Retinal phagocytes in age-related macular degeneration

Macrophages coordinate immune response to laser-induced injury via extracellular traps

BACKGROUND

Retinal degeneration results from disruptions in retinal homeostasis due to injury, disease, or aging and triggers peripheral leukocyte infiltration. Effective immune responses rely on coordinated actions of resident microglia and recruited macrophages, critical for tissue remodeling and repair. However, these phagocytes also contribute to chronic inflammation in degenerated retinas, yet the precise coordination of immune response to retinal damage remains elusive. Recent investigations have demonstrated that phagocytic cells can produce extracellular traps (ETs), which are a source of self-antigens that alter the immune response, which can potentially lead to tissue injury.

METHODS

Innovations in experimental systems facilitate real-time exploration of immune cell interactions and dynamic responses. We integrated in vivo imaging with ultrastructural analysis, transcriptomics, pharmacological treatments, and knockout mice to elucidate the role of phagocytes and their modulation of the local inflammatory response through extracellular traps (ETs). Deciphering these mechanisms is essential for developing novel and enhanced immunotherapeutic approaches that can redirect a specific maladaptive immune response towards favorable wound healing in the retina.

RESULTS

Our findings underscore the pivotal role of innate immune cells, especially macrophages/monocytes, in regulating retinal repair and inflammation. The absence of neutrophil and macrophage infiltration aids parenchymal integrity restoration, while their depletion, particularly macrophages/monocytes, impedes vascular recovery. We demonstrate that macrophages/monocytes, when recruited in the retina, release chromatin and granular proteins, forming ETs. Furthermore, the pharmacological inhibition of ETosis support retinal and vascular repair, surpassing the effects of blocking innate immune cell recruitment. Simultaneously, the absence of ETosis reshapes the inflammatory response, causing neutrophils, helper, and cytotoxic T-cells to be restricted primarily in the superficial capillary plexus instead of reaching the damaged photoreceptor layer.

CONCLUSIONS

Our data offer novel insights into innate immunity's role in responding to retinal damage and potentially help developing innovative immunotherapeutic approaches that can shift the immune response from maladaptive to beneficial for retinal regeneration.

The impact of early RPE cell junction loss on VEGF, Ang-2, and TIMP secretion in vitro

Purpose

The retinal pigment epithelium (RPE) is an important tissue for maintaining a healthy retina. Retinal pigment epithelial cells help regulate nutrient transport to photoreceptors and are heavily pigmented to prevent light scattering. These cells also have junction proteins to form monolayers. Monolayers are key players in pathologies such as age-related macular degeneration (AMD), a leading cause of vision loss in older adults. During AMD, RPE cell detachment can occur, resulting in a loss of junctions. Losing junctions can increase the expression of pro-angiogenic vascular endothelial growth factor (VEGF). This overexpression can cause abnormal blood vessel growth or angiogenesis in the retina. Age-related macular degeneration treatments target VEGF to slow angiogenesis progression. However, other proteins, such as angiopoietin-2 (Ang-2) and the tissue inhibitor of metalloproteinase-1 (TIMP-1), may also play important roles, making them potential targets for treatment. Controlling RPE junction formation will help elucidate the relationship between RPE cell detachment and additional angiogenic factor secretion, lead to more therapeutics, and increase the efficacy of current treatments.

Methods

Micropatterning was used to control the spatial arrangement of primary porcine RPE cells using polydimethylsiloxane (PDMS) stencils. Patterns were formed into PDMS stencils to mimic 10%, 25%, and 50% overall detachment of the RPE monolayer. Zonula-occludens-1 (ZO-1), Ang-2, and VEGF were visualized using immunocytochemical (ICC) staining. An enzyme-linked immunosorbent assay (ELISA) was used to quantify extracellular Ang-2, VEGF, TIMP-1, and TIMP-2 levels. A rod outer segment (OS) phagocytosis assay was performed to determine how RPE junction loss directly affects photoreceptor support.

Results

The growth of primary porcine RPE cells was successfully controlled using stencils. Morphological changes and a decrease in pigmentation were observed, showing a decline in barrier and light absorption functions as degeneration increased. One day after stencil removal, junction proteins were delocalized, and angiogenic factor secretions were correlated with increased levels of detachment. Secretion levels of Ang-2 and TIMP-1 were significantly increased, whereas VEGF and TIMP-2 concentrations were not as affected by varying levels of detachment. OS phagocytosis appeared lower in RPE cells when ZO-1 was affected.

Conclusions

These results suggest a correlation between loss of junctions, abnormal angiogenic protein secretion, and reduced OS phagocytosis. Furthermore, Ang-2 and TIMP-1 proteins might be beneficial targets for AMD treatments, and their roles in retinal diseases deserve further investigation.

