Regulation and Privilege in Transplantation Tolerance

Switching off autoimmunity

Discovered 30 years ago, CD40L antagonists are proving to be powerful autoimmune drugs.

The epigenetic promise to improve prognosis of heart failure and heart transplantation

Heart transplantation is still the only possible life-saving treatment for end-stage heart failure, the critical epilogue of several cardiac diseases. Epigenetic mechanisms are being intensively investigated because they could contribute to establishing innovative diagnostic and predictive biomarkers, as well as ground-breaking therapies both for heart failure and heart transplantation rejection. DNA methylation and histone modifications can modulate the innate and adaptive immune response by acting on the expression of immune-related genes that, in turn, are crucial determinants of transplantation outcome. Epigenetic drugs acting on methylation and histone-modification pathways may modulate Treg activity by acting as immunosuppressive agents. Moreover, the identification of non-invasive and reliable epigenetic biomarkers for the prediction of allograft rejection and for monitoring immunosuppressive therapies represents an attractive perspective in the management of transplanted patients. MiRNAs seem to fit particularly well to this purpose because they are differently expressed in patients at high and low risk of rejection and are detectable in biological fluids besides biopsies. Although increasing evidence supports the involvement of epigenetic tags in heart failure and transplantation, further short and long-term clinical studies are needed to translate the possible available findings into clinical setting.

Differential sensitivity of regulatory and effector T cells to cell death: a prerequisite for transplant tolerance

Despite significant progress achieved in transplantation, immunosuppressive therapies currently used to prevent graft rejection are still endowed with severe side effects impairing their efficiency over the long term. Thus, the development of graft-specific, non-toxic innovative therapeutic strategies has become a major challenge, the goal being to selectively target alloreactive effector T cells while sparing CD4⁺Foxp3⁺ regulatory T cells (Tregs) to promote operational tolerance. Various approaches, notably the one based on monoclonal antibodies or fusion proteins directed against the TCR/CD3 complex, TCR coreceptors, or costimulatory molecules, have been proposed to reduce the alloreactive T cell pool, which is an essential prerequisite to create a therapeutic window allowing Tregs to induce and maintain allograft tolerance. In this mini review, we focus on the differential sensitivity of Tregs and effector T cells to the depleting and inhibitory effect of these immunotherapies, with a particular emphasis on CD3-specific antibodies that beyond their immunosuppressive effect, also express potent tolerogenic capacities.

Interpretation of transplant biopsies and immune responses following Treg cell therapy

PURPOSE OF REVIEW

Regulatory T cells (Treg) are now well established as vital participants in maintaining self-tolerance and preventing autoimmunity. Tregs have already been shown to be effective in preventing graft-versus-host disease in clinical bone marrow transplantation, and numerous animal studies have suggested a therapeutic role for Treg in solid organ transplantation. Recent advances in Treg isolation and expansion have the field poised to perform trials of therapeutic Treg infusion in solid organ transplantation worldwide. An important component of these trials will be the detection of infused cells and the assessment of Treg activity after infusion.

RECENT FINDINGS

Several animal studies have demonstrated that infused Treg migrate to transplanted tissue in the early period after transplantation. This finding has important implications for the interpretation of biopsy results in human trials. Recent refinements in Treg identification, quantification, and functional assays will be discussed in the context of immune monitoring.

SUMMARY

Understanding the migration/localization and persistence of infused Treg into transplanted tissues as well as how they impact the peripheral immune response will be critical to the interpretation of early Treg trials.

New generation CD3 monoclonal antibodies: are we ready to have them back in clinical transplantation?

PURPOSE OF REVIEW

The continuing problem of late graft loss and immunosuppressive drug toxicity forces us to explore new treatments for the induction of transplant tolerance. Monoclonal antibodies targeting molecules implicated in lymphocyte activation, in particular CD3/TCR, constitute a promising strategy.

RECENT FINDINGS

Promising results were obtained from the use of antibodies targeting CD3/TCR, coreceptors or costimulatory pathways as tolerance-promoting tools in experimental transplantation. These antibodies do not uniformly depress the immune system but act in an antigen-specific manner by preferentially targeting effector T cells while preserving regulatory T cells. However, translation to the clinic proved to be more difficult than expected. New generation CD3 antibodies, currently used in phase II/III trials in autoimmunity, constitute a promising approach as, beside their immunosuppressive effect, they also express potent tolerogenic capacities. Importantly, CD3 therapy is effective especially when applied in primed hosts, highlighting the importance of the therapeutic window for tolerance induction.

