Retinal immunopathology in horses with uveitis

Background: Equine uveitis is a spontaneous disorder of horses that can ser ve as a model for the study of human uveitis. Although the initial presenta tion is that of an anterior uveitis, retinal involvement has been noted in some cases. We report here the immunohistopathology of retinas from horses with uveitis. Methods: Sections of eyes recovered from horses with naturall y occurring uveitis and from Shetland ponies with experimental leptospira-i nduced uveitis were stained by hematoxylin and eosin for histopathological evaluation. Immunohistochemistry was used to evaluate retinas for MHC Class II antigen expression and infiltration of T and B lymphocytes. Results: Hi stopathological abnormalities in retinas from horses with uveitis ranged fr om minimal to total loss of retinal tissue. MHC Class II antigen-positive r ound and dendritiform cells were seen in these retinas, but were not seen i n retinas from horses without uveitis. There was no significant reactivity noted in the retinal pigment epithelial cells or Muller cells. Numbers of M HC Class II antigen-expressing cells and T lymphocytes correlated with the extent of retinal histopathology, B lymphocytes were seen primarily in reti nas from horses that were seroreactive for Leptospira interrogans serovar p omona. Retinas from ponies with experimental uveitis had changes similar to those from horses with spontaneous uveitis. Conclusions: These results sug gest that retinal pathology may be a primary immunological event in equine uveitis, provide evidence that leptospira-associated uveitis may be a disti nct subset of equine uveitides, underscore the relevance of the study of eq uine uveitis to human uveitis, and support the plausibility of a post-infec tious immunopathogenesis of some naturally occurring uveitides in both huma ns and horses.