The structure and function of the human choroid

In this manuscript, the structure of the human choroid is reviewed with emphasis of the macro- and microscopic anatomy including Bruch's membrane, choriocapillaris, Sattler's and Haller's layer, and the suprachoroid. We here discuss the development of the choroid, as well as the question of choroidal lymphatics, and further the neuronal control of this tissue, as well as the pathologic angiogenesis. Wherever possible, functional aspects of the various structures are included and reviewed.

PI3K signaling promotes formation of lipid-laden foamy macrophages at the spinal cord injury site

After spinal cord injury (SCI), infiltrating macrophages undergo excessive phagocytosis of myelin and cellular debris, forming lipid-laden foamy macrophages. To understand their role in the cellular pathology of SCI, investigation of the foamy macrophage phenotype in vitro revealed a pro-inflammatory profile, increased reactive oxygen species (ROS) production, and mitochondrial dysfunction. Bioinformatic analysis identified PI3K as a regulator of inflammation in foamy macrophages, and inhibition of this pathway decreased their lipid content, inflammatory cytokines, and ROS production. Macrophage-specific inhibition of PI3K using liposomes significantly decreased foamy macrophages at the injury site after a mid-thoracic contusive SCI in mice. RNA sequencing and in vitro analysis of foamy macrophages revealed increased autophagy and decreased phagocytosis after PI3K inhibition as potential mechanisms for reduced lipid accumulation. Together, our data suggest that the formation of pro-inflammatory foamy macrophages after SCI is due to the activation of PI3K signaling, which increases phagocytosis and decreases autophagy.

Endothelial progenitor cell-derived exosomes promote anti-inflammatory macrophages via SOCS3/JAK2/STAT3 axis and improve the outcome of spinal cord injury

BACKGROUND

Macrophage in the spinal cord injury (SCI) area imparts a chronic pro-inflammation effect that challenges the recovery of SCI. Previously, endothelial progenitor cell-produced exosomes (EPC-EXOs) have been noticed to facilitate revascularization and inflammation control after SCI. However, their effects on macrophage polarization remained unclear. This study aimed to investigate the EPC-EXOs' role in macrophage polarization and reveal its underlying mechanism.

METHODS

We extracted the macrophages and EPC from the bone marrow suspension of C57BL/L mice by centrifugation. After cell identification, the EPC-EXOs were collected by ultra-high-speed centrifugation and exosome extraction kits and identified by transmission electron microscopy and nanoparticle tracking analysis. Then, macrophages were cultured with EPC-EXOs in different concentrations. We labeled the exosome to confirm its internalization by macrophage and detected the macrophage polarization marker level both in vitro and in vivo. We further estimated EPC-EXOs' protective effects on SCI by mice spinal cord tissue H&E staining and motor behavior evaluation. Finally, we performed RT-qPCR to identify the upregulated miRNA in EPC-EXOs and manipulate its expression to estimate its role in macrophage polarization, SOCS3/JAK2/STAT3 pathway activation, and motor behavior improvement.

RESULTS

We found that EPC-EXOs decreased the macrophages' pro-inflammatory marker expression and increased their anti-inflammatory marker expression on the 7 and 14 days after SCI. The spinal cord H&E staining results showed that EPC-EXOs raised the tissue-sparing area rate significantly after 28 days of SCI and the motor behavior evaluation indicated an increased BMS score and motor-evoked potential by EPC-EXOs treatment after SCI. The RT-qPCR assay identified that miR-222-3P upregulated in EPC-EXOs and its miRNA-mimic also decreased the pro-inflammatory macrophages and increased the anti-inflammatory macrophages. Additionally, miR-222-3P mimic activated the SOCS3/JAK2/STAT3 pathway, and SOCS3/JAK2/STAT3 pathway inhibition blocked miR-2223P's effects on macrophage polarization and mouse motor behavior.

CONCLUSION

Comprehensively, we discovered that EPC-EXOs-derived miR-222-3p affected macrophage polarization via SOCS3/JAK2/STAT3 pathway and promoted mouse functional repair after SCI, which reveals EPC-EXOs' role in modulation of macrophage phenotype and will provide a novel interventional strategy to induce post-SCI recovery.

Parabiosis reveals the correlation between the recruitment of circulating antigen presenting cells to the retina and the induction of spontaneous autoimmune uveoretinitis

BACKGROUND

Characterizing immune cells and conditions that govern their recruitment and function in autoimmune diseases of the nervous system or in neurodegenerative processes is an area of active investigation. We sought to analyze the origin of antigen presenting cells associated with the induction of retinal autoimmunity using a system that relies on spontaneous autoimmunity, thus avoiding uncertainties associated with immunization with adjuvants at remotes sites or adoptive transfer of in vitro activated T cells.

METHODS

R161H mice (B10.RIII background), which spontaneously and rapidly develop severe spontaneous autoimmune uveoretinitis (SAU), were crossed to CD11cDTR/GFP mice (B6/J) allowing us to track the recruitment to and/or expansion within the retina of activated, antigen presenting cells (GFPhi cells) in R161H+/- × CD11cDTR/GFP F1 mice relative to the course of SAU. Parabiosis between R161H+/- × CD11cDTR/GFP F1 mice and B10.RIII × B6/J F1 (wild-type recipient) mice was done to explore the origin and phenotype of antigen presenting cells crucial for the induction of autoimmunity. Analysis was done by retinal imaging, flow cytometry, and histology.

RESULTS

Onset of SAU in R161H+/- × CD11cDTR/GFP F1 mice was delayed relative to B10.RIII-R161H+/- mice revealing a disease prophase prior to frank autoimmunity that was characterized by expansion of GFPhi cells within the retina prior to any clinical or histological evidence of autoimmunity. Parabiosis between mice carrying the R161H and CD11cDTR/GFP transgenes and transgene negative recipients showed that recruitment of circulating GFPhi cells into retinas was highly correlative with the occurrence of SAU.

CONCLUSIONS

Our results here contrast with our previous findings showing that retinal antigen presenting cells expanding in response to either sterile mechanical injury or neurodegeneration were derived from myeloid cells within the retina or optic nerve, thus highlighting a unique facet of retinal autoimmunity.

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.

Hyalocyte origin, structure, and imaging

Introduction

Hyalocytes have been recognized as resident tissue macrophages of the vitreous body since the mid-19^th century. Despite this, knowledge about their origin, turnover, and dynamics is limited.

Areas covered

Historically, initial studies on the origin of hyalocytes used light and electron microscopy. Modern investigations across species including rodents and humans will be described. Novel imaging is now available to study human hyalocytes in vivo. The shared ontogeny with retinal microglia and their eventual interdependence as well as differences will be discussed.

Expert opinion

Owing to a common origin as myeloid cells, hyalocytes and retinal microglia have similarities, but hyalocytes appear to be distinct as resident macrophages of the vitreous body.

