Ocular surface immunity: homeostatic mechanisms and their disruption in dry eye disease

The tear film, lacrimal glands, corneal and conjunctival epithelia and Meibomian glands work together as a lacrimal functional unit (LFU) to preserve the integrity and function of the ocular surface. The integrity of this unit is necessary for the health and normal function of the eye and visual system. Nervous connections and systemic hormones are well known factors that maintain the homeostasis of the ocular surface. They control the response to internal and external stimuli. Our and others' studies show that immunological mechanisms also play a pivotal role in regulating the ocular surface environment. Our studies demonstrate how anti-inflammatory factors such as the expression of vascular endothelial growth factor receptor-3 (VEGFR-3) in corneal cells, immature corneal resident antigen-presenting cells, and regulatory T cells play an active role in protecting the ocular surface. Dry eye disease (DED) affects millions of people worldwide and negatively influences the quality of life for patients. In its most severe forms, DED may lead to blindness. The etiology and pathogenesis of DED remain largely unclear. Nonetheless, in this review we summarize the role of the disruption of afferent and efferent immunoregulatory mechanisms that are responsible for the chronicity of the disease, its symptoms, and its clinical signs. We illustrate current anti-inflammatory treatments for DED and propose that prevention of the disruption of immunoregulatory mechanisms may represent a promising therapeutic strategy towards controlling ocular surface inflammation.

The Controlled-Environment Chamber: A New Mouse Model of Dry Eye

PURPOSE

To develop a controlled-environment chamber (CEC) for mice and verify the effects of a low-humidity setting on ocular surface signs in normal mice.

METHODS

Eight- to 12-week-old BALB/c mice were used in a controlled-environment chamber (CEC) where relative humidity (RH), temperature (T), and airflow (AF) are regulated and monitored. Mice were placed into the CEC and exposed to specific environmentally controlled conditions (RH = 18.5% +/- 5.1%, AF = 15 L/min, T = 21-23 degrees C) for 3, 7, 14, and 28 days. Control mice were kept in a normal environment (RH = 50%-80%, no AF, T = 21-23 degrees C) for the same duration. Aqueous tear production by means of the cotton thread test, corneal fluorescein staining (score, 0-15), and goblet cell density in the superior and inferior conjunctiva were measured by a masked observer.

RESULTS

No statistically significant differences between the groups were found at baseline. Decreased tear secretion and increased corneal fluorescein staining were significantly present on day 3, 7, 14, and 28 in animals kept in the CEC. Goblet cell density was significantly decreased in the superior conjunctiva on day 7, and on day 3, 7, and 14 in the inferior conjunctiva in the CEC-kept mice compared with control animals.

CONCLUSIONS

This study indicates that exposure of normal mice to a low-humidity environment in a CEC can lead to significant alterations in tear secretion, goblet cell density, and acquisition of dry eye-related ocular surface signs.

Antisense probing of the human U4/U6 snRNP with biotinylated 2'-OMe RNA oligonucleotides

We have used antisense 2'-OMe RNA oligonucleotides carrying four 5'-terminal biotin residues to probe the structure and function of the human U4/U6 snRNP. Nine oligonucleotides, complementary to multiple regions of U4 and U6 snRNAs, bound stably and specifically to U4/U6 snRNP. This allowed for efficient and selective removal of U4/U6 from HeLa cell nuclear extracts. Binding of oligonucleotides to certain snRNA domains inhibited splicing and affected the U4-U6 interaction. Pre-mRNA and splicing products could also be affinity-selected through binding of the oligonucleotides to U4/U6 snRNPs in splicing complexes. The results suggest that U4 snRNP is not released during spliceosome assembly.

Systemic linoleic and gamma-linolenic acid therapy in dry eye syndrome with an inflammatory component

PURPOSE

To evaluate the efficacy and anti-inflammatory activity of systemic linoleic (LA) and gamma-linolenic acid (GLA), which decrease chronic inflammation in rheumatoid arthritis, on the ocular surface of patients with keratoconjunctivitis sicca.

