The retinal pigment epithelium as a gateway for monocyte trafficking into the eye

Replenishing Age-Related Decline of IRAK-M Expression in Retinal Pigment Epithelium Attenuates Outer Retinal Degeneration

Unchecked, chronic inflammation is a constitutive component of age-related diseases, including age-related macular degeneration (AMD). Here we identified interleukin-1 receptor-associated kinase (IRAK)-M as a key immunoregulator in retinal pigment epithelium (RPE) that declines with age. Rare genetic variants of IRAK-M increased the likelihood of AMD. IRAK-M expression in RPE declined with age or oxidative stress and was further reduced in AMD. IRAK-M-deficient mice exhibited increased incidence of outer retinal degeneration at earlier ages, which was further exacerbated by oxidative stressors. The absence of IRAK-M disrupted RPE cell homeostasis, including compromised mitochondrial function, cellular senescence, and aberrant cytokine production. IRAK-M overexpression protected RPE cells against oxidative or immune stressors. Subretinal delivery of AAV-expressing IRAK-M rescued light-induced outer retinal degeneration in wild-type mice and attenuated age-related spontaneous retinal degeneration in IRAK-M-deficient mice. Our data support that replenishment of IRAK-M expression may redress dysregulated pro-inflammatory processes in AMD, thereby treating degeneration.

The Value of White Cell Inflammatory Biomarkers as Potential Predictors for Diabetic Retinopathy in Type 2 Diabetes Mellitus (T2DM)

The pathogenesis of diabetic retinopathy is still challenging, with recent evidence proving the key role of inflammation in the damage of the retinal neurovascular unit. This study aims to investigate the predictive value of the neutrophil-to-lymphocyte ratio (NLR), platelet-to-lymphocyte (PLR), lymphocyte-to-monocyte ratio (LMR), and systemic inflammation index (SII) for diabetic retinopathy (DR) and its severity. We performed a retrospective study on 129 T2DM patients, divided into three groups: without retinopathy (NDR), non-proliferative DR (NPDR), and proliferative DR (PDR). NLR, MLR, and SII were significantly higher in the PDR group when compared to NDR and NPDR (3.2 ± 1.6 vs. 2.4 ± 0.9 and 2.4 ± 1.1; p = 0.005; 0.376 ± 0.216 vs. 0.269 ± 0.083 and 0.275 ± 0.111, p = 0.001; 754.4 ± 514.4 vs. 551.5 ± 215.1 and 560.3 ± 248.6, p = 0.013, respectively). PDR was correlated with serum creatinine (OR: 2.551), NLR (OR: 1.645), MPV (OR: 1.41), and duration of diabetes (OR: 1.301). Logistic regression analysis identified three predictive models with very good discrimination power for PDR (AUC ROC of 0.803, 0.809, and 0.830, respectively): combining duration of diabetes with NLR, MLR, and, respectively, PLR, MPV, and serum creatinine. NLR, MPV, SII, and LMR were associated with PDR and could be useful when integrated into comprehensive risk prediction models.

The correlation between peripheral complete blood count parameters and diabetic macular edema in proliferative diabetic retinopathy patients: a cross-sectional study

Background

Numerous studies have demonstrated that retinal chronic inflammation plays a critical role in the pathogenesis of diabetic macular edema (DME). However, studies about the association between peripheral complete blood count, an inexpensive and easily measurable laboratory index, and DME are limited.

Research design and methods

The current study was a hospital-based, cross-sectional study. The participants were inpatients with type 2 diabetes who underwent vitrectomy for PDR, and the contralateral eyes in these PDR patients meeting the criteria were included in the study. Central macular thickness (CMT) was measured automatically and the DME was characterized as CMT ≥ 300 μm.

