Interaction between nitric oxide and prostaglandin synthesis in the acute phase of allergic conjunctivitis

Ocular redness - I: Etiology, pathogenesis, and assessment of conjunctival hyperemia

The translucent appearance of the conjunctiva allows for immediate visualization of changes in the circulation of the conjunctival microvasculature consisting of extensive branching of superficial and deep arterial systems and corresponding drainage pathways, and the translucent appearance of the conjunctiva allows for immediate visualization of changes in the circulation. Conjunctival hyperemia is caused by a pathological vasodilatory response of the microvasculature in response to inflammation due to a myriad of infectious and non-infectious etiologies. It is one of the most common contributors of ocular complaints that prompts visits to medical centers. Our understanding of these neurogenic and immune-mediated pathways has progressed over time and has played a critical role in developing targeted novel therapies. Due to a multitude of underlying etiologies, patients must be accurately diagnosed for efficacious management of conjunctival hyperemia. The diagnostic techniques used for the grading of conjunctival hyperemia have also evolved from descriptive and subjective grading scales to more reliable computer-based objective grading scales.

The role of nitric oxide in ocular surface physiology and pathophysiology

Nitric oxide (NO) has a wide array of biological functions including the regulation of vascular tone, neurotransmission, immunomodulation, stimulation of proinflammatory cytokine expression and antimicrobial action. These functions may depend on the type of isoform that is responsible for the synthesis of NO. NO is found in various ocular tissues playing a pivotal role in physiological mechanisms, namely regulating vascular tone in the uvea, retinal blood circulation, aqueous humor dynamics, neurotransmission and phototransduction in retinal layers. Unregulated production of NO in ocular tissues may result in production of toxic superoxide free radicals that participate in ocular diseases such as endotoxin-induced uveitis, ischemic proliferative retinopathy and neurotoxicity of optic nerve head in glaucoma. However, the role of NO on the ocular surface in mediating physiology and pathophysiological processes is not fully understood. Moreover, methods used to measure levels of NO in the biological samples of the ocular surface are not well established due to its rapid oxidation. The purpose of this review is to highlight the role of NO in the physiology and pathophysiology of ocular surface and propose suitable techniques to measure NO levels in ocular surface tissues and tears. This will improve the understanding of NO's role in ocular surface biology and the development of new NO-based therapies to treat various ocular surface diseases. Further, this review summarizes the biochemistry underpinning NO's antimicrobial action.

Elevated levels of prostaglandin E2 in the tears of patients with severe allergic conjunctivitis and primary cultured conjunctival cells are suppressed by ketotifen and dexamethasone

Objective

We examined the production of prostaglandin E₂ (PGE₂), which is the key prostaglandin involved in inflammatory disorders of the ocular surface. Tears and conjunctival fibroblasts were evaluated in order to assess allergic inflammation and the effect of specific drugs.

Methods and analysis

PGE₂ was measured in tears from both patients and normal volunteers. Primary cultures of human conjunctival fibroblasts were incubated with interleukin (IL)-4 and tumour necrosis factor (TNF)-α with or without ketotifen fumarate or dexamethasone. The culture supernatants were removed 24 hours after exposure and the concentrations of PGE₂ were quantified by ELISA.

Results

Significantly higher levels of PGE₂ were observed in the tears of patients with severe allergic conjunctivitis than in those with post-surgical inflammation (p=0.02), and this production was reduced by eye drops. Stimulation with IL-4 and TNF-α induced the generation of PGE₂ in supernatants of conjunctival fibroblasts, and this production was significantly downregulated by ketotifen fumarate or steroids.

Conclusion

PGE₂ may participate in the pathogenesis of severe ocular allergic disease, and both ketotifen fumarate and steroid reduce the production of PGE₂.

Profile of tear lipid mediator as a biomarker of inflammation for meibomian gland dysfunction and ocular surface diseases: Standard operating procedures

Human tear is a biological fluid rich in lipids that is increasingly collected in clinical and biological research. The repertoire of small lipids or lipid mediators (often termed eicosanoids or oxylipins) found in human tear provides insight into metabolism of fatty acids and physiology of the ocular surface and Meibomian glands. Disturbances in the tear lipid mediators profile also occur during inflammation of the ocular surface that is not directly linked to lipid metabolism. The changes in the levels of pro-inflammatory and pro-resolution lipid mediators in the tear help assess the severity and stage of inflammation in ocular surface tissues. Mass spectrometry, used in the evaluation of tear lipid mediators, is an emerging tool in clinical diagnostics and personalized medicine. Here we describe the reproducibility, accuracy, and precision of quantifying lipid mediators in human tears, with a suggested method for tear collection and sample handling. The ranges of lipid mediators concentrations in tear fluid of healthy and diseased individuals with Meibomian gland dysfunction are reported, as well as the impact of age and disease on individual lipid mediators. We would like to recommend a set of guidelines, which can be further discussed in workshops. This will facilitate harmonization of future tear lipid mediators data across different instrument platforms in various laboratories. We hope that other fields requiring lipid mediators assays will also benefit from such an effort.

