Changes in ocular mast cell numbers and histamine distribution during experimental autoimmune uveitis.

Choroidal mast cells have been implicated in experimental autoimmune uveitis (EAU), an ocular inflammatory disease induced by S-antigen. Our data confirm that choroidal mast cell numbers decrease with clinical onset of S-antigen-induced EAU in Lewis rats, and establish that the decrease is statistically significant. In addition, we find that the numbers of limbal mast cells also decrease during S-antigen-induced EAU, and that this decrease occurs earlier in the course of the disease than that observed for choroidal mast cells. Activation and degranulation of mast cells, as evidenced by decreases in mast cell number, result in the synthesis and/or release of large quantities of mast cell mediators, such as histamine. Histamine levels in EAU were found to change significantly, decreasing in the anterior portion of the eye and increasing in the choroid and retina, in concert with changes in mast cell number over the course of EAU. Mast cell mediators may actively contribute to the pathogenesis of EAU through direct enhancement of the inflammation, by stimulation of other elements of the immune system, and/or through facilitation of the blood-retinal barrier breakdown that occurs in EAU. Overall, these results add to the evidence for a mast cell role in EAU, and, in addition, show that the mast cell involvement in EAU includes the mast cells of the limbus.