Results

A total of 239 PDR participants were enrolled. The average age was 55.46 ± 10.08 years old, and the average CMT was 284.23 ± 122.09 μm. In the fully adjusted model, for CMT, the results revealed a significantly negative association between CMT and both white blood cell (WBC) count and neutrophil count (β = -11.95, 95% CI: -22.08, -1.82; p = 0.0218; β = -14.96, 95% CI: -28.02, -1.90; p = 0.0259, respectively); for DME, the results showed an inverse association between DME and WBC count, monocyte count, and eosinophil count (OR = 0.75, 95% CI: 0.59, 0.95; p = 0.0153; OR = 0.07, 95% CI: 0.00, 0.92; p = 0.0431; OR = 0.03, 95% CI: 0.00, 0.88; p = 0.0420, respectively).

Conclusions

In conclusion, our results suggest that WBC and its subtypes in circulation may play an important role in the pathogenesis of DME in PDR patients.

Temporal single-cell atlas of non-neuronal retinal cells reveals dynamic, coordinated multicellular responses to central nervous system injury

Non-neuronal cells are key to the complex cellular interplay that follows central nervous system insult. To understand this interplay, we generated a single-cell atlas of immune, glial and retinal pigment epithelial cells from adult mouse retina before and at multiple time points after axonal transection. We identified rare subsets in naive retina, including interferon (IFN)-response glia and border-associated macrophages, and delineated injury-induced changes in cell composition, expression programs and interactions. Computational analysis charted a three-phase multicellular inflammatory cascade after injury. In the early phase, retinal macroglia and microglia were reactivated, providing chemotactic signals concurrent with infiltration of CCR2+ monocytes from the circulation. These cells differentiated into macrophages in the intermediate phase, while an IFN-response program, likely driven by microglia-derived type I IFN, was activated across resident glia. The late phase indicated inflammatory resolution. Our findings provide a framework to decipher cellular circuitry, spatial relationships and molecular interactions following tissue injury.

Evaluation of Lymphocyte-to-Monocyte Ratio and Mean Platelet Volume-to-Platelet Ratio in Rhegmatogenous Retinal Detachment

Purpose

To assess the predictive value of inflammatory blood biomarkers in rhegmatogenous retinal detachment (RRD) patients and its correlation with proliferative retinopathy (PVR) grade.

Methods

82 RRD patients and 1 : 1 age- and gender-randomly matched cataract patients as the control group were included. The clinical features and laboratory parameters of all participants were collected and recorded, and the comprehensive index of inflammatory blood and its correlation with PVR were calculated.

Results

Monocytes and monocyte-to-high-density lipoprotein ratio (MHR) were significantly lower (P=0.005, P=0.044), while mean platelet volume (MPV), lymphocyte-to-monocyte ratio (LMR), and MPV-to-platelet ratio (MPR) were significantly higher in RRD patients as compared with the control group (P=0.013, P=0.019, P=0.037). LMR and MPR might be the predictors of RRD. The receiver operating characteristics analysis showed that the values of MPV, LMR, and MPR in RRD patients were 0.612, 0.606, and 0.594, respectively. PVR grade was not associated with inflammatory blood biomarkers.

Conclusion

The increase in MPV, LMR, and MPR were associated with increased odds of RRD. LMR and MPR may be useful as inexpensive and effortless biomarkers for assessing the occurrence of RRD.

Correlation between ultrawide-field fluorescence contrast results and white blood cell indexes in diabetic retinopathy

BACKGROUND

Diabetic retinopathy (DR) is one of the most common microvascular complications of diabetes. DR involves a state of systemic inflammation, and chronic inflammation can promote microvascular and macrovascular diseases in diabetic patients and accelerate disease progression. Ultrawide-field FFA (UWFA) systems are increasingly being used to examine a wider retina. The aim of this study was to explore the correlation between the different manifestations of retinopathy under UWFA and the systemic indicators of white blood cells in patients with diabetic retinopathy.

