Relevance of regulatory T cell promotion of donor-specific tolerance in solid organ transplantation

Graft-Specific Regulatory T Cells for Long-Lasting, Local Tolerance Induction

BACKGROUND

Solid organ transplantation is hindered by immune-mediated chronic graft dysfunction and the side effects of immunosuppressive therapy. Regulatory T cells (Tregs) are crucial for modulating immune responses post-transplantation; however, the transfer of polyspecific Tregs alone is insufficient to induce allotolerance in rodent models.

METHODS

To enhance the efficacy of adoptive Treg therapy, we investigated different immune interventions in the recipients. By utilizing an immunogenic skin transplant model and existing transplantation medicine reagents, we facilitated the clinical translation of our findings. Specifically, antigen-specific Tregs were used.

RESULTS

Our study demonstrated that combining the available induction therapies with drug-induced T-cell proliferation due to lymphopenia effectively increased the Treg/T effector ratios. This results in significant Treg accumulation within the graft, leading to long-term tolerance after the transfer of antigen-specific Tregs. Importantly, all the animals achieved operational tolerance, which boosted the presence of adoptively transferred Tregs within the graft.

CONCLUSIONS

This protocol offers a means to establish tolerance by utilizing antigen-specific Tregs. These results have promising implications for future trials involving adoptive Treg therapy in organ transplantation.

New insights in immunomodulation for intestinal transplantation

Tolerance is the Holy Grail of solid organ transplantation (SOT) and remains its primary challenge since its inception. In this topic, the seminal contributions of Thomas Starzl at Pittsburgh University outlined foundational principles of graft acceptance and tolerance, with chimerism emerging as a pivotal factor. Immunologically, intestinal transplantation (ITx) poses a unique hurdle due to the inherent characteristics and functions of the small bowel, resulting in increased immunogenicity. This necessitates heavy immunosuppression (IS) while IS drugs side effects cause significant morbidity. In addition, current IS therapies fall short of inducing clinical tolerance and their discontinuation has been proven unattainable in most cases. This underscores the unfulfilled need for immunological modulation to safely reduce IS-related burdens. To address this challenge, the Leuven Immunomodulatory Protocol (LIP), introduced in 2000, incorporates various pro-tolerogenic interventions in both the donor to the recipient, with the aim of facilitating graft acceptance and improving outcome. This review seeks to provide an overview of the current understanding of tolerance in ITx and outline recent advances in this domain.

Chimeric antigen receptor T cell and regulatory T cell therapy in non-oncology diseases: A narrative review of studies from 2017 to 2023

Recently, the remarkable success of chimeric antigen receptor T cell (CAR-T) therapy in treating certain tumors has led to numerous studies exploring its potential application to treat non-oncology diseases. This review discusses the progress and evolution of CAR-T cell therapies for treating non-oncology diseases over the past 5 years. Additionally, we summarize the advantages and disadvantages of CAR-T cell therapy in treating non-oncological diseases and identify any difficulties that should be overcome. After conducting an in-depth analysis of the most recent studies on CAR-T technology, we discuss the key elements of CAR-T therapy, such as developing an effective CAR design for non-oncological diseases, controlling the rate and duration of response, and implementing safety measures to reduce toxicity. These studies provide new insights into different delivery strategies, the discovery of new target molecules, and improvements in the safety of CAR-T therapy for non-oncological diseases.

Immunologic risk stratification of pediatric heart transplant patients by combining HLA-EMMA and PIRCHE-II

Human leukocyte antigen (HLA) molecular mismatch is a powerful biomarker of rejection. Few studies have explored its use in assessing rejection risk in heart transplant recipients. We tested the hypothesis that a combination of HLA Epitope Mismatch Algorithm (HLA-EMMA) and Predicted Indirectly Recognizable HLA Epitopes (PIRCHE-II) algorithms can improve risk stratification of pediatric heart transplant recipients. Class I and II HLA genotyping were performed by next-generation sequencing on 274 recipient/donor pairs enrolled in the Clinical Trials in Organ Transplantation in Children (CTOTC). Using high-resolution genotypes, we performed HLA molecular mismatch analysis with HLA-EMMA and PIRCHE-II, and correlated these findings with clinical outcomes. Patients without pre-formed donor specific antibody (DSA) (n=100) were used for correlations with post-transplant DSA and antibody mediated rejection (ABMR). Risk cut-offs were determined for DSA and ABMR using both algorithms. HLA-EMMA cut-offs alone predict the risk of DSA and ABMR; however, if used in combination with PIRCHE-II, the population could be further stratified into low-, intermediate-, and high-risk groups. The combination of HLA-EMMA and PIRCHE-II enables more granular immunological risk stratification. Intermediate-risk cases, like low-risk cases, are at a lower risk of DSA and ABMR. This new way of risk evaluation may facilitate individualized immunosuppression and surveillance.

Dendritic cells originating exosomal miR-193b-3p induces regulatory T cells to alleviate liver transplant rejection

BACKGROUND

Exosomes exert considerable influence in mediating regulatory T (Treg) cells differentiation, which attach great importance to attenuating acute cellular rejection after liver transplantation (LT). And, miRNAs are known to play essential roles in cell-cell communication delivered by exosomes. However, the function of exosomal miRNAs in regulating Treg cells after LT remains unknown. Here, we performed an expression profiling analysis of exosome-miRNAs from human plasma after LT and investigated their immunoregulatory effects on Treg cells.

METHODS

Fifty-eight LT patients and nine donors were included in this report. miRNA profiles in plasma exosomes were analyzed using next-generation sequencing. Flow cytometry, HE and multiplex immunofluorescent staining were used to identify Treg cells in the liver and peripheral blood. A lentiviral vector system was used to overexpress miR-193b-3p in dendritic cells (DCs), and exosomes isolated from these transfected cells were co-cultured with spleen lymphocytesin vitro. A quantitative Real-time PCR and enzyme-linked immunosorbent assay were used to detect the expression of cytokines.

RESULTS

Treg cell infiltration was increased in the liver along with Th17 and CD8+ T cell, and it was down-regulated in peripheral blood in the acute rejection group. High-throughput sequencing revealed that miR-193b-3p was markedly up-regulated in plasma exosomes of non-rejection LT patients. The NLRP3 inflammasome was screened as a target for miR-193b-3p based on target prediction and functional enrichment analyses. Exosomal miR-193b-3p derived from DCs increased Treg cells as demonstrated in vitro. miR-193b-3p overexpression down-regulated NLRP3 as well as the inflammatory cytokines IL-1β and IL-17A while increasing levels of the cytokines IL-10 and TGF-β.

CONCLUSION

DC derived exosomal miR-193b-3p promoted Treg cells by inhibiting NLRP3 expression. These findings not only provide a new perspective on the mechanisms, but also hold great promise for the treatment or prevention of liver allograft rejection.

