Topical flagellin protects the injured corneas from Pseudomonas aeruginosa infection

Neutrophil subsets enhance the efficacy of host-directed therapy in pneumococcal pneumonia

Host-directed therapy, using nasal administration of the Toll-like receptor 5 agonist flagellin in combination with antibiotics, has proven effective against pneumococcal pneumonia. In this study, we investigated the immune mechanisms underlying the therapy-induced protective effects. Transcriptomic analysis of lung tissue during infection revealed that flagellin not only enhanced pathways associated with myeloid cell infiltration into the airways and antimicrobial functions, but also promoted the early and transient mobilization of neutrophils and inflammatory monocytes. Neutrophils were identified as crucial for the protective effects of flagellin. The adjunct activity of flagellin correlated with the increased recruitment of neutrophils into airways, their localization at the periphery of bronchi, alveoli, and lung vessels, along with alterations in phagocytic activity. Clustering analysis identified seven neutrophil subsets; notably, flagellin adjunct treatment expanded clusters involved in recruitment and antibacterial activity, and primed augmented functionality. In conclusion, this study highlights specific neutrophil subsets as a promising target for host-directed therapy in infection.

More than Antibiotics: Latest Therapeutics in the Treatment and Prevention of Ocular Surface Infections

Ocular surface infections have been common issues for ophthalmologists for decades. Traditional strategies for infection include antibiotics, antiviral agents, and steroids. However, multiple drug-resistant bacteria have become more common with the prevalence of antibiotic use. Furthermore, an ideal treatment for an infectious disease should not only emphasize eliminating the microorganism but also maintaining clear and satisfying visual acuity. Immunogenetic inflammation, tissue fibrosis, and corneal scarring pose serious threats to vision, and they are not attenuated or prevented by traditional antimicrobial therapeutics. Herein, we collected information about current management techniques including stem-cell therapy, probiotics, and gene therapy as well as preventive strategies related to Toll-like receptors. Finally, we will introduce the latest research findings in ocular drug-delivery systems, which may enhance the bioavailability and efficiency of ocular therapeutics. The clinical application of improved delivery systems and novel therapeutics may support people suffering from ocular surface infections.

IL-36α Enhances Host Defense against Pseudomonas aeruginosa Keratitis in C57BL/6 Mouse Corneas

The IL-36 cytokines are known to play various roles in mediating the immune response to infection in a tissue- and pathogen-dependent manner. The present study seeks to investigate the role of IL-36R signaling in C57BL/6 mouse corneas in response to Pseudomonas aeruginosa infection. IL-36α-/-, IL-36γ-/-, and IL-36R-/- mice had significantly more severe keratitis than wild-type mice. At six hours postinfection, IL-36α pretreatment augmented P. aeruginosa-induced expression of IL-1Ra, IL-36γ, LCN2, and S100A8/A9. At one day postinfection, exogenous IL-36α suppressed, whereas IL-36α deficiency promoted, the expression of IL-1β. At three days postinfection, exogenous IL-36α suppressed Th1 but promoted Th2 immune response. IL-36α stimulated the infiltration of IL-22-expressing immune cells, and IL-22 neutralization resulted in more severe keratitis. IL-36α alone stimulated dendritic cell infiltration in B6 mouse corneas. Taken together, our study suggests that IL-36R signaling plays a protective role in the pathogenesis of P. aeruginosa keratitis by promoting the innate immune defense, Th2, and/or Th22/IL-22 immune responses. Exogenous IL-36α might be a potential therapy for improving the outcome of P. aeruginosa keratitis.

Conjunctival Goblet Cell Responses to TLR5 Engagement Promote Activation of Local Antigen-Presenting Cells

Conjunctival epithelium forms a barrier between the ocular surface microbial flora and the ocular mucosa. In addition to secreting gel-forming mucins, goblet cells, located in the conjunctival epithelium, help maintain local immune homeostasis by secreting active TGFβ2 and promoting tolerogenic phenotype of dendritic cells in the vicinity. Although dendritic cell subsets, characteristic of mucosal tissues, are found in the conjunctiva, previous studies provided limited information about their location within the tissue. In this study, we examine immunostained conjunctiva explants to determine the location of CD11c-positive dendritic cells in the context of MUC5AC-positive goblet cells. Considering that conjunctival goblet cells are responsive to signaling induced by pathogen recognition receptors, we also assess if their responses to microbial product, flagellin, can contribute to the disruption of ocular mucosal homeostasis that promotes activation of dendritic cells and results in chronic ocular surface inflammation. We find that dendritic cells in the conjunctiva with an increased microbial colonization are located adjacent to goblet cells. While their cell bodies in the stromal layer are immediately below the epithelial layer, several extensions of dendritic cells are projected across the epithelium towards the ocular surface. Such trans-epithelial dendrites are not detectable in healthy ocular mucosa. In response to topically applied flagellin, increased proportion of CD11c-positive cells in the conjunctiva strongly express MHC class II relative to the untreated conjunctiva. This change is accompanied by reduced immunoreactivity to TGFβ-activating Thrombospondin-1 in the conjunctival epithelium. These findings are supported by in vitro observations in primary cultures of goblet cells that respond to the TLR5 stimulation with an increased expression of IL-6 and reduced level of active TGFβ. The observed changes in the conjunctiva after flagellin application correspond with the development of clinical signs of chronic ocular mucosal inflammation including corneal epitheliopathy. Collectively, these findings demonstrate the ability of ocular mucosal dendritic cells to extend trans-epithelial dendrites in response to increased microbial colonization at the ocular surface. Moreover, this study provides key insight into how goblet cell responses to microbial stimuli may contribute to the disruption of ocular mucosal homeostasis and chronic ocular mucosal inflammation.

Systemic diseases and the cornea

There is a number of systemic diseases affecting the cornea. These include endocrine disorders (diabetes, Graves' disease, Addison's disease, hyperparathyroidism), infections with viruses (SARS-CoV-2, herpes simplex, varicella zoster, HTLV-1, Epstein-Barr virus) and bacteria (tuberculosis, syphilis and Pseudomonas aeruginosa), autoimmune and inflammatory diseases (rheumatoid arthritis, Sjögren's syndrome, lupus erythematosus, gout, atopic and vernal keratoconjunctivitis, multiple sclerosis, granulomatosis with polyangiitis, sarcoidosis, Cogan's syndrome, immunobullous diseases), corneal deposit disorders (Wilson's disease, cystinosis, Fabry disease, Meretoja's syndrome, mucopolysaccharidosis, hyperlipoproteinemia), and genetic disorders (aniridia, Ehlers-Danlos syndromes, Marfan syndrome). Corneal manifestations often provide an insight to underlying systemic diseases and can act as the first indicator of an undiagnosed systemic condition. Routine eye exams can bring attention to potentially life-threatening illnesses. In this review, we provide a fairly detailed overview of the pathologic changes in the cornea described in various systemic diseases and also discuss underlying molecular mechanisms, as well as current and emerging treatments.

