Chimeric donor cells play an active role in both induction and maintenance phases of transplantation tolerance induced by mixed chimerism

Donor hemopoietic cell engraftment is considered to be an indicator of allograft tolerance. We depleted chimerism with cells specifically presensitized to the bone marrow donor to investigate its role in mixed chimera-induced tolerance. Three experimental models were used: model A, B10.A cells presensitized to B6 (a anti-b cells) were injected into (B6 x D2)F-1 --> B10.A mixed chimeras grafted with DBA/2 skin; model B, anti-B6 presensitized cells prepared in DBA/2 --> B10.A mixed chimeras, thus unresponsive to DBA/2 (a anti-b/tol-d cells), were injected into (B6 x D2)F-1 --> B10.A mixed chimeras grafted with DBA/2 skin; and model C, (BALB/c X B6)F-1 cells presensitized to CBA (d/b anti-k cells) were injected into (B6 x CBA)F-1 --> BALB/c mixed chimeras grafted with B6 skin. Skin was grafted on day 30. Injection of each cell type before skin grafting abolished hemopoietic cell engraftment and prevented allograft acceptance. Injection of presensitized cells after skin grafting resulted in different outcomes depending on the models. In model A, injection of a anti-b cells completely depleted chimerism and caused allograft rejection. In model B, injection of a anti-b/tol-d cells markedly reduced, but did not deplete, peripheral chimerism and maintained skin allograft survival. In model C, d/b anti-k cells reduced chimerism to the background levels but failed to cause graft rejection,, probably due to persistence of injected cells which share MHC with skin grafts. Together, the results show that presence of chimeric donor cells is essential in both the induction and maintenance phases of tolerance induced by mixed chimerism.