Antigen-driven bystander suppression after oral administration of antigens

Abstract

Suppression of experimental autoimmune encephalomyelitis (EAE) in Lewis rats by the oral administration of myelin basic protein (MBP) is mediated by CD8+ T cells that can be isolated from the spleens of MBP-fed animals. These cells adoptively transfer protection to naive animals subsequently immunized with MBP and complete Freund's adjuvant (CFA) and suppress in vitro MBP proliferative responses. Using a transwell system in which the modulator spleen cells from MBP-fed animals are separated by a semipermeable membrane from responder cells, MBP, or OVA-specific T cell lines, we have found that cell contact is not required for in vitro suppression to occur. In vitro suppression is dependent, however, upon antigen-specific triggering of modulator T cells. Once antigen-specific triggering occurs, suppression across the transwell is mediated by an antigen-nonspecific soluble factor that equally suppresses an MBP line or an ovalbumin (OVA) line. This phenomenon of antigen-driven bystander suppression was also demonstrated in vivo. Specifically, Lewis rats fed OVA which were then immunized with MBP/CFA plus OVA given separately subcutaneously were protected from EAE. Animals fed OVA and then immunized with MBP/CFA without OVA given subcutaneously were not protected. The protective effect of feeding OVA could be adoptively transferred by CD8+ T cells from OVA-fed animals into MBP/CFA plus OVA-injected animals. Feeding bovine serum albumin (BSA) or keyhole limpet hemocyanin did not suppress EAE in animals immunized with MBP/CFA plus OVA. EAE was suppressed, however, if BSA was fed and animals then immunized with MBP/CFA plus BSA given subcutaneously. Antigen-driven bystander suppression appears to be an important mechanism by which antigen-driven peripheral tolerance after oral administration of antigen is mediated, and presumably occurs in the microenvironment accounting for the antigen specificity of suppression generated by oral tolerization to antigens.