The Effect of Cytokine-Stimulation and Pharmacologic Intervention on PGE2 Production in Primary Human Conjunctival and Corneal Cells

We studied the production of PGE2 by human conjunctival and corneal cells in response to inflammation, and reduction of inflammation with non-steroidal anti-inflammatory drugs. Primary cultures of human conjunctival epithelial cells, fibroblasts, corneal epithelial cells, and keratocytes were incubated with IL-4 and TNF-α. PGE2 and COX-2 levels were analyzed. Effects of anti-inflammatory and anti-immune drugs on PGE2 production were also investigated. IL-4 and TNF-α induced the generation of PGE2 and COX-2 in conjunctival and corneal cells. Epithelial PGE2 production was significantly lower than in keratocytes and fibroblasts, which was down-regulated by aspirin. IL-4 and TNF-α enhanced the inflammatory response via prostaglandin production which contributed to ocular surface inflammation. Prostaglandin production was higher in stromal cells than epithelial cells. These results suggest that the epithelial barrier disruption may contribute to ocular allergic inflammation by the PGE2 production from stromal cells. Moreover, NSAIDs were effective in suppressing PGE2 production in our experiment.

Late-phase reaction in ocular allergy

PURPOSE OF REVIEW

To determine if the late-phase reaction, which commonly occurs in allergic rhinitis and asthma, is also found in ocular allergy.

RECENT FINDINGS

Using PubMed, 542 articles were found; 18 articles in the allergy and ophthalmology literature were specifically related to late-phase reaction. Ocular late-phase reaction is clinically seen in 50-100% of allergic rhinoconjunctivitis patients, is associated with progression to systemic atopic disorders that is allergic rhinoconjunctivitis and occurs in several forms including biphasic, multiphasic and a prolonged response.

SUMMARY

The existing literature demonstrates that an ocular late-phase reaction also exists and has implications in the development severity of disease, change of reactivity and progression of the atopic disease state from a localized target organ, such as the nose or eye, to a more systemic atopic disorder. The existence of the clinically relevant allergic late-phase response is not only limited to the nose, skin and lungs but also includes the eyes. The appreciation that the late-phase response may be clinically very important as there is a continuum of ocular mast-cell activation during the waking hours of the day, a better understanding of its clinical impact may be a more appropriate focus in the development of future treatments.

Immunohistochemical detection and Western blot analysis of nitrated protein in stored human corneal epithelium

While the production of nitric oxide by human corneas in storage has recently been demonstrated, protein nitration as a result of this production has not been demonstrated. In this study, nitrated protein accumulation in the epithelium of stored human corneas was assessed. One half of five donor corneas maintained in storage media for 3 days were prepared for immunohistochemical studies. The other halves remained in storage media for 7 additional days and were also processed for immunohistochemistry. Mouse monoclonal antibody to nitrotyrosine adducts was used to define the localisation of these epitopes. The density of antibody staining was observed and quantified on a digital camera system and statistically analysed. Immunostaining in the epithelium was greater in tissues recovered after 10 days in storage compared to the intensity of staining after 3 days of storage (p<0.0001). No staining was evident in the epithelium in sections exposed to non-immune mouse IgG. Western blot analysis was performed on epithelial cells scraped from corneal surfaces of one-half of four donor corneas in storage for 3 days and from the other half at 10 days of storage. Nitrated BSA was used as a positive control. After extraction and homogenisation, identical protein concentrations of each sample were loaded per lane on 10% gels and subjected to SDS-PAGE. Proteins were blotted and probed with the anti-nitrotyrosine antibody. Western blot immunoreactivity was detected in epithelial samples at the 3 and 10 day recovery times with the latter samples showing greater staining intensity. Nitrated protein, thought to indicate toxic peroxynitrite formation, accumulates in the human corneal epithelium with time of storage. Our study shows that there is an association between increased nitrated protein and storage time.

