Phenotype, distribution and alloreactive properties of memory T cells from cynomolgus monkeys

T-cell exhaustion and stemness in antitumor immunity: Characteristics, mechanisms, and implications

T cells play a crucial role in the regulation of immune response and are integral to the efficacy of cancer immunotherapy. Because immunotherapy has emerged as a promising treatment for cancer, increasing attention has been focused on the differentiation and function of T cells in immune response. In this review, we describe the research progress on T-cell exhaustion and stemness in the field of cancer immunotherapy and summarize advances in potential strategies to intervene and treat chronic infection and cancer by reversing T-cell exhaustion and maintaining and increasing T-cell stemness. Moreover, we discuss therapeutic strategies to overcome T-cell immunodeficiency in the tumor microenvironment and promote continuous breakthroughs in the anticancer activity of T cells.

Analysis of T-Cell Receptor Repertoire in Transplantation: Fingerprint of T Cell-mediated Alloresponse

T cells play a key role in determining allograft function by mediating allogeneic immune responses to cause rejection, and recent work pointed their role in mediating tolerance in transplantation. The unique T-cell receptor (TCR) expressed on the surface of each T cell determines the antigen specificity of the cell and can be the specific fingerprint for identifying and monitoring. Next-generation sequencing (NGS) techniques provide powerful tools for deep and high-throughput TCR profiling, and facilitate to depict the entire T cell repertoire profile and trace antigen-specific T cells in circulation and local tissues. Tailing T cell transcriptomes and TCR sequences at the single cell level provides a full landscape of alloreactive T-cell clones development and biofunction in alloresponse. Here, we review the recent advances in TCR sequencing techniques and computational tools, as well as the recent discovery in overall TCR profile and antigen-specific T cells tracking in transplantation. We further discuss the challenges and potential of using TCR sequencing-based assays to profile alloreactive TCR repertoire as the fingerprint for immune monitoring and prediction of rejection and tolerance.

d-Amino acid substitutions and dimerization increase the biological activity and stability of an IL-15 antagonist peptide

Interleukin (IL)-15 plays an important role in several inflammatory diseases. We have previously identified an IL-15 antagonist called P8 peptide, which binds specifically to IL-15 receptor alpha subunit. However, the P8 peptide rapidly degraded by proteases, limiting its therapeutic application. Thus, we replaced each P8 peptide l-amino acid by its corresponding d-isomers. First, we determined the biological activity of the resulting peptides in a proliferation assay by using CTLL-2 cells. The substitution of l-Ala by d-Ala ([A4a]P8 peptide) increased the inhibitory effect of the P8 peptide in CTLL-2 cells in five-fold. In addition to that, the [A4a]P8 peptide dimer showed the most inhibitory effect. To protect the [A4a]P8 peptide and its dimer against exopeptidase activity, we acetylated the N-terminal of these peptides. At least a three-fold reduction in antagonist activity of acetylated peptides was exhibited. However, the substitution of the N-terminal l-Lys residue of [A4a]P8 peptide and its dimer by d-Lys ([K1k;A4a]P8 peptide) did not affect the antagonist effect of the aforementioned peptides. The [K1k;A4a]P8 peptide dimer was stable to the degradation of trypsin, chymotrypsin, and pepsin up until 48 min. Also, the safety and immunogenicity studies in healthy BALB/c mice demonstrated that the administration of this peptide did not affect the clinical parameters of the animals nor generated antipeptide antibodies. Our findings reveal that two distinct d-amino acid substitutions and dimerization increase the biological activity and stability of P8 peptide. The resulting peptide constitutes a novel IL-15 antagonist with potential applicability in inflammatory diseases.

Impact of CMV Reactivation, Treatment Approaches, and Immune Reconstitution in a Nonmyeloablative Tolerance Induction Protocol in Cynomolgus Macaques

BACKGROUND

Cytomegalovirus (CMV) infection is a serious complication in immunosuppressed patients, specifically transplant recipients. Here, we describe the development and use of an assay to monitor the incidence and treatment of CMV viremia in a Cynomolgus macaque model of bone marrow transplantation (BMT) for tolerance induction. We address the correlation between the course of viremia and immune reconstitution.

