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

Transcriptome profiling of immune rejection mechanisms in a porcine vascularized composite allotransplantation model

Background

Vascularized composite allotransplantation (VCA) offers the potential for a biological, functional reconstruction in individuals with limb loss or facial disfigurement. Yet, it faces substantial challenges due to heightened immune rejection rates compared to solid organ transplants. A deep understanding of the genetic and immunological drivers of VCA rejection is essential to improve VCA outcomes.

Methods

Heterotopic porcine hindlimb VCA models were established and followed until reaching the endpoint. Skin and muscle samples were obtained from VCA transplant recipient pigs for histological assessments and RNA sequencing analysis. The rejection groups included recipients with moderate pathological rejection, treated locally with tacrolimus encapsulated in triglycerol-monostearate gel (TGMS-TAC), as well as recipients with severe end-stage rejection presenting evident necrosis. Healthy donor tissue served as controls. Bioinformatics analysis, immunofluorescence, and electron microscopy were utilized to examine gene expression patterns and the expression of immune response markers.

Results

Our comprehensive analyses encompassed differentially expressed genes, Gene Ontology, and Kyoto Encyclopedia of Genes and Genomes pathways, spanning various composite tissues including skin and muscle, in comparison to the healthy control group. The analysis revealed a consistency and reproducibility in alignment with the pathological rejection grading. Genes and pathways associated with innate immunity, notably pattern recognition receptors (PRRs), damage-associated molecular patterns (DAMPs), and antigen processing and presentation pathways, exhibited upregulation in the VCA rejection groups compared to the healthy controls. Our investigation identified significant shifts in gene expression related to cytokines, chemokines, complement pathways, and diverse immune cell types, with CD8 T cells and macrophages notably enriched in the VCA rejection tissues. Mechanisms of cell death, such as apoptosis, necroptosis and ferroptosis were observed and coexisted in rejected tissues.

Conclusion

Our study provides insights into the genetic profile of tissue rejection in the porcine VCA model. We comprehensively analyze the molecular landscape of immune rejection mechanisms, from innate immunity activation to critical stages such as antigen recognition, cytotoxic rejection, and cell death. This research advances our understanding of graft rejection mechanisms and offers potential for improving diagnostic and therapeutic strategies to enhance the long-term success of VCA.

Low-dose IL-2 therapy in autoimmune diseases: An update review

Regulatory T (Treg) cells are essential for maintaining self-immune tolerance. Reduced numbers or functions of Treg cells have been involved in the pathogenesis of various autoimmune diseases and allograft rejection. Therefore, the approaches that increase the pool or suppressive function of Treg cells in vivo could be a general strategy to treat different autoimmune diseases and allograft rejection. Interleukin-2 (IL-2) is essential for the development, survival, maintenance, and function of Treg cells, constitutively expressing the high-affinity receptor of IL-2 and sensitive response to IL-2 in vivo. And low-dose IL-2 therapy in vivo could restore the imbalance between autoimmune response and self-tolerance toward self-tolerance via promoting Treg cell expansion and inhibiting follicular helper T (Tfh) and IL-17-producing helper T (Th17) cell differentiation. Currently, low-dose IL-2 treatment is receiving extensive attention in autoimmune disease and transplantation treatment. In this review, we summarize the biology of IL-2/IL-2 receptor, the mechanisms of low-dose IL-2 therapy in autoimmune diseases, the application in the progress of different autoimmune diseases, including Systemic Lupus Erythematosus (SLE), Type 1 Diabetes (T1D), Rheumatoid Arthritis (RA), Autoimmune Hepatitis (AIH), Alopecia Areata (AA), Immune Thrombocytopenia (ITP) and Chronic graft-versus-host-disease (GVHD). We also discuss the future directions to optimize low-dose IL-2 treatments.

The intragraft vascularized bone marrow induces secondary donor-specific mystacial pad allograft tolerance

Introduction

Vascularized bone marrow (VBM) is essential in tolerance induction through chimerism. We hypothesized that the inclusion of VBM contributes to the induction of mystacial pad allotransplantation tolerance.

