Bone marrow-derived chimerism in non-irradiated, cyclosporin-treated rats receiving microvascularized limb transplants: evidence for donor-derived dendritic cells in recipient lymphoid tissues.

Tolerance to donor transplantation antigens develops when recipients are made chimeric with donor bone marrow. To establish chimerism, the haemopoietic system of recipients typically is severely compromised. We report on a system in which chimerism develops without ablative therapies. Immunosuppression with cyclosporin A allowed the lower limb of a rat to be replaced by a microvascularized transplant from a fully allogeneic donor. Many donor-derived cells populated recipient lymph nodes and spleen, and most had the large size, irregular shape and strong major histocompatibility complex (MHC) class II expression that typify dendritic cells. Donor cells were not found in the macrophage-rich regions of lymphoid tissues, but instead occupied splenic white pulp and lymph node cortex. The donor cells were derived from radiosensitive marrow precursors, as chimerism was abolished if the grafted limb was irradiated, or if muscle and skin flaps devoid of bone were grafted. Donor cells were rare or not detectable in blood, thymus and liver. Whereas lymphoid chimerism was prominent following limb transfer, donor cells were not detected 1-2 weeks after an injection of two femur equivalents of a marrow suspension. We suggest that dendritic cells that undergo rapid turnover in lymphoid organs are replaced from allogeneic precursors in bone grafts. The combination of cyclosporin and vascularized bone provides a means for inducing chimerism in lymphoid tissues of non-irradiated recipients.