Separated parabiont reveals the fate and lifespan of peripheral-derived immune cells in normal and ischaemia-induced injured kidneys

Immune cell infiltration plays a key role in acute kidney injury (AKI) to chronic kidney disease (CKD) progression. T lymphocytes, neutrophils, monocytes/macrophages and other immune cells regulate inflammation, tissue remodelling and repair. To determine the kinetics of accumulation of various immune cell populations, we established an animal model combining parabiosis and separation surgery to explore the fate and lifespan of peripheral leucocytes that migrate to the kidney. We found that peripheral T lymphocytes could survive for a long time (more than 14 days), whereas peripheral neutrophils survived for a short time in both healthy and ischaemia-induced damaged kidneys. Nearly half of the peripheral-derived macrophages disappeared after 14 days in normal kidneys, while their existing time in the inflammatory kidneys was prolonged. A fraction of F4/80^high macrophages were renewed from the circulating monocyte pool. In addition, we found that after renal ischaemia reperfusion, neutrophils increased significantly in the early phase, and T lymphocytes mainly accumulated in the late stage, whereas macrophages infiltrated throughout AKI-CKD progression and were sustained longer in injured as opposed to normal kidneys. In conclusion, peripheral-derived macrophages, T lymphocytes and neutrophils exhibit different lifespans in the kidney, which may play different roles during AKI-CKD progression.

Melanoblasts Populate the Mouse Choroid Earlier in Development Than Previously Described

Purpose

Human choroidal melanocytes become evident in the last trimester of development, but very little is known about them. To better understand normal and diseased choroidal melanocyte biology we examined their precursors, melanoblasts (MB), in mouse eyes during development, particularly their relation to the developing vasculature and immune cells.

Methods

Naïve B6(Cg)-Tyrc-2J/J albino mice were used between embryonic (E) day 15.5 and postnatal (P) day 8, with adult controls. Whole eyes, posterior segments, or dissected choroidal wholemounts were stained with antibodies against tyrosinase-related protein 2, ionized calcium binding adaptor molecule-1 or isolectin B4, and examined by confocal microscopy. Immunoreactive cell numbers in the choroid were quantified with Imaris. One-way ANOVA with Tukey's post hoc test assessed statistical significance.

Results

Small numbers of MB were present in the presumptive choroid at E15.5 and E18.5. The density significantly increased between E18.5 (381.4 ± 45.8 cells/mm2) and P0 (695.2 ± 87.1 cells/mm2; P = 0.032). In postnatal eyes MB increased in density and formed multiple layers beneath the choriocapillaris. MB in the periocular mesenchyme preceded the appearance of vascular structures at E15.5. Myeloid cells (Ionized calcium binding adaptor molecule-1-positive) were also present at high densities from this time, and attained adult-equivalent densities by P8 (556.4 ± 73.6 cells/mm2).

Conclusions

We demonstrate that choroidal MB and myeloid cells are both present at very early stages of mouse eye development (E15.5). Although MB and vascularization seemed to be unlinked early in choroidal development, they were closely associated at later stages. MB did not migrate into the choroid in waves, nor did they have a consistent relationship with nerves.

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.

Microglia in Retinal Degeneration

The retina is a complex tissue with multiple cell layers that are highly ordered. Its sophisticated structure makes it especially sensitive to external or internal perturbations that exceed the homeostatic range. This necessitates the continuous surveillance of the retina for the detection of noxious stimuli. This task is mainly performed by microglia cells, the resident tissue macrophages which confer neuroprotection against transient pathophysiological insults. However, under sustained pathological stimuli, microglial inflammatory responses become dysregulated, often worsening disease pathology. In this review, we provide an overview of recent studies that depict microglial responses in diverse retinal pathologies that have degeneration and chronic immune reactions as key pathophysiological components. We also discuss innovative immunomodulatory therapy strategies that dampen the detrimental immunological responses to improve disease outcome.

Systemic exposure to CpG-ODN elicits low-grade inflammation in the retina

Previous studies have reported that topical exposure to the toll-like receptor (TLR) 9 ligand CpG-ODN causes widespread ocular inflammation, including retinal microglial activation and posterior segment inflammation. Here we sought to determine the effects of systemic exposure to CpG-ODN in the retina and whether this inflammatory response was altered with Cx₃cr1 deficiency or hyperglycemia. Male non-diabetic Cx₃cr1^+/gfp and Cx₃cr1^gfp/gfp littermates (normoglycemic controls) and Cx₃cr1^+/gfpIns2^Akitaand Cx₃cr1^gfp/gfpIns2^Akita diabetic mice were injected intraperitoneally with 40 μg CpG-ODN. Immunofluorescence staining was performed 1 week later to assess the expression of MHC Class II and glial fibrillary acidic protein (GFAP), as well as to identify morphological changes to microglia and changes in retinal macrophage cell density. Systemic exposure to CpG-ODN induced the upregulated expression of both GFAP on retinal Müller cells and MHC Class II on the retinal vasculature. Additionally, there was an increased accumulation of macrophages in the subretinal space 1 week after exposure to systemic CpG-ODN as well as characteristic morphological changes to microglia indicative of an activated phenotype. These preliminary studies demonstrate that low-grade inflammatory changes were not enhanced in Cx₃cr1-deficient or diabetic mice, indicating that the inflammatory response to systemic CpG-ODN in the retina is unaltered in the context of Cx₃cr1 deficiency or prolonged hyperglycemia.

The origin, fate, and contribution of macrophages to spinal cord injury pathology

Virtually all phases of spinal cord injury pathogenesis, including inflammation, cell proliferation and differentiation, as well as tissue remodeling, are mediated in part by infiltrating monocyte-derived macrophages. It is now clear that these infiltrating macrophages have distinct functions from resident microglia and are capable of mediating both harmful and beneficial effects after injury. These divergent effects have been largely attributed to environmental cues, such as specific cytokines, that influence the macrophage polarization state. In this review, we also consider the possibility that different macrophage origins, including the spleen, bone marrow, and local self-renewal, may also affect macrophage fate, and ultimately their function that contribute to the complex pathobiology of spinal cord injury.

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.

The Role of Microglia and Peripheral Monocytes in Retinal Damage after Corneal Chemical Injury

Eyes that have experienced alkali burn to the surface are excessively susceptible to subsequent severe glaucoma and retinal ganglion cell loss, despite maximal efforts to prevent or slow down the disease. Recently, we have shown, in mice and rabbits, that such retinal damage is neither mediated by the alkali itself reaching the retina nor by intraocular pressure elevation. Rather, it is caused by the up-regulation of tumor necrosis factor-α (TNF-α), which rapidly diffuses posteriorly, causing retinal ganglion cell apoptosis and CD45⁺ cell activation. Herein, we investigated the involvement of peripheral blood monocytes and microglia in retinal damage. Using CX3CR1^+/EGFP::CCR2^+/RFP reporter mice and bone marrow chimeras, we show that peripheral CX3CR1⁺CD45^hiCD11b⁺MHC-II⁺ monocytes infiltrate into the retina from the optic nerve at 24 hours after the burn and release further TNF-α. A secondary source of peripheral monocyte response originates from a rare population of patrolling myeloid CCR2⁺ cells of the retina that differentiate into CX3CR1⁺ macrophages within hours after the injury. As a result, CX3CR1⁺CD45^loCD11b⁺ microglia become reactive at 7 days, causing further TNF-α release. Prompt TNF-α inhibition after corneal burn suppresses monocyte infiltration and microglia activation, and protects the retina. This study may prove relevant to other injuries of the central nervous system.

