Background: A mixed-chimerism protocol that promotes long-term renal allograft acceptance has proven unsuccessful for cardiac allografts. Here we test whether a protocol additionally targeting CD28 and CD2 is sufficient for cardiac allograft acceptance or promotes expansion of regulatory T-cells.
Methods: Donor bone marrow transplantation (BMT) was administered to recipients of MHC-mismatched heterotopic heart allografts after a 4-month delay period under TNX-1500 (anti-CD154).  BMT induction was comprised of one (Group 1) or two (Group 2) doses of total body irradiation, thymic irradiation, and horse anti-thymocyte globulin (ATG) followed by two (Group 1) or five (Group 2) weekly doses of anti-CD2 and five weekly treatments with anti-CD28 and TNX-1500.
Results: One graft in Group 1 was rejected during the delay period; one in Group 1 and one in Group 2 with normal graft function exhibited moderate rejection on protocol biopsy prior to BMT, while five others exhibited normal histology. Lymphocyte chimerism >10% was observed in three of the five Group 2 animals but not in either Group 1 recipient. One Group 1 animal rejected 44 days after BMT, while another succumbed to disseminated CMV infection. In Group 2, two monkeys succumbed to CMV disease or bacterial infection during the post-BMT treatment period. Two of three Group 2 animals developed >35% lymphocyte chimerism but succumbed to post-transplantation lymphoproliferative disease (PTLD) at 37, 41, and 51 days after BMT, with normal graft function and histology in all three at euthanasia.
Conclusion: The combination of anti-CD28 with multi-dose anti-CD2 often promotes lymphocyte chimerism in this delayed BMT model at levels that predict prolonged graft survival or long-term acceptance in kidney recipients. However, the high incidence of PTLD and opportunistic infection prevented the assessment of the regimen’s effectiveness in promoting alloimmune tolerance. Strategies to improve CMV control and PTLD prophylaxis merit investigation in this model.