Human corneal epithelial cells respond to ocular-pathogenic, but not to nonpathogenic-flagellin

Culture- and non-culture-based approaches reveal unique features of the ocular microbiome in dry eye patients

PURPOSE

The purpose of this study was to investigate the ocular microbiome in individuals with dry eye disease and to identify features of their ocular microbiome of possible health and diagnostic significance.

METHODS

Conjunctival samples were collected from both eyes in duplicate from 91 individuals (61 dry eye, 30 healthy) and used for both culture-dependent and culture-independent analyses. Samples were either analysed using next generation sequencing (V3-V4 16S rDNA) or inoculated on a wide range of agar types and grown under a broad range of conditions to maximize recovery. Isolates were identified by partial sequencing of the 16S rDNA and rpoB genes and tested for antibiotic susceptibility. We applied a L2-regularized logistic regression model on the next generation sequencing data to investigate any potential association between severe dry eye disease and the ocular microbiome.

RESULTS

Culture-dependent analysis showed the highest number of colony forming units in healthy individuals. The majority of isolates recovered from the samples were Corynebacterium, Micrococcus sp., Staphylococcus epidermidis, and Cutibacterium acnes. Culture independent analysis revealed 24 phyla, of which Actinobacteria, Firmicutes and Proteobacteria were the most abundant. Over 405 genera were detected of which Corynebacterium was the most dominant, followed by Staphylococcus and Cutibacterium. The L2-regularized logistic regression model indicated that Blautia and Corynebacterium sp. may be associated with severe DED.

CONCLUSIONS

Our study indicates that the ocular microbiome has characteristic features in severe DED patients. Certain Corynebacterium species and Blautia are of particular interest for future studies.

Anti-inflammatory and wound healing potential of medicinal maggot excretions/secretions at the ocular surface

PURPOSE

In the skin, Lucilia sericata maggot excretions/secretions (ES) accelerate wound healing and limit inflammation. This study aimed to determine whether ES have similar beneficial effects at the ocular surface.

METHODS

Human corneal epithelial cells (HCEC) were cultured with ES and cell viability was determined by the MTT assay. Additionally, mRNA expression of growth factors, antimicrobial peptides (AMPs) and cytokines was assessed by qPCR. ES ability to modulate TLR-induced IL-6 and IL-8 expression was determined by qPCR and ELISA. ES potential to promote corneal healing was evaluated in vitro by a migration assay in HCEC, and in vivo using a mouse model.

RESULTS

ES did not impair HCEC viability up to 25 μg/ml. Among the factors evaluated, only hBD-2 was upregulated (2.5-fold) by 1.5 μg/ml ES after 6 hrs (P = 0.04). In HCEC, ES reduced Poly I:C-induced IL-6 and IL-8 mRNA (P ≤ 0.001) and protein (P ≤ 0.0001) expression. A similar effect was observed with Flagellin (TLR5 agonist) but it was less robust for FSL-1 (TLR2/6 agonist) and Pam3CSK4 (TLR1/2 agonist). The greatest in vitro migration effect was observed with 6.2 μg/ml ES after 44 hrs where gap area compared to vehicle was 53.3 ± 3.7% vs. 72.6 ± 5.4% (P = 0.001). In the mouse model, the maximum healing effect was present with 1.5 μg/ml ES after 12 hrs with a wound area of 19.0 ± 2.7% vs. 60.1 ± 21.6% (P = 0.003) or 77% reduction of the wound area compared to the negative control.

CONCLUSIONS

ES significantly reduce in vitro TLR-induced production of inflammatory cytokines and promote corneal wound healing.

