Factors Affecting the Induction of Transplantation Tolerance in Bone Marrow Transplantation
Creators: name: Mari, Elisabeth Rose
Publisher: name: Imperial College London
Year: 2012

description: The engraftment of allogeneic haematopoietic stem cells (HSC) relies on the use partially or fully myeloablative regimens to condition the transplant recipient in order to “make space” in the bone marrow microenvironment and establish immunological tolerance to donor alloantigens. In murine models of bone marrow transplantation where donor-recipient pair differ for a single minor histocompatibility (H) antigen, HY, engraftment can be obtained using low-dose irradiation. Such conditioning favours the homeostatic expansion of regulatory (Treg) cells that play a crucial role in generating host versus graft tolerance (HvG). However, the HY model has some limitations because, in the clinical setting, HLA-matched donor and recipient still differ for several minor H antigens. Although there is evidence that immune responses across minor H differences concentrate on immunodominant epitopes, it was fundamental to understand whether increasing the number of donor antigenic disparities necessitated a proportional increment in the dose of conditioning to achieve engraftment. I utilised a bone marrow transplantation model in which donor and recipient differed for multiple minor H antigens and whereby immune responses were prominently skewed against a single immunodominant epitope. Recipient mice were C57BL/6 and bone marrow was obtained from BALB.B donors, whereby the immunodominant epitope was H60. My results showed that low-dose irradiation was not sufficient to obtained BALB.B donor cell engraftment but a fully myeloablative dose of (850cGy) was required. When the amount of antigenic determinants was decreased, by transplanting (BALB.BxC57BL/6) F1 cells, donor cell engraftment was achieved at an irradiation dose of 500cGy. These data show that the dose of conditioning regimen required for engraftment is proportional to the magnitude of the antigenic differences across donor and recipient. Different doses of myeloablation certainly bear different impacts on the depletion of lymphocyte subsets and lympho-haemopoietic reconstitution. Therefore, I investigated the kinetics and extent of Treg expansion. In all groups of transplanted mice the engrafted mice had significantly increased proportions of Treg cells which peaked during the second week post-transplant. When host Treg cells were depleted prior to transplantation with male C57BL/6 or (BALB.BxC57BL/6) F1 donor bone marrow under irradiated at 500cGy or 600cGy, the level of donor cell engraftment was not affected. However, there was a delay in the engraftment of (BALB.BxC57BL/6) F1 bone marrow, thus suggesting a marginal role for Treg cells using higher doses of conditioning. These data imply that the magnitude of antigenic disparities between the donor and recipient deeply impacts on the dose of irradiation required to obtain durable engraftment. The dose of irradiation does not correlate with the level of Treg cell expansion and Treg depletion only marginally affects engraftment. These data indicate that, in the presence of multiple antigenic disparities, depletion of T cell effecting HvG responses rather than induction of immune regulation is necessary.