SUMMARY

Further investigations are required for adapting to the clinic monoclonal antibodies as substitutes for current immunosuppression. Our aim is to show that development of new therapeutic strategies/molecules may come from transversal-type research, in particular from experience in autoimmunity, as immune responses leading to autoimmunity and graft rejection involve similar pathways.

Attenuation of donor-reactive T cells allows effective control of allograft rejection using regulatory T cell therapy

Regulatory T cells (Tregs) are essential for the establishment and maintenance of immune tolerance, suggesting a potential therapeutic role for Tregs in transplantation. However, Treg administration alone is insufficient in inducing long-term allograft survival in normal hosts, likely due to the high frequency of alloreactive T cells. We hypothesized that a targeted reduction of alloreactive T effector cells would allow a therapeutic window for Treg efficacy. Here we show that preconditioning recipient mice with donor-specific transfusion followed by cyclophosphamide treatment deleted 70-80% donor-reactive T cells, but failed to prolong islet allograft survival. However, infusion of either 5 × 10(6) Tregs with direct donor reactivity or 25 × 10(6) polyclonal Tregs led to indefinite survival of BALB/c islets in more than 70% of preconditioned C57BL/6 recipients. Notably, protection of C3H islets in autoimmune nonobese diabetic mice required islet autoantigen-specific Tregs together with polyclonal Tregs. Treg therapy led to significant reduction of CD8(+) T cells and concomitant increase in endogenous Tregs among graft-infiltrating cells early after transplantation. Together, these results demonstrate that reduction of the donor-reactive T cells will be an important component of Treg-based therapies in transplantation.

PARP-1 controls immunosuppressive function of regulatory T cells by destabilizing Foxp3

Poly (ADP-ribose) polymerase-1 (PARP-1) is a nuclear enzyme and transcription factor that is involved in inflammatory response, but its role in T cell response remains largely unknown. We show here that PARP-1 regulates the suppressive function of CD4⁺CD25⁺Foxp3⁺ regulatory T cells (Tregs). Specifically, Tregs in mice with a null mutation of the PARP-1 gene (PARP-1^–/–) showed significantly stronger suppressive activity than did wild-type Tregs in culture. We elucidate that this enhanced suppressive function is attributed to sustained higher expression of Foxp3 and CD25 in PARP-1^−/− Tregs. Furthermore, in PARP-1^−/− Tregs, Foxp3 protein shows substantially higher levels of binding to the conserved non-coding DNA sequence 2 (CNS2) at the foxp3 gene, a region important in maintaining Foxp3 gene expression in Tregs. Thus, our data reveal a role for PARP-1 in controlling the function of Tregs through modulation of the stable expression of Foxp3.

Regulatory T-cell therapy for transplantation: how many cells do we need?

PURPOSE OF REVIEW

As regulatory T-cell (Treg) therapy begins to enter the clinic and more clinical trials of Treg therapy are being actively planned for solid organ transplantations, a thorough quantitative assessment of therapeutic dosing is essential for the design of an effective Treg-therapy trial in the solid organ transplant setting.

RECENT FINDINGS

Considering the requirement for a high percentage of Tregs to control transplant rejection in mouse models of transplantation and the total cellularity of the human T-cell compartment, we estimate that it would take billions of Tregs, preferably alloantigen-reactive Tregs, to effectively control transplant rejection in humans. Donor dendritic cells and B cells can be used to selectively expand donor alloantigen-reactive Tregs. Recent improvements in manufacturing alloantigen-reactive Tregs demonstrate that billions of alloantigen-reactive T cells can be manufactured in short-term cultures.

SUMMARY

It is feasible to grow human alloantigen-reactive Tregs up to billions, an optimal number to achieve therapeutic efficacy. Better understanding of Treg lineage commitment and further technological investments are needed to ease the implementation and ensure consistency in Treg manufacturing.

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).