METHODS

In a randomized clinical trial, 26 patients with aqueous-deficient keratoconjunctivitis sicca were consecutively selected from patients presenting to Department of Neurosciences, Ophthalmology and Genetics, University of Genoa. The diagnosis was based on dry eye symptom survey score, Schirmer-1 test values, positive vital staining with lissamine green, and fluorescein break-up time (FBUT). All patients had ocular surface inflammation based on HLA-DR expression, a major histocompatibility class II antigen, on epithelial bulbar conjunctiva samples. The subjects were randomly divided into two groups of 13 patients each. The study group received tablets containing LA (28.5 mg) and GLA (15 mg) twice daily for 45 days and used tears; the control group received a tear substitute and a placebo tablet for 45 days.

RESULTS

Statistically significant changes in symptoms (p < 0.005), lissamine green staining (p < 0.005), and ocular surface inflammation (p < 0.05) occurred in the study group compared with controls. HLA-DR expression varied from 58.5 +/- 14.1% positive conjunctival cells to 41.3 +/- 18.9% in the treated group and from 61.4 +/- 21.9% to 58.0 +/- 13.3% in the controls. No statistically significant difference between groups was found for FBUT and the Schirmer-1 test.

CONCLUSIONS

Therapy with LA and GLA and tear substitutes reduces ocular surface inflammation and improves dry eye symptoms. Long-term studies are needed to confirm the role of this new therapy for keratoconjunctivitis sicca.

A multicentre, double-masked, randomized, controlled trial assessing the effect of oral supplementation of omega-3 and omega-6 fatty acids on a conjunctival inflammatory marker in dry eye patients

PURPOSE

To determine whether oral supplementation with omega-3 and omega-6 fatty acids can reduce conjunctival epithelium expression of the inflammatory marker human leucocyte antigen-DR (HLA-DR) in patients with dry eye syndrome (DES).

METHODS

This 3-month, double-masked, parallel-group, controlled study was conducted in nine centres, in France and Italy. Eligible adult patients with mild to moderate DES were randomized to receive a placebo containing medium-chain triglycerides or treatment supplement containing omega-3 and omega-6 fatty acids, vitamins and zinc. Treatment regimen was three capsules daily. Impression cytology (IC) was performed at baseline and at month 3 to assess the percentage of cells expressing HLA-DR and to evaluate fluorescence intensity, an alternate measure of HLA-DR. Dry eye symptoms and objective signs were also evaluated. Analyses were performed on the full analysis set (FAS) and per-protocol set (PPS).

RESULTS

In total, 138 patients were randomized; 121 patients with available IC were included in the FAS, and of these, 106 patients had no major protocol deviations (PPS). In the PPS, there was a significant reduction in the percentage of HLA-DR-positive cells in the fatty acids group (p = 0.021). Expression of HLA-DR as measured by fluorescence intensity quantification was also significantly reduced in the fatty acids group [FAS (p = 0.041); PPS (p = 0.017)]. No significant difference was found for the signs and symptoms, but there was a tendency for improvement in patients receiving the fatty acids treatment.

CONCLUSION

This study demonstrates that supplementation with omega-3 and omega-6 fatty acids can reduce expression of HLA-DR conjunctival inflammatory marker and may help improve DES symptoms.

Distribution of Conjunctival HLA-DR Expression and the Pathogenesis of Damage in Early Dry Eyes

PURPOSE

To examine the expression of HLA-DR, a marker of inflammation, in the early stages of dry eye disease and to locate the appearance of this marker on specific areas of the bulbar conjunctiva.

METHODS

Dry eye patients were identified and their condition classified as mild (n = 16) or moderate (n = 16) based on Schirmer testing, vital staining, tear break-up time, and symptom questionnaire scores. Brush cytology was used to collect epithelial cells from the nasal, temporal, and superior conjunctivae of patients and age-matched controls. HLA-DR positive cells were detected by immunohistochemical staining and quantified.

RESULTS

Patients with moderate dry eye had the highest rate of conjunctival HLA-DR-positive cells, with significantly higher rates than controls regardless of which region of the conjunctiva was sampled (P < 0.01). The mild dry eye group had similar rates of HLA-DR-positive cells in the superior conjunctival region compared with controls. However, in the nasal and temporal regions, they displayed a significantly higher rate of HLA-DR-positive cells than controls (P < 0.01) and the nasal region showed a significant difference (P < 0.01) when compared with the temporal one. Some of these mild dry eyes had no vital staining.

