Inhibition of inflammatory cells delays retinal degeneration in experimental retinal vein occlusion in mice

Exploring laser-induced acute and chronic retinal vein occlusion mouse models: Development, temporal in vivo imaging, and application perspectives

Photodynamic venous occlusion is a commonly accepted method for establishing mouse models of retinal vein occlusion (RVO). However, existing model parameters do not distinguish between acute and chronic RVO subtypes. Large variations in laser energy seem to correlate with fluctuating retinopathy severity and high rates of venous recanalization during the acute phase, along with the variable levels of retinal perfusion during the chronic phase. After optimizing the modeling procedure and defining success and exclusion criteria, laser energy groups of 80mW, 100mW, and 120mW were established. Multimodal imaging confirmed that higher energy levels increased the incidence of retinal cystoid edema and intraretinal hemorrhage, exacerbated the severity of exudative retinal detachment, and reduced the venous recanalization rate. For the acute model, 100mW was considered an appropriate parameter for balancing moderate retinopathy and venous recanalization. Continuous imaging follow-up revealed that day 1 after RVO was the optimal observation point for peaking of retinal thickness and intensive occurrence of retinal cystic edema and intraretinal hemorrhage. After excluding the influence of venous recanalization on retinal thickness, acute retinal edema demonstrated a positive response to standard anti-vascular endothelial growth factor therapy, validating the clinical relevance of the acute RVO model for further study in pathogenic mechanisms and therapeutic efficacy. For the chronic model, the 120mW parameter with the lowest venous recanalization rate was applied, accompanied by an increase in both photocoagulation shots and range to ensure sustained vein occlusion. Imaging follow-up clarified non-ischemic retinopathy characterized by tortuosity and dilation of the distal end, branches, and adjacent veins of the occluded vein. These morphological changes are quantifiable and could be combined with electrophysiological functional assessment for treatment effectiveness evaluation. Moreover, the stable state of venous occlusion may facilitate investigations into response and compensation mechanisms under conditions of chronic retinal hypoperfusion.

Rate and associations of inner retinal thinning in eyes with retinal vein occlusion and regressed macular oedema

PURPOSE

To investigate the associations between visual acuity (VA) and structural optical coherence tomography (OCT) features in retinal vein occlusion (RVO) eyes after cystoid macular oedema (CMO) regression and to assess whether inner retinal thinning is progressive.

METHODS

Retrospective observational study of RVO eyes with regressed CMO for at least 6 months. OCT scans at CMO regression were analysed, and features were correlated with VA at that visit. The inner retinal thickness was longitudinally compared between RVO and unaffected fellow eyes (controls) with linear mixed models. The rate of inner retinal thinning was obtained as the interaction term between disease status and time. Associations between inner retinal thinning and clinical characteristics were explored.

RESULTS

Thirty-six RVO eyes were followed for 34.2 ± 21.1 months after CMO regression. The presence of ellipsoid zone disruption (regression estimate[standard error(SE)] = 0.16[0.04] LogMAR vs. intact, p < 0.001) and lower inner retinal thickness (regression estimate[SE] = -0.25[0.12] LogMAR for 100-μm increase, p = 0.01) were associated with worse VA. The inner retinal thickness decreased faster in RVO than controls (rate of retinal thinning -0.27 ± 0.09 μm/month vs. -0.08 ± 0.11 μm/month, p = 0.01). Macular ischaemia was associated with a faster rate of retinal thinning (interaction term macular ischaemia*follow-up time, p = 0.04).

CONCLUSION

Inner retinal and photoreceptors' layers integrity are associated with better visual acuity once CMO resolves. RVO eyes undergo progressive inner retinal thinning after CMO regression, faster in eyes with macular ischaemia.

Microglia: The breakthrough to treat neovascularization and repair blood-retinal barrier in retinopathy

Microglia are the primary resident retinal macrophages that monitor neuronal activity in real-time and facilitate angiogenesis during retinal development. In certain retinal diseases, the activated microglia promote retinal angiogenesis in hypoxia stress through neurovascular coupling and guide neovascularization to avascular areas (e.g., the outer nuclear layer and macula lutea). Furthermore, continuously activated microglia secrete inflammatory factors and expedite the loss of the blood-retinal barrier which causes irreversible damage to the secondary death of neurons. In this review, we support microglia can be a potential cellular therapeutic target in retinopathy. We briefly describe the relevance of microglia to the retinal vasculature and blood-retinal barrier. Then we discuss the signaling pathway related to how microglia move to their destinations and regulate vascular regeneration. We summarize the properties of microglia in different retinal disease models and propose that reducing the number of pro-inflammatory microglial death and conversing microglial phenotypes from pro-inflammatory to anti-inflammatory are feasible for treating retinal neovascularization and the damaged blood-retinal barrier (BRB). Finally, we suppose that the unique properties of microglia may aid in the vascularization of retinal organoids.

