L. Vécsei (ed.), Frontiers in Clinical Neuroscience
`© Springer Science+Business Media New York 2004
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`deterioration associated with the disease. The fact that axonal loss is irreversible has
`important implications for when, and what therapeutic intervention should be used.
`
`Multiple Sclerosis
`
`Neurodegenerative disease
`
`Inflammatory
`demyelinating
`
`loss
`
`relapses 5 Axonal
`
`
`
`Neuronalloss
`
`
`Course of disease
`
`Course of disease
`
`
`
`Tlflarlydiagnosispossible
`
`Only late diagnosis? possible
`
`Figure 1: Early diagnosis and therapy of the demyelinating neurodegenerative disorder MS is possible in
`comparison to other neurodegenerative diseases like M. Parkinson or Alzheimer
`
`It is likely that various mechanisms contribute to axonal damage during different stages
`of disease. In active lesions, the extent of axonal transsection correlates with inflammatory
`activity while even there seems to be an inflammation-independent axonal losss. Hence,
`axonal loss may be caused by inflammatory products of activated immune and glial cells,
`including proteolytic enzymes, cytokines, oxidative products and free radicals, although the
`precise molecular mechanisms of axonal damage are poorly understood. In addition the
`magnitude of axonal
`loss in chronic MS lesions without pronounced inflammatory
`infiltrates suggests that mechanisms other than inflammatory demyelination contribute to
`the degeneration of axons. Several conditions interfere with attemps of axonal regrowth
`afier lesions develop. These include the lack of neurotrophic factors that support growth,
`the presence of a glial scar (depending on the site of lesion) or the presence of inhibitory
`molecules that impede axonal growth. Recent evidence shows that axon degeneration
`following injury has similarities with the cellular mechanisms underlying programmed cell
`deaths.
`
`The concept of MS as an inflammatory neurodegenerative disease highlights the
`importance of an active therapeutic approach. But since lesions outnumber clinical relapses
`by much as 10:1 and, in addition, inflammation-independent axonal degeneration may
`occur and add to the considerable continuing subclinical pathophysiological process, tissue
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`NEUROPROTECTION AND GLATIRAMER ACETATE
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`damage even in the absence of clinical manifestations may take place. Because MS has its
`characteristic clinical phenotype with clinical relapses and remissions, MS could be
`diagnosed and treated at an earlier
`timepoint of the disease in contrast
`to other
`neurodegenerative disorders like M. Parkinson or M. Alzheimer (Figure 1). Theoretically,
`an antiinflammatory and neuroprotective treatment could be started early in MS before
`most axons and neurons would be lost.
`
`The early and continuous application of disease-modifying therapies offers the
`possibility that accumulating axonal degeneration and permanent fimctional disability can
`be prevented or delayed. The clinical challenge in this respect is therefore the early decision
`for an individual MS patient on anti-inflarnmatory and neuroprotective MS therapy.
`
`2. DETECTING AXONAL DAMAGE IN MS
`
`It is well known that there is axonal loss within chronic MS lesions. The use of
`immunocytochemical methods that stain axonal end-bulbs demonstrates evidence of axonal
`injury even in acute and early lesions‘. Axonal injury in MS lesions will lead to both
`Wallerian degeneration of the axon and also retrograde degeneration of the cell body. The
`functional consequences of the axon injury will depend on the numbers of injured axons
`and the topographical organization of the fibres coursing through the lesions.
`Besides neuropathology, the other technical advance that has drawn attention to axonal
`loss in MS is the use of magnetic resonance imaging (MRI) and spectroscopy (MRS)°. The
`use of these techniques is helpful in characterizing the underlying pathologic processes in
`multiple sclerosis.-There is consensus that T2—weighted MRI reflects the broad spectrum of
`pathological changes, including inflammation, edema, demyelination, gliosis and axonal
`loss. Changes in the number and volume of lesions on T2—weighted MRI (lesion load) are
`sensitive but nonspecific indicators of disease activity and the response to treatment. There
`is evidence that — below the detection treshold - the normal-appearing white matter is not
`normal at all in patients with MS.
