The immune system and glaucoma

Bioinformatic and statistical analysis of the optic nerve head in a primate model of ocular hypertension

BACKGROUND

The nonhuman primate model of glaucomatous optic neuropathy most faithfully reproduces the human disease. We used high-density oligonucleotide arrays to investigate whole genome transcriptional changes occurring at the optic nerve head during primate experimental glaucoma.

RESULTS

Laser scarification of the trabecular meshwork of cynomolgus macaques produced elevated intraocular pressure that was monitored over time and led to varying degrees of damage in different samples. The macaques were examined clinically before enucleation and the myelinated optic nerves were processed post-mortem to determine the degree of neuronal loss. Global gene expression was examined in dissected optic nerve heads with Affymetrix GeneChip microarrays. We validated a subset of differentially expressed genes using qRT-PCR, immunohistochemistry, and immuno-enriched astrocytes from healthy and glaucomatous human donors. These genes have previously defined roles in axonal outgrowth, immune response, cell motility, neuroprotection, and extracellular matrix remodeling.

CONCLUSION

Our findings show that glaucoma is associated with increased expression of genes that mediate axonal outgrowth, immune response, cell motility, neuroprotection, and ECM remodeling. These studies also reveal that, as glaucoma progresses, retinal ganglion cell axons may make a regenerative attempt to restore lost nerve cell contact.

Immunological mechanisms in glaucoma

Glaucoma is one of the most frequent causes of blindness worldwide. The elevated intraocular pressure does not explain glaucoma in all patients but can be considered as a risk factor of the disease. There are some evidences that autoimmune mechanisms may be involved in this disorder. This review attempts to demonstrate the findings about autoimmune mechanisms in glaucoma patients. Consistent up- and down-regulations in the autoantibody profiles against ocular antigens are present in glaucoma patients. These changes in natural autoimmunity could be found in independent study populations and might be a promising tool for glaucoma detection.

Axons of retinal ganglion cells are insulted in the optic nerve early in DBA/2J glaucoma

Here, we use a mouse model (DBA/2J) to readdress the location of insult(s) to retinal ganglion cells (RGCs) in glaucoma. We localize an early sign of axon damage to an astrocyte-rich region of the optic nerve just posterior to the retina, analogous to the lamina cribrosa. In this region, a network of astrocytes associates intimately with RGC axons. Using BAX-deficient DBA/2J mice, which retain all of their RGCs, we provide experimental evidence for an insult within or very close to the lamina in the optic nerve. We show that proximal axon segments attached to their cell bodies survive to the proximity of the lamina. In contrast, axon segments in the lamina and behind the eye degenerate. Finally, the Wld(s) allele, which is known to protect against insults to axons, strongly protects against DBA/2J glaucoma and preserves RGC activity as measured by pattern electroretinography. These experiments provide strong evidence for a local insult to axons in the optic nerve.

Longitudinal evaluation of retinal ganglion cell function and IOP in the DBA/2J mouse model of glaucoma

PURPOSE

To characterize progressive changes of retinal ganglion cell (RGC) function and intraocular pressure (IOP) in the DBA/2J mouse model of spontaneous glaucoma.

METHODS

Serial pattern electroretinograms (PERGs) and IOPs measures were obtained from both eyes of 32 anesthetized DBA/2J mice over an age range of 2 to 12 months at 1-month intervals. Cone-driven flash-ERGs (FERGs) were also recorded. The endpoint was defined as the age at which the PERG amplitude reached the noise level in at least one eye. At that point, both eyes were histologically processed to evaluate the thickness of the retinal fiber layer (RNFL).

RESULTS

IOP increased moderately between 2 and 6 months ( approximately 14-17 mm Hg) and then more steeply, until it leveled off at approximately 28 mm Hg by 9 to 11 months. The mean PERG amplitude decreased progressively after 3 months of age to reach the noise level (85% reduction of normal amplitude) at approximately 9 to 12 months in different animals. When the PERG was at noise level, the RNFL showed a relatively smaller reduction (40%) in normal thickness. The FERG displayed minor changes throughout the observation period. IOP and PERG changes were highly correlated (r(2) = 0.51, P < 0.001).

CONCLUSIONS

Results indicate that inner retina function in DBA/2J mice progressively decreases after 3 months of age, and it is nearly abolished by 10 to 11 months, whereas outer retina function shows little change and the RNFL thickness is relatively spared. This result suggests that surviving RGCs may not be functional. Progression of inner retinal dysfunction is strongly associated with increased IOP.

