Aberrant Migratory Behavior of Immune Cells in Recurrent Autoimmune Uveitis in Horses

Septin and actin contributions to endothelial cell-cell junctions and monolayer integrity

Septins in endothelial cells (ECs) have important roles supporting the integrity of the endothelial monolayer. Cell-cell junctions in EC monolayers are highly dynamic, with continuous retractions and protrusions. Depletion of septins in ECs leads to disruption of cell-cell junctions, which are composed of VE-cadherin and other junctional proteins. In EC monolayers, septins are concentrated at the plasma membrane at sites of cell-cell contact, in curved- and scallop-shaped patterns. These membrane-associated septin accumulations are located in regions of positive membrane curvature, and those regions are often associated with and immediately adjacent to actin-rich protrusions with negative membrane curvature. EC septins associate directly with plasma membrane lipids, based on findings with site-specific mutations of septins in ECs, which is consistent with biochemical and cell biological studies in other systems. Loss of septins leads to disruption of the EC monolayer, and gaps form between cells. The number and breadth of cell-cell contacts and junctions decreases, and the number and frequency of retractions, ruffles, and protrusions at cell edges also decreases. In addition, loss of septins leads to decreased amounts of F-actin at the cortical membrane, along with increased amounts of F-actin in stress fibers of the cytoplasm. Endothelial monolayer disruption from loss of septins is also associated with decreased transendothelial electric resistance (TEER) and increased levels of transendothelial migration (TEM) by immune and cancer cells, owing to the gaps in the monolayer. A current working model is that assembly of septin filaments at regions of positive membrane curvature contributes to a mechanical footing or base for actin-based protrusive forces generated at adjoining regions of the membrane. Specific molecular interactions between the septin and actin components of the cytoskeleton may also be important contributors. Regulators of actin assembly may promote and support the assembly of septin filaments at the membrane, as part of a molecular feedback loop between the assembly of septin and actin filaments.

Ex Vivo and In Vitro Analysis Identify a Detrimental Impact of Neutrophil Extracellular Traps on Eye Structures in Equine Recurrent Uveitis

Equine recurrent uveitis (ERU) is a common ocular disease of horses and described as a model for human autoimmune uveitis. This immune-mediated, inflammatory condition progressively destroys the eye, ultimately leading to blindness. Genetic and autoimmune factors, next to infections with Leptospira, are discussed as key factors in the pathogenesis. Furthermore, a release of neutrophil extracellular traps (NETs) by activated neutrophils is involved. NETs are composed of decondensed chromatin and proteins that can immobilize invading pathogens. However, if NETs accumulate, they can contribute to detrimental autoimmune processes. Thus, we aimed to investigate the impact of NETs in ERU patients. Therefore, we quantified several NET-markers (cell-free DNA, nucleosomes, citrullinated histone H3, histone-myeloperoxidase complexes, interleukin-17, equine cathelicidin 1 and DNase I activity) and NET-autoantibodies in sera and vitreous body fluids (VBF) of ERU-diseased horses and correlated the data with the disease status (signalment, ERU scores and Leptospira infection status). NET markers were detected to varying degrees in VBF of diseased horses, and partially correlated to disease severity and the presence of Leptospira spp. Cell-free DNA and nucleosomes as NET markers correlate with ERU severity in total and VBF scores, despite the presence of active DNases. Additionally, a significant correlation between fundus affection in the eye and NET autoantibodies was detectable. Therefore, we further investigated the influence of VBF samples from equine patients and isolated NETs on the blood-retina barrier in a cell culture model. VBF of diseased horses significantly induced cytotoxicity in retinal pigment epithelial cells. Moreover, partially digested NETs also resulted in cytotoxic effects. In the presence of lipopolysaccharide (LPS), the main component of the leptospiral surface, both undigested and completely digested NETs were cytotoxic. Correlations between the ERU-scores and Leptospira were also calculated. Detection of leptospiral DNA, and antibody titers of the serovar Grippotyphosa correlated with disease severity. In addition, a correlation between Leptospira and several NET markers was observed in VBF. Altogether, our findings suggest a positive correlation between NET markers with disease severity and involvement of Leptospira in the VBF of ERU-diseased horses, as well as a cytotoxic effect of NETs in eyes.