METHODS

This retrospective study included the hospitalized DR patients in the Department of Ophthalmology and Endocrinology of the Affiliated Hospital 2 of Nantong University between January 2016 and March 2019. This study examined the correlations between the UWFA examination results and glycated hemoglobin (HbA1c), routine blood tests,and the neutrophil-to-lymphocyte ratio of patients with clinically diagnosed DR during hospitalization.

RESULTS

A total of 115 patients with DR (53 females and 62 males) were included (199 eyes: 102 right eyes and 97 left eyes). UWFA revealed that most eyes (77.4%) had grade 4 microvascular leakage, 52.8% had grade 0 capillary non-perfusion area, 59.3% had grade 0 neovascularization, and 92.0% had grade 0 fibrous proliferative membranes. Microvascular leakage was correlated with the NLR (r = 0.186, P = 0.027). Capillary non-perfusion area was correlated with the monocyte ratio (r = 0.144, P = 0.042) and the eosinophil ratio (r = 0.123, P = 0.044). Neovascularization was correlated to the monocyte ratio (r = 0.324, P = 0.018). Finally, the fibrous proliferative membrane was correlated to the monocyte ratio (r = 0.418, P = 0.002). Only the eosinophil ratio was independently associated with proliferative DR (odds ratio = 1.25, 95% confidence interval: 1.04-1.51, P = 0.018).

CONCLUSION

The results of UWFA imaging in patients with DR are correlated with white blood cell population indexes. The eosinophil ratio was independently associated with proliferative DR.

Association of monocyte-lymphocyte ratio and proliferative diabetic retinopathy in the U.S. population with type 2 diabetes

OBJECTIVE

Diabetic retinopathy (DR), especially proliferative diabetic retinopathy (PDR) is a common cause of blindness and visual impairment. Early prediction of its occurrence and progression is important to improved patient outcomes. Inflammation-related markers may play important roles, and the monocyte-lymphocyte ratio (MLR) can act as a novel inflammatory marker. However, the association between MLR and PDR remains unclear. The aim of the present study was to investigate the association between MLR and PDR in the U.S. population with type 2 diabetes (T2D) based on DR data from NHANES in 2005-2008.

METHODS

This cross-sectional study was conducted in the National Health and Nutrition Examination Survey (NHANES) from 2005 to 2008. DR was defined by the criteria of the Early Treatment for Diabetic Retinopathy Study based on nonmydriatic fundus photography. The MLR is the monocyte count/lymphocyte count. The lymphocyte count and monocyte count can be obtained directly from laboratory data files. Logistic regression was used to explore the association between MLR and PDR. Stratified analyses were also conducted according to age, sex, hemoglobin, and glycated hemoglobin categories. We applied the duration of diabetes with multiple imputations of missing data.

RESULTS

A total of 367 participants were included, among whom the PDR prevalence was 7% (27/367). Multivariate regression models revealed that PDR was significantly associated with 0.1 unit increase in MLR (adjusted OR = 1.46, 95% CI: 1.08-1.96) after all covariates were adjusted. In the subgroup analysis, effect size of MLR on the presence of PDR in subgroups were stable (all P values > 0.05).

CONCLUSIONS

MLR was significantly associated with PDR in participants with T2D. Assessing the MLR might be a valuable part of follow-up visits for patients with T2D.

Osteopontin is a biomarker for early autoimmune uveoretinitis

Osteopontin (OPN) is an extracellular matrix protein with a diverse range of functions, including roles in cell adhesion, migration, and immunomodulation, which are associated with the modulation of neuroinflammation in the central nervous system. The present study was performed to evaluate the involvement of OPN in the eyes of an experimental autoimmune uveoretinitis (EAU) model. The EAU model was developed by immunization of Lewis rats with interphotoreceptor retinoid-binding protein. The results showed the OPN level was remarkably upregulated in the eye of EAU rats on day 9 post-immunization. The level of CD44, a ligand of OPN, was increased in the ciliary body of EAU rats. Furthermore, OPN was also detected in the ciliary body and activated microglia/macrophages in the EAU retina. The results suggest that OPN was significantly upregulated in the eyes of EAU rats, and that it may be useful as an early biomarker of ocular autoimmune diseases. All animal experiments were approved by the Institutional Animal Care and Use Committee of Jeju National University (approval No. 2020-0012) on March 11, 2020.