Method

In this study, 19 VBM, nine mystacial pad, and six sequential VBM and mystacial pad allografts were transplanted from Brown Norway (BN) rats to Lewis (LEW) rats to test our hypothesis. The VBM recipients were divided into antilymphocyte serum (ALS) monotherapy group (two doses of ALS on day 3 pretransplantation and day 1 posttransplantation), immunosuppressant group [a week of 2 mg/kg/day tacrolimus (Tac) and 3 weeks of 3 mg/kg/day rapamycin (RPM)], and combined therapy group. The mystacial pad recipients were divided into VBM and non-VBM transplantation groups, and both groups were treated with an immunosuppression regimen that consists of ALS, Tac, and RPM. For the recipients of sequential VBM and mystacial pad allotransplantations, additional Tac was given 1 week after mystacial pad transplantation. Allograft survival, donor-specific tolerance, and chimerism level were evaluated.

Results

With the administration of ALS and short-term Tac and RPM treatments, VBM recipients demonstrated long-term graft survival (>120 days) with persistent chimerism for 30 days. CD3+ T cells from tolerant rats showed donor-specific hyporesponsiveness and tolerance to donor skin grafts but not to third-party counterparts. Furthermore, mystacial pad graft recipients with VBM transplantation exhibited a higher allograft survival rate than those without VBM transplantation [median survival time (MST) >90 days vs. 70 days, p < 0.05].

Conclusion

This study demonstrated that VBM transplantation is an efficient strategy to induce and maintain donor-specific tolerance for an osseous-free allograft.

Skin Immunity and Tolerance: Focus on Epidermal Keratinocytes Expressing HLA-G

Although the role of epidermal cells in skin regeneration has been extensively documented, their functions in immunity and tolerance mechanisms are largely underestimated. The aim of the present review was to outline the state of knowledge on resident immune cells of hematopoietic origin hosted in the epidermis, and then to focus on the involvement of keratinocytes in the complex skin immune networks acting in homeostasis and regeneration conditions. Based on this knowledge, the mechanisms of immune tolerance are reviewed. In particular, strategies based on immunosuppression mediated by HLA-G are highlighted, as recent advances in this field open up perspectives in epidermis-substitute bioengineering for temporary and permanent skin replacement strategies.

Advances in Liver Transplantation: where are we in the pursuit of transplantation tolerance?

Liver transplantation is the ultimate treatment option for end-stage liver disease. Breakthroughs in surgical practice and immunosuppression have seen considerable advancements in survival after transplantation. However, the intricate management of immunosuppressive regimens, balancing desired immunological quiescence while minimizing toxicity has proven challenging. Diminishing improvements in long-term morbidity and mortality have been inextricably linked with the protracted use of these medications. As such, there is now enormous interest to devise protocols that will allow us to minimize or completely withdraw immunosuppressants after transplantation. Immunosuppression withdrawal trials have proved the reality of tolerance following liver transplantation, however, without intervention will only occur after several years at the risk of potential cumulative immunosuppression-related morbidity. Focus has now been directed at accelerating this phenomenon through tolerance-inducing strategies. In this regard, efforts have seen the use of regulatory cell immunotherapy. Here we focus particularly on regulatory T cells, discussing preclinical data that propagated several clinical trials of adoptive cell therapy in liver transplantation. Furthermore, we describe efforts to further optimize the specificity and survival of regulatory cell therapy guided by concurrent immunomonitoring studies and the development of novel technologies including chimeric antigen receptors and co-administration of low-dose IL-2.

Antigen Specific Regulatory T Cells in Kidney Transplantation and Other Tolerance Settings

Kidney transplantation is the most common solid organ transplant and the best current therapy for end-stage kidney failure. However, with standard immunosuppression, most transplants develop chronic dysfunction or fail, much of which is due to chronic immune injury. Tregs are a subset of T cells involved in limiting immune activation and preventing autoimmune disease. These cells offer the potential to provide tolerance or to allow reduction in immunosuppression in kidney transplants. The importance of Tregs in kidney transplantation has been shown in a number of seminal mouse and animal studies, including those with T cell receptors (TCRs) transgenic Tregs (TCR-Tregs) or Chimeric Antigen Receptor (CAR) Tregs (CAR-Tregs) showing that specificity increases the potency of Treg function. Here we outline the animal and human studies and clinical trials directed at using Tregs in kidney transplantation and other tolerance settings and the various modifications to enhance allo-specific Treg function in vivo and in vitro.

