Immunological and inflammatory mechanisms in ocular allergy with special reference to vernal keratoconjunctivitis : clinical and experimental studies

Abstract

Ocular allergy encompasses a wide range of conditions, most of which implicate IgE in the pathogenesis. In contrast to the benign nature of seasonal allergic conjunctivitis (SAC), the less common chronic eye diseases vernal and atopic keratoconjunctivitis (VKC and AKC) are severe and debilitating disorders. The factors that are responsible for the triggering and maintenance of these chronic diseases are largely unknown. The following objectives were pursued in this project: to characterise VKC in epidemiological and immunologic terms; to delineate the cellular infiltration and to determine the expression of mRNA for various cytokines in conjunctival biopsies from VKC patients with reference to atopic status; to investigate the role of the eosinophilic granulocyte in general ocular allergy by assessing cosinophil cationic protein (ECP) in tear fluid; to determine the kinetics of ECP release into tears after a single allergen provocation in sensitised volunteers with SAC; and finally, to explore the possible delaying effect of ECP in a corneal wound healing model in rabbits.

In a cohort of VKC subjects, mostly children, only 60% of the cases were classed as atopic by conventional testing, i.e. presence of positive skin prick tests or RAST. An examination of specific IgE in tears did not reveal any additional atopic subjects. By retrieving information from a social database, it was found that VKC affects disproportionately high numbers of children of Asian or African origin. Since the majority of these children were born in Sweden, genetic rather than environmental factors should explain the unequal distribution of VKC among different ethnic groups. Expression of immunocytochemical markers and mRNA for cytokines were investigated in papillary tissue from atopic and non-atopic subjects with VKC. Salient differences with respect to the numbers of infiltrating cells and mRNA signals for cytokines pathognomonic for allergy: e.g. IgE cells, T-cells, mast cells, eosinophils, interleukin (IL)-5, and IL-13 were found when disease and control samples were compared. In contrast, no differences were detectable between the atopic and non-atopic subjects with VKC. Non-atopic and atopic VKC thus share common immunological and inflammatory disease mechanisms that seem to be Th2-polarised.

Tear samples from symptomatic patients with SAC, AKC, VKC, and healthy controls were analysed with respect to ECP levels. All three allergic conditions were associated with a significant ECP increase as compared with controls. Moreover, there was a significant correlation between symptom severity and ECP levels, particularly in VKC, substantiating the concept of eosinophil involvement in ocular allergic disease processes and a possible cause and effect relationship with keratopathy. ECP release into tears and its relationship to the acute and late phase reaction in a 72-hour time course in the conjunctiva was studied after relevant local allergen provocation in non-symptomatic subjects with SAC. Tear ECP was elevated in the late phase allergic reaction and was significantly correlated with symptoms at 24 hours after the challenge. To explore the possibility that ECP is instrumental in corneal disease in VKC, ECP was administered in eyes of rabbits that had been subjected to excimer laser operations to create standard size wounds. ECP significantly delayed the the process and was related to a changed cell distribution in the sliding epithelial sheets and a reduced expression of anti-apoptotic protein in keratocytes. ECP may thus be partially responsible for the keratopathy seen in chronic allergic eye disease.