Choroidal mast cell dynamics during experimental autoimmune uveoretinitis in rat strains of differing susceptibility

Choroidal mast cells have been implicated in the pathogenesis of experimental autoimmune uveoretinitis (EAU). The aim of the present study was to examine the dynamics of choroidal mast cells during the course of EAU in rat strains of varying susceptibility. Histochemical staining showed choroidal mast cell degranulation in Lewis rats, a highly susceptible strain, commenced nine days post-immunisation, and peaked at day 11, at which time the percentage of degranulated choroidal mast cells (32. 5± 4%) was significantly higher than controls (15. 3 ± 3%; p <0.05). At day 14, mean choroidal mast cell density was significantly reduced (from 23. 6 ± 1/mm(2) tot 16. 2 ± 2/mm(2); p<0.05) and early signs of choroidal mast cell regeneration were evident. Immunisation of PVG/C (moderate susceptibility) and Brown Norway (very low susceptibility) rats produced a similar pattern of morphological changes. Onset of clinical signs in Lewis rats, which possess approximately 1100 to 1800 choroidal mast cells per eye, occurred one day following commencement of choroidal mast cell degranulation but prior to the peak of degranulation. However, in PVG/C and Brown Norway rats, which possess only approximately 70 and 110 choroidal mast cells per eye respectively, onset of disease was not temporally linked to commencement of degranulation. Production of antigen-specific IgE during the course of EAU was extremely low in all three strains. These results indicate that choroidal mast cells may be important in the pathogenesis of EAU in Lewis rats but not in PVG/C or Brown Norway rats and that non-IgE mediated degranulation may play a role in disease induction.