Immunotherapeutic strategy based on anti-OX40L and low dose of IL-2 to prolong graft survival in sensitized mice by inducing the generation of CD4+ and CD8+ Tregs

Alloreactive memory cells play a critical role after a second transplant and are difficult to suppress. This study investigated the effect of an immunotherapeutic strategy that combines anti-OX40L, rapamycin (Rapa), and a low dose of IL-2 in a memory cell-based adoptive model. In this model, the median survival time (MST) of the grafts of the combined treatment group was significantly extended compared to that of the control group and other treatment groups. A similar effect was observed regarding a reduction in memory T cells (Tm) and inflammatory cytokines production. Also, the percentages of Foxp3⁺ regulatory T cells (Tregs) increased in our model. In addition, mounting evidence has shown CD8⁺CD122⁺ T cells are also Tregs. We found that the group of CD8⁺CD122⁺PD1⁺ T cells was markedly increased in the combined treatment group, especially in the graft. We further demonstrated that CD8⁺CD122⁺PD1⁺ T cells could suppress activated T cells. Our data suggest that anti-OX40L combined with Rapa and a low dose of IL-2 can suppress Tm, modulate CD4 and CD8 Tregs, and induce long-term heart allograft survival in sensitized mice.

Treg Therapies Revisited: Tolerance Beyond Deletion

Induction of immune tolerance is the Holy Grail in transplantation medicine and autoimmunity. Currently, patients are required to use immunosuppressive drugs for the rest of their lives, resulting in unwanted side effects and complication from global suppression of the immune response. It is well established that regulatory T cells (Tregs) are critical for the maintenance of immune tolerance towards self-antigens by several mechanisms of immune regulation, in parallel with intrathymic deletion of self-reactive T cells during ontogeny. Therefore, approaches for increasing Treg numbers or function in vivo could provide an all-purpose solution for tolerance induction. Currently, most state-of-the-art therapeutics for treating autoimmune diseases or preventing allograft rejection work either by general immunosuppression or blocking inflammatory reactions and are non-specific. Hence, these approaches cannot provide satisfactory long-term results, let alone a cure. However, in animal models the therapeutic potential of Treg expansion for inducing effective tolerance has now been demonstrated in various models of autoimmunity and allogeneic transplantation. Here, we focus on therapies for increasing the size of the Treg pool by expanding endogenous Treg numbers in vivo or by adoptive transfer of Tregs. In particular, we discuss IL-2 based approaches (low dose IL-2, IL-2 complexes) for inducing Treg expansion in vivo as well as cell-based approaches (polyclonal, antigen specific, or cell engineered) for adoptive Treg therapy. We also mention new questions arising from the first clinical studies on Treg therapy in the fields of transplantation and autoimmunity.

Current Preventions and Treatments of aGVHD: From Pharmacological Prophylaxis to Innovative Therapies

Graft versus host disease (GVHD) is one of the main causes of mortality and the reason for up to 50% of morbidity after hematopoietic stem cell transplantations (HSCT) which is the treatment of choice for many blood malignancies. Thanks to years of research and exploration, we have acquired a profound understanding of the pathophysiology and immunopathology of these disorders. This led to the proposition and development of many therapeutic approaches during the last decades, some of them with very promising results. In this review, we have focused on the recent GVHD treatments from classical chemical and pharmacological prophylaxis to more innovative treatments including gene therapy and cell therapy, most commonly based on the application of a variety of immunomodulatory cells. Furthermore, we have discussed the advantages and potentials of cell-free therapy as a newly emerging approach to treat GVHD. Among them, we have particularly focused on the implication of the TNFα-TNFR2 axis as a new immune checkpoint signaling pathway controlling different aspects of many immunoregulatory cells.

Low-Dose Interleukin-2 Combined With Rapamycin Led to an Expansion of CD4+CD25+FOXP3+ Regulatory T Cells and Prolonged Human Islet Allograft Survival in Humanized Mice