Membrane-bound IL-2 improves the expansion, survival, and phenotype of CAR Tregs and confers resistance to calcineurin inhibitors

Background

Regulatory T cells (Tregs) play an important role in the maintenance of immune homeostasis and the establishment of immune tolerance. Since Tregs do not secrete endogenous IL-2, they are especially dependent on external IL-2. IL-2 deficiency leads to lower Treg numbers, instability of the Treg phenotype and loss of immune regulation. After organ transplantation, patients are treated with calcineurin inhibitors (CNIs), which further limits available IL-2. Application of low-dose IL-2 expands Tregs but also activates NK and CD8+ T cells. It was recently shown that graft-specific Tregs recognizing mismatched MHC I molecules via a chimeric antigen receptor were far more potent than polyclonal Tregs in the regulation of immune responses after solid organ transplantation in a humanized mouse model.

Methods

Therefore, our aim was to enhance the function and stability of transferred CAR-Tregs via expression of membrane-associated IL-2 (mbIL-2).

Results

mbIL-2 promoted higher survival, phenotypic stability, and function among CAR-Tregs than observed in clinical trials. The cells were also more stable under inflammatory conditions. In a preclinical humanized mouse model, we demonstrated that mbIL-2 CAR Tregs survive better in the Treg niche than control CAR Tregs and are even resistant to CNI therapy without affecting other Tregs, thus acting mainly in cis.

Discussion

The functional and phenotypic improvements observed after membrane-attached IL-2 expression in CAR-Tregs will be important step for enhancing CAR-Treg therapies currently being tested in clinical trials for use after kidney and liver transplantation as well as in autoimmune diseases.

T cell receptor repertoires within liver allografts are different to those in the peripheral blood

BACKGROUND & AIMS

T cells are the main mediators of allogeneic immune responses. Specific T cell clones can be tracked by their unique T cell receptor (TCR), but specificity and function remain elusive and have not been investigated in human liver biopsies thus far.

METHODS

TCR repertoire analysis of CD4⁺, CD8⁺, and regulatory T cells of the peripheral blood and liver graft was performed in 7 liver transplant recipients with either stable course (non-rejector, NR), subclinical cellular rejection (SCR), or acute cellular rejection (ACR) during an observation period from pre-transplant to 6 years post-transplant. Furthermore, donor-reactive T cells, identified by their expression of CD154 and glycoprotein A repetitions predominant (GARP) after allogeneic activation, were tracked longitudinally in peripheral blood and within the liver allograft.

RESULTS

Although overall clonality of the TCR repertoire did not increase in peripheral blood after liver transplantation, clonality of donor-reactive CD4⁺ and regulatory T cells increased and these clones accumulated within the liver graft. Surprisingly, the TCR repertoires between the liver graft and the periphery were distinct and showed only limited overlap. Notably, during ACR, TCR repertoires aligned suggesting either graft-specific homing or release of activated T cells from the graft.

CONCLUSIONS

This is the first study comparing TCR repertoires between liver grafts and blood in patients with NR, SCR, and ACR. Moreover, we attribute specificity and function to a subgroup of intragraft T cell populations. Given the limited overlap between peripheral blood and intragraft repertoires, future studies investigating function and specificities of T cells after liver transplantation should focus on the intragraft immune response.

LAY SUMMARY

In solid organ transplantation, T cells are key mediators of the recipient's immune response directed at the transplanted organ. In our study, we characterised the T cell repertoire in a cohort of 7 liver transplant recipients. We demonstrate that donor-specific T cells expand clonally and accumulate in the transplanted liver. Moreover, we show that the composition of T cells in peripheral blood differs from the T cells in the liver allograft, only aligning in the context of acute cellular rejection but not in normal graft or subclinical cellular rejection. This indicates that the intragraft immune response is not mirrored in the peripheral blood. Our findings clarify the importance of protocol liver biopsies in identifying intragraft immune responses for future investigations of allo-directed immune responses.

Progress in Translational Regulatory T Cell Therapies for Type 1 Diabetes and Islet Transplantation

Regulatory T cells (Tregs) have become highly relevant in the pathophysiology and treatment of autoimmune diseases, such as type 1 diabetes (T1D). As these cells are known to be defective in T1D, recent efforts have explored ex vivo and in vivo Treg expansion and enhancement as a means for restoring self-tolerance in this disease. Given their capacity to also modulate alloimmune responses, studies using Treg-based therapies have recently been undertaken in transplantation. Islet transplantation provides a unique opportunity to study the critical immunological crossroads between auto- and alloimmunity. This procedure has advanced greatly in recent years, and reports of complete abrogation of severe hypoglycemia and long-term insulin independence have become increasingly reported. It is clear that cellular transplantation has the potential to be a true cure in T1D, provided the remaining barriers of cell supply and abrogated need for immune suppression can be overcome. However, the role that Tregs play in islet transplantation remains to be defined. Herein, we synthesize the progress and current state of Treg-based therapies in T1D and islet transplantation. We provide an extensive, but concise, background to understand the physiology and function of these cells and discuss the clinical evidence supporting potency and potential Treg-based therapies in the context of T1D and islet transplantation. Finally, we discuss some areas of opportunity and potential research avenues to guide effective future clinical application. This review provides a basic framework of knowledge for clinicians and researchers involved in the care of patients with T1D and islet transplantation.

Increased CD40L+PD-1+ follicular helper T cells (Tfh) as a biomarker for predicting calcineurin inhibitor sensitivity against Tfh-mediated B-cell activation/antibody production after kidney transplantation

It is unclear to what extent the development of follicular helper T cells (Tfh) and de novo donor-specific human leukocyte antigen antibody (DSA) production could be influenced by immunosuppressive agents, particularly calcineurin inhibitor (CNI; cyclosporine or tacrolimus), after kidney transplantation. Here, the effects of immunosuppressive agents on Tfh-mediated B-cell activation and antibody production were investigated. In vitro circulating Tfh (cTfh; memory CD4+CXCR5+)/B-cell (CD19+) co-culture assays revealed that CNI considerably inhibited cTfh-mediated B-cell activation and IgG antibody secretion through the suppression of IL-21 and IL-2. Both IL-21 and CD40L up-regulated IL-2 receptors (CD25) on B cells, and anti-CD25 antibody induced apoptosis of activated B cells, resulting in the inhibition of IgG production. The frequency of cTfh-expressed CD40L and PD-1 was elevated in patients with de novo DSA 1 year after transplantation. The degree of inhibition by CNI was dependent on Staphylococcal enterotoxin B-induced CD40L+PD-1+ cTfh up-regulation level. Our data demonstrate that CD40L+PD-1+cTfh could be a marker to implicate individual difference in CNI sensitivity for Tfh-mediated B-cell activation in kidney transplantation.

