Dendritic cells in ocular allergy

Background: The prevalence of ocular allergy is increasing, and dendritic cells (DC) are responsible for the initiation of the allergic response. Although it has been established that DC density increases in vernal keratoconjunctivitis, this is yet to be explored in other more prevalent forms of allergic conjunctivitis. A subclinical state termed “minimal persistent inflammation” has been described in some allergic diseases (e.g., rhinitis, asthma, vernal keratoconjunctivitis) where subjects maintain a base level of subclinical inflammation when not in contact with allergens but display increased susceptibility to developing symptoms in the presence of allergens, this even at subthreshold doses. Minimal persistent inflammation may similarly be a feature of allergic eye disease. Furthermore, most human studies have focused on characterising central corneal DC with little attention to the topographical characteristics of corneal and conjunctival DC or to DC morphology, both of which are important factors to consider in understanding DC activation and migration. This thesis aims to assess corneal and conjunctival epithelial DC (distribution, density, and morphology) in ocular allergy during the active and asymptomatic phases of allergy in humans. Methods: A retrospective study of ocular surface DC density and morphology in participants with systemic allergies, characterised by a positive skin prick test to common allergens compared with those negative to a skin prick test, was undertaken. To enable in vivo confocal microscopy (IVCM) images of DC to be reliably assessed in a clinical setting, a grading system for DC morphology at the ocular surface was developed and validated. Diurnal and topographical changes in DC density and morphology were assessed. Inter and intra-rater repeatability of corneal and conjunctival DC density and morphology was also described. A series of cross-sectional studies in the active and asymptomatic phases of allergic conjunctivitis based in Australia and in the active phase of allergic conjunctivitis and vernal keratoconjunctivitis based in Iran were conducted. Topographical characteristics of corneal and conjunctival DC density and morphology were assessed using IVCM. Results: Longer DC dendrites were observed in systemic allergic participants despite reduced DC density, suggesting an immune response at the ocular surface. A morphology grading system involving cell body size, presence of dendrites, and presence of long and thick dendrites was repeatable and could provide a user-friendly pictorial reference in a clinical setting. Diurnal variation was not observed in ocular surface DC density and morphology, and they could be reliably measured using IVCM. Both allergic conjunctivitis and vernal keratoconjunctivitis were associated with an increase in DC density and changes in morphology, indicating immune activation at the ocular surface. DC with larger cell bodies and a higher proportion of long dendrites at the corneal locations of people with allergic conjunctivitis were suggestive of increased antigen capture capacity of DC. Elevated DC density persisted at all corneal locations in the asymptomatic phase of allergic conjunctivitis but not in the conjunctiva, an indication of persistent inflammation. However, morphology analysis suggested these cells were not activated for antigen capture. In vernal keratoconjunctivitis, compared to non-allergic participants, DC bodies were larger, and more DC with long dendrites were observed at corneal locations. Participants with vernal keratoconjunctivitis had higher DC density at the corneal limbus and larger DC bodies at the corneal centre and periphery compared to those with allergic conjunctivitis. Conclusion: This research established the reliability of assessing ocular surface DC density and morphology using IVCM. Changes observed in DC density and morphology at the ocular surface might serve as biomarkers for the diagnosis and management of active inflammation in ocular allergy. The presence of corneal inflammation during the asymptomatic phase of allergy may have relevance for practitioners aiming to optimise treatment outcomes for their ocular allergy patients. The applicability of the morphology grading system should be explored in different ocular surface inflammatory and immune diseases