Switching on immune regulation to prevent graft rejection.

Achieving immune tolerance to allografts is an important goal of clinical transplantation. Although remarkable achievements in graft survival have been made with the use of immunosuppressive drugs, the “operational” tolerance that these molecules provide is, unfortunately, associated with a cohort of complications that develop in permanently immunosuppressed patients. More suitable approaches to pharmaceutical immunosuppression should, ideally, provide longterm, graft-specific tolerance, while preserving an intact immunity to pathogens and tumors. Alternative strategies have been devised from the study of animal models of T-cell tolerance to self-antigens. Notably, it has been shown that healthy immune systems use at least two complementary mechanisms to harness the autoreactivity of T lymphocytes: deletion of high-affinity T-cell clones and down-regulation of the less-reactive subset. The adaptation of the deletional mechanism to experimental transplantation has led to the induction of donorspecific tolerance through the creation of donor/recipient bone marrow chimera (1). In contrast, regulatory pathways have been invoked through the recruitment of specialized regulatory T-cell subsets (2). Although effective at inducing donor-specific tolerance in mice, the mixed chimerism approach remains difficult to apply to clinical settings because of complications associated with the conditioning regimen used and the development of graft-versus-host disease. Thus, the recent characterization of regulatory cell subsets, as powerful repressors of alloreactive T cells, has renewed the interest in the use of T-cell regulation as a means to harnessing graft rejection (3). The principal subpopulation of regulatory T lymphocytes (Tregs) includes CD4 CD25 cells that have been subdivided into two functional subsets. The T-regulatory type 1 (Tr1)/T-helper (Th) 3 subset, also known as adaptive/induced Tregs, displays T-cell receptor (TCR) specificities for nominal antigens (i.e., the same TCR specificities as carried by the T cells that they suppress). It differentiates peripherally and represses effector T-cell functions by local release of inhibitory cytokines such as transforming growth factorand interleukin-10. Conversely, the TCR specificity of natural Tregs (the second subset) is still unknown, although they differentiate in the thymus and act in the periphery through an initial cell contact with activated effector cells. In addition to being CD4 CD25 , both the natural and adaptive Tregs shared a number of surface and intracellular markers, including cytotoxic T lymphocyte antigen (CTLA)-4 and glucocorticoid-induced tumor necrosis factor receptor (GITR), which are not specific to these regulatory subsets. The understanding of the mechanism of Treg-mediated suppression and the therapeutic manipulation of these cells to prevent rejection of allogeneic grafts requires identification of Treg-specific markers and further knowledge of their TCR specificity. Recently, two Treg-specific markers have been described: the LAG-3 surface molecule, a CD4 homolog that also binds major histocompatibility complex class II (4), and the FoxP3 nuclear protein, a transcriptional repressor of the forkhead/winged-helix family. Considerable attention has been given to FoxP3 when it was discovered that its forced expression in naı̈ve T cells converted them into functional Tregs (5). This seminal observation also suggested that it was possible to customize T effector cells to become functional Tregs with predefined TCR specificity. Tregs, created by FoxP3 gene transfer, have been successfully used to prevent and alleviate wasting disease (6) and colitis (5), but have not been tested for their ability to repress responses to allografts. In this regard, the report of J.-G. Chai et al. in this issue of Transplantation constitutes an important step in the understanding of the mechanism of regulatory tolerance to transplants. The model uses female mice that express a transgenic TCR directed at the male H-Y antigen and are recipients of male (H-Y ) skin grafts. Without treatment, host animals rejected their transplants in 20 to 25 days, essentially through H-Y responding T cells. As observed in other models of FoxP3 gene transfer, the authors showed that gene transfer into CD4 CD25 , TCR transgenic cells converted these cells into anti–H-Y Tregs that exhibited the phenotypic (CD25 , GITR ) and functional (anergy and suppression in vitro) characteristics of “true” Tregs. The most impressive result was that H-Y skin grafts were accepted on female anti–H-Y recipients after the injection of the FoxP3-customized Tregs. At first sight, this study seems to be a duplicate of previously published work on colitis and wasting disease; in fact, it is an original contribution to the field of transplantation. Indeed, it is the first clear demonstration that FoxP3-induced Tregs can control rejection processes in an antigen-specific manner. It Address correspondence to: Christian LeGuern, Transplantation Biology Research Center, Building 149 –9019, 13th Street, Boston, MA, 02129. E-mail: leguern@helix.mgh. harvard.edu. Received 3 January 2005. Accepted 5 January 2005. Copyright © 2005 by Lippincott Williams & Wilkins ISSN 0041-1337/05/7910-1308 DOI: 10.1097/01.TP. 0000159146.85824. 5B