Human ocular mast cells.

Abstract

Seasonal allergic conjunctivitis is a common and clinically significant type I hypersensitivity response, in which the mast cell is considered to play a pivotal role in causing the signs and symptoms, including ocular itching, hyperaemia, lacrimation and chemosis. This review focuses on the biology of the human mast cell, particularly that of the human conjunctiva. The ocular mast cell not only releases histamine and eicosanoids into the extravascular environment when activated, but also synthesizes the cytokines IL-4 and TNF-alpha. The number of IL-4 messenger RNA-positive mast cells found in the conjunctival submucosa increases threefold in seasonal allergic conjunctivitis 'in season' compared with 'out of season', suggesting a role in disease. Treatment of the eye with 2% nedocromil sodium eye drops twice a day for 2 weeks reduced the tear concentrations of both histamine and prostaglandin D(2) by more than 70% at 30 min after challenge (both <0.05) illustrating an effective mast cell stabilizing effect in the conjunctiva. Mast cells are a heterogeneous family of cells that are pivotal in initiating the signs and symptoms of seasonal allergic conjunctivitis. The expression of cytokines also endows them with the ability to initiate the inflammatory cascade, resulting in eosinophil accumulation associated with vernal conjunctivitis. Drug modulation of mast cell activity, although reducing the acute symptoms of active disease, also reduces the cytokine stimulus for the development of chronic allergic inflammation.