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From The Weizmann Institute of Science, Rehovot, Israel.
Glaucoma can be viewed as a neurodegenerative disease that is often associated with high intraocular pressure (IOP), and in which at any time there are fibers (optic nerve axons) and cell bodies (retinal ganglion cells) that are vulnerable to degeneration and amenable to protection. According to this view, patients receiving antihypertensive therapy to control an increase in intraocular pressure should also receive neuroprotective treatment that will circumvent, bypass, or reduce the threat to the neurons imposed by the degenerating neurons. Vaccination, proposed herein as a therapeutic measure, slows down disease propagation, in much the same way as it reduces secondary degeneration after acute insult to the rat optic nerve. The proposed vaccination is based on the unexpected discovery of "protective autoimmunity," according to which a mechanical injury (optic nerve crush) or biochemical insult (glutamate toxicity in retinal ganglion cells) evokes a physiological autoimmune response which is specific to self-antigens residing in the site of damage and protects the nerve against the degenerative effects of glutamate and other destructive self-compounds. Protection was found to be boosted, without risk of autoimmune disease development, by vaccination with Cop-1, a synthetic antigenic copolymer which weakly cross-reacts with a broad spectrum of self-reactive (autoimmune) T cells, thus safely activating them for self repair. The proposed vaccination can therefore be viewed as a way of boosting the bodys physiological defense and repair mechanisms. Once its regimen and formulation are optimized for protection of retinal ganglion cells against death induced by an increase in IOP, Cop-1 can be immediately developed as a therapy for glaucoma.
Background
Regeneration and Neuroprotection of the Injured Central Nervous System
Injury to the central nervous system (CNS) causes irreversible functional loss, as there is little or no neurogenesis (because adult neurons cannot proliferate and repopulate the site of injury), little or no regeneration (because neurons with axonal damage have limited capacity for spontaneous regrowth), and ongoing secondary degeneration (because the primary neuronal loss creates an environment hostile to neurons that escaped direct injury, causing them to degenerate).
Research in my laboratory over the past two decades has been aimed at finding ways to promote the recovery of damaged nerve fibers. In the course of these studies, it became clear that only part of the functional loss after an injury is due to neuronal losses caused by primary transection of nerve fibers and for which the appropriate therapy would be nerve regeneration. A significant partand sometimes the major partof the loss of function is due to delayed degeneration of fibers that escaped the primary injury. This secondary loss is a consequence of numerous injury-related processes, which were found to be common to many acute and chronic neurodegenerative disorders (Fig. 1) . The appropriate therapy for preventing or minimizing the degeneration of neurons that escaped direct injury is neuroprotection. Neuroprotective therapy is a general term referring to any therapeutic approach that neutralizes, circumvents, and prevents neuronal losses caused by self-destructive processes. Our research focuses on achieving recovery by both neuroprotection and neuroregeneration.
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2-adrenoreceptor agonists, Ca2+ blockers, scavengers of free radicals) and molecular intervention (for example, the use of anti-apoptotic or survival genes to increase neuronal resistance to injurious conditions). The neuroprotective strategy developed by my research group, and presented in this lecture, is based on the assumption (borne out by experimental evidence) that the body harnesses the immune system to help cope with the stressful conditions imposed by an injury. In practical terms, this requires reinforcement of the immune response by boosting the bodys own mechanisms of defense and repair, while avoiding the risk of autoimmune disease.
Partial-Crush Injury of the Rat Optic Nerve: Relevance for Glaucoma
At an early stage of our research, we developed a model of a well-controlled partial injury of the rat optic nerve as a way to assess the posttraumatic spread of damage, identify the agents mediating it, and screen potential methods of neuroprotective intervention (Fig. 3) 1 2 The model was used to demonstrate that an optic nerve insult is followed by secondary degeneration, to identify mediators of secondary degeneration (nitric oxide, glutamate), to identify physiological self-protective mechanisms, and to screen potential agents for therapeutic protection (e.g., N-methyl-D-aspartate [NMDA] antagonists,
2-adrenoreceptor agonists).1 2 3 4 5
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Glaucoma is a disease characterized by degeneration of optic nerve axons and death of retinal ganglion cells (RGCs), and is frequently associated with high IOP. The loss of vision often continues despite anti-hypertensive treatment. We pointed out that the progressive loss of neurons in glaucoma is reminiscent of secondary degeneration after an acute or chronic insult to the CNS.7 Glaucoma can therefore be viewed as a neurodegenerative disease, in which at any time some fibers are undergoing acute degeneration, whereas other fibers are still healthy and are amenable to neuroprotection.6 The work of Quigley et al.7 showed that secondary degeneration also occurs in monkeys.
