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Glaucoma is a neurodegenerative disease of the optic nerve characterized by progressive loss of retinal ganglion cells (RGCs), which can lead to irreversible blindness. Numerous factors have been implicated in the disease, with high intraocular pressure (IOP), coupled with advanced age, being major risk factors. Other factors include ischemia, generation of reactive oxygen species, a genetic pre-disposition and neurotrophin deprivation in the retina or optic nerve (Figure 1).1 The pathogenesis of glaucoma is challenging to understand since it is a multifactorial neurodegenerative disease.2,3 It is even more confounding what causes normal tension glaucoma (NTG). Despite a normal IOP, patients still suffer optic nerve degeneration. The immune system is a probable player in both high tension glaucoma (HTG) and normal tension glaucoma (NTG). Findings from numerous studies support the notion that both the innate and adaptive immune responses are involved in the pathogenesis of glaucoma.1,3-18 Yet, the precise mechanisms of immune responses and the specific cell interactions contributing to the disease process are still not fully understood. Many questions remain about the role of the immune system in glaucoma (Figure 1)
Keywords: glaucoma; numerous
THE OPHTHALMOLOGY OPEN JOURNAL Mini Review Insights into the Immune System and * *Corresponding author Glaucoma Jennifer L. Kielczewski, PhD National Eye Institute National Institutes of Health * Biological Imaging Core Jennifer L. Kielczewski, PhD* Bethesda, MD 20892, USA Tel. (301) 594-0297 E-mail: jennifer.kielczewski@nih.gov National Eye Institute, National Institutes of Health, Biological Imaging Core, Bethesda, MD 20892, USA Volume 2 : Issue 2 Article Ref. #: 1000OOJ2112 THE IMMUNE SYSTEM HAS BEEN IMPLICATED IN THE PATHOGENESIS OF GLAUCOMA Glaucoma is a neurodegenerative disease of the optic nerve characterized by progressive loss of Article History retinal ganglion cells (RGCs), which can lead to irreversible blindness. Numerous factors have st Received: September 1st, 2017 been implicated in the disease, with high intraocular pressure (IOP), coupled with advanced th Accepted: October 4th, 2017 age, being major risk factors. Other factors include ischemia, generation of reactive oxygen th Published: October 9th, 2017 species, a genetic pre-disposition and neurotrophin deprivation in the retina or optic nerve (Figure 1).1 The pathogenesis of glaucoma is challenging to understand since it is a multifac- 2,3 torial neurodegenerative disease.2,3 It is even more confounding what causes normal tension Citation Kielczewski JL. Insights into the im- glaucoma (NTG). Despite a normal IOP, patients still suffer optic nerve degeneration. The mune system and glaucoma. Ophthal- immune system is a probable player in both high tension glaucoma (HTG) and normal tension mol Open J. 2017; 2(2): 38-44. doi: glaucoma (NTG). Findings from numerous studies support the notion that both the innate and 10.17140/OOJ-2-112 1,3-18 adaptive immune responses are involved in the pathogenesis of glaucoma. Yet, the precise mechanisms of immune responses and the specific cell interactions contributing to the disease process are still not fully understood. Many questions remain about the role of the immune system in glaucoma (Figure 1).
Glaucoma High Tension Glaucoma (HTG) and Normal Tension Glaucoma (NTG) Factors involved (combination of genetic, environmental, immunological, and biological stressors): - Oxidative damage/tissue stress (mitochondrial dysfunction, reactive oxygen species/light damage) - Mechanical stress (high IOP, lamina cribrosa damage, extracellular matrix remodeling, increased TGF-β) - Genetic predisposition/epigenetics (myocilin, optineurin) - Autoantibodies/autoimmunity (HSP’s) - Immune dysfunction (increased TNF-α, microglial activation, complement system activation) - Protein aggregates/accumulation of misfolded proteins (synuclein) - Hypoxia/ischemia - Excitotoxicity (glutamate) - Growth factor deprivation (NGF, BDNF) and disrupted axonal transport (trkA, trkB) Copyright ©2017 Kielczewski JL. This is an open access article distributed un- RCG loss in retina and optic nerve head cupping and atrophy der the Creative Commons Attribu- tion 4.0 International License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction What injury happens first to the optic nerve and/or RGCs that initiates an immune in any medium, provided the origi- response? Is it in the early or late stage of the disease? Is the immune response systemic nal work is properly cited.
or localized only within the eye? What immune cells and cytokines influence RGC loss? Low-level inflammation is important for biologi- IOP is experimentally raised versus in animals subjected to a cal homeostasis. Immune responses are necessary for proper non-pressure injury to the RGCs. Strain differences33 can also 19,20 tissue cleaning, maintenance, and repair. It has also been influence a particular immune response due to differences in the proposed that protective or beneficial autoimmunity may help major histocompatibility complex (MHC) and/or human leuko- protect against neurodegeneration in the central nervous sys- cyte antigen (HLA) complexes. Animal models are important 11,21-25 tem (CNS). This concept suggests that the immune sys- in studying immune responses associated with RGC injury and tem plays a key role in the ability of the CNS, like the optic death, but teasing out significant correlations or trends between nerve and retina, to withstand neurodegeneration, by recruiting specific immune responses and RGC degeneration can be daunt- both innate (resident and blood-borne macrophages) and adap- ing, due to the inherent experimental variation present in animal tive (self-antigen specific T-cells) cells that together promote a models. Thus, translating the complex immune responses from protective niche and hinder disease progression under a well- glaucomatous animal models to human glaucoma is complicated 23,24,26-29 controlled response. However, excessively uncontrolled for researchers, yet the immune system has been implicated in immune stimulation can lead to a neurotoxic environment in the pathogenesis of glaucoma.