Mesenchymal Stromal Cells for Transplant Tolerance

In solid organ transplantation lifelong immunosuppression exposes transplant recipients to life-threatening complications, such as infections and malignancies, and to severe side effects. Cellular therapy with mesenchymal stromal cells (MSC) has recently emerged as a promising strategy to regulate anti-donor immune responses, allowing immunosuppressive drug minimization and tolerance induction. In this review we summarize preclinical data on MSC in solid organ transplant models, focusing on potential mechanisms of action of MSC, including down-regulation of effector T-cell response and activation of regulatory pathways. We will also provide an overview of available data on safety and feasibility of MSC therapy in solid organ transplant patients, highlighting the issues that still need to be addressed before establishing MSC as a safe and effective tolerogenic cell therapy in transplantation.

Unveiling the Role of DNA Methylation in Kidney Transplantation: Novel Perspectives toward Biomarker Identification

The burden of chronic kidney disease is dramatically rising, making it a major public health concern worldwide. Kidney transplantation is now the best treatment for patients with end-stage renal disease. Although kidney transplantation may improve survival and quality of life, its long-term results are hampered by immune- and/or non-immune-mediated complications. Thus, the identification of transplanted patients with a higher risk of posttransplant complications has become a big challenge for public health. However, current biomarkers of posttransplant complications have a poor predictive value, rising the need to explore novel approaches for the management of transplant patient. In this review we summarize the emerging literature about DNA methylation in kidney transplant complications, in order to highlight its perspectives toward biomarker identification. In the forthcoming future the monitoring of DNA methylation in kidney transplant patients could become a plausible strategy toward the prevention and/or treatment of kidney transplant complications.

Adoptive transfer of xenoantigen‑stimulated T cell receptor Vβ‑restricted human regulatory T cells prevents porcine islet xenograft rejection in humanized mice

Polyclonal expansion of human regulatory T cells (Tregs) prevents xenogeneic rejection by suppressing effector T cell responses in vitro and in vivo. However, a major limitation to using polyclonally expanded Tregs is that they may cause pan‑immunosuppressive effects. The present study was conducted to compare the ability of ex vivo expanded human xenoantigen‑stimulated Tregs (Xeno‑Treg) and polyclonal Tregs (Poly‑Treg) to protect islet xenografts from rejection in NOD‑SCID interleukin (IL)‑2 receptor (IL2r)γ‑/‑ mice. Human cluster of differentiation (CD)4+CD25+CD127lo Tregs, expanded either by stimulating with porcine peripheral blood mononuclear cells (PBMCs) or anti‑CD3/CD28 beads, were characterized by immune cell phenotyping, T cell receptor (TCR) Vβ CDR3 spectratyping and performing suppressive activity assays in vitro. The efficiency of adoptively transferred ex vivo human Tregs was evaluated in vivo using neonatal porcine islet cell clusters (NICC) transplanted into NOD‑SCID IL‑2rγ‑/‑ mice, which received human PBMCs with or without Xeno‑Treg or Poly‑Treg. Xeno‑Treg, which expressed increased levels of human leukocyte antigen‑DR and secreted higher levels of IL‑10, demonstrated enhanced suppressive capacity in a pig‑human mixed lymphocyte reaction. Spectratypes of TCR Vβ4, Vβ10, Vβ18 and Vβ20 in Xeno‑Treg showed restriction and expanded clones at sizes of 205, 441, 332 and 196 respectively, compared to those of Poly‑Treg. Reconstitution of mice with human PBMCs and Poly‑Treg resulted in NICC xenograft rejection at 63 days. Adoptive transfer with human PBMCs and Xeno‑Treg prolonged islet xenograft survival beyond 84 days, with grafts containing intact insulin‑secreting cells surrounded by a small number of human CD45+ cells. This study demonstrated that adoptive transfer of ex vivo expanded human Xeno‑Treg may potently prevent islet xenograft rejection in humanized NOD‑SCID IL2rγ‑/‑ mice compared with Poly‑Treg. These findings suggested that adoptive Treg therapy may be used for immunomodulation in islet xenotransplantation by minimizing systemic immunosuppression.

Peptide Tk-PQ induces immunosuppression in skin allogeneic transplantation via increasing Foxp3+ Treg and impeding nuclear translocation of NF-κB

Solid organ transplantation is used as the last resort for patients with end-stage disease, but allograft rejection is an unsolved problem. Here, we showed that Tk-PQ, a peptide derived from trichosanthin, had an immune-suppressive effect without obvious cytotoxicity in vitro and in a mouse skin allo-transplantation model. In vitro, treatment of Tk-PQ administrated type 2 T helper cell (Th2)/regulatory T-cell (Treg) cytokines, and increased the ratio of CD4⁺CD25⁺Foxp3⁺ Treg by repressing the PI3K/mTOR pathway. In addition, Tk-PQ decreased NF-κB activation to downregulate pro-inflammatory cytokines. Tk-PQ treatment in the mouse skin transplantation model also caused the similar molecular and cellular phenotypes. Furthermore, Tk-PQ enhanced the suppressive function of Treg by increasing Foxp3 expression, and substantially improved allograft survival. These finding demonstrate that Tk-PQ has the potential to be used in clinical allogeneic transplantation.

Corneal Tissue From Dry Eye Donors Leads to Enhanced Graft Rejection

PURPOSE

To assess the effect of dry eye disease (DED) in graft donors on dendritic cell (DC) maturation, host T-cell sensitization, and corneal allograft rejection.

METHODS

Corneas of control (healthy donor) and DED mice (C57BL/6) were transplanted onto fully allogeneic naive BALB/c recipients (n = 10 mice/group). Long-term allograft survival was evaluated for 8 weeks. Corneas and draining lymph nodes (dLNs) were harvested at posttransplantation day 14 (n = 5 mice/group). The frequencies of MHCII CD11c DCs in the donor corneas and host dLNs and the frequencies of interferon (IFN)-γ and IL-17 CD4 T cells and Foxp3 expression by Tregs in host dLNs were investigated using flow cytometry. The enzyme-linked immunospot assay was used to assess host T-cell allosensitization through direct and indirect pathways (n = 3/group).

RESULTS

Recipients of DED donor corneas showed significantly reduced graft survival (10%) compared with control mice (50% survival, P = 0.022), and had significantly increased frequencies of mature DCs in the grafted cornea (DED donor 44.0% ± 0.36% vs. healthy donor 35.4 ± 0.5%; P < 0.0001) and host dLNs (DED donor 25.1% ± 0.66% vs. healthy donor 13.7% ± 1.6%; P = 0.005). Frequencies of IFN-γ and IL-17 T cells were increased in the dLNs of recipients of DED corneas, whereas the expression (mean fluorescence intensity) of Foxp3 in Tregs was decreased significantly in these mice (DED donor 6004 ± 193 vs. healthy donor 6806 ± 81; P = 0.0002). Enzyme-linked immunospot analysis showed that the direct pathway of allosensitization was significantly amplified in recipients of grafts with DED (P = 0.0146).