Differential susceptibility of airway and ocular surface cell lines to FlhDC-mediated virulence factors PhlA and ShlA from Serratia marcescens

Introduction. Serratia marcescens is a bacterial pathogen that causes ventilator-associated pneumonia and ocular infections. The FlhD and FlhC proteins complex to form a heteromeric transcription factor whose regulon, in S. marcescens, regulates genes for the production of flagellum, phospholipase A and the cytolysin ShlA. The previously identified mutation, scrp-31, resulted in highly elevated expression of the flhDC operon. The scrp-31 mutant was observed to be more cytotoxic to human airway and ocular surface epithelial cells than the wild-type bacteria and the present study sought to identify the mechanism underlying the increased cytotoxicity phenotype.Hypothesis/Gap Statement. Although FlhC and FlhD have been implicated as virulence determinants, the mechanisms by which these proteins regulate bacterial cytotoxicity to different cell types remains unclear.Aim. This study aimed to evaluate the mechanisms of FlhDC-mediated cytotoxicity to human epithelial cells by S. marcescens.Methodology. Wild-type and mutant bacteria and bacterial secretomes were used to challenge airway and ocular surface cell lines as evaluated by resazurin and calcein AM staining. Pathogenesis was further tested using a Galleria mellonella infection model.Results. The increased cytotoxicity of scrp-31 bacteria and secretomes to both cell lines was eliminated by mutation of flhD and shlA. Mutation of the flagellin gene had no impact on cytotoxicity under any tested condition. Elimination of the phospholipase gene, phlA, had no effect on bacteria-induced cytotoxicity to either cell line, but reduced cytotoxicity caused by secretomes to airway epithelial cells. Mutation of flhD and shlA, but not phlA, reduced bacterial killing of G. mellonella larvae.Conclusion. This study indicates that the S. marcescens FlhDC-regulated secreted proteins PhlA and ShlA, but not flagellin, are cytotoxic to airway and ocular surface cells and demonstrates differences in human epithelial cell susceptibility to PhlA.

ISG15 Acts as a Mediator of Innate Immune Response to Pseudomonas aeruginosa Infection in C57BL/6J Mouse Corneas

Purpose

IFN-stimulated gene (ISG) 15 is a type 1 IFN-induced protein and known to modify target proteins in a manner similar to ubiquitylation (protein conjugation by ISG15 is termed ISGylation). We sought to determine the role of ISG15 and its underlying mechanisms in corneal innate immune defense against Pseudomonas aeruginosa keratitis.

Methods

ISG15 expression in cultured human corneal epithelial cells (HCECs) and mouse corneas was determined by PCR and Western blot analysis. Gene knockout mice were used to define the role of ISG15 signaling in controlling the severity of P. aeruginosa keratitis, which was assessed with photographing, clinical scoring, bacterial counting, myeloperoxidase assay, and quantitative PCR determination of cytokine expression. Integrin LFA-1 inhibitor was used to assess its involvement of ISG15 signaling in P. aeruginosa-infected corneas.

Results

Heat-killed P. aeruginosa induced ISG15 expression in cultured HCECs and accumulation in the conditioned media. Isg15 deficiency accelerated keratitis progress, suppressed IFNγ and CXCL10, and promoted IL-1β while exhibiting no effects on IFNα expression. Moreover, exogenous ISG15 protected the corneas of wild-type mice from P. aeruginosa infection while markedly reducing the severity of P. aeruginosa keratitis in type 1 IFN-receptor knockout mice. Exogenous ISG15 increased bacteriostatic activity of B6 mouse corneal homogenates, and inhibition of LFA-1 exacerbated the severity of and abolished protective effects of ISG15 on P. aeruginosa keratitis.

Conclusions

Type 1 INF-induced ISG15 regulates the innate immune response and greatly reduces the susceptibility of B6 mouse corneas to P. aeruginosa infection in an LFA-1-dependent manner.

IL-17 Promotes Pseudomonas aeruginosa Keratitis in C57BL/6 Mouse Corneas

The aim of this study was to elucidate the expression and functions of IL-17 in C57BL/6 mouse corneas in response to Pseudomonas aeruginosa infection. We found that P. aeruginosa infection induced and increased signaling of IL-23/23R/17/17R in mouse corneas. Targeting IL-17A or the IL-17A-specific receptor IL-17RA/IL-17RC with neutralizing Abs resulted in a significant decrease in the severity of P. aeruginosa keratitis, including a decrease in bacterial burden and polymorphonuclear leukocyte infiltration. IL-17A-signaling blockade also significantly reduced the expression of the proinflammatory cytokines L-1β, IL-24, and MMP-13 and increased the expression of the anti-inflammatory cytokine IL-1RA in mouse corneal epithelium. The presence of mouse IL-17A exacerbated P. aeruginosa-mediated tissue destruction. A cytokine protein array revealed that the expression of osteoprotegerin (OPG) was regulated by IL-17A, and OPG neutralization also resulted in a decrease in the severity of P. aeruginosa keratitis. Although both IL-17 and OPG affected the balanced expression of IL-1β and IL-1RA, only IL-17 inhibited the expression of TH2 cytokines. Taken together, our results revealed that IL-17A, along with its downstream factor OPG, plays a detrimental role in the pathogenesis of P. aeruginosa keratitis. Targeting IL-17A and/or the OPG/RANKL/RANK/TRAIL system is a potential therapeutic strategy in controlling the outcome of P. aeruginosa keratitis, which was demonstrated by concurrent topical application of IL-17A-neutralizing Ab and ciprofloxacin in B6 mice.

Pseudomonas aeruginosa keratitis: passive immunotherapy with antibodies raised against divalent flagellin

Objective(s):

Pseudomonas aeruginosa infections such as keratitis are considered among the major health problems worldwide due to the complexity of pathogenesis and antibiotic resistance crisis, thus, finding new effective approaches for prevention and treatment of the infections seem to be still vital. In this report, we aimed to investigate the therapeutic effects of topical administration of the antibodies against type a and b-flagellin (FLA and FLB) in Pseudomonas keratitis model of infection in mice.

Materials and Methods:

Scratched corneas of mice were treated with approximately 10⁷ CFUs/eye of PAK and/or PAO1 strains of P. aeruginosa. Specific IgG to FLA, FLB or divalent flagellin were topically applied to the infected corneas for 20 min, 24, and 36 hr post-infection. The bacterial burden and myeloperoxidase activity (as a marker for polymorphonuclears (PMNs) infiltration) were determined in the corneas. The biological activity of the anti-FLA and FLB IgG was evaluated in vitro by opsonophagocytosis test.

Results:

Compared to other treated corneas, divalent anti-flagellin IgG treatment showed a significant decrease in the bacterial CFUs and myeloperoxidase activity in the infected corneas (P<0.05). Results of opsonophagocytosis revealed that the specific antibodies raised against FLA and FLB had more potent opsonic killing activity on their homologous strains as compared with control group (P<0.05).

Conclusion:

It appears that in P. aeruginosa keratitis, topical administration of the combined antibodies likely via decreasing the bacterial load, and PMNs infiltration as well as increasing opsonophagocytosis could lead to dramatic improvement of the infected corneas.

