Requirement of a higher degree of chimerism for skin allograft tolerance in cyclophosphamide-induced tolerance.

By using a cyclophosphamide (CP)-induced tolerance system, we previously raised the possibility that the degree of chimerism might determine the induction of heart and skin allograft tolerance. When C3H (H-2k; Thy1.2, Mls-1b) mice were intravenously primed with 1 x 10(8) spleen cells (SCs) from H-2 matched AKR (H-2k; Thy1.1, Mls-1a) mice and then treated intraperitoneally with 200 mg/kg CP, the survival of AKR skin grafts was permanently prolonged in a tolerogen-specific fashion. After this treatment, a minimal degree of mixed chimerism and the clonal destruction of Mls-1a-reactive CD4+Vbeta6+ T cells in the periphery were observed. When AKR SCs and 100 mg/kg CP were used for conditioning, the survival of the AKR skin grafts was mildly prolonged. The clonal destruction of CD4+Vbeta6+ T cells in the periphery was induced and a minimal degree of mixed chimerism was detectable. The degree of mixed chimerism induced with AKR SCs and 200 mg/kg CP was significantly higher than that with AKR SCs and 100 mg/kg CP during the observation. On the other hand, neither skin allograft prolongation nor permanent mixed chimerism could be induced when C3H mice were treated with AKR SCs and 50 mg/kg CP. In order to increase the degree of mixed chimerism, we injected 1 x 10(8) tolerant AKR SCs on day 3 into the recipient C3H mice that had been treated with AKR SCs on day 0 and with 100 mg/kg CP on day 2. The reason that we used tolerant SCs was that untreated AKR SCs caused graft-versus-host disease in most of the recipients. Tolerant AKR SCs were harvested from AKR mice that had been treated with C3H SCs and 200 mg/kg CP 2 weeks earlier, and did not contain regulatory cells. By adoptive transfer, the degree of chimerism was stably and significantly increased in all recipients, and AKR skin graft tolerance was induced in half of the recipients. T-cell-depleted bone marrow cells (BMCs) from untreated AKR mice induced skin allograft tolerance in 83% of recipients. Thus, the present study strongly confirmed the hypothesis that a higher degree of chimerism is required for the induction of skin allograft tolerance in CP-induced tolerance.