Role of regulatory T cells in the promotion of transplant tolerance

Liver transplantation is now recognized as the most effective therapy for patients with end-stage acute and chronic liver failure. Despite outstanding short-term graft and patient survival, liver transplantation continues to face several major challenges, including poor long-term graft survival due to chronic rejection and major side effects of long-term immunosuppressive therapy (which is required for the prevention of rejection). The ability to produce a state of tolerance after transplantation would potentially obviate long-term immunosuppression. Self-tolerance and immune homeostasis involve both central and peripheral immunoregulatory mechanisms. To date, studies have shown that many subsets of regulatory T cells (Tregs) control immune responses to foreign and alloantigens. The identification of Tregs that are positive for CD4, CD25, and the transcription factor forkhead box (Foxp3) has resulted in major advances in our understanding of the immunology of rejection and the development of transplant tolerance. In this article, we focus on the importance of Tregs in tolerance induction in experimental models of liver transplantation. Furthermore, we discuss the therapeutic potential of Tregs for the promotion of tolerance in transplant patients and highlight recent clinical trials of Treg-based therapies.

CD4(+)Foxp3(+) regulatory T cell therapy in transplantation

Regulatory T cells (Tregs) are long-lived cells that suppress immune responses in vivo in a dominant and antigen-specific manner. Therefore, therapeutic application of Tregs to control unwanted immune responses is an active area of investigation. Tregs can confer long-term protection against auto-inflammatory diseases in mouse models. They have also been shown to be effective in suppressing alloimmunity in models of graft-versus-host disease and organ transplantation. Building on extensive research in Treg biology and preclinical testing of therapeutic efficacy over the past decade, we are now at the point of evaluating the safety and efficacy of Treg therapy in humans. This review focuses on developing therapy for transplantation using CD4(+)Foxp3(+) Tregs, with an emphasis on the studies that have informed clinical approaches that aim to maximize the benefits while overcoming the challenges and risks of Treg cell therapy.

Role of the Fc region in CD70-specific antibody effects on cardiac transplant survival

BACKGROUND

The role of the CD70-specific antibody and the mechanisms by which it extends transplant survival are not known.

METHODS

Fully major histocompatibility complex-mismatched heterotopic heart transplantation (BALB/c to C57BL/6) was performed. Treated mice received intraperitoneal injections of wild-type (WT) CD70-specific antibody (FR70) or IgG1 or IgG2a chimeric antibodies on days 0, 2, 4, and 6 posttransplantation.

RESULTS

WT FR70 antibody significantly extended heart transplant survival to 19 days compared with untreated mice (median survival time [MST]=10 days). Graft survival using the nondepleting IgG1 antibody was significantly shorter (MST=14 days), whereas the survival using depleting IgG2a antibody (MST=18) was similar to that using WT FR70. The FR70 and IgG2a antibodies demonstrated a greater efficiency of fixing mouse complement over the IgG1 variant in vitro. CD4 and CD8 T-cell graft infiltration was reduced with treatment; however, this was most pronounced with WT FR70 and IgG2a antibody therapy compared with the IgG1 chimeric variant. Circulating donor-specific IgG alloantibodies were initially reduced with WT FR70 treatment (day 8 posttransplantation) but increased at days 15 and 20 posttransplantation to the level detected in untreated controls.

CONCLUSION

We conclude that WT (FR70) and the IgG2a depleting variant of CD70-specific antibody reduce graft infiltrating CD4 and CD8 T cells, transiently reduce serum alloantibody levels, and extend graft survival. In contrast, the nondepleting IgG1 variant of this antibody showed lower efficacy. These data suggest that a depleting mechanism of action and not merely costimulation blockade plays a substantial role in the therapeutic effects of CD70-specific antibody.

Stimulation of α7 nicotinic acetylcholine receptor by nicotine increases suppressive capacity of naturally occurring CD4+CD25+ regulatory T cells in mice in vitro

α7 Nicotinic acetylcholine receptor (α7 nAChR) has been found in several non-neuronal cells and is described as an important regulator of cellular function. Naturally occurring CD4(+)CD25(+) regulatory T cells (Tregs) are essential for the active suppression of autoimmunity. The present study investigated whether naturally occurring Tregs expressed α7 nAChR and investigated the functionary role of this receptor in controlling suppressive activity of these cells. We found that CD4(+)CD25(+) Tregs from naive C57BL/6J mice positively expressed α7 nAChR, and its activation by nicotine enhanced the suppressive capacity of Tregs. Nicotine stimulation up-regulated the expression of cytotoxic T-lymphocyte-associated antigen (CTLA)-4 and forkhead/winged helix transcription factor p3 (Foxp3) on Tregs but had no effect on the production of interleukin (IL)-10 and transforming growth factor-β1 by Tregs. In the supernatants of CD4(+)CD25(+) Tregs/CD4(+)CD25(-) T-cell cocultures, we observed a decrease in the concentration of IL-2 in nicotine-stimulated groups, but nicotine stimulation had no effect on the ratio of IL-4/interferon (IFN)-γ, which partially represented T-cell polarization. The above-mentioned effects of nicotine were reversed by a selective α7 nAChR antagonist, α-bungarotoxin. In addition, the ratio of IL-4/IFN-γ was increased by treatment with α-bungarotoxin. We conclude that nicotine might increase Treg-mediated immune suppression of lymphocytes via α7 nAChR. The effect is related to the up-regulation of CTLA-4 as well as Foxp3 expression and decreased IL-2 secretion in CD4(+)CD25(+) Tregs/CD4(+)CD25(-) T-cell coculture supernatants. α7 nAChR seems to be a critical regulator for immunosuppressive function of CD4(+)CD25(+) Tregs.

