP MS.
`
`Glucocorticosteroids
`Methylprednisolone and prednisoloneare the mainstays of
`treatment for acute attacks in MS [30]. Becauseof side-
`effects in long-term treatment regimens and superior
`efficacy compared with oral application, steroids are
`mostly delivered in intravenous pulses. Most of the
`immunologicaleffects of glucocorticosteroids are mediated
`by specific, ubiquitously distributed intracellular recep-
`tors. These form a multiprotein complex with two
`http://tips.trends.com
`
`molecules of heat shock protein 90 (HSP90), one molecule
`of HSP70, one molecule of HSP56, one so-called immuno-
`philin, andother, less well characterized, proteins. After
`binding lipophilic glucocorticosteroid, the receptor dis-
`sociates irreversibly from the rest of the complex, and the
`steroid—receptor complex ligates specific glucocorticoid-
`responsive elements in nuclear DNA.This either upregu-
`lates or downregulates the transcription of target genes.
`The nuclearfactor kB (NF-«B) transcription factor induces
`the expression of multiple inflammatory and immune
`genes [31]. Glucocorticosteroids inhibit NF-«B, both by
`direct binding of the activated receptor to NF-KB and by
`inducing expression of the specific inhibitory protein IkB
`in lymphocytes.
`effects of glucocorticosteroids
`The immunological
`include: (1) inhibition of T-cell activation and production
`of pro-inflammatory cytokines, such as interleukin 2 (IL-2)
`and IFN-y; (2) increased production of anti-inflammatory
`Th2 cytokines by T cells; (3) inhibition of IFN-y-induced
`major histocompatibility complex (MHC) class IT expres-
`sion on macrophages; (4) decreased production of pro-
`inflammatory cytokines, prostaglandins and leukotrienes
`by macrophages;(5) diminished adhesion of neutrophils to
`endothelial cells; and (6) inhibition of endothelial-cell
`activation, and expression of MHCclass II and adhesion
`molecules. Taken together,
`these actions underlie the
`sealing effect of steroids on the blood—brain barrier, which
`prevents further access of immunecells and molecules to
`the brain.
`In addition to the genomic effects mediated by steroid
`receptor activation, there is recent evidence that high
`doses of glucocorticosteroids induce apoptosis of target
`cells by a direct, non-genomic effect
`[32]. In this case,
`apoptosis is thought to be mediated by a direct effect on
`cellular membranes, which influences transmembranous
`ion transport and, subsequently, reduces the availability of
`ATP. Steroid-induced apoptosis of autoreactive T cells is
`important for the termination of an acute MSattack [32].
`
`

`

`TRENDSin Pharmacological Sciences Vol.24 No.3 March 2003
`
`133
`
`IFN-B
`IFN-8 is highly species specific. In humans, the IFN-B
`polypeptide is produced andsecreted by fibroblasts, but
`virtually all mammalian cells can produce IFN-f on stimu-
`lation. Two recombinant IFN-B preparations, IFN-B1a and
`IFN-81b, are approved for treatment of MS. The optimal
`dosage and route of administration (subcutaneously
`versus intramuscularly), the resulting pharmacodynamic
`properties of IFN-f and therole of neutralizing antibodies
`are controversial [33-37].