`Besides inflammatory markers, techniques to quantify images from MR1 have revealed
`significant tissue atrophy in the spinal cord and brain in MS patients. These measures of
`whole tissue cross-sections or volume do not, however, discriminate between myelin and
`axonal loss. A powerful technique for the analysis of the biochemical components of tissue
`in life is magnetic resonance spectroscopy (MRS). The normal proton spectrum in brain
`tissue is dominated by a signal from N-acetyl aspartate (NAA), an amino acid that appears
`to be specifically localized to neuronal cell bodies and axons7. There are a number of
`studies demonstrating that the amount of NAA is decreased in MS lesions but importantly
`also in apparently normal white matter”.
`Hypointense lesions on enhanced Tl-weighted images (“Tl black holes”) have been
`reported to correspond to areas where chronic severe tissue disruption has occurred"). When
`one considers the evolution of a MS lesion, the T1 black hole usually represents the end
`stage of the process, when significant demyelination, axonal loss, and reactive gliosis have
`occurred. The degree of hypointensity appears to correlate with a decrease in the
`magnetization transfer ratio (MTR) and with axonal loss as quantified by a reduction in the
`NAA peak on MR spectroscopy or in histopathology”"2 (Figure 2). This measure is
`beginning to show a better correlation with disease progression than either T2 disease
`burden or Gadolinium (Gd)-enhancing lesions in patients with secondary progressive MS.
`At the time of Gd-enhancement, a significant proportion of lesions will demonstrate some
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`T. ZIEMSSEN
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`hypointensity on Tl-weighted images. Over a period of time, most enhancing lesions
`become isointense to white matter and their MTR returns to that of normal-appearing white
`matter. A proportion of Gd-enhancing lesions will remain hypointense and eventually
`become Tl black holes.
`
`.Biacl< §}{3§iif5"‘ in the Mitt
`
`
`
`Ti~weights3t§ .\’§R¥
`.,.\\\»».zw\m.~»\\.\~.»m\».\«\~_x\\»
`
`Figure 2: Tl hypointense lesions (black holes) are strongly associated with axonal density, emphasizing their role
`in monitoring progression in multiple sclerosis.
`
`Roughly 30% to 40% of new lesions will evolve into persistent black holes over short
`time periods (5 to 12 months) and represent severe and irreversible tissue disruption.
`Evidence from a large number of postmortem and in vivo MRI studies have substantiated
`that permanent Tl hypointensive lesions correspond to areas of severe axonal damage and
`myelin loss. Consistent with these findings, data also indicate that the overall extent of
`hypointense brain lesions correlates with the degree of MS-related disability”.
`
`3. NEUROTROPHIC FACTORS AS SPECIAL NEUROTROPHIC AGENTS
`
`Damaged neurons in the CNS attempt to repair themselves although these atternps are
`usually not succesful. An important component of restoration of function cxpecially in a
`disorder like MS in which axons are darrtaged, must include the potential of axonal repair.
`One of the more promising approaches to encourage axonal growth is the administration of
`growth-supporting molecules, particulary specific growth factors.
`Historically, nerve growth factor (NGF) was first and for a long time, the only known
`neurotrophic factor which was primarily best characterized by its anti-apoptotic function on
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`neurons during development. Following the discovery of structurally related proteins with a
`similar neurotrophic function,
`the term “neurotrophin” was introduced for this protein
`family of homodimers with a conserved region containing a cystein bond in the core of the
`molecule and with duplicate sites for receptor binding”.
`The neurotrophins of the NGF family are not the only proteins with neurotrophic
`function (Table 1). In recent years, two additional families of protein growth factors have
`been characterized, which exert strong neurotrophic activity on developing neurons. The
`first one is the family of the glial-cell-derived neurotrophic factor (GDNF) ligands (GFLS)
`including GDNF and three related proteins". The second family is
`formed by the
`neuropoietic cytokines, which besides other more pleiotropic cytokines includes ciliary
`neurotrophic factor (CNTF) and leukemia inhibitory factor (LIF)15. Although structurally
`different, these three families are now collectively referred to as neurotrophic factors. In
`addition, neutroprotective activity has been reported for growth factors not belonging to any
`of the three neurotrophic factor families, one prominent example beeing insulin-like growth
`factor (IGF)-1”.
`
`Table 1: Different protein families with neurotrophic function
`
`NGF-related neurotrophins Nerve growth factor (NGF)
`
`Brain-derived neurotrophic factor (BDNF)
`
`Neurotrophin (NT)-3
`
`Neurotrophin (NT)-4/5
`
`GDNF family ligands
`
`Glial-cell-derived neurotrophic factor (GDNF)
`Neurturin
`
`Neuregulins (GGF-2)
`
`Artemin
`
`Persephin
`
`Neuropoetic cytokines
`
`Ciliary neurotrophic factor (CNTF)
`
`Leukemia inhibitory factor (LIF)
`
`Miscellaneous factors
`
`Insulin-like growth factor (IGF)-l
`
`In a therapeutical context it is important to note that the functions of neurotrophic
`factors are not restricted to neural development. It is evident that neurotrophins act on
`mature neurons, most prominently on injured and degenerating nerve
`cellsms.