TSLP Protects Corneas From Pseudomonas aeruginosa Infection by Regulating Dendritic Cells and IL-23-IL-17 Pathway

Purpose

We sought to determine the role of epithelium-produced thymic stromal lymphopoietin (TSLP) and its underlying mechanisms in corneal innate immune defense against Pseudomonas (P.) aeruginosa keratitis.

Methods

The expression of TSLP and TSLPR in cultured human corneal epithelial cells (HCECs) and mouse corneas was determined by PCR, Western, and/or ELISA. Cellular localization of TSLP receptor (TSLPR) was determined by whole mount confocal microscopy. TSLP-TSLPR signaling was downregulated by neutralizing antibodies and/or small interfering (si)RNA; their effects on the severity of P. aeruginosa–keratitis and cytokine expression were assessed using clinical scoring, bacterial counting, PMN infiltration, and real-time PCR. The role of dendritic cells (DCs) in corneal innate immunity was determined by local DC depletion using CD11c-DTR mice.

Results

P. aeruginosa–infection induced the expression of TSLP and TSLPR in both cultured primary HCECs and in C57BL/6 mouse corneas. While TSLP was mostly expressed by epithelial cells, CD11c-positive cells were positive for TSLPR. Targeting TSLP or TSLPR with neutralizing antibodies or TSLPR with siRNA resulted in more severe keratitis, attributable to an increase in bacterial burden and PMN infiltration. TSLPR neutralization significantly suppressed infection-induced TSLP and interleukin (IL)-17C expression and augmented the expression of IL-23 and IL-17A. Local depletion of DCs markedly increased the severity of keratitis and exhibited no effects on TSLP and IL-23 expression while suppressing IL-17A and C expression in P. aeruginosa–infected corneas.

Conclusions

The epithelium-expressed TSLP plays a protective role in P. aeruginosa keratitis through targeting of DCs and in an IL-23/IL-17 signaling pathway-related manner.

Differential Expression of Antimicrobial Peptides in Streptococcus pneumoniae Keratitis and STAT3-Dependent Expression of LL-37 by Streptococcus pneumoniae in Human Corneal Epithelial Cells

Streptococcus pneumoniae is the leading cause of bacterial keratitis in the developing world with a growing trend of acquiring resistance against various antibiotics. In the current study, we determined the expression of different antimicrobial peptides (AMPs) in response to S. pneumoniae in patients, as well as in primary and immortalized human corneal epithelial cells. We further focused on LL-37 and determined its expression in human cornea infected with S. pneumoniae and studied the killing ability of LL-37 against S. pneumoniae. The expression of AMPs was determined by quantitative PCR and the phosphorylation of signaling proteins was evaluated by immunoblot analysis. LL-37 expression was also determined by immunofluorescence and Western blot method and the killing ability of LL-37 against S. pneumoniae was determined by colony-forming units. Differential expression of antimicrobial peptides was observed in patients with S. pneumoniae keratitis. Although S. pneumoniae induced expression of the AMPs in human corneal epithelial cells (HCEC), it did not induce AMP expression in U937, a human monocyte cell line. S. pneumoniae also caused activation of nuclear factor kappa-light-chain enhancer of activated B cells (NF-κB)and mitogen activated protein kinase (MAPK) pathways in corneal epithelial cells. LL-37 was found to be effective against both laboratory and clinical strains of S. pneumoniae. LL-37 induction by S. pneumoniae in human corneal epithelial cells was mediated by signal transducer and activator of transcription 3 (STAT3) activation, and inhibition of STAT3 activation significantly reduced LL-37 expression. Our study determines an extensive profile of AMPs expressed in the human cornea during S. pneumoniae infection, and suggests the potential of LL-37 to be developed as an alternative therapeutic intervention to fight increasing antibiotic resistance among bacteria.

Stimulator of Interferon Genes Promotes Host Resistance Against Pseudomonas aeruginosa Keratitis

Pseudomonas aeruginosa (PA) is the leading cause of bacterial keratitis, especially in those who wear contact lens and who are immunocompromised. Once the invading pathogens are recognized by pattern recognition receptors expressed on the innate immune cells, the innate immune response is stimulated to exert host defense function, which is the first line to fight against PA infection. As a converging point of cytosolic DNA sense signaling, stimulator of interferon genes (STING) was reported to participate in host–pathogen interaction. However, the role of STING in regulating PA-induced corneal inflammation and bacterial clearance remains unknown. Our data demonstrated that STING was activated in murine model of PA keratitis and in in vitro-cultured macrophages, indicated by Western blot, immunostaining, and flow cytometry. To explore the role of STING in PA keratitis, we used siRNA to silence STING and 2′,3′-cGAMP to activate STING in vivo and in vitro, and the in vivo data found out that STING promoted host resistance against PA infection. To investigate the reason why STING played a protective role in PA keratitis, the inflammatory cytokine secretion and bacterial load were measured by using real-time PCR and bacterial plate count, respectively. Our data demonstrated that STING suppressed the production of inflammatory cytokines and enhanced bacterial elimination in murine model of PA keratitis and in PA-infected macrophages. To further investigate the mechanism beneath, the phosphorylation of mitogen-activated protein kinase, the nuclear translocation of nuclear factor-κB (NF-κB) and the bactericidal mechanism were measured by western-blot, immunofluorescence, and real-time PCR, respectively. Our data indicated that STING suppressed inflammatory cytokine expressing via restraining NF-κB activity and enhanced inducible NO synthase expression, an oxygen-dependent bactericidal mechanism. In conclusion, this study demonstrated that STING promoted host resistance against PA keratitis and played a protective role in PA-infected corneal disease, via inhibiting corneal inflammation and enhancing bacterial killing.

ISG15 in Host Defense Against Candida albicans Infection in a Mouse Model of Fungal Keratitis

Purpose

ISG15, a di-ubiquitin-like protein, is critical for controlling certain viral and bacterial infections. We sought to determine if ISG15 plays a role in corneal innate immunity against Candida albicans (C. albicans) using a C57BL/6 (B6) mouse model of human fungal keratitis.

Methods

Scarified corneas of adult B6 mice were pretreated with TLR5 ligand flagellin and then inoculated with C. albicans. The expression of ISG15 and other genes involved in ISG15 conjugation (ISGylation) was determined by real-time PCR. ISG15 expression and distribution in infected corneas were assessed by immunohistochemistry. ISGylation was examined by Western blotting. siRNA knockdown and recombinant ISG15 were used to elucidate the effects of ISG15 on controlling fungal keratitis by clinical scoring, fungal number plate counting, ELISA cytokine determination, and polymorphonuclear leukocytes (PMN) infiltration measurement.

Results

Heat-killed C. albicans induced expression of ISG15, and hBD2 was markedly enhanced by flagellin-pretreatment in cultured human primary corneal epithelial cells (CECs). In vivo, C. albicans infection induced the expression of ISG15, ISGylation-associated genes (UBE1L, UBCH8, and HERC5), and ISGylation in mouse CECs, all of which were enhanced by flagellin-pretreatment. siRNA knockdown of ISG15 increased keratitis severity, dampened flagellin-induced protection, and greatly suppressed the expressions of ISGylation enzymes, IFN-γ, but not CXCL2 in B6 mouse CECs. Recombinant ISG15, on the other hand, enhanced corneal innate immunity against C. albicans and suppressed infection-induced IL-1β, but not IL-Ra expression. ISG15 alone induced the expression of IL-1Ra, CXCL10, and CRAMP in mouse CECs. ISG15 was upregulated and secreted in cultured human CECs in response to challenge in a type 1 IFN-dependent manner.

