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.

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 CD4⁺CD25⁺Foxp3⁺ 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.

Antigen-presenting cells (APCs) play an important role in transplant rejection and tolerance. In high-risk corneal transplantation, where the graft bed is inflamed and vascularized, immature APCs in the donor corneal stroma quickly mature and migrate to lymphoid tissues to sensitize host T cells. In this study, using a mouse model of corneal transplantation, we investigated whether enrichment of tolerogenic APCs (tolAPCs) in donor corneas can enhance graft survival in corneal allograft recipients with inflamed graft beds. Treatment of donor corneas with interleukin-10 (IL-10) and transforming growth factor-β1 (TGFβ1) altered the phenotype and function of tissue-residing APCs. Transplantation of these tolAPC-enriched corneas decreased frequencies of interferon gamma (IFNγ)⁺ effector T cells (Teffs), as well as allosensitization in the hosts, diminished graft infiltration of CD45⁺ and CD4⁺ cells, and significantly improved corneal allograft survival compared to saline-injected controls. These data provide a novel approach for tolAPC-based immunotherapy in transplantation by direct cytokine conditioning of the donor tissue.

Background Corneal neovascularization increases the risk of T cell–mediated allograft rejection. Here, we investigate whether T cells promote angiogenesis in transplantation. Methods Conventional effector T cells were collected from draining lymph nodes of allogeneic or syngeneic corneal transplanted BALB/c mice. T cells were either cocultured with vascular endothelial cells (VECs) to assess VEC proliferation or used in a mixed lymphocyte reaction assay. Messenger RNA (mRNA) expression of vascular endothelial growth factor (VEGF)-A, -C, and VEGF receptor 2 (VEGF-R2) in VECs was assessed by real-time PCR. VEGF-A protein expression was determined by enzyme-linked immunosorbent assay. Flow cytometry was used to analyze VEGF-R2 expression in corneal CD31⁺ cells, and VEGF-A and IFNγ expression in corneal CD4⁺ T cells. Results Allogeneic T cells from high-risk (HR) grafted mice induced more VEC proliferation than those from syngeneic transplant recipients (P = 0.03). Vascular endothelial growth factor-A mRNA and protein expression were higher in T cells from draining lymph nodes (P = 0.03 and P = 0.04, respectively) and cornea (protein; P = 0.04) of HR compared with low-risk (LR) grafted hosts. Vascular endothelial growth factor-A, VEGF-C, and VEGF-R2 mRNA expression were increased in VECs when cocultured with T cells from HR transplants compared with LR transplants and naive mice. In addition, IFNγ blockade in T cell/VEC coculture increased VEC proliferation and VEGF-A protein expression, whereas blocking VEGF-A significantly reduced VEC proliferation (P = 0.04). Conclusions Allogeneic T cells from corneal transplant hosts promote VEC proliferation, probably via VEGF-A signaling, whereas IFNγ shows an antiangiogenic effect. Our data suggest that T cells are critical mediators of angiogenesis in transplantation.

Disorders of the cornea are responsible for a significant portion of preventable blindness worldwide. Various types of corneal transplantation procedures have successfully restored vision in many individuals; however, they carry the risk of graft failure due to immune rejection, endothelial cell dysfunction, infections and limbal stem cell deficiency. Thus, regenerative therapies of the cornea serve as promising alternatives or adjunct therapies. With improved understanding of limbal stem cell function and advancement in limbal stem cell culture technologies, major progress has been made in the in vivo and ex vivo cell-based therapies for treatment of corneal diseases. In this review, we summarize the recent developments achieved in cell-based therapeutics to target corneal epithelial, stromal, and endothelial cell disorders.