`Neurotrophic factors can protect and rescue neurons in a large number of experimental
`models. Of particular relevance to MS is the demonstrated ability of BDNF and NT—3 to
`promote regeneration of long tracts in the spinal cord”. In addition, the expression levels of
`neurotrophins and their receptors are strongly regulated in pathological conditions, thus
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`arguing for a role of these proteins in the response of neurons to traumatic or degenerative
`processes.
`
`Neurotrophins are also involved in the development and maintenance of glia, including
`oligodendroglia. NT—3 stimulates the proliferation of oligodendrocyte progenitors in vitro,
`and botzh NT—3 and NGF enhance the survival of differentiated oligodendrocytes in
`cultures °.
`
`The role of the neurotrophins in demyelinating diseases is presently unclear. Levels of
`NGF are increased in the CSF and the optic nerve of MS patients and in the brains of
`animals with the MS model disease,
`the experimental autoimmune encephalomyelitis
`(EAE)2l. There is no information available concerning BDNF or NT—3 in animal and
`human demyelinating disease. Exogenous NGF can prevent autoimmune demyelination in
`marmorsets 2. Whether this is due to inhibition of the immune attack or elicitation of
`protective responses in oligodendrocytes remains to be determined.
`
`4. THE JANUS FACE OF CNS-DIRECTED AUTOIMMUNE INFLAMMATION
`
`Inflammation is considered to be a key feature in MS pathogenesis. The neurotoxic
`effects of inflammation are well established and thought to be at least partially responsible
`for the observed axonal damage. Recently, an increasing body of experimental ecidence
`supports the view of a dual role of ‘the immune system in CNS-directed autoimmune
`inflammation. A number of recent studies have proposed that autoimmune inflammation
`may have neuroprotective effects in the CNS.
`On the one hand, MS and its animal models, representing neuroimmunological
`diseases which arise when immune cells attack the nervous system, provide the paradigm
`for the deleterious interaction between cells of the immune and nervous system. EAE, the
`MS animal model, can be induced by active immunization with CNS autoantigens [e.g.,
`rnyeljn basic protein (MBP)], or by the transfer of autoantigen-specific T cells into naive
`syngeneic recipients”. On the other hand, it was recently demonstrated that MBP-specific,
`encephalitogenic T cells may have
`seemingly neuroprotective (side-)effects. The
`neuroprotective and regenerative potential of immune cells was first coined by Michal
`Schwartz and her colleagues".
`They demonstrated that autoimmune T cells could protect neurons in an animal model
`of secondary degeneration after a partial crush injury of the optic nerve”. In several
`experiments, T cells with different specificities (specific for MBP, for the control antigen
`ovalbumin (OVA), or a heatshock protein (hsp) peptide) were activated by restimulation
`with their respective antigens in vitro, and then injected into rats immediately after an
`unilateral optic nerve injury. Seven days after injury, the optic nerves were analyzed by
`immunohistochernistry for the presence of T cells. Small numbers of T cells could be found
`in the intact (uninjured) optic nerves of rats injected with anti-MBP T cells which is
`consistent with previous observations that activated MBP-specific T cells home to intact
`CNS white matter.
`
`A much more pronounced accumulation of T cells, however, was observed in the
`crushed optic nerves of the rats injected with T cells specific for MBP, hsp peptide, or
`OVA. The degree of primary and secondary damage to the optic nerve axons and their
`attached retinal ganglion cells was measured by injecting a neurotracer distal to the site of
`the optic nerve lesion immediately alter the injury, and again after two weeks. The number
`of labeled retinal ganglion cells as marker for viable axons was significantly greater in the
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`retinas of the rats injected with anti-MBP T cells than in the retinas of rats injected with
`anti-OVA or anti-hsp peptide T cells. Thus, although all three T-cell lines accumulated at
`the site of injury, only the MBP-specific, autoimmune T cells had a substantial effect in
`limiting the extent of secondary degeneration. The neuroprotective effect was confirmed by
`electrophysiological studies.