Altered Metabolic Phenotype of Immune Cells in a Spontaneous Autoimmune Uveitis Model

As one of the leading causes of blindness worldwide, uveitis is an important disease. The exact pathogenesis of autoimmune uveitis is not entirely elucidated to date. Equine recurrent uveitis (ERU) represents the only spontaneous animal model for autoimmune uveitis in humans. As the metabolism of immune cells is an emerging field in research and gains more and more significance to take part in the pathogenesis of various diseases, we conducted experiments to investigate the metabolism of immune cells of ERU cases and healthy controls. To our knowledge, the link between a deviant immunometabolism and the pathogenesis of autoimmune uveitis was not investigated so far. We showed that PBMC of ERU cases had a more active metabolic phenotype in basal state by upregulating both the oxidative phosphorylation and the glycolytic pathway. We further revealed an increased compensatory glycolytic rate of PBMC and CD4⁺ T cells of ERU cases under mitochondrial stress conditions. These findings are in line with metabolic alterations of immune cells in other autoimmune diseases and basic research, where it was shown that activated immune cells have an increased need of energy and molecule demand for their effector function. We demonstrated a clear difference in the metabolic phenotypes of PBMC and, more specifically, CD4⁺ T cells of ERU cases and controls. These findings are another important step in understanding the pathogenesis of ERU and figuratively, human autoimmune uveitis.

NEU1 is more abundant in uveitic retina with concomitant desialylation of retinal cells

Desialylation of cell surface glycoproteins carried out by sialidases affects various immunological processes. However, the role of neuraminidase 1 (NEU1), one of four mammalian sialidases, in inflammation and autoimmune disease is not completely unraveled to date. In this study, we analyzed retinal expression of NEU1 in equine recurrent uveitis (ERU), a spontaneous animal model for autoimmune uveitis. Mass spectrometry revealed significantly higher abundance of NEU1 in retinal Müller glial cells (RMG) of ERU-diseased horses compared to healthy controls. Immunohistochemistry uncovered NEU1 expression along the whole Müller cell body in healthy and uveitic state and confirmed higher abundance in inflamed retina. Müller glial cells are the principal macroglial cells of the retina and play a crucial role in uveitis pathogenesis. To determine whether higher expression levels of NEU1 in uveitic RMG correlate with desialylation of retinal cells, we performed lectin binding assays with sialic acid-specific lectins. Through these experiments we could demonstrate a profound loss of both α2-3- and α2-6-linked terminal sialic acids in uveitis. Hence, we hypothesize that higher abundance of NEU1 in uveitic RMG plays an important role in the pathogenesis of uveitis by desialylation of retinal cells. As RMG become activated in the course of uveitis and actively promote inflammation, we propose that NEU1 might represent a novel activation marker for inflammatory RMG. Our data provide novel insights in the expression and implication of NEU1 in inflammation and autoimmune disease.

Forchlorfenuron (CPPU) causes disorganization of the cytoskeleton and dysfunction of human umbilical vein endothelial cells, and abnormal vascular development in zebrafish embryos

Forchlorfenuron (CPPU) has been used worldwide, to boost size and improve quality of various agricultural products. CPPU and its metabolites are persistent and have been detected frequently in fruits, water, sediments, and organisms in aquatic systems. Although the public became aware of CPPU through the exploding watermelon scandal of 2011 in Zhenjiang, China, little was known of its potential effects on the environment and wildlife. In this study, adverse effects of CPPU on developmental angiogenesis and vasculature, which is vulnerable to insults of persistent toxicants, were studied in vivo in zebrafish embryos (Danio rerio). Exposure to 10 mg CPPU/L impaired survival and hatching, while development was hindered by exposure to 2.5 mg CPPU/L. Developing vascular structure, including common cardinal veins (CCVs), intersegmental vessels (ISVs) and sub-intestinal vessels (SIVs), were significantly restrained by exposure to CPPU, in a dose-dependent manner. Also, CPPU caused disorganization of the cytoskeleton. In human umbilical vein endothelial cells (HUVECs), CPPU inhibited proliferation, migration and formation of tubular-like structures in vitro. Results of Western blot analyses revealed that exposure to CPPU increased phosphorylation of FLT-1, but inhibited phosphorylation of FAK and its downstream MAPK pathway in HUVECs. In summary, CPPU elicited developmental toxicity to the developing endothelial system of zebrafish and HUVECs. This was do, at least in part due to inhibition of the FAK/MAPK signaling pathway rather than direct interaction with the VEGF receptor (VEGFR).