Intermittent antibody-based combination therapy removes alloantibodies and achieves indefinite heart transplant survival in presensitized recipients

BACKGROUND

It is well established that primed/memory T cells play a critical role in heart transplant rejection. This contributes to the challenges faced in the transplant clinic because current treatments that are efficient in controlling naïve T cell alloresponses have limited efficacy on primed T cell responders.

METHODS

Fully MHC-mismatched heart transplantation was performed from BALB/c to C57BL/6 mice presensitized with BALB/c splenocytes 14 days pretransplantation. A combination therapy comprising CD70-, CD154-, and CD8-specific antibodies (Abs) was administered at day 0 and 4 posttransplantation with rapamycin on days 0 to 4.

RESULTS

The Ab combination therapy extended heart transplant survival in presensitized recipients from median survival time 8 days (MST) to MST 78 days. A decrease in the number of splenic interferon-gamma-secreting cells measured by ELISpot assay was seen in the treated group compared with the untreated controls. However, graft-infiltrating CD8+ and CD4+ T cells persisted despite treatment and the number of intragraft CD4+ T cells increased at day 30 posttransplantation. When an additional "rescue therapy" comprising the same Abs was readministered at days 30, 60, and 90 posttransplantation, T cell infiltration was reduced and indefinite graft survival was observed. Furthermore, rescue therapy resulted in gradual decrease in titer and, by day 90 posttransplantation, the complete loss of the preexisting, donor-specific Abs.

CONCLUSION

We conclude that our Ab combination therapy extends allograft survival in presensitized recipients. When combined with intermittent Ab-mediated rescue therapy, this results in indefinite allograft survival and a loss of the preexisting, donor-specific Abs from the circulation.

Antigen-non-specific regulation centered on CD25+Foxp3+ Treg cells

CD4(+)CD25(+)Foxp3(+) regulatory T cells (Tregs) are of special interest in immunology because of their potent inhibitory function. Many fundamental aspects of Tregs, including their antigenic profile, development and peripheral homeostasis, remain highly controversial. Here, we propose a Treg-centered antigen-non-specific immunoregulation model focused on the T-cell system, particularly on CD4(+) T cells. The T-cell pool consists of naive T cells (Tnais), Tregs and effector T cells (Teffs). Regardless of antigen specificity, the ratio of the activated T-cell subsets (Treg/Teff/Tnai) and their temporal and spatial uniformity dictate the differentiation of Tnais. Activated Tregs inhibit the activation, proliferation, induction and activity of Teffs; in contrast, activated Teffs inhibit the induction of Tregs from Tnais but cooperate with Treg-specific antigens to promote the proliferation and activity of Tregs. In many cases, these interactions are antigen-non-specific, whereas the activation of both Tregs and Teffs is antigen-specific. Memory T-cell subsets are essential for the maintenance of adaptive immune responses, but the antigen-non-specific interactions among T-cell subsets may be more important during the establishment of the adaptive immune system to a newly encountered antigen. This is especially important when new and memory antigens are presented closely-both temporally and spatially-to T cells, because there are always baseline levels of activated Tregs, which are usually higher than levels of memory T cells for new antigens. Based on this hypothesis, we further infer that, under physiological conditions, Tregs in lymph nodes mainly recognize antigens frequently released from draining tissues, and that these self-reactive Tregs are commonly involved in the establishment of adaptive immunity to new antigens and in the feedback control of excessive responses to pathogens.