Immune Activity in the CNS after Injury
The primary role of the immune system is to protect the body against destructive elements, clear it of threatening material, and facilitate tissue repair. The CNS, however, partly because of its status as a site of "immune privilege," partly because of its high incidence of autoimmune diseases (such as multiple sclerosis), and partly because inflammation is often seen in conjunction with acute and chronic degenerative conditions, has traditionally been considered off-limits to these immune activities. As a result, immune activity in general, and autoimmunity in particular, have been viewed as harmful in the CNS, and the therapeutic strategy for acute CNS injuries and chronic degenerative conditions has therefore often been one of immune suppression.
Our studies suggested, however, that an inflammatory response in the injured CNS, provided that it is well-regulated, is essential in helping the damaged tissue cope with the injurious conditions. Our early work in this connection showed that traumatic injury to the optic nerve is followed by an accumulation of T cells at the site of the lesion.4 According to the accepted view, such an accumulation would be interpreted as having a negative effect on nerve recovery. In light of our earlier experience with macrophages,8 however, we thought it possible that the accumulated T cells in the damaged optic nerve might have a beneficial function, but that their activity is not strong enough to have a perceptible effect. This suspicion was borne out when we discovered that increasing the number of T cells that home to the lesion site has a positive impact on nerve recovery (i.e., it reduces the neuronal loss resulting from secondary degeneration), provided that the systemically injected T cells include at least some that are specifically directed to myelin-associated antigens.4 9 These myelin-specific T cells were found to be neuroprotective in function, and their beneficial effect on nerve recovery was manifested both morphologically and functionally.4 9 10 11 Subsequent studies showed that this T-cell-mediated protection against neuronal death is not restricted to injury of optic nerve axons but is also evident after spinal cord injury. Thus, passive transfer of myelin-specific T cells into the contused spinal cord of rats protected viable axons from secondary degeneration.12 13 14 The most pronounced effect of the T cells in the injured spinal cord, in addition to the rescue of neurons from otherwise inevitable death, is the reduction of cavity formation.12
It is important to note that in the injured optic nerve or spinal cord, the beneficial effect of the T cells is discernible in spite of the transient appearance of symptoms of a monophasic experimental autoimmune disease caused by the transferred T cells.4 Thus, the benefit apparently outweighs the cost. We subsequently discovered that similar benefits can be obtained if we use T cells that recognize a nondominant epitope within the myelin antigens. In this way we can obtain the protective benefit unaccompanied by symptoms of the disease.9
These and other findings raised an important question: Is the T-cell-mediated neuroprotective activity a physiological response or merely the result of an experimental manipulation?7 15 16 17 Our studies demonstrated that the injury evokes a protective autoimmunity, which is a physiological response to an insult, at least in the CNS.18 This was shown by the finding that in the absence of T cells, fewer neurons survive a crush injury to the optic nerve or exposure of RGCs to glutamate toxicity, and that recovery in the optic nerve is better if the insult is preceded by another CNS injury (e.g., spinal cord contusion).18 19
Our working hypothesis was therefore that if protective autoimmunity is a physiological mechanism designed to cope with stressful conditions in the CNS, individuals might differ in their ability to withstand the degenerative consequences of a CNS insult. Also, the absence of T cells would wipe out this difference between individuals, and tolerance (defined as nonresponsiveness) to myelin would eliminate the ability to resist the consequences of axonal injury. We found that not all animals are equally capable of manifesting this protective response, although all can manifest anti-self (i.e., autoimmune) activity. We showed, moreover, that the ability to manifest a protective response correlates with the ability, when challenged with a myelin antigen, to resist the development of an autoimmune disease.19 We further demonstrated that the absence of T cells indeed wipes out the differences between strains that are "resistant" and those that are "susceptible" to injurious conditions, eliminating the advantage of strains that normally recover better.18 19 Neonatal immunization of rats with myelin abolishes the ability of the adult rat to respond to myelin or to withstand injurious conditions imposed by axonal injury.20
Taken together, the above findings led us to formulate the concept of protective autoimmunity as a physiological response to a CNS insult and to suggest that individuals differ in their ability to manifest this response. In the case of injury to myelinated axons the response is specific to myelin antigens, and is amenable to boosting by peptides related to or derived from myelin. A T cell-dependent protective mechanism was also found to operate in response to glutamate toxicity imposed directly on RGCs.19 21 22 23 In this case, myelin antigens were unable to boost protection. We therefore reasoned that to be effective, the T cells must be activated by their specific antigens presented to them at the site where protection is neededthat is, at the site of the lesion. Our experiments indeed showed that RGCs damaged by direct exposure to toxic amounts of glutamate are beneficially affected by antigens that reside in the eye. We further postulated that these antigens are identical with the immunodominant proteins causing the ocular autoimmune disease that develops in strains susceptible to autoimmune disease development. This hypothesis is in line with our perception of an autoimmune disease as a failure in the mechanism controlling the purposeful autoimmune response needed for defense against self-destructive compounds.24
Boosting of Protective Autoimmunity as a Therapeutic Approach for Glaucoma
Based on the results summarized above, we suggest that T cells orchestrate the local immune response to destructive self-compounds. We further suggest that: antigenic specificity is needed both for homing of T cells and for their local activation. The activated T cells augment and regulate the local cellular immune response needed to clear the lesioned site of cell debris and other potentially destructive materials. The same cells (i.e., T cells with identical phenotypes and antigenic specificities) might be responsible for both destruction and protection, and the difference in their effects lies in their regulation.20 Thus, the essence of the beneficial response is a dialogue between T cells and resident microglia. Once activated, the microglia are better equipped to buffer toxicity (e.g., glutamate) in a receptor-dependent fashion (Shaked et al., unpublished observations, 2003), to function more efficiently as phagocytic cells capable of eliminating toxic self-compounds (Shaked et al., unpublished observations, 2003), and to act as a source of neurotrophic factors and cytokines25 (Fig. 4) .