CONCLUSIONS

The HLA-DR expression pattern in mild and moderate dry eyes appears to reflect disease progression. Overexpression of HLA-DR in mild dry eyes showing no vital staining suggests that inflammation may be a primary cause of ocular surface damage. These data support the use of immunomodulatory drugs in the treatment of dry eye disease.

TFOS DEWS II Clinical Trial Design Report

The development of novel therapies for Dry Eye Disease (DED) is formidable, and relatively few treatments evaluated have been approved for marketing. In this report, the Subcommittee reviewed challenges in designing and conducting quality trials, with special reference to issues in trials in patients with DED and present the regulatory perspective on DED therapies. The Subcommittee reviewed the literature and while there are some observations about the possible reasons why so many trials have failed, there is no obvious single reason other than the lack of correlation between signs and symptoms in DED. Therefore the report advocates for conducting good quality studies, as described, going forward. A key recommendation for future studies is conduct consistent with Good Clinical Practice (GCP), including use of Good Manufacturing Practice (GMP) quality clinical trial material. The report also recommends that the design, treatments, and sample size be consistent with the investigational treatment, the objectives of the study, and the phase of development. Other recommendations for pivotal studies are a priori selection of the outcome measure, and an appropriate sample size.

Corneal epithelial proliferation and thickness in a mouse model of dry eye

Although several studies have previously focused on the conjunctival epithelial response to surface dryness, little is known about the effect of a dry environment on corneal epithelium, which is the most clinically significant tissue affected in dry eye. The aim of this study was to quantitatively evaluate the effect of desiccating stress on the number of proliferating corneal epithelial cells and corneal epithelial thickness in mice placed in a controlled-environment chamber (CEC) that induces dry eye. Corneal epithelial cell proliferation and thickness were studied in 8- to 12-week-old female BALB/c mice placed in the CEC (temperature: 22.3+/-0.7 degrees C; relative humidity: 22.5+/-4.5%; airflow: 15 L/min) for 7 days and compared to a control group of mice with no dry eye. Actively proliferating cells were identified by immunofluorescence using a FITC-conjugated antibody against the Ki-67 protein, a cell proliferation marker expressed during active phases of the cell cycle. To detect the spatial distribution of proliferative cells, Ki-67(+) cells were counted in three areas of the epithelium: center, periphery, and limbus. Corneal epithelial thickness was evaluated in the central cornea after staining with hematoxylin-eosin. Results from each experimental group were compared using the Mann-Whitney test. The number of Ki-67(+) cells observed in the corneal epithelium of mice exposed to the CEC was significantly higher in each area (center: 32.1+/-1.1; periphery: 94.2+/-5.3; limbus: 4.0+/-1.5) than in the control group (center: 13.2+/-1.0, p=0.02; periphery: 42.9+/-2.3, p=0.02; limbus: 0.0, p=0.01). In mice subjected to desiccating stress, a significant number of Ki-67(+) positive cells were detected in the basal and suprabasal cell layers (central area 46%; periphery 30.8%: limbus 0%), whereas in the control group the cells were exclusively distributed through the basal cell layer. Ki-67(+) cells were not found in the corneal stroma or endothelium in any group. The corneal epithelium was found to be significantly thicker in dry eye mice (54.94+/-6.09 microm) as compared to the controls (43.9+/-6.23 microm, p<0.0001) by a mean of 25%. These results demonstrate that desiccating stress increases corneal epithelial turnover and thickness, similar to what is observed in other chronic inflammatory states of other epithelialized surfaces. The CEC can facilitate the study of the regulation of epithelial cell function and turnover at the molecular and cellular levels under desiccating stress conditions.

The ocular microbiome and microbiota and their effects on ocular surface pathophysiology and disorders

The ocular surface flora perform an important role in the defense mechanisms of the ocular surface system. Its regulation of the immunological activity and the barrier effect against pathogen invasion are remarkable. Composition of the flora differs according to the methods of investigation, because the microbiome, composed of the genetic material of bacteria, fungi, viruses, protozoa, and eukaryotes on the ocular surface, differs from the microbiota, which are the community of microorganisms that colonize the ocular surface. The observed composition of the ocular surface flora depends on harvesting and examining methods, whether with traditional culture or with more refined genetic analysis based on rRNA and DNA sequencing. Environment, diet, sex, and age influence the microbial flora composition, thus complicating the analysis of the baseline status. Moreover, potentially pathogenic organisms can affect its composition, as do various disorders, including chronic inflammation, and therapies applied to the ocular surface. A better understanding of the composition and function of microbial communities at the ocular surface could bring new insights and clarify the epidemiology and pathology of ocular surface dynamics in health and disease. The purpose of this review is to provide an up-to-date overview of knowledge about this topic.

