Retinal immunopathology in horses with uveitis

Equine uveitis is a spontaneous disorder of horses that can serve as a model for the study of human uveitis. Although the initial presentation 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.

Sections of eyes recovered from horses with naturally occurring uveitis and from Shetland ponies with experimental leptospira-induced 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.

Histopathological abnormalities in retinas from horses with uveitis ranged from minimal to total loss of retinal tissue. MHC Class II antigen-positive round and dendritiform cells were seen in these retinas, but were not seen in retinas from horses without uveitis. There was no significant reactivity noted in the retinal pigment epithelial cells or Muller cells. Numbers of MHC Class II antigen-expressing cells and T lymphocytes correlated with the extent of retinal histopathology. B lymphocytes were seen primarily in retinas from horses that were seroreactive for Leptospira interrogans serovar pomona. Retinas from ponies with experimental uveitis had changes similar to those from horses with spontaneous uveitis.

These results suggest that retinal pathology may be a primary immunological event in equine uveitis, provide evidence that leptospira-associated uveitis may be a distinct subset of equine uveitides, underscore the relevance of the study of equine uveitis to human uveitis, and support the plausibility of a post-infectious immunopathogenesis of some naturally occurring uveitides in both humans and horses.