Hyperoxia impairs airway relaxation in immature rats via a cAMP-mediated mechanism

Hyperoxic exposure enhances airway reactivity in newborn animals, possibly due to altered relaxation. We sought to define the role of prostaglandinand nitric oxide-mediated mechanisms in impaired airway relaxation induced by hyperoxic stress. We exposed 7-day-old rat pups to either room air or hyperoxia (>95% O2) for 7 days to assess airway relaxation and cAMP and cGMP production after electrical field stimulation (EFS). EFS-induced relaxation of preconstricted trachea was diminished in hyperoxic vs. normoxic animals (P < 0.05). Indomethacin (a cyclooxygenase inhibitor) reduced EFS-induced airway relaxation in tracheae from normoxic (P < 0.05), but not hyperoxic, rat pups; however, in the presence of NG-nitro-L-arginine methyl ester (a nitric oxide synthase inhibitor) EFS-induced airway relaxation was similarly decreased in tracheae from both normoxic and hyperoxic animals. After EFS, the increase from baseline in the production of cAMP was significantly higher in tracheae from normoxic than hyperoxic rat pups, and this was accompanied by greater prostaglandin E2 release only in the normoxic group. cGMP production after EFS stimulation did not differ between normoxic and hyperoxic groups. We conclude that hyperoxia impairs airway relaxation in immature animals via a mechanism primarily involving the prostaglandin-cAMP signaling pathway with an impairment of prostaglandin E2 release and cAMP accumulation.

Animal models of allergic and inflammatory conjunctivitis

Allergic eye diseases are complex inflammatory conditions of the conjunctiva with an increasing prevalence and incidence. The diseases are often concomitant with other allergic diseases such as allergic rhinitis, atopic dermatitis and allergic asthma. Despite the disabling and prominent symptoms of ocular allergies, they are less well studied and further insights into the molecular basics are still required. To establish new therapeutic approaches and assess immunological mechanisms, animal models of ocular allergies have been developed in the past years. The major forms of allergic ocular diseases, seasonal and perennial allergic conjunctivitis, vernal and atopic keratoconjunctivitis and giant papillary conjunctivitis, each have different pathophysiological and immunological components. In contrast to these distinct entities, the current animal models are based on the sensitization against a small number of allergens such as ovalbumin, ragweed pollen or major cat allergens and consecutive challenge. Different animal species have been used so far. Starting with guinea-pig models of allergic conjunctivitis to assess pharmacological aspects, new models including rats and mice have been developed which mimic major features of ocular allergy. The presently preferred species for the investigation of the immunological basis of the disease is represented by murine models of allergic conjunctivitis. In the future, combined ocular, nasal and aerosolic challenges with allergens may provide a model of allergy that encompasses simultaneously the target organs eye, nose and airways with conjunctivitis, rhinitis and asthma.

Beneficial effects of heme oxygenase-1 up-regulation in the development of experimental inflammation induced by zymosan

Heme oxygenase-1 (HO-1) is part of the integrated response to oxidative stress. This enzyme may exert anti-inflammatory effects in some animal models, although the precise mechanisms are not fully understood. We have examined the role of HO-1 in the inflammatory response induced by zymosan in the mouse air pouch. Zymosan administration induced HO-1 protein expression in leukocytes migrating to exudates, with maximal levels in the late phase of this response (24-48 h). This was accompanied by ferritin induction and bilirubin accumulation, indicating that this enzyme is active in our model. HO-1 expression by zymosan treatment was partly reduced by aminoguanidine, suggesting the participation of endogenous nitric oxide in the mechanisms leading to HO-1 synthesis in the zymosan-injected mouse air pouch. Up-regulation of HO-1 by hemin administration resulted in inhibition of nitric-oxide synthase-2 activity, cellular infiltration into the air pouch exudate, and plasmatic exudation. Leukotriene B4 levels in exudates were significantly decreased in the early phase of this response (4 h), whereas interleukin-1beta and tumor necrosis factor-alpha were inhibited at all time points. Inhibition of HO-1 activity by zinc protoporphyrin IX prevented most of the effects caused by hemin administration. Our results indicate that HO-1 exerts anti-inflammatory effects on the response to zymosan in the mouse air pouch and support a role for this enzyme in the modulation of inflammatory processes.

A synthetic heparin-mimicking polyanionic compound inhibits central nervous system inflammation

The immunomodulating capacity of heparin led us to test the effect of the synthetic heparin-mimicking and low anticoagulant compound RG-13577 on the course of experimental autoimmune encephalomyelitis (EAE) and central nervous system (CNS) inflammation. EAE was induced in SJL mice by inoculation with whole mouse spinal cord homogenate. RG-13577, delivered intraperitoneally, inhibited the clinical signs of acute EAE and markedly ameliorated inflammation in the spinal cord, primarily by inhibiting heparanase activity in lymphocytes and astrocytes and thus impairing lymphocyte traffic. RG-13577 treatment was effective when started on day of disease induction or day 7 after induction. The low molecular weight heparin, enoxaparin, tested under the same conditions, exerted only a minor insignificant inhibitory effect. RG-13577 also inhibited the tyrosine phosphorylation of several proteins, particularly Erk1 and Erk2 of the MAP kinase signaling pathways associated with inflammation and cell proliferation. RG-13577 blocked the activity of sPLA(2) and inhibited CNS PGE(2) production both in vivo and in vitro.

