NEUROPROTECTION: BACKGROUND AND AIMS
During the last decade, scientists and clinicians have begun to categorize glaucoma as a part of the large group of neurodegenerative disorders of the central nervous system that are characterized by a primary loss of neurons and a subsequent, ongoing process of secondary degeneration.1 This view of glaucoma has significantly altered the nature of glaucoma research, the way in which clinicians perceive the disease, and their approach to therapy. Instead of focusing on primary risk factors and their effect on optic neurons, today's researchers emphasize the hostile environment created by the primary insult and the consequential progression of degeneration (even if that particular etiological factor is adequately neutralized). Most current study targets neuroprotection; it involves identifying the compounds and factors responsible for mediating the self-perpetuating degeneration and seeking ways to circumvent or block them.
My colleagues and I, as well as other investigators, have postulated that factors contributing to the ongoing cell death in glaucoma include the emission of physiological compounds in toxic quantities from the injured fibers, their cell bodies, and neighboring supportive cells.2-6 Some of the compounds identified in the pathogenesis of glaucoma are also known to be active in other neurodegenerative diseases.7-11 Our studies12-17 of acute and chronic injuries of the rodent's optic nerve led us to the unexpected discovery that the immune system plays a key role in the ability of the optic nerve and retina to withstand injurious conditions.
IS GLAUCOMA A SYSTEMIC DISEASE?
Our first observations that the immune system (in the form of T cells directed to specific antigens of the central nervous system [ie, “autoimmune” T cells]) can protect injured neurons from death came from our studies in rodents. We found that the passive transfer of T cells specific to myelin basic protein reduced the loss of retinal ganglion cells after trauma to the optic nerve.15 The myelin-specific T cells were also effective when directed (1) to either cryptic or pathogenic epitopes of myelin basic protein and (2) to other myelin antigens or their epitopes.18,19
Our findings raised some fundamental questions. First, are myelin antigens capable of protecting the visual system from all types of acute and chronic insult? Second, is the observed neuroprotective activity by immune cells merely an experimental artifact? Alternatively, does it reflect the critical participation of the immune system in fighting off injurious conditions in the central nervous system in general and in the visual system in particular? Third, if the immune system combats injury to the visual system, is glaucoma a systemic disease? If so, can this finding be translated into a systemic therapy that will protect the eye?
From our research over the last few years, we have learned that an insult to the central nervous system physiologically evokes a protective T-cell response, but we found that reaction may not be sufficiently effective when the insult is severe and that it may not always be optimally controlled. Moreover, the specificity of the protective T cells depends on the site of the insult. For example, the protective effect of vaccination with myelin-associated antigens is restricted to injuries of the white matter (ie, myelinated axons).16,19,20 These antigens would have no effect on an injury to the retina, which contains no myelin.
In addition, we found that the observed injury-induced behavior of autoimmune T cells is a spontaneous physiological response.17 We therefore sought to identify the antigenic specificity and the phenotype of the beneficial autoimmune T cells and to understand what determines the balance between a beneficial (ie, neuroprotective) outcome of the T-cell–mediated response to a central nervous system injury and a destructive effect, which causes autoimmune disease. We also examined ways of translating the beneficial response into a therapy for glaucoma.
We had to address several issues during the course of our research. First, we verified that the loss of retinal ganglion cells in a rat model of high IOP, simulating some types of hypertensive glaucoma, depends on the immune system's integrity.13 Second, we attempted to determine whether the specific T cells that are harnessed in our rat model of chronic glaucoma (and that can be used for therapeutic boosting of the beneficial T-cell–mediated response) are directed to self-antigens that reside in the retina or the optic nerve.12 Lastly, we searched for an antigen that would safely boost the physiological response without causing an autoimmune disease.
Using a rat model of elevated IOP, we showed that a protective response could be obtained only with an antigen residing in the retina. This finding suggests that, at least in this experimental model, the site at which the self-perpetuating degeneration is initiated—and therefore the site primarily in need of protection—is not the optic nerve but the retina.12,16 We further determined that, in T-cell–deficient rats, the number of retinal ganglion cells that survive a high-IOP insult is significantly lower than in matched controls with an intact immune system. The ability to withstand insults to the optic nerve or retina may therefore depend on the integrity of the peripheral immune system—specifically, on immune cells that recognize site-specific self-antigens.
Interestingly, the use of steroids after an IOP-induced injury in rats, as well as in the healthy animals, caused a significant loss of retinal ganglion cells.12 Moreover, in a model of retinal ganglion cell loss caused by conditions other than IOP (eg, uveitis), steroids that reduced the inflammation not only failed to rescue the retinal ganglion cells, but they actually caused these cells' death. These results led us to suggest that boosting a well-controlled level of T cells might protect retinal ganglion cells in patients with glaucomas associated with high IOPs and in those with normal-tension glaucoma.