Suppression of Toll-like receptor-mediated innate immune responses at the ocular surface by the membrane-associated mucins MUC1 and MUC16

Membrane-associated mucins (MAMs) expressed on the ocular surface epithelium form a dense glycocalyx that is hypothesized to protect the cornea and conjunctiva from external insult. In this study, the hypothesis that the MAMs MUC1 and MUC16, expressed on the apical surface of the corneal epithelium, suppress Toll-like receptor (TLR)-mediated innate immune responses was tested. Using an in vitro model of corneal epithelial cells that are cultured to express MAMs, we show that reduced expression of either MUC1 or MUC16 correlates with increased message and secreted protein levels of the proinflammatory cytokines interleukin (IL)-6, IL-8, and tumor necrosis factor-α (TNF-α) following exposure of cells to the TLR2 and TLR5 agonists, heat-killed Listeria monocytogenes and flagellin, respectively. As mice express Muc1 (but not Muc16) in the corneal epithelium, a Muc1(-/-) mouse model was used to extend in vitro findings. Indeed, IL-6 and TNF-α message levels were increased in the corneal epithelium of Muc1(-/-) mice, in comparison with wild-type mice, following exposure of enucleated eyes to the TLR2 and TLR5 agonists. Our results suggest that the MAMs MUC1 and MUC16 contribute to the maintenance of immune homeostasis at the ocular surface by limiting TLR-mediated innate immune responses.

Expression and localization of epithelial stem cell and differentiation markers in equine skin, eye and hoof

BACKGROUND

The limited characterization of equine skin, eye and hoof epithelial stem cell (ESC) and differentiation markers impedes the investigation of the physiology and pathophysiology of these tissues.

HYPOTHESIS/OBJECTIVES

To characterize ESC and differentiation marker expression in epithelial tissues of the equine eye, haired skin and hoof capsule.

METHODS

Indirect immunofluorescence microscopy and immunoblotting were used to detect expression and tissue localization of keratin (K) isoforms K3, K10, K14 and K124, the transcription factor p63 (a marker of ESCs) and phosphorylated p63 [pp63; a marker of ESC transition to transit-amplifying (TA) cell] in epithelial tissues of the foot (haired skin, hoof coronet and hoof lamellae) and the eye (limbus and cornea).

RESULTS

Expression of K14 was restricted to the basal layer of epidermal lamellae and to basal and adjacent suprabasal layers of the haired skin, coronet and corneal limbus. Coronary and lamellar epidermis was negative for both K3 and K10, which were expressed in the cornea/limbus epithelium and haired skin epidermis, respectively. Variable expression of p63 with relatively low to high levels of phosphorylation was detected in individual basal and suprabasal cells of all epithelial tissues examined.

CONCLUSIONS

To the best of the author's knowledge, this is the first report of the characterization of tissue-specific keratin marker expression and the localization of putative epithelial progenitor cell populations, including ESCs (high p63 expression with low pp63 levels) and TA cells (high expression of both p63 and pp63), in the horse. These results will aid further investigation of epidermal and corneal epithelial biology and regenerative therapies in horses.

Role of TLR5 and flagella in bacillus intraocular infection

B. cereus possesses flagella which allow the organism to migrate within the eye during a blinding form of intraocular infection called endophthalmitis. Because flagella is a ligand for Toll-like receptor 5 (TLR5), we hypothesized that TLR5 contributed to endophthalmitis pathogenesis. Endophthalmitis was induced in C57BL/6J and TLR5−/− mice by injecting 100 CFU of B. cereus into the mid-vitreous. Eyes were analyzed for intraocular bacterial growth, retinal function, and inflammation by published methods. Purified B. cereus flagellin was also injected into the mid-vitreous of wild type C57BL/6J mice and inflammation was analyzed. TLR5 activation by B. cereus flagellin was also analyzed in vitro. B. cereus grew rapidly and at similar rates in infected eyes of C57BL/6J and TLR5−/− mice. A significant loss in retinal function in both groups of mice was observed at 8 and 12 hours postinfection. Retinal architecture disruption and acute inflammation (neutrophil infiltration and proinflammatory cytokine concentrations) increased and were significant at 8 and 12 hours postinfection. Acute inflammation was comparable in TLR5−/− and C57BL/6J mice. Physiological concentrations of purified B. cereus flagellin caused significant inflammation in C57BL/6J mouse eyes, but not to the extent of that observed during active infection. Purified B. cereus flagellin was a weak agonist for TLR5 in vitro. These results demonstrated that the absence of TLR5 did not have a significant effect on the evolution of B. cereus endophthalmitis. This disparity may be due to sequence differences in important TLR5 binding domains in B. cereus flagellin or the lack of flagellin monomers in the eye to activate TLR5 during infection. Taken together, these results suggest a limited role for flagellin/TLR5 interactions in B. cereus endophthalmitis. Based on this and previous data, the importance of flagella in this disease lies in its contribution to the motility of the organism within the eye during infection.