Tear film and ocular surface tests in animal models of dry eye: uses and limitations

Many ocular surface tests have been developed for the clinical diagnosis of keratoconjunctivitis sicca, and several have been used to 'validate' animal models of Sjogren's and non-Sjogren's dry eye. However, many of these tests themselves have not been systematically studied or standardized, and yet their use in animal models of dry eye is common. This review provides a rational approach and systematic review of the tear film and ocular surface tests described in the literature that may be applicable to the current animal models of dry eye, with particular emphasis on their limitations, along with some suggestions regarding their standardization and applications in eye research.

Role of amniotic membrane transplantation for conjunctival reconstruction in ocular-cicatricial pemphigoid

PURPOSE

To evaluate the role and the effectiveness over time of amniotic membrane transplantation (AMT) as a first-step procedure to treat conjunctival reconstruction in late-stage ocular-cicatricial pemphigoid (OCP).

DESIGN

Prospective interventional noncomparative case series.

PARTICIPANTS

Nine eyes (9 patients) with advanced OCP.

METHODS

Preoperatively, the ocular surface conditions were evaluated by immunohistochemistry of conjunctival biopsy and impression cytology specimens. The amniotic membrane was obtained during cesarean section from women who were 39 weeks pregnant and seronegative for human immunodeficiency virus, hepatitis B and C, and syphilis; it was processed, histologically tested, and stored at -80 degrees C. After scar tissue was removed, the preserved amniotic membrane was placed over the cornea, the bulbar, and tarsal conjunctiva, and was secured with 8-0 Vicryl sutures to the conjunctival edges and the deep fornices with double-armed 6-0 silk sutures. In 2 cases a double layer of amniotic membrane was transplanted. All patients received immunosuppressive systemic therapy and preservative-free tear substitutes and steroids topically for at least 6 months. During follow-up (average, 48 weeks; range, 28-96 weeks), a new standardized method was used to evaluate the fornix depth, and impression cytology testing was performed and conjunctival inflammation recorded and used as parameters for monitoring disease activity.

MAIN OUTCOME MEASURES

Symblepharon, increased inferior fornix depth, presence of conjunctival goblet cells, and the degree of conjunctival inflammation.

RESULTS

The conjunctival surface was free from symblepharon in all subjects for the first 16 weeks. At the week 28 examination, a small area of symblepharon was present in four eyes (44.4%). The depth of the fornix was significantly (P < 0.0001, analysis of variance) improved at weeks 4, 16, and 28. The normal conjunctival epithelium with goblet cells was restored in 6 of 9 eyes (66.7%) at the week 4 examination and in 4 eyes (44.4%) at the week 28 examination. Conjunctival inflammation was clinically but not statistically reduced. The visual acuity improved in 5 subjects.

CONCLUSIONS

AMT can be a first-step procedure for ocular surface reconstruction in OCP, but its effectiveness deteriorates slightly over time.

Novel expression and characterization of lymphatic vessel endothelial hyaluronate receptor 1 (LYVE-1) by conjunctival cells

PURPOSE

Lymphatic vessel endothelial hyaluronic acid receptor (LYVE-1) is a newly discovered lymphatic-specific marker. To date, there is no report of its expression on conjunctival cells. The purpose of this study was to investigate the expression of LYVE-1 in normal conjunctiva, to phenotype LYVE-1+ cells, and to study changes in their expression levels during corneal inflammation.

METHODS

Flat-mounted conjunctivae or cross sections of eyeballs were harvested from BALB/c mice (6-8 weeks of age) for immunofluorescent confocal microscopic studies.