Islet transplantation is an emerging therapy for type 1 diabetes and hypoglycemic unawareness. However, a key challenge for islet transplantation is cellular rejection and the requirement for long-term immunosuppression. In this study, we established a diabetic humanized NOD-scidIL2Rγ^null (NSG) mouse model of T-cell-mediated human islet allograft rejection and developed a therapeutic regimen of low-dose recombinant human interleukin-2 (IL-2) combined with low-dose rapamycin to prolong graft survival. NSG mice that had received renal subcapsular human islet allografts and were transfused with 1 × 10⁷ of human spleen mononuclear cells reconstituted human CD45⁺ cells that were predominantly CD3⁺ T cells and rejected their grafts with a median survival time of 27 days. IL-2 alone (0.3 × 10⁶ IU/m² or 1 × 10⁶ IU/m²) or rapamycin alone (0.5-1 mg/kg) for 3 weeks did not prolong survival. However, the combination of rapamycin with IL-2 for 3 weeks significantly prolonged human islet allograft survival. Graft survival was associated with expansion of CD4⁺CD25⁺FOXP3⁺ regulatory T cells (Tregs) and enhanced transforming growth factor-β production by CD4⁺ T cells. CD8⁺ T cells showed reduced interferon-γ production and reduced expression of perforin-1. The combination of IL-2 and rapamycin has the potential to inhibit human islet allograft rejection by expanding CD4⁺FOXP3⁺ Tregs in vivo and suppressing effector cell function and could be the basis of effective tolerance-based regimens.

Limited efficacy of rapamycin monotherapy in vascularized composite allotransplantation

BACKGROUND

Vascularized composite allotransplantation (VCA) is a novel and life-enhancing procedure to restore a patient's function and/or appearance. Current immunosuppression in VCA recipients is based on calcineurin inhibitor (CNI) therapy that can lead to severe complications, such that inducing immune tolerance is a major goal of VCA research. In contrast to CNI, rapamycin (RPM) is thought to be beneficial to the development of immune tolerance by suppressing T-effector cells (Teffs) and expanding T-regulatory (Treg) cells. However, we found high dose RPM monotherapy prolonged VCA survival by only a few days, leading us to explore the mechanisms responsible.

METHODS

A mouse orthotopic forelimb transplantation model (BALB/c- > C57BL/6) was established using WT mice, as well as C57BL/6 recipients with conditional deletion of T-bet within their Treg cells. Events in untreated VCA recipients or those receiving RPM or FK506 therapy were analyzed by flow-cytometry, histopathology and real-time qPCR.

RESULTS

Therapy with RPM (2 mg/kg/d, p < .005) or FK506 (2 mg/kg/d, p < .005) each prolonged VCA survival. In contrast to FK506, RPM increased the ratio of splenic Treg to Teff cells (p < .05) by suppressing Teff and expanding Treg cells. While the proportion of activated splenic CD4 + Foxp3- T cells expressing IFN-γ were similar in control and RPM-treated groups, RPM decreased the proportions ICOS+ and CD8+ IFN-γ + splenic T cells. However, RPM also downregulated CXCR3+ expression by Tregs, and forelimb allografts had reduced infiltration by CXCR3+ Treg cells. In addition, allograft recipients whose Tregs lacked T-bet underwent accelerated rejection compared to WT mice despite RPM therapy.

CONCLUSIONS

We demonstrate that while RPM increased the ratio of Treg to Teff cells and suppressed CD8+ T cell allo-activation, it failed to prevent CD4 Teff cell activation and impaired CXCR3-dependent Treg graft homing, thereby limiting the efficacy of RPM in VCA recipients.

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.

IL-2 enhances ex vivo-expanded regulatory T-cell persistence after adoptive transfer

As regulatory T cell (Treg) adoptive therapy continues to develop clinically, there is a need to determine which immunomodulatory agents pair most compatibly with Tregs to enable persistence and stabilize suppressor function. Prior work has shown that mechanistic target of rapamycin inhibition can increase the stability of thymic Tregs. In this study, we investigated the transcriptomic signatures of ex vivo-expanded Tregs after adoptive transfer in the setting of clinically relevant immunosuppression using a nonhuman primate (NHP) model as a prelude to future transplant studies. Here, we found that adding interleukin-2 (IL-2) to rapamycin in vivo supported a logarithmic increase in the half-life of adoptively transferred carboxyfluorescein diacetate succinimidyl ester-labeled, autologous NHP Tregs, effectively doubling the number of cells in the peripheral blood Treg compartment compared with Treg infusion when rapamycin was given alone. Using single-cell transcriptomics, we found that transferred ex vivo-expanded Tregs initially exhibit a gene expression signature consistent with an activated state. Moreover, those cells with the highest levels of activation also expressed genes associated with p53-mediated apoptosis. In contrast, transferred Tregs interrogated at day +20 posttransfer demonstrated a gene signature more similar to published profiles of resting Tregs. Together, these preclinical data further support combining IL-2 and rapamycin in vivo as adjunctive therapy for ex vivo-expanded adoptively transferred Tregs and suggest that the activation status of ex vivo-expanded Tregs is critical to their persistence.