Knockout of Ccr2 alleviates photoreceptor cell death in rodent retina exposed to chronic blue light

Age-related macular degeneration (AMD), the leading cause of visual loss after the age of 60 years, is a degenerative retinal disease involving a variety of environmental and hereditary factors. Although it has been implicated that immune system is involved in the disease progression, the exact role that microglia has is still unclear. Here we demonstrated that knockout of Ccr2 gene could alleviate photoreceptor cell death in mice retinas exposed to chronic blue light. In Ccr2^−/− mice, a damaged microglia recruitment was shown in retina and this could protect the visual function in electroretinogram and alleviate the photoreceptor apoptosis, which thus helped attenuate the blue light-induced retinopathy. We further found an increased co-location of NLRP3, Iba-1, and IL-1β in fluorescence and a concomitant increased protein expression of NLRP3, caspase-1, and IL-1β in western blotting in chronic blue light-induced retinopathy. Moreover, the activation of microglia and their cellular NLRP3 inflammasomes occurred as an earlier step before the structural and functional damage of the mice retinas, which collectively supported that microglial NLRP3 inflammasome might be the key to the chronic blue light-induced retinopathy.

Turning Down the Thermostat: Modulating the Endocannabinoid System in Ocular Inflammation and Pain

The endocannabinoid system (ECS) has emerged as an important regulator of both physiological and pathological processes. Notably, this endogenous system plays a key role in the modulation of pain and inflammation in a number of tissues. The components of the ECS, including endocannabinoids, their cognate enzymes and cannabinoid receptors, are localized in the eye, and evidence indicates that ECS modulation plays a role in ocular disease states. Of these diseases, ocular inflammation presents a significant medical problem, given that current clinical treatments can be ineffective or are associated with intolerable side-effects. Furthermore, a prominent comorbidity of ocular inflammation is pain, including neuropathic pain, for which therapeutic options remain limited. Recent evidence supports the use of drugs targeting the ECS for the treatment of ocular inflammation and pain in animal models; however, the potential for therapeutic use of cannabinoid drugs in the eye has not been thoroughly investigated at this time. This review will highlight evidence from experimental studies identifying components of the ocular ECS and discuss the functional role of the ECS during different ocular inflammatory disease states, including uveitis and corneal keratitis. Candidate ECS targeted therapies will be discussed, drawing on experimental results obtained from both ocular and non-ocular tissue(s), together with their potential application for the treatment of ocular inflammation and pain.

The impact of ranibizumab on the level of intercellular adhesion molecule type 1 in the vitreous of eyes with proliferative diabetic retinopathy

PURPOSE

This study was to investigate the impact of ranibizumab on the level of intercellular adhesion molecule type 1 (ICAM-1) in the vitreous of eyes with PDR.

METHODS

This is an interventional case-control study. A total of 82 eyes from 82 patients who had undergone vitreous surgery for the treatment of retinal disorders were included. Twenty-two eyes with PDR received an intravitreal ranibizumab injection (IVR) 3-7 days before vitrectomy and were grouped as 'PDR with recent IVR' or Group 1. Sixteen eyes with PDR received IVR more than 7 days before vitrectomy and were grouped as 'PDR with remote IVR' or Group 2. Twenty-two matched PDR eyes did not receive IVR before vitrectomy and were grouped as 'PDR without IVR' or Group 3. Finally, 22 eyes from 22 patients with idiopathic macular pucker (IMP) served as the 'non-diabetic control' group, or Group 4. Vitreous samples were obtained at the time of vitrectomy from all eyes, and the levels of vascular endothelium growth factor (VEGF) and ICAM-1 were analysed using ELISA.

RESULTS

PDR without IVR (Group 3) had the highest vitreous VEGF concentration; the difference was significant compared with those in the PDR with recent IVR (Group 1), PDR with remote IVR (Group 2) and the non-diabetic control group (Group 4) (p < 0.001). Group 2 had a lower vitreous VEGF level than Group 1 (p = 0.041). Group 1 had the highest vitreous ICAM-1 levels (p < 0.001 versus. Groups 2, 3 and 4); Group 2 had a lower vitreous ICAM-1 level than Group 3 (p = 0.028).

CONCLUSION

The vitreous fluid level of ICAM-1 was significantly increased within 1 week of IVR administration, but markedly decreased after a week of administration in eyes with PDR. This suggests that leucostasis, vascular leakage and endothelial dysfunction may be amplified in the early days after IVR, but that a therapeutic effect of IVR in these processes may appear after 1 week of ranibizumab administration in eyes with PDR.

Macrophages and Uveitis in Experimental Animal Models

Resident and infiltrated macrophages play relevant roles in uveitis as effectors of innate immunity and inductors of acquired immunity. They are major effectors of tissue damage in uveitis and are also considered to be potent antigen-presenting cells. In the last few years, experimental animal models of uveitis have enabled us to enhance our understanding of the leading role of macrophages in eye inflammation processes, including macrophage polarization in experimental autoimmune uveoretinitis and the major role of Toll-like receptor 4 in endotoxin-induced uveitis. This improved knowledge should guide advantageous iterative research to establish mechanisms and possible therapeutic targets for human uveitis resolution.

Differences in the distribution, phenotype and gene expression of subretinal microglia/macrophages in C57BL/6N (Crb1 rd8/rd8) versus C57BL6/J (Crb1 wt/wt) mice

Background

Microglia/macrophages (MG/MΦ) are found in the subretinal space in both mice and humans. Our goal was to study the spatial and temporal distribution, the phenotype, and gene expression of subretinal MG/MΦ in mice with normal retinas and compare them to mice with known retinal pathology.

Methods

We studied C57BL/6 mice with (C57BL/6N), or without (C57BL/6J) the rd8 mutation in the Crb1 gene (which, in the presence of yet unidentified permissive/modifying genes, leads to a retinal degeneration), and documented their fundus appearance and the change with aging. Immunostaining of retinal pigment epithelium (RPE) flat mounts was done for 1) Ionized calcium binding adaptor (Iba)-1, 2) FcγIII/II Receptor (CD16/CD32, abbreviated as CD16), and 3) Macrophage mannose receptor (MMR). Reverse-transcription quantitative PCR (RT-qPCR) was done for genes involved in oxidative stress, complement activation and inflammation.

Results

The number of yellow fundus spots correlated highly with subretinal Iba-1+ cells. The total number of subretinal MG/MΦ increased with age in the rd8 mutant mice, but not in the wild-type (WT) mice. There was a centripetal shift in the distribution of the subretinal MG/MΦ with age. Old rd8 mutant mice had a greater number of CD16+ MG/MΦ. CD16+ cells had morphological signs of activation, and this was most prominent in old rd8 mutant mice (P <1×10⁻⁸ versus old WT mice). Subretinal MG/MΦ in rd8 mutant mice also expressed iNOS and MHC-II, and had ultrastructural signs of activation. Finally, rd8 mutant mouse RPE/ MG/MΦ RNA isolates showed an upregulation of Ccl2, CFB, C3, NF-kβ, CD200R and TNF-alpha. The retinas of rd8 mutant mice showed upregulation of HO-1, C1q, C4, and Nrf-2.

Conclusions

When compared to C57BL/6J mice, C57BL/6N mice demonstrate increased accumulation of subretinal MG/MΦ, displaying phenotypical, morphological, and gene-expression characteristics consistent with a pro-inflammatory shift. These changes become more prominent with aging and are likely due to the combination of the rd8 mutation and yet unidentified permissive/modulatory genes in the C57BL/6N mice. In contrast, aging leads to a scavenging phenotype in the C57BL/6J subretinal microglia/macrophages.

