Understanding How Tregs Are Regulated: Therapeutic Implications (2009)

Natural and antigen-driven regulatory T cells (Tregs) are important for the induction and maintenance of transplantation tolerance, and developing methods to induce graft-protective Tregs might allow for long-term allograft acceptance, while reducing or eliminating the need for long-term immunosuppression. Targeting the interaction of glucocorticoid-induced TNFR-related gene (GITR) and its ligand could represent one way to induce Tregs that are able to protect allografts. Mouse GITR was first cloned in 1997, while human GITR and its ligand were cloned in 1999 by two different groups, and, later, the murine ortholog of GITR-L was cloned in 2003 (1–7). GITR is predominantly expressed on activated T-effector cells and CD4+Foxp3+ regulatory T cells, as well as B cells, NK cells, and macrophages (8, 9). GITR-L is expressed by dendritic cells, vascular endothelial cells, and other parenchymal cells (9). GITR-GITR-L interactions have a significant effect on T-cells activation, they promote T-cell proliferation and cytokine production (10). GITR can also activate signaling pathways that trigger apoptotic cell death (11, 12). The precise role of the GITR-GITR-L pathway in transplantation has not been elucidated. However, some reports have suggested that blocking this pathway could promote tolerance through regulation of Foxp3+ Treg (13) because GITR signaling is known to interfere with Treg function, and it seems to regulate Treg-suppressor function or the sensitivity of T-effector cells to suppression by Treg (8, 14, 15). In this issue of Transplantation, Sonawane and colleagues report that the acute phase of skin transplant rejection is accompanied by enhanced GITR-L expression on antigen presenting cells concentrated in the draining lymph node. They postulate that the GITR-GITR-L interaction is a crucial mediator of counter-regulatory processes invoked by innate inflammatory signaling that accompanies the acute phase of transplantation. To test this hypothesis, they developed a novel recombinant molecule that combines 129 amino acids of the extracellular domain of the activation-inducible TNF receptor (AITRL) and the human ortholog of GITR-L fused to the human IgG1 constant region (AITRL-Fc). Unlike DTA-1, an anti-GITR antibody that has been shown to suppress the inhibitory function of GITR+Tregs (8, 13) and to block the ability of activated GITR+T-effector cells to be suppressed by Tregs through GITR (16), AITRL-Fc preserves Treg suppression and prolongs skin allograft survival in an adoptive cell transfer model. There have been conflicting reports on the mechanism by which blocking the GITR pathway prolongs allograft survival in terms of whether inhibition occurs by inhibiting T-effector or by promoting Treg function. Using AITRL-Fc, the authors provide evidence that supports the later. They argued that blocking GITR-GITR-L interactions with AITRL-Fc represents an attractive means by which graft survival can be improved in a Treg dependent manner. The generation and function of Tregs is intimately regulated by costimulatory signals. In depth study of various T-cell costimulatory pathways has revealed that T-cell costimulation is much more complicated than initially expected when costimulatory pathway were first identified (17). Some costimulatory molecules and their ligands are constitutively expressed, whereas others are expressed at various time points after immune activation, indicating differences in temporal involvement of different pathways. In addition, some costimulatory molecules are preferentially expressed on specific T-cell subsets, and some of costimulatory molecule ligands are expressed on lymphoid lineages other than T cells and even on parenchymal cells (18). Importantly, the same receptor-ligand interaction can have similar or distinct effects on different T-cell subsets. Although many pathways deliver positive signals to activate T cells, some pathways induce negative signal to inhibit T-cell activation. Finally, there is redundancy in costimulatory pathways. For example, B7-1 can bind CD28 and PDL1 (19). Perhaps because of the complexity and redundancy of costimulatory pathways, targeting single costimulatory molecule pathway or single mechanism has provided disappointing results when translated preclinical or clinical trials. Indeed, the fate and the functional status of the T-cell response is ultimately determined by many factors, including the integration of both positive and negative costimulatory signals, the size of the effector versus regulatory cell pool, the type of graft, and local graft environment. Although the results shown are indeed compelling, graft prolongation is relatively easy to achieve in an adoptive cell transfer model than in classic transplant models. Moreover, the authors chose to use a donor mismatched for a single minor antigen and have not tested the efficacy of this approach on MHC mismatched or fully mismatched allografts. Therefore, although the findings described in this article will help to further understand the function of GITR-GITR-L costimulation in alloimmune responses and in regulation of T-cell activity, the clinical relevance of this publication is unclear because of the model used. Nevertheless, these studies set the stage for testing whether GITR-GITR-L blockade can show a synergistic effect when combined with other costimulatory molecule blockade strategies or immunosuppressive drugs.