Conclusions

Our data, for the first time, demonstrate that ISG15 acts as an immunomodulator in the cornea and plays a critical role in controlling fungal keratitis.

IL-24 Promotes Pseudomonas aeruginosa Keratitis in C57BL/6 Mouse Corneas

The aim of this study was to elucidate the expression and functions of IL-24 in C57BL/6 mouse corneas in response to Pseudomonas aeruginosa infection. Among IL-20R cytokines, only IL-24 was induced at both mRNA and protein levels by infection at early time points. The upregulation of IL-24 was dampened by flagellin pretreatment, which protects the corneas from microbial infection. Time course studies revealed bimodal early and later peaks of IL-24 expression, a pattern shared with suppressor of cytokine signaling (SOCS)3 but not IL-1β or IL-6. Silencing of IL-24 enhanced S100A8/A9 expression and suppressed SOCS3, IL-1β, IL-1RN, and matrix metalloproteinase 13 expression at 6 h postinfection. Downregulation of the IL-24 signaling pathway significantly reduced the severity of keratitis, whereas rIL-24 exacerbated P. aeruginosa-mediated tissue destruction. In vitro, rIL-1β induced the expression of SOCS3, IL-24, IL-1β, and IL-6 in primary cultured human corneal epithelial cells. rIL-24, alternatively, stimulated the expression of SOCS3, but not the others. In conclusion, IL-24 promotes P. aeruginosa keratitis through the suppression of early protective mucosal immunity, culminating in increased severity of P. aeruginosa keratitis.

Bacterial Evasion of Host Antimicrobial Peptide Defenses

Antimicrobial peptides (AMPs), also known as host defense peptides, are small naturally occurring microbicidal molecules produced by the host innate immune response that function as a first line of defense to kill pathogenic microorganisms by inducing deleterious cell membrane damage. AMPs also possess signaling and chemoattractant activities and can modulate the innate immune response to enhance protective immunity or suppress inflammation. Human pathogens have evolved defense molecules and strategies to counter and survive the AMPs released by host immune cells such as neutrophils and macrophages. Here, we review the various mechanisms used by human bacterial pathogens to resist AMP-mediated killing, including surface charge modification, active efflux, alteration of membrane fluidity, inactivation by proteolytic digestion, and entrapment by surface proteins and polysaccharides. Enhanced understanding of AMP resistance at the molecular level may offer insight into the mechanisms of bacterial pathogenesis and augment the discovery of novel therapeutic targets and drug design for the treatment of recalcitrant multidrug-resistant bacterial infections.

Protective efficacy of a peptide derived from a potential adhesin of Pseudomonas aeruginosa against corneal infection

Dissecting the interactions between Pseudomonas aeruginosa and corneal cells is important to identify a novel target for prevention and treatment of Pseudomonas keratitis. The current study began with a peptide identified by phage display, and was to investigate the protective efficacy against P. aeruginosa infection in cornea. The original peptide Pc-E, with high homology to a hypothetical membrane protein (HmpA) in P. aeruginosa, and the derived peptide Pc-EP, with the same sequence as a region in HmpA, were synthesized. Peptide Pc-EP could directly bind to HCEC, stronger than Pc-E, and specifically activate toll-like receptor 5, and thereby significantly induce the production of pro-inflammatory factors, such as IL-1β, IL-6, IFN-γ and IL-17. Moreover, Pc-EP could act as an antagonist to inhibit the adhesion of wild-type P. aeruginosa to HCEC and mouse corneas. No inhibitory effect was observed on the adhesion of the strain loss of HmpA. When compared to the wild-type strain, the adhesion of the hmpA mutant to corneal cells was significantly decreased. Treatment of infected mouse corneas with Pc-EP before infection significantly decreased the bacterial load in the cornea and attenuated the corneal pathology. These results indicate that Pc-EP can be a useful prophylactic agent for P. aeruginosa keratitis.

The Toll-Like Receptor 5 Agonist Entolimod Mitigates Lethal Acute Radiation Syndrome in Non-Human Primates

There are currently no approved medical radiation countermeasures (MRC) to reduce the lethality of high-dose total body ionizing irradiation expected in nuclear emergencies. An ideal MRC would be effective even when administered well after radiation exposure and would counteract the effects of irradiation on the hematopoietic system and gastrointestinal tract that contribute to its lethality. Entolimod is a Toll-like receptor 5 agonist with demonstrated radioprotective/mitigative activity in rodents and radioprotective activity in non-human primates. Here, we report data from several exploratory studies conducted in lethally irradiated non-human primates (rhesus macaques) treated with a single intramuscular injection of entolimod (in the absence of intensive individualized supportive care) administered in a mitigative regimen, 1-48 hours after irradiation. Following exposure to LD50-70/40 of radiation, injection of efficacious doses of entolimod administered as late as 25 hours thereafter reduced the risk of mortality 2-3-fold, providing a statistically significant (P<0.01) absolute survival advantage of 40-60% compared to vehicle treatment. Similar magnitude of survival improvement was also achieved with drug delivered 48 hours after irradiation. Improved survival was accompanied by predominantly significant (P<0.05) effects of entolimod administration on accelerated morphological recovery of hematopoietic and immune system organs, decreased severity and duration of thrombocytopenia, anemia and neutropenia, and increased clonogenic potential of the bone marrow compared to control irradiated animals. Entolimod treatment also led to reduced apoptosis and accelerated crypt regeneration in the gastrointestinal tract. Together, these data indicate that entolimod is a highly promising potential life-saving treatment for victims of radiation disasters.

Chitinase 3-Like 1 Promotes Candida albicans Killing and Preserves Corneal Structure and Function by Controlling Host Antifungal Responses

Chitinase 3-like 1 (CHI3L1) has been shown to play a role in promoting antibacterial responses, decreasing tissue injury, and enhancing pulmonary repair. This study sought to elucidate the role of CHI3L1 in augmenting the corneal innate immune response to Candida albicans infection in an animal model of fungal keratitis. Flagellin applied topically 24 h prior to C. albicans inoculation significantly protected the corneal from C. albicans and induced CHI3L1 expression in C57BL/6 mouse corneas. CHI3L1, however, played a detectable but minor role in flagellin-induced protection. While C. albicans keratitis was more severe in the corneas treated with Chi3l1 small interfering RNA (siRNA), corneas treated with recombinant CHI3L1 before C. albicans inoculation had markedly ameliorated keratitis, reduced fungal load, and decreased polymorphonucleocyte (PMN) infiltration in an interleukin 13 receptor α2 (IL-13Rα2)-dependent manner. CHI3L1 treatment resulted in the induction of the antimicrobial peptides β-defensin 3, CRAMP, and chemokine CXCL10 and its receptor CXCR3 in corneal epithelial cells. Importantly, CHI3L1 administered after C. albicans inoculation also had strong protection against fungal keratitis, suggesting a therapeutic window. This is the first report demonstrating that CHI3L1 is induced during fungal infection, where it acts as an immunomodulator to promote fungal clearance and to regulate antifungal innate immune responses in the cornea.