Rebamipide suppresses PolyI:C-stimulated cytokine production in human conjunctival epithelial cells

PURPOSE

We previously documented that ocular surface epithelial cells could regulate ocular surface inflammation and suggested that, while Toll-like receptor 3 upregulates, EP3, one of the prostaglandin E2 receptors, downregulates ocular surface inflammation. Others reported that rebamipide, a gastroprotective drug, could not only increase the gastric mucus production, but also suppressed gastric mucosal inflammation and that it was dominantly distributed in mucosal tissues. The eyedrop form of rebamipide, approved in Japan for use in the treatment of dry eye diseases, upregulates mucin secretion and production, thereby suppressing superficial punctate keratopathy on the ocular surface of patients with this disease. In the current study, we investigated whether rebamipide has anti- inflammatory effects on the ocular surface.

METHODS

To examine the effects of rebamipide on polyI:C-induced cytokine expression by primary human conjunctival epithelial cells, we used enzyme-linked immunosorbent assay and quantitative reverse transcription-polymerase chain reaction assay. We studied the effects of rebamipide on ocular surface inflammation in our murine experimental allergic conjunctivitis (EAC) model.

RESULTS

Rebamipide could suppress polyI:C-induced cytokine production and the expression of mRNAs for CXCL10, CXCL11, RANTES, MCP-1, and IL-6 in human conjunctival epithelial cells. In our EAC model, the topical administration of rebamipide suppressed conjunctival allergic eosinophil infiltration.

CONCLUSIONS

The topical application of rebamipide on the ocular surface might suppress ocular surface inflammation by suppressing the production of cytokines by ocular surface epithelial cells.

Innate immune regulation of Serratia marcescens-induced corneal inflammation and infection

PURPOSE

Serratia marcescens is frequently isolated from lenses of patients with contact lens-associated corneal infiltrates. In the current study, we examined the role of toll-like receptors (TLRs) and interleukin-1 receptor type 1 (IL-1R1) in S. marcescens-induced corneal inflammation and infection.

METHODS

The central corneal epithelium of C57BL/6 and gene knockout mice was abraded, and 1 × 10(7) S. marcescens were added in the presence of a silicone hydrogel contact lens, and we examined corneal inflammation by confocal microscopy and neutrophil enumeration. Viable bacteria were quantified by colony-forming units (CFU).

RESULTS

S. marcescens induced neutrophil recruitment to the corneal stroma, and increased corneal thickness and haze in C57BL/6 mice. Conversely, CFU was significantly lower by 48 hours post infection. In contrast, MyD88(-/-), IL-1R(-/-), TLR4(-/-), and TLR4/5(-/-) corneas infected with S. marcescens had significantly increased CFU, indicating impaired clearance. However, there was no significant difference in CFU among C57BL/6, TIRAP(-/-), and TRIF(-/-) mice. Tobramycin-killed S. marcescens induced corneal inflammation in C57BL/6 mice, which was impaired significantly in MD-2(-/-) mice and in C57BL/6 mice pretreated topically with the MD-2 antagonist eritoran tetrasodium.

CONCLUSIONS

S. marcescens induces corneal inflammation by activation of TLR4/MD-2/MyD88 and the IL-1R1/MyD88 pathways, which are potential therapeutic targets for inhibition of S. marcescens-induced corneal inflammation.