CONCLUSIONS

Our results indicate that DED in the donor is a significant risk factor for subsequent corneal allograft rejection.

Early Growth Response Gene 2-Expressing CD4+LAG3+ Regulatory T Cells: The Therapeutic Potential for Treating Autoimmune Diseases

Regulatory T cells (Tregs) are necessary for the maintenance of immune tolerance. Tregs are divided into two major populations: one is thymus derived and the other develops in the periphery. Among these Tregs, CD4⁺CD25⁺ Tregs, which mainly originate in the thymus, have been extensively studied. Transcription factor Foxp3 is well known as a master regulatory gene for the development and function of CD4⁺CD25⁺ Tregs. On the other hand, peripheral Tregs consist of distinct cell subsets including Foxp3-dependent extrathymically developed Tregs and interleukin (IL)-10-producing type I regulatory T (Tr1) cells. Lymphocyte activation gene 3 (LAG3) and CD49b are reliable cell surface markers for Tr1 cells. CD4⁺CD25⁻LAG3⁺ Tregs (LAG3⁺ Tregs) develop in the periphery and produce a large amount of IL-10. LAG3⁺ Tregs characteristically express the early growth response gene 2 (Egr2), a zinc-finger transcription factor, and exhibit its suppressive activity in a Foxp3-independent manner. Although Egr2 was known to be essential for hindbrain development and myelination of the peripheral nervous system, recent studies revealed that Egr2 plays vital roles in the induction of T cell anergy and also the suppressive activities of LAG3⁺ Tregs. Intriguingly, forced expression of Egr2 converts naive CD4⁺ T cells into IL-10-producing Tregs that highly express LAG3. Among the four Egr gene family members, Egr3 is thought to compensate for the function of Egr2. Recently, we reported that LAG3⁺ Tregs suppress humoral immune responses via transforming growth factor β3 production in an Egr2- and Egr3-dependent manner. In this review, we focus on the role of Egr2 in Tregs and also discuss its therapeutic potential for the treatment of autoimmune diseases.

Clinical Grade Regulatory CD4+ T Cells (Tregs): Moving Toward Cellular-Based Immunomodulatory Therapies

Regulatory T cells (Tregs) are CD4⁺ T cells that are key players of immune tolerance. They are powerful suppressor cells, able to impact the function of numerous immune cells, including key effectors of inflammation such as effector T cells. For this reason, Tregs are an ideal candidate for the development of cell therapy approaches to modulate immune responses. Treg therapy has shown promising results so far, providing key knowledge on the conditions in which these cells can provide protection and demonstrating that they could be an alternative to current pharmacological immunosuppressive therapies. However, a more comprehensive understanding of their characteristics, isolation, activation, and expansion is needed to be able design cost effective therapies. Here, we review the practicalities of making Tregs a viable cell therapy, in particular, discussing the challenges faced in isolating and manufacturing Tregs and defining what are the most appropriate applications for this new therapy.

Strategies for long-term preservation of kidney graft function

Kidney transplantation has become a routine procedure in the treatment of patients with kidney failure, and requires collaboration of experts from different disciplines, such as nephrology, surgery, immunology, pathology, infectious disease medicine, cardiology, and oncology. Grafts can be obtained from deceased or living donors, with different logistical requirements and implications for long-term graft patency. 1-year graft survival rates are greater than 95% in many centres but improvement of long-term function remains a challenge. New developments in molecular immunology and computational biology have increased precision of donor and recipient matching of HLA and non-HLA compatibility. Individual omics-wide molecular diagnostics, extracorporeal therapies, and drug developments allow for precise individual decision making and treatment. Tolerance induction by mixed chimerism without toxic conditioning and with a low risk of graft versus host disease is a visionary but realistic goal. Some of these innovations are already used in modern transplant centres and will allow advancement in long-term allograft preservation.

Expression of a Chimeric Antigen Receptor Specific for Donor HLA Class I Enhances the Potency of Human Regulatory T Cells in Preventing Human Skin Transplant Rejection

Regulatory T cell (Treg) therapy using recipient-derived Tregs expanded ex vivo is currently being investigated clinically by us and others as a means of reducing allograft rejection following organ transplantation. Data from animal models has demonstrated that adoptive transfer of allospecific Tregs offers greater protection from graft rejection compared to polyclonal Tregs. Chimeric antigen receptors (CAR) are clinically translatable synthetic fusion proteins that can redirect the specificity of T cells toward designated antigens. We used CAR technology to redirect human polyclonal Tregs toward donor-MHC class I molecules, which are ubiquitously expressed in allografts. Two novel HLA-A2-specific CARs were engineered: one comprising a CD28-CD3ζ signaling domain (CAR) and one lacking an intracellular signaling domain (ΔCAR). CAR Tregs were specifically activated and significantly more suppressive than polyclonal or ΔCAR Tregs in the presence of HLA-A2, without eliciting cytotoxic activity. Furthermore, CAR and ΔCAR Tregs preferentially transmigrated across HLA-A2-expressing endothelial cell monolayers. In a human skin xenograft transplant model, adoptive transfer of CAR Tregs alleviated the alloimmune-mediated skin injury caused by transferring allogeneic peripheral blood mononuclear cells more effectively than polyclonal Tregs. Our results demonstrated that the use of CAR technology is a clinically applicable refinement of Treg therapy for organ transplantation.

Treg therapy in transplantation: a general overview

Solid organ transplantation remains the treatment of choice for end-stage organ failure. Whilst the short-term outcomes post-transplant have improved in the last decades, chronic rejection and immunosuppressant side effects remain an ongoing concern. Hematopoietic stem cell transplantation is a well-established procedure for the treatment of patients with haematological disorders. However, donor T cells are continually primed and activated to react against the host causing graft-versus-host disease (GvHD) that leads to tissue damages and death. Regulatory T cells (Tregs) play an essential role in maintaining tolerance to self-antigens, preventing excessive immune responses and abrogating autoimmunity. Due to their suppressive properties, Tregs have been extensively studied for their use as a cellular therapy aiming to treat GvHD and limit immune responses responsible for graft rejection. Several clinical trials have been conducted or are currently ongoing to investigate safety and feasibility of Treg-based therapy. This review summarizes the general understanding of Treg biology and presents the methods used to isolate and expand Tregs. Furthermore, we describe data from the first clinical trials using Tregs, explaining the limitations and future application of these cells.

