Immunosuppression and regulation: cast in new light?

R esearch and clinical trials aimed at prolonging allograft survival have historically been viewed as promoting either immunosuppression or tolerance. However, new studies reveal that some immunosuppressive agents may also promote tolerance through enhanced generation of regulatory T cells (Treg). These findings lead to new ways of thinking about these agents and how they might be combined to promote tolerance while suppressing the immune response. This requires a new view of tolerance and immunosuppression as a continuum rather than a dichotomy. The outcome of kidney transplantation continues to improve through judicious use of potent drugs that prevent rejection by “nonspecifically” suppressing the immune system while trying to avoid deleterious consequences of overimmunosuppression. In contrast, research, including pilot clinical trials, continues to focus on achieving immunologic tolerance, where the immune system is rendered specifically unresponsive to the allograft in the absence of ongoing therapy. Tolerance promises to reduce rejection and improve long-term outcome, while avoiding the dangers of chronic immunosuppression. Conventional wisdom holds that tolerance has only been achieved in rodents and that standard immunosuppressive agents may actually hinder its development. However, a truth lies between these extremes. First, tolerance may be viewed as a balance rather than “all or none” and intermediate levels are still clinically useful. Whatever the mechanisms, most allograft recipients require much less immunosuppression after six months than they did initially. Patients whose immune systems lean toward tolerance may suffer fewer rejections and require less maintenance immunosuppression. Second, some patients, albeit a minority, on standard therapy do actually develop tolerance. If we could only identify these patients, immunosuppression could be stopped or tapered with anticipation of a favorable outcome rather than by trial and error. Indeed, development of assays to identify tolerant recipients are the subject of a review on immunological monitoring by Najafian et al. in this issue of JASN (1). Also in this issue, Lopez et al., and other reports noted below, suggest that certain standard immunosuppressive agents may directly promote tolerance through generation of regulatory T cells (Tregs) (2). This challenges previous thinking about immunosuppression and tolerance, and provides new insight into Treg biology. Tolerance capitalizes on the immune system’s endogenous regulatory mechanisms. Key among these are CD4 Treg cells. Five to ten percent of CD4 cells emerging from the thymus constitutively express CD25 (IL-2R -chain) and other activation markers (3). Depletion of these cells, dubbed natural Treg cells (NTregs), precipitates systemic autoimmunity in mice. Unlike CD25 and other markers, the Foxp3 transcription factor is not expressed by activated murine effector T cells, and is specific for NTregs. Indeed, humans and mice deficient in Foxp3, lack NTregs and develop severe systemic autoimmunity. NTregs normally respond to antigenic challenge in vivo and dampen the effector response. While other regulatory populations (both expressing and lacking Foxp3) may be induced in the periphery under certain conditions, the details of this induction and the role of induced versus NTregs remains unclear (Figure 1). In experimental models, Tregs can prevent autoimmunity, prevent graft-versus-host disease (GVHD), and play an important role in both the induction and maintenance of allograft tolerance (3–5). Given their important role in immune homeostasis, there is great interest in trying to manipulate Tregs for therapeutic advantage. While NTregs are normally present even in transplant recipients on conventional immunosuppressives, there is hope that boosting levels of such cells may promote tolerance. In humans, CD25 Tregs have been shown to be increased in transplant patients with stable graft function (6,7). Moreover, a recent study notes that although Foxp3 mRNA increases in urine of patients undergoing acute renal allograft rejection, patients with the highest increases were most responsive to treatment (8). Based on such findings, several groups are attempting ex vivo expansion of pre-existent NTregs followed by reinfusion, for clinical trials in prevention/treatment of GVHD and type 1 diabetes (5). However, cell culture approaches present a significant challenge and it is important to identify agents that can boost the number of endogenous Tregs and/or induce conversion of nonregulatory T cells into Tregs in vivo. Conversion is particularly attractive because it targets 90 to 95% (rather than 5 to 10%) of the CD4 population (Figure 1). Enter the study of Lopez et al., which shows that rabbit antithymocyte globulin (ATG), widely believed to immunosuppress through T cell depletion, can induce a five-fold increase in CD4 cells expressing CD25 and Foxp3 after just 24 h in vitro. Although the number of CD25 CD4 cells decreased after ATG, this was accompanied by a similar increase in the number Published online ahead of print. Publication date available at www.jasn.org.