Early and late phases of ocular anaphylaxis in actively immunized guinea pigs

Changes in sensory activity of ocular surface sensory nerves during allergic keratoconjunctivitis

Peripheral neural mechanisms underlying the sensations of irritation, discomfort, and itch accompanying the eye allergic response have not been hitherto analyzed. We explored this question recording the changes in the electrical activity of corneoconjunctival sensory nerve fibers of the guinea pig after an ocular allergic challenge. Sensitization was produced by i.p. ovalbumin followed by repeated application in the eye of 10% ovalbumin on days 14 to 18. Blinking and tearing rate were measured. Spontaneous and stimulus-evoked (mechanical, thermal, chemical) impulse activity was recorded from mechanonociceptor, polymodal nociceptor and cold corneoscleral sensory afferent fibers. After a single (day 14) or repeated daily exposures to the allergen during the following 3 to 4days, tearing and blinking rate increased significantly. Also, sensitization was observed in mechanonociceptors (transient reduction of mechanical threshold only on day 14) and in polymodal nociceptors (sustained enhancement of the impulse response to acidic stimulation). In contrast, cold thermoreceptors showed a significant decrease in basal ongoing activity and in the response to cooling. Treatment with the TRPV1 and TRPA1 blockers capsazepine and HC-030031 reversed the augmented blinking. Only capsazepine attenuated tearing rate increase and sensitization of the polymodal nociceptors response to CO2. Capsazepine also prevented the decrease in cold thermoreceptor activity caused by the allergic challenge. We conclude that changes in nerve impulse activity accompanying the ocular allergic response, primarily mediated by activation of nociceptor's TRPV1 and to a lesser degree by activation of TRPA1 channels, explain the eye discomfort sensations accompanying allergic episodes.

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.

Objective measurement of allergic reactions in the eye

PURPOSE OF REVIEW

Conjunctival allergen challenge has become a useful model of human ocular allergy and an accepted method for studying the effects of topical antiinflammatory medications. Conjunctival allergen challenge reactions are typically evaluated in a subjective fashion. Erythema and edema are graded by observation, using a 0 to 4 scale. Itching is graded on a 0 to 4 scale by questioning the subject. We have reviewed the recent literature on conjunctival allergen challenge, including recently developed methods aimed at objective assessment of ocular allergic reactions, such as the erythema, edema, sensation, or EES, method.

RECENT FINDINGS

Conjunctival erythema can be measured objectively using a spectroradiometer, or colorimeter, a device which measures the chromaticity of reflected light, and provides the x, or u', and y, or v', coordinates of any color. Edema of the eyelids and conjunctiva can be measured using a fractional millimeter reticule in the eyepiece of a slit lamp microscope. Ocular surface sensation can be evaluated using the aesthesiometer of Cochet and Bonnet, or by using a 'sensation index', a measurement that takes into account the intensity and duration of itching.

SUMMARY

CAC reactions can be measured objectively using the erythema, edema, sensation method, as well as subjectively by observation and questioning.

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.

Animal models of ocular allergy and their clinical correlations

Ocular allergic diseases represent a wide spectrum of disorders, from the acute self-limited, mild form of seasonal allergic conjunctivitis to the chronic, severe, sight-threatening atopic keratoconjunctivitis. The least problematic forms are the most prevalent, and several animal models have contributed to elucidate their etiopathogenetic mechanisms and have served to test numerous anti-allergic compounds. The most severe and chronic, although less prevalent, ocular allergic problems have not benefited from a similar advance, with the subsequent lack of full understanding and a limited therapeutic armamentarium. Research in this field is currently concentrating efforts in developing more protracted models of ocular allergic inflammation involving the cornea and mimicking more closely the human disease caused by chronic ocular allergy. Most recent experimental models are demonstrating that inhibiting Th2 cells and their secreted cytokines might be one important therapeutic target for inhibiting chronic allergic inflammation in the ocular surface.

Conjunctival provocation testing: overview of recent clinical trials in ocular allergy

PURPOSE OF REVIEW

The conjunctival provocation test is a human model of ocular allergy that has been used to study the ocular response to allergenic stimuli and to evaluate antiallergic therapy. The review will discuss recent studies using the conjunctival provocation test and other models of ocular allergy, and evaluate the relative merits of different models.

RECENT FINDINGS

The conjunctival provocation test has allowed investigators to recruit large numbers of allergic individuals who, although asymptomatic, can be challenged with the conjunctival administration of allergen. Observations of the eye can be made before and after challenge, and cells and mediators may be sampled from the ocular surface. In addition, the effectiveness of antiallergic therapy can be evaluated, usually by pretreating the two eyes with different forms or doses of drug.

SUMMARY

Considerable useful information has been gained about the ocular allergic response and drug efficacy using the conjunctival provocation test and naturally occurring seasonal allergic conjunctivitis.

A new murine model of allergic conjunctivitis and effectiveness of nedocromil sodium

BACKGROUND

Allergic conjunctivitis is the most common atopic disease affecting the eye. To study the pathophysiology and effectiveness of antiallergic drugs, it is necessary to develop animal models that closely mimic human allergic conjunctivitis.

OBJECTIVE

The study was performed to develop an experimental murine model of ocular allergic conjunctivitis to an airborne allergen.

METHODS

SWR/J mice were divided into the following groups: group 1, untreated, experimental; group 2, phosphate-buffered saline-treated; group 3, nedocromil sodium-treated; and group 4, unmanipulated controls. Groups 1, 2, and 3 were exposed to ragweed by topical contact with the nasal and conjunctival mucosae. Allergic conjunctivitis was evaluated by scoring of clinical signs, serum IgE levels, and histologic findings.

RESULTS

Mice exposed to ragweed had clinicopathologic signs of allergic conjunctivitis and specific anti-ragweed IgE. Allergic conjunctivitis was modulated by nedocromil sodium. Treated mice had fewer clinical signs of allergy, lower levels of ragweed-specific IgE, reduction of conjunctival eosinophil infiltration, decrease in the number of intact and degranulating mast cells, and reduction of cytokine release.

CONCLUSION

This is the first report of a murine model of allergic conjunctivitis to an airborne allergen that can be used to study the disease pathophysiology and its response to treatment.

Experimental model of allergic conjunctivitis to ragweed in guinea pig

An experimental model of ocular allergy was developed in the guinea pig by exposing these animals to ragweed through topical contact on nasal and conjunctival mucosae, followed by subsequent challenge with ragweed contact on the conjunctiva. Animals developed clinical and histological signs of allergy with and without the use of interleukin 4 as adjuvant. This model mimics human hay fever conjunctivitis more naturally than do animal models previously reported, and it may be subsequently more valuable in the study of the response of allergic conjunctivitis to different therapeutic approaches.