Impaired Function of Peripherally Induced Regulatory T Cells in Hosts at High Risk of Graft Rejection

Regulatory T cells (Tregs) are crucial for allograft survival. Tregs can be divided into thymus-derived natural Tregs (tTregs) and peripherally-derived induced Tregs (pTregs). Here, we determine whether the suppressive function of Treg subsets is hampered in hosts who are at high risk for rejecting their graft. To induce graft beds that promote high risk of transplant rejection, intrastromal corneal sutures were placed two weeks prior to the transplant procedure in mice. We demonstrate that in high-risk recipients the frequencies and function of pTregs (but not tTregs) are suppressed. Reduced function of pTregs correlated with decreased expression of CTLA-4, interleukin-10, and transforming growth factor-β. Adoptive transfer of pTregs from mice at low risk of subsequent graft rejection is able to rescue graft survival in recipients that are at high risk of rejecting their grafts. Our data suggest that impaired function of pTregs, but not tTregs, mediates the loss of immune tolerance and promotes allograft rejection.

MiR-142-5p and miR-486-5p as biomarkers for early detection of chronic antibody-mediated rejection in kidney transplantation

De novo donor-specific HLA antibody (DSA) would not necessarily contribute to chronic antibody-mediated rejection (CAMR) in kidney transplantation. Here, we investigated whether PBMC miRNAs could be predictable biomarkers for CAMR. Microarray profiling of 435 mature miRNAs in pooled samples was conducted. Individual analysis revealed that miR-142-5p was significantly (p < 0.01) underexpressed in patients with DSA. After DSA production, miR-486-5p and its target PTEN/foxO3 mRNA were significantly overexpressed (p < 0.01) and underexpressed (p < 0.01), respectively, in patients with biopsy-proven CAMR, compared with non-CAMR. Our studies suggest that miRNA expression patterns may serve as noninvasive diagnostic biomarkers to evaluate immune response and kidney allograft status.

Autoantigen-specific immunosuppression with tolerogenic peripheral blood cells prevents relapses in a mouse model of relapsing-remitting multiple sclerosis

BACKGROUND

Dendritic cells (DCs) rendered suppressive by treatment with mitomycin C and loaded with the autoantigen myelin basic protein demonstrated earlier their ability to prevent experimental autoimmune encephalomyelitis (EAE), the animal model for multiple sclerosis (MS). This provides an approach for prophylactic vaccination against autoimmune diseases. For clinical application such DCs are difficult to generate and autoantigens hold the risk of exacerbating the disease.

METHODS

We replaced DCs by peripheral mononuclear cells and myelin autoantigens by glatiramer acetate (Copaxone(®)), a drug approved for the treatment of MS. Spleen cells were loaded with Copaxone(®), incubated with mitomycin C (MICCop) and injected into mice after the first bout of relapsing-remitting EAE. Immunosuppression mediated by MICCop was investigated in vivo by daily assessment of clinical signs of paralysis and in in vitro restimulation assays of peripheral immune cells. Cytokine profiling was performed by enzyme-linked immunosorbent assay (ELISA). Migration of MICCop cells after injection was examined by biodistribution analysis of (111)Indium-labelled MICCop. The number and inhibitory activity of CD4(+)CD25(+)FoxP3(+) regulatory T cells were analysed by histology, flow cytometry and in vitro mixed lymphocyte cultures. In order to assess the specificity of MICCop-induced suppression, treated EAE mice were challenged with the control protein ovalbumin. Humoral and cellular immune responses were then determined by ELISA and in vitro antigen restimulation assay.

RESULTS

MICCop cells were able to inhibit the harmful autoreactive T-cell response and prevented mice from further relapses without affecting general immune responses. Administered MICCop migrated to various organs leading to an increased infiltration of the spleen and the central nervous system with CD4(+)CD25(+)FoxP3(+) cells displaying a suppressive cytokine profile and inhibiting T-cell responses.

CONCLUSION

We describe a clinically applicable cell therapeutic approach for controlling relapses in autoimmune encephalomyelitis by specifically silencing the deleterious autoimmune response.

Epigenetics in Kidney Transplantation: Current Evidence, Predictions, and Future Research Directions

Epigenetic modifications are changes to the genome that occur without any alteration in DNA sequence. These changes include cytosine methylation of DNA at cytosine-phosphate diester-guanine dinucleotides, histone modifications, microRNA interactions, and chromatin remodeling complexes. Epigenetic modifications may exert their effect independently or complementary to genetic variants and have the potential to modify gene expression. These modifications are dynamic, potentially heritable, and can be induced by environmental stimuli or drugs. There is emerging evidence that epigenetics play an important role in health and disease. However, the impact of epigenetic modifications on the outcomes of kidney transplantation is currently poorly understood and deserves further exploration. Kidney transplantation is the best treatment option for end-stage renal disease, but allograft loss remains a significant challenge that leads to increased morbidity and return to dialysis. Epigenetic modifications may influence the activation, proliferation, and differentiation of the immune cells, and therefore may have a critical role in the host immune response to the allograft and its outcome. The epigenome of the donor may also impact kidney graft survival, especially those epigenetic modifications associated with early transplant stressors (e.g., cold ischemia time) and donor aging. In the present review, we discuss evidence supporting the role of epigenetic modifications in ischemia-reperfusion injury, host immune response to the graft, and graft response to injury as potential new tools for the diagnosis and prediction of graft function, and new therapeutic targets for improving outcomes of kidney transplantation.

Hepatic Infiltrates in Operational Tolerant Patients After Liver Transplantation Show Enrichment of Regulatory T Cells Before Proinflammatory Genes Are Downregulated

Immunosuppression can be discontinued from selected and stable patients after liver transplantation resulting in spontaneous operational tolerance (SOT), although the underlying mechanisms remain elusive. Thus, we analyzed serial liver biopsy specimens from adult liver recipients enrolled in a prospective multicenter immunosuppression withdrawal trial that used immunophenotyping and transcriptional profiling. Liver specimens were collected before the initiation of weaning, at the time of rejection, or at 1 and 3 years after complete drug discontinuation. Unexpectedly, the tolerated grafts developed portal tract expansion with increased T cell infiltration after immunosuppression withdrawal. This was associated with transient and preferential accumulation of CD4(+) FOXP3(+) cells and a trend toward upregulation of immune activation and regulatory genes, without signs of rejection. At the same time, no markers of endothelial damage or activation were noted. Portal infiltrates persisted at 3 years but were characterized by decreased expression of genes associated with chronic immunological damage. Further, SOT was not associated with a progressive liver fibrosis up to 5 years. These data suggest that SOT involves several mechanisms: a long-lasting local immune cell persistence with a transient regulatory T cells accumulation followed by a downregulation of immune-activated genes over years. These results have important implications for designs and follow-up of weaning trials.

