Induction of donor-type chimerism and transplantation tolerance across major histocompatibility barriers in sublethally irradiated mice by Sca-1(+)Lin(-) bone marrow progenitor cells: synergism with non-alloreactive (host x donor)F(1) T cells.

Induction of transplantation tolerance by means of bone marrow (BM) transplantation could become a reality if it was possible to achieve engraftment of hematopoietic stem cells under nonlethal preparatory cytoreduction of the recipient. To that end, BM facilitating cells, veto cells, or other tolerance-inducing cells, have been extensively studied. In the present study, we show that BM cells within the Sca-1(+)Lin(-) cell fraction, previously shown to be enriched for early hematopoietic progenitors, are capable of reducing specifically antidonor CTL-p frequency in vitro and in vivo, and of inducing split chimerism in sublethally 7-Gy-irradiated recipient mice across major histocompatibility complex barriers. The immune tolerance induced by the Sca-1(+)Lin(-) cells was also associated with specific tolerance toward donor-type skin grafts. The minimal number of cells required to overcome the host immunity remaining after 7 Gy total body irradiation is very large and, therefore, it may be very difficult to harvest sufficient cells for patients. This challenge was further addressed in our study by demonstrating that non-alloreactive (host x donor)F(1) T cells, previously shown to enhance T-cell-depleted BM allografts in lethally irradiated mice, synergize with Sca-1(+)Lin(-) cells in their capacity to overcome the major transplantation barrier presented by the sublethal mouse model.