Electronic supplementary material

The online version of this article (doi:10.1186/s12974-014-0221-4) contains supplementary material, which is available to authorized users.

Retinal microglia: just bystander or target for therapy?

Resident microglial cells can be regarded as the immunological watchdogs of the brain and the retina. They are active sensors of their neuronal microenvironment and rapidly respond to various insults with a morphological and functional transformation into reactive phagocytes. There is strong evidence from animal models and in situ analyses of human tissue that microglial reactivity is a common hallmark of various retinal degenerative and inflammatory diseases. These include rare hereditary retinopathies such as retinitis pigmentosa and X-linked juvenile retinoschisis but also comprise more common multifactorial retinal diseases such as age-related macular degeneration, diabetic retinopathy, glaucoma, and uveitis as well as neurological disorders with ocular manifestation. In this review, we describe how microglial function is kept in balance under normal conditions by cross-talk with other retinal cells and summarize how microglia respond to different forms of retinal injury. In addition, we present the concept that microglia play a key role in local regulation of complement in the retina and specify aspects of microglial aging relevant for chronic inflammatory processes in the retina. We conclude that this resident immune cell of the retina cannot be simply regarded as bystander of disease but may instead be a potential therapeutic target to be modulated in the treatment of degenerative and inflammatory diseases of the retina.

c-Src function is necessary and sufficient for triggering microglial cell activation

Microglial cells are the resident macrophages of the central nervous system. Their function is essential for neuronal tissue homeostasis. After inflammatory stimuli, microglial cells become activated changing from a resting and highly ramified cell shape to an amoeboid-like morphology. These morphological changes are associated with the release of proinflammatory cytokines and glutamate, as well as with high phagocytic activity. The acquisition of such phenotype has been associated with activation of cytoplasmic tyrosine kinases, including those of the Src family (SFKs). In this study, using both in vivo and in vitro inflammation models coupled to FRET-based time-lapse microscopy, lentiviruses-mediated shRNA delivery and genetic gain-of-function experiments, we demonstrate that among SFKs c-Src function is necessary and sufficient for triggering microglia proinflammatory signature, glutamate release, microglia-induced neuronal loss, and phagocytosis. c-Src inhibition in retinal neuroinflammation experimental paradigms consisting of intravitreal injection of LPS or ischemia-reperfusion injury significantly reduced microglia activation changing their morphology to a more resting phenotype and prevented neuronal apoptosis. Our data demonstrate an essential role for c-Src in microglial cell activation.

Microglia in mouse retina contralateral to experimental glaucoma exhibit multiple signs of activation in all retinal layers

Background

Glaucomatous optic neuropathy, a leading cause of blindness, can progress despite control of intraocular pressure - currently the main risk factor and target for treatment. Glaucoma progression shares mechanisms with neurodegenerative disease, including microglia activation. In the present model of ocular hypertension (OHT), we have recently described morphological signs of retinal microglia activation and MHC-II upregulation in both the untreated contralateral eyes and OHT eyes. By using immunostaining, we sought to analyze and quantify additional signs of microglia activation and differences depending on the retinal layer.

Methods

Two groups of adult Swiss mice were used: age-matched control (naïve, n = 12), and lasered (n = 12). In the lasered animals, both OHT eyes and contralateral eyes were analyzed. Retinal whole-mounts were immunostained with antibodies against Iba-1, MHC-II, CD68, CD86, and Ym1. The Iba-1+ cell number in the plexiform layers (PL) and the photoreceptor outer segment (OS), Iba-1+ arbor area in the PL, and area of the retina occupied by Iba-1+ cells in the nerve fiber layer-ganglion cell layer (NFL-GCL) were quantified.

Results

The main findings in contralateral eyes and OHT eyes were: i) ameboid microglia in the NFL-GCL and OS; ii) the retraction of processes in all retinal layers; iii) a higher level of branching in PL and in the OS; iv) soma displacement to the nearest cell layers in the PL and OS; v) the reorientation of processes in the OS; vi) MHC-II upregulation in all retinal layers; vii) increased CD68 immunostaining; and viii) CD86 immunolabeling in ameboid cells. In comparison with the control group, a significant increase in the microglial number in the PL, OS, and in the area occupied by Iba-1+ cells in the NFL-GCL, and significant reduction of the arbor area in the PL. In addition, rounded Iba-1+ CD86+ cells in the NFL-GCL, OS and Ym1+ cells, and rod-like microglia in the NFL-GCL were restricted to OHT eyes.

Conclusions

Several quantitative and qualitative signs of microglia activation are detected both in the contralateral and OHT eyes. Such activation extended beyond the GCL, involving all retinal layers. Differences between the two eyes could help to elucidate glaucoma pathophysiology.

VEGF-production by CCR2-dependent macrophages contributes to laser-induced choroidal neovascularization

Age-related macular degeneration (AMD) is the most prevalent cause of blindness in the elderly, and its exsudative subtype critically depends on local production of vascular endothelial growth factor A (VEGF). Mononuclear phagocytes, such as macrophages and microglia cells, can produce VEGF. Their precursors, for example monocytes, can be recruited to sites of inflammation by the chemokine receptor CCR2, and this has been proposed to be important in AMD. To investigate the role of macrophages and CCR2 in AMD, we studied intracellular VEGF content in a laser-induced murine model of choroidal neovascularisation. To this end, we established a technique to quantify the VEGF content in cell subsets from the laser-treated retina and choroid separately. 3 days after laser, macrophage numbers and their VEGF content were substantially elevated in the choroid. Macrophage accumulation was CCR2-dependent, indicating recruitment from the circulation. In the retina, microglia cells were the main VEGF+ phagocyte type. A greater proportion of microglia cells contained VEGF after laser, and this was CCR2-independent. On day 6, VEGF-expressing macrophage numbers had already declined, whereas numbers of VEGF+ microglia cells remained increased. Other sources of VEGF detectable by flow cytometry included in dendritic cells and endothelial cells in both retina and choroid, and Müller cells/astrocytes in the retina. However, their VEGF content was not increased after laser. When we analyzed flatmounts of laser-treated eyes, CCR2-deficient mice showed reduced neovascular areas after 2 weeks, but this difference was not evident 3 weeks after laser. In summary, CCR2-dependent influx of macrophages causes a transient VEGF increase in the choroid. However, macrophages augmented choroidal neovascularization only initially, presumably because VEGF production by CCR2-independent eye cells prevailed at later time points. These findings identify macrophages as a relevant source of VEGF in laser-induced choroidal neovascularization but suggest that the therapeutic efficacy of CCR2-inhibition might be limited.

Vascular associations and dynamic process motility in perivascular myeloid cells of the mouse choroid: implications for function and senescent change

PURPOSE

Immune and vascular alterations in the choroid are implicated in age-related macular degeneration (AMD). As choroidal immune cells are incompletely understood with regard to their physiology and interactions with choroidal vessels, we examined the associations between myeloid and vascular components of the choroid in young and aged mice.