`significant
`can mediate
`autoimmunity
`that T-cell
`The
`results
`demonstrate
`neuroprotection after CNS injury. The authors speculate that after injury, ‘cryptic’ epitopes
`might become available and might be recognized by endogenous non-encephalitogenic
`(benign) T cells. After local stimulation, these protective autoreactive T cells could exert
`their neuroprotective effect. The findings fiuther substantiate the idea t.hat
`‘natural
`autoimmunity’ can be benign and may even function as a protective mechanism“
`Macrophages seem to represent another type of immune cell
`that
`is capable of
`mediating neuroprotection and/or stimulating recovery of CNS lesions: The injection of
`activated macrophages into transected rat spinal cord stimulated tissue repair and partial
`recovery of motor function”. The neuroprotective activity of immune cells is not restricted
`to the CNS. Afier experimental axotomy of the facial nerve of immunodeficient SCID
`mice,
`the
`survival of facial motor neurons was
`severely impaired compared to
`immunocompetent wild-type mice. Reconstitution of SCID mice with wild-type
`splenocytes containing T and B cells restored the survival of facial motor neurons in these
`mice to the level of the wild-type controls”.
`
`Proinflammatory and
`neurotoxic factors
`
`\
`
`Antiinflammatory and
`neuroprotective factors
`
`
`
`TH] cymkines
`TNF
`HA
`Osteopomin
`Leukotrienes
`Chemokines
`MMP
`Plasminogen activators
`Nitric oxide
`Reactive oxygen species
`Glutamate
`Antibody +/- complement
`Cell-mediated cytotoxicity
`
`~14
`
`Destruction
`
`TH2 cytokines
`TGF
`TNF ‘.7
`Soluble TNF receptor
`Ifolluble 2"} reéeptoi t
`' Weep or an giims
`Some prostaglandms
`some chemokines
`L'TpI°$;'S
`A t.th
`b.
`'1 1 mm m
`Neummiphic factors
`like BDNF’ NGF
`
`~|v
`
`Protection
`
`Figure 3: The Janus face of CNS-directed autoimmune inflammation (adapted from 2’)
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`It is equally evident that a large number of neurotoxic and proinflammatory mediators
`are produced and released by immune cells (Figure 3)”. The neutralization of toxic
`inflammatory mediators may improve the outcome of MS and experimental models of CNS
`damage. On the other hand we know that
`immune cells can supply a number of
`neuroprotective mediators including neurotrophic factors, anti-inflammatory cytokines and
`prostaglandins which may reduce tissue damage”.
`Anti-myelin specific antibodies serve as a good example for the Janus face of
`inflammatory responses in the CNS. On the one hand,
`they are key players in the
`demyelinating process“. On the other hand,
`they can promote remyelination and
`neuroregenerationn. There seems to exist a balance between destructive and protective
`components of inflammation in the CNS.
`The idea that
`inflammatory reactions may not always be harmful under certain
`conditions even confer neuroprotection and repair has important consequences for the
`design of immunomodulatory therapies for MS”. Undebatably,
`there is convincing
`rationale for immunosuppressive treatment when the noxious effects of the inflammatory
`reaction prevail. Because nonselective immunosuppressive treatments will suppress both
`destructive and beneficial components of inflammation, therapy is likely to fail when the
`beneficial effects of CNS inflammation outweigh its negative consequences. It seems for
`example possible that the lack of beneficial (side) effects of inflammation during the late
`phase of MS with little inflammation but ongoing axonal
`loss contributes to the
`pathogenesis of neurodegeneration. In MS it is unfortunately unclear whether there is a
`stage of the disease when the inflammatory reaction is more beneficial than harmful.
`The concept of the neuroprotective role of inflammation can be extended to
`neurodegenerative,
`ischemic
`and traumatic lesions of the CNS,
`considering that
`inflammation is a universal tissue reaction crucial for defense and repair”.
`
`5. NEUROTROPHIC FACTORS ARE RELEASED BY DIFFERENT IMMUNE
`CELLS
`
`The precise mechanisms involved in irnmune—mediated neuroprotection remain to be
`clarified. A number of recent studies have shown that several types of immune cells and
`hernatogenic progenitor cells express one or more neurotrophic factors. For example, nerve
`growth factor (NGF) is produced by B cells, which also express the trkA receptor and p75
`NGF receptor“. Because neutralization of endogenous NGF caused apoptosis of memory B
`cells, it was concluded that NGF is an autocrine growth factor for memory B cells.