`IFN-8 has immunomodulatory properties, antiviral and
`anti-proliferative effects, and promotescell differentiation
`[38]. Although the mechanismsofaction of IFN-B are not
`fully understood, there is agreement that the majoreffects
`are mediated by activation of a transmembrane IFN
`receptor, which leads to either the upregulation or down-
`regulation of target genes [39,40]. Unlike IFN-vy, the ‘type
`II interferon’, IFN-8 and IFN-a (type I interferons) share
`a receptor, which consists of two chains, IFN-aR1 and
`IFN-aR2, or several subvariants. Binding of IFN-8 (and
`other type I interferons) to the extracellular domain of the
`receptor induces an intracellular signal
`transduction
`cascade that involves: (1) recruitment and activation of
`the cytoplasmic tyrosine kinase 2 by IFN-aR1, and Janus
`kinase 1 by IFN-aR2; (2) subsequent phosphorylation and
`recruitment of signal transducers andactivators of tran-
`scription (STAT1 and STAT2) to form a STAT1-—STAT2
`heterodimer; (3) migration of the STAT1—STAT2 hetero-
`dimer to the nucleus; (4) association of STAT1—STAT2
`with the p48 protein, to form the active ‘IFN-stimulated
`genefactor 3’; (5) binding of IFN-stimulated genefactor 3
`to promoter elements andinitiation of the transcription
`of target genes. The variations in this rough scheme
`ofsignal transduction thatleadto different effects ofIFN-B
`in different target cells are based on variations in each
`of the steps outlined above, which are not yet fully
`understood[39].
`The range ofimmuneeffects attributed to IFN-B is wide
`and ever broadening [38,41]. It suppresses T-cell pro-
`liferation, diminishes IFN-y-induced upregulation of MHC
`class II expression, induces the production of Th2 cyto-
`kines andreduces synthesis ofTh1 cytokines, and inhibits
`monocyte activation. In addition, IFN-8 downregulates
`matrix metalloproteinases (MMPs), decreases surface-
`expressed adhesion molecules and increases the release
`ofsoluble adhesion molecules, which combineto reduce the
`migratory potential of T cells.
`
`GA
`GA is the acetate salt of a standardized, randomized
`mixture of synthetic polypeptides. After subcutaneous
`administration, GA is quickly degradedto free aminoacids
`and small oligopeptides and, thus, most probablyinitiates
`its major immunological effects in the periphery. Unlike
`the multiple immunological effects of IFN-8, which are
`antigen nonspecific, the immunomodulatory potential of
`GAis based on immunecells that are specific for myelin
`basic protein (MBP)and, probably, other myelin antigens
`[38,42]. Four major mechanismsof GA activity have been
`identified:
`(1) competition between GA and MBP for
`binding to MHC molecules;
`(2) competition between
`http://tips.trends.com
`
`GA-MHCcomplexes and MBP—MHCcomplexesfor bind-
`ing to the T-cell receptor (TCR); (3) activation andtoler-
`ance induction in MBP-specific T cells through an altered
`peptide ligand; and (4) induction of GA-reactive, Th2-like
`regulatory cells that mediate local bystander suppression.
`Whereas the first two effects are thought to take
`place only in vitro, the latter two are also likely to occur
`in vivo and could contribute to the anti-inflammatory
`effects of GA [42].
`
`Intravenous immunoglobulins
`Intravenous immunoglobulins (IVIgs) are pooled, purified,
`human Igs with virtually unlimited specificities. Consist-
`ent with their composition, several mechanismsof action
`of IVIgs have been suggested [43,44]. These include:
`(1) anti-idiotype antibodies (binding and inactivation of
`pathogenic antibodies by IVIgs);
`(2) blockade of Fc
`receptors on mononuclear phagocytes; (3) downregulation
`of the endogenous production of Igs; (4) attenuation and
`abrogation of complement-mediated effects (partially by
`‘consumption’ of complement components); (5) neutraliz-
`ation of molecules (TCR, MHC, costimulatory molecules
`and cytokines)
`that are involved in inflammation;
`(6) induction ofanti-inflammatory cytokines; and (7) induc-
`tion of apoptosis. Experimental evidence indicates that
`IVIgs mightalso be involved in myelin repair, but clinical
`proofof this is lacking [44]. Currently, the role of IVIgs in
`treating MS is undecided.
`
`Immunosuppressive agents
`Mitoxantroneis effective in the treatment of severe active
`forms of MS [45,46]. Although mitoxantrone suppresses
`both T cells and B cells,
`in vitro experiments indicate
`that major sites of action in MS are antigen-presenting
`cells, which are induced to undergo apoptosis, and macro-
`phages, the major effector cells of demyelination, which
`are deactivated [47].