METHODS

Twenty-one animals received a nonmyeloablative conditioning regimen. Seven received cyclosporine A for 28 days and 14 received rapamycin. A CMV polymerase chain reaction assay was developed and run twice per week to monitor viremia. Nineteen recipients were CMV seropositive before BMT. Immune reconstitution was monitored through flow cytometry and CMV viremia was tracked via quantitative polymerase chain reaction.

RESULTS

Recipients developed CMV viremia during the first month post-BMT. Two animals developed uncontrollable CMV disease. CMV reactivation occurred earlier in cyclosporine A-treated animals compared with those receiving rapamycin. Post-BMT, T-cell counts remained significantly lower compared with pretransplant levels until CMV reactivation, at which point they increased during the viremic phase and approached pretransplant levels 3 months post-BMT. Management of CMV required treatment before viremia reached 10 000 copies/mL; otherwise clinical symptoms were observed. High doses of ganciclovir resolved the viremia, which could subsequently be controlled with valganciclovir.

CONCLUSIONS

We developed an assay to monitor CMV in Cynomolgus macaques. CMV reactivation occurred in 100% of seropositive animals in this model. Rapamycin delayed CMV reactivation and ganciclovir treatment was effective at high doses. As in humans, CD8 T cells proliferated during CMV viremia.

Transient-mixed Chimerism With Nonmyeloablative Conditioning Does Not Induce Liver Allograft Tolerance in Nonhuman Primates

BACKGROUND

Although short-term outcomes for liver transplantation have improved, patient and graft survival are limited by infection, cancer, and other complications of immunosuppression. Rapid induction of tolerance after liver transplantation would decrease these complications, improving survival and quality of life. Tolerance to kidneys, but not thoracic organs or islets, has been achieved in nonhuman primates and humans through the induction of transient donor chimerism. Since the liver is considered to be tolerogenic, we tested the hypothesis that the renal transplant transient chimerism protocol would induce liver tolerance.

METHODS

Seven cynomolgus macaques received immune conditioning followed by simultaneous donor bone marrow and liver transplantation. The more extensive liver surgery required minor adaptations of the kidney protocol to decrease complications. All immunosuppression was discontinued on postoperative day (POD) 28. Peripheral blood chimerism, recipient immune reconstitution, liver function tests, and graft survival were determined.

RESULTS

The level and duration of chimerism in liver recipients were comparable to those previously reported in renal transplant recipients. However, unlike in the kidney model, the liver was rejected soon after immunosuppression withdrawal. Rejection was associated with proliferation of recipient CD8 T effector cells in the periphery and liver, increased serum interleukin (IL)-6 and IL-2, but peripheral regulatory T cell (Treg) numbers did not increase. Antidonor antibody was also detected.

CONCLUSIONS

These data show the transient chimerism protocol does not induce tolerance to livers, likely due to greater CD8 T cell responses than in the kidney model. Successful tolerance induction may depend on greater control or deletion of CD8 T cells in this model.

Chimerism, Transplant Tolerance, and Beyond

The present review discusses current developments in tolerance induction for solid organ transplantation with a particular emphasis on chimerism-based approaches. It explains the basic mechanisms of chimerism-based tolerance and provides an update on ongoing clinical tolerance trials. The concept of "delayed tolerance" is presented, and ongoing preclinical studies in the nonhuman primate setting-including current limitations and hurdles regarding this approach-are illustrated. In addition, a brief overview and update on cell-based tolerogenic clinical trials is provided. In a critical approach, advantages, limitations, and potential implications for the future of these different regimens are discussed.

Transplant research in nonhuman primates to evaluate clinically relevant immune strategies in organ transplantation

Research in transplant immunology using non-human primate (NHP) species to evaluate immunologic strategies to prevent rejection and prolong allograft survival has yielded results that have translated successfully into human organ transplant patient management. Other therapies have not proceeded to human translation due to failure in NHP testing, arguably sparing humans the futility and risk of such testing. The NHP transplant models are ethically necessary for drug development in this field and provide the closest analogue to human transplant patients available. The refinement of this resource with respect to colony MHC typing, reagent and assay development, and availability to the research community has greatly enhanced knowledge about transplant immunology and drug development.