Association of the Monocyte-to-High-Density Lipoprotein Cholesterol Ratio With Diabetic Retinopathy

Background: Chronic inflammation in type 2 diabetes mellitus (T2DM) is an essential contributor to the development of diabetic retinopathy (DR). The monocyte–to–high-density lipoprotein cholesterol (HDL-C) ratio (MHR) is a novel and simple measure related to inflammatory and oxidative stress status. However, little is known regarding the role of the MHR in evaluating the development of DR.

Methods: A total of 771 patients with T2DM and 607 healthy controls were enrolled in this cross-sectional study. MHR determination and eye examination were performed. The association of MHR with the prevalence of DR in T2DM patients was analyzed.

Results: The MHR in patients with DR was significantly higher than that in both non-DR diabetic patients (P < 0.05) and healthy controls (P < 0.01). No significance was observed in the MHR of different DR severity grades. Moreover, the MHR was similar between patients with non-macular oedema and those with macular oedema. Logistic regression analysis demonstrated that MHR was independently associated with the prevalence of DR in diabetic patients [odds ratio (OR) = 1.438, 95% confidence interval (CI): 1.249–1.655, P < 0.01]. After additional stratification by HbA1c level and diabetic duration, the MHR was still independently associated with the prevalence of DR.

Conclusions: Our study suggests that the MHR can be used as a marker to indicate the prevalence of DR in patients with T2DM.

Exploring the systemic delivery of a poorly water-soluble model drug to the retina using PLGA nanoparticles

During the drug development process, many pharmacologically active compounds are discarded because of poor water solubility, but nanoparticle-based formulations are increasingly proposed as a solution for this problem. We therefore studied the distribution of nanoparticulate carriers and the delivery of their poorly water-soluble cargo to a structure of the central nervous system, the retina, under naive and pathological conditions. The lipophilic fluorescent dye coumarin 6 (Cou6) was encapsulated into poly(lactic-co-glycolic acid) PLGA nanoparticles (NPs). After intravenous administration in rats, we analyzed the distribution of cargo Cou6 and of the NP carrier covalently labeled with Cy5.5 in healthy animals and animals with optic nerve crush (ONC). In vivo real-time retina imaging revealed that Cou6 was rapidly released from PLGA NPs and penetrated the inner blood-retina barrier (BRB) within 15 min and PLGA NPs were gradually eliminated from the retinal blood circulation. Ex vivo microscopy of retinal flat mounts indicated that the Cou6 accumulated predominantly in the extracellular space and to a lesser extent in neurons. While the distribution of Cou6 in healthy animals and post ONC was comparable at early time point post-operation, the elimination of the NPs from the vessels was faster on day 7 post ONC. These results demonstrate the importance of considering different kinetics of nano-carrier and poorly water-soluble cargo, emphasizing the critical role of their parenchymal distribution, i.e. cellular/extracellular, and function of different physiological and pathological conditions.

Congenital Anomalies Programmed by Maternal Diabetes and Obesity on Offspring of Rats