Augmentation of allergic inflammation in prostanoid IP receptor deficient mice

1 To evaluate the role of prostaglandin I(2) (PGI(2)) in allergic inflammation, allergic responses in the airway, skin and T cells were studied in mice lacking the receptor for PGI(2) (the prostanoid IP receptor) through gene disruption. 2 Three inhalations of antigen caused an increase in plasma extravasation, leukocyte accumulation and cytokine (interleukin (IL)-4 and IL-5) production in the airway of sensitized mice. These airway inflammatory responses were significantly greater in IP receptor deficient mice than in wild-type mice. 3 The vascular leakage caused by passive cutaneous anaphylaxis, substance P and 5-hydroxytryptamine was markedly increased in the skin of IP receptor deficient mice, compared with comparably treated wild-type mice. 4 The inhalation of antigen in sensitized mice resulted in increased serum antigen specific IgE, total IgE and IgG levels. The magnitude of the elevations of each immunoglobulin level in IP receptor deficient mice is notably higher than that in wild-type mice. To elucidate the mechanism of an enhancement of immunoglobulin production, the activity of T cells in sensitized and non-sensitized mice was studied by means of the production of cytokines. The antigen-induced IL-4 production by spleen cells from sensitized IP receptor deficient mice was almost three times greater than that in wild-type mice. On the contrary, the anti-CD3 antibody-induced interferon-gamma production by CD4(+) T cells from non-sensitized IP receptor deficient mice was significantly lower than that in wild-type mice. 5 The present data indicate that IP receptor deficiency reinforced an allergic airway and skin inflammation by augmentation of vascular permeability increase and the T helper 2 cell function. These findings suggest a regulatory role of PGI(2) in allergic inflammation.

Nitric oxide generated by corneas in corneal storage media

PURPOSE

The purpose of the study was to quantify nitric oxide release by human corneal buttons in storage media over time.

METHODS

Group 1 consisted of six chambers of Optisol GS corneal storage media, each containing a viable human corneal button with an attached scleral rim (unsuitable for transplantation), sampled at 1-day intervals for at least 17 days (range, 17-28 days). Group 2 consisted of 34 chambers of Optisol GS media, each used to store a corneal button for penetrating keratoplasty, sampled immediately after each surgery. An unused vial of Optisol GS storage medium was sampled daily for 17 days to serve as a background medium control. The total amount of nitrite and nitrate in each sample was determined by a spectrophotometric method based on the Griess reaction.

RESULTS

Data from the daily sampling in group 1 showed that nitrite and nitrate concentrations in storage media containing human corneas increase from a baseline level (beginning at the time the corneas are placed in the media) to an equilibrium concentration of 2.77 microM in a mean time of 6.15 days. Seventy-six percent of the data points from group 2 fell within the 80% predictive interval derived from group 1. No nitrite or nitrate was detected in background medium control samples.

CONCLUSION

The progressive increase in nitrite and nitrate in corneal storage media over time suggests that nitric oxide is continuously released by corneas during storage before transplantation. Given the toxic free radical properties of nitric oxide, corneas in storage media may be subjected to the cumulative toxic effects of nitric oxide.

Mechanism for substance P-induced relaxation of precontracted airway smooth muscle during development

Release of substance P (SP) from sensory nerve endings of the tracheobronchial system modulates airway smooth muscle contraction and may cause relaxation of precontracted airways. We sought to elucidate the effect of postnatal maturation on SP-induced relaxation of precontracted airways and determine the roles of endogenously generated nitric oxide (NO) and prostaglandins (PGs). Cylindrical airway segments were isolated from the midtrachea of rats at four different ages, 1, 2, and 4 wk and 3 mo, and contracted to 50-75% of the maximum response induced by bethanechol. SP was then administered in the absence and presence of the NO synthase inhibitor NG-nitro-L-arginine methyl ester (L-NAME), the PG inhibitor indomethacin, or both. Relaxation of airways with SP decreased significantly with advancing postnatal age. SP-induced tracheal relaxation was consistently attenuated by pretreatment with L-NAME, indomethacin, or both. In a different group of animals, L-NAME significantly attenuated the relaxant response of airways to PGE2 exposure, but indomethacin had no significant effect on the relaxant response to exogenous NO. We conclude that SP induces a relaxant effect on precontracted airway smooth muscle, which decreases with advancing age and is mediated via SP-induced release of NO and/or PG.