Macrophage-like Cells Are Increased in Retinal Vein Occlusion and Correlate with More Intravitreal Injections and Worse Visual Acuity Outcomes

Macrophage-like cells (MLCs) are an emerging retinal biomarker. MLCs are increased in retinal vein occlusion (RVO) eyes, but their predictive value is unknown. This study investigated if MLCs can predict meaningful clinical outcomes. This prospective, cross-sectional study involved 46 eyes from 23 patients with unilateral RVO. Patients' unaffected eyes were used as matched controls. MLCs were quantified to determine MLC density and percent image area. We collected demographic, clinical, ocular, and imaging characteristics at the time of MLC imaging. We additionally recorded best corrected visual acuity (BCVA) and number of intravitreal injections at 6 months and 12 months post-imaging. MLC density and percent area increased by 1.86 (p = 0.0266)- and 1.94 (p = 0.0415)-fold in RVO compared to control eyes. We found no significant correlation between MLC parameters and any baseline characteristic. MLC density was positively correlated with the number of intravitreal injections at 6 months (n = 12, r = 0.62, p = 0.03) and 12 months (n = 9, r = 0.80, p = 0.009) post-imaging. MLC percent area was correlated with LogMAR BCVA change over 12 months (n = 17, r = 0.57, p = 0.02). High MLC counts correlated with more future intravitreal injections and worse visual acuity outcomes, suggesting that MLCs are a biomarker for treatment resistant RVO eyes.

More than meets the eye: The role of microglia in healthy and diseased retina

Microglia are the main resident immune cells of the nervous system and as such they are involved in multiple roles ranging from tissue homeostasis to response to insults and circuit refinement. While most knowledge about microglia comes from brain studies, some mechanisms have been confirmed for microglia cells in the retina, the light-sensing compartment of the eye responsible for initial processing of visual information. However, several key pieces of this puzzle are still unaccounted for, as the characterization of retinal microglia has long been hindered by the reduced population size within the retina as well as the previous lack of technologies enabling single-cell analyses. Accumulating evidence indicates that the same cell type may harbor a high degree of transcriptional, morphological and functional differences depending on its location within the central nervous system. Thus, studying the roles and signatures adopted specifically by microglia in the retina has become increasingly important. Here, we review the current understanding of retinal microglia cells in physiology and in disease, with particular emphasis on newly discovered mechanisms and future research directions.

Neurovascular injury associated non-apoptotic endothelial caspase-9 and astroglial caspase-9 mediate inflammation and contrast sensitivity decline

Retinal neurovascular injuries are a leading cause of vision loss in young adults presenting unmet therapeutic needs. Neurovascular injuries damage homeostatic communication between endothelial, pericyte, glial, and neuronal cells through signaling pathways that remain to be established. To understand the mechanisms that contribute to neuronal death, we use a mouse model of retinal vein occlusion (RVO). Using this model, we previously discovered that after vascular damage, there was non-apoptotic activation of endothelial caspase-9 (EC Casp9); knock-out of EC Casp9 led to a decrease in retinal edema, capillary ischemia, and neuronal death. In this study, we aimed to explore the role of EC Casp9 in vision loss and inflammation. We found that EC Casp9 is implicated in contrast sensitivity decline, induction of inflammatory cytokines, and glial reactivity. One of the noted glial changes was increased levels of astroglial cl-caspase-6, which we found to be activated cell intrinsically by astroglial caspase-9 (Astro Casp9). Lastly, we discovered that Astro Casp9 contributes to capillary ischemia and contrast sensitivity decline after RVO (P-RVO). These findings reveal specific endothelial and astroglial non-apoptotic caspase-9 roles in inflammation and neurovascular injury respectively; and concomitant relevancy to contrast sensitivity decline.

Distinguished Functions of Microglia in the Two Stages of Oxygen-Induced Retinopathy: A Novel Target in the Treatment of Ischemic Retinopathy

Microglia is the resident immune cell in the retina, playing the role of immune surveillance in a traditional concept. With the heated focus on the mechanisms of microglia in pathological conditions, more and more functions of microglia have been discovered. Although the regulating role of microglia has been explored in ischemic retinopathy, little is known about its mechanisms in the different stages of the pathological process. Here, we removed microglia in the oxygen-induced retinopathy model by PLX5622 and revealed that the removal of activated microglia reduced pathological angiogenesis in the early stage after ischemic insult and alleviated the over-apoptosis of photoreceptors in the vessel remodeling phase. Our results indicated that microglia might play distinguished functions in the angiogenic and remodeling stages, and that the inhibition of microglia might be a promising target in the future treatment of ischemic retinopathy.