Embryo-fetal exposure to maternal disorders during intrauterine life programs long-term consequences for the health and illness of offspring. In this study, we evaluated whether mild diabetic rats that were given high-fat/high-sugar (HF/HS) diet presented maternal and fetal changes at term pregnancy. Female rats received citrate buffer (non-diabetic-ND) or streptozotocin (diabetic-D) after birth. According to the oral glucose tolerance test (OGTT), the experimental groups (n = 11 animals/group) were composed of non-diabetic and diabetic receiving standard diet (S) or HF/HS diet. High-fat/high-sugar diet (30% kcal of lard) in chow and water containing 5% sucrose and given 1 month before mating and during pregnancy. During and at the end of pregnancy, obesity and diabetes features were determined. After laparotomy, blood samples, periovarian fat, and uterine content were collected. The diabetic rats presented a higher glycemia and percentage of embryonic losses when compared with the NDS group. Rats DHF/HS presented increased obesogenic index, caloric intake, and periovarian fat weight and reduced gravid uterus weight in relation to the other groups. Besides, this association might lead to the inflammatory process, confirmed by leukocytosis. Obese rats (NDHF/HS and DHF/HS) showed higher triglyceride levels and their offspring with lower fetal weight and ossification sites, indicating intrauterine growth restriction. This finding may contribute to vascular alterations related to long-term hypertensive disorders in adult offspring. The fetuses from diabetic dams showed higher percentages of skeletal abnormalities, and DHF/HS dams still had a higher rate of anomalous fetuses. Thus, maternal diabetes and/or obesity induces maternal metabolic disorders that contribute to affect fetal development and growth.

The immunological link between neonatal lung and eye disease

Bronchopulmonary dysplasia (BPD) and retinopathy of prematurity (ROP) are two neonatal diseases of major clinical importance, arising in large part as a consequence of supplemental oxygen therapy used to promote the survival of preterm infants. The presence of coincident inflammation in the lungs and eyes of neonates receiving oxygen therapy indicates that a dysregulated immune response serves as a potential common pathogenic factor for both diseases. This review examines the current state of knowledge of immunological dysregulation in BPD and ROP, identifying similarities in the cellular subsets and inflammatory cytokines that are found in the alveoli and retina during the active phase of these diseases, indicating possible mechanistic overlap. In addition, we highlight gaps in the understanding of whether these responses emerge independently in the lung and retina as a consequence of oxygen exposure or arise because of inflammatory spill-over from the lung. As BPD and ROP are anatomically distinct, they are often considered discreet disease entities and are therefore treated separately. We propose that an improved understanding of the relationship between BPD and ROP is key to the identification of novel therapeutic targets to treat or prevent both conditions simultaneously.

The unique association between the level of peripheral blood monocytes and the prevalence of diabetic retinopathy: a cross-sectional study

OBJECTIVE

The attraction and influx of monocytes into the retina has been considered a critical step in the development of diabetic retinopathy (DR). However, large population studies about the association between peripheral blood monocyte levels, an inexpensive and easily measurable laboratory index, and DR are limited. Thus, we aimed to investigate the association between peripheral blood monocyte levels and DR.

METHODS

A total of 3223 participants out of 3277 adults with diabetes were enrolled from seven communities in China in this cross-sectional survey. Participants underwent several medical examinations, including the measurement of anthropometric factors, blood pressure, routinely analyzed leukocyte characteristics, glucose, lipid profiles, urine albumin/creatinine ratio and fundus photographs.

RESULTS

The prevalence of DR among the participants in the highest quartile of peripheral blood monocyte levels significantly decreased by 41% (OR 0.59; 95% CI 0.43, 0.81) compared with the participants in the first quartile (P for trend < 0.05). However, there were no associations between the monocyte level and the prevalence of cardiovascular and cerebrovascular diseases (CVD) and diabetic kidney disease (DKD) (both P for trend > 0.05). Associations between leukocyte, neutrophil and lymphocyte levels and DR were also not found (all P for trend > 0.05). These associations were all fully adjusted for age, sex, education status, duration of diabetes history, current smoking, BMI, HbA1c, dyslipidemia, systolic blood pressure and insulin therapy.

CONCLUSION

Decreased peripheral blood monocyte levels were associated with increased odds of DR after adjusting for potential confounders in diabetic adults. However, causation remains to be demonstrated.

