Histamine H4 receptors in normal conjunctiva and in vernal keratoconjunctivitis

Background: While it is known that histamine is the primary mediator of ocular allergy, the presence and distribution of histamine receptors are not well documented in the human eye. Our aim was to evaluate histamine receptor expression in normal and vernal keratoconjunctivitis conjunctiva. Methods: Mucosal biopsies were obtained from conjunctiva of healthy donors and from tarsal conjunctiva of vernal patients. Immunostaining and semi-quantitative reverse-transcriptase polymerase chain reaction for H ₁, H ₂, H ₃, and H ₄ receptors were performed. Histamine receptor expression was also evaluated in conjunctival cell cultures exposed to histamine, interleukin-4, interleukin-5, interferon-γ and tumor necrosis factor-α. Results: Immunostaining for H ₁ and H ₂ receptors was slightly positive in normal and over-expressed in vernal tissues. H ₃ receptors were rarely present in normal and inflamed conjunctiva. In striking contrast to control tissues, H ₄ receptors were highly expressed in all inflamed tissues, particularly by stromal inflammatory cells. Semi-quantitative reverse-transcriptase polymerase chain reaction demonstrated an over-expression of H ₁, H ₂, and H ₄ receptors in vernal vs control tissues. Notably, H ₄ receptors were five times more expressed in vernal vs control tissues. In cell cultures, H ₂ receptor expression was stimulated eight times the normal levels by interleukin-4 and three times by histamine, but the H ₄ receptor was only slightly affected by stimulation with these mediators. Conclusions: Increased expression of H1, and particularly of H ₂ and H ₄ receptors in vernal keratoconjunctival tissues indicate their important role in the pathogenesis of this disease. H ₄ receptors may be a target in the treatment of allergic inflammation.