Neurotoxin from Naja naja atra venom inhibits skin allograft rejection in rats

BACKGROUND

Recent studies reported that Naja naja atra venom (NNAV) regulated immune function and had a therapeutic effect on adjunctive arthritis and nephropathy. We hypothesized that NNAV and its active component, neurotoxin (NTX), might inhibit skin allograft rejection.

METHODS

Skin allografts were used to induce immune rejection in rats. In addition, mixed lymphocyte culture (MLC) was used to mimic immune rejection reaction in vitro. Both NNAV and NTX were orally given starting from 5days prior to skin allograft surgery.

RESULTS

The results showed that oral administration of NNAV or NTX prolonged the survival of skin allografts and inhibited inflammatory response. The production of Th1 cytokines (IFN-γ, IL-2) was also suppressed. NTX inhibited T-cell proliferation and CD4(+) T cell division induced by skin allografts. NTX also showed immunosuppressive activity in mixed lymphocyte culture. Atropine alone inhibited Con A-induced proliferation of T cells and potentiated NTX' s inhibitory effects on T cells, while pilocarpine only slightly enhanced Con A-induced T cell proliferation and partially reversed the inhibitory effect of NTX. On the other hand, neither nicotine nor mecamylamine had an influence on NTX's inhibitory effects on Con A-induced T cell proliferation in vitro. NTX inhibited T cell proliferation by arresting the cell cycle at the G0/G1 phase.

CONCLUSIONS

The present study revealed that NNAV and NTX suppressed skin allograft rejection by inhibiting T cell-mediated immune responses. These findings suggest both NNAV and NTX as potential immunosuppressants for preventing the immune response to skin allografts.

Rapamycin and CTLA4Ig synergize to induce stable mixed chimerism without the need for CD40 blockade

The mixed chimerism approach achieves donor-specific tolerance in organ transplantation, but clinical use is inhibited by the toxicities of current bone marrow (BM) transplantation (BMT) protocols. Blocking the CD40:CD154 pathway with anti-CD154 monoclonal antibodies (mAbs) is exceptionally potent in inducing mixed chimerism, but these mAbs are clinically not available. Defining the roles of donor and recipient CD40 in a murine allogeneic BMT model, we show that CD4 or CD8 activation through an intact direct or CD4 T cell activation through the indirect pathway is sufficient to trigger BM rejection despite CTLA4Ig treatment. In the absence of CD4 T cells, CD8 T cell activation via the direct pathway, in contrast, leads to a state of split tolerance. Interruption of the CD40 signals in both the direct and indirect pathway of allorecognition or lack of recipient CD154 is required for the induction of chimerism and tolerance. We developed a novel BMT protocol that induces mixed chimerism and donor-specific tolerance to fully mismatched cardiac allografts relying on CD28 costimulation blockade and mTOR inhibition without targeting the CD40 pathway. Notably, MHC-mismatched/minor antigen-matched skin grafts survive indefinitely whereas fully mismatched grafts are rejected, suggesting that non-MHC antigens cause graft rejection and split tolerance.

Overcoming immunological barriers in regenerative medicine

Regenerative therapies that use allogeneic cells are likely to encounter immunological barriers similar to those that occur with transplantation of solid organs and allogeneic hematopoietic stem cells (HSCs). Decades of experience in clinical transplantation hold valuable lessons for regenerative medicine, offering approaches for developing tolerance-induction treatments relevant to cell therapies. Outside the field of solid-organ and allogeneic HSC transplantation, new strategies are emerging for controlling the immune response, such as methods based on biomaterials or mimicry of antigen-specific peripheral tolerance. Novel biomaterials can alter the behavior of cells in tissue-engineered constructs and can blunt host immune responses to cells and biomaterial scaffolds. Approaches to suppress autoreactive immune cells may also be useful in regenerative medicine. The most innovative solutions will be developed through closer collaboration among stem cell biologists, transplantation immunologists and materials scientists.

Advances and challenges in immunotherapy for solid organ and hematopoietic stem cell transplantation

Although major advances have been made in solid organ and hematopoietic stem cell transplantation in the last 50 years, big challenges remain. This review outlines the current immunological limitations for hematopoietic stem cell and solid organ transplantation and discusses new immune-modulating therapies in preclinical development and in clinical trials that may allow these obstacles to be overcome.

CTLA4-Ig preserves thymus-derived T regulatory cells

BACKGROUND

CTLA-4 immunoglobulin fusion proteins (CTLA4-Ig) suppress immune reactions by blocking the T-cell costimulatory CD28-CD80-86 pathway and are used in clinical trials for diseases featuring exaggerated T-cell reactivity including autoimmune diseases and allograft rejection. However, because CTLA4-Ig has been suspected to interfere with T regulatory (Treg) cell homeostasis and function, recently, substantial concerns on CTLA4-Ig's potentially antitolerogenic effects have been raised.

METHODS

We tested immunoregulatory CTLA4-Ig explicitly for its effect on Treg cell numbers, frequencies and function in an in vitro murine major histocompatibility complex mismatched setting using C57BL/6 bone marrow-derived dendritic cells as stimulators of allogeneic Balb/c Foxp3 T cells, which allowed for tracing Treg cells in a straightforward fashion.

RESULTS

The presence of CTLA4-Ig in mixed leukocyte reactions-while dampening the global proliferative response of allostimulated Balb/c T cells-resulted in a relative increase of the frequency of thymus-derived CD4CD25Foxp3 Treg cells with intact suppressive activity. This relative increase was caused by a selective inhibitory effect of CTLA4-Ig on proliferating conventional T cells, whereas the proliferative capacity of Treg cells in cell cultures remained unaffected. Additionally, in the presence of CTLA4-Ig, the frequency of apoptosis was decreased in these cells.

CONCLUSION

Our findings unequivocally demonstrate that CTLA4-Ig does not negatively affect Treg cell frequencies and function in vitro.

Regulatory and Effector Helper T-Cell Profile after Nerve Xenografting in the Toll-Like Receptor-Deficient Mice

INTRODUCTION

The balance between regulatory T cells (Tregs) and effector T help cells (Th cells) is critical for the control of adaptive immune response during nerve transplantation. However, whether the homeostasis of immune regulation between Tregs and Th cells requires toll-like receptor (TLR) signaling is unclear. The aim of this study is to profile the distribution of spleen Tregs and Th cells in a mouse model of nerve xenografting in the TLR2 and NF-κB gene knockout mice.

METHODS

The sciatic nerve was taken from a SD rat or an allogeneic mouse and transplanted to a right back leg of recipient C57BL/6, TLR2(-/-), or NF-κB(-/-) mice by subcutaneous transplantation. After 7 days, the T lymphocytes were then isolated from spleen, stained with phenotyping kits, and analyzed by flow cytometry.