Arsenic trioxide inhibits proliferation of retinal pigment epithelium by downregulating expression of extracellular matrix and p27

The present study aimed to investigate the effect of arsenic trioxide (ATO) on the proliferation of retinal pigment epithelium (RPE) and its mechanism. RPE cells were cultivated with 0.5-11 μmol/L ATO for 24, 48, and 72 h and their survival and growth were measured by MTT assay. The expression of p27 and proliferating cell nuclear antigen (PCNA) in RPE cells was detected using cell immunofluorescence and western blotting. Dose-dependency was evident in both the experimental and control groups. The 50% inhibitory concentration was obtained at a concentration of 6 mol/L with cells treated for 3 days. The optimum concentration of ATO was 6 μmol/L based on the result of MTT. After the third day of ATO treatment, the number of cells was significantly lower in the experimental group compared with the control group. The expression of extracellular matrix (ECM) components decreased relative to the control group. The expression of p27 and PCNA declined gradually in cells treated for 72 h at 6 μmol/L ATO compared with the control group. The difference between the experimental and control groups was significant (P=0.005). ATO has the ability to inhibit the growth and proliferation of RPE cells by regulating the expression of the ECM components' p27 and PCNA, in a time- and dose-dependent manner. Thus, ATO may lead to an innovative method for the treatment of proliferative retinopathy.

Neuronal integrity and complement control synaptic material clearance by microglia after CNS injury

Norris et al. show that microglia are the key phagocytes in removal of synaptic debris in the dorsal lateral geniculate nucleus after optic nerve injury. This microglial function is dependent on recognition of neurodegeneration and is mediated by the complement system.

Phagocytosis of synaptic material by microglia is critical for central nervous system development. Less well understood is this microglial function in the injured adult brain. Assay of microglial phagocytosis is challenging, because peripheral myeloid cells engraft the site of injury, which could obscure interpretation of microglial roles. The model used here, optic nerve crush injury, results in degeneration of synapses in the dorsal lateral geniculate nucleus (dLGN), which stimulates rapid activation and engulfment of synaptic material by resident microglia without myeloid cell engraftment. Pharmacological depletion of microglia causes postinjury accumulation of synaptic debris, suggesting that microglia are the dominant postinjury phagocytes. Genetic or pharmacological manipulations revealed that neuronal activity does not trigger microglia phagocytosis after injury. RNA sequencing reveals C1q and CD11b/CR3 involvement in clearance of debris by dLGN-resident microglia. Indeed, C1qa^−/− and Itgam^−/− mice exhibit impaired postinjury debris clearance. Our results show how neurodegenerative debris is cleared by microglia and offers a model for studying its mechanisms and physiological roles.

New concepts in macrophage ontogeny in the adult neural retina

The number of neurons dedicated to vision itself is thought to be greater than the sum of the four other senses combined. Yet, little attention has been payed to the retina as compared to elsewhere in the central nervous system with respect to microglia, the macrophages of the neural parenchyma. Indeed, major advancements in the understanding of microglial ontogeny and maintenance in brain and spinal cord are now widely appreciated, whereas less notice has been given to the neural retina in this regard. The current Review covers topical concepts on adult microglia and perivascular macrophage ontogenies in the steady state retina, as well as parallels made with these macrophages in other areas of the central nervous system. The subject of recruited monocytes and their descendant monocyte-derived macrophages in degenerative diseases of the retina is also integrated into this Review. Key experiments that have led to the theories covered are highlighted throughout, as are the knowledge gaps that remain unresolved.

Serine racemase deficiency attenuates choroidal neovascularization and reduces nitric oxide and VEGF levels by retinal pigment epithelial cells