RESULTS

The data demonstrate, for the first time, that in addition to its expression on lymphatic vessels, LYVE-1 was expressed on CD45+, CD11b+, and CD31- conjunctival cells, indicating a bone-marrow-derived monocytic lineage. Surprisingly, the number of cells that expressed LYVE-1 decreased during corneal inflammation, in conjunction with ingrowth of lymphatics into the cornea.

CONCLUSIONS

A new population of monocytic cells has been found to express LYVE-1 in normal conjunctiva. These cells that normally express LYVE-1 may act as a reservoir for lymphangiogenesis and cell recruitment when the immune system is challenged.

Artificial Tears: Biological Role of Their Ingredients in the Management of Dry Eye Disease

Dry eye disease (DED) is the most common ocular surface disease, characterized by insufficient production and/or instability of the tear film. Tear substitutes are usually the first line of treatment for patients with DED. Despite the large variety of tear substitutes available on the market, few studies have been performed to compare their performance. There is a need to better understand the specific mechanical and pharmacological roles of each ingredient composing the different formulations. In this review, we describe the main categories of ingredients composing tear substitutes (e.g., viscosity-enhancing agents, electrolytes, osmo-protectants, antioxidants, lipids, surfactants and preservatives) as well as their effects on the ocular surface, and we provide insight into how certain components of tear substitutes may promote corneal wound healing, and/or counteract inflammation. Based on these considerations, we propose an approach to select the most appropriate tear substitute formulations according to the predominant etiological causes of DED.

Effect of the ocular microenvironment in regulating corneal dendritic cell maturation

OBJECTIVE

To determine whether the ocular anterior segment (aqueous humor and cornea) actively inhibits dendritic cell (DC) maturation.

METHODS

Dendritic cells were injected into syngeneic corneas or conjunctivae, and their surface major histocompatibility complex class II expression in response to the local milieu was assessed using confocal microscopy. Immature DCs were cocultured with corneal supernatant or with aqueous humor to evaluate their regulation of DC phenotypic and functional maturity.

RESULTS

In contrast to conjunctivally injected DCs, DCs injected into the cornea resisted up-regulation in expression of surface major histocompatibility complex class II. Corneal supernatant-treated and aqueous humor-treated DCs retained their immaturity, as reflected by high antigen uptake but low costimulatory molecule (CD80 and CD86) expression and poor T-cell stimulation. Anti-transforming growth factor beta(2) treatment of aqueous humor and of corneal supernatant led to complete and partial blockade of their inhibition of DC maturation, respectively. However, alpha-melanocyte-stimulating hormone and calcitonin gene-related peptide had no demonstrable effect on DC maturation.

CONCLUSION

Cornea and aqueous humor, principally through transforming growth factor beta(2,) promote generation of phenotypically and functionally immature DCs. Clinical Relevance Our results indicate that relative immune quiescence in the cornea and in the anterior segment is actively maintained in part by the inhibitory effect of transforming growth factor beta(2) on resident DCs and by their suppression of T-cell-mediated immune and inflammatory responses.

Dry eye syndromes

Over the past 20 years it has become clear that dry eye syndrome (DES) or keratoconjunctivitis sicca (KCS) is a complex multifactorial disease characterized by an immune and inflammatory process that affects the lacrimal glands and ocular surface. In this paradigm, inflammation is seen as both the cause and consequence of conjunctival and corneal cell damage. In this chapter, we identify the unique characteristics of the lacrimal gland, the role of epithelial cells, regulatory T cells, and cytokines in maintaining ocular surface homeostasis and tear secretion function. We analyze the factors inducing loss of the lacrimal gland homeostasis and its consequences, and in so doing hope to provide a picture of the role of the immune system in the pathophysiology of KCS and useful information to help understand the complexity of DES.

