Novel discovery of LYVE-1 expression in the hyaloid vascular system

Redefining the ontogeny of hyalocytes as yolk sac-derived tissue-resident macrophages of the vitreous body

BACKGROUND

The eye is a highly specialized sensory organ which encompasses the retina as a part of the central nervous system, but also non-neural compartments such as the transparent vitreous body ensuring stability of the eye globe and a clear optical axis. Hyalocytes are the tissue-resident macrophages of the vitreous body and are considered to play pivotal roles in health and diseases of the vitreoretinal interface, such as proliferative vitreoretinopathy or diabetic retinopathy. However, in contrast to other ocular macrophages, their embryonic origin as well as the extent to which these myeloid cells might be replenished by circulating monocytes remains elusive.

RESULTS

In this study, we combine transgenic reporter mice, embryonic and adult fate mapping approaches as well as parabiosis experiments with multicolor immunofluorescence labeling and confocal laser-scanning microscopy to comprehensively characterize the murine hyalocyte population throughout development and in adulthood. We found that murine hyalocytes express numerous well-known myeloid cell markers, but concomitantly display a distinct immunophenotype that sets them apart from retinal microglia. Embryonic pulse labeling revealed a yolk sac-derived origin of murine hyalocytes, whose precursors seed the developing eye prenatally. Finally, postnatal labeling and parabiosis established the longevity of hyalocytes which rely on Colony Stimulating Factor 1 Receptor (CSF1R) signaling for their maintenance, independent of blood-derived monocytes.

CONCLUSION

Our study identifies hyalocytes as long-living progeny of the yolk sac hematopoiesis and highlights their role as integral members of the innate immune system of the eye. As a consequence of their longevity, immunosenescence processes may culminate in hyalocyte dysfunction, thereby contributing to the development of vitreoretinal diseases. Therefore, myeloid cell-targeted therapies that convey their effects through the modification of hyalocyte properties may represent an interesting approach to alleviate the burden imposed by diseases of the vitreoretinal interface.

Oxidative stress induces inflammation of lens cells and triggers immune surveillance of ocular tissues

Recent reports have challenged the notion that the lens is immune-privileged. However, these studies have not fully identified the molecular mechanism(s) that promote immune surveillance of the lens. Using a mouse model of targeted glutathione (GSH) deficiency in ocular surface tissues, we have investigated the role of oxidative stress in upregulating cytokine expression and promoting immune surveillance of the eye. RNA-sequencing of lenses from postnatal day (P) 1-aged Gclcf/f;Le-CreTg/- (KO) and Gclcf/f;Le-Cre-/- control (CON) mice revealed upregulation of many cytokines (e.g., CCL4, GDF15, CSF1) and immune response genes in the lenses of KO mice. The eyes of KO mice had a greater number of cells in the aqueous and vitreous humors at P1, P20 and P50 than age-matched CON and Gclcw/w;Le-CreTg/- (CRE) mice. Histological analyses revealed the presence of innate immune cells (i.e., macrophages, leukocytes) in ocular structures of the KO mice. At P20, the expression of cytokines and ROS content was higher in the lenses of KO mice than in those from age-matched CRE and CON mice, suggesting that oxidative stress may induce cytokine expression. In vitro administration of the oxidant, hydrogen peroxide, and the depletion of GSH (using buthionine sulfoximine (BSO)) in 21EM15 lens epithelial cells induced cytokine expression, an effect that was prevented by co-treatment of the cells with N-acetyl-l-cysteine (NAC), a antioxidant. The in vivo and ex vivo induction of cytokine expression by oxidative stress was associated with the expression of markers of epithelial-to-mesenchymal transition (EMT), α-SMA, in lens cells. Given that EMT of lens epithelial cells causes posterior capsule opacification (PCO), we propose that oxidative stress induces cytokine expression, EMT and the development of PCO in a positive feedback loop. Collectively these data indicate that oxidative stress induces inflammation of lens cells which promotes immune surveillance of ocular structures.