RESULTS

The results showed that Tregs were decreased after nerve xenografting in the recipient C57BL/6 mouse. In addition, nerve xenografting also increased the Th1 and Th17 but not the Th2 cell populations. In contrast, amelioration of the Tregs elimination was found in TLR2(-/-) and NF-κB(-/-) mice after transplantation of the nerve xenograft. Moreover, the mice lacking TLR2 or NF-κB showed attenuation of the increase in Th1 and Th17 cells after nerve xenografting.

CONCLUSIONS

TLR signaling is involved in T cell population regulation during tissue transplantation. Knock-out of TLR2 and NF-κB prevented Tregs elimination and inhibited Th1- and Th17-driven immune response after nerve xenografting. This study highlighted the potential of inhibiting TLR signaling to modulate T cell-mediated immune regulation to facilitate tolerance to nerve transplantation.

Synergic silencing of costimulatory molecules prevents cardiac allograft rejection

Background

While substantial progress has been made in blocking acute transplant rejection with the advent of immune suppressive drugs, chronic rejection, mediated primarily by recipient antigen presentation, remains a formidable problem in clinical transplantation. We hypothesized that blocking co-stimulatory pathways in the recipient by induction of RNA interference using small interference RNA (siRNA) expression vectors can prolong allogeneic heart graft survival.

Method

Vectors expressing siRNA specifically targeting CD40 and CD80 were prepared. Recipients (BALB/c mice) were treated with CD40 and/or CD80 siRNA expression vectors via hydrodynamic injection. Control groups were injected with a scrambled siRNA vector and sham treatment (PBS). After treatment, a fully MHC-mismatched (BALB/c to C57/BL6) heart transplantation was performed.

Result

Allogeneic heart graft survival (>100 days) was approximately 70% in the mice treated simultaneously with CD40 and CD80 siRNA expression vectors with overall reduction in lymphocyte interstitium infiltration, vascular obstruction, and edema. Hearts transplanted into CD40 or CD80 siRNA vector-treated recipients had an increased graft survival time compared to negative control groups, but did not survive longer than 40 days. In contrast, allogenic hearts transplanted into recipients treated with scrambled siRNA vector and PBS stopped beating within 10–16 days. Real-time PCR (RT-PCR) and flow cytometric analysis showed an upregulation of FoxP3 expression in spleen lymphocytes and a concurrent downregulation of CD40 and CD80 expression in splenic dendritic cells of siRNA-treated mice. Functional suppressive activity of splenic dendritic cells (DCs) isolated from tolerant recipients was demonstrated in a mixed lymphocyte reaction (MLR). Furthermore, DCs isolated from CD40- and CD80-treated recipients promoted CD4 + CD25 + FoxP3+ regulatory T cell differentiation in vitro.

Conclusion

This study demonstrates that the simultaneous silencing of CD40 and CD80 genes has synergistic effects in preventing allograft rejection, and may therefore have therapeutic potential in clinical transplantation.

CD4+CD25+Foxp3+ Treg cells and liver transplant tolerance

Several subgroups of regulatory T (Treg) cells play crucial roles in both induction and maintenance of immune tolerance to self-antigens and alloantigens. The most extensively and comprehensively studied regulatory T cell subgroup is CD4⁺CD25⁺Foxp3⁺ Treg cells. Numerous studies indicate that insufficiency or dysfunction of CD4⁺CD25⁺Foxp3⁺ Treg cells is responsible for the development of many autoimmune diseases and rejections after organ transplantation. Therefore, harnessing CD4⁺CD25⁺Foxp3⁺ Treg cells may provide a promising approach for inducing and maintaining liver transplant tolerance. In this review, we will focus on the history and classification of regulatory T cells, the mechanisms by which regulatory T cells induce transplant tolerance and their roles in liver transplant tolerance.

Prospects for therapeutic tolerance in humans

PURPOSE OF REVIEW

To provide an overview of recent advances and future possibilities for therapeutic tolerance.

RECENT FINDINGS

Allograft survival despite complete immunosuppressant withdrawal has been demonstrated in selected renal-transplant recipients with haematopoietic chimerism. Early clinical trials of mesenchymal stromal cell therapy have shown promising results in several autoimmune diseases. Regulatory T cells show potential benefit in graft versus host disease, although challenges to ex-vivo expansion remain. Targeted modulation of T-cell function in vivo with monoclonal antibodies has shown beneficial effects in phase II/III trials of multiple sclerosis (alemtuzumab) and type I diabetes mellitus (teplizumab, otelixizumab). Emerging data from animal models suggest an important role for the commensal microbiome in the maintenance and disruption of immune tolerance with parallels in human studies.

SUMMARY

After years of slow progress, recent research has reduced the translational gap between animal models and clinical therapeutic tolerance. Early detection of autoimmunity, potentially at preclinical stages, offers a window of opportunity for tolerogenic therapy. Reliable biomarkers of tolerance are urgently needed to provide objective measurements of the effectiveness of tolerogenic therapies, and to allow intelligent immunosuppressant withdrawal in patients whose autoimmune disease is stable.

VIDEO ABSTRACT AVAILABLE

See the Video Supplementary Digital Content 1 (http://links.lww.com/COR/A8).

Infiltrating Foxp3(+) regulatory T cells from spontaneously tolerant kidney allografts demonstrate donor-specific tolerance

Foxp3(+) regulatory T cells (Tregs) have an essential role in immune and allograft tolerance. However, in both kidney and liver transplantation in humans, FOXP3(+) Tregs have been associated with clinical rejection. Therefore, the role and function of graft infiltrating Tregs have been of great interest. In the studies outlined, we demonstrated that Foxp3(+) Tregs were expanded in tolerant kidney allografts and in draining lymph nodes in the DBA/2 (H-2(d) ) to C57BL/6 (H-2(b) ) mouse spontaneous kidney allograft tolerance model. Kidney allograft tolerance was abrogated after deletion of Foxp3(+) Tregs in DEpletion of REGulatory T cells (DEREG) mice. Kidney allograft infiltrating Foxp3(+) Tregs (K-Tregs) expressed elevated levels of TGF-β, IL-10, interferon gamma (IFN-γ), the transcriptional repressor B lymphocyte-induced maturation protein-1 (Blimp-1) and chemokine receptor 3 (Cxcr3). These K-Tregs had the capacity to transfer dominant tolerance and demonstrate donor alloantigen-specific tolerance to skin allografts. This study demonstrated the crucial role, potency and specificity of graft infiltrating Foxp3(+) Tregs in the maintenance of spontaneously induced kidney allograft tolerance.