`Azathioprine is widely used in organ transplantation
`and autoimmunedisorders [48] and is considered a second-
`line drug in MS.It mainly targets the activation, pro-
`liferation and differentiation of both T cells and B cells by
`competition between its metabolites and DNA nucleotides.
`Specific immunomodulatory properties have not been
`reported to date.
`Cyclophosphamideis usedto treat severe and rapidly
`progressive forms of MS, although evidenceofits efficacy
`in clinical trials is conflicting. In addition to strong
`immunosuppression, cyclophosphamide exerts immuno-
`modulatory effects that shift immuneresponses from Th1
`towards Th2 by an unknown mechanism [49].
`One study has shown methotrexate to convey some
`therapeutic benefit in progressive forms of MS [10], but its
`mechanismsof action in autoimmunediseasesare largely
`unknown.
`
`Potential new agents
`Anti-adhesion molecules
`Adhesion molecules comprise several families of mol-
`ecules that are essential in virtually all cellular inter-
`actions ofimmunecells [50,51]. The selectins (e.g. L-selectin,
`E-selectin and P-selectin), which have a lectin-binding
`
`

`

`134
`
`TRENDSin Pharmacological Sciences Vol.24 No.3 March 2003
`
`
`
`(a) Glucocorticosteroids
`
`(g) Cyclophosphamide
`
`—C|
`
`
`
`HoCS~ cy,
`
`QQ. N—~cH,
`a
`O~
`~NH HeC~o,
`
`279.1 Da
`
`(h) Methotrexate
`
`H2N
`
`N
`
`NH»
`
`7 . aNn @Z
`
`N
`
`N'
`
` CH»NCH3
`
`Hooc
`HOOC~-CH)-CH2-CH—NH-CO
`454.4 Da
`
`(i) Anti-adhesion molecule antibodies
`150 kDa
`
`Murine
`Human
`
`150 kDa
`
`(j) Statins
`
`Simvastatin (418.6 Da)
`
`(k) Neurotrophic factors
`
`NH,
`
`COOH
`
`BDNF
`(13.5 kDa)
`
`Disulfide bridges:
`Cys13-80; Cys58—109; Cys68-111
`
`(I) Neuroprotective agents
`
`oO
`
`CF3~ CrN
`
`s
`
`Riluzole (234.2 Da)
`
`
`
`Methylprednisolone (374.5 Da)
`
`(b) Interferon B (IFN-B)
`
`
`Cys31 Cys17
`
`NH»)
`Met1
`
`E
`
`COOH
`Asn166
`
`Sugar
`residues
`Disulfide bridge:
`Cys31-141
`
`IFN-B1a
`(22.5kDa)
`
`Cys31
`
`Cys141
`
`Ser17
`
`A
`
`B
`
`D
`
`E
`
`NH
`Ser2
`
`COOH
`Asn166
`
`IFN-B1b (18.5 kDa)
`
`(c) Glatiramer acetate (GA)
`COLSON acid
`GOBHBOOGLLOBO
`oeDESAISUSENLSAIDC
`HOSASAAN
`CRDSADADESLSCNING
`SSIES
`et tat tet tet
`
`4.7-11.0 kDa
`
`(d) Intravenous immunoglobulins (IVIgs)
`
`Vv,
`
`Gy
`
`C
`
`L
`
`(\y,,
`
`Fab fragment<
`.
`:
`Hinge region
`
`Fc fragment
`
`~— Light chain
`<<— Heavy chain
`
`:
`,
`Variable region
`Constant region
`.
`.