METHODS

Albino CX3CR1(GFP/+) transgenic mice, whose choroidal myeloid cells possess green fluorescence, were perfused intraluminally with the vital dye DiI to label choroidal vessels. The distribution, morphology, behavior, and vascular associations of resident myeloid cells were examined using time-lapse live confocal imaging and immunohistochemical analysis.

RESULTS

Dendritiform myeloid cells, comprising most of the resident immune cell population in the choroid, were widely distributed across the choroid and demonstrated close associations with choroidal vessels that varied with their position in the vascular tree. Notably, myeloid cells associated with choroidal arteries and arterioles appeared as elongated cells flanking the long axes of vessels, whereas those associated with the choriocapillaris were distributed as a layer of stellate cells on the scleral but not vitreal choriocapillaris surface. While stationary in position, dendritiform myeloid cells demonstrated the rapid process dynamism well suited to comprehensive immunosurveillance of the perivascular space. Myeloid cells also increased in density as a function of aging, correlating locally with greater choroidal vascular attenuation.

CONCLUSIONS

Resident myeloid cells demonstrated close but dynamic physical interactions with choroidal vessels, indicative of constitutive immune-vascular interactions in the normal choroid. These interactions may alter progressively with aging, providing a basis for understanding age-related choroidal dysfunction underlying AMD.

Subretinal infiltration of monocyte derived cells and complement misregulation in mice with AMD-like pathology

We have characterized a naturally-occurring mutation in mice that causes slow, progressive photoreceptor degeneration, white fundus flecks, and late-onset RPE atrophy. These animals predictably lose visual function as photoreceptors degenerate. Genetic studies identified a deletion in the 5' coding sequence of Mfrp, designated Mfrp (174delG) , which essentially results in a complete knockout at the protein level. We have shown in Mfrp (174delG) mice that these white fundus flecks are due to the presence of F4/80+ inflammatory cells in the subretinal space. Here we expand on our initial description of the cells with additional markers and by determining their origin. We have also begun an analysis of complement factors in the RPE and found decreased levels of C3d, suggesting that the alternative complement pathway may be misregulated. Finally, we compare and contrast the characteristics of fundus images in Mfrp (174delG) mice with those of other mutations that cause similar irregularities, including Crb1 (rd8) and RDH5, and discuss the structural differences that may underlie them.

Characterization and inflammatory response of perivascular-resident macrophage-like melanocytes in the vestibular system

A large number of perivascular cells expressing both macrophage and melanocyte characteristics (named perivascular-resident macrophage-like melanocytes, PVM/Ms), previously found in the intra-strial fluid–blood barrier, are also found in the blood–labyrinth barrier area of the vestibular system in normal adult cochlea, including in the three ampullae of the semicircular canals (posterior, superior, and horizontal), utricle, and saccule. The cells were identified as PVM/Ms, positive for the macrophage and melanocyte marker proteins F4/80 and GSTα4. Similar to PVM/Ms present in the stria vascularis, the PVM/Ms in the vestibular system are closely associated with microvessels and structurally intertwined with endothelial cells and pericytes, with a density in normal (unstimulated) utricle of 225 ± 43/mm²; saccule 191 ± 25/mm²; horizontal ampullae 212 ± 36/mm²; anterior ampullae 238 ± 36/mm²; and posterior ampullae 223 ± 64/mm². Injection of bacterial lipopolysaccharide into the middle ear through the tympanic membrane causes the PVM/Ms to activate and arrange in an irregular pattern along capillary walls in all regions within a 48-h period. The inflammatory response significantly increases vascular permeability and leakage. The results underscore the morphological complexity of the blood barrier in the vestibular system, with its surrounding basal lamina, pericytes, as well as second line of defense in PVM/Ms. PVM/Ms may be important to maintain blood barrier integrity and initiating local inflammatory response in the vestibular system.

The effects of CX3CR1 deficiency and irradiation on the homing of monocyte-derived cell populations in the mouse eye

This study examined whether CX3CR1 deficiency altered monocytic cell replenishment dynamics in ocular tissues in the context of radiation chimeras. Long-term effects of irradiation and effects of sublethal irradiation on ocular macrophages were also assessed. Bone marrow from BALB/c Cx 3 cr1 (+/gfp) or Cx 3 cr1 (gfp/gfp) mice was used to reconstitute full body irradiated WT mice and donor cell densities in the uveal tract were compared at 4 and 8 weeks post-transplantation. BALB/c and C57BL/6J chimeric mice were examined at 6 months of age to determine strain-related differences in microglial replenishment and radiation sensitivity. A separate cohort of mice were sublethally irradiated (5.5 Gy) and retinal tissue assessed 8 and 12 weeks later. CX3CR1 deficiency altered the early replenishment of monocytes in the posterior iris but not in the iris stroma, choroid or retina. In six month old chimeric mice, there were significantly higher GFP(+) cell densities in the uveal tract when compared to non-irradiated 8-12 week old Cx 3 cr1 (+/gfp) mice. Additionally, MHC Class II expression was upregulated on hyalocytes and GFP(+) cells in the peripheral retina and the repopulation of microglia appeared to be more rapid in C57BL/6J mice compared to BALB/c mice. Transient expression of MHC Class II was observed on retinal vasculature in sublethally irradiated mice. These data indicate CX3CR1-deficiency only slightly alters monocyte-derived cell replenishment in the murine uveal tract. Lethal irradiation leads to long-term increase in monocytic cell density in the uveal tract and retinal microglial activation, possibly as a sequelae to local irradiation induced injury. Microglial replenishment in this model appears to be strain dependent.

VEGF induces neuroglial differentiation in bone marrow-derived stem cells and promotes microglia conversion following mobilization with GM-CSF

PURPOSE

Evaluation of potential tropic effects of vascular endothelial growth factor (VEGF) on the incorporation and differentiation of bone-marrow-derived stem cells (BMSCs) in a murine model of anterior ischemic optic neuropathy (AION).

METHODS

In the first approach, small-sized subset of BMCs were isolated from GFP donors mice by counterflow centrifugal elutriation and depleted of hematopoietic lineages (Fr25lin(-)). These cells were injected into a peripheral vein (1 × 10(6) in 0.2 ml) or inoculated intravitreally (2 × 10(5)) to syngeneic mice, with or without intravitreal injection of 5 μg/2μL VEGF, simultaneously with AION induction. In a second approach, hematopoietic cells were substituted by myelablative transplant of syngeseic GFP + bone marrow cells. After 3 months, progenitors were mobilized with granulocyte-macrophage colony-stimulating factor (GM-CSF) followed by VEGF inoculation into the vitreous body and AION induction . Engraftment and phenotype were examined by immunohistochemistry and FISH at 4 and 24 weeks post-transplantation, and VEGF receptors were determined by real time PCR.

RESULTS

VEGF had no quantitative effect on incorporation of elutriated cells in the injured retina, yet it induced early expression of neuroal markers in cells incorporated in the RGC layer and promoted durable gliosis, most prominent perivascular astrocytes. These effects were mediated by VEGF-R1/Flt-1, which is constitutively expresses in the elutriated fraction of stem cells. Mobilization with GM-CSF limited the differentiation of bone marrow progenitors to microglia, which was also fostered by VEGF.

CONCLUSIONS

VEGF signaling mediated by Flt-1 induces early neural and sustained astrocytic differentiation of stem cells elutriated from adult bone-marrow, with significant contribution to stabilization retinal architecture following ischemic injury.