Relationships between Cytokine Levels and Disease Parameters during the Development of a Collagen-induced Arthritis Model in Cynomolgus Macaques (Macaca fascicularis)

In rheumatoid arthritis research, NHP models of collagen-induced arthritis are important because these species share many immunologic and pathologic features with humans. In addition, serum levels of various cytokines in patients with rheumatoid arthritis have been studied as immune markers for disease prediction, early diagnosis, and effective therapeutic management. The purpose of this study was to identify changes in cytokine levels that occur during the development of collagen-induced arthritis in female cynomolgus macaques (n = 8) and to assess the relationships between these changes and various disease parameters. Blood samples were collected weekly before (week 0) and after (weeks 1 through 7) immunization with type II collagen; clinicopathologic and cytokine data from those samples and other clinical parameters were used in correlation analysis. Serum levels of IFN γ, chemokine (C-C motif) ligand 2 (CCL2), and IL6 showed significant changes after generation of collagen-induced arthritis. IFNγ levels showed a strong negative correlation with body weight (an indicator of general body condition), and CCL2 and IL6 showed moderate negative correlation with body weight. Serum IL6 levels showed moderate positive correlation with the soft tissue swelling score and strong positive correlation with serum C-reactive protein levels in our NHP model of collagen-induced arthritis. In addition, serum levels of matrix metalloproteinase 3 increased significantly after inoculation with type II collagen and showed a moderate positive correlation with serum levels of C-reactive protein, IL6, and IL15. These results suggest close correlations between various cytokines and disease parameters in NHP models of rheumatoid arthritis. These cytokines therefore potentially could be used as markers for monitoring the efficacy of novel treatments in NHP models of rheumatoid arthritis.

Maintaining T cell tolerance of alloantigens: Lessons from animal studies

Achieving host immune tolerance of allogeneic transplants represents the ultimate challenge in clinical transplantation. It has become clear that different cells and mechanisms participate in acquisition versus maintenance of allograft tolerance. Indeed, manipulations which prevent tolerance induction often fail to abrogate tolerance once it has been established. Hence, elucidation of the immunological mechanisms underlying maintenance of T cell tolerance to alloantigens is essential for the development of novel interventions that preserve a robust and long lasting state of allograft tolerance that relies on T cell deletion in addition to intra-graft suppression of inflammatory immune responses. In this review, we discuss some essential elements of the mechanisms involved in the maintenance of naturally occurring or experimentally induced allograft tolerance, including the newly described role of antigen cross-dressing mediated by extracellular vesicles.

Pilot Study of Delayed ICOS/ICOS-L Blockade With αCD40 to Modulate Pathogenic Alloimmunity in a Primate Cardiac Allograft Model

Supplemental digital content is available in the text.

Background

Inducible costimulator (ICOS) is rapidly upregulated with T-cell stimulation and may represent an escape pathway for T-cell costimulation in the setting of CD40/CD154 costimulation blockade. Induction treatment exhibited no efficacy in a primate renal allograft model, but rodent transplant models suggest that the addition of delayed ICOS/ICOS-L blockade may prolong allograft survival and prevent chronic rejection. Here, we ask whether ICOS-Ig treatment, timed to anticipate ICOS upregulation, prolongs NHP cardiac allograft survival or attenuates pathogenic alloimmunity.

Methods

Cynomolgus monkey heterotopic cardiac allograft recipients were treated with αCD40 (2C10R4, d0-90) either alone or with the addition of delayed ICOS-Ig (d63-110).

Results

Median allograft survival was similar between ICOS-Ig + αCD40 (120 days, 120-125 days) and αCD40 (124 days, 89-178 days) treated animals, and delayed ICOS-Ig treatment did not prevent allograft rejection in animals with complete CD40 receptor coverage. Although CD4⁺ T_EM cells were decreased in peripheral blood (115 ± 24) and mLNs (49 ± 1.9%) during ICOS-Ig treatment compared with monotherapy (214 ± 27%, P = 0.01; 72 ± 9.9%, P = 0.01, respectively), acute and chronic rejection scores and kinetics of alloAb elaboration were similar between groups.

Conclusions

Delayed ICOS-Ig treatment with the reagent tested is probably ineffective in modulating pathogenic primate alloimmunity in this model.

