Orally induced bystander suppression in experimental autoimmune uveoretinitis occurs only in the periphery and not in the eye

Oral administration of retinal soluble antigen (S‐Ag) suppresses the induction of S‐Ag‐mediated experimental autoimmune uveitis (EAU) in Lewis rats. EAU induced with interphotoreceptor retinoid‐binding protein (IRBP), another retinal autoantigen, can also be suppressed by oral administration of IRBP. It has been speculated that feeding with one retinal autoantigen could suppress induction of uveitis with the other retinal protein by means of bystander suppression. Both uveitogenic effector and suppressor cells should find their antigens within the retina, where the suppressor cells would be expected to act on the effector cells. However, reciprocal combinations of antigens used for induction and suppression of uveitis failed to prevent onset of disease, demonstrating that bystander suppression obviously does not occur in the eye. To investigate further the localization of suppressor mechanisms, we fed Lewis rats either with retinal S‐Ag or with ovalbumin (OVA) and then immunized the animals either with a mixture of S‐Ag and OVA or with each antigen separately, injected into contralateral hind legs. Feeding of S‐Ag prior to immunization led to suppression of uveitis, whereas feeding of OVA had no tolerizing effect when S‐Ag and OVA were injected into different legs. Nevertheless, immunizing rats with a mixture of S‐Ag and OVA after OVA feeding suppressed uveitis to a high degree. These findings suggest that orally induced bystander suppression might not occur in the target organ, but rather peripherally at the site of induction of the autoimmune T cells.