Ocular surface inflammation is regulated by innate immunity

On the ocular surface, as in the intestines and airway, the surface epithelium serves a critical function as the front-line defense of the mucosal innate immune system. Although the detection of microbes is arguably the most important task of the immune system, an exaggerated epithelial host defense reaction to endogenous bacteria may initiate and perpetuate inflammatory mucosal responses. In this review we first describe commensal bacteria found on the ocular surface, which is in contact with the ocular surface epithelium. We also discuss the innate immunity of the ocular surface epithelium and we present the allergic reaction regulated by ocular surface epithelial cells. We address ocular surface inflammation due to disordered innate immunity and we present our hypothesis that the onset of Stevens-Johnson syndrome (SJS) with severe ocular surface complications, a devastating ocular surface inflammatory disease, is strongly associated with abnormality of the innate immune system. In this review we raise the possibility that some ocular surface inflammatory diseases are pathogenetically related with a disordered innate immune response. Focusing on the innate immunity of the ocular surface might help to elucidate the pathogenesis of various ocular surface diseases.

Expression of interleukin-4 receptor α in human corneal epithelial cells

PURPOSE

We previously reported that human conjunctival epithelial cells expressed functioning interleukin-4 receptor α (IL-4Rα). In this study, we investigated whether human corneal epithelial cells also express functioning IL-4Rα.

METHODS

The presence of IL-4Rα mRNA and protein in human corneal epithelium was examined by reverse-transcriptase polymerase chain reaction (RT-PCR) and immunohistology, respectively. The cell surface expression of IL-4Rα and the transcripts upregulated upon IL-4Rα ligand (IL-4 or IL-13) stimulation were examined by flow cytometry and quantitative RT-PCR, respectively, using immortalized human corneal-limbal epithelial (HCLE) cells.

RESULTS

The mRNA and protein of IL-4Rα were detected in human corneal epithelium. Flow cytometry analysis showed the cell surface expression of IL-4Rα protein. Quantitative RT-PCR assay of HCLE cells showed the upregulation of the transcripts tumor necrosis factor alpha-induced protein 6 (TNFAIP6), RAS guanyl-releasing protein 1 (RASGRP1), carbonic anhydrase II (CA2), cytokine-inducible SH2-containing protein (CISH), hyaluronan synthase 3 (HAS3), calpain 14 (CAPN14), endothelin receptor type A (EDNRA), cathepsin C (CTSC), and lecithin retinol acyltransferase (LRAT) as well as human conjunctival epithelial cells.

CONCLUSION

Human corneal epithelial cells expressed functioning IL-4Rα, and stimulation of its ligands, IL-4 and IL-13, could induce the expression of various genes, e.g., antiinflammatory molecule genes such as TNFAIP6 and CISH and cellular differentiation and proliferation-related molecule genes such as RASGRP1, HAS3, EDNRA, and LRAT.

Toll-like receptors in ocular surface diseases: overview and new findings

The ocular surface is the first line of defence in the eye against environmental microbes. The ocular innate immune system consists of a combination of anatomical, mechanical and immunological defence mechanisms. TLRs (Toll-like receptors), widely expressed by the ocular surface, are able to recognize microbial pathogens and to trigger the earliest immune response leading to inflammation. Increasing evidence highlights the crucial role of TLRs in regulating innate immune responses during ocular surface infective and non-infective inflammatory conditions. In addition, recent observations have shown that TLRs modulate the adaptive immune response, also playing an important role in ocular autoimmune and allergic diseases. One of the main goals of ocular surface treatment is to control the inflammatory reaction in order to preserve corneal integrity and transparency. Recent experimental evidence has shown that specific modulation of TLR pathways induces an improvement in several ocular inflammatory conditions, such as allergic conjunctivitis, suggesting new therapeutic anti-inflammatory strategies. The purpose of the present review is to summarize the current knowledge of TLRs at the ocular surface and to propose them as potential targets of therapy for ocular inflammatory conditions.