Immunological Insights in Equine Recurrent Uveitis

Horses worldwide suffer from equine recurrent uveitis (ERU), an organ-specific, immune-mediated disease with painful, remitting-relapsing inflammatory attacks alternating with periods of quiescence, which ultimately leads to blindness. In course of disease, both eyes can eventually be affected and since blind horses pose a threat to themselves and their surroundings, these animals have to be killed. Therefore, this disease is highly relevant for veterinary medicine. Additionally, ERU shows strong clinical and pathological resemblance to autoimmune uveitis in man. The exact cause for the onset of ERU is unclear to date. T cells are believed to be the main effector cells in this disease, as they overcome the blood retinal barrier to invade the eye, an organ physiologically devoid of peripheral immune cells. These cells cause severe intraocular inflammation, especially in their primary target, the retina. With every inflammatory episode, retinal degeneration increases until eyesight is completely lost. In ERU, T cells show an activated phenotype, with enhanced deformability and migration ability, which is reflected in the composition of their proteome and downstream interaction pathways even in quiescent stage of disease. Besides the dysregulation of adaptive immune cells, emerging evidence suggests that cells of the innate immune system may also directly contribute to ERU pathogenesis. As investigations in both the target organ and the periphery have rapidly evolved in recent years, giving new insights on pathogenesis-associated processes on cellular and molecular level, this review summarizes latest developments in ERU research.

Deviant proteome profile of equine granulocytes associates to latent activation status in organ specific autoimmune disease

Equine recurrent uveitis (ERU) is a spontaneous, remitting-relapsing autoimmune disease driven by the adaptive immune system. Although T cells are described as the main effector cells in pathogenesis, granulocytes have also emerged as possible disease mediators. To explore the role of these innate immune cells, we investigated the whole cell proteome of granulocytes from equine recurrent uveitis cases and healthy controls. Among the 2362 proteins identified by mass spectrometry, we found 96 proteins with significantly changed abundance between groups (p < 0.05, fold change >1.2), representing 4.1% of total granulocyte proteome. Within these differential identifications, calgranulin B, a protein associated with pathogenesis in other autoimmune diseases, showed highest abundance in equine recurrent uveitis (18 fold). For a better interpretation of the results from our hypothesis-generating approach, we added a threshold for biological significance (ratio ERU/controls >2: 36 proteins) to the proteins with increased abundance in equine recurrent uveitis and analyzed their allocation to the subsets within the Immune System superpathway. The 36 differentially abundant proteins predominantly associated to RAF/MAP kinase cascade, MHC-I-mediated antigen presentation and neutrophil degranulation, suggesting a latently activated phenotype of these innate immune cells in disease. Raw data are available via ProteomeXchange with identifier PXD013648. SIGNIFICANCE: Our study provides new insights into the protein repertoire of primary equine granulocytes and identifies protein abundance changes associated to equine recurrent uveitis (ERU), an organ specific, spontaneously occurring autoimmune disease. We show that granulocyte proteins with increased abundance in ERU strongly associate to RAF/MAP kinase signaling, MHC-I antigen presentation and neutrophil degranulation, pointing to a more activated state of these cells in ERU cases. Since cells were obtained in quiescent stage of disease, latent activation of granulocytes underlines the role of these innate immune cells in ERU. These findings are highly relevant for veterinary medicine, further establishing the importance of granulocytes in this T cell-driven autoimmune disease. Moreover, they have translational quality for autoimmune uveitis in man, due to strong similarity in disease occurrence, progression and pathogenesis.