`More recently, another neurotrophin, brain-derived neurotrophic factor (BDNF), was
`found to be expressed in immune cells. BDNF was originally cloned in 1989 as the second
`member of the neurotrophin family which includes nerve growth factor (NGF) and
`neurotrophins (NT)-3, -4/5, -6 and -735. Since then,
`the important role of BDNF in
`regulating the survival and differentiation of various neuronal populations including
`sensory neurons, cerebellar neurons and spinal motor neurons has been firmly established3 .
`Neurons are the major source of BDNF in the nervous system”. BDNF binds to different
`types of receptors: the tyrosine kinase receptor B (trkB) which exists in two isoforrns — the
`fiill length receptor {gpl4StrkB) and the truncated receptor (gp95trkB) lacking the tyrosine
`kinase domain — and the p75 neurotrophin receptor”. It is thought that BDNF and NT4/5
`exert their biological function via the fixll-length form of tIkB receptor which expression
`seems to be restricted to neuronal cell populations.
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`Immune cells can be a potent source of the neuroprotective factor BDNF in
`neuroinflammatory disease”. Activated human T cells, B cells and monocytes are able to
`secrete bioactive BDNF after in vitro activation”. The BDNF secreted by immune cells is
`bioactive as it supports neuronal survival in vitro. In histology, BDNF-immunoreactivity
`was found in T cells and macrophages in active and inactive MS lesions.
`Similar observations were recently reported by several other groups of investigators.
`After experimental
`injury of the striatum activated macrophages and mieroglia cells
`transcribe mRNA for glial cell line-derived neurotrophic factor (GDNF) and BDNF“. This
`could help to explain the sprouting of doparninergic neurons observed after experimental
`injury. Transcripts for BDNF and NT-3 and their receptors trkB and trkC were found in
`subpopulations of human peripheral blood cells”. BDNF protein was secreted by cultured
`T cell clones. Several neurotrophins and their receptors have recently been demonstrated in
`human bone marrow“). More types of immune cells need to be examined for possible
`functional effects of neurotrophins.
`Besides BDNF,
`there is a robust expression of the full-length BDNF receptor
`gp145trkB in neurones in the immediate vicinity of multiple sclerosis plaques“. Single
`neurones with clearly pronounced trkB-immunoreactivity close to multiple sclerosis lesions
`can be observed, suggesting an upregulation of trkB in a proportion of damaged neurones.
`Additionally, full-length trkB irnmunoreactivity is present in reactive astrocytes within the
`lesions. The restriction of trkB expression to neural cell types in multiple sclerosis
`underscores the possibility that there may be BDNF signalling from infiltrating cells to
`neurones in neuroinflammatory lesions, as would be necessary for immune cells to support
`neuronal survival or to provide axonal protection.
`
`Table 2: Cell-type specific expression of BDNF and BDNF full-length receptor TrkB in MS lesionsn“
`
`
`
`
`
`
`
`+
`
`— MB
`
`
`
`
`
`
`
`—
`
` Inflammatory cells
`
`The appeal of neurotrophin-mediated neuroprotection in MS lies in the pleiotropy
`inherent to neurotrophin actions”. It is well established that BDNF can prevent neuronal
`cell death after various pathological insults including experimental transection of axons in
`the spinal cord42‘". Moreover, BDNF can also protect axons against elimination during
`development,
`as well
`as
`against degeneration after axotomy or
`in experimental
`neurodegenerative diseases“’45. Furthermore, the preservation of axons provides the basis
`for neuroregenerative attempts including axonal regeneration and sprouting, which are
`directly supported by BDNF“"6. Apart from influencing survival and regeneration of
`neuronal elements, BDNF supports remyelination after both peripheral and CNS injury”.
`Finally, BDNF has been shown to downregulate the expression of MHC (major
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`T. ZIEMSSEN
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`histocompatibility complex) molecules in hippocampal slices, and may thus also act as an
`immunomodulator“.
`Within the multiple sclerosis lesion immune cells seem to be the major source of
`BDNF“. They are likely to release this substance in the immediate vicinity of nerve cell
`processes, which — according to the observed trkB expression - are likely to be responsive to
`the neuroprotective effects of BDNF. This neurotrophin-mediated neuroimmune signalling
`network could be a major factor that helps to preserve axons in a microenvironrnent that is
`clearly capable of exerting significant neurotoxicity. Thus, it should be considered as a
`beneficial aspect of neuroinflammation that could be Worth preserving therapeutically, or
`even reinforcing using tailored immunomodulatory treatment strategies.