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Cop-1, a random copolymer of four amino acids, was originally synthesized to mimic myelin basic protein, and was subsequently approved by the FDA as an effective treatment for multiple sclerosis. We discovered that this copolymer can activate a wide range of low-affinity self-reactive T cells. In this way it can circumvent the tissue specificity barrier needed for T cell-mediated neuroprotection.28 Using a rat model of raised IOP, we recently discovered that the ability of these animals to resist a pressure-induced loss of RGCs is immune-dependent and varies among strains.29 In the absence of mature T cells, RGC losses are greater in strains that are better equipped to cope with the stress.29
We found that Cop-1 vaccination protects rats and mice from the consequences of optic nerve injury.21 We further found that vaccination with Cop-1, unlike immunization with myelin antigens, is neuroprotective in a model of glutamate toxicity in the eye, a myelin-free site.22 In a rat model of high IOP, vaccination with Cop-1 significantly reduces the pressure-induced death of RGCs22 (Fig. 5) . An interesting finding was that RGC death in this model was amenable to treatment by antigens residing in the retina but not in the myelin (Bakalash et al., unpublished data).
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Authors Note
This paper covers part of the material presented in the Friedenwald Award lecture, and is not intended as a comprehensive review. The list of cited publications, therefore, refers mainly to work done in the authors laboratory.
Acknowledgements
The author wishes to thank Shirley Smith for scientific editing.
Footnotes
Supported by grants from The Glaucoma Research Foundation, Proneuron Biotechnologies Ltd., Teva Pharmaceutical Industries Ltd., and the Israel Defense Force.
Submitted for publication June 17, 2002; accepted July 23, 2002.
The publication costs of this article were defrayed in part by page charge payment. This article must therefore be marked "advertisement" in accordance with 18 U.S.C.
1734 solely to indicate this fact.
Corresponding author: Michal Schwartz, The Weizmann Institute of Science, 76100 Rehovot, Israel; michal.schwartz{at}weizmann.ac.il.
References
This article has been cited by other articles:
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J. L. Kielczewski, M. E. Pease, and H. A. Quigley The Effect of Experimental Glaucoma and Optic Nerve Transection on Amacrine Cells in the Rat Retina Invest. Ophthalmol. Vis. Sci., September 1, 2005; 46(9): 3188 - 3196. [Abstract] [Full Text] [PDF] |
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H. A. Quigley Glaucoma: Macrocosm to Microcosm The Friedenwald Lecture Invest. Ophthalmol. Vis. Sci., August 1, 2005; 46(8): 2663 - 2670. [Full Text] [PDF] |
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D. E. Brooks, M. E. Kallberg, R. L. Cannon, A. M. Komaromy, F. J. Ollivier, O. E. Malakhova, W. W. Dawson, M. B. Sherwood, E. E. Kuekuerichkina, and G. N. Lambrou Functional and Structural Analysis of the Visual System in the Rhesus Monkey Model of Optic Nerve Head Ischemia Invest. Ophthalmol. Vis. Sci., June 1, 2004; 45(6): 1830 - 1840. [Abstract] [Full Text] [PDF] |
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E. Wehrwein, S. A. Thompson, S. F. Coulibaly, D. M. Linn, and C. L. Linn Acetylcholine Protection of Adult Pig Retinal Ganglion Cells from Glutamate-Induced Excitotoxicity Invest. Ophthalmol. Vis. Sci., May 1, 2004; 45(5): 1531 - 1543. [Abstract] [Full Text] [PDF] |
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