Indirect effects of oral tolerance cannot be ascribed to bystander suppression.

The addition of tolerated antigens to immunizing doses of unrelated antigens blocks antibody responses to these unrelated antigens. This inhibition, which the authors have called the indirect effects of tolerated antigens, occurs even when the mixture of proteins is injected as soon as 24 h after the oral tolerance induction. The indirect effects also do not require the simultaneous injection of the two proteins: they are still present 72 h after an injection of Ova in Ova tolerant mice, but do not occur if the unrelated protein is injected 24 h before the tolerated protein. In addition, indirect effects do not block secondary responses to unrelated proteins if the primary immunization is made in the absence of the tolerated protein. These results cannot be explained by innocent bystander suppression, which is believed to result from the action of suppressive cytokines released by specific tolerant lymphocytes upon unrelated lymphocytes that would otherwise respond to the second, non-tolerated antigen. Indirect effects may be better understood in terms of network models.