Recombinant human ADAMTS13 treatment and anti-NET strategies enhance skin allograft survival in mice

Enhancing skin allograft longevity lessens the need for new allografts before optimal intervention is available. Reduced activity of ADAMTS13 (an enzyme that cleaves the pro-thrombotic and proinflammatory von Willebrand factor) and presence of neutrophil extracellular traps (NETs) have been implicated in liver and lung allograft failures. The effect of ADAMTS13 treatment and the impact of NETs on skin allografts, however, remain unexplored. Here, we adopted a murine model of complete mismatch full-thickness skin transplant by grafting dorsal skin from BALB/c mice to C57BL/6J background mice. Recombinant human ADAMTS13 (rhADAMTS13) treatment of graft recipients increased allograft survival. Western blot and immunofluorescence microscopy revealed the presence of NETs in allografts of vehicle, but surprisingly, not in rhADAMTS13-treated mice, 3 days after surgery. Recapitulating the observations in mice, NETs were also observed in all the examined allografts from burn patients. Intriguingly, knocking out peptidylarginine deiminase 4 (PAD4, a key enzyme for NET formation) or DNase 1 treatment (which cleaves NETs) also prolonged allograft survival. In summary, rhADAMTS13 lessens inflammation in allografts by reducing NET burden, resulting in enhanced allograft survival. RhADAMTS13 and anti-NET treatments could be new therapeutic strategies to promote skin allograft longevity and, hence, the survival of patients with severe burns.

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.

The promise of CD4+FoxP3+ regulatory T-cell manipulation in vivo: applications for allogeneic hematopoietic stem cell transplantation

CD4⁺FoxP3⁺ regulatory T cells (Tregs) are a non-redundant population critical for the maintenance of self-tolerance. Over the past decade, the use of these cells for therapeutic purposes in transplantation and autoimmune disease has emerged based on their capacity to inhibit immune activation. Basic science discoveries have led to identifying key receptors on Tregs that can regulate their proliferation and function. Notably, the understanding that IL-2 signaling is crucial for Treg homeostasis promoted the hypothesis that in vivo IL-2 treatment could provide a strategy to control the compartment. The use of low-dose IL-2 in vivo was shown to selectively expand Tregs versus other immune cells. Interestingly, a number of other Treg cell surface proteins, including CD28, CD45, IL-33R and TNFRSF members, have been identified which can also induce activation and proliferation of this population. Pre-clinical studies have exploited these observations to prevent and treat mice developing autoimmune diseases and graft-versus-host disease post-allogeneic hematopoietic stem cell transplantation. These findings support the development of translational strategies to expand Tregs in patients. Excitingly, the use of low-dose IL-2 for patients suffering from graft-versus-host disease and autoimmune disease has demonstrated increased Treg levels together with beneficial outcomes. To date, promising pre-clinical and clinical studies have directly targeted Tregs and clearly established the ability to increase their levels and augment their function in vivo. Here we review the evolving field of in vivo Treg manipulation and its application to allogeneic hematopoietic stem cell transplantation.

Galectin-1-induced tolerogenic dendritic cells combined with apoptotic lymphocytes prolong liver allograft survival

Donor-derived tolerogenic dendritic cells (DCs) and apoptotic lymphocytes (ALs) are practical tools for controlling rejection after transplantation by targeting direct and indirect allorecognition pathways, respectively. To date, few studies have investigated the combination of donor-derived tolerogenic DCs and ALs infusion in organ transplantation protection. In the present study, we generated galectin-1-induced tolerogenic DCs (DC_gal-1s) and ultraviolet irradiation-induced ALs with stable immune characteristics in vitro and potential immune regulatory activity in vivo. A rat model of acute liver transplant rejection was established, and the intrinsic tolerogenic profiles associated with the short-term alleviation of rejection and the long-term maintenance of tolerance in the absence of immunosuppressive drugs were evaluated. The DC_gal-1-AL treatment prolonged allograft survival more significantly than a transfusion of DC_gal-1s or ALs alone. This benefit was associated with CD4⁺ Treg cell expansion and decreased interferon (IFN)-γ⁺ T cell levels. Moreover, DC_gal-1-AL treatment led to different cytokine/chemokine changes in the allograft and peripheral blood, that indicated an alleviation of local and systemic inflammation on day 7 post-transplantation. TGF-β1 and TGF-β2 were significantly increased in the long-term surviving allografts after DC_gal-1-AL treatment. Our results indicate that the combination of DC_gal-1s with ALs effectively prolongs liver allograft survival and represents a novel therapeutic strategy for liver transplant rejection.