Activated microglia-induced neuroinflammatory cytokines lead to photoreceptor apoptosis in Aβ-injected mice

Age-related macular degeneration (AMD) is mainly characterized by the progressive accumulation of drusen deposits and loss of photoreceptors and retinal pigment epithelial (RPE) cells. Because amyloid β (Aβ) is the main component of drusen, Aβ-induced activated microglia most likely lead to neuroinflammation and play a critical role in the pathogenesis of AMD. However, the relationship between activated microglia-mediated neuroinflammatory cytokines and photoreceptor death has not been clarified. By subretinal injection of Aβ₄₂ in mice, we mimicked an inflammatory milieu of AMD to better understand how activated microglia-induced neuroinflammatory cytokines lead to photoreceptor apoptosis in the AMD progression. We demonstrated that subretinal injection of Aβ₄₂ induces microglial activation and increases inflammatory cytokine release, which gives rise to photoreceptor apoptosis in mice. Our results were verified in vitro by co-culture of Aβ₄₂ activated primary microglia and the photoreceptor cell line 661W. We also demonstrated that the p38 mitogen-activated protein kinase (MAPK) signaling pathway was involved in Aβ₄₂-induced microglial activation and inflammatory cytokine release. Overall, our findings indicate that activated microglia-derived neuroinflammatory cytokines could contribute to photoreceptor apoptosis under the stimulation of Aβ₄₂. Moreover, this study may provide a potential therapeutic approach for AMD. KEY MESSAGES: Further explore the association between activated microglia-derived neuroinflammatory cytokine secretion and photoreceptor apoptosis under the stimulation of Aβ₄₂. Subretinal injection of Aβ₄₂ induces the activation of microglia and increases proinflammatory cytokines IL-1β and COX-2 expression in the retina, which could give rise to the deterioration of visual function and aggravate photoreceptor apoptosis in mice. Primary microglial are activated and the levels of proinflammatory cytokines are increased by Aβ₄₂ stimulation, which could increase the apoptosis of photoreceptor cell line 661W in vitro. The p38 MAPK signaling pathway is involved in microglial activation and photoreceptor apoptosis under Aβ₄₂ treatment.

RPE phagocytic function declines in age-related macular degeneration and is rescued by human umbilical tissue derived cells

Background

Age-related macular degeneration (AMD) is a leading cause of blindness among the elderly characterized by retinal pigment epithelium (RPE) degeneration with accumulation of abnormal intracellular deposits (lipofuscin) and photoreceptor death. RPE is vital for the retina and integrity of photoreceptors through its phagocytic function which is closely linked to formation of lipofuscin through daily phagocytosis of discarded photoreceptor outer segments (POS). Although phagocytosis has been implicated in AMD, it has not been directly shown to be altered in AMD. RPE phagocytic defect was previously shown to be rescued by subretinal injection of human umbilical tissue derived cells (hUTC) in a rodent model of retinal degeneration (RCS rat) through receptor tyrosine kinase (RTK) ligands and bridge molecules. Here, we examined RPE phagocytic function directly in the RPE from AMD patients and the ability and mechanisms of hUTC to affect phagocytosis in the human RPE.

Methods

Human RPE was isolated from the post-mortem eyes of normal and AMD-affected subjects and cultured. RPE phagocytic function was measured in vitro using isolated POS. The effects of hUTC conditioned media, recombinant RTK ligands brain-derived neurotrophic factor (BDNF), hepatocyte growth factor (HGF), and glial cell-derived neurotrophic factor (GDNF), as well as bridge molecules milk-fat-globule-EGF-factor 8 (MFG-E8), thrombospondin (TSP)-1, and TSP-2 on phagocytosis were also examined in phagocytosis assays using isolated POS. RNA was isolated from normal and AMD RPE treated with hUTC conditioned media and subjected to transcriptome profiling by RNA-Seq and computational analyses.

Results

RPE phagocytosis, while showing a moderate decline with age, was significantly reduced in AMD RPE, more than expected for age. hUTC conditioned media stimulated phagocytosis in the normal human RPE and significantly rescued the phagocytic dysfunction in the AMD RPE. RTK ligands and bridge molecules duplicated the rescue effect. Moreover, multiple molecular pathways involving phagocytosis, apoptosis, oxidative stress, inflammation, immune activation, and cholesterol transport were affected by hUTC in the RPE.

Conclusions

We demonstrated for the first time RPE phagocytic dysfunction in AMD, highlighting its likely importance in AMD, and the ability of hUTC to correct this dysfunction, providing insights into the therapeutic potential of hUTC for AMD.

Electronic supplementary material

The online version of this article (10.1186/s12967-018-1434-6) contains supplementary material, which is available to authorized users.