Characterization of Macrophage-Like Cells in Retinal Vein Occlusion Using En Face Optical Coherence Tomography

Purpose

To investigate the clinical features of a macrophage-like cell (MLC) obtained by en face optical coherence tomography (OCT) in retinal vein occlusion (RVO).

Methods

The study involved 36 patients with treatment-naïve unilateral acute RVO, including 21 branch RVO (BRVO) and 15 central RVO. Vessel density and macular thickness were quantified using OCT angiography. A 3-μm en face OCT slab on the inner limiting membrane in the optic nerve head (ONH) region or macular region was used to visualize the MLCs. The MLCs were binarized and quantified using a semiautomated method. The unaffected fellow eyes served as the control group.

Results

The morphology of MLCs appeared larger and plumper in RVO eyes. The mean MLC density in the ONH and macular regions was 2.46 times and 2.86 times higher than their fellow eyes, respectively (p < 0.001). The macular MLC density of the occlusive region was significantly lower than that of the unaffected region in BRVO (p = 0.01). The ONH and macular MLC densities in the non-perfused region were significantly lower than those in the perfused region in all RVO eyes (p < 0.001). The ONH MLC density in RVO eyes was negatively correlated with radial peripapillary capillary vessel density (r = -0.413, p = 0.012). Both ONH and macular MLC densities were positively correlated with macular thickness (r = 0.505, p = 0.002; r = 0.385, p = 0.02, respectively).

Conclusion

The increased density and changes of morphology characterized by OCT may indicate generalized activation and aggregation of MLCs in RVO. More MLCs are recruited in the perfused region rather than the non-perfused region. RVO eyes with a higher density of MLCs tend to suffer from the thicker macula.

Retinal ganglion cell survival after severe optic nerve injury is modulated by crosstalk between Jak/Stat signaling and innate immune responses in the zebrafish retina

Visual information is transmitted from the eye to the brain along the optic nerve, a structure composed of retinal ganglion cell (RGC) axons. The optic nerve is highly vulnerable to damage in neurodegenerative diseases, such as glaucoma, and there are currently no FDA-approved drugs or therapies to protect RGCs from death. Zebrafish possess remarkable neuroprotective and regenerative abilities. Here, utilizing an optic nerve transection (ONT) injury and an RNA-seq-based approach, we identify genes and pathways active in RGCs that may modulate their survival. Through pharmacological perturbation, we demonstrate that Jak/Stat pathway activity is required for RGC survival after ONT. Furthermore, we show that immune responses directly contribute to RGC death after ONT; macrophages/microglia are recruited to the retina and blocking neuroinflammation or depleting these cells after ONT rescues survival of RGCs. Taken together, these data support a model in which crosstalk between macrophages/microglia and RGCs, mediated by Jak/Stat pathway activity, regulates RGC survival after optic nerve injury.

Diphtheria toxin induced but not CSF1R inhibitor mediated microglia ablation model leads to the loss of CSF/ventricular spaces in vivo that is independent of cytokine upregulation

Background

Two recently developed novel rodent models have been reported to ablate microglia, either by genetically targeting microglia (via Cx3cr1-creER: iDTR + Dtx) or through pharmacologically targeting the CSF1R receptor with its inhibitor (PLX5622). Both models have been widely used in recent years to define essential functions of microglia and have led to high impact studies that have moved the field forward.

Methods

Using either Cx3cr1-iDTR mice in combination with Dtx or via the PLX5622 diet to pharmacologically ablate microglia, we compared the two models via MRI and histology to study the general anatomy of the brain and the CSF/ventricular systems. Additionally, we analyzed the cytokine profile in both microglia ablation models.