Effect of statin drugs and aspirin on progression in open-angle glaucoma suspects using confocal scanning laser ophthalmoscopy

PURPOSE

To determine the effect of statins and aspirin on the rate of progression of optic nerve parameters in open-angle glaucoma (OAG) suspects, as defined by confocal scanning laser ophthalmoscopy (CSLO).

METHODS

Data of OAG suspects who had undergone at least two CSLO tests at the Beckman Vision Center at UCSF from January 2001 to June 2006 was collected. We conducted a retrospective chart review of 149 eyes from 76 patients considered suspect for glaucoma based on a cup-to-disc ratio >0.5, but with normal intraocular pressures (IOP) and visual fields. Subjects included glaucoma suspects who took statin drugs or aspirin for greater than 23 months. The control group consisted of suspects who never used statins or aspirin. The data were analysed using mixed effects regression.

RESULTS

When comparing controls with the statin group there were significant differences in the progression of multiple CSLO parameters per year, including rim volume (-13.7% controls, +26.7% statin only; P = 0.0156), retinal nerve fibre layer cross-sectional area (-12.2% controls, +24.3% statin only; P = 0.0051), and mean global retinal nerve fibre layer thickness (-10.3% controls, +26.6% statin only; P = 0.0114), with adjustment for age, gender, race, IOP, central corneal thickness, refractive error and multiple systemic comorbidities. No significant differences were found when comparing subjects taking a statin plus aspirin or aspirin alone with the controls.

CONCLUSIONS

Statin drugs may be associated with slowed progression of optic nerve parameters in glaucoma suspects as measured by CSLO.

Development of spontaneous optic neuropathy in NF-kappaBetap50-deficient mice: requirement for NF-kappaBetap50 in ganglion cell survival

Although the transcription factor NF-kappaBeta is known to regulate cell death and survival, its precise role in cell death within the central nervous system remains unknown. The purpose of this study was to investigate the role of NF-kappaBetap50 in the age-related survival of retinal ganglion cells (RGCs). Eyes of mice with a deleted NF-kappaBetap50 gene and its wild-type mice at each of age were studied by histopathological studies. The number of RGCs was counted using retrograde labelling methods. Mice were subjected to intravitreous injection of N-methyl-D aspartate (NMDA) to induce RGC death. In p50-deficient mice, the number of RGCs significantly decreased with age in total independence of intraocular pressure measurement. Optic nerves of p50-deficient mice showed hypertrophy astrocytes and enlargement of the axons, together with a decreased number of axons. Immunohistochemistry showed a strong expression of glial fibrillary acidic protein. The histological results show obvious excavation of the optic nerve head in p50-deficient mice at 10 months of age. Intravitreal injection of NMDA in young p50-deficient mice damaged RGCs more intensively than in control animals. We further noticed that autoantibodies against RGCs were produced in p50-deficient mice. Our results show that p50 deficiency induced age-related RGC death, indicating a new insight into the role of p50 in the pathophysiology of neuropathy, and further experiments with p50-deficient mice may provide new targets for therapeutic intervention for human glaucoma.

Optic Nerve Restoration

Neural regeneration and repair in the central nervous system are currently hot topics in neuroscience. For many years there has been a hope that neurodegenerative diseases which are resistant to current therapies may be treated by the selective replacement of cells. Yet it is only recently that we have started to acquire the knowledge, tools, and techniques that may translate such optimism into new therapies. In this article, we will consider the potential to restore function to the damaged optic nerve. We will consider the technical issues involved and suggest a strategy for research progress.

Accelerated aging in glaucoma: immunohistochemical assessment of advanced glycation end products in the human retina and optic nerve head

PURPOSE

This study aimed to determine the association between advanced glycation end products (AGEs) and glaucoma based on the known synergism between oxidative stress with AGEs and the evidence of oxidative stress during glaucomatous neurodegeneration.

METHODS

The extent and cellular localization of immunolabeling for AGEs and their receptor, RAGE, were determined in histologic sections of the retina and optic nerve head obtained from 38 donor eyes with glaucoma and 30 eyes from age-matched donors without glaucoma.

RESULTS

The extent of AGE and RAGE immunolabeling was greater in older than in younger donor eyes. However, compared with age-matched controls, an enhanced accumulation of AGEs and an up-regulation of RAGE were detectable in the glaucomatous retina and optic nerve head. Although some retinal ganglion cells (RGCs) and glia exhibited intracellular immunolabeling for AGEs, increased AGE immunolabeling in glaucomatous eyes was predominantly extracellular and included laminar cribriform plates in the optic nerve head. Some RAGE immunolabeling was detectable on RGCs; however, increased RAGE immunolabeling in glaucomatous eyes was predominant on glial cells, primarily Müller cells.

