Optimization of a histamine-induced allergic conjunctivitis model in Guinea pigs

Atopic dermatitis and ocular allergy: common mechanisms and uncommon questions

PURPOSE OF REVIEW

Atopic dermatitis (AD) and ocular allergy aka allergic eye disease (AED) are two common conditions that often coexist in patients. However, molecular connections between these two conditions are incompletely understood. While common etiologic components including Th2 immune signaling have been suggested for AD and AED, the mechanism how current Th2-targetd therapies (dupilumab, tralokinumab) for AD can augment conjunctivitis is not well understood.

RECENT FINDINGS

Differentially regulated genes and pathways relevant for AD disease manifestation are known. In contrast, similar information is not yet available for AED, which could be largely addressed by emerging noninvasive ocular sampling techniques. Emerging evidence indicated a reduction in goblet cell number and mucin production in a subpopulation of AD patients with AD leading to adverse ocular outcomes, while other potential mechanisms could also be involved. Involvement of particular barrier function protein(s) in AED needs further investigation.

SUMMARY

Modern cytokine-targeted therapies for AD showed elevated risk for developing conjunctivitis. Recently developed noninvasive sampling techniques should be leveraged to identify AD endotypes associated with AED and with dupilumab-associated ocular outcomes.

Alleviation of allergic conjunctivitis by (±)5(6)-dihydroxy-8Z,11Z,14Z,17Z-eicosatetraenoic acid in mice

Background: Allergic conjunctivitis (AC) is a common ophthalmologic disorder that causes symptoms that often reduces a patient's quality of life (QOL). We investigated the effects of the eicosapentaenoic acid metabolite (±)5(6)-dihydroxy-8Z,11Z,14Z,17Z-eicosatetraenoic acid ((±)5(6)-DiHETE) on AC using a mouse model. Methods: BALB/c mice were sensitized with two injections of short ragweed pollen in alum, challenged fifth with pollen in eyedrops. The clinical signs and tear volume were evaluated at 15 min after the final challenge. Histamine-induced ocular inflammation model was prepared by instilling histamine onto the surface of the eye. Fifteen minutes after histamine application, tear volume was measured using the Schirmer tear test. Miles assay was performed to investigate vascular permeability. To cause scratching behavior 10 μg of serotonin was injected in the cheek. Results: Repeated topical application of pollen induced conjunctivitis, accompanied by eyelid edema and tearing in mice. Pollen application typically degranulates mast cells and recruits eosinophils to the conjunctiva. Intraperitoneal administration of 300 μg/kg of (±)5(6)-DiHETE significantly inhibited pollen-induced symptoms. The administration of (±)5(6)-DiHETE also attenuated mast cell degranulation and eosinophil infiltration into the conjunctiva. To assess the effects of (±)5(6)-DiHETE on the downstream pathway of mast cell activation in AC, we used a histamine-induced ocular inflammation model. Topical application of 4 μg/eye histamine caused eyelid edema and tearing and increased vascular permeability, as indicated by Evans blue dye extravasation. Intraperitoneal administration of 300 μg/kg or topical administration of 1 μg/eye (±)5(6)-DiHETE inhibited histamine-induced manifestations. Finally, we assessed the effects of (±)5(6)-DiHETE on itching. An intradermal injection of 10 μg serotonin in the cheek caused scratching behavior in mice. Intraperitoneal administration of 300 μg/kg (±)5(6)-DiHETE significantly inhibited serotonin-induced scratching. Conclusion: Thus, (±)5(6)-DiHETE treatment broadly suppressed AC pathology and could be a novel treatment option for AC.

Characteristics of histamine H4 receptor agonist-induced allergic conjunctivitis model in Guinea pigs

Histamine is strongly associated with the onset of allergic conjunctivitis. The most recent cloned histamine H₄ receptor antagonist is highly expected as a new therapeutic drug candidate. As a model for a therapeutic drug targeting the histamine H₄ receptor, a mouse model in which conjunctivitis symptoms are induced by instilling 4-methylhistamine, a histamine H₄ receptor agonist, has been reported. However, the affinity of the H₄ receptor for histamine varies in species, and it is known that the histamine binding affinity for the guinea pig H₄ receptor is closer to that for human receptor than mice receptor. In this paper, we investigated a possibility that a guinea pig model would become a drug efficacy evaluation model with higher evaluation accuracy than the mouse model. As a result, hyperemia was observed in the conjunctivae and iris of guinea pigs after instillation of 4-methylhistamine and specifically suppressed by the histamine H₄ receptor antagonist. Unlikely to the previously reported mouse model, however, none of edema, increased vascular permeability or scratching behavior was observed, suggesting that there may be differences between mice and guinea pigs not only in the binding affinity of histamine to the H₄ receptor but also in the biological reaction to 4-methylhistamine. Although the symptoms of allergic conjunctivitis do not appear comprehensively in the guinea pig model, results of this study indicated a possibility that this model can be used as a simple screening model in the early stages of drug development.