NEUROPROTECTIVE ACTIVITIES OF THE ANTI-SELF T CELLS
To be protective, autoimmune T cells must travel to the site of damage and become activated by the encounter with their specific antigens there. For that reason, only antigens located at that site are relevant for the purposes of vaccination. Once activated, the T cells provide a source of cytokines and growth factors that, in turn, activate the microglia to exert defensive activities that the eye can tolerate (eg, an uptake of glutamate, the removal of debris, the production of growth factors). Importantly, these factors do not produce the poorly tolerated toxic agents that are part of their microbe-killing apparatus, such as tumor necrosis factor-alpha.21-25 Such T cells are constitutively controlled by naturally occurring regulatory T cells, which are themselves amenable to physiological or therapeutic control.26,27
CHOICE OF ANTIGENS FOR THERAPEUTIC PURPOSES
Among the compounds we tested in our search for a safe, suitable antigen for neuroprotection was glatiramer acetate (also known as Cop-1). This synthetic 4-amino-acid copolymer is currently used in a regimen for multiple sclerosis. We chose to test this FDA-approved compound, because it is known to be safe and because our studies16,28 have demonstrated its low-affinity cross-reaction with a wide range of self-antigens. In our rat model of chronically high IOP, vaccination with Cop-1 significantly reduced the loss of retinal ganglion cells even if the IOP remained high. Although the vaccination did not prevent the onset of disease, the therapy can slow its progression by controlling the local extracellular environment of the nerve and retina. In other words, vaccination makes the environment friendlier to neuronal survival and helps the retinal ganglion cells to withstand the stress.16,27-29
Chronic conditions that are not autoimmune diseases occasionally require boosting of T cells directed to or cross-reactive with the relevant self-antigens. Bakalash et al30 established that the frequency of vaccination requires long-lasting neuroprotection in a rat model of elevated IOP. A search by our group is now on for antigens besides Cop-1, some of which are being tested and developed. Because numerous factors participate in self-perpetuating degeneration, it seems likely that a multidimensional therapy based on the immune system will be superior to any treatment involving the use of any single neuroprotective drug.
Michal Schwartz, PhD, is Professor and the Maurice and Isle Katz Professorial Chair in Neuroimmunology, Department of Neurobiology at the Weizmann Institute of Science in Rehovot, Israel. She stated that the research described herein was supported in part by TEVA Pharmaceutical Industries, Ltd., and Proneuron Biotechnologies, Inc. Dr. Schwartz may be reached at +972 8 9342467; michal.schwartz@weizmann.ac.il.
1. Schwartz M, Belkin M, Yoles E, Solomon A. Potential treatment modalities for glaucomatous neuropathy: neuroprotection and neuroregeneration. J Glaucoma. 1996;5:427-432.
2. Liu B, Neufeld AH. Expression of nitric oxide synthase-2 in specific cells of human glaucomatous optic nerve head [in Chinese]. Zhonghua Yan Ke Za Zhi. 2001;37:381-383.
3. Lipton SA. Paradigm shift in NMDA receptor antagonist drug development: molecular mechanism of uncompetitive inhibition by memantine in the treatment of Alzheimer's disease and other neurologic disorders. J Alzheimers Dis. 2004;6(suppl 6):61-74.
4. Schmidt KG, Pillunat LE, Osborne NN. Ischemia and hypoxia. An attempt to explain the different rates of retinal ganglion cell death in glaucoma [in German]. Ophthalmologe. 2004;101:1071-1075.
5. Shen F, Chen B, Danias J, et al. Glutamate-induced glutamine synthetase expression in retinal Muller cells after short-term ocular hypertension in the rat. Invest Ophthalmol Vis Sci. 2004;45:3107-3112.
6. McIlnay TR, Gionfriddo JR, Dubielzig RR, et al. Evaluation of glutamate loss from damaged retinal cells of dogs with primary glaucoma. Am J Vet Res. 2004;65:776-786.
7. Weishaupt JH, Rohde G, Polking E, et al. Effect of erythropoietin axotomy-induced apoptosis in rat retinal ganglion cells. Invest Ophthalmol Vis Sci. 2004;45:1514-1522.
8. Bayer AU, Keller ON, Ferrari F, Maag KP. Association of glaucoma with neurodegenerative diseases with apoptotic cell death: Alzheimer's disease and Parkinson's disease. Am J Ophthalmol. 2002;133:135-137.