`— Disulfide bridge
`
`150 kDa
`
`(e) Mitoxantrone
`
`OH
`
`© H-N-CH)-CH)-NH-CH)-CH,-OH
`
`e 2 HCI
`
`OH
`
`O H-N-CHj-CH2-NH-CH)-CH2-OH
`517.4 Da
`
`Azathioprine
`
`(fy
`
`—
`
`OxLO
`vsLy
`
`7
`
`H3c’
`
`N
`
`H
`NeN-N
`
`‘T
`
`277.3 Da
`
`TRENDSin Pharmacological Sciences
`
`http://tips.trends.com
`
`

`

`TRENDSin Pharmacological Sciences Vol.24 No.3 March 2003
`
`135
`
`[55]. In addition, statins
`transactivator IV promoter
`curtail T-cell proliferation, lower expression of activation
`surface markers and induce production of the cytokine
`IL-4 [56]. Currently, simvastatin is being tested in a Phase
`II clinical trial in MS. The oral administration, extensive
`safety data, possible effects synergistic with IFN-8 and
`simultaneous treatment of co-morbidity make statins
`particularly attractive candidate agents.
`
`domain that binds to glycosylated andsialylated ligands,
`are involved in the ‘rolling’ of leukocytes. Integrins
`comprise a4B1, «581, a681 integrins [also known as
`very late antigen 4 (VLA-4) VLA-5 and VLA-6, respec-
`tively] and leukocyte function antigen 1 (LFA-1). VLA-4
`and LFA-1 are present on circulating leukocytes and
`mediate their migration across the vascular endothelium;
`in addition, LFA-1 is a costimulatory molecule in T-cell
`activation. Cadherins
`(including classical cadherins,
`desmosomal cadherins and protocadherins) form molecu-
`lar links between adjacentcells by zipper-like structures.
`Members of the Ig superfamily, such as intercellular
`adhesion molecule 1 (ICAM-1),
`ICAM-2 and ICAM-3,
`vascular cell adhesion molecule (VCAM), and LFA-2 and
`LFA-3 have Ig-like domains. ICAM-1 and VCAM-1 are the
`counter-receptors of LFA-1 and VLA-4, respectively, and
`are thus involved in leukocyte migration.
`Antibodies against single adhesion molecules can
`potently inhibit crucial steps in the pathogenesis of MS,
`especially lymphocyte migration. Currently,
`the most
`promising candidate is natalizumab [52], a humanized
`monoclonal antibody against the a4 chain of 0481 integrin
`that has been effective in PhaseII clinical trials [53].
`‘Humanized’ meansthat a murine antibody clone has been
`Neuroprotective agents
`grafted toa human IgG4 framework at the complementary
`Recent evidence indicates that axonal and neuronal
`determining region to reduce its immunogenicity [52]. Two
`
`degeneration occur early in MS and-—as the disease
`large-scale, PhaseIII clinical trials in RR MS have been
`evolves — predominate the underlying pathogenetic
`initiated recently and small-molecule antagonists, which
`mechanisms [4,17,24]. This paradigm shift has obvious
`offer advantages over monoclonal antibodies (oral avail-
`therapeutic implications.
`In addition to neurotrophic
`ability and lack of antigenicity) will certainly be explored
`in the future [54].
`factors, other chemically defined neuroprotective agents
`that save neuronsfrom toxic stress [64], such asriluzole (a
`potent K* channel activator used in amyotrophiclateral
`sclerosis) are either being investigatedor are likely to be
`tested in clinical trials.
`
`Neurotrophic factors
`Neurotrophic factors are secreted proteins that regulate
`the survival anddifferentiation ofnervecells [59]. They act
`via specific neurotrophin receptors [60]. Neurotrophic
`factors have been observed to shift the CNS cytokine
`balance from Th1 to Th2 by an undefined mechanism [61].
`In addition, they might promote survival ofneurons in MS
`lesions [6] and pharmacological neuroprotection by the
`exogenous application of neurotrophic factors provides
`a promising therapeutic approach [59,62]. Insulin-like
`growth factor 1 is currently explored in a PhaseI/II study.