A Toll-Like Receptor 5 Agonist Improves the Efficacy of Antibiotics in Treatment of Primary and Influenza Virus-Associated Pneumococcal Mouse Infections

Prophylactic intranasal administration of the Toll-like receptor 5 (TLR5) agonist flagellin protects mice against respiratory pathogenic bacteria. We hypothesized that TLR5-mediated stimulation of lung immunity might improve the therapeutic index of antibiotics for the treatment of Streptococcus pneumoniae respiratory infections in mice. Intranasal administration of flagellin was combined with either oral administration of amoxicillin or intraperitoneal injection of trimethoprim-sulfamethoxazole to treat S. pneumoniae-infected animals. Compared with standalone treatments, the combination of antibiotic and flagellin resulted in a lower bacterial load in the lungs and greater protection against S. pneumoniae dissemination and was associated with an early increase in neutrophil infiltration in the airways. The antibiotic-flagellin combination treatment was, however, not associated with any exacerbation of inflammation. Moreover, combination treatment was more efficacious than standalone antibiotic treatments in the context of post-influenza virus pneumococcal infection. Lastly, TLR5 signaling was shown to be mandatory for the efficacy of the combined antibacterial therapy. This report is the first to show that combining antibiotic treatment with the stimulation of mucosal innate immunity is a potent antibacterial strategy against pneumonia.

Unexpected Roles for Toll-Like Receptor 4 and TRIF in Intraocular Infection with Gram-Positive Bacteria

Inflammation caused by infection with Gram-positive bacteria is typically initiated by interactions with Toll-like receptor 2 (TLR2). Endophthalmitis, an infection and inflammation of the posterior segment of the eye, can lead to vision loss when initiated by a virulent microbial pathogen. Endophthalmitis caused by Bacillus cereus develops as acute inflammation with infiltrating neutrophils, and vision loss is potentially catastrophic. Residual inflammation observed during B. cereus endophthalmitis in TLR2(-/-) mice led us to investigate additional innate pathways that may trigger intraocular inflammation. We first hypothesized that intraocular inflammation during B. cereus endophthalmitis would be controlled by MyD88- and TRIF-mediated signaling, since MyD88 and TRIF are the major adaptor molecules for all bacterial TLRs. In MyD88(-/-) and TRIF(-/-) mice, we observed significantly less intraocular inflammation than in eyes from infected C57BL/6J mice, suggesting an important role for these TLR adaptors in B. cereus endophthalmitis. These results led to a second hypothesis, that TLR4, the only TLR that signals through both MyD88 and TRIF signaling pathways, contributed to inflammation during B. cereus endophthalmitis. Surprisingly, B. cereus-infected TLR4(-/-) eyes also had significantly less intraocular inflammation than infected C57BL/6J eyes, indicating an important role for TLR4 in B. cereus endophthalmitis. Taken together, our results suggest that TLR4, TRIF, and MyD88 are important components of the intraocular inflammatory response observed in experimental B. cereus endophthalmitis, identifying a novel innate immune interaction for B. cereus and for this disease.

2-O-Sulfated Domains in Syndecan-1 Heparan Sulfate Inhibit Neutrophil Cathelicidin and Promote Staphylococcus aureus Corneal Infection

Ablation of syndecan-1 in mice is a gain of function mutation that enables mice to significantly resist infection by several bacterial pathogens. Syndecan-1 shedding is induced by bacterial virulence factors, and inhibition of shedding attenuates bacterial virulence, whereas administration of purified syndecan-1 ectodomain enhances virulence, suggesting that bacteria subvert syndecan-1 ectodomains released by shedding for their pathogenesis. However, the pro-pathogenic functions of syndecan-1 ectodomain have yet to be clearly defined. Here, we examined how syndecan-1 ectodomain enhances Staphylococcus aureus virulence in injured mouse corneas. We found that syndecan-1 ectodomain promotes S. aureus corneal infection in an HS-dependent manner. Surprisingly, we found that this pro-pathogenic activity is dependent on 2-O-sulfated domains in HS, indicating that the effects of syndecan-1 ectodomain are structure-based. Our results also showed that purified syndecan-1 ectodomain and heparan compounds containing 2-O-sulfate motifs inhibit S. aureus killing by antimicrobial factors secreted by degranulated neutrophils, but does not affect intracellular phagocytic killing by neutrophils. Immunodepletion of antimicrobial factors with staphylocidal activities demonstrated that CRAMP, a cationic antimicrobial peptide, is primarily responsible for S. aureus killing among other factors secreted by degranulated neutrophils. Furthermore, we found that purified syndecan-1 ectodomain and heparan compounds containing 2-O-sulfate units potently and specifically inhibit S. aureus killing by synthetic CRAMP. These results provide compelling evidence that a specific subclass of sulfate groups, and not the overall charge of HS, permits syndecan-1 ectodomains to promote S. aureus corneal infection by inhibiting a key arm of neutrophil host defense.

In Vivo Role of TLR2 and MyD88 Signaling in Eliciting Innate Immune Responses in Staphylococcal Endophthalmitis

PURPOSE

The purpose of this study was to investigate the protective mechanisms evoked by TLR2 and MyD88 signaling in bacterial endophthalmitis in vivo.

METHODS

Endophthalmitis was induced in wild-type (WT), TLR2(-/-), MyD88(-/-), and Cnlp(-/-) mice by intravitreal injections of a laboratory strain (RN6390) and two endophthalmitis isolates of Staphylococcus aureus. Disease progression was monitored by assessing corneal and vitreous haze, bacterial burden, and retinal tissue damage. Levels of inflammatory cytokines/chemokines were determined using quantitative RT-PCR (qRT-PCR) and ELISA. Flow cytometry was used to assess neutrophil infiltration. Cathelicidin-related antimicrobial peptide (CRAMP) expression was determined by immunostaining and dot blot.

RESULTS

Eyes infected with either laboratory or clinical isolates exhibited higher levels of inflammatory mediators at the early stages of infection (≤24 hours) in WT mice than in TLR2(-/-) or MyD88(-/-) mice. However, their levels surpassed that of WT mice at the later stages of infection (>48 hours), coinciding with increased bacterial burden and retinal damage. Both TLR2(-/-) and MyD88(-/-) retinas produced reduced levels of CRAMP, and its deficiency (Cnlp(-/-)) rendered the mice susceptible to increased bacterial burden and retinal tissue damage as early as 1 day post infection. Analyses of inflammatory mediators and neutrophil levels in WT versus Cnlp(-/-) mice showed a trend similar to that observed in TLR2 and MyD88 KO mice. Furthermore, we observed that even a 10-fold lower infective dose of S. aureus was sufficient to cause endophthalmitis in TLR2(-/-) and MyD88(-/-) mice.

CONCLUSIONS

TLR2 and MyD88 signaling plays an important role in protecting the retina from staphylococcal endophthalmitis by production of the antimicrobial peptide CRAMP.

