Transplanted Donor APC Co-Stimulatory Molecule Expression Is Required To Elicit Host T Cell Resistance in MHC-Matched Allogeneic BMT Recipients Following Reduced Intensity Conditioning.

Controlling host resistance to hematopoietic progenitor cell grafts is crucial for successful engraftment and the induction and maintenance of immune tolerance. We are studying the induction and regulation of the pathways that lead to T cell resistance in a well characterized minor histocompatibility antigen (MiHA) matched allogeneic transplant model under reduced intensity conditioning (RIC). B6 recipients transplanted with MHC matched allogeneic TCD-BM following RIC resist donor grafts as evidenced by expansion of anti-MiHA specific CD8 T cells and transient peripheral blood donor chimerism. To initially examine the involvement of CD80/CD86 expression in this resistance, mabs to CD80 and CD86 were administered at the time of transplant of BALB.B (H2^b, 4x10⁶ TCD-BMC) BMC into 5.5 Gy TBI conditioned B6-wt (H2^b) mice. Donor (Ly9.1⁺) cells were not rejected in such recipients. We next investigated the requirement of CD80/86 expression on recipient cells with respect to development of resistance. C3H.SW (H2^b) TCD-BMC were transplanted into B6-wt or B6-CD80^−/−86^−/− (H2^b) mice as above. Donor chimerism in both B6-wt and B6-CD80^−/−86^−/− recipients was transient indicating resistance had been elicited against the donor BM. BM deficient in both CD80 and CD86 was then transplanted into MHC matched recipients following RIC. In contrast to B6-wt BMC grafts, transplantation of B6-CD80^−/−86^−/− BM failed to result in rejection leading to a sustained and high level of donor cell chimerism. These results suggested direct recognition of MiHA on donor APC was sufficient to elicit resistance. Since wt BMC transplants into CD4^−/− recipients demonstrated CD4⁺ T cell function is critical for resistance in this model, we hypothesized that activated host CD4 T cell recognition of donor APC resulted in the upregulation of CD80/CD86 co-stimulatory molecules. B6-wt and B6-CD40L deficient mice were transplanted with BALB.B (H2^b) TCD-BM. As predicted, B6-wt recipients again rejected the donor BM within 2–3 wks of transplant. The resistance was evidenced by the generation of a host CD8⁺ T cell response against the immunodominant donor MiHA H60 (elevated % of tetramer staining host H60⁺CD8⁺ T cells). In contrast, B6-CD40L deficient recipients demonstrated stable chimerism for >50 days post-transplant and did not generate H60⁺CD8⁺ T cells. To specifically examine the requirement of CD40L on CD4 T cells in the host, B6-CD4^−/− mice (5.5Gy) were transplanted with BALB.B TCD-BM together with CD4⁺ T cells from B6-wt or B6-CD40L^−/− mice. Recipients co-transplanted with B6-wt CD4⁺ T cells rejected their grafts 2–3 wks post-transplant whereas recipients of CD4⁺ T cells from B6-CD40L^−/− donors expressed donor cell chimerism and failed to resist the MHC-matched marrow allograft. In total, we interpret the results to demonstrate a requirement for host CD4 T cells to recognize donor MiHA and undergo alloantigen induced activation resulting in CD40L induction of CD80/86 on donor APC. These interactions then result in host CD8 T cell effector activity which inhibits donor engraftment. Since recipients containing anti-donor specific CD8 memory cells were found to resist MHC-matched HCT containing CD80/86 deficient APC, the present findings support the notion that direct recognition of donor APC is crucial to elicit T cell mediated resistance to hematopoietic engraftment in ‘naive’ RIC MHC-matched allogeneic recipients.