Impact of Interleukin-2–expanded Regulatory T Cells in Various Allogeneic Combinations on Mouse Skin Graft Survival

Low-dose interleukin-2 promotes immune regulation in face transplantation: A pilot study

Face transplantation is a life-changing procedure for patients with severe composite facial defects. However, it is hampered by high acute rejection rates due to the immunogenicity of skin allograft and toxicity linked to high doses of immunosuppression. To reduce immunosuppression-associated complications, we, for the first time in face transplant recipients, used low-dose interleukin 2 (IL-2) therapy to expand regulatory T cells (Tregs) in vivo and to enhance immune modulation, under close immunological monitoring of peripheral blood and skin allograft. Low-dose IL-2 achieved a sustained expansion (∼4-fold to 5-fold) of circulating Tregs and a reduction (∼3.5-fold) of B cells. Post-IL-2 Tregs exhibited greater suppressive function, characterized by higher expression of TIM-3 and LAG3co-inhibitory molecules. In the skin allograft, Tregs increased after low-dose IL-2 therapy. IL-2 induced a distinct molecular signature in the allograft with reduced cytotoxicity-associated genes (granzyme B and perforin). Two complications were observed during the trial: one rejection event and an episode of autoimmune hemolytic anemia. In summary, this initial experience demonstrated that low-dose IL-2 therapy was not only able to promote immune regulation in face transplant recipients but also highlighted challenges related to its narrow therapeutic window. More specific targeted Treg expansion strategies are needed to translate this approach to the clinic.

Biased IL-2 signals induce Foxp3-rich pulmonary lymphoid structures and facilitate long-term lung allograft acceptance in mice

Transplantation of solid organs can be life-saving in patients with end-stage organ failure, however, graft rejection remains a major challenge. In this study, by pre-conditioning with interleukin-2 (IL-2)/anti-IL-2 antibody complex treatment biased toward IL-2 receptor α, we achieved acceptance of fully mismatched orthotopic lung allografts that remained morphologically and functionally intact for more than 90 days in immunocompetent mice. These allografts are tolerated by the actions of forkhead box p3 (Foxp3)⁺ regulatory T (Treg) cells that home to the lung allografts. Although counts of circulating Treg cells rapidly return to baseline following cessation of IL-2 treatment, Foxp3⁺ Treg cells persist in peribronchial and peribronchiolar areas of the grafted lungs, forming organized clusters reminiscent of inducible tertiary lymphoid structures (iTLS). These iTLS in lung allografts are made of Foxp3⁺ Treg cells, conventional T cells, and B cells, as evidenced by using microscopy-based distribution and neighborhood analyses. Foxp3-transgenic mice with inducible and selective deletion of Foxp3⁺ cells are unable to form iTLS in lung allografts, and these mice acutely reject lung allografts. Collectively, we report that short-term, high-intensity and biased IL-2 pre-conditioning facilitates acceptance of vascularized and ventilated lung allografts without the need of immunosuppression, by inducing Foxp3-controlled iTLS formation within allografts.

Receptor-gated IL-2 delivery by an anti-human IL-2 antibody activates regulatory T cells in three different species

Stimulation of regulatory T (Treg) cells holds great promise for the treatment of autoimmune, chronic inflammatory, and certain metabolic diseases. Recent clinical trials with low-dose interleukin-2 (IL-2) to expand Treg cells led to beneficial results in autoimmunity, but IL-2 immunotherapy can activate both Treg cells and pathogenic T cells. Use of IL-2 receptor α (IL-2Rα, CD25)–biased IL-2/anti–IL-2 antibody complexes improves IL-2 selectivity for Treg cells; however, the mechanism of action of such IL-2 complexes is incompletely understood, thus hampering their translation into clinical trials. Using a cell-based and dynamic IL-2R platform, we identified a particular anti-human IL-2 antibody, termed UFKA-20. When bound to UFKA-20, IL-2 failed to stimulate cells expressing IL-2Rβ (CD122) and IL-2Rγ (CD132), unless these cells also expressed high amounts of CD25. CD25 allowed IL-2/UFKA-20 complexes to bind, and binding to CD25 in the presence of CD122 and CD132 was followed by rapid dissociation of UFKA-20 from IL-2, delivery of IL-2 to CD122 and CD132, and intracellular signaling. IL-2/UFKA-20 complexes efficiently and preferentially stimulated CD4+ Treg cells in freshly isolated human T cells ex vivo and in mice and rhesus macaques in vivo. The crystal structure of the IL-2/UFKA-20 complex demonstrated that UFKA-20 interfered with IL-2 binding to CD122 and, to a lesser extent, also CD25. Together, we translated CD25-biased IL-2 complexes from mice to nonhuman primates and extended our mechanistic understanding of how CD25-biasing anti-human IL-2 antibodies work, which paves the way to clinical trials of CD25-biased IL-2 complexes.