`
`6. NEUROTROPHIC FACTORS AS THERAPY OPTION IN MS
`
`the treatment of multiple sclerosis has two major
`According to current opinion,
`objectives, namely (a) suppression of the inflammatory process, and (b) restoration and
`protection of glial and neuronal function“. The potential neuroprotective function of
`inflammatory cells is relevant to both these treatment goals.
`A number of studies have shown that the administration of BDNF protein or the BDNF
`gene can rescue injured or degenerating neurons and induce axonal outgrowth and
`regeneration42'43‘". Furthermore, BDNF had beneficial effects in several animal models of
`neurodegenerative diseases”. Difficulties in delivering sutficient amounts of BDNF to the
`site of CNS lesions have so far hampered the successful application of BDNF and other
`neurotrophic factors for treatment of human diseases”. To the best of our knowledge,
`systemically administered neurotrophic factors do not cross the blood brain barrier. For the
`treatment of MS, it would seem that they must be administered directly into the CNS.
`One promising novel strategy for the delivery of neuroprotective factors relies on the
`(retroviral) transduction of one or several neurotrophic factors into antigen—specif1c T cell
`lines“. As the transduced T cells are specific for an autoantigen expressed in the nervous
`system, they home to the sites where the relevant autoantigen is expressed, recognize their
`antigen and are then stimulated locally to secrete neurotrophic factor(s)52. Unfortunately,
`these autoimmune T cells could lead to tissue injury like in EAE.
`This experimental strategy has a natural counterpart in the secretion of neurotrophic
`factors by activated immune cells. However, it appears that the neurotrophins secreted by
`immune cells under natural conditions are oflen insufficient to prevent
`injury. It will
`therefore be worthwhile to further refine the strategies to enhance the production of
`neurotrophic factors by immune cells and to exploit the homing properties of the immune
`cells for targeting neuroprotective factors into the nervous system5“52.
`
`7. DRUG DEVELOPMENT OF GLATIRAMER ACETATE
`
`Glatiramer acetate (Copaxone®, GA), formerly known as copolymer 1, is the acetate
`salt of a standardized mixture of synthetic polypeptides containing the four amino acids L-
`alanine, L-glutamic acid, L-lysine and L-tyrosine with a defined molar ratio of 0.14 : 0.34 :
`0.43 : 0.09 and an average molecular mass of 4.7 — 11.0 kDa, i.e. an average length of 45-
`100 amino acids53'5".
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`
`In the 1960s Drs Sela, Amon and their colleagues at the Weizmann Institute, Israel
`were involved in studies on the immunological properties of a series of polymers and
`copolymers which were developed to resemble myelin basic protein (MBP), a myelin
`protein. MBP in Freund's complete adjuvant induces EAE, the best animal model of MS.
`They were interested in evaluating whether these polypeptides could simulate the ability of
`MBP and of fragments and regions of the MBP molecule to induce EAE55“. None of these
`series was capable of inducing EAE, but several polypeptides were able to suppress EAE in
`guinea pigs. Copolymerl, later known as GA was shown to be the most effective polymer
`in preventing or decreasing the severity of EAE. The suppressive effect is a general
`phenomenon and not restricted to a particular species, disease type or encephalitogen used
`for EAE induction”.
`Abramsky et al. were the first who treated a group of severe relapsing-remitting MS
`patients with intramuscular GA 2-3 mg every 2-3 days for 3 weeks, then weekly for 2-5
`months”. No conclusions could be drawn regarding drug efficacy but there were no
`significant undesirable side effects. Three clinical
`trials in the 1980s were performed
`showing some evidence of efficacy that was adequate for support of the Food and Drug
`Administration (FDA) approval and a good safety profile51'53. However the results of these
`studies must be inte reted with caution because before 1991 the production of the drug
`was not standardized 4'“. Different batches had variable suppressive effects on EAE which
`could also imply variable effects in MS patients.
`In 1991 a phase III multicentre trial with a daily 20 mg dose of a s.c. administered,
`highly standardized GA preparation was started in the USA. This double-blind placebo
`controlled study demonstrated that GA significantly reduced the relapse rate without
`significant side effects“.