Selective CD28 blockade attenuates CTLA-4-dependent CD8+ memory T cell effector function and prolongs graft survival

Memory T cells pose a significant problem to successful therapeutic control of unwanted immune responses during autoimmunity and transplantation, as they are differentially controlled by cosignaling receptors such as CD28 and CTLA-4. Treatment with abatacept and belatacept impede CD28 signaling by binding to CD80 and CD86, but they also have the unintended consequence of blocking the ligands for CTLA-4, a process that may inadvertently boost effector responses. Here, we show that a potentially novel anti-CD28 domain antibody (dAb) that selectively blocks CD28 but preserves CTLA-4 coinhibition confers improved allograft survival in sensitized recipients as compared with CTLA-4 Ig. However, both CTLA-4 Ig and anti-CD28 dAb similarly and significantly reduced the accumulation of donor-reactive CD8+ memory T cells, demonstrating that regulation of the expansion of CD8+ memory T cell populations is controlled in part by CD28 signals and is not significantly impacted by CTLA-4. In contrast, selective CD28 blockade was superior to CTLA-4 Ig in inhibiting IFN-γ, TNF, and IL-2 production by CD8+ memory T cells, which in turn resulted in reduced recruitment of innate CD11b+ monocytes into allografts. Importantly, this superiority was CTLA-4 dependent, demonstrating that effector function of CD8+ memory T cells is regulated by the balance of CD28 and CTLA-4 signaling.

Role of Memory T Cells in Allograft Rejection and Tolerance

Memory T cells are characterized by their low activation threshold, robust effector functions, and resistance to conventional immunosuppression and costimulation blockade. Unlike their naïve counterparts, memory T cells reside in and recirculate through peripheral non-lymphoid tissues. Alloreactive memory T cells are subdivided into different categories based on their origins, phenotypes, and functions. Recipients whose immune systems have been directly exposed to allogeneic major histocompatibility complex (MHC) molecules display high affinity alloreactive memory T cells. In the absence of any prior exposure to allogeneic MHC molecules, endogenous alloreactive memory T cells are regularly generated through microbial infections (heterologous immunity). Regardless of their origin, alloreactive memory T cells represent an essential element of the allograft rejection process and a major barrier to tolerance induction in clinical transplantation. This article describes the different subsets of alloreactive memory T cells involved in transplant rejection and examine their generation, functional properties, and mechanisms of action. In addition, we discuss strategies developed to target deleterious allospecific memory T cells in experimental animal models and clinical settings.

Allorecognition by T Lymphocytes and Allograft Rejection

Recognition of donor antigens by recipient T cells in secondary lymphoid organs initiates the adaptive inflammatory immune response leading to the rejection of allogeneic transplants. Allospecific T cells become activated through interaction of their T cell receptors with intact allogeneic major histocompatibility complex (MHC) molecules on donor cells (direct pathway) and/or donor peptides presented by self-MHC molecules on recipient antigen-presenting cells (APCs) (indirect pathway). In addition, recent studies show that alloreactive T cells can also be stimulated through recognition of allogeneic MHC molecules displayed on recipient APCs (MHC cross-dressing) after their transfer via cell-cell contact or through extracellular vesicles (semi-direct pathway). The specific allorecognition pathway used by T cells is dictated by intrinsic and extrinsic factors to the allograft and can influence the nature and magnitude of the alloresponse and rejection process. Consequently, various organs and tissues such as skin, cornea, and solid organ transplants are recognized differently by pro-inflammatory T cells through these distinct pathways, which may explain why these grafts are rejected in a different fashion. On the other hand, the mechanisms by which anti-inflammatory regulatory T cells (Tregs) recognize alloantigen and promote transplantation tolerance are still unclear. It is likely that thymic Tregs are activated through indirect allorecognition, while peripheral Tregs recognize alloantigens in a direct fashion. As we gain insights into the mechanisms underlying allorecognition by pro-inflammatory and Treg cells, novel strategies are being designed to prevent allograft rejection in the absence of ongoing immunosuppressive drug treatment in patients.

Adoptive Cell Therapy with Tregs to Improve Transplant Outcomes: The Promise and the Stumbling Blocks

The contribution of regulatory T cells (Treg) to the induction and maintenance of tolerance is well-recognized in rodents and may contribute to long-term human organ allograft survival. The therapeutic efficacy of adoptively-transferred Treg in promoting tolerance to organ allografts is well-recognized in mouse models. Early phase 1/2 clinical studies of Treg therapy have been conducted in patients with type-1 (autoimmune) diabetes and refractory Crohn's disease, and for inhibition of graft-versus-host disease following bone marrow transplantation with proven safety. The feasibility of adoptive Treg therapy in the clinic is subject to various parameters, including optimal cell source, isolation procedure, expansion, target dose, time of infusion, as well as generation of a GMP-cell product. Several phase 1/2 Treg dose-escalation studies are underway in organ transplantation. Recent evidence suggests that additional factors are critical to ensure Treg safety and efficacy in allograft recipients, including Treg characterization, stability, longevity, trafficking, concomitant immunosuppression, and donor antigen specificity. Accordingly, Treg therapy in the context of organ transplantation may prove more challenging in comparison to other prospective clinical settings of Treg immunotherapy, such as type-1 diabetes.