`Other attractive candidates are brain-derived neuro-
`trophic factor, glial growth factor and ciliary neurotrophic
`factor [63].
`
`Statins
`Statins are effective lipid-lowering agents. Recent findings
`indicate that they have additional immunomodulatory
`effects in vivo in the animal model experimental auto-
`immune encephalomyelitis (EAE), and in vitro [55-57].
`Oneproposed immunomodulatory mechanism ofstatins is
`basedon theselective inhibition of the adhesion molecule
`LFA-1, an integrin that is involved in inflammation [58].
`Furthermore,statins reduce IFN-y-induced MHCclassII
`expression by blocking transcription of the class II
`
`Similarities and peculiarities
`there are
`Given the complexity of MS pathogenesis,
`multiple sites where immunomodulatory agents, either
`alone or in combination, might be effective. Current
`knowledge of the sites of action obtained from in vitro
`and in vivo data is summarized in Fig. 3. Although the
`
`Fig. 2. Structures of immunomodulators. Interferon f (IFN-f), glatiramer acetate (GA) and mitoxantrone are currently approved for use in multiple sclerosis (MS). (a) Gluco-
`corticosteroids are derived from cortisol, a naturally occurring adrenal hormone. Methylprednisolone is commonly usedto treat acute relapses in MS.(b) IFN-8 contains
`166 residuesthat form five a-helices (A-E). Two preparations of recombinantIFN-B are approved for MStreatment. IFN-B1a is produced in Chinese hamsterovarycells and
`is pharmacologically identical to the natural form (i.e. it is glycosylated by oligosaccharides at Asn80). Compared with natural IFN-f, IFN-81b is not glycosylated, it lacks
`Met1 (i.e. IFN-81b has 165 residues) and thereis a Cys to Ser substitution at residue 17. In both forms of recombinant IFN-B,there is a disulfide bridge between Cys31 and
`Cys141. Structurally important amino acids are shownin green.(c) GAis the acetate salt of a standardized, randomized mixture of synthetic polypeptides (average length
`45-100 aminoacids) that consist of L-glutamic acid, L-lysine, L-alanine and L-tyrosine in the molar ratio of 0.14:0.34:0.43:0.09. An example sequence is shown. (d) Intra-
`venous immunoglobulins (IVIgs) are pooled, purified, human Igs prepared by cold ethanol fractionation of human plasma derived from 3000-10 000 donors. Igs share a
`commonY-shapedstructure, which consists of two heavy chains (50 < hsp sp = 0.25 > kDa each) and twolight chains (25 < hsp sp = 0.25 > kDa each) connectedbydisul-
`fide bridges. The variable region is responsible for antigen recognition. The five heavy-chain isotypes determine the immunoglobulin classes. IVigs contain ~ 95% IgG,
`2.5% IgA and a minority of IgM. (e) Mitoxantrone is an anthracenedionederivative that is related to the anthracyclins doxorubicine and daunorubicine.It interacts with
`topoisomerase-2 and causessingle and double string breaks by intercalating with DNA. (f) Azathioprine is a purine analoguethat is metabolized rapidly to the cytotoxic
`and immunosuppressantderivatives 6-mercaptopurine and thioinosine acid, the latter competes with DNAnucleotides. (g) Cyclophosphamideis an alkylating agent of the
`nitrogen mustard group;its active metabolites, formed by the activity of hepatic cytochrome P450, induce DNA-string breaks. (h) Methotrexate interferes with DNA syn-
`thesis by inhibiting dihydrofolate reductase and, thus, thymidine biosynthesis. It reduces 1-carbontransfers to purines. (i) Monoclonal antibodies against adhesion mol-
`ecules inhibit homing of T cells to the CNS. To reduce immunogenicity, newer approachesuse ‘humanized’ antibodies, which are chimeras of murine variable regions and
`humanconstant regions connected at the complementary determining region.(j) Statins inhibit 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase. Statins are
`effective lipid-lowering agents because HMG-CoAreductaseis essential to the cholesterol biosynthesis pathway, but they also have immunomodulatory properties. (k) The
`structure of neurotrophic factors is dominated by four antiparallel pairs of 8 strands (green arrows) held in place by three disulfide bridges. Neurotrophic factors are active
`as homodimersandact via specific neurotrophin receptors. Brain-derived neurotrophic factor (BDNF) is shown,with the position of the cysteine residues that form the dis-
`ulfide bridges in green.(1) Neuroprotective agents comprise a heterogeneousgroup thatis characterized by their ability to exert protection from neurotoxic stress. Riluzole
`acts by activating Kt channels.