Matrix Metalloproteinase-13 as a Target for Suppressing Corneal Ulceration Caused by Pseudomonas aeruginosa Infection

PURPOSE

Pseudomonas aeruginosa keratitis is characterized by severe corneal ulceration. This study investigated whether matrix metalloproteinase-13 (MMP13) is involved in P. aeruginosa-induced corneal ulceration and whether it therefore can be targeted for preventing P. aeruginosa keratitis.

METHODS

MMP13 expression in P. aeruginosa-infected C57BL/6 mouse corneas was assessed by quantitative polymerase chain reaction and immunohistochemistry analyses. An MMP13-inhibitor (MMP13i) was either injected subconjunctivally prior to or coapplied topically with gatifloxacin 16 hours after infection. Disease severity was assessed by corneal imaging, clinical scoring, bacterial burden, neutrophil infiltration, and CXCL2 expression. Corneal damage and infiltration were also determined by immunohistochemistry analysis and whole-mount confocal microscopy.

RESULTS

P. aeruginosa infection induced an increased expression of MMP13 in mouse corneas from 6 to 24 hours after infection in a Toll-liked receptor 5-dependent manner. Subconjunctival injection of MMP13i prior to P. aeruginosa inoculation significantly decreased keratitis severity, as evidenced by preserved epithelium integrity and intact basement membrane, leading to reduced bacterial dissemination to the stroma. Furthermore, topical coapplication of MMP13i with gatifloxacin greatly improved disease outcomes, including accelerated opacity dissolution; decreased inflammation, cellular infiltration, and collagen disorganization; and basement membrane preservation.

CONCLUSIONS

Elevated MMP13 activity may contribute to P. aeruginosa keratitis through basement membrane degradation, and its inhibition could potentially be used as an adjunctive therapy to treat microbial keratitis and other mucosal infections.

Flagellin-induced expression of CXCL10 mediates direct fungal killing and recruitment of NK cells to the cornea in response to Candida albicans infection

We previously showed that topical flagellin induces profound mucosal innate protection in the cornea against microbial infection, a response involving multiple genes and cell types. In this study, we used a Candida albicans (CA)-C57BL/6 mouse keratitis model to delineate the contribution of CXCL10- and CXCR3-expressing cells in flagellin-induced protection. Flagellin pretreatment markedly enhanced CXCL10 expression at 6 h post CA infection (hpi), but significantly dampened CXCL10 expression at 24 hpi. At the cellular level, CXCL10 was expressed in the epithelia at 6 hpi in flagellin-pretreated corneas, and concentrated at lesion sites 24 hpi. CXCR3-expressing cells were detected in great numbers at 24 hpi, organized within clusters at the lesion sites in CA-infected corneas. CXCL10 or CXCR3 neutralization increased keratitis severity and dampened flagellin-induced protection. CXCR3-positive cells were identified as NK cells, the depletion of which resulted in severe CA keratitis. Contributions from NK T-cells were excluded by finding no change in flagellin-induced protection in Rag1 KO mice. Recombinant CXCL10 inhibited CA growth in vitro and accelerated fungal clearance and inflammation resolution in vivo. Taken together, our data indicate that epithelium-expressed CXCL10 plays a critical role in fungal clearance and that CXCR3-expressing NK cells contribute to CA eradication in mouse corneas.

Pathogenic strains of Acanthamoeba are recognized by TLR4 and initiated inflammatory responses in the cornea

Free-living amoebae of the Acanthamoeba species are the causative agent of Acanthamoeba keratitis (AK), a sight-threatening corneal infection that causes severe pain and a characteristic ring-shaped corneal infiltrate. Innate immune responses play an important role in resistance against AK. The aim of this study is to determine if Toll-like receptors (TLRs) on corneal epithelial cells are activated by Acanthamoeba, leading to initiation of inflammatory responses in the cornea. Human corneal epithelial (HCE) cells constitutively expressed TLR1, TLR2, TLR3, TLR4, and TLR9 mRNA, and A. castellanii upregulated TLR4 transcription. Expression of TLR1, TLR2, TLR3, and TLR9 was unchanged when HCE cells were exposed to A. castellanii. IL-8 mRNA expression was upregulated in HCE cells exposed to A. castellanii. A. castellanii and lipopolysaccharide (LPS) induced significant IL-8 production by HCE cells as measured by ELISA. The percentage of total cells positive for TLR4 was higher in A. castellanii stimulated HCE cells compared to unstimulated HCE cells. A. castellanii induced upregulation of IL-8 in TLR4 expressing human embryonic kidney (HEK)-293 cells, but not TLR3 expressing HEK-293 cells. TLR4 neutralizing antibody inhibited A. castellanii-induced IL-8 by HCE and HEK-293 cells. Clinical strains but not soil strains of Acanthamoeba activated TLR4 expression in Chinese hamster corneas in vivo and in vitro. Clinical isolates but not soil isolates of Acanthamoeba induced significant (P< 0.05) CXCL2 production in Chinese hamster corneas 3 and 7 days after infection, which coincided with increased inflammatory cells in the corneas. Results suggest that pathogenic species of Acanthamoeba activate TLR4 and induce production of CXCL2 in the Chinese hamster model of AK. TLR4 may be a potential target in the development of novel treatment strategies in Acanthamoeba and other microbial infections that activate TLR4 in corneal cells.

Crystal structure of FliC flagellin from Pseudomonas aeruginosa and its implication in TLR5 binding and formation of the flagellar filament

Pseudomonas aeruginosa is one of leading opportunistic pathogens in humans and its movement is driven by a flagellar filament that is constituted through the polymerization of a single protein, FliC flagellin (paFliC). paFliC is an essential virulence factor for the colonization of P. aeruginosa. paFliC activates innate immune responses via its recognition by Toll-like receptor 5 (TLR5) and adaptive immunity in the host. Thus, paFliC has been a vaccine candidate to prevent P. aeruginosa infection, particularly for cystic fibrosis patients. To provide structural information on paFliC and its flagellar filament, we have determined the crystal structure of paFliC, which contains the conserved D1 and variable D2 domains, at 2.1 Å resolution. As observed for Salmonella FliC, the paFliC D1 domain is folded into a rod-shaped structure, and paFliC was demonstrated by gel filtration and native PAGE analyses to directly interact with TLR5. Moreover, a structural model of the paFliC-TLR5 complex suggests that paFliC D1 would provide major TLR5-binding sites, similar to Salmonella FliC. In contrast to the D1 domain, the paFliC D2 domain exhibits a unique structure of two β-sheets and one α-helix that has not been found in other flagellins. An in silico construction of a flagellar filament based on the packing of paFliC in the crystal suggests that the D2 domain would be exposed to solution and could play an important role in immunogenicity. Our biophysical and structure-based modeling study on paFliC, the paFliC-TLR5 complex, and the paFliC filament could contribute to the improvement of vaccine design to control P. aeruginosa infection.

