Ocular allergic inflammation: interaction between the cornea and conjunctiva

Severe ocular allergic diseases such as vernal keratoconjunctivitis are characterized not only by conjunctival allergic inflammation, including infiltration of T helper 2 cells and eosinophils into the conjunctiva, but also by various corneal disorders such as persistent epithelial defects and shield ulcer. Although the cornea and conjunctiva are thought to influence each other during ocular allergic inflammation, direct evidence for interaction between these tissues in vivo has been lacking. Eosinophils and eosinophil-derived factors are implicated in the pathogenesis of corneal lesions associated with ocular allergy, with cytotoxic granule proteins such as major basic protein and matrix metalloproteinase 9 derived from eosinophils having been detected in shield ulcer. Major basic protein exhibits cytotoxic effects in cultured corneal epithelial cells and inhibits corneal epithelial wound healing in organ culture, whereas matrix metalloproteinase 9 can degrade the corneal epithelial basement membrane. In vitro studies have revealed that cytokines and other inflammatory mediators directly impair the barrier function of corneal epithelial cells and increase the expression of chemokines and adhesion molecules by corneal stromal fibroblasts, effects that may enhance allergic inflammation. We have recently shown that removal of the corneal epithelium augmented late-phase clinical signs and conjunctival eosinophilia, whereas conjunctival inflammation delayed corneal epithelial wound healing, in a rat model of ocular allergy. Conjunctival allergic inflammation and corneal epithelial disorders thus interact with each other in vivo to generate a vicious cycle, interruption of which might provide the basis for novel approaches to the treatment of severe ocular allergy.