Anti-CD154 mAb and rapamycin induce T regulatory cell mediated tolerance in rat-to-mouse islet transplantation

Background

Anti-CD154 (MR1) monoclonal antibody (mAb) and rapamycin (RAPA) treatment both improve survival of rat-to-mouse islet xenograft. The present study investigated the effect of combined RAPA/MR1 treatment on rat-to-mouse islet xenograft survival and analyzed the role of CD4⁺CD25⁺Foxp3⁺ T regulatory cells (Treg) in the induction and maintenance of the ensuing tolerance.

Methodology/Principal Findings

C57BL/6 mice were treated with MR1/RAPA and received additional monoclonal anti-IL2 mAb or anti CD25 mAb either early (0–28 d) or late (100–128 d) post-transplantation. Treg were characterised in the blood, spleen, draining lymph nodes and within the graft of tolerant and rejecting mice by flow cytometry and immunohistochemistry. Fourteen days of RAPA/MR1 combination therapy allowed indefinite islet graft survival in >80% of the mice. Additional administration of anti-IL-2 mAb or depleting anti-CD25 mAb at the time of transplantation resulted in rejection (100% and 89% respectively), whereas administration at 100 days post transplantation lead to lower rejection rates (25% and 40% respectively). Tolerant mice showed an increase of Treg within the graft and in draining lymph nodes early post transplantation, whereas 100 days post transplantation no significant increase of Treg was observed. Rejecting mice showed a transient increase of Treg in the xenograft and secondary lymphoid organs, which disappeared within 7 days after rejection.

Conclusions/Significances

These results suggest a critical role for Treg in the induction phase of tolerance early after islet xenotransplantation. These encouraging data support the need of developing further Treg therapy for overcoming the species barrier in xenotransplantation.

CD40-activated B cells are more potent than immature dendritic cells to induce and expand CD4(+) regulatory T cells

CD4(+) regulatory T cells (Tregs) play an important role in maintaining immune tolerance by suppressing pathologic immune responses. The generation of large numbers of antigen-specific Tregs ex vivo is critical for the development of clinical immunotherapy based on the adoptive transfer of Tregs. Both CD40-activated B cells (CD40-B) and immature dendritic cells (imDCs) have been used as professional antigen-presenting cells (APCs) to generate antigen-specific Tregs. However, the efficiencies of CD40-B and imDCs to generate CD4(+) Tregs have not been compared directly and the mechanism driving the generation of these Tregs remains largely unknown. In this study, we found that CD40-B exhibited mature phenotypes and were more able to induce and expand CD4(high)CD25(+) Tregs than imDCs. Moreover, Tregs induced by CD40-B had greater suppressive capacity than those induced by imDCs. The generation of CD4(high)CD25(+) Tregs by CD40-B and imDCs is cell-cell contact dependent and partially relies on the expression of human leukocyte antigen (HLA)-DR and CD80/86. Differences in CD4(high)CD25(+) Treg generation efficiency were largely explained by the production of endogenous IL-2 by CD40-B. Our results suggest that CD40-B is better able to generate large numbers of antigen-specific Tregs than imDCs. Additionally, using CD40-B to generate Tregs may accelerate the clinical use of Treg-based immunotherapy in the treatment of allograft rejection, graft versus host disease (GVHD) and autoimmune diseases.

Induction of tolerance in renal transplantation using splenic transplantation: experimental study in a canine model

OBJECTIVE

To evaluate in a canine model the induction of tolerance to renal transplantation after splenectomy and splenosis.

MATERIALS AND METHODS

This prospective, experimental, comparative, longitudinal study included 4 experimental groups, each comprising 4 dogs. Group 1 (control group) underwent renal transplantation only; group 2 underwent renal transplantation and splenectomy; group 3 underwent renal transplantation and splenosis; and group 4 underwent renal transplantation, splenectomy, and splenosis. Survival and degree of rejection were compared between the 4 groups.

RESULTS

Splenosis improved renal function after transplantation, as indicated by increased serum creatinine concentration (group 3, 6.2 mg/dL vs group 1, 12.9 mg/dL). Comparison of weighted survival curves (corrected for degree of rejection) demonstrated a significant difference between group 2 (66.0 days) and group 4 (66.2 days) vs group 1 (52.7 days) and group 3 (41.2 days) (P = .05, Wilcoxon rank sum test).

CONCLUSION

These results suggest that in this experimental model of renal transplantation, splenosis and splenectomy induce clinical tolerance, as indicated by improved renal function and prolonged recipient survival.