`http://tips.trends.com
`
`

`

`136
`
`TRENDSin Pharmacological Sciences Vol.24 No.3 March 2003
`
`
`
`
`(Cc)T-cell migration, adhesion andpenetration
`Steroids, IFN-B, IVigs, anti-adhesion molecule
`antibodies, statins
`
`(@) Release of pro-inflammatory cytokines
`Steroids, IFN-B, IVigs
`
`(h)Effectorcell activation: macrophages
`Steroids, IFN-B, mitoxantrone
`
`
`
`
`
` (k) Axonal damage
`
`
`
`
`(a) Antigen-presenting cell (d)T-cell reactivation (i) Effectorcell activation:Tcells
`Steroids, IFN-B, IVigs, mitoxantrone,
`IFN-B, IVigs, azathioprine
`Steroids, IFN-B, IVigs, azathioprine
`Neurotrophins, neuroprotective agents
`statins
`
`
`
` TRENDSin Pharmacological Sciences
`
`
`
`
`
`(b) Autoreactive T cell
`IFN-B, GA, mitoxantrone,
`azathioprine, cyclophosphamide,
`methotrexate, statins
`
`
`
`(9)
`
`GA
`o=(©)
`TCR
`MHC-II
`
`The
`
`GA-reactive
`regulatory T cell
`
`
`
`
`
`
`(j)Effectorcellactivation:Bcells,
`.
`roan
`plasmacells and demyelinating antibodies
`IVIgs, mitoxantrone, azathioprine
`
`
`
`(I) Demyelination
`+ Remyelination as neuroconstructive approach?
`+ Stem cells as source?
`+ Schwanncell preparations?
`
`
`’
`’ Bystander
`suppression
`effect
`
`/
`
`/
`
`/
`
`v
`/
`7 IL-4
`‘CNS Ag
`Ee f
`
`TCR =
`MHC-II
`
`(f) Cytokineshift
`Th1 —> Th2
`Steroids, IFN-B,
`GA-reactive T cells,
`cyclophosphamide,
`factors
`
`statins,neurotrophic
`
`Fig. 3. lmmunomodulatory agents have commonsites of action in multiple sclerosis (MS) and interfere at other sites individually. (a) Antigen presentation by antigen-pre-
`senting cells is inhibited. (b) Proliferation of autoreactive T cells is blocked or tolerance is induced.(c) T-cell migration, adhesion and penetration into the CNS are abro-
`gated. (d) T-cell reactivation in the CNS is blocked. (e) Release of pro-inflammatory cytokines by autoreactive T cells is diminished. (f) Several agents induce a shift from
`pro-inflammatory Th1 to anti-inflammatory Th2 cytokines. (g) Uniquely, GA induces regulatory T cells that cross-react to both GA and CNSAgs.After reactivation inside
`the CNS,these regulatory T cells exert a local bystander suppression effect through the release of Th2 cytokines. (h) Activation of macrophagesorsecretion of pro-inflam-
`matory mediators are curtailed. (i) Apoptosis of autoreactive T cells is induced. (j) Activation of B cells and their differentiation to plasma cells is blocked; demyelinating
`antibodies are neutralized by several mechanisms. (k) Axonal damage is reduced by neuroprotective approaches.(I) Therapeutical approaches that target demyelination
`arestill at an experimental stage. Abbreviations: Ag, antigen; GA, glatirameracetate; IFN, interferon; IL, interleukin; IVigs, intravenous immunoglobulins; MHC, majorhisto-
`compatibility complex; TCR, T-cell receptor; Th1, T helper1.