Organogenesis and distribution of the ocular lymphatic vessels in the anterior eye

Glaucoma surgeries, such as trabeculectomy, are performed to lower intraocular pressure to reduce risk of vision loss. These surgeries create a new passage in the eye that reroutes the aqueous humor outflow to the subconjunctival space, where the fluid is presumably absorbed by the conjunctival lymphatics. Here, we characterized the development and function of the ocular lymphatics using transgenic lymphatic reporter mice and rats. We found that the limbal and conjunctival lymphatic networks are progressively formed from a primary lymphatic vessel that grows from the nasal-side medial canthus region at birth. This primary lymphatic vessel immediately branches out, invades the limbus and conjunctiva, and bidirectionally encircles the cornea. As a result, the distribution of the ocular lymphatics is significantly polarized toward the nasal side, and the limbal lymphatics are directly connected to the conjunctival lymphatics. New lymphatic sprouts are produced mainly from the nasal-side limbal lymphatics, posing the nasal side of the eye as more responsive to fluid drainage and inflammatory stimuli. Consistent with this polarized distribution of the ocular lymphatics, a higher drainage efficiency was observed in the nasal side than the temporal side of the eye when injected with a fluorescent tracer. In contrast, blood vessels are evenly distributed at the anterior surface of the eyes. Also, we found that these distinct vascular distribution patterns were conserved in human eyes. Together, our study demonstrated that the ocular surface lymphatics are more densely present in the nasal side and uncovered the potential clinical benefits in selecting the nasal side as a glaucoma surgery site to improve fluid drainage.

Hemmule: A Novel Structure with the Properties of the Stem Cell Niche

Stem cells are nurtured and regulated by a specialized microenvironment known as stem cell niche. While the functions of the niches are well defined, their structure and location remain unclear. We have identified, in rat bone marrow, the seat of hematopoietic stem cells—extensively vascularized node-like compartments that fit the requirements for stem cell niche and that we called hemmules. Hemmules are round or oval structures of about one millimeter in diameter that are surrounded by a fine capsule, have afferent and efferent vessels, are filled with the extracellular matrix and mesenchymal, hematopoietic, endothelial stem cells, and contain cells of the megakaryocyte family, which are known for homeostatic quiescence and contribution to the bone marrow environment. We propose that hemmules are the long sought hematopoietic stem cell niches and that they are prototypical of stem cell niches in other organs.

Regression of fetal vasculature and visual improvement in nonsurgical persistent hyperplastic primary vitreous: a case report

BACKGROUND

Persistent hyperplastic primary vitreous (PHPV) is a rare congenital developmental ocular disorder caused by incomplete regression of the embryonic hyaloid vasculature. Here we report a case of nonsurgical unilateral anterior PHPV that was managed by amblyopia treatment and resulted in an improvement of visual acuity and regression of the fetal vasculature.

CASE PRESENTATION

A three-year-old girl was diagnosed with unilateral anterior PHPV in the left eye, manifested with posterior pole cataract, posterior capsule opacification, tunica vasculosa lentis, and a floating hyaloid artery connected to the retrolental mass. The plaque was not large enough to fill the pupil, and conservative management along with amblyopia treatment was conducted. Nineteen months later, the visual acuity in the affected eye improved from 20/100 to 20/50 with correction, and the fetal vasculature regressed gradually and finally into a nonperfusion ghost vessel.

CONCLUSIONS

In PHPV-affected children, regression of the fetal vasculature may be observed, and conservative management and amblyopia treatment may be helpful for visual improvement.

Functional OCT angiography reveals early physiological dysfunction of hyaloid vasculature in developing mouse eye

Hyaloid vascular system (HVS) is a transient capillary network nourishing developing eye. Better study of the HVS regression correlated with eye development is essential for in-depth understanding of the nature of vision system. In this study, we demonstrate the feasibility of longitudinal optical coherence tomography (OCT) and OCT angiography (OCTA) monitoring of the HVS in C57BL/6J mice. OCT enables morphological monitoring of the HVS regression, and OCTA allows physiological assessment of the HVS involution correlated with eye development. Functional OCTA reveals early physiological dysfunction before morphological regression of the hyaloid vasculature in developing mouse eye. We anticipate that noninvasive, simultaneous OCT/OCTA observation of morphological regression and physiological degradation in normal and diseased animal models will be valuable to unravel the complex mechanisms of the HVS regression correlated with normal eye development and abnormal persistent hyaloid conditions.