Results

We discovered that the genetic ablation (Cx3cr1-iDTR + Dtx), but not the pharmacological microglia ablation (PLX5622), displays a surprisingly rapid pathological condition in the brain represented by loss of CSF/ventricles without brain parenchymal swelling. This phenotype was observed both in MRI and histological analysis. To our surprise, we discovered that the iDTR allele alone leads to the loss of CSF/ventricles phenotype following diphtheria toxin (Dtx) treatment independent of cre expression. To examine the underlying mechanism for the loss of CSF in the Cx3cr1-iDTR ablation and iDTR models, we additionally investigated the cytokine profile in the Cx3cr1-iDTR + Dtx, iDTR + Dtx and the PLX models. We found increases of multiple cytokines in the Cx3cr1-iDTR + Dtx but not in the pharmacological ablation model nor the iDTR + Dtx mouse brains at the time of CSF loss (3 days after the first Dtx injection). This result suggests that the upregulation of cytokines is not the cause of the loss of CSF, which is supported by our data indicating that brain parenchyma swelling, or edema are not observed in the Cx3cr1-iDTR + Dtx microglia ablation model. Additionally, pharmacological inhibition of the KC/CXCR2 pathway (the most upregulated cytokine in the Cx3cr1-iDTR + Dtx model) did not resolve the CSF/ventricular loss phenotype in the genetic microglia ablation model. Instead, both the Cx3cr1-iDTR + Dtx ablation and iDTR + Dtx models showed increased activated IBA1 + cells in the choroid plexus (CP), suggesting that CP-related pathology might be the contributing factor for the observed CSF/ventricular shrinkage phenotype.

Conclusions

Our data, for the first time, reveal a robust and global CSF/ventricular space shrinkage pathology in the Cx3cr1-iDTR genetic ablation model caused by iDTR allele, but not in the PLX5622 ablation model, and suggest that this pathology is not due to brain edema formation but to CP related pathology. Given the wide utilization of the iDTR allele and the Cx3cr1-iDTR model, it is crucial to fully characterize this pathology to understand the underlying causal mechanisms. Specifically, caution is needed when utilizing this model to interpret subtle neurologic functional changes that are thought to be mediated by microglia but could, instead, be due to CSF/ventricular loss in the genetic ablation model.

Mesenchymal Stem Cell-Based Therapy for Retinal Degenerative Diseases: Experimental Models and Clinical Trials

Retinal degenerative diseases, such as age-related macular degeneration, retinitis pigmentosa, diabetic retinopathy or glaucoma, represent the main causes of a decreased quality of vision or even blindness worldwide. However, despite considerable efforts, the treatment possibilities for these disorders remain very limited. A perspective is offered by cell therapy using mesenchymal stem cells (MSCs). These cells can be obtained from the bone marrow or adipose tissue of a particular patient, expanded in vitro and used as the autologous cells. MSCs possess potent immunoregulatory properties and can inhibit a harmful inflammatory reaction in the diseased retina. By the production of numerous growth and neurotrophic factors, they support the survival and growth of retinal cells. In addition, MSCs can protect retinal cells by antiapoptotic properties and could contribute to the regeneration of the diseased retina by their ability to differentiate into various cell types, including the cells of the retina. All of these properties indicate the potential of MSCs for the therapy of diseased retinas. This view is supported by the recent results of numerous experimental studies in different preclinical models. Here we provide an overview of the therapeutic properties of MSCs, and their use in experimental models of retinal diseases and in clinical trials.

Therapeutic Targeting of Retinal Immune Microenvironment With CSF-1 Receptor Antibody Promotes Visual Function Recovery After Ischemic Optic Neuropathy

Retinal ischemia/reperfusion injury (RI) is a common cause of irreversible visual impairment and blindness in elderly and critical unmet medical need. While no effective treatment is available for RI, microglial activation and local immune responses in the retina are thought to play important roles in the pathophysiology of neurodegeneration. While survival and activation of microglia depend critically on colony-stimulating factor receptor (CSF-1R) signaling, it remains unclear if targeting the retinal immune microenvironments by CSF-1RAb after RI is sufficient to rescue vision and present a potentially effective therapy. Here we used rodent models of RI and showed that retinal ischemia induced by acute elevation of intraocular pressure triggered an early activation of microglia and macrophages in the retina within 12 h. This was followed by lymphocyte infiltration and increased production of pro-inflammatory cytokines. Intravitreal injection of CSF-1R neutralizing antibody (CSF-1RAb) after RI significantly blocked microglial activation and the subsequent T cell recruitment. This also led to improved retinal ganglion cell survival and function measured by cell quantification and electroretinogram positive scotopic threshold responses, as well as increased visual acuity and contrast sensitivity as assessed by optomotor reflex-based assays, when compared to the isotype-treated control group. Moreover, the administration of CSF-1RAb efficiently attenuated inflammatory responses and activation of human microglia in culture, suggesting a therapeutic target with human relevance. These results, together with the existing clinical safety profiles, support that CSF-1RAb may present a promising therapeutic avenue for RI, a currently untreatable condition, by targeting microglia and the immune microenvironment in the retina to facilitate neural survival and visual function recovery.