the optic nerve and retina, resulting in the neurodegeneration of RGCs. Immune dysregulation and immune activation in glau-
Cell Loss.(A) Cross-section Stained with Toluidine Blue Showing a Normal Mouse Optic Nerve. The Black Arrows Depict Healthy Axons. (B) Transmis- sion Electron Microscopy (TEM) Micrograph Illustrating Healthy Axons, which LIMITATIONS OF STUDYING IMMUNE RESPONSES IN are Myelinated in a Normal Mouse Optic Nerve. (C) Cross-Section Stained EXPERIMENTAL GLAUCOMATOUS ANIMAL MODELS with Toluidine Blue Showing a 4 Day Optic Nerve Crush Injured Mouse Nerve. The Black Boxes Represent Areas Where Axons have Undergone Loss and/or Degeneration. (D) TEM Micrograph Representing a 4 Day Optic Nerve Crush 31,32 Experimental glaucoma animal models are quite valuable. Injured Mouse Nerve. The Yellow Arrows Illustrate Sick and/or Dying Axons, which are Demyelinated. (E) Normal Mouse Retinal Flatmount with Fluoro- They can be useful in studying the early changes in pathologi- Gold Labeled RGCs. The White Arrows Show Healthy RGCs. (F) 4 Day Optic cal and molecular changes in the optic nerve and retina. Since Nerve Crush Injured Mouse Retinal Flatmount with Dying RGCs. The Red Ar- glaucoma is considered “the thief of sight” it can go undetected row Shows a Possible Astrocytic Glia Cell and the Blue Arrows Show Possible Microglia. The White Arrows Show Sick and/or Dying RGCs. for many years in patients, making it hard to study in the clinic. Likewise, post-mortem eye tissue from human patients is typi- cally limited to late onset and highly diseased glaucoma eyes, while early onset diseased eyes are not abundantly available for research studies. Studying glaucoma in humans is complicated; therefore, animal models are beneficial in dissecting out mo- lecular mechanisms, especially for immune dysregulation and immune responses involved in the early stages of RGC injury. There are several experimental animal models for glau- 31,32 coma. Some are based on chemical injury, such as intravitreal injection of neurotoxic concentrations of glutamate, while oth- ers utilize elevated pressure with laser injury to the trabecular meshwork or intravitreal injection of microbeads to raise IOP (injection of foreign beads into eye may not be an ideal glau- coma model to study immune responses). There are also animal models for mechanical-induced injury, where the optic nerve is transected or the optic nerve is crushed (Figure 2). Genetic models also exist such as the DBA/2J.5 But all of these models have limitations, especially with studying immune system ef- fects. No animal model can fully recapitulate human glaucoma due to its heterogeneous nature and each model can provide dif- ferent insight into immune mechanisms and responses. The type of injury, whether biochemical or mechanical, may dictate a dif- ferent immune response at a different time and location and may not only depend upon the severity, but also the chronicity of the injury. Moreover, some RGC cell types may be more suscep- tible to a particular injury; hence, certain RGCs may be more prone to immune attack or destruction than others. The immune responses are likely to be different in animal models, where GLIAL INTERACTIONS IN GLAUCOMA AUTOIMMUNITY AND GLAUCOMA Many studies have shown immune cell responses are involved There is evidence that autoimmune mechanisms are involved in 1,2,17,18 in glaucoma (Figure 3). This is especially true of glia (astro- glaucoma. Studies have shown the presence of autoanti- cytes and microglia), which are resident cells in the retina and bodies against ocular antigens such a rhodopsin and glycosami- 19,34,35 1,7,38-40 optic nerve that can initiate an immune response. Glia are noglycans (GAGs). It has been revealed that patients with important in immune surveillance, cleaning, as well as remov- normal tension glaucoma have increased levels of heat shock ing tissue debris. However, once astrocytes become reactive and proteins, such as heat shock protein 60 (HSP 60), due to exces- 17,18,41-43 microglia take on activated state, both cell types can increase sive tissue stress and damage. But there is no direct evi- production of cytokines (IL-6), reactive oxygen species (ROS), dence to confirm that RGC loss is due to humoral immunity in nitric oxide (NO), and tumor necrosis factor-α (TNF-α), creating glaucoma. It is possible that retinal or optic nerve specific auto- 34,36 a highly neurotoxic environment in the eye. Microglia can antigens are present in some glaucoma patients in a manner sim- express MHC molecules and function as resident antigen pre- ilar to organ specific autoantigens present in other autoimmune 34-36 senting cells. It is possible microglia become dysregulated diseases. Autoantibodies in glaucoma patients may be generated due to excessive activation in glaucoma. In the activated state, as a secondary consequence to disease pathogenesis or they they recruit other immune cells leading to an uncontrolled adap- could be generated directly due to RGC death. Likewise, there is tive immune response, resulting in increased antigenicity and evidence aberrant immune activity glaucoma is due to molecular increased antigen presentation. Microglia could be a therapeutic mimicry, through misguided immune responses to self-proteins target and modulating their behavior in glaucoma may decrease resulting in injury. A host response is mistakenly directed at a RGC loss. Likewise, studying the interactions between RGCs self-protein because it shares high homology to a specific protein and microglia may shed light on their tropic, cytokine, and im- found on the surface of a pathogen, like a bacteria or virus.17 One mune cell interactions. There are over 20 types of RGCs and specific study showed increased Heliobacter pylori titers among certain subtypes may interact more closely with microglia than glaucoma patients, which may represent molecular mimicry to others, which may influence their immune response to injury or this bacterium.44 Furthermore, epitope mapping has revealed the 30,37 stress.