Interferon regulatory factor-1 in flagellin-induced reprogramming: potential protective role of CXCL10 in cornea innate defense against Pseudomonas aeruginosa infection

PURPOSE

We previously showed that pre-exposure of the cornea to Toll-like receptor (TLR)5 ligand flagellin induces strong protective innate defense against microbial pathogens and hypothesized that flagellin modulates gene expression at the transcriptional levels. Thus, we sought to determine the role of one transcription factor, interferon regulatory factor (IRF1), and its target gene CXCL10 therein.

METHODS

Superarray was used to identify transcription factors differentially expressed in Pseudomonas aeruginosa-challenged human corneal epithelial cells (CECs) with or without flagellin pretreatment. The expression of CXCL10, IRF1, LI-8(CXCL2), and IFNγ was determined by PCR, immunohistochemistry, Western/dot blotting, and/or ELISA. IRF1 knockout mice, CXCL10 and IFNγ neutralization, and NK cell depletion were used to define in vivo regulation and function of CXCL10. The severity of P. aeruginosa was assessed using clinical scoring, slit-lamp microscopy, bacterial counting, polymorphonuclear leukocytes (PMN) infiltration, and macrophage inflammatory protein 2/Chemokine (C-X-C motif) ligand 2 (MIP-2/CXCL2) expression.

RESULTS

Flagellin pretreatment drastically affected P. aeruginosa-induced IRF1 expression in human CECs. However, flagellin pretreatment augmented the P. aeruginosa-induced expression of Irf1 and its target gene Cxcl10 in B6 mouse corneas. Irf1 deficiency reduced infection-triggered CXCL10 expression, increased keratitis severity, and attenuated flagellin-elicited protection compared to values in wild-type (WT) controls. CXCL10 neutralization in the cornea of WT mice displayed pathogenesis similar to that of IRF1⁻/⁻ mice. IFNγ receptor neutralization and NK cell depletion prevented flagellin-augmented IRF1 and CXCL10 expression and increased the susceptibility to P. aeruginosa infection in mouse corneas.

CONCLUSIONS

IRF1 plays a role in the corneal innate immune response by regulating CXCL10 expression. IFNγ-producing NK cells augment the epithelial expression of IRF1 and CXCL10 and thus contribute to the innate defense of the cornea against P. aeruginosa infection.

Targeting toll-like receptor signaling as a novel approach to prevent ocular infectious diseases

Toll-like receptors (TLRs) play a key role in the innate immune response to invading pathogens. Thus, their discovery has opened up a wide range of therapeutic possibilities for various infectious and inflammatory diseases. In the last several years, extensive research efforts have provided a considerable wealth of information on the expression and function of TLRs in the eye, with significant implications for better understanding of pathogenesis of infectious eye diseases affecting the cornea, uvea, and the retina. In this review, by using bacterial keratitis and endophthalmitis as examples, we discuss the possibilities of targeting TLR signaling for the prevention or treatment of ocular infectious diseases.

Genome-wide transcriptional analysis of differentially expressed genes in flagellin-pretreated mouse corneal epithelial cells in response to Pseudomonas aeruginosa: involvement of S100A8/A9

We previously showed that pre-exposure of the cornea to Toll-like receptor 5 ligand flagellin induces profound mucosal innate protection against infections by modifying gene expression. Taking advantage of easily procurable epithelial cell population, this study is the first report to use genome-wide cDNA microarray approach to document genes associated with flagellin-induced protection against Pseudomonas aeruginosa in corneal epithelial cells (CECs). Infection altered the expression of 675 genes (497 up and 178 down), while flagellin pretreatment followed by infection resulted in a great increase in 890 gene upregulated and 37 genes downregulated. Comparing these two groups showed 209 differentially expressed genes (157 up, 52 down). Notably, among 114 genes categorized as defense related, S100A8/A9 are the two most highly induced genes by flagellin, and their expression in the corneal was confirmed by realtime PCR and immunohistochemistry. Neutralization of S100A8 and, to a less extent, A9, resulted in significantly increased bacterial burden and severe keratitis. Collectively, our study identifies many differentially expressed genes by flagellin in CECs in response to Pseudomonas. These novel gene expression signatures provide new insights and clues into the nature of protective mechanisms established by flagellin and new therapeutic targets for reducing inflammation and for controlling microbial infection.

Host defense at the ocular surface

Microbial infections of the cornea frequently cause painful, blinding and debilitating disease that is often difficult to treat and may require corneal transplantation. In addition, sterile corneal infiltrates that are associated with contact lens wear cause pain, visual impairment and photophobia. In this article, we review the role of Toll-Like Receptors (TLR) in bacterial keratitis and sterile corneal infiltrates, and describe the role of MD-2 regulation in LPS responsiveness by corneal epithelial cells. We conclude that both live bacteria and bacterial products activate Toll-Like Receptors in the cornea, which leads to chemokine production and neutrophil recruitment to the corneal stroma. While neutrophils are essential for bacterial killing, they also cause tissue damage that results in loss of corneal clarity. These disparate outcomes, therefore, represent a spectrum of disease severity based on this pathway, and further indicate that targeting the TLR pathway is a feasible approach to treating inflammation caused by live bacteria and microbial products. Further, as the P. aeruginosa type III secretion system (T3SS) also plays a critical role in disease pathogenesis by inducing neutrophil apoptosis and facilitating bacterial growth in the cornea, T3SS exotoxins are additional targets for therapy for P. aeruginosa keratitis.

Extracellular matrix protein lumican promotes clearance and resolution of Pseudomonas aeruginosa keratitis in a mouse model

Lumican is an extracellular protein that associates with CD14 on the surface of macrophages and neutrophils, and promotes CD14-TLR4 mediated response to bacterial lipopolysaccharides (LPS). Lumican-deficient (Lum(-/-)) mice and macrophages are impaired in TLR4 signals; raising the possibility that lumican may regulate host response to live bacterial infections. In a recent study we showed that invitro Lum(-/-) macrophages are impaired in phagocytosis of gram-negative bacteria and in a lung infection model the Lum(-/-) mice showed poor survival. The cornea is an immune privileged barrier tissue that relies primarily on innate immunity to protect against ocular infections. Lumican is a major component of the cornea, yet its role in counteracting live bacteria in the cornea remains poorly understood. Here we investigated Pseudomonas aeruginosa infections of the cornea in Lum(-/-) mice. By flow cytometry we found that 24 hours after infection macrophage and neutrophil counts were lower in the cornea of Lum(-/-) mice compared to wild types. Infected Lum(-/-) corneas showed lower levels of the leukocyte chemoattractant CXCL1 by 24-48 hours of infection, and increased bacterial counts up to 5 days after infection, compared to Lum(+/-) mice. The pro-inflammatory cytokine TNF-α was comparably low 24 hours after infection, but significantly higher in the Lum(-/-) compared to Lum(+/-) infected corneas by 2-5 days after infection. Taken together, the results indicate that lumican facilitates development of an innate immune response at the earlier stages of infection and lumican deficiency leads to poor bacterial clearance and resolution of corneal inflammation at a later stage.