Endothelial activation of caspase-9 promotes neurovascular injury in retinal vein occlusion

Central nervous system ischemic injury features neuronal dysfunction, inflammation and breakdown of vascular integrity. Here we show that activation of endothelial caspase-9 after hypoxia-ischemia is a critical event in subsequent dysfunction of the blood-retina barrier, using a panel of interrelated ophthalmic in vivo imaging measures in a mouse model of retinal vein occlusion (RVO). Rapid nonapoptotic activation of caspase-9 and its downstream effector caspase-7 in endothelial cells promotes capillary ischemia and retinal neurodegeneration. Topical eye-drop delivery of a highly selective caspase-9 inhibitor provides morphological and functional retinal protection. Inducible endothelial-specific caspase-9 deletion phenocopies this protection, with attenuated retinal edema, reduced inflammation and preserved neuroretinal morphology and function following RVO. These results reveal a non-apoptotic function of endothelial caspase-9 which regulates blood-retina barrier integrity and neuronal survival, and identify caspase-9 as a therapeutic target in neurovascular disease.

Retinal vein occlusion can cause blindness, and features neuronal dysfunction, inflammation and breakdown of vascular integrity. Here the authors report a non-apoptotic role of endothelial caspase-9 in regulating blood-retina barrier integrity and neuronal survival, which can be therapeutically targeted in a mouse model of retinal vein occlusion.

Effects of age on retinal macrophage responses to acute elevation of intraocular pressure

There is accumulating evidence that aging shifts the central nervous system milieu towards a proinflammatory state, with increased reactivity of microglia in the aging eye and brain having been implicated in the development of age-related neurodegenerative conditions. Indeed, alterations to microglial morphology and function have been recognized as a part of normal aging. Here, we sought to assess the effects of age on the retinal microglial and macrophage response to acute intraocular pressure (IOP) elevation. Further, we performed experiments whereby bone marrow from young or middle-aged mice was used to reconstitute the bone marrow of whole-body irradiated 12 month old mice. Bone marrow chimeric mice then underwent cannulation and IOP elevation 8 weeks after whole-body irradiation and bone marrow transplantation in order to determine whether the age of bone marrow alters the macrophage response to retinal injury. Our data show retinal macrophage reactivity and microglial morphological changes were enhanced in older mice when compared to younger mice in response to injury. When IOP elevation was performed after whole-body irradiation and bone marrow rescue, we noted subretinal macrophage accumulation and glial reactivity was reduced compared to non-irradiated mice that had also undergone IOP elevation. This effect was evident in both groups of chimeric mice that had received either young or middle-aged bone marrow, suggesting irradiation itself may alter the macrophage and glial response to injury rather than the age of bone marrow.

Inhibition of inflammatory cells delays retinal degeneration in experimental retinal vein occlusion in mice

The role of microglia in retinal inflammation is still ambiguous. Branch retinal vein occlusion initiates an inflammatory response whereby resident microglia cells are activated. They trigger infiltration of neutrophils that exacerbate blood–retina barrier damage, regulate postischemic inflammation and irreversible loss of neuroretina. Suppression of microglia‐mediated inflammation might bear potential for mitigating functional impairment after retinal vein occlusion (RVO). To test this hypothesis, we depleted microglia by PLX5622 (a selective tyrosine kinase inhibitor that targets the colony‐stimulating factor‐1 receptor) in fractalkine receptor reporter mice (Cx3cr1^gfp/+) subjected to various regimens of PLX5622 treatment and experimental RVO. Effectiveness of microglia suppression and retinal outcomes including retinal thickness as well as ganglion cell survival were compared to a control group of mice with experimental vein occlusion only. PLX5622 caused dramatic suppression of microglia. Despite vein occlusion, reappearance of green fluorescent protein positive cells was strongly impeded with continuous PLX5622 treatment and significantly delayed after its cessation. In depleted mice, retinal proinflammatory cytokine signaling was diminished and retinal ganglion cell survival improved by almost 50% compared to nondepleted animals 3 weeks after vein occlusion. Optical coherence tomography suggested delayed retinal degeneration in depleted mice. In summary, findings indicate that suppression of cells bearing the colony‐stimulating factor‐1 receptor, mainly microglia and monocytes, mitigates ischemic damage and salvages retinal ganglion cells. Blood–retina barrier breakdown seems central in the disease mechanism, and complex interactions between different cell types composing the blood–retina barrier as well as sustained hypoxia might explain why the protective effect was only partial.