Low-Dose IL-2 Therapy in Transplantation, Autoimmunity, and Inflammatory Diseases

Regulatory T cells (Tregs) play a central role in the induction and maintenance of immune homeostasis and self-tolerance. Tregs constantly express the high-affinity receptor to IL-2. IL-2 is a pleiotropic cytokine and a key survival factor for Tregs. It maintains Tregs' suppressive function by promoting Foxp3 expression and subsequent production of immunoregulatory cytokines. Administration of low-dose IL-2 is shown to be a promising approach to prevent allograft rejection and to treat autoimmune and inflammatory conditions in experimental models. The combination of IL-2 with its mAb (JES6-1) has also been shown to increase the t 1/2 of IL-2 and further enhance Treg frequencies and function. Low-dose IL-2 therapy has been used in several clinical trials to treat conditions such as hepatitis C vasculitis, graft-versus-host disease, type 1 diabetes, and systemic lupus erythematosus. In this paper, we summarize our findings on low-dose IL-2 treatment in corneal allografting and review recent studies focusing on the use of low-dose IL-2 in transplantation, autoimmunity, and other inflammatory conditions. We also discuss potential areas of further investigation with the aim to optimize current low-dose IL-2 regimens.

Treg-mediated prolonged survival of skin allografts without immunosuppression

Injection of Interleukin-2 (IL-2) complexed with a particular anti-IL-2 monoclonal antibody (mab) JES6-1 has been shown to selectively expand CD4⁺Foxp3⁺ T regulatory T cells (Tregs) in vivo. Although the potency of this approach with regard to transplantation has already been proven in an islet transplantation model, skin graft survival could not be prolonged. Since the latter is relevant to human allograft survival, we sought to improve the efficiency of IL-2 complex (cplx) treatment for skin allograft survival in a stringent murine skin graft model. Here, we show that combining low doses of IL-2 cplxs with rapamycin and blockade of the inflammatory cytokine IL-6 leads to long-term (>75 d) survival of major histocompatibility complex-different skin allografts without the need for immunosuppression. Allograft survival was critically dependent on CD25⁺FoxP3⁺ Tregs and was not accompanied by impaired responsiveness toward donor alloantigens in vitro after IL-2 cplx treatment was stopped. Furthermore, second donor-type skin grafts were rejected and provoked rejection of the primary graft, suggesting that operational tolerance is not systemic but restricted to the graft. These findings plus the lack of donor-specific antibody formation imply that prolonged graft survival was largely a reflection of immunological ignorance. The results may represent a potentially clinically translatable strategy for the development of protocols for tolerance induction.

In Vivo Expansion of Regulatory T Cells by Low-Dose Interleukin-2 Treatment Increases Allograft Survival in Corneal Transplantation

BACKGROUND

Corneal allograft survival dramatically decreases in hosts with inflamed or vascularized recipient beds. We have previously shown that in rejected corneal allografts regulatory T cells (Treg) demonstrate diminished Foxp3 expression and immunoregulatory function. Treatment with low doses of IL-2 selectively expands Treg and has been proposed for the treatment of autoimmune diseases. In this study, we investigated the effect of low-dose IL-2 administration on Treg function and corneal allograft survival.

METHODS

Allogeneic corneal transplantation was performed on inflamed host beds. Low-dose systemic IL-2 was administered starting 3 days before grafting until 6 weeks after transplantation. Frequencies of Treg and their immunosuppressive function and antigen specificity were assessed using flow cytometry, in vitro proliferation assays, and adoptive transfer experiments. Frequencies of effector T cells (Teff) and graft infiltrating immune cells were measured at 2 weeks posttransplantation. Long-term allograft survival was evaluated for up to 9 weeks using Kaplan-Meier survival analysis.