CONCLUSIONS

Given that the generation of AGEs is an age-dependent event, increased AGE accumulation in glaucomatous tissues supports that an accelerated aging process accompanies neurodegeneration in glaucomatous eyes. One of the potential consequences of AGE accumulation in glaucomatous eyes appears to be its contribution to increased rigidity of the lamina cribrosa. The presence of RAGE on RGCs and glia also makes them susceptible to AGE-mediated events through receptor-mediated signaling, which may promote cell death or dysfunction during glaucomatous neurodegeneration.

Mechanisms of immune system activation in glaucoma: oxidative stress-stimulated antigen presentation by the retina and optic nerve head glia

PURPOSE

Evidence supports the immune system activity accompanying glaucomatous neurodegeneration. This study aimed to determine the in vitro effects of reactive oxygen species (ROS) on the phenotype and antigen-presenting function of the retina and optic nerve head glia.

METHODS

Cultures of rat retina and optic nerve head glia were treated with a mixture of ROS-generating compounds for 24 and 48 hours. Pretreated glial cells were then coincubated with syngeneic CD4(+) T cells for 48 hours. ROS generation and cell viability were assessed with the use of dihydroethidium and calcein assays, respectively. Flow cytometry and immunocytochemistry were used to determine major histocompatibility complex (MHC) class II molecules. In addition, functional experiments were performed to determine the proliferation and cytokine secretion of T cells using [(3)H]-thymidine incorporation and TNF-alpha assays, respectively.

RESULTS

MHC class II molecules were upregulated on glial cells exposed to ROS. Compared with the control glia, glial cells in ROS-generating systems were found to be more potent inducers of T-cell activation in a cell density- and time-dependent manner, as assessed by increased T-cell proliferation (approximately threefold) and TNF-alpha secretion (approximately sixfold; P < 0.01). When an ROS scavenging treatment was applied, MHC class II upregulation on glial cells persisted, but antigen-mediated T-cell activation was significantly decreased (P < 0.01), indicating an additional costimulatory function of ROS during antigen presentation.

CONCLUSIONS

These in vitro findings support that ROS regulate the immune response by stimulating the antigen-presenting ability of glial cells and functioning as costimulatory molecules for antigen presentation.

Tumor necrosis factor-alpha mediates oligodendrocyte death and delayed retinal ganglion cell loss in a mouse model of glaucoma

Glaucoma is a widespread ocular disease characterized by a progressive loss of retinal ganglion cells (RGCs). Previous studies suggest that the cytokine tumor necrosis factor-alpha (TNF-alpha) may contribute to the disease process, although its role in vivo and its mechanism of action are unclear. To investigate pathophysiological mechanisms in glaucoma, we induced ocular hypertension (OH) in mice by angle closure via laser irradiation. This treatment resulted in a rapid upregulation of TNF-alpha, followed sequentially by microglial activation, loss of optic nerve oligodendrocytes, and delayed loss of RGCs. Intravitreal TNF-alpha injections in normal mice mimicked these effects. Conversely, an anti-TNF-alpha-neutralizing antibody or deleting the genes encoding TNF-alpha or its receptor, TNFR2, blocked the deleterious effects of OH. Deleting the CD11b/CD18 gene prevented microglial activation and also blocked the pathophysiological effects of OH. Thus TNF-alpha provides an essential, although indirect, link between OH and RGC loss in vivo. Blocking TNF-alpha signaling or inflammation, therefore, may be helpful in treating glaucoma.

Oxidative stress in glaucomatous neurodegeneration: mechanisms and consequences

Reactive oxygen species (ROS) are generated as by-products of cellular metabolism, primarily in the mitochondria. Although ROS are essential participants in cell signaling and regulation, when their cellular production overwhelms the intrinsic antioxidant capacity, damage to cellular macromolecules such as DNA, proteins, and lipids ensues. Such a state of "oxidative stress" is thought to contribute to the pathogenesis of a number of neurodegenerative diseases. Growing evidence supports the involvement of oxidative stress as a common component of glaucomatous neurodegeneration in different subcellular compartments of retinal ganglion cells (RGCs). Besides the evidence of direct cytotoxic consequences leading to RGC death, it also seems highly possible that ROS are involved in signaling RGC death by acting as a second messenger and/or modulating protein function by redox modifications of downstream effectors through enzymatic oxidation of specific amino acid residues. Different studies provide cumulating evidence, which supports the association of ROS with different aspects of the neurodegenerative process. Oxidative protein modifications during glaucomatous neurodegeneration increase neuronal susceptibility to damage and also lead to glial dysfunction. Oxidative stress-induced dysfunction of glial cells may contribute to spreading neuronal damage by secondary degeneration. Oxidative stress also promotes the accumulation of advanced glycation end products in glaucomatous tissues. In addition, oxidative stress takes part in the activation of immune response during glaucomatous neurodegeneration, as ROS stimulate the antigen presenting ability of glial cells and also function as co-stimulatory molecules during antigen presentation. By discussing current evidence, this review provides a broad perspective on cellular mechanisms and potential consequences of oxidative stress in glaucoma.

