Similarity and difference in the mechanisms of neonatally induced tolerance and cyclophosphamide-induced tolerance in mice

The mechanisms of cyclophosphamide (CP)-induced tolerance were investigated by comparing with those of neonatally induced tolerance. When C3H/He Slc (C3H; H-2^k, Mls-1^b) mice were given i.v. either AKR/J Sea (AKR; H-2^k, Mls-1^a) or (AKR x C3H)F1 (AKC3F1; H-2^k, Mls-1^a/b) spleen cells and treated i.p. with CP 2 days later, a long-lasting skin allograft tolerance to AKR was induced in each case without any signs of graft-vs-host disease (GVHD). However, typical signs of GVHD were observed in the C3H mice neonatally tolerized with AKR spleen cells, but not in those tolerized with AKC3F1 spleen cells. The expression of TCR Vβ6, which is strongly correlated with the reactivity to Mls-1^a Ag (of donor AKR origin), in the periphery was quite different between the two types of tolerant C3H mice. Namely, in the lymph nodes of the C3H mice tolerized with AKR spleen cells and CP, only CD4⁺-Vβ6⁺, but not CD8⁺-Vβ6⁺, T cells selectively disappeared, whereas both of them were abrogated in the lymph nodes of the C3H mice neonatally tolerized of AKR. By contrast, in the thymus of the two types of tolerant C3H mice, both CD4⁺CD8^- and CD4^-CD8⁺ single-positive thymocytes expressing TCR Vβ6 were clonally deleted, suggesting that the thymic involvement was the same in each type of tolerance. These results suggest that the preferential disappearance of the CD4⁺-Vβ6⁺ T cells (of host origin) and the effector T cells of GVHD (of donor origin) occurred only in the periphery of the C3H mice tolerized with AKR spleen cells plus CP and was attributable to the destruction of Ag-stimulated T cells by the CP treatment. In contrast, the intrathymic clonal deletion of immature Vβ6⁺ T cells was a common mechanism for both of the tolerance induction systems.