Purpose: We hypothesized that a vascularized bone marrow transplant (VBMT) can reconstitute the immune system of a lethally irradiated semiallogeneic recipient and induce antigen-specific tolerance without immunosuppression and GVHD.

Methods: Lewis (LEW) male rats served as donors, and Lewis-Brown-Norway (LBN) as recipients (n=4) of VBMTs. Nontransplanted, irradiated LBNs served as controls. Animals were irradiated with 900cGy, followed by heterotopic VBMT. After 75 days, recipients received LEW and ACI skin grafts. Flow cytometry and polymerase chain reaction (PCR) assessed chimerism, and suppressor cell studies evaluated host-specific immune tolerance.

Results: Radiation controls expired after 2 weeks. All VBMT animals demonstrated long-term survival (greater than 100 days) without GVHD. Histology showed repopulation of lymphoid and hemopoietic systems. PCR and flow cytometry demonstrated 98.5 % donor chimerism in all recipients. Suppressor cell assays showed BN-antigen specific immune unresponsiveness and vigorous responses to third-party determinants, supporting the skin graft results showing acceptance of LEW grafts while fully rejecting ACI grafts. Flow cytometry showed a CD4:CD8 ratio of 1.12 and 23% double-positive cells, consistent with a developing immune response and tolerance induction.

Conclusion: The VBMT was capable of hemopoietically reconstituting lethally irradiated recipients with absence of GVHD, and inducing antigen specific immune tolerance, without immunosuppressants.