Ocular surface inflammation mediated by innate immunity

This review addresses three subjects: the innate immunity of the ocular surface epithelium, innate immunity and ocular surface inflammation, and Stevens-Johnson syndrome (SJS) and abnormality of innate immunity. In innate immunity of the ocular surface epithelium, ocular surface epithelial cells respond selectively to microbial components and induce limited inflammation, whereas immune-competent cells such as macrophages can recognize various microbial components through Toll-like receptors (TLRs) and induce inflammation to exclude the microbes. The difference between macrophages and ocular surface epithelial cells may be caused by the dissimilarity in the degree of coexistence with commensal bacteria. The unique innate immune response of ocular surface epithelium might contribute to coexistence with commensal bacteria. In innate immunity and ocular surface inflammation, we speculate that an abnormality in the proper innate immunity of the ocular surface may result in ocular surface inflammation. Our investigation shows that TLR3 positively regulates the late-phase reaction of experimental allergic conjunctivitis, which causes reduced eosinophilic conjunctival inflammation in TLR3KO (knockout) mice and pronounced eosinophilic conjunctival inflammation in TLR3Tg mice. We also demonstrate that human ocular surface epithelial cells can be induced to express many transcripts, including antiviral innate immune response-related genes and allergy-related genes, through polyI:C stimulation. Furthermore, we show that IkappaBzeta KO mice exhibit severe, spontaneous ocular surface inflammation accompanied by the eventual loss of almost all goblet cells and spontaneous perioral inflammation. IkappaBzeta is induced by diverse pathogen-associated molecular patterns and regulates nuclear factor-kappaB activity, possibly to prevent excessive inflammation in the presence of bacterial components. The spontaneous ocular surface inflammation observed in IkappaBzeta KO mice suggested that dysfunction/abnormality of innate immunity can play a role in ocular surface inflammation. In SJS and abnormality of innate immunity, we considered the possibility that there may be an association between SJS and a disordered innate immune response. In gene expression analysis of CD14 cells, we found that IL4R gene expression was different in patients with SJS/toxic epidermal necrolysis (TEN) and controls on lipopolysaccharide stimulation, being downregulated in patients with SJS/TEN and slightly upregulated in the controls. The expression of IkappaBzeta- and interleukin (IL)-1alpha-specific mRNA in patients with SJS/TEN was lower than in normal controls after 1-hour culture. Although SJS/TEN can be induced by drugs, not all individuals treated with these drugs developed SJS/TEN. Because the incidence of SJS/TEN is very low, we suspected a genetic predisposition and performed single-nucleotide polymorphism (SNP) association analysis using candidate genes associated with innate immunity, apoptosis, or allergy. We found that TLR3 SNP rs.3775296 and IL4R SNP rs.1801275 (Gln551Arg) were strongly associated (P<0.0005) with SJS/TEN with ocular surface complications, FasL rs.3830150 SNP was mildly associated (P<0.005), and IL13 rs.20541 (Arg110Gln) and IkappaBzeta SNP rs.595788G/A exhibited a weak association (P<0.05). Genetic and environmental factors may play a role in an integrated cause of SJS, and there is the possibility of an association between SJS and a disordered innate immunity.

Toll-like receptor activation modulates antimicrobial peptide expression by ocular surface cells