Targeting Regulatory T Cells to Treat Patients With Systemic Lupus Erythematosus

Regulatory T cells (Tregs) are central in integration and maintenance of immune homeostasis. Since breakdown of self-tolerance is a major culprit in the pathogenesis of systemic lupus erythematosus (SLE), restoration of the immune tolerance through the manipulation of Tregs can be exploited to treat patients with SLE. New information has revealed that Tregs besides their role in suppressing the immune response are important in tissue protection and regeneration. Expansion of Tregs with low-dose IL-2 represents an approach to control the autoimmune response. Moreover, control of Treg metabolism can be exploited to restore or improve their function. Here, we summarize the function and diversity of Tregs and recent strategies to improve their function in patients with SLE.

Transplantation Tolerance through Hematopoietic Chimerism: Progress and Challenges for Clinical Translation

The perception that transplantation of hematopoietic stem cells can confer tolerance to any tissue or organ from the same donor is widely accepted but it has not yet become a treatment option in clinical routine. The reasons for this are multifaceted but can generally be classified into safety and efficacy concerns that also became evident from the results of the first clinical pilot trials. In comparison to standard immunosuppressive therapies, the infection risk associated with the cytotoxic pre-conditioning necessary to allow allogeneic bone marrow engraftment and the risk of developing graft-vs.-host disease (GVHD) constitute the most prohibitive hurdles. However, several approaches have recently been developed at the experimental level to reduce or even overcome the necessity for cytoreductive conditioning, such as costimulation blockade, pro-apoptotic drugs, or Treg therapy. But even in the absence of any hazardous pretreatment, the recipients are exposed to the risk of developing GVHD as long as non-tolerant donor T cells are present. Total lymphoid irradiation and enriching the stem cell graft with facilitating cells emerged as potential strategies to reduce this peril. On the other hand, the long-lasting survival of kidney allografts, seen with transient chimerism in some clinical series, questions the need for durable chimerism for robust tolerance. From a safety point of view, loss of chimerism would indeed be favorable as it eliminates the risk of GVHD, but also complicates the assessment of tolerance. Therefore, other biomarkers are warranted to monitor tolerance and to identify those patients who can safely be weaned off immunosuppression. In addition to these safety concerns, the limited efficacy of the current pilot trials with approximately 40-60% patients becoming tolerant remains an important issue that needs to be resolved. Overall, the road ahead to clinical routine may still be rocky but the first successful long-term patients and progress in pre-clinical research provide encouraging evidence that deliberately inducing tolerance through hematopoietic chimerism might eventually make it from dream to reality.

Negative modulation of suppressive HIV-specific regulatory T cells by IL-2 adjuvanted therapeutic vaccine

The potential benefit in using IL-2 in immunotherapy for cancer and autoimmunity has been linked to the modulation of immune responses, which partly relies on a direct effect on Tregs populations. Here, we revisited the role of IL-2 in HIV infection and investigated whether its use as an adjuvant with therapeutic vaccination, impacts on HIV-specific responses. Antiretroviral therapy treated-patients were randomized to receive 4 boosts of vaccination (ALVACHIV/Lipo-6T, weeks 0/4/8/12) followed by 3 cycles of IL-2 (weeks 16/24/32) before treatment interruption (TI) at week40. IL-2 administration increased significantly HIV-specific CD4⁺CD25⁺CD134⁺ T-cell responses, which inversely correlated with viral load after TI (r = -0.7, p <0.007) in the vaccine/IL-2 group. IL-2 increased global CD25⁺CD127^lowFoxP3⁺Tregs (p <0.05) while it decreased HIV- but not CMV- specific CD39⁺FoxP3⁺CD25⁺CD134⁺Tregs (p <0.05). HIV-specific Tregs were inversely correlated with IFN-γ producing specific-effectors (p = 0.03) and positively correlated with viral load (r = 0.7, p = 0.01), revealing their undesired presence during chronic infection. Global Tregs, but not HIV-specific Tregs, inversely correlated with a decrease in exhausted PD1⁺CD95⁺ T-cells (p = 0.001). Altogether, our results underline the negative impact of HIV-specific Tregs on HIV-specific effectors and reveal the beneficial use of IL-2 as an adjuvant as its administration increases global Tregs that impact on T-cell exhaustion and decreases HIV-specific CD39⁺Tregs by shifting the balance towards effectors.

