A given number of effector T cells can only destroy a limited number of target cells in graft rejection

Oridonin suppresses transplant rejection by depleting T cells from the periphery

Oridonin is a natural compound purified from Rabdosia rubescens that has remarkable anti-inflammatory and antitumor activities. Although oridonin has been used in traditional Chinese medicine for many years to treat inflammatory diseases, the underlying mechanisms of these activities are not well understood. In this paper, we explored whether oridonin could be used in transplantation and the mechanisms of its immunosuppression. Oridonin efficacy in transplantation is manifested by prolonged graft survival and decreased graft infiltration. Oridonin induces T cell apoptosis in a concentration- and time-dependent manner. In vivo, oridonin depleted large numbers of T cells from the spleen and peripheral blood. Decreased T cell numbers resulted from apoptosis, which was accompanied by increased phagocyte numbers in the periphery. Reduction in the number of thymocytes was observed in mice treated for 8days, and CD4+CD8+ cells were more sensitive to apoptosis induced by oridonin. Additionally, successive treatment with oridonin for 16days resulted in a considerable reduction in the total number of spleen cells and spleen volume. Thus, T cell depletion may play an essential role in prolonged graft survival and immunosuppression induced by oridonin.

Chimerism but not neonatal antigen exposure induces transplant tolerance

Acquired transplant tolerance could be readily induced during foetal or neonatal period through donor cell infusion, but it is not the case in adults. This phenomenon has been attributed to the variation of immune system development in neonatal and adult periods. To investigate the role of immature immune system and chimerism in neonatal transplant tolerance, irradiated spleen cells or cell fraction from F1 (BALB/c × C57BL/6) or GFP-F1 mice were injected intravenously into neonatal C57BL/6 mice to induce tolerance. Irradiated cells or cell fraction could not induce chimerism and transplant tolerance in neonatal mice, even increasing the dose of donor cells to 5 × 10(7). Living donor cells induced tolerance in neonatal mice, and the quantity of living cells was correlated with the degree of chimerism and tolerance. At the amount of 3 × 10(7) F1 spleen cells, skin grafts were survived permanently in more than 80% of treated mice. However, the amount of 0.7 × 10(7) F1 spleen cells could only slightly prolong allografts' survival. The more donor cells were infused, the higher level of chimerism was achieved, and the higher frequency of alloreactive T cells was deleted. Chimerism is prerequisite for the induction and maintenance of tolerance. Chimerism in long-term tolerant mice was significantly higher than that in chronic graft rejected mice, with 6.48 ± 4.02% versus 1.41 ± 0.77%. It implies that transplant tolerance depends on the establishment of chimerism, but not on antigen exposure to immature immune system in foetus or neonates.