Gene therapy and transplantation in CNS repair: The visual system

Normal visual function in humans is compromised by a range of inherited and acquired degenerative conditions, many of which affect photoreceptors and/or retinal pigment epithelium. As a consequence the majority of experimental gene- and cell-based therapies are aimed at rescuing or replacing these cells. We provide a brief overview of these studies, but the major focus of this review is on the inner retina, in particular how gene therapy and transplantation can improve the viability and regenerative capacity of retinal ganglion cells (RGCs). Such studies are relevant to the development of new treatments for ocular conditions that cause RGC loss or dysfunction, for example glaucoma, diabetes, ischaemia, and various inflammatory and neurodegenerative diseases. However, RGCs and associated central visual pathways also serve as an excellent experimental model of the adult central nervous system (CNS) in which it is possible to study the molecular and cellular mechanisms associated with neuroprotection and axonal regeneration after neurotrauma. In this review we present the current state of knowledge pertaining to RGC responses to injury, neurotrophic and gene therapy strategies aimed at promoting RGC survival, and how best to promote the regeneration of RGC axons after optic nerve or optic tract injury. We also describe transplantation methods being used in attempts to replace lost RGCs or encourage the regrowth of RGC axons back into visual centres in the brain via peripheral nerve bridges. Cooperative approaches including novel combinations of transplantation, gene therapy and pharmacotherapy are discussed. Finally, we consider a number of caveats and future directions, such as problems associated with compensatory sprouting and the reformation of visuotopic maps, the need to develop efficient, regulatable viral vectors, and the need to develop different but sequential strategies that target the cell body and/or the growth cone at appropriate times during the repair process.

Un vaccin contre le glaucome, mythe ou réalité ?

The cellular mechanisms involved in the loss of ganglion cells observed in glaucomatous neuropathy are based on a phenomenon of apoptosis: a primary apoptosis, related to the initial hypertonic process whatever its mechanism, and a secondary apoptosis via the free oxygen radicals or nitrogen monoxide, responsible for neuronal degeneration even after the initial factor has disappeared. In addition, glaucoma appears to be characterized by an increase in both TNF-alpha of the glia in the optic-nerve head and its type 1 receptor in the ganglion cells of the retina, which makes them particularly easy to stimulate by TNF-alpha. T lymphocytes also provide a certain neuroprotection by freeing neurotrophins or growth factors when neuronal lesion occurs, according to a specific and active process involving antigen-presenting cells. The T lymphocyte response was stimulated by sensitizing them by epitopes sequestered in the nervous system, notably myelinic proteins, in animal models of ganglion degeneration (obtained via a secondary apoptosis similar to that found during glaucoma). Prevention of ganglion cells loss was also observed by prior immunization of animals using a synthetic polymer close to myelin (COP1), capable of stimulating a specific lymphocyte reaction of neuronal impairment without inducing uveitis. Finally, glial cells, both activated during glaucoma and by TNF-alpha, and secreting TNF-alpha, could serve as antigen-presenting cells and thus constitute the keys to neuroprotection, using an original pathway independent of intraocular pressure control.

High-dose radiation with bone marrow transfer prevents neurodegeneration in an inherited glaucoma

Here, we show that high-dose gamma-irradiation accompanied with syngeneic bone marrow transfer can confer complete protection against glaucoma in a mouse model. Because bone marrow genotype was unaltered by this procedure, it was not the causative agent. The neuroprotection is robust and highly reproducible. Glaucoma-prone DBA/2J mice received a single treatment at 5-8 weeks of age and were protected from glaucomatous retinal ganglion cell degeneration out to 14 months of age (oldest assessed). By 12-14 months, retinal ganglion cell degeneration is usually very severe and essentially complete in the majority of untreated DBA/2J mice. To assess reproducibility, three groups of mice were treated at different times, and the results were essentially the same each time. Considering all experiments, the vast majority of treated mice had no detectable glaucomatous neurodegeneration. A beneficial effect of treatment including high-dose radiation is unprecedented, and we are not aware of any other neuroprotective effects this substantial. Because of the robust protective effect, this treatment offers another tool for studying mechanisms of neuroprotection.