Tipping the Balance (2018)

Naturally occurring FOXP3 positive regulatory T (nTreg) cells play an important role in controlling immune responses in autoimmunity and transplantation. In transplantation, increased Treg cell numbers have been associated with better graft outcome1 and nTreg cells accumulate within tolerant liver transplants after intentional weaning immunosuppression.2 During cell-mediated rejection nTreg cells increase over-proportionally within liver grafts to control immune responses.3 Animal experiments revealed that an adoptive transfer of nTreg cells can establish tolerance for completely MHC-mismatched grafts. However, these experimental attempts have been way more successful with graft specific nTreg cells as compared with polyspecific nTreg cells.4,5 Those data led to the initiation of clinical trials with Treg cells supporting weaning of immunosuppression after liver transplantation. Todo and coworkers6 have shown in a proof of concept study that early weaning of immunosuppression is feasible after transfer of a Treg cell–enriched cell product. In this trial, a cell product enriched in Treg cells had been administered in parallel to conditioning recipients with cyclophosphamide and a splenectomy. Moving forward, a combinatorial approach that combines the transfer of graft-specific naturally occurring FOXP3 positive regulatory T cells with the application of IL-2c could be envisioned. Although Treg cell expansion in vitro is cumbersome and expensive, making Treg cell function less effective, alternatively nTreg cells may be expanded in vivo by low doses of IL-2. This approach is of particular interest as the natural Treg cell repertoire contains up to 8% to 15% of allospecific nTreg cells.7 IL-2 signals are operative through a trimeric receptor consisting of IL2-Rα (CD25), IL-2Rβ (CD122), and IL-2Rγ (CD133). Although nTreg cells have a high, constitutive expression of CD25, therefore signaling through the high-affinity trimeric complex, conventional T (Tconv) cells initially signal via the lower affinity dimeric complex compromised of IL2Rβ and IL-2Rγ. IL-2 has been initially been used in cancer immunology to boost antitumor NK and T-cell responses. However, it has later been shown that low doses of IL-2 might preferentially act on nTreg cells as they constitutively express high-affinity IL-2Rα (CD25).8 Interestingly, it turned out that IL-2 is the major rheostat controlling Treg cell numbers with a lack of IL-2 leading to Treg cell apoptosis in vivo and in vitro. Because it has been shown clinically that low doses of IL-2 can considerably expand nTreg cells, IL-2 therapy is currently tested in a broad variety of autoimmune and autoinflammatory diseases.9 Early success has been reported for graft-versus-host disease (GvHD),10 autoimmune vasculitis and early trials in lupus erythematosus looked promising.11 In addition, IL-2 supported weaning trials after liver transplantation are currently being tested at King's College (LITE trial, www.clinicaltrials.gov). Notably, even low doses of IL-2 represent a fine balance between Treg cell expansion versus an activation of T and NK cells.12 It has also been shown that low-dose IL-2 can enhance autoimmunity. Moreover, a trial of low-dose IL-2 in type 1 diabetes showed a decline of C-peptide levels.13 Attempts have therefore been made to enhance the specificity of IL-2 for Treg cell activations. In mice, IL-2 has been complexed with a monoclonal antibody (JES6-1) (IL-2c), resulting in an augmented effectivity.8 Initially believed that the greater effectivity had been linked to a prolonged half-life of the IL-2 complex, it became clear later that specific properties of the antibody changed affinity and binding properties of IL-2, favoring the activation of nTreg cells. As human anti-IL-2 antibodies have not shown comparable effects thus far, clinical trials have used low doses of pure, uncomplexed IL-2. Current studies have shown clinical safety in autoimmunity and transplantation. This is an important and relevant information, as other attempts augmenting nTreg cells in vivo by an agonistic anti-CD28 antibody had to be discontinued linked to severe cytokine release syndrome. The work by Trotta and coworkers14 recently in Nature Medicine might therefore represent a “game changer” for IL-2 therapies. The authors developed a new set of human anti–IL-2 antibodies from a phage display library, and complexed those with human IL-2 (IL-2c). Antibodies were selected to inhibit binding to IL-2Rα or IL-2Rβ while maintaining STAT5 phosphorylation in nTreg cells after activation with IL-2c. Due to their inhibitory effect on IL-2 signaling, IL-2–dependent genes were not upregulated in CD8+ T cells at low IL-2c concentrations. The improved effectivity on nTreg cells could partially be explained by a marked upregulation of CD25. Moreover, in NOD scid gamma mice (NSG) reconstituted with activated human peripheral blood mononuclear cells (PBMCs), IL-2c lead to a marked expansion of nTreg cells and a smaller expansion of CD4+ and CD8+ T cells. Those effects resulted into twofold to threefold increased Treg/Tconv cells and Treg/CD8 cell ratios. Yet some minor increase of Th1, Th2, and Th17 cells was also noted in the CD4+ population, whereas some CD8+ cells showed a slight increase in cytolytic molecules and effector cytokines.14 Finally, the authors demonstrated the therapeutic capacity of the human IL-2c through a partial reversal of recent onset diabetes in NOD mice (nonobese diabetic mouse) and a slight amelioration of experimental autoimmune encephalitis in C57BL/6 mice. Moreover, IL-2c could dampen a xenogeneic GvHD in immune reconstituted humanized NSG mice. These results are very promising and could lead to new and more effective approaches enhancing nTreg cells. Does that mean that clinical trials with IL-2c in solid organ transplantation are around the corner? The much awaited safety results of the IL-2 supported weaning trial from London, United Kingdom, will certainly be helpful on the way. Moreover, control of GvHD in humanized mice represents a rather weak immune response. It is also possible that IL-2c (complexed with JES6-1) may just “just” prolong graft survival without leading to tolerance.15 Although human IL-2c may lean the balance between tolerance and immunity into the ”right direction,” effects are by no means clear-cut with an expansion and activation of conventional T cells albeit to a lesser degree as compared with nTreg cells.14 Yet, under clinical transplant conditions with long-lasting memory immune responses, effects might be different. In addition, IL-2 must dissociate from the antibody complex to gain activity at the receptor side and, consequently, several groups are trying to augment the specificity IL-2 to nTreg cells by using mutated IL-2 proteins. Transducing nTreg cells with a mutated IL-2R that binds a specific designer IL-2 reacting with the mutated receptor may be an alternative approach. That way, adoptively transferred nTreg cells could potentially be expanded without coactivation of conventional T cells. Finally, neither IL-2c nor adoptive transfer of polyspecific nTreg cells has thus far been able to prevent rejection in models with strong histoincompatibility. Moving forward, a combinatorial approach that combines the transfer of graft-specific nTreg cells with the application of IL-2c could be envisioned. This strategy could minimize the need for prolonged in vitro expansion and potentially override the inhibitory effects of calcineurin inhibitors on nTreg cells. Ideally, tissue-specific tolerance could be achieved without compromising the overall immune competence. The balance to achieve this goal has been tipped to the “right” side. The future will show if the exciting data by Trotta and coworkers will be a “clinical” game changer.