Impact statement

Hyaloid vascular system (HVS) is known to have an essential role in the eye development. However, established knowledge of the HVS largely relies on end-point studies with biochemically fixed tissues, lacking a full description of the natural dynamics of the HVS correlated with eye development. An imaging methodology for noninvasive, longitudinal, and high-resolution monitoring of the HVS is important not only for better understanding of the nature of the vision system and is also valuable for better study of abnormal eye conditions. Here, we report the feasibility of in vivo optical coherence tomography (OCT) and OCT angiography (OCTA) imaging of the HVS regression in developing mouse eye. OCT enables morphological imaging of the HVS structure, and OCTA allows functional assessment of the HVS physiology correlated with eye development.

Bush-like integrin filament networks associated with hyaloid vasculature in murine neonate eyes

The vitreous of perinatal mice temporarily develops a unique vascular system, called the vasa hyaloidea propria (VHP). Observations showed the vessels possessed an extracellular matrix including the basement membrane in their entire length. Immunostaining of whole mount preparations of VHP with integrin β₁ antibody displayed a bush-like network consisting of long and straight fibers which were associated with the VHP but extended apart from the blood vessels. Electron microscopically, each fiber was composed of a bundle of thin filaments different from collagen fibrils. Macrophages associated with the VHP appeared to be arrested by the integrin bushes. The integrin bushes fragmented and disappeared by postnatal day 10, just before the regression of the VHP. Macrophages were involved in the digestion and clearance of integrin bushes. The vitreous integrin bushes appear to provide a scaffold for architectural maintenance of the hyaloid vessels and macrophages.

Involvement of premacular mast cells in the pathogenesis of macular diseases

We previously reported on the elevated intravitreal activities of tryptase and chymase in association with idiopathic epiretinal membrane (ERM) and idiopathic macular hole (MH). In this present study, we investigated the potential intraocular production of these serine proteases, and measured and compared tryptase and chymase activities in the vitreous body and serum in ERM, MH, proliferative diabetic retinopathy (PDR), and rhegmatogenous retinal detachment (RRD) patients. In addition, nuclear staining with hematoxylin and eosin (H&E) and mast-cell staining with toluidine blue were performed on samples of the vitreous core and bursa premacularis (BPM) of MH. We also performed immunostaining on the above two regions of vitreous samples for MH with anti-tryptase antibody, anti-chymase antibody, anti-podoplanin antibody, anti-lymphatic vessel endothelial hyaluronan receptor 1 (LYVE-1) antibody, and anti-fibroblast antibody. Moreover, we performed immunostaining with anti-tryptase antibody and anti-chymase antibody on ERMs collected intraoperatively. Tryptase activity in the vitreous body was significantly higher in ERM and MH than in PDR. However, no significant differences were observed in the tryptase activity in the serum among these four diseases. Chymase activity in the vitreous body was significantly higher in MH than in the other three diseases, yet chymase activity in the serum was below detection limit in any of the diseases. Nuclear staining with H&E revealed an abundance of nuclei in the BPM region, but few in the surrounding area. Mast-cell staining with toluidine blue revealed that the BPM showed metachromatic staining. In immunostaining with anti-fibroblasts antibody, anti-tryptase antibody, anti-chymase antibody, anti-podoplanin antibody, and anti-LYVE-1 antibody, the BPM stained more strongly than the vitreous core. Tryptase and chymase-positive cells were also observed in ERM. These findings revealed that the presence of mast cells in the BPM potentially represent the source of these serine proteases. Moreover, the BPM, as a lymphatic tissue, may play an important role in the pathogenesis of macular disease.