Choroidal neovascularization (CNV) is a leading cause of blindness in age-related macular degeneration. Production of vascular endothelial growth factor (VEGF) and macrophage recruitment by retinal pigment epithelial cells (RPE) significantly contributes to the process of CNV in an experimental CNV model. Serine racemase (SR) is expressed in retinal neurons and glial cells, and its product, d-serine, is an endogenous co-agonist of N-methyl-d-aspartate receptor. Activation of the receptor results in production of nitric oxide (^. NO), a molecule that promotes retinal and choroidal neovascularization. These observations suggest possible roles of SR in CNV. With laser-injured CNV mice, we found that inactivation of SR-coding gene (Srr_null ) significantly reduced CNV volume, neovascular density, and invading macrophages. We exploited the underlying mechanism in vivo and ex vivo. RPE from wild-type (WT) mice expressed SR. To explore the possible downstream target of SR inactivation, we showed that choroid/RPE homogenates extracted from laser-injured Srr_null mice contained less inducible nitric oxide synthase and decreased phospho-VEGFR2 compared to amounts in WT mice. In vitro, inflammation-primed WT RPEs expressed more inducible NOS, produced more^. NO and VEGF than did inflammation-primed Srr_null RPEs. When co-cultured with inflammation-primed Srr_null RPE, significantly fewer RF/6A-a cell line of choroidal endothelial cell, migrated to the opposite side of the insert membrane than did cells co-cultured with pre-treated WT RPE. Altogether, SR deficiency reduces RPE response to laser-induced inflammatory stimuli, resulting in decreased production of a cascade of pro-angiogenic cytokines, including^. NO and VEGF, and reduced macrophage recruitment, which contribute synergistically to attenuated angiogenesis.

Foxp3+ Tregs are recruited to the retina to repair pathological angiogenesis

Neovascular retinopathies are major causes of vision loss; yet treatments to prevent the condition are inadequate. The role of regulatory T cells in neovascular retinopathy is unknown. Here we show that in retinopathy regulatory T cells are transiently increased in lymphoid organs and the retina, but decline when neovascularization is established. The decline is prevented following regulatory T cells expansion with an IL-2/anti-IL-2 mAb complex or the adoptive transfer of regulatory T cells. Further, both approaches reduce vasculopathy (vaso-obliteration, neovascularization, vascular leakage) and alter the activation of Tmem119⁺ retinal microglia. Our in vitro studies complement these findings, showing that retinal microglia co-cultured with regulatory T cells exhibit a reduction in co-stimulatory molecules and pro-inflammatory mediators that is attenuated by CTLA-4 blockade. Collectively, we demonstrate that regulatory T cells are recruited to the retina and, when expanded in number, repair the vasculature. Manipulation of regulatory T cell numbers is a previously unrecognized, and promising avenue for therapies to prevent blinding neovascular retinopathies.

The local immune responses in the eye are attenuated to preserve sight. Surprisingly, Deliyanti et al. show that regulatory T cells (Tregs) take an active role in protecting the eye from neovascularization in oxygen-induced retinopathy, and that interventions that augment the retinal Treg numbers reduce neovascular retinopathy in mice.

Newly Formed Endothelial Cells Regulate Myeloid Cell Activity Following Spinal Cord Injury via Expression of CD200 Ligand

The central nervous system (CNS) is endowed with several immune-related mechanisms that contribute to its protection and maintenance in homeostasis and under pathology. Here, we discovered an additional mechanism that controls inflammatory responses within the CNS milieu under injurious conditions, involving CD200 ligand (CD200L) expressed by newly formed endothelial cells. We observed that CD200L is constitutively expressed in the mouse healthy CNS by endothelial cells of the blood-cerebrospinal fluid barrier and of the spinal cord meninges, but not by the endothelium of the blood-spinal cord barrier. Following spinal cord injury (SCI), newly formed endothelial cells, located only at the epicenter of the lesion site, expressed CD200L. Moreover, in the absence of CD200L expression by CNS-resident cells, functional recovery of mice following SCI was impaired. High throughput single-cell flow cytometry image analysis following SCI revealed CD200L-dependent direct interaction between endothelial and local CD200R⁺ myeloid cells, including activated microglia and infiltrating monocyte-derived macrophages (mo-MΦ). Absence of CD200L signaling, both in vitro and in vivo, resulted in a higher inflammatory response of the encountering macrophages, manifested by elevation in mRNA expression of Tnfα and Il1β, increased intracellular TNFα immunoreactivity, and reduced expression levels of macrophage factors that are associated with resolution of inflammation, Dectin-1, CD206 (mannose receptor), and IL-4R. Collectively, our results highlight the importance of CD200-mediated immune dialogue between endothelial cells and the local resident microglia and infiltrating mo-MΦ within the lesion area, as a mechanism that contributes to regulation of inflammation following acute CNS injury.