Cell-based therapies for ocular inflammation

Since the plasticity and the potential for re-programming cells has become widely accepted, there has been great interest in cell-based therapies. These are being applied to a range of diseases, not least ocular diseases, where it is assumed that there is a reduced risk of immune rejection although this may be more perceived than real. There are two broad classes of cell-based therapies: those aimed at restoring structure and function of specific tissues and cells; and those directed towards restoring immunological homeostasis by controlling the damaging effects of inflammatory disease. Stem cells of all types represent the first group and prototypically have been used with the aim of regenerating failing cells. In contrast, immune cells have been suggested as potential modulators of inflammation. However, there is functional overlap in these two applications, with some types of stem cells, such as mesenchymal stem cells, demonstrating a potent immunomodulatory effect. This review summarises recent information on cell based therapies for ocular disease, with special emphasis on ocular inflammatory disease, and explores current uses, potential and limitations.

Interferon γ-dependent migration of microglial cells in the retina after systemic cytomegalovirus infection

Microglial cells are the resident macrophages of the central nervous system and participate in both innate and adaptive immune responses but can also lead to exacerbation of neurodegenerative pathologies after viral infections. Microglia in the outer layers of the retina and the subretinal space are thought to be involved in retinal diseases where low-grade chronic inflammation and oxidative stress play a role. This study investigated the effect of systemic infection with murine cytomegalovirus on the distribution and dynamics of retinal microglia cells. Systemic infection with murine cytomegalovirus elicited a significant increase in the number of microglia in the subretinal space and an accumulation of iris macrophages, along with morphological signs of activation. Interferon γ (IFN-γ)-deficient mice failed to induce changes in microglia distribution. Bone marrow chimera experiments confirmed that microglial cells in the subretinal space were not recruited from the circulating monocyte pool, but rather represented an accumulation of resident microglial cells from within the retina. Our results demonstrate that a systemic viral infection can lead to IFN-γ-mediated accumulation of microglia into the outer retinal layers and offer proof of concept that systemic viral infections alter the ocular microenvironment and therefore, may influence the course of diseases such as macular degeneration, diabetic retinopathy, or autoimmune uveitis, where low-grade inflammation is implicated.

CX3CL1 expression in the conjunctiva is involved in immune cell trafficking during toxic ocular surface inflammation

Inappropriate expression of the chemokine CX3CL1 is reportedly known to act on inflammatory conditions in extraocular immune diseases. We studied the expression and effects of CX3CL1 in human patients, cultured human conjunctival cells, and transgenic mice exposed to benzalkonium chloride (BAC), a commonly used preservative in ophthalmic medications despite its proinflammatory properties, to determine whether CX3CL1 is involved in conjunctival inflammation. We report that CX3CL1 expression is increased in the conjunctiva of patients receiving BAC-containing medication, and correlates with clinical inflammation. BAC enhances the production of CX3CL1 in a conjunctival epithelial cell line, through the tumor-necrosis factor-α pathway, which attracts specific leukocyte subsets. In vivo, BAC-induced macrophage infiltration and subsequent inflammation of the conjunctiva is decreased in CX3CR1-deficient mice as compared with CX3CR1(+/+) controls. This translational study opens new avenue to investigate ocular surface disorders by focusing on chemokine-related inflammation and immune cell trafficking in the ocular conjunctival mucosa.

Fas determines differential fates of resident and recruited macrophages during resolution of acute lung injury

RATIONALE

During acute lung injury (ALI) the macrophage pool expands markedly as inflammatory monocytes migrate from the circulation to the airspaces. As inflammation resolves, macrophage numbers return to preinjury levels and normal tissue structure and function are restored.

OBJECTIVES

To determine the fate of resident and recruited macrophages during the resolution of ALI in mice and to elucidate the mechanisms responsible for macrophage removal.

METHODS

ALI was induced in mice using influenza A (H1N1; PR8) infection and LPS instillation. Dye labeling techniques, bone marrow transplantation, and surface immunophenotyping were used to distinguish resident and recruited macrophages during inflammation and to study the role of Fas in determining macrophage fate during resolving ALI.

MEASUREMENTS AND MAIN RESULTS

During acute and resolving lung injury from influenza A and LPS, a high proportion of the original resident alveolar macrophages persisted. In contrast, recruited macrophages exhibited robust accumulation in early inflammation, followed by a progressive decline in their number. This decline was mediated by apoptosis with local phagocytic clearance. Recruited macrophages expressed high levels of the death receptor Fas and were rapidly depleted from the airspaces by Fas-activating antibodies. In contrast, macrophage depletion was inhibited in mice treated with Fas-blocking antibodies and in chimeras with Fas-deficient bone marrow. Caspase-8 inhibition prevented macrophage apoptosis and delayed the resolution of ALI.

CONCLUSIONS

These findings indicate that Fas-induced apoptosis of recruited macrophages is essential for complete resolution of ALI.

Endotoxin-induced uveitis is primarily dependent on radiation-resistant cells and on MyD88 but not TRIF

TLR4 activation by LPS (endotoxin) is mediated by the MyD88 and TRIF intracellular signaling pathways. We determined the relative activation of these pathways in murine ocular tissue after LPS exposure. Additionally, we explored whether BM-derived or non-BM-derived cells were the major contributors to EIU. Mice deficient in TRIF or MyD88 and their congenic (WT) controls received 250 ng ultrapure LPS ivt at 0 h. Ocular inflammation was assessed by histological analysis at 4, 6, and 24 h, and additionally, in MyD88(-/-) mice, intravital microscopy was performed at 4 h and 6 h to assess adherent, rolling, and infiltrating cells in the iris vasculature and tissue. Cytokines associated with the MyD88 and TRIF intracellular signaling pathways were analyzed in ocular tissue at 4 h. BM chimeric mice (WT→WT, TLR4(-/-)→WT, WT→TLR4(-/-)) received 250 ng LPS by ivt injection, and ocular tissues were examined by histology at 6 h. Lack of MyD88 resulted in a markedly diminished cellular response and reduced production of MyD88-related cytokines 4 h post-LPS treatment. In contrast, lack of TRIF led to reduced production of TRIF-related cytokines and no change in the cellular response to LPS. Therefore, the MyD88 pathway appears to be the dominant TLR4 pathway in EIU. Only WT → TLR4(-/-) chimeric mice were resistant to EIU, and this suggests, surprisingly, that non-BM-derived (radiation-resistant) cells in the eye play a greater role than BM-derived cells.

Accumulation of murine subretinal macrophages: effects of age, pigmentation and CX3CR1

Macrophages or activated microglia in the subretinal space are considered a hallmark of some retinal pathologies. We investigated the effects of age, pigmentation and CX(3)CR1 deficiency on the accumulation of macrophages/activated microglia in the outer retina of young and old Cx(3)cr1(gfp/gfp) (CX(3)CR1-deficient) or Cx(3)cr1(gfp/+) mice on either a pigmented (C57BL/6) or albino (BALB/c) background. Quantitative analysis of immunostained retinal-choroidal whole mounts revealed an increase in subretinal macrophage (SRMΦ) numbers in young Cx(3)cr1(gfp/gfp) mice compared with Cx(3)cr1(gfp/+) mice, however the increase was more marked in albino Cx(3)cr1(gfp/gfp) mice. In aged mice, large numbers of SRMΦ/activated microglia replete with autofluorescent debris were noted in both old pigmented Cx(3)cr1(gfp/gfp) and Cx(3)cr1(gfp/+) mice proving this accumulation was not CX(3)CR1-dependent. While CX(3)CR1 deficiency leads to an early onset of SRMΦ accumulation, our data reveal that this change occurs in both aged Cx(3)cr1(gfp/+) and Cx(3)cr1(gfp/gfp) pigmented mice in the absence of marked retinal degeneration and is likely a normal response to aging.

