The Function And The Mechanism Of CD47 In Transplantation Immunology

Organ transplantation is the most effective method to end-stage organ failure. At present, although the application of new immunosuppressive can be effective in the prevention and treatment of acute rejection, graft rejection in organ transplant is still an important issue need to be faced. Therefore, how to induce and maintain immune tolerance and how to maximize the protection of graft function, have become the key part of transplantation immunology research topics.CD47 (integrin-associated protein, IAP) is a cell surface glycoprotein widely expressed in various types of cells. CD47 participates in a variety of physiological activities of organisms. CD47 through its ligand SIRPα(singal regulatory proteinα), transducs inhibition signal, changes the function and activity status of cells, inhibits of phagocytosis and activation of macrophage and DC; CD47 can also be activated by another ligand TSP (thrombospondin), changes the endothelial permeability, cell-mediated and cell-matrix interactions, and regulated cell adhesion, motility, activation, and platelet aggregation. The species-specific differences between CD47 and SIRPα, resulting in xenogeneic cells were quickly cleared by host macrophages. However, the mechanisms of CD47 in antigen-specific tolerance induction, and the significance of CD47 expressed on non-hematopoietic cells, have not to see a clear report.Aims: (1) To evaluate of the mechanisms of CD47 expressed on hematopoietic cells in antigen-specific tolerance induction, and its effects on host immune system; (2) To evaluate the significance of CD47 expressed on non-hematopoietic cells in transplantation immunology.Methods: (1) Using MHC-I mismatched skin graft model, DST induced skin graft tolerance as a means to study the mechanism of CD47 expressed on donor hematopoietic cells in tolerance induction under the MHC-I mismatch condition. (2) Using MHC fully- mistached heart transplantation as a model, to evaluate the mechanism of CD47 expressed on donor cells in tolerance induction and effects on host immune system by using DST plus CD154 co-stimulator blockade therapy. (3) Using fetal thymus transplantation model, to evaluste the significance of CD47 expressed on non-hematopoietic cells in syngeneic transplantation immunology.Results: (1) In the MHC-I mismatched skin graft model, CD47 KO DST could not extend WTB6 skin graft survival time (MST= 16days), and the survival time of WTB6 skin grafts which received CD47 WT DST treatment have been significantly prolonged(MST =42.5days, p<0.001); moreover, all groups rejected third-party(B10.A) skin grafts at similar time points. Antigen-specific T cell response were significantly increased in CD47 KO DST group, and mainly CD8+T cell response; antigen- specific T cell response can not be found in CD47 WT DST group; and all groups have the similar anti third party (B10.A) T cell response. Single CD47 WT DST could induced long-term survival of skin grafts (MST = 38 days), single or multiple CD47 KO DST even accelerated skin grafts rejection; mixed DST extended skin grafts survival time compared with Non-DST group or CD47 KO DST group (p<0.05), but still significantly shorter than CD47 WT DST group (p<0.05). CD47 heterozygote cells have the half level of CD47 expression on the cells surface compared with CD47 WT cells, but its enough to inhibit the phagocytosis and donor-specific T cell responses by host; there were no significent difference of the survival time compared CD47 WT cells and CD47 heterozygote cells, moreover, the skin grafts from CD47 heterozygote DST group have been prolonged significently, but still significantly shorter than CD47 WT DST group, (MST = 47days, p<0.05). CD47 KO DST induced host DC up-regulated activation marker CD86 and I-Ab expression on cells surface, and the percentages of CD11chiCD86+DC and CD11chiI-AbhiDC in total CD11c+DC increased significantly. (2) In MHC fully-mismatched heart transplan- tation model, CD47-/-DST cells induced rapid clearance by BALB/C receipients, and anti-donor antigen-specific T cells response more stronger compared with CD47 WT DST group (p<0.01); and CD47 heterozygote cells have the half level of CD47 expression on the cells surface compared with the normal cells, but but its enough to inhibit the phagocytosis and donor-specific T cell responses by host; CD47 WT DST plus MR1 induced cardiac allografts long-term of survival (MST=150 days); CD47 KO DST/MR1 treatment can also extend cardiac allografts survival (MST= 42days), but still significantly shorter compared with CD47 WT DST group(p<0.01); CD47 heterozygote DST plus MR1 prolonged the cardiac allografts survival compared with CD47 KO DST group(MST=90days, p<0.05), but its still significantly shorter than CD47 WT DST group(p<0.05). Again, CD47 KO DST increased CD86 and the I-Ad expression on host DC, and the percentages of CD86 +CD11c+DC and I-Adhi CD11c+ DC in total CD11c+ DC significantly increased. A large number of lymphocytes infiltretion can be found in rejected heart grafts in each group at different time points, and Treg infiltrated at the same area with lymphocyte infiltration. (3) In fetal thymus transplantation model, T cells dispeared in host peripheral blood of KO→WT group 3 weeks after thymus transplantation; only CD47+T cells could be found in host peripheral blood of this group 6 weeks after transplantation; Similarly with the result of peripheral blood, only CD47+T cells could be detected in recipients splenocytes. Almost all the macrophages (CD11b+SIRPa+) from thymus grafts came from host 7 weeks after thymus transplantation. Both CD47 KO thymus grafts and CD47 WT thymus grafts have the similar CD4+CD8-, CD8 + CD4-, CD4 +CD8 + thymocyte subsets and percentages; and almost all the thymocytes came from host 20 weeks after transplantation. CD47 KO thymus grafts have the normal tissue structure and integrate endothelial cells compared the CD47 WT thymus grafts. Conclusions: (1) In MHC-? mismatched mouse skin transplantation model, lack of CD47 expression on donor hematopoietic cells lead to this kind of cells quickly cleared, while normal hematopoietic cells survival time were prolonged. CD47 WT DST inhibited donor-specific T cell response and induced donor-specific skin grafts tolerance, while CD47 KO DST activated host CD4+ DC and could not extend grafts survival. Although CD47 heterozygote cells have half amounts of CD47 on the surface, but its sufficient to inhibit the host phagocytosis, and induce considerable antigen-specific immune tolerance. Directly explain the amount of CD47 expressed on donor DST cells positively correlated to the intensity of tolerance. (2) Similar results could be found in MHC fully-mistached heart transplant model in mice, lack of CD47 epression on donor DST cells induced faster phagocytosis by host. Activated DC may contributed to anti-donor specific T cells proliferation and failed to induce cardiac allografts antigen-specific tolerance. CD47 heterozygote DST cells have the novel role on heart grafts survival time prolongation. Again, these resluts suggested that the strength of tolerance positive related to CD47 expression on DST cells. (3)Unlike CD47 on hematopoietic cells, thymus grafts which lack of CD47 expression supported the precursor cells normal differentiation and mature; macrophages infiltrated in the thymus grafts are the source of recipients. Lack of CD47 expression on thymus grafts can not induce macrophages mediated rejection by host. Thymus grafts which lack of CD47 expression have long-term survival in CD47 WT recipients, and maintain the normal structure and function of the region. Endothelial cells of CD47 KO thymus grafts have normal structure and function as normal.Significance: This subject can support a new theoretical foundation by fully understand the mechanisms of CD47 in transplantation immunology; provide a new idea on clinical tolerance induction and maintenance; and have a good clinical application for allo-or xeno-grafts long-term survival.