Regulatory T-cell therapy in transplantation: moving to the clinic

Regulatory T cells (Tregs) are essential to transplantation tolerance and their therapeutic efficacy is well documented in animal models. Moreover, human Tregs can be identified, isolated, and expanded in short-term ex vivo cultures so that a therapeutic product can be manufactured at relevant doses. Treg therapy is being planned at multiple transplant centers around the world. In this article, we review topics critical to effective implementation of Treg therapy in transplantation. We will address issues such as Treg dose, antigen specificity, and adjunct therapies required for transplant tolerance induction. We will summarize technical advances in Treg manufacturing and provide guidelines for identity and purity assurance of Treg products. Clinical trial designs and Treg manufacturing plans that incorporate the most up-to-date scientific understanding in Treg biology will be essential for harnessing the tolerogenic potential of Treg therapy in transplantation.

Clinical application of regulatory T cells in type 1 diabetes

Regulatory T cells (Tregs) are responsible for the maintenance of peripheral tolerance. Animal studies have shown that administration of Tregs can prevent type 1 diabetes (DM1). Several clinical trials attempted to induce Tregs with various agents, and thus provide long-term tolerance of β cells in DM1. Nevertheless, most of these studies have focused on clinical parameters (e.g. C-peptide) and not Treg numbers nor their function after treatment. Therefore, it is not possible to conclude if the majority of these therapies failed because the drugs did not induce Tregs, or if they failed despite Treg expansion. The current knowledge regarding Tregs, along with our experience in Treg therapy of patients with graft versus host disease, prompted us to use ex vivo expanded Tregs in 10 children with recent-onset DM1. No adverse effects in the treated individuals were observed. There was a significant increase in Treg number in peripheral blood immediately after the treatment administration, while the first clinical differences between treated and control patients were observed 4 months after Treg injection. Treated individuals had higher C-peptide levels and lower insulin requirements than non-treated children. Eleven months after diagnosis of DM1, there are still 2 individuals who are independent of exogenous insulin. These results indicate that autologous Tregs are a safe and well-tolerated therapy in children with DM1, which can inhibit or delay the destruction of pancreatic β cells. Additionally, Tregs can be a useful tool for local protection of transplanted pancreatic islets. Isolation and expansion of antigen-specific Tregs is one of the directions for future studies on cellular therapy of DM1.

Epigenetic modulation of the immune function: a potential target for tolerance

Great efforts in the field of solid organ transplantation are being devoted to identifying biomarkers that allow a transplanted patient's immune status to be established. Recently, it has been well documented that epigenetic mechanisms like DNA methylation and histone modifications regulate the expression of immune system-related genes, modifying the development of the innate and adaptive immune responses. An in-depth knowledge of these epigenetic mechanisms could modulate the immune response after transplantation and to develop new therapeutic strategies. Epigenetic modifiers, such as histone deacetylase (HDAC) inhibitors have considerable potential as anti-inflammatory and immunosuppressive agents, but their effect on transplantation has not hitherto been known. Moreover, the detection of epigenetic marks in key immune genes could be useful as biomarkers of rejection and progression among transplanted patients. Here, we describe recent discoveries concerning the epigenetic regulation of the immune system, and how this knowledge could be translated to the field of transplantation.

Fetal regulatory T cells and peripheral immune tolerance in utero: implications for development and disease

The developing fetus must actively learn to tolerate benign antigens or suffer the consequences of broken tolerance. Tolerance of self-antigens prevents development of autoimmune diseases and is achieved by both deletion of autoreactive T cell clones in the thymus (central tolerance) and by the suppressive influence of CD4(+)  CD25(+)  FoxP3(+) regulatory T cells (Tregs) in the periphery. Fetal CD4(+) T cells have a strong predisposition to differentiate into tolerogenic Tregs that actively promote self-tolerance, as well as tolerance to non-inherited antigens on chimeric maternal cells that reside in fetal tissues. As the fetus nears birth, a crucial transition must occur between the tolerogenic fetal immune system and a more defensive adult-type immune system that is able to combat pathogens. This paper will review the unique tolerogenic nature of fetal T cells and will examine evidence for a novel model of fetal immune development: the layered immune system hypothesis.

Cell-based therapies for ocular inflammation

Since the plasticity and the potential for re-programming cells has become widely accepted, there has been great interest in cell-based therapies. These are being applied to a range of diseases, not least ocular diseases, where it is assumed that there is a reduced risk of immune rejection although this may be more perceived than real. There are two broad classes of cell-based therapies: those aimed at restoring structure and function of specific tissues and cells; and those directed towards restoring immunological homeostasis by controlling the damaging effects of inflammatory disease. Stem cells of all types represent the first group and prototypically have been used with the aim of regenerating failing cells. In contrast, immune cells have been suggested as potential modulators of inflammation. However, there is functional overlap in these two applications, with some types of stem cells, such as mesenchymal stem cells, demonstrating a potent immunomodulatory effect. This review summarises recent information on cell based therapies for ocular disease, with special emphasis on ocular inflammatory disease, and explores current uses, potential and limitations.

Non-myeloablative conditioning is sufficient to induce mixed chimerism and subsequent acceptance of donor specific cardiac and skin grafts

Organ transplant recipients have elevated cancer and viral infection risks due to immunosuppression and long-term results of organ transplantation remain unsatisfactory, mainly because of chronic rejection. The purpose of the current study is to establish a nonmyeloablative perioperative regimen, able to induce mixed chimerism and tolerance of allografts. To establish a nonmyeloablative perioperative regimen, we used Busulfan, an important component of many bone marrow transplantation preparative regimens for a variety of non-neoplastic diseases as an alternative to total body irradiation (TBI), and FTY720, a unique immunosuppression agent, inhibition lymphocyte homing. We found that creating a lymphohematopoietic chimera in which donor and recipient hematopoiesis coexist resulted in prolongation of the donor specific heart and skin allografts. Consistent with graft survival, pathological analysis indicated that the allografts from tolerant recipients were free of myocardial injury and had only a few interstitial infiltrates, and obliterative vasculopathy was not observed. Furthermore, we found that Treg cells were increased in the long-term graft acceptance recipients. Our data revealed that the therapeutic potential for using hematopoietic chimerism in non-myeloablated recipients hope the advances in rodent models described above in the development of minimal, nontoxic host conditioning regimens for mixed chimerism induction and subsequent acceptance of donor specific grafts.

The where and when of T cell regulation in transplantation

Multiple cell types contribute to the peripheral regulation of T cell alloresponses in haematopoieitc cell transplantation (HCT) and solid organ transplantation (SOT). Of these, regulatory T cells (Tregs) are the principal players and have shown the greatest success in the therapeutic control of detrimental immune responses. Investigations into the induction, location, and mechanism of suppression utilised by Tregs to control alloreactive responses are ongoing. The activation and homing characteristics of Tregs are important to their regulatory capabilities, with activation and homing occurring in the same time and space as conventional T cells. This review discusses these characteristics and recent advances in the field as we move closer to the ultimate goal of utilising Tregs as treatment for allograft rejection and graft-versus-host disease (GvHD).