Lymphatic markers in the human optic nerve

Tissues of the central nervous system (CNS), including the optic nerve (ON), are considered a-lymphatic. However, lymphatic structures have been described in the dura mater of human ON sheaths. Since it is known that lymphatic markers are also expressed by single non-lymphatic cells, these results need confirmation according to the consensus statement for the use of lymphatic markers in ophthalmologic research. The aim of this study was to screen for the presence of lymphatic structures in the adult human ON using a combination of four lymphatic markers. Cross and longitudinal cryo-sections of human optic nerve tissue (n = 12, male and female, postmortem time = 15.8 ± 5.5 h, age = 66.5 ± 13.8 years), were obtained from cornea donors of the Salzburg eye bank, and analyzed using immunofluorescence with the following markers: FOXC2, CCL21, LYVE-1 and podoplanin (PDPN; lymphatic markers), Iba1 (microglia), CD68 (macrophages), CD31 (endothelial cell, EC), NF200 (neurofilament), as well as GFAP (astrocytes). Human skin sections served as positive controls and confocal microscopy in single optical section mode was used for documentation. In human skin, lymphatic structures were detected, showing a co-localization of LYVE-1/PDPN/FOXC2 and CCL21/LYVE-1. In the human ON however, single LYVE-1+ cells were detected, but were not co-localized with any other lymphatic marker tested. Instead, LYVE-1+ cells displayed immunopositivity for Iba1 and CD68, being more pronounced in the periphery of the ON than in the central region. However, Iba1+/LYVE-1- cells outnumbered Iba1+/LYVE-1+ cells. PDPN, revealed faint labeling in human ON tissue despite strong immunoreactivity in rat ON controls, showing co-localization with GFAP in the periphery. In addition, pronounced autofluorescent dots were detected in the ON, showing inter-individual differences in numbers. In the adult human ON no lymphatic structures were detected, although distinct lymphatic structures were identified in human skin tissue by co-localization of four lymphatic markers. However, single LYVE-1+ cells, also positive for Iba1 and CD68 were present, indicating LYVE-1+ macrophages. Inter-individual differences in the number of LYVE-1+ as well as Iba1+ cells were obvious within the ONs, most likely resulting from diverse medical histories of the donors.

Histochemical characteristics of regressing vessels in the hyaloid vascular system of neonatal mice: Novel implication for vascular atrophy

The hyaloid vasculature constitutes a transitory system nourishing the internal structures of the developing eye, but the mechanism of vascular regression and its cell biological characteristics are not fully understood. The present study aimed to reveal the specificity of the hyaloid vessels by a systematic immunohistochemical approach for marker substances of myeloid cells and the extracellular matrix (ECM) in neonatal mice. Macrophages immunoreactive for F4/80, cathepsin D, and LYVE-1 gathered around the vasa hyaloidea propria (VHP), while small round cells in vascular lumen of VHP were selectively immunoreactive for galectin-3; their segmented nuclei and immunoreactivities for Ly-6G, CD11b, and myeloperoxidase indicated their neutrophilic origin. VHP possessed thick ECM and a dense pericyte envelope as demonstrated by immunostaining for laminin, type IV collagen, integrin β1, and NG2. The galectin-3⁺ cells loosely aggregated with numerous erythrocytes in the lumen of hyaloid vessels in a manner reminiscent of vascular congestion. Galectin-3 is known to polymerize and form a complex with ECM and NG2 as well as recruit leukocytes on the endothelium. Observation of galectin-3 KO mice implicated the involvement of galectin-3 in the regression of hyaloid vasculature. Since macrophages may play central roles including blocking of the blood flow and the induction of apoptosis in the regression, galectin-3⁺ neutrophils may play a supportive role in the macrophage-mediated involution of the hyaloid vascular system.

Hyaluronic Acid in Vascular and Immune Homeostasis during Normal Pregnancy and Preeclampsia

Preeclampsia (PE) is a multisystem pathologic state that clinically manifests itself after the 20th week of pregnancy. It is characterized by high maternal and perinatal morbidity and mortality. According to modern concepts, the impairment of trophoblast invasion into maternal spiral arteries, leading to the development of ischemia in placenta, is considered to be the major pathogenetic factor of PE development. Ischemic lesions initiate the development of a systemic inflammatory response (SIR) and endothelial dysfunction, which is the main cause of the multiple organ failure in PE. Some data has appear indicating the importance of a glycans-forming endothelial glycocalyx and extracellular matrix (ECM) for placenta morphogenesis, as well as their role in the regulation of vascular permeability and vascular tone in hypertension disorders and, in particular, PE. Since intact glycocalyx and ECM are considered to be the major factors that maintain the physiological vascular tone and adequate intercellular interactions, their value in PE pathogenesis is underestimated. This review is focused on hyaluronic acid (HA) as the key glycan providing the organization and stabilization of the ECM and glycocalyx, its distribution in tissues in the case of presence or absence of placental pathology, as well as on the regulatory function of hyaluronic acids of various molecular weights in different physiological and pathophysiological processes. The summarized data will provide a better understanding of the PE pathogenesis, with the main focus on glycopathology.