9. Kimura A, Ando E, Fukushima M, et al. Secondary glaucoma in patients with familial amyloidotic polyneuropathy. Arch Ophthalmol. 2003;121:351-356.
10. McKinnon SJ. Glaucoma: ocular Alzheimer's disease? Front Biosci. 2003;8(suppl): 1140-1156.
11. Karseras AG. Progressive glaucoma in patients with Alzheimer's disease. Eye. 2004;18:229.
12. Bakalash S, Kessler A, Mizrahi T, et al. Antigenic specificity of immunoprotective therapeutic vaccination for glaucoma. Invest Ophthalmol Vis Sci. 2003;44:3374-3381.
13. Bakalash S, Kipnis J, Yoles E, Schwartz M. Resistance of retinal ganglion cells to an increase in intraocular pressure is immune-dependent. Invest Ophthalmol Vis Sci. 2002;43:2648-2653.
14. Kipnis J, Yoles E, Schori H, et al. Neuronal survival after CNS insult is determined by a genetically encoded autoimmune response. J Neurosci. 2001;21:4564-4571.
15. Moalem G, Leibowitz-Amit R, Yoles E, et al. Autoimmune T cells protect neurons from secondary degeneration after central nervous system axotomy. Nat Med. 1999;5:49-55.
16. Schori H, Kipnis J, Yoles E, et al. Vaccination for protection of retinal ganglion cells against death from glutamate cytotoxicity and ocular hypertension: implications for glaucoma. Proc Natl Acad Sci USA. 2001;98:3398-3403.
17. Yoles E, Hauben E, Palgi O, et al. Protective autoimmunity is a physiological response to CNS trauma. J Neurosci. 2001;21:3740-3748.
18. Fisher J, Levkovitch-Verbin H, Schori H, et al. Vaccination for neuroprotection in the mouse optic nerve: implications for optic neuropathies. J Neurosci. 2001;21:136-142.
19. Mizrahi T, Hauben E, Schwartz M. The tissue-specific self-pathogen is the protective self-antigen: the case of uveitis. J Immunol. 2002;169:5971-5977.
20. Avidan H, Kipnis J, Butovsky O, et al. Vaccination with autoantigen protects against aggregated beta-amyloid and glutamate toxicity by controlling microglia: effect of CD4+CD25+ T cells. Eur J Immunol. 2004;34:3434-3445.
21. Butovsky O, Hauben E, Schwartz M. Morphological aspects of spinal cord autoimmune neuroprotection: colocalization of T cells with B7—2 (CD86) and prevention of cyst formation. FASEB J. 2001;15:1065-1067.
22. Butovsky O, Talpalar AE, Ben-Yaakov K, Schwartz M. Aggregated beta-amyloid renders microglia cytotoxic but T cell-derived IFN-gamma and IL-4 make them protective. Mol Cell Neurosci. In press.
23. Barouch R, Schwartz M. Autoreactive T cells induce neurotrophin production by immune and neural cells in injured rat optic nerve: implications for protective autoimmunity. FASEB J. 2002;16:1304-1306.
24. Moalem G, Gdalyahu A, Shani Y, et al. Production of neurotrophins by activated T cells: implications for neuroprotective autoimmunity. J Autoimmun. 2000;15:331-345.
25. Shaked I, Tchroesh D, Gersner R, et al. Protective autoimmunity: Interferon-gamma enables microglia to remove glutamate without evoking inflammatory mediators. J Neurochem. 2005;92:997-1009.
26. Kipnis J, Cardon M, Avidan H, et al. Dopamine, through the extracellular signal-regulated kinase pathway, downregulates CD4+CD25+ regulatory T-cell activity: implications for neurodegeneration. J Neurosci. 2004;24:6133-6143.
27. Kipnis J, Mizrahi T, Hauben E, et al. Neuroprotective autoimmunity: naturally occurring CD4+CD25+ regulatory T cells suppress the ability to withstand injury to the central nervous system. Proc Natl Acad Sci USA. 2002;99:15620-15625.
28. Kipnis J, Yoles E, Porat Z, et al. T cell immunity to copolymer 1 confers neuroprotection on the damaged optic nerve: possible therapy for optic neuropathies. Proc Natl Acad Sci USA. 2000;97:7446-7451.
29. Benner EJ, Mosley RL, Destache CJ, et al. Therapeutic immunization protects dopaminergic neurons in a mouse model of Parkinson's disease. Proc Natl Acad Sci USA. 2004;101:9435-9440.
30. Bakalash S, Ben Shlomo G, Aloni E, et al. T cell-based vaccination for morphological and functional neuroprotection in a rat model of chronically elevated intraocular pressure. J Molec Med. In press.
