Syngeneic bone marrow expressing a single donor class I MHC molecule permits acceptance of a fully allogeneic cardiac allograft

Bone marrow cells may be a useful vehicle for pretransplant delivery of alloantigen to induce tolerance in vivo. However, infusion of fully allogeneic bone marrow cells carries the risk. of graft-versus-host disease. In order to reduce this risk while retaining the tolerogenic potential of the bone marrow infusion, we have investigated the ability of recipient bone marrow cells expressing a single donor MHC class I antigen to induce specific unresponsiveness in vivo. We show that 5 x 10(7) and 5 x 10(6) bone marrow cells from a transgenic strain of CBA mice, CBK, that express a single donor class I MHC gene, H2K(b) (H2(k) + H2K(b)), were able to induce long term survival of a fully allogeneic C57BL/10 (H2(b)) cardiac allograft in 80% and 20% of unmanipulated CBA (H2(k)) recipients, respectively, when administered intravenousely on the day of transplantation. In contrast, the same doses of fully allogeneic C57BL/10 donor bone marrow were completely ineffective at prolonging graft survival. When the interval between bone marrow infusion and transplantation was increased to 14 days, CBK bone marrow at either dose (5 x 10(6) and 5 x 10(7)) induced long term survival of C57BL/10 cardiac allografts in all recipients (MST>100 days) while fully allogeneic donor bone marrow was ineffective (MST=7, 5 x 10(6) cells; MST=6, 5 x 10(7)). Only when 27 or 42 days had elapsed between bone marrow infusion and transplantation did fully allogeneic bone marrow exert a beneficial effect on graft survival. Administration of 5 x 10(6) C57BL/10 bone marrow cells 27 and 42 days before transplantation resulted in long term survival of C57BL/10 hearts in 67% and 75% of CBA recipients. Next, we investigated whether manipulating the periphery of the recipient with a depleting anti-Cd4(4) monoclonal antibody before bone marrow infusion would facilitate the induction of unresponsiveness. When pretreatment with bone marrow cells was combined with anti-Cd4 monoclonal antibody 28 days before transplantation, a 10-fold reduction in the number of either C57BL/10 or CBK bone marrow cells required to induce tolerance was observed. These data confirm that bone marrow is a suitable vehicle for alloantigen delivery at the time of, or before, transplantation, on its own or in combination with anti-Cd4. The use of recipient type bone marrow cells expressing one or more donor MHC genes may be more effective than fully allogeneic, donor bone marrow cells in inducing tolerance in vivo. This difference may have important clinical implications for the current trials of donor bone marrow given at the time of transplantation in order to augment chimerism and to prolong graft survival.