The genetic control of avascular area in mouse oxygen-induced retinopathy

PURPOSE

The C57BL/6ByJ and BALB/cByJ inbred strains of mice are, respectively, susceptible and resistant to oxygen-induced retinopathy (OIR). The purpose of this work was to investigate the genetic control of the retinal avascular area in mouse OIR using a mapping cross.

METHODS

The central retinal avascular area was measured on postnatal day 16 (P16) in C57BL/6ByJ, BALB/cByJ, 101 (C57BL/6ByJ x BALB/cByJ)F₂, and 116 (BALB/cByJ x C57BL/6ByJ)F₂ mice that had been subjected to the OIR protocol. A genome-wide scan was performed of selected albino and non-albino mice to determine quantitative trait loci associated with weight and avascular area.

RESULTS

C57BL/6ByJ mice had significantly larger avascular areas than BALB/cByJ ones. Albino mice of the F₂ generation had smaller avascular areas than the non-albino mice. Genotyping was performed at 856 informative single nucleotide polymorphisms approximately evenly distributed across the genome from each of 85 selected F₂ mice. Weight, sex, and the paternal grandmother were found to act as additive covariates associated with the avascular area on P16; mapping analyses that used a model incorporating these covariates found a quantitative trait locus on chromosome 7 related to avascular area. Mapping analyses that used a model that did not incorporate covariates found a quantitative trait locus on chromosome 9 related to avascular area. A quantitative trait locus for bodyweight on P16 was mapped to chromosome 5.

CONCLUSIONS

The retinal avascular area in the mouse OIR model is under genetic control. Revascularization in OIR is related to the weight, strain of paternal grandmother, sex, and albinism. Our data support the existence of a quantitative trait locus on chromosome 5 that influences weight after exposure to hyperoxia, as well as quantitative trait loci on chromosomes 7 and 9 that modify susceptibility to OIR.

Spontaneous lymphatic vessel formation and regression in the murine cornea

PURPOSE

Lymphatic dysfunctions are associated with many diseases, ranging from cancer metastasis to transplant rejection, for which there is little effective treatment. To date, there is no natural model with which to study lymphatic regression. This study was conducted to investigate whether murine cornea, an extensively exploited tissue for vascular studies, derives its lymphatic-free status from a natural regression mechanism. The differential behaviors between the lymphatic and blood vessels under normal development and inflammation conditions are also compared.

METHODS

Normal mouse eyeballs or whole-mount corneas encompassing the entire course of corneal development and maturation and adult inflamed corneas were used for immunofluorescent microscopic studies.

RESULTS

The data demonstrated, for the first time, that mouse cornea was endowed with a significant number of lymphatic vessels that underwent spontaneous formation and regression during a critical period after birth, which was not observed for blood vessels. Because lymphatic growth can be reactivated in the adult cornea after inflammatory stimulation, the cornea thereby becomes the first tissue ever identified to have a full range of lymphatic plasticity.

CONCLUSIONS

These novel findings not only provide a new concept in defining the cornea and its related diseases, they also reveal a completely natural model with which to study both lymphatic regression and formation. It is hoped that further studies will divulge novel and potent pro- or anti-lymphatic factors to treat lymphatic disorders inside and outside the eye.

Specific role of lymphatic marker podoplanin in retinal pigment epithelial cells

Podoplanin is a small transmembrane glycoprotein widely known to be a marker for lymphatic endothelial cells. In this study, we identify a novel localization of podoplanin in the retinal pigment epithelium (RPE), a cellular monolayer critically involved in the visual process. Using a small interfering RNA (siRNA)-mediated gene silencing approach, we have also demonstrated, for the first time, that podoplanin depletion in human RPE cells leads to a marked reduction of cell aggregates and tight junctions. Additionally, the podoplanin-depleted cells also exhibit a significantly lower rate of proliferation. These data together indicate that podoplanin plays a crucial role in RPE cell functions. Further investigation on this factor may reveal novel mechanisms and therapeutic strategies for RPE-related eye diseases, such as proliferative retinopathy and age-related macular degeneration.