immunogenicity of rhodopsin antibodies detected in glaucoma
Glaucoma & the Immune System Resident cells in optic Infiltrating cells into optic Complement Autoantibody nerve and retina nerve and retina System activation production and and upregulation in deposition of retina (classical and immunoglobulin in alternative pathways) retina or optic nerve Microglia and Astrocytes from an adaptive Monocytes/macrophages which They can have innate-like immune response are innate cells function and their activation can lead to release of cytotoxic and C1qa, C3 complement pro- inflammatory mediators components in the retina Could be neuroprotective or RGC apoptosis create a pro-inflammatory environment for RGCs Can release IL-6, TNF-α, NO, Degeneration or synaptic chemokines Activated microglia dysfunction of RGCs upregulate CD11c, MHC I& MHC II molecules and act as antigen presenting cells (APCs), which may activate T-cells for an adaptive immune response Neurodegeneration of RGCs due to neurotoxicity patient sera is shared with epitope proteins found on common different roles and responses in the primary degeneration and 14,17 bacterial and viral pathogens. Lastly, microbial flora whether secondary degeneration stages of glaucoma.
oral, gastrointestinal, or ocular, may contribute to the pathogen- esis of glaucoma.45 Overall, autoimmunity may play a role in There are key areas in glaucoma research that require glaucoma, but it is still an area of open investigation. further development to study immune responses. For example, high contrast optical coherence tomography (OCT) imaging COMPLEMENT PATHWAY AND GLAUCOMA and adaptive optics imaging techniques of the optic nerve and retina can facilitate studying the role of the immune system in The complement system is a key pathway in the innate immune glaucoma patients. Additionally, studies are needed to determine system. Various components in the complement system are up- which biomarkers or serum antigens can be indicators of glauco- regulated in human glaucoma and in animal models of glauco- ma.51 It has already been mentioned that HSP are autoantibodies 15,46-49 ma. The complement pathway can be activated in either the found in glaucoma patients. It is likely there are other autoanti- 20 10,52,53 optic nerve head or in the inner plexiform layer of the retina. It bodies generated in response to tissue damage and stress. has been shown that RGCs sense damage or stress and respond Macrophages, have been shown to play a role in the pathogen- by activating C1, which is part of the classical complement path- esis of glaucoma, but whether they are protective or beneficial 22,54,55 way. Once C1 is activated, C3 convertase is triggered by glial is still open for debate. Identification of specific immune cells, which can amplify RGC damage by recruiting other im- cells, whether they are pathogenic T-cells subsets or monocytes/ 20,47 mune cells like monocytes. It is still unknown whether the macrophages may be prognostic indicators of disease progres- complement pathway is directly responsible for RGC degenera- sion.56 Elucidation of epigenetic and genetic alterations, as well tion.
as age-related factors and susceptibility factors associated with glaucoma, is another area of open research inquiry.
T-CELLS AND GLAUCOMA In summary, glaucoma is likely a disease that is influ- Clinical studies have shown that glaucoma patients exhibit dif- enced by an uncontrollable immune response, due to an over- 3,34,50 ferences in immune cell populations, such as T-cells subsets. whelming burden of constant tissue insults. After a certain length A study showed significant alterations in Th1 and Th2 cytokine of time and disease progression, the immune system may be un- levels in human glaucoma patient serum.50 In animal models, T- able to provide protection and only offer destruction to neurons. cell migration into the optic nerve has been observed. In immune The question is how, when, and what specific immunotherapeu- deficient Rag1 knockout mice, which lack mature B- and T-cells, tics, cellular therapies (stem cells), or immunomodulators can these mice have reduced RGC loss compared to immune compe- be used to slow or reduce the loss of RGCs and prevent disease tent wild type mice with experimental glaucoma injury.3 But this progression, resulting in the precious preservation of sight in has not been definitively shown in human glaucoma. T-cell mi- glaucoma patients.
gration into the human retina or optic nerve can be transient and it cannot be ruled out that T-cells can be cytotoxic to RGCs, even