RESULTS

Treatment with low-dose IL-2 significantly increased frequencies of CD4CD25Foxp3 Treg and their immunosuppressive function. It also suppressed alloimmune response as shown by the decreased CD4 IFNγ T cell frequencies and graft infiltration of CD45 and CD4 cells. Clinical evaluation of the grafts showed significant improvement in long-term corneal allograft survival in the IL-2 treated group compared with controls.

CONCLUSIONS

Our study is the first to report that treatment with low-dose IL-2 increases survival of corneal allografts. We propose that IL-2-mediated Treg expansion can be an effective tool to prevent alloimmunity and to improve long-term allograft survival in transplantation.

New Molecular and Cellular Mechanisms of Tolerance: Tolerogenic Actions of IL-2

Interleukin-2 (IL-2) is an old molecule with brand new functions. Indeed, IL-2 has been first described as a T-cell growth factor but recent data pointed out that its main function in vivo is the maintenance of immune tolerance. Mechanistically, IL-2 is essential for the development and function of CD4(+) Foxp3(+) regulatory T cells (Treg cells) that are essential players in the control of immune responded to self, tumors, microbes and grafts. Treg cells are exquisitely sensitive to IL-2 due to their constitutive expression of the high affinity IL-2 receptor (IL-2R) and the new paradigm suggests that low-doses of IL-2 could selectively boost Treg cells in vivo. Consequently, a growing body of clinical research is aiming at using IL-2 at low doses as a tolerogenic drug to boost endogenous Treg cells in patients suffering from autoimmune or inflammatory conditions. In this manuscript, we briefly review IL-2/IL-2R biology and the role of IL-2 in the development, maintenance, and function of Treg cells; and also its effects on other immune cell populations such as CD4(+) T helper cells and CD8(+) memory T cells. Then, focusing on type 1 diabetes, we review the preclinical studies and clinical trials supporting the use of low-doses IL-2 as a tolerogenic immunotherapy. Finally, we discuss the limitations and future directions for IL-2 based immunotherapy.

Administration of low doses of IL-2 combined to rapamycin promotes allogeneic skin graft survival in mice

Human CD4(+) CD25(+) FoxP3(+) regulatory T cells (Tregs) prevent allogeneic graft rejection by inhibiting T cell activation, as has been shown in mouse models. Recently, low-dose IL-2 administration was shown to specifically activate Tregs but not pathogenic conventional T cells, leading to resolution of type 1 diabetes in nonobese diabetic mice. We therefore tested the ability of low-dose IL-2 to prevent allogeneic skin graft rejection. We found that while IL-2 alone was inefficient in preventing rejection, combined with rapamycin, IL-2 treatment promoted skin graft survival both in minor disparate and semi-allogeneic skin graft combinations. Tregs are activated by this combined treatment while conventional CD4(+) cell expansion and activation are markedly inhibited. Co-administration of anti-CD25 antibodies dramatically reduces the effect of the IL-2/rapamycin treatment, strongly supporting a central role for Treg activation. Thus, we provide the first preclinical data showing that low-dose IL-2 combined with rapamycin can significantly delay transplant rejection in mice. These findings may form the rational for clinical evaluation of this novel approach for the prevention of transplant rejection.

Helminths and immunological tolerance

Current immunosuppression regimens for solid-organ transplantation have shown disappointing efficacy in the prevention of chronic allograft rejection and carry unacceptable risks including toxicity, neoplasia, and life-threatening infection. Achievement of immunological tolerance (long-term antigen unresponsiveness in an immunocompetent host) presents the exciting prospect of freedom from immunosuppression for transplant recipients. It is now 60 years since the first demonstration of immunological tolerance in animal models of transplantation, but translation into routine clinical practice remains elusive. Helminth parasites may provide novel strategies toward achieving this goal. Helminths are remarkably successful parasites: they currently infect more than one quarter of the world’s population. It is now well established that the parasites’ success is the result of active immunomodulation of their hosts’ immune response. Although this primarily secures ongoing survival of the parasites, helminth-induced immunomodulation can also have a number of benefits for the host. Significant reductions in the prevalence of allergy and autoimmune conditions among helminth-infected populations are well recognized and there is now a significant body of evidence to suggest that harmful immune responses to alloantigens may be abrogated as well. Here, we review all existing studies of helminth infection and transplantation, explore the mechanisms involved, and discuss possible avenues for future translation to clinical practice.