Interleukin-2 (IL-2) has been used in immunotherapy for cancer and autoimmunity and its beneficial effect has been linked to the modulation of immune responses, which partly relies on a direct effect on Tregs populations. In this study, we assessed the role of IL-2 in HIV infection and investigated whether its use as an adjuvant with therapeutic vaccination, impacts on HIV-specific responses. We show that IL-2 administration increased HIV-specific CD4⁺CD25⁺CD134⁺ T-cell responses which inversely correlated with viral load after treatment interruption in the vaccine/IL-2 group. We also show that IL-2 increased global CD25⁺CD127^lowFoxP3⁺Tregs while it decreased HIV- but not CMV- specific CD39⁺FoxP3⁺CD25⁺CD134⁺Tregs. Moreover, we show that HIV-specific Tregs were inversely correlated with IFN-γ-producing specific-effectors and positively correlated with viral load. Moreover, we show that global Tregs, but not HIV-specific Tregs, inversely correlated with a decrease in exhausted PD1⁺CD95⁺ T-cells. Altogether, our results underline the negative impact of HIV-specific Tregs on HIV-specific effectors and reveal the beneficial use of IL-2 as an adjuvant as its administration increases global Tregs that impact on T-cell exhaustion and decreases HIV-specific CD39⁺Tregs by shifting the balance towards effectors.

Dying to protect: cell death and the control of T-cell homeostasis

T cells play a critical role in immune responses as they specifically recognize peptide/MHC complexes with their T-cell receptors and initiate adaptive immune responses. While T cells are critical for performing appropriate effector functions and maintaining immune memory, they also can cause autoimmunity or neoplasia if misdirected or dysregulated. Thus, T cells must be tightly regulated from their development onward. Maintenance of appropriate T-cell homeostasis is essential to promote protective immunity and limit autoimmunity and neoplasia. This review will focus on the role of cell death in maintenance of T-cell homeostasis and outline novel therapeutic strategies tailored to manipulate cell death to limit T-cell survival (eg, autoimmunity and transplantation) or enhance T-cell survival (eg, vaccination and immune deficiency).

In vitro skin models and tissue engineering protocols for skin graft applications

In this review, we present a brief introduction of the skin structure, a concise compilation of skin-related disorders, and a thorough discussion of different in vitro skin models, artificial skin substitutes, skin grafts, and dermal tissue engineering protocols. The advantages of the development of in vitro skin disorder models, such as UV radiation and the prototype model, melanoma model, wound healing model, psoriasis model, and full-thickness model are also discussed. Different types of skin grafts including allografts, autografts, allogeneic, and xenogeneic are described in detail with their associated applications. We also discuss different tissue engineering protocols for the design of various types of skin substitutes and their commercial outcomes. Brief highlights are given of the new generation three-dimensional printed scaffolds for tissue regeneration applications.

Immunosuppression for in vivo research: state-of-the-art protocols and experimental approaches

Almost every experimental treatment strategy using non-autologous cell, tissue or organ transplantation is tested in small and large animal models before clinical translation. Because these strategies require immunosuppression in most cases, immunosuppressive protocols are a key element in transplantation experiments. However, standard immunosuppressive protocols are often applied without detailed knowledge regarding their efficacy within the particular experimental setting and in the chosen model species. Optimization of such protocols is pertinent to the translation of experimental results to human patients and thus warrants further investigation. This review summarizes current knowledge regarding immunosuppressive drug classes as well as their dosages and application regimens with consideration of species-specific drug metabolization and side effects. It also summarizes contemporary knowledge of novel immunomodulatory strategies, such as the use of mesenchymal stem cells or antibodies. Thus, this review is intended to serve as a state-of-the-art compendium for researchers to refine applied experimental immunosuppression and immunomodulation strategies to enhance the predictive value of preclinical transplantation studies.

