Alloresistance to K locus class I-mismatched bone marrow engraftment is mediated entirely by CD4+ and CD8+ T cells.

Clinical application of approaches to inducing transplantation tolerance that involve bone marrow reconstitution will require achievement of engraftment without major toxicity to the recipient. These requirements are likely to vary according to the type of histoincompatibility between donor and recipient. We have attempted to determine the minimal conditioning required to achieve lasting mixed allogeneic chimerism and tolerance in the presence of a class I MHC disparity by evaluating the host elements that resist alloengraftment. We based our approach on a regimen that was shown to induce mixed chimerism in fully MHC-mismatched strain combinations. Recipient B10.AKM (KkIkDq) mice were treated with 7 Gy thymic irradiation (TI) and 3 Gy whole body irradiation (WBI) and received either anti-CD8 mAb alone or anti-CD4 plus anti-CD8 mAbs before transplantation of K locus-disparate B10.MBR (KbIkDq) marrow. All (27 of 27) animals receiving both mAbs showed lasting multi-lineage mixed chimerism and donor-specific tolerance. In contrast, five of 22 (23%) recipients pre-treated with anti-CD8 mAb alone in the same experiments failed to develop lasting multilineage mixed chimerism, suggesting that the CD4 T cell subset also participates in resistance to class I-mismatched marrow engraftment. We next attempted to determine whether or not host non-T cell elements resist allogeneic engraftment by comparing the minimum number of syngeneic vs allogeneic BMC required to achieve lasting multilineage mixed chimerism. Titrated numbers (10(6) to 10(7)) of B10.MBR (KbIkDq) bone marrow cells were administered to B10.AKM recipients treated with anti-CD4 and -CD8 mAbs, 3 Gy WBI and 7 Gy TI. All recipients of each marrow dose developed lasting multilineage mixed chimerism and showed specific tolerance to B10.MBR skin grafts. The level of donor-type repopulation in recipients of each dose was not lower than that observed in similarly irradiated recipients in an Ly5 congenic, otherwise syngeneic, BMT system. Together, our results suggest that CD4+ T cells contribute to resistance to K locus class I-mismatched marrow allografts and that resistance is mediated only by CD4 and CD8 T cells, with no role for non-T cell host elements.