Response To: Cyclosporine Abrogates De Novo Generation of Tregs Independently of IL-2 (2007)

To the Editor: In their letter, Zuber et al. noted that the adaptation of the tolerance‐inducing protocol cyclosporine (CsA) plus a long‐lived IL‐2/Fc, which was successful in mice (1Zuber J, Brodin-Sartorius A, Thervet E, Legendre C. Cyclosporine abrogates de novo generation of Tregs independently of IL-2. Am J Transplant 2007 (In press).Google Scholar), should be taken with caution in humans. This is because: 1.CsA inhibits new synthesis of Foxp3 protein and the formation of NFAT:Foxp3 complex required for Treg cell programming; and2.Humans may have a much larger pool of extra‐thymically generated Foxp3+ cells crucial for transplant tolerance. These authors expressed doubts that CsA could permit de novo generation of Tregs even with the rescue by exogenous IL‐2/Fc. Their point is supported by our own data obtained with the Foxp3GFP knockin mice, where excessive amount of IL‐2/Fc failed to revert CsA blockade on de novo generation of Tregs in vivo (2Kang HG Zhang D Degauque N Mariat C Alexopoulos S Zheng XX Effects of cyclosporine on transplant tolerance: The role of IL‐2.Am J Transplant. 2007; 7: 1907-1916Abstract Full Text Full Text PDF PubMed Scopus (53) Google Scholar, 3Gao W Lu Y El Essawy B Oukka M Kuchroo VK Strom TB Contrasting effects of cyclosporine and rapamycin in de novo generation of alloantigen‐specific regulatory T cells.Am J Transplant. 2007; 7: 1722-1732Abstract Full Text Full Text PDF PubMed Scopus (220) Google Scholar). Several groups, including our own, have demonstrated an inability to obtain allograft tolerance in mice depleted of CD4+ and CD25+ natural Tregs prior to transplantation, indicating thymus‐derived Tregs play important roles in tolerance induction in these models. Other studies also suggested the existence of alloantigen‐specific Tregs induced in the periphery. The natural and induced Tregs might be interrelated by an ‘infectious’ tolerance mode first proposed by Dr. Herman Waldmann in 1993, where CD4+ T cells from tolerant mice disable naïve lymphocytes, which then become tolerant and, in turn, develop the capacity to antigen‐specifically disable other naïve CD4+ cohorts (4Qin S Cobbold SP Pope H et al.“Infectious” transplantation tolerance.Science. 1993; 259: 974-977Crossref PubMed Scopus (789) Google Scholar). Although we agree with Zuber et al. (1Zuber J, Brodin-Sartorius A, Thervet E, Legendre C. Cyclosporine abrogates de novo generation of Tregs independently of IL-2. Am J Transplant 2007 (In press).Google Scholar) on the concerns of using calcineurin inhibitors in tolerance induction, we do not agree with the insistence that the peripheral de novo generated Tregs are more important for the induction of tolerance in human than in mice. In fact, so far there are no real data regarding the relative importance of naturally‐occurring versus de novo generated Tregs in the induction and maintenance of tolerance in either rodents or humans. While we and colleagues within our Transplant Research Center do not prefer calcineurin inhibitor based protocols as the best means to gain peripheral type transplant tolerance, our study does suggest an interim means by which calcineurin inhibitors may be used in conjunction with IL‐2/Fc in an attempt to create tolerance. The key for the success of this protocol in rodents may lie in the fact that CsA was given in a 14‐day short‐term course, not as a maintenance therapy. During this 2‐week period, the CsA plus IL‐2/Fc regimen allows exaggerated activation‐induced cell death (AICD) of effector T cells and enhances allospecific immunoregulatory function of Tregs that are prerequisite for tolerance induction. After the short course of CsA and IL‐2/Fc treatment in the tolerance induction phase, secession of CsA treatment would permit the ‘infectious’ tolerance to take place and recruit new army of induced Tregs to achieve dominant tolerance. Given the difference between species in terms of stringency of Foxp3 expression and stability as well as Treg cell homeostasis, the role of natural and de novo generated Tregs in the induction and maintenance of transplant tolerance in humans warrants further studies before combined treatment with CsA and IL‐2/Fc can be clinically adapted.