Unique patterns of CD8+ T-cell-mediated organ damage in the Act-mOVA/OT-I model of acute graft-versus-host disease

T-cell receptor (TCR)-transgenic models of acute graft-versus-host disease (aGvHD) offer a straightforward and highly controlled approach to study the mechanisms and consequences of T-cell activation following allogeneic hematopoietic stem cell transplantation (aHSCT). Here, we report that aHSCT involving OT-I mice as donors, carrying an ovalbumin-specific CD8+ TCR, and Act-mOVA mice as recipients, expressing membrane-bound ovalbumin driven by the β-actin promoter, induces lethal aGvHD in a CD8+ T-cell-dependent, highly reproducible manner, within 4-7 days. Tracking of UBC-GFP/OT-I graft CD8+ T cells disclosed heavy infiltration of the gastrointestinal tract, liver, and lungs at the onset of the disease, and histology confirmed hallmark features of gastrointestinal aGVHD, hepatic aGvHD, and aGvHD-associated lymphocytic bronchitis in infiltrated organs. However, T-cell infiltration was virtually absent in the skin, a key target organ of human aGvHD, and histology confirmed the absence of cutaneous aGVHD, as well. We show that the model allows studying CD8+ T-cell responses in situ, as selective recovery of graft CD45.1/OT-I CD8+ T cells from target organs is simple and feasible by automated tissue dissociation and subsequent cell sorting. Assessment of interferon-gamma production by flow cytometry, granzyme-B release by ELISA, TREC assay, and whole-genome gene expression profiling confirmed that isolated graft CD8+ T cells remained intact, underwent clonal expansion, and exerted effector functions in all affected tissues. Taken together, these data demonstrate that the OT-I/Act-mOVA model is suitable to study the CD8+ T-cell-mediated effector mechanisms in a disease closely resembling fatal human gastrointestinal and hepatic aGVHD that may develop after aHSCT using HLA-matched unrelated donors.

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.

Antigen-Specific Regulatory T Cells and Low Dose of IL-2 in Treatment of Type 1 Diabetes

Regulatory T cells (Tregs) play an important role in preventing effector T-cell (Teff) targeting of self-antigens that can lead to tissue destruction in autoimmune settings, including type 1 diabetes (T1D). Autoimmunity is caused in part by an imbalance between Teff and Tregs. Early attempts to treat with immunosuppressive agents have led to serious side effects, thus requiring a more targeted approach. Low-dose IL-2 (LD IL-2) can provide immunoregulation with few side effects by preferentially acting on Tregs to drive tolerance. The concept of LD IL-2 as a therapeutic approach is supported by data in mouse models where autoimmunity is cured and further strengthened by success in human clinical studies in hepatitis C virus-induced vasculitis, chronic graft-versus-host disease, and Alopecia areata. Treatment will require identification of a safe therapeutic window, which is a difficult task given that patients are reported to have deficient or defective IL-2 production or signaling and have experienced mild activation of NK cells and eosinophils with LD IL-2 therapy. In T1D, an LD IL-2 clinical trial concluded that Tregs can be safely expanded in humans; however, the study was not designed to address efficacy. Antigen-specific therapies have also aimed at regulation of the autoimmune response but have been filled with disappointment despite an extensive list of diverse islet antigens tested in humans. This approach could be enhanced through the addition of LD IL-2 to the antigenic treatment regimen to improve the frequency and function of antigen-specific Tregs, without global immunosuppression. Here, we will discuss the use of LD IL-2 and islet antigen to enhance antigen-specific Tregs in T1D and focus on what is known about their immunological impact, their safety, and potential efficacy, and need for better methods to identify therapeutic effectiveness.

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.

Preventing Allograft Rejection by Targeting Immune Metabolism

Upon antigen recognition and co-stimulation, T lymphocytes upregulate the metabolic machinery necessary to proliferate and sustain effector function. This metabolic reprogramming in T cells regulates T cell activation and differentiation but is not just a consequence of antigen recognition. Although such metabolic reprogramming promotes the differentiation and function of T effector cells, the differentiation of regulatory T cells employs different metabolic reprogramming. Therefore, we hypothesized that inhibition of glycolysis and glutamine metabolism might prevent graft rejection by inhibiting effector generation and function and promoting regulatory T cell generation. We devised an anti-rejection regimen involving the glycolytic inhibitor 2-deoxyglucose (2-DG), the anti-type II diabetes drug metformin, and the inhibitor of glutamine metabolism 6-diazo-5-oxo-L-norleucine (DON). Using this triple-drug regimen, we were able to prevent or delay graft rejection in fully mismatched skin and heart allograft transplantation models.