The ability of the ocular surface to respond to pathogens is in part attributed to toll-like receptors (TLRs) that recognize conserved motifs on various microbes. This study examines TLR expression on various ocular surface cells, if TLR agonists can modulate the expression of antimicrobial peptides (AMPs), human beta defensins (hBD) and cathelicidin (hCAP-18/LL-37) which maybe functionally active against Pseudomonas aeruginosa (PA) and if TLR agonists or AMPs can modulate TLR mRNA expression. TLR1-10 mRNA expression was examined in corneal epithelial, corneal stromal cells and conjunctival epithelial cells by RT-PCR. To confirm protein expression flow cytometry or immunostaining was performed for selected TLRs on some cell cultures. Ocular surface cells were cultured with a range of TLR agonists and then hBD-1, 2, 3, or hCAP-18 mRNA and protein expression was determined by RT-PCR and immunoblotting. In some experiments, cells were cultured with a cocktail of agonists for TLR3, 5 and 6/2 and the antimicrobial activity of the culture media was tested against PA. TLR mRNA expression was also examined in primary human corneal epithelial cells (HCEC) treated with either 3 μg/ml of hBD-2, 5 μg/ml of LL-37 or TLR4, 5 and 9 agonists. Overall, the ocular surface cells expressed mRNA for most of the TLRs but some differences were found. TLR2 was not detected in corneal fibroblasts, TLR4 was not detected in primary cultured or freshly isolated HCEC, TLR5 was not detected in conjunctival epithelial cells (IOBA-NHC) and corneal fibroblasts, TLR7 was not detected in freshly isolated HCEC and TLR10 was not detected in HCEC and IOBA-NHC. TLR8 mRNA was not expressed by any of the samples tested. Immunostaining of cadaver corneas revealed TLR5 and 9 expression throughout the cornea while TLR3 was significantly expressed only in the epithelium. Flow cytometry and immunostaining revealed cultured fibroblasts expressed TLR9 but had no significant TLR3 expression. hBD-2 expression was upregulated by TLR1/2, 3, 4, 5 and 6/2 agonists depending on the cell type, whereas only the TLR3 agonist upregulated the expression of hCAP-18 in primary HCEC. The combination of TLR3, 5 and 6/2 agonists in primary HCEC, upregulated hBD-2 and hCAP-18 mRNA and peptide expression and secretion into the culture media, which significantly killed PA. This antimicrobial activity was primarily attributed to LL-37. TLR agonists did not modulate TLR expression itself, however, LL-37 or hBD-2 downregulated TLR5, 7 and/or 9 mRNA depending on the cell type. TLRs are expressed on the ocular surface and TLR agonists trigger the production of LL-37 and hBD-2, with LL-37 being particularly important for protecting the ocular surface against PA infection.

Toll-like receptors in ocular surface disease

The ability of the ocular surface to mount an immune response is in part attributed to a family of proteins called toll-like receptors (TLRs). The latter are evolutionary conserved receptors that recognize and respond to various microbes and endogenous ligands. In addition to their recognition function, TLR activation triggers a complex signal transduction cascade that induces the production of inflammatory cytokines and co-stimulatory molecules, thus initiating innate and adaptive immunity. Toll-like receptor expression at the ocular surface is modulated during infection (e.g. Herpes simplex, bacterial keratitis and fungal keratitis) as well as during various inflammatory conditions (allergic conjunctivitis and dry-eye syndrome). Here recent findings regarding TLR expression and their involvement in various ocular surface diseases are discussed.

Innate immunity of the ocular surface

The ocular surface epithelium serves a critical function as the defensive front line of the innate immune system. While the detection of microbes is arguably its most important task, an exaggerated host defense reaction to endogenous bacterial flora may initiate and perpetuate inflammatory mucosal responses. The ability of cells to recognize pathogen-associated molecular patterns (PAMPs) mainly depends on the expression of a family of Toll-like receptors (TLRs). A healthy ocular surface is not inflammatory, even though ocular surface epithelium is in constant contact with bacteria and bacterial products. In this study, we show that human ocular surface epithelial cells, both corneal and conjuctival epithelial cells, respond to viral double-stranded RNA mimic polyI:C to produce pro-inflammatory cytokines through TLR3, while they fail to respond functionally to lipopolysaccharide, a TLR4 ligand. Moreover, human ocular surface epithelium responds to flagellins from ocular pathogenic, but not ocular non-pathogenic bacteria, to produce pro-inflammatory cytokines through TLR5. Thus, ocular surface epithelial cells selectively respond to microbial components and induce limited inflammation; immune-competent cells can recognize microbial components through TLRs and induce the inflammation. The unique innate immune response of the ocular surface epithelium may contribute to its coexistence with commensal bacteria. Inflammatory bowel disease is thought to result from an abnormal response to the gut microbiota. Thus, we also considered the possibility of an association between ocular surface inflammation and a disordered innate immune response. IkappaBzeta is important for TLR signaling, in mice, its knock-out produced severe, spontaneous ocular surface inflammation, the eventual loss of goblet cells, and spontaneous perioral inflammation, suggesting that dysfunction/abnormality of innate immunity can lead to ocular surface inflammation.