SIGNIFICANCE STATEMENT

This manuscript focuses on a novel mechanism of inflammation-regulation following spinal cord injury (SCI), orchestrated by CD200-ligand (CD200L) expressed by newly formed endothelial cells within the lesion site. Our study reveals that, in homeostasis, CD200L is expressed by endothelial cells of the mouse blood-cerebrospinal fluid barrier and of the blood-leptomeningeal barrier, but not by endothelial cells of the blood-spinal cord barrier. Following SCI, newly formed endothelial cells located within the epicenter of the lesion site were found to express CD200L at time points that were shown to be critical for repair. Our results reveal a direct interaction between CD200L⁺ endothelial cells and CD200R⁺ microglia and macrophages, resulting in attenuated inflammation, biasing macrophage phenotype toward inflammation-resolving cells, and promotion of functional recovery following SCI.

MMP-3 Deficiency Alleviates Endotoxin-Induced Acute Inflammation in the Posterior Eye Segment

Matrix metalloproteinase-3 (MMP-3) is known to mediate neuroinflammatory processes by activating microglia, disrupting blood-central nervous system barriers and supporting neutrophil influx into the brain. In addition, the posterior part of the eye, more specifically the retina, the retinal pigment epithelium (RPE) and the blood-retinal barrier, is affected upon neuroinflammation, but a role for MMP-3 during ocular inflammation remains elusive. We investigated whether MMP-3 contributes to acute inflammation in the eye using the endotoxin-induced uveitis (EIU) model. Systemic administration of lipopolysaccharide induced an increase in MMP-3 mRNA and protein expression level in the posterior part of the eye. MMP-3 deficiency or knockdown suppressed retinal leukocyte adhesion and leukocyte infiltration into the vitreous cavity in mice subjected to EIU. Moreover, retinal and RPE mRNA levels of intercellular adhesion molecule 1 (Icam1), interleukin 6 (Il6), cytokine-inducible nitrogen oxide synthase (Nos2) and tumor necrosis factor α (Tnfα), which are key molecules involved in EIU, were clearly reduced in MMP-3 deficient mice. In addition, loss of MMP-3 repressed the upregulation of the chemokines monocyte chemoattractant protein (MCP)-1 and (C-X-C motif) ligand 1 (CXCL1). These findings suggest a contribution of MMP-3 during EIU, and its potential use as a therapeutic drug target in reducing ocular inflammation.

The retinal pigment epithelium as a gateway for monocyte trafficking into the eye

The choroid plexus epithelium within the brain ventricles orchestrates blood-derived monocyte entry to the central nervous system under injurious conditions, including when the primary injury site is remote from the brain. Here, we hypothesized that the retinal pigment epithelium (RPE) serves a parallel role, as a gateway for monocyte trafficking to the retina following direct or remote injury. We found elevated expression of genes encoding leukocyte trafficking determinants in mouse RPE as a consequence of retinal glutamate intoxication or optic nerve crush (ONC). Blocking VCAM-1 after ONC interfered with monocyte infiltration into the retina and resulted in a local pro-inflammatory cytokine bias. Live imaging of the injured eye showed monocyte accumulation first in the RPE, and subsequently in the retina, and peripheral leukocytes formed close contact with the RPE Our findings further implied that the ocular milieu can confer monocytes a phenotype advantageous for neuroprotection. These results suggest that the eye utilizes a mechanism of crosstalk with the immune system similar to that of the brain, whereby epithelial barriers serve as gateways for leukocyte entry.