Induction of bystander suppression by feeding antigen occurs despite normal clonal expansion of the bystander T cell population.

The induction of bystander suppression, whereby the response against one Ag is suppressed when it is presented in the context of an Ag to which tolerance is already established, would be an important property of oral tolerance, because it would allow treatment of autoimmune and hypersensitivity responses where the initiating Ag is not known. Although bystander suppression has been described in oral tolerance, it is not known how its effects are mediated at the level of the bystander T cells. In addition, previous studies have not compared regimes in which Ag is fed in a tolerogenic or immunogenic manner, meaning that the possible effects of Ag competition have not been excluded. In this study we have used two populations of Ag-specific TCR transgenic CD4(+) T cells to examine the cellular basis of bystander suppression associated with oral tolerance in mice in vitro and in vivo. Our results show that bystander responses can be inhibited by feeding Ag and that these effects are more pronounced in mice fed protein in tolerogenic form than after feeding Ag with mucosal adjuvant. However, the expansion of the bystander-specific CD4(+) T cells is not influenced by the presence of oral tolerance. Thus, bystander suppression does not reflect clonal deletion or reduced clonal expansion of the bystander T cells, but may act by altering the functional differentiation of bystander T cells.