Multimodal fundus imaging of pseudoxanthoma elasticum

PURPOSE

To investigate spectral domain optical coherence tomography and autofluorescence findings in eyes with pseudoxanthoma elasticum.

METHODS

A retrospective analysis of visual acuity, spectral domain optical coherence tomography, and autofluorescence findings of consecutive patients with pseudoxanthoma elasticum was performed. The spectral domain optical coherence tomography was evaluated for retinal architecture, subretinal accumulations, and photoreceptor layer thickness. Autofluorescence and near-infrared reflectance images were reviewed for correlative findings.

RESULTS

There were 21 patients (42 eyes) with pseudoxanthoma elasticum with a mean age of 56.1 ± 12.4 years. Subretinal fluid was found in 14 eyes, 7 of which had no signs of choroidal neovascularization. In six of the seven eyes with a history of choroidal neovascularization controlled with antivascular endothelial growth factor injections, there were areas of subretinal fluid that were not contiguous with the choroidal neovascularization and did not seem responsive to antivascular endothelial growth factor injections. Two types of formed material were observed in the subretinal space and outer retina. The first was hypoautofluorescent deposits above the retinal pigment epithelium resembling subretinal drusenoid deposits (reticular pseudodrusen). The second was yellow to brown hyperautofluorescent aggregates in the subretinal space and outer retina similar to those seen in pattern dystrophies and was found in 19 eyes. There was an apparent association between the presence of subretinal fluid and pattern dystrophy-like findings.

CONCLUSION

Subretinal fluid in patients with pseudoxanthoma elasticum is not always indicative of active leakage from underlying choroidal neovascularization and can be resistant to antivascular endothelial growth factor injections. This fluid is associated with pattern dystrophy-like findings and may indicate abnormal retinal pigment epithelial function.

Localized retinal neuropeptide regulation of macrophage and microglial cell functionality

The functionality of immune cells is manipulated within the ocular microenvironment to protect the sensitive and non-regenerating light-gathering tissue from the collateral damage of inflammation. This is mediated partly by the constitutive presence of immunomodulating neuropeptides. Treating primary resting macrophages with soluble factors produced by the posterior eye induced co-expression of Arginase1 and NOS2. The neuropeptides alpha-melanocyte stimulating hormone and Neuropeptide Y alternatively activated the macrophages to co-express Arginase1 and NOS2 like myeloid suppressor cells. Similar co-expressing cells were found within healthy, but not in wounded retinas. Therefore, the healthy retina regulates macrophage functionality to the benefit of ocular immune privilege.

Novel characterization of monocyte-derived cell populations in the meninges and choroid plexus and their rates of replenishment in bone marrow chimeric mice

The mouse dura mater, pia mater, and choroid plexus contain resident macrophages and dendritic cells (DCs). These cells participate in immune surveillance, phagocytosis of cellular debris, uptake of antigens from the surrounding cerebrospinal fluid and immune regulation in many pathologic processes. We used Cx3cr1 knock-in, CD11c-eYFP transgenic and bone marrow chimeric mice to characterize the phenotype, density and replenishment rate of monocyte-derived cells in the meninges and choroid plexus and to assess the role of the chemokine receptor CX3CR1 on their number and tissue distribution. Iba-1 major histocompatibility complex (MHC) Class II CD169 CD68 macrophages and CD11c putative DCs were identified in meningeal and choroid plexus whole mounts. Comparison of homozygous and heterozygous Cx3cr1 mice did not reveal CX3CR1-dependancy on density, distribution or phenotype of monocyte-derived cells. In turnover studies, wild type lethally irradiated mice were reconstituted with Cx3cr1/-positive bone marrow and were analyzed at 3 days, 1, 2, 4 and 8 weeks after transplantation. There was a rapid replenishment of CX3CR1-positive cells in the dura mater (at 4 weeks) and the choroid plexus was fully reconstituted by 8 weeks. These data provide the foundation for future studies on the role of resident macrophages and DCs in conditions such as meningitis, autoimmune inflammatory disease and in therapies involving irradiation and hematopoietic or stem cell transplantation.

Resident macrophages in the cochlear blood-labyrinth barrier and their renewal via migration of bone-marrow-derived cells

A large population of perivascular cells was found to be present in the area of the blood-labyrinth barrier in the stria vascularis of normal adult cochlea. The cells were identified as perivascular resident macrophages (PVMs), as they were positive for several macrophage surface molecules including F4/80, CD68, and CD11b. The macrophages, which were closely associated with microvessels and structurally intertwined with endothelial cells and pericytes, constitutively expressed scavenger receptor classes A(1) and B(1) and accumulated blood-borne proteins such as horseradish peroxidase and acetylated low-density lipoprotein. The PVMs were demonstrated to proliferate slowly, as evidenced by the absence of 5-bromo-2-deoxyuridine (BrdU)-positive PVMs at 3-14 days in normal mice injected with BrdU. However, in irradiated mice, the majority of the PVMs turned over via bone-marrow-cell migration within a 10-month time-frame. The existence of PVMs in the vascular wall of the blood-labyrinth barrier might therefore serve as a source for progenitor cells for postnatal vasculogenesis and might contribute to the repair of damaged vessels in the context of a local inflammatory response.

Bone marrow chimeric mice reveal a role for CX₃CR1 in maintenance of the monocyte-derived cell population in the olfactory neuroepithelium

Macrophages in the olfactory neuroepithelium are thought to play major roles in tissue homeostasis and repair. However, little information is available at present about possible heterogeneity of these monocyte-derived cells, their turnover rates, and the role of chemokine receptors in this process. To start addressing these issues, this study used Cx₃cr1(gfp) mice, in which the gene sequence for eGFP was knocked into the CX₃CR1 gene locus in the mutant allele. Using neuroepithelial whole-mounts from Cx₃cr1(gfp/+) mice, we show that eGFP(+) cells of monocytic origin are distributed in a loose network throughout this tissue and can be subdivided further into two immunophenotypically distinct subsets based on MHC-II glycoprotein expression. BM chimeric mice were created using Cx₃cr1(gfp/+) donors to investigate turnover of macrophages (and other monocyte-derived cells) in the olfactory neuroepithelium. Our data indicate that the monocyte-derived cell population in the olfactory neuroepithelium is actively replenished by circulating monocytes and under the experimental conditions, completely turned over within 6 months. Transplantation of Cx₃cr1(gfp/gfp) (i.e., CX₃CR1-deficient) BM partially impaired the replenishment process and resulted in an overall decline of the total monocyte-derived cell number in the olfactory epithelium. Interestingly, replenishment of the CD68(low)MHC-II(+) subset appeared minimally affected by CX₃CR1 deficiency. Taken together, the established baseline data about heterogeneity of monocyte-derived cells, their replenishment rates, and the role of CX₃CR1 provide a solid basis to further examine the importance of different monocyte subsets for neuroregeneration at this unique frontier with the external environment.

