Phenotypic analysis of oral tolerance to alloantigens: Evidence that the indirect pathway of antigen presentation is involved

Background. Oral administration of alloantigens induces down-regulation of Th1 immune responses and reduces the incidence of corneal graft rejection. This study examined the role of Th1 and Th2 cytokines, accessory cells, and lymphoid organs that are known to be instrumental in other forms of antigen-specific tolerance. Methods. Allogeneic dendritic cells (DC) were administered orally using a protocol that is known to reduce the incidence of corneal allograft rejection and prevent the generation of allospecific delayed-type hypersensitivity (DTH). Hosts included normal mice and gene knockout (KO) mice, including B cell-deficient μMT, interleukin (IL)-4 KO, IL-10 KO, and interferon (IFN)-γ KO mice. The requirement for either an intact spleen or thymus was also examined. Orally administered paraformaldehyde-fixed, UVB-treated, or sonicated allogeneic cells were tested to determine if dead cells were capable of inducing tolerance. Results. Studies on gene KO mice indicated that a Th1 cytokine (IFN-γ) and a Th2 cytokine (IL-4) were needed for the development of oral tolerance to alloantigens. By contrast, IL-10 was not required. Although an intact spleen was necessary for the development of tolerance, removal of the thymus did not affect down-regulation of DTH. Conclusions. Oral tolerance induced with allogeneic cells shares characteristics with antigen-specific unresponsiveness induced by other routes, yet there are some noteworthy differences. The capacity of killed or sonicated allogeneic cells to induce oral tolerance and enhance corneal graft survival indicates that oral tolerance to alloantigens can occur via the indirect pathway of alloantigen presentation. These results also emphasize the remarkable redundancy in the mechanisms that the immune system employs to produce antigen-specific unresponsiveness.