Dry eye modulates the expression of toll-like receptors on the ocular surface

We aimed to determine if toll-like receptor (TLR) expression is modulated in response to dry eye-associated conditions and in dry eye syndrome (DES). Primary human corneal epithelial cells (HCEC), an SV40 HCEC cell line or a normal human conjunctival epithelial cell line (IOBA-NHC) were cultured under hyperosmolar stress (HOS) (400-500 mOsm/kg) or with DES associated cytokines (IL-1α/β, TNFα or TGFβ) at concentrations ranging from 1 to 1000 ng/ml for up to 24 h. Epithelial cells were harvested from a human cornea organ culture model following 24 h of desiccation. Conjunctival impression cytology samples were harvested from subjects with DES and age and gender-matched normal subjects. TLR4, TLR5 or TLR9 mRNA or protein was examined by quantitative RT-PCR, western blotting or flow cytometry. TLR functionality was evaluated in terms of addition of TLR agonists and quantitation of secreted inflammatory cytokines by the use of ELISA and Luminex assays. In SV40 HCEC, HOS significantly increased TLR4 by 8.18 fold, decreased TLR9 by 0.58 fold, but had no effect on TLR5 mRNA expression. TLR4 and TLR9 protein were decreased by 67.7% and 72% respectively. TLR4 mRNA was also significantly up-regulated by up to 9.70 and 3.36 fold in primary HCEC and IOBA-NHC respectively. DES associated cytokines had no effect on TLR4, TLR5 and TLR9 expression. In response to desiccation, TLR4 and TLR5 mRNA were significantly up-regulated by 4.81 and 2.51 fold respectively, while TLR9 mRNA was down-regulated by 0.86 fold in HCEC. A similar trend for TLR4 and TLR9 protein was observed. TLR9 mRNA was significantly down-regulated by almost 59.5% in DES subjects. In conclusion, changes in TLR expression occur in dry eye and could have an important role in ocular surface susceptibility to inflammation and infection.

Very late antigen 1 blockade markedly promotes survival of corneal allografts

OBJECTIVE

To investigate the role of very late antigen 1 (VLA-1) (also known as integrin receptor alpha(1)beta(1)) in corneal transplantation inflammation and allograft survival.

METHODS

Cell infiltration and vasculogenesis (both angiogenesis and lymphangiogenesis) associated with allodisparate corneal transplantation were assessed in VLA-1-deficient conditions and controls by immunofluorescent microscopic studies. Corneal allograft survival was also assessed after anti-VLA-1 antibody treatment and in VLA-1 knockout recipient mice.

RESULTS

Anti-VLA-1 antibody treatment leads to a profound reduction in the granulocytic, monocytic, and T-cell infiltration after corneal transplantation. In addition, corneal angiogenesis and lymphangiogenesis were both significantly suppressed in VLA-1 knockout mice. Remarkably, universal graft survival was observed in both anti-VLA-1 antibody treatment and knockout mice.

CONCLUSIONS

Very late antigen 1 blockade markedly reduces inflammation and inflammation-induced tissue responses, including vasculogenic responses, associated with corneal transplantation and promotes allograft survival.

CLINICAL RELEVANCE

These studies offer insights into important integrin-mediated mechanisms of corneal transplant-related inflammation and provide possible new integrin-based immunotherapies for transplant rejection.

The role of systemic and topical fatty acids for dry eye treatment

Dry eye is a prevalent condition and one of the main reasons for patients to seek ophthalmic medical care. A low systemic level of omega fatty acids is a risk factor for dry eye disease (DED). There are two groups of essential fatty acids (EFAs): the omega-6 (n-6) family and the omega-3 (n-3) family. Humans evolved on a diet in which the n-6:n-3 ratio was approximately 1:1, however the current Western diet tends to be deficient in n-3 EFAs and this ratio is typically much higher (approaching 17:1). The metabolism of EFAs generates four new families of local acting mediators: lipoxins, resolvins, protectins, and maresins. These molecules have anti-inflammatory and pro-resolution properties. We present a critical overview of animal model studies and human clinical trials that have shown that dietary modification and oral supplementation could be complementary therapeutic strategies for the treatment of dry eye. Furthermore, we discuss preliminary results of the topical application of n-3 and n-6 EFAs because these molecules may act as natural anti-inflammatory agents with positive changes of the entire ocular surface system.

Radiotherapy-Induced Ocular Surface Disease

Today radiation is routinely used as a therapeutic modality for select tumors of the orbit, adnexa, paranasal sinus, and nasopharynx. Despite significant improvements in mechanisms of delivery and protective shielding, acute and chronic complications of radiation can affect different segments of the eye. In this report, we provide an overview of ocular damage secondary to radiotherapy. We identify the characteristic clinical changes and underlying pathophysiologic mechanisms involving the ocular surface and provide a rational approach to their prevention and treatment.