`
`immunomodulatory agents presented share some common
`features, many differ markedly in their mechanisms and
`sites of action. General properties to be considered are
`nonspecific versus specific antigen effects (e.g.
`IFN-B
`versus GA), polydirectional versus unidirectional immuno-
`modulatory properties(e.g. the pleiotropic effects of IFN-B
`versus
`the monoclonal
`antibody directed against
`a4-integrin)
`and
`pure
`immunomodulators
`versus
`immunosuppressants with additional immunomodulatory
`properties (e.g. IFN-B versus mitoxantrone).
`The interactions between the various drugs arestill
`widely unknown [65,66]. However, gaining more know]-
`edge of the individual targets might allow combination
`therapies to be developed that have either additive or
`synergistic effects.
`
`Concluding remarks
`Although an increasing arsenal of immunomodulatory
`agents is available, many questions remain about each
`single approach [67]. These include whento initiate and
`whento stop treatment, the optimal dose, frequency and
`route ofadministration, the long-term effects oftreatment,
`the occurrence and relevanceof neutralizing antibodies to
`biological agents, and the cost utility.
`http://tips.trends.com
`
`It is likely that differential therapy in MS will gain in
`importance in the near future. As evidence mounts that
`MS comprises several distinct subforms[5], the choice of
`treatmentfor individual patients will, ideally, be determined
`by the knowledgeof the specific underlying pathomechan-
`ism and the respective optimal drug or combination ofdrugs.
`The goalis tailor-made immunotherapy.
`Recent evidenceindicates that autoimmunecells have
`neuroprotective properties that are mediated, at least in
`part, by neurotrophic factors [6,7]. Based on this, a novel
`therapeutic approach would be to ‘import’ neurotrophic
`factors into the CNS via immunecells [68]. However, these
`findings also provide an impetusto shift the rationale of
`nonselective immunosuppressive therapies towards a more
`selective immunomodulatory regimen, which would pre-
`serve or even enhance the neuroprotective functions of
`autoimmuneT cells[6].
`In a contrasting approach, bone marrow transplan-
`tation (the most aggressive strategy to eradicate a
`disordered immune system) is being tested clinically
`[69,70], but this remains controversial [71].
`Taking neurobiological approaches further, the next
`step on the horizon (which is being investigated in
`animal models and in vitro) is neuroregeneration. This
`
`

`

`TRENDSin Pharmacological Sciences Vol.24 No.3 March 2003
`
`137
`
`Box 1. Currentclinical trials in multiple sclerosis
`
`http.//www.nationalmssociety.org/pdf/research/agents.pdf
`http.//www.nationalmssociety.org/pdf/research/clinicaltrials. pdf
`
`neuroconstructive therapy encompassesinhibition of scar
`formation and promotion of axonal regrowth and remye-
`lination [72-74]. The contentious issue of the use of
`embryonic stem cells as source of progenitors for remye-
`lination must be discussed in this context. Alternatively,
`retrieval of multipotential neuronal stem cells from the
`adult brain has been suggested [75]. Recently, oligo-
`dendrocyte progenitors have been identified, even in
`chronic MS lesions [76]. Transplantation of peripheral
`Schwanncells is being studied in Phase II clinical trials
`anddefined chemical agents for CNS myelination, such as
`eliprodil (a high-affinity ligand of a so-called o-receptor
`that mediates neuroprotection) are being explored experi-
`mentally [77]. B