Antimicrobial peptide elicitors: New hope for the post-antibiotic era

Antimicrobial peptides or host defense peptides are fundamental components of human innate immunity. Recent and growing evidence suggests they have a role in a broad range of diseases, including cancer, allergies and susceptibility to infection, including HIV/AIDS. Antimicrobial peptide elicitors (APEs) are physical, biological or chemical agents that boost human antimicrobial peptide expression. The current knowledge of APEs and their potential use in the treatment of human infectious diseases are reviewed, and a classification system for APEs is proposed. The efficient use of APEs in clinical practice could mark the beginning of the urgently needed post-antibiotic era, but further trials assessing their efficacy and safety are required.

Toll-like receptor 2 ligand pretreatment attenuates retinal microglial inflammatory response but enhances phagocytic activity toward Staphylococcus aureus

Staphylococcus aureus is a leading cause of severe endophthalmitis, which often results in vision loss in some patients. Previously, we showed that Toll-like receptor 2 (TLR2) ligand pretreatment prevented the development of staphylococcal endophthalmitis in mice and suggested that microglia might be involved in this protective effect (Kumar A, Singh CN, Glybina IV, Mahmoud TH, Yu FS. J. Infect. Dis. 201:255-263, 2010). The aim of the present study was to understand how microglial innate response is modulated by TLR2 ligand pretreatment. Here, we demonstrate that S. aureus infection increased the CD11b(+) CD45(+) microglial/macrophage population in the C57BL/6 mouse retina. Using cultured primary retinal microglia and a murine microglial cell line (BV-2), we found that these cells express TLR2 and that its expression is increased upon stimulation with bacteria or an exclusive TLR2 ligand, Pam3Cys. Furthermore, challenge of primary retinal microglia with S. aureus and its cell wall components peptidoglycan (PGN) and lipoteichoic acid (LTA) induced the secretion of proinflammatory mediators (tumor necrosis factor alpha [TNF-α] and MIP-2). This innate response was attenuated by a function-blocking anti-TLR2 antibody or by small interfering RNA (siRNA) knockdown of TLR2. In order to assess the modulation of the innate response, microglia were pretreated with a low dose (0.1 or 1 μg/ml) of Pam3Cys and then challenged with live S. aureus. Our data showed that S. aureus-induced production of proinflammatory mediators is dramatically reduced in pretreated microglia. Importantly, microglia pretreated with the TLR2 agonist phagocytosed significantly more bacteria than unstimulated cells. Together, our data suggest that TLR2 plays an important role in retinal microglial innate response to S. aureus, and its sensitization inhibits inflammatory response while enhancing phagocytic activity.

Retinal Muller glia initiate innate response to infectious stimuli via toll-like receptor signaling

Ocular surgeries and trauma predispose the eye to develop infectious endophthalmitis, which often leads to vision loss. The mechanisms of initiation of innate defense in this disease are not well understood but are presumed to involve retinal glial cells. We hypothesize that retinal Muller glia can recognize and respond to invading pathogens via TLRs, which are key regulators of the innate immune system. Using the mouse retinal sections, human retinal Muller cell line (MIO-M1), and primary mouse retinal Muller cells, we show that they express known human TLR1-10, adaptor molecules MyD88, TRIF, TRAM, and TRAF6, and co-receptors MD2 and CD14. Consistent with the gene expression, protein levels were also detected for the TLRs. Moreover, stimulation of the Muller glia with TLR 2, 3, 4, 5, 7 and 9 agonists resulted in an increased TLR expression as assayed by Western blot and flow cytometry. Furthermore, TLR agonists or live pathogen (S. aureus, P. aeruginosa, & C. albicans)-challenged Muller glia produced significantly higher levels of inflammatory mediators (TNF-α, IL-1β, IL-6 and IL-8), concomitantly with the activation of NF-κB, p38 and Erk signaling. This data suggests that Muller glia directly contributes to retinal innate defense by recognizing microbial patterns under infectious conditions; such as those in endophthalmitis.

Topical flagellin-mediated innate defense against Candida albicans keratitis

PURPOSE

This study was conducted to investigate whether flagellin, the sole ligand of Toll-like receptor-5 (TLR5), induces an innate defense that is sufficient to protect injured corneas from Candida albicans.

METHODS

Scarified corneas of adult B6, TLR5(-/-), Camp(-/-) (cathelicidin-related antimicrobial peptide), or PMN-depleted mice were pretreated with Pseudomonas aeruginosa flagellin or a mutant and then were inoculated with C. albicans. The corneas were compared for disease progression, cytokine and Camp expression, and PMN infiltration before and after C. albicans infection. Disease progress was recorded by digital photography and clinical scoring, cytokine levels were determined by ELISA, the levels of Camp gene product were assessed by Western blot, and PMN infiltration was measured by MPO determination and immunohistochemistry.

RESULTS

Topical application of flagellin induced profound protection against Candida keratitis in a TLR5-dependent manner. The improved disease outcome including reduced tissue inflammation and rapid functional recovery can be attributed to a marked decrease in fungal burden at the early stage of C. albicans infection in flagellin-exposed B6 mouse corneas. Although both PMN infiltration and Camp upregulation contributed to corneal innate defense against fungal infection, Camp ablation totally, and PMN depletion partially, abrogated flagellin-induced fungal clearance in B6 mouse corneas.

CONCLUSIONS

Flagellin induces a strong innate defense and promotes robust resistance to C. albicans infection in the cornea. Topical flagellin or its mimetic may become a new prophylactic agent for preventing contact lens or trauma/injury-associated microbial keratitis.

Role of defensins in corneal epithelial barrier function against Pseudomonas aeruginosa traversal

Studies have shown that epithelium-expressed antimicrobial peptides (AMPs), e.g., β-defensins, play a role in clearing bacteria from mouse corneas already infected with Pseudomonas aeruginosa. Less is known about the role of AMPs in allowing the cornea to resist infection when healthy. We previously reported that contact lens exposure, a major cause of P. aeruginosa keratitis, can inhibit the upregulation of human β-defensin 2 (hBD-2) by corneal epithelial cells in response to P. aeruginosa antigens in vitro. Here, we studied the role of AMPs in maintaining the corneal epithelial barrier to P. aeruginosa penetration using both in vitro (human) and in vivo (mouse) experiments. Results showed that preexposing human corneal epithelial multilayers to bacterial antigens in a culture supernatant (known to upregulate AMP expression) reduced epithelial susceptibility to P. aeruginosa traversal up to 6-fold (P < 0.001). Accordingly, small interfering RNA (siRNA) knockdown of any one of four AMPs expressed by human epithelia promoted P. aeruginosa traversal by more than 3-fold (P < 0.001). The combination knockdown of AMPs further enhanced susceptibility to bacterial traversal by ∼8-fold (P < 0.001). In vivo experiments showed that the loss of murine β-defensin 3 (mBD-3), a murine ortholog of hBD-2, enhanced corneal susceptibility to P. aeruginosa. The uninjured ocular surface of mBD-3(-/-) mice showed a reduced capacity to clear P. aeruginosa, and their corneal epithelia were more susceptible to bacterial colonization, even when inoculated ex vivo to exclude tear fluid effects. Together, these in vitro and in vivo data show functional roles for AMPs in normal corneal epithelial cell barrier function against P. aeruginosa.