MHC class II compatibility between donor bone marrow and secondary organ grafts is required for tolerance based on mixed chimerism

The importance of MHC class II in allogeneic bone marrow transplantation and chimerism-based solid organ tolerance in a rat model

Mixed hematopoietic chimerism enables donor-specific tolerance for solid organ grafts. This study evaluated the influence of different serological major histocompatibility complex disparities on chimerism development, graft-versus-host disease incidence and subsequently on solid organ tolerance in a rat model. For bone marrow transplantation conditioning total body irradiation was titrated using 10, 8 or 6 Gray. Bone marrow transplantation was performed across following major histocompatibility complex mismatched barriers: complete disparity, MHC class II, MHC class I or non-MHC mismatch. Recipients were clinically monitored for graft-versus-host disease and analyzed for chimerism using flow cytometry. After a reconstitution of 100 days, composition of peripheral leukocytes was determined. Mixed chimeras were challenged with heart grafts from allogeneic donor strains to define the impact of donor MHC class disparities on solid organ tolerance on the basis of stable chimerism. After myeloablation with 10 Gray of total body irradiation, chimerism after bone marrow transplantation was induced independent of MHC disparity. MHC class II disparity increased the incidence of graft-versus-host disease and reduced induction of stable chimerism upon myelosuppressive total body irradiation with 8 and 6 Gray, respectively. Stable mixed chimeras showed tolerance towards heart grafts from donors with MHC matched to either bone marrow donors or recipients. Isolated matching of MHC class II with bone marrow donors likewise led to stable tolerance as opposed to matching of MHC class I. In summary, MHC class II disparity was critically associated with the onset of graft-versus host disease and was identified as obstacle for successful development of chimerism after bone marrow transplantation and subsequent donor-specific solid organ tolerance.

Application of allogeneic bone marrow cells in view of residual alloreactivity: sirolimus but not cyclosporine evolves tolerogenic properties

BACKGROUND

Application of bone marrow cells (BMC) is a promising strategy for tolerance induction, but usually requires strong depletion of the host immune system. This study evaluates the ability of immunosuppressants to evolve tolerogenic properties of BMC in view of residual alloreactivity.

METHODS

The rat model used a major histocompatibility complex (MHC) class II disparate bone marrow transplantation (BMT) setting (LEW.1AR1 (RT1auu) → LEW.1AR2 (RT1aau)). Heart grafts (LEW.1WR1 (RT1uua)) were disparate for the complete MHC to recipients and for MHC class I to BMC donors. Limited conditioning was performed by total body irradiation of 6 Gy. Cyclosporine (CsA) or Sirolimus (Srl) were administered for 14 or 28 days. Transplantation of heart grafts (HTx) was performed at day 16 or at day 100 after BMT. Chimerism and changes in the T cell pool were detected by flow cytometry.

RESULTS

Mixed chimeras accepted HTx indefinitely, although the composition of the regenerated T cell pool was not changed to a basically donor MHC class II haplotype. Non-chimeric animals rejected HTx spontaneously. BMC recipients, who received HTx during T cell recovery at day 16, accepted HTx only after pre-treatment with Srl, although chimerism was lost. CsA pre-treatment led to accelerated HTx rejection as did isolated application of BMC.

CONCLUSION

Srl evolves tolerogenic properties of allogeneic BMC to achieve indefinite acceptance of partly MHC disparate HTx despite residual alloreactivity and in particular loss of chimerism.

Permanent acceptance of both cardiac and skin allografts using a mild conditioning regimen for the induction of stable mixed chimerism in mice

Patients who are receiving an organ transplant nowadays are sentenced to the life-long administration of immunosuppressive drugs, which have serious side effects. The reliable induction of donor-specific tolerance therefore remains a major goal in organ transplantation. Previously, we have developed a sublethal, non-myeloablative murine model in which permanent mixed, multilineage chimerism and donor-specific tolerance are established. Our model involves engraftment of fully allogeneic T cell depleted donor bone marrow cells in low dose irradiated and anti-CD3 treated major histocompatibility complex (MHC)-disparate recipient mice. To investigate whether vascularized organ grafts are accepted in our model, we performed heterotopic heart transplantations in our mixed chimeric mice. Chimeric mice permanently accepted hearts from the bone marrow donor (>130 days) and rapidly rejected third party-type allografts (median survival time 9 days). Untreated control recipient mice rejected both donor- and third party-type allografts. In addition, mice that accepted their cardiac grafts, donor-specific acceptance of skin grafts was observed. In conclusion, the establishment of stable mixed chimerism with this low-toxicity regimen resulted in permanent donor-specific acceptance of vascularized organ as well as skin grafts across a full MHC barrier.