Microglia in the healthy and degenerating retina: insights from novel mouse models

In contrast to the tremendous amount of research data from the central nervous system, relatively little is known about microglial homeostasis in the retina. This may be explained by a strong research bias towards important brain pathologies including Alzheimer's disease, Parkinson's disease, and Multiple Sclerosis. In addition, there are specific technical limitations which hampered the analysis of retinal microglia, including their relatively small number in ocular tissue. The lack of experimental tools also prevented direct visualization and molecular analysis of this specialized neuronal macrophage population. Over the last few years, this situation has changed considerably as more and more retinal disorders have come into focus. Many rare monogenic forms as well as more prevalent complex disorders, in particular the age-related macular degeneration involves innate immune mechanisms. As a consequence, new genetic and experimental mouse models have been developed that mimic various forms of human retinal degeneration. In conjunction with these disease models, novel macrophage/microglia-specific reporter mice were established that allow the monitoring of retinal microglia in situ and in vivo. This review summarizes recent findings from these mouse models and thereby provides an overview of microglial homeostasis in the healthy and degenerating retina. Based on this knowledge, microglia-targeted therapies are envisioned which could delay or attenuate degenerative retinal disease.

A common progenitor for retinal astrocytes and oligodendrocytes

Developing neural tissue undergoes a period of neurogenesis followed by a period of gliogenesis. The lineage relationships among glial cell types have not been defined for most areas of the nervous system. Here we use retroviruses to label clones of glial cells in the chick retina. We found that almost every clone had both astrocytes and oligodendrocytes. In addition, we discovered a novel glial cell type, with features intermediate between those of astrocytes and oligodendrocytes, which we have named the diacyte. Diacytes also share a progenitor cell with both astrocytes and oligodendrocytes.

Microglial response to light-induced photoreceptor degeneration in the mouse retina

The microglial response elicited by degeneration of retinal photoreceptor cells was characterized in BALB/c mice exposed to bright light for 7 hours and then kept in complete darkness for survival times ranging from 0 hours to 10 days. Photodegeneration resulted in extensive cell death in the retina, mainly in the outer nuclear layer (ONL), where the photoreceptor nuclei are located. Specific immunolabeling of microglial cells with anti-CD11b, anti-CD45, anti-F4/80, anti-SRA, and anti-CD68 antibodies revealed that microglial cells were activated in light-exposed retinas. They migrated to the ONL, changed their morphology, becoming rounded cells with short and thick processes, and, finally, showed immunophenotypic changes. Specifically, retinal microglia began to strongly express antigens recognized by anti-CD11b, anti-CD45, and anti-F4/80, coincident with cell degeneration. In contrast, upregulation of the antigen recognized by anti-SRA was not detected by immunocytochemistry until 6 hours after light exposure. Differences were also observed at 10 days after light exposure: CD11b, CD45, and F4/80 continued to be strongly expressed in retinal microglia, whereas the expression of CD68 and SRA had decreased to near-normal values. Therefore, microglia did not return to their original state after photodegeneration and continued to show a degree of activation. The accumulation of activated microglial cells in affected regions simultaneously with photoreceptor degeneration suggests that they play some role in photodegeneration.

The immune privileged retina mediates an alternative activation of J774A.1 cells

PURPOSE

We have previously found that retinal pigment epithelial (RPE) cells suppressed endotoxin-stimulated macrophages; moreover, it induced expression of anti-inflammatory cytokines. We further assessed the possibility that the RPE is alternatively activating macrophages.

METHODS

J774A.1 cells were stimulated with endotoxin and treated with the conditioned media (CM) of RPE, or neuroretinal eyecups from healthy mouse eyes. The supernatant was assayed for IL-1 beta, TNF-alpha, IL-6, IL-12(p70), and IL-10, and for nitric-oxide generation. The RPE conditioned media (RPE CM) was absorbed of known soluble factors to identify the factor that augments nitric-oxide generation.

RESULTS

We found the RPE CM suppressed all cytokine production except IL-10, and augmented nitric-oxide generation. The augmented nitric-oxide levels were mediated by RPE derived alpha-melanocyte stimulating hormone (alpha-MSH).

CONCLUSIONS

Healthy RPE not only suppresses inflammatory activity, it promotes an alternative activation of macrophages that can further promote immune privilege.

Genetics of human iris colour and patterns

The presence of melanin pigment within the iris is responsible for the visual impression of human eye colouration with complex patterns also evident in this tissue, including Fuchs' crypts, nevi, Wolfflin nodules and contraction furrows. The genetic basis underlying the determination and inheritance of these traits has been the subject of debate and research from the very beginning of quantitative trait studies in humans. Although segregation of blue-brown eye colour has been described using a simple Mendelian dominant-recessive gene model this is too simplistic, and a new molecular genetic perspective is needed to fully understand the biological complexities of this process as a polygenic trait. Nevertheless, it has been estimated that 74% of the variance in human eye colour can be explained by one interval on chromosome 15 that contains the OCA2 gene. Fine mapping of this region has identified a single base change rs12913832 T/C within intron 86 of the upstream HERC2 locus that explains almost all of this association with blue-brown eye colour. A model is presented whereby this SNP, serving as a target site for the SWI/SNF family member HLTF, acts as part of a highly evolutionary conserved regulatory element required for OCA2 gene activation through chromatin remodelling. Major candidate genes possibly effecting iris patterns are also discussed, including MITF and PAX6.

Regulation of dynamic behavior of retinal microglia by CX3CR1 signaling

PURPOSE

Microglia in the central nervous system display a marked structural dynamism in their processes in the resting state. This dynamic behavior, which may play a constitutive surveying role in the uninjured neural parenchyma, is also highly responsive to tissue injury. The role of CX3CR1, a chemokine receptor expressed in microglia, in regulating microglia morphology and dynamic behavior in the resting state and after laser-induced focal injury was examined.

METHODS

Time-lapse confocal imaging of retinal explants was used to evaluate the dynamic behavior of retinal microglia labeled with green fluorescent protein (GFP). Transgenic mice in which CX3CR1 signaling was ablated (CX3CR1(GFP/GFP)/CX3CR1(-/-)) and preserved (CX3CR1(+/GFP)/CX3CR1(+/-)) were used.

RESULTS

Retinal microglial density, distribution, cellular morphology, and overall retinal tissue anatomy were not altered in young CX3CR1(-/-) animals. In the absence of CX3CR1, retinal microglia continued to exhibit dynamic motility in their processes. However, rates of process movement were significantly decreased, both under resting conditions and in response to tissue injury. In addition, microglia migration occurring in response to focal laser injury was also significantly slowed in microglia lacking CX3CR1.

CONCLUSIONS

CX3CR1 signaling in retinal microglia, though not absolutely required for the presence of microglial dynamism, plays a role in potentiating the rate of retinal microglial process dynamism and cellular migration. CX3CL1 signaling from retinal neurons and endothelial cells likely modulates dynamic microglia behavior so as to influence the level of microglial surveillance under basal conditions and the rate of dynamic behavior in response to tissue injury.