Nonmyeloablative conditioning to induce bilateral tolerance after allogeneic bone marrow transplantation in mice

We recently described a new nonmyeloablative method to induce stable and specific transplantation tolerance to allogeneic tissues in adult mice. It included total lymphoid irradiation (TLI) of recipients with six fractions of 200 cGy each, inoculation with donor bone marrow (BM) cells and cyclophosphamide (Cy) for selective elimination or inactivation of residual donor-reactive cells of the host, and infusion with T-cell depleted donor BM cells after Cy. Here, we investigated the possibility to induce stable bilateral graft-vs-host and host-vs-graft transplantation tolerance using non–T-cell depleted allogeneic BM. Our results show that the dose of BM required for the induction of transplantation tolerance was inversely correlated with the intensity of the conditioning. Transfer of a low dose (3 × 106) of total donor BM cells to recipients preconditioned with a less intensive regimen (two or three TLI fractions instead of six) diminished graft-vs-host disease (GVHD)-related mortality of recipients to 40% and converted 89% of the survivors into GVHD-free mixed hematopoietic chimeras that maintained donor skin allografts >180 days. A tenfold increase in the number of donor BM cells (3 × 107 instead of 3 × 106) reduced the rate of GVHD-related mortality of recipients to 20% and resulted in bilateral transplantation tolerance in 100% of nonirradiated survivors.