Blockade of microglial adenosine A2A receptor impacts inflammatory mechanisms, reduces ARPE-19 cell dysfunction and prevents photoreceptor loss in vitro

Age-related macular degeneration (AMD) is characterized by pathological changes in the retinal pigment epithelium (RPE) and loss of photoreceptors. Growing evidence has demonstrated that reactive microglial cells trigger RPE dysfunction and loss of photoreceptors, and inflammasome pathways and complement activation contribute to AMD pathogenesis. We and others have previously shown that adenosine A_2A receptor (A_2AR) blockade prevents microglia-mediated neuroinflammatory processes and mediates protection to the retina. However, it is still unknown whether blocking A_2AR in microglia protects against the pathological features of AMD. Herein, we show that an A_2AR antagonist, SCH58261, prevents the upregulation of the expression of pro-inflammatory mediators and the alterations in the complement system triggered by an inflammatory challenge in human microglial cells. Furthermore, blockade of A_2AR in microglia decreases the inflammatory response, as well as complement and inflammasome activation, in ARPE-19 cells exposed to conditioned medium of activated microglia. Finally, we also show that blocking A_2AR in human microglia increases the clearance of apoptotic photoreceptors. This study opens the possibility of using selective A_2AR antagonists in therapy for AMD, by modulating the interplay between microglia, RPE and photoreceptors.

Correlative light and immuno-electron microscopy of retinal tissue cryostat sections

Correlative light-electron microscopy (CLEM) is a powerful technique allowing localisation of specific macromolecules within fluorescence microscopy (FM) images to be mapped onto corresponding high-resolution electron microscopy (EM) images. Existing methods are applicable to limited sample types and are technically challenging. Here we describe novel methods to perform CLEM and immuno-electron microscopy (iEM) on cryostat sections utilising the popular FM embedding solution, optimal cutting temperature (OCT) compound. Utilising these approaches, we have (i) identified the same phagosomes by FM and EM in the retinal pigment epithelium (RPE) of retinal tissue (ii) shown the correct localisation of rhodopsin on photoreceptor outer segment disc like-structures in iPSC derived optic cups and (iii) identified a novel interaction between peroxisomes and melanosomes as well as phagosomes in the RPE. These data show that cryostat sections allow easy characterisation of target macromolecule localisation within tissue samples, thus providing a substantial improvement over many conventional methods that are limited to cultured cells. As OCT embedding is routinely used for FM this provides an easily accessible and robust method for further analysis of existing samples by high resolution EM.

Contributions of age-related alterations of the retinal pigment epithelium and of glia to the AMD-like pathology in OXYS rats

Age-related macular degeneration (AMD) is a major cause of blindness in developed countries, and the molecular pathogenesis of early events of AMD is poorly understood. It is known that age-related alterations of retinal pigment epithelium (RPE) cells and of glial reactivity are early hallmarks of AMD. Here we evaluated contributions of the age-related alterations of the RPE and of glia to the development of AMD-like retinopathy in OXYS rats. We showed that destructive alterations in RPE cells are a primary change during the development of retinopathy in OXYS rats. Furthermore, a defect of retinal maturation and decreased immune function at the preclinical stage of retinopathy were observed in OXYS rats in addition to the impairment of RPE cell proliferation and of their capacity for division. At the active stage of the disease, the atrophic alterations increased, and reactive gliosis was observed when disease progressed, but immune function stayed weakened. Unexpectedly, we did not observe migration of microglia and macrophages into the photoreceptor layer. These results and the wide spectrum of age-related retinal alterations in humans as well as individual differences in the risk of AMD may be attributed to genetic factors and to differences in the underlying molecular events.

Photoreceptor disc shedding in the living human eye

Cone photoreceptors undergo a daily cycle of renewal and shedding of membranous discs in their outer segments (OS), the portion responsible for light capture. These physiological processes are fundamental to maintaining photoreceptor health, and their dysfunction is associated with numerous retinal diseases. While both processes have been extensively studied in animal models and postmortem eyes, little is known about them in the living eye, in particular human. In this study, we report discovery of the optical signature associated with disc shedding using a method based on adaptive optics optical coherence tomography (AO-OCT) in conjunction with post-processing methods to track and monitor individual cone cells in 4D. The optical signature of disc shedding is characterized by an abrupt transient loss in the cone outer segment tip (COST) reflection followed by its return that is axially displaced anteriorly. Using this signature, we measured the temporal and spatial properties of shedding events in three normal subjects. Average duration of the shedding event was 8.8 ± 13.4 minutes, and average length loss of the OS was 2.1 μm (7.0% of OS length). Prevalence of cone shedding was highest in the morning (14.3%) followed by the afternoon (5.7%) and evening (4.0%), with load distributed across the imaged patch. To the best of our knowledge these are the first images of photoreceptor disc shedding in the living retina.