`In 1996 GA was approved by the US FDA as a treatment for ambulatory patients with
`active relapsing-remitting (RR) MS. Since then GA has been licensed for approval in many
`other countries”.
`
`8. GLATIRAMER ACETATE: MECHANISM OF ACTION
`
`Until recently the effects of GA on the human immune system and the mechanisms of
`action of GA were largely unknown. Most of the data have been obtained in animal models
`so far. Several new papers, however, have shed light on the mechanisms of GA in MS und
`suggest
`several major effects of GA on human T cells“.
`In contrast
`to other
`immunomodulatory MS therapies which exert its effects in an antigen-nonspecific way, GA
`appears to preferentially affect immune cells specific for GA, MBP and possibly other
`myelin autoantigens implicated in the MS disease process.
`In contrast to the lack of effect of GA on immune cells isolated from untreated animals,
`GA induces vigorous polyclonal proliferation of peripheral blood lymphocytes (PBL) from
`untreated (unprimed) human donors67'72. In GA—treated patients, the proliferative response
`to GA decreases with time”. Recent results from our group indicate that this decrease is
`specific to GA, as it
`is not observed with recall antigens like tetanus toxoid and
`tuberculin“. Theoretically, the observed decrease in GA-reactive T cells could be due to
`anergy induction or activation-induced cell death of GA-specific T cells.
`GA binds to major histocornpatibility complex (MHC) class II and perhaps to MHC
`class I molecules, thereby competing with the MHC binding of other antigens75’76. This
`effect, which by its nature is antigen-nonspecific, is unlikely to play a role in vivo, since
`
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`122
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`T. ZIEMSSEN
`
`after subcutaneous (s.c.) administration, GA is quickly degraded and thus it is not likely to
`reach the CNS where it could compete with the relevant auto-antigens for MHC binding“.
`Complexes of GA/MHC can compete with MBP/MHC for binding to the antigen—s ecific
`surface receptor of MBP-specific T cells (T cell receptor (TCR) antagonism)7 . The
`experimental evidence supporting this effect is controversial”. If it occurs, it is unlikely to
`be relevant in vivo, since GA is unlikely to reach sites where it could compete with MBP.
`
`Table 3: Major immunologic effects of glatiramer acetate (adapted from 66.74)
`
`Suppression of proliferation of MBP-reactive T cells in
`T cell proliferation in vitro
`vitro
`
`Proliferation of PBL during
`treatment with GA
`
`Decreased proliferation of PBL to GA during treatment
`
`MHC (Major histocompati-
`Direct and promiscuous binding of GA to different HLA-
`DR alleles
`bility complex) binding
`
`T cell migration:
`
`TH1 -> TH2 shift in PBL
`
`TH1 - > TH2 shift in GA-
`
`specific T cells
`Cross-stimulation of T cells
`with MBP
`
`Cross-inhibition
`
`of MBP-
`
`specific T cell lines
`
`Reduced migration of PBL from GA-treated patients
`(unknown mechanism); no effect on adhesion molecule
`expression on human brain microvascular endothelial cells
`Increased levels of IL-10 in serum and of mRNA for TGF-
`B and IL-4; reduction of mRNA for TNF-ot in PBL
`
`Shift of GA-reactive T cells from TH1 towards TH2
`phenotype during GA treatment
`
`Induction by GA of cytokine production in MBP-specific
`T cells and vice versa
`
`Inhibition of proliferation of T cells specific for MBP and
`some other antigens
`
`receptor
`cell
`T
`antagonism
`
`(TCR)
`
`TCR antagonism with MBP 82—lO0
`findings)
`
`(contIoversial
`
`Altered peptide ligand effect
`on MBP—specific T cells
`
`GA-specific antibodies
`
`Induction of anergy in MBP-specific T cell clones
`
`Anti-GA-antibodies of IgG2 >> IgG1 isotype without
`MBP crossreactivity and without neutralizing effect
`(maximum titer at month 4 after onset of GA-treatment);
`anti-GA IgG4 antibodies only detectable in GA-treated
`patients
`
`GA-specific CD8 T cells
`
`Disrninished GA-specific CD8+ T cell proliferation in
`untreated MS patients which is restored by GA treatment
`
`Effects on antigen presenting
`cells
`
`Inhibition of INF-<1 and cathepsin-B production in a
`monocytic cell line
`
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`NEUROPROTECTION AND GLATIRAMER ACETATE
`
`123
`
`On the other hand, GA could act in the