Residual APCs Are Critical for the More Potent Immune Response Following the Engraftment of Older Organs

Abstract

Background: Organs from old donors are increasingly utilized. Here, we tested mechanisms of donor age-dependent immune responses in vivo and in vitro. Methods and Results: Fully MHC-mismatched hearts from young (3 mo) or old (18 mo) C57BL/6 (B6) donor mice were grafted into young (3 mo) DBA/2J recipients. Old hearts were rejected more rapidly (MST 9 vs. 11 d; n=7; p=0.002) and demonstrated increased MNC infiltrates (n=12, p< 0.05) when compared to young hearts. The more rapid rejection of old grafts was accompanied by a more potent immune response in recipient animals which included increased frequencies of alloreactive IFN-γ (Fig 1A) and IL-6 producing splenocytes (ELISPOT), increased percentages of CD8+ effector/memory and CD8+ IFN-γ +- T cells (Flow Cytometry; p< 0.05 for all experiments). To distinguish whether the more rapid rejection is initiated by the age of passenger leukocytes or the myocardial tissue itself, we generated chimeric animals by transplanting bone marrow from young B6 mice (107 cells) into lethally irradiated (1100 rad) old or young B6 mice. After confirming the repopulation with CD11c+ passenger leukocytes (IHC by wk 6), chimeric hearts were transplanted into young DBA mice. Graft survival of old and young chimeric grafts was now comparable (MST 10 vs. 10 d, p>0.05) and structural changes were indistinguishable suggesting a critical role of passenger leukocytes as instigators of the more rapid immune response when transplanting older organs. Moreover, the systemic immune response (assessed as above) did not differ between groups (Fig. 1B, p>0.05). Next, we explored the role of residual APCs. Donor animals where pretreated with liposomal clodronate which reliably depletes APCs, mainly CD11c+ dendritic cells and F4/80+ macrophages. After donor pre-treatment with liposomal clodronate and transplantation of either young or old B6 cardiac allografts into young DBA recipients we observed age-independent survival rates and systemic alloimmune responses (assessed as above) were not different between groups (n=6, p>0.05 for all groups). We then went on and tested the immunogeneic capacity of young and old flow-sorted splenic CD11c+ DCs. Consistent with a higher expression of MHC-II molecules, CD11c+ DCs from old B6 mice were more potent in priming allogenic responder cells than their young counterparts in vitro (Fig. 1C).[Fig]Conclusion: Antigen presenting cells are critical for the observed enhanced alloimmune responses subsequent to the transplantation of older grafts. Our mechanistic studies may allow developing strategies to interfere with the more potent immune response to organs from older donors.