Regulatory T Cell Infusion Can Enhance Memory T Cell and Alloantibody Responses in Lymphodepleted Nonhuman Primate Heart Allograft Recipients

The ability of regulatory T cells (Treg) to prolong allograft survival and promote transplant tolerance in lymphodepleted rodents is well established. Few studies, however, have addressed the therapeutic potential of adoptively transferred, CD4(+) CD25(+) CD127(-) Foxp3(+) (Treg) in clinically relevant large animal models. We infused ex vivo-expanded, functionally stable, nonselected Treg (up to a maximum cumulative dose of 1.87 billion cells) into antithymocyte globulin-lymphodepleted, MHC-mismatched cynomolgus monkey heart graft recipients before homeostatic recovery of effector T cells. The monkeys also received tacrolimus, anti-interleukin-6 receptor monoclonal antibodies and tapered rapamycin maintenance therapy. Treg administration in single or multiple doses during the early postsurgical period (up to 1 month posttransplantation), when host T cells were profoundly depleted, resulted in inferior graft function compared with controls. This was accompanied by increased incidences of effector memory T cells, enhanced interferon-γ production by host CD8(+) T cells, elevated levels of proinflammatory cytokines, and antidonor alloantibodies. The findings caution against infusion of Treg during the early posttransplantation period after lymphodepletion. Despite marked but transient increases in Treg relative to endogenous effector T cells and use of reputed "Treg-friendly" agents, the host environment/immune effector mechanisms instigated under these conditions can perturb rather than favor the potential therapeutic efficacy of adoptively transferred Treg.

Persistence of Indirect but Not Direct T Cell Xenoresponses in Baboon Recipients of Pig Cell and Organ Transplants

We investigated the contributions of direct and indirect T cell antigen recognition pathways to the immune response to porcine antigens in naïve baboons and baboon recipients of pig xenografts. In naïve baboons, in vitro culture of peripheral blood T cells with intact pig cells (direct xenorecognition pathway) or pig cell sonicates and baboon antigen-presenting cells (indirect xenorecognition pathway) induced the activation and expansion of xenoreactive T cells producing proinflammatory cytokines, interleukin-2 and interferon-γ. Primary indirect xenoresponses were mediated by preexisting memory T cells, whose presence is not typically observed in primary alloresponses. Next, baboons were conditioned with a nonmyeloablative regimen before short-term immunosuppression and transplantation of xenogeneic peripheral blood progenitor cells and a kidney, heart, or pancreatic islets from a miniature swine. All transplants were rejected acutely within 30 days after their placement. Posttransplantation, we observed an inhibition of the direct xenoresponse but a significant expansion of indirectly activated proinflammatory T cells. These results suggest that additional treatment to suppress indirect T cell immunity in primates may be required to achieve tolerance of pig xenografts through hematopoietic chimerism.

Evaluation of assay interference and interpretation of CXCR4 receptor occupancy results in a preclinical study with MEDI3185, a fully human antibody to CXCR4

Background

Receptor occupancy (RO) assays provide a means to measure the direct interaction of therapeutics with their cell surface targets. Free receptor assays quantify cell‐surface receptors not bound by a therapeutic while total receptor assays quantify the amount of target on the cell surface.

Methods

We developed both a flow cytometry‐based free RO assay to detect free surface CXCR4, and a total surface CXCR4 assay. In an effort to evaluate potential displacement interference, we performed in vitro experiments to compare on‐cell affinity with the IC₅₀ values from in vitro and in vivo from the free CXCR4 assay. We determined free and total surface CXCR4 on circulating blood cells in cynomolgus monkeys dosed with MEDI3185, a fully human monoclonal antibody to CXCR4.

Results

We devised an approach to evaluate displacement interference during assay development and showed that our free assay demonstrated little to no displacement interference. After dosing cynomolgus monkeys with MEDI3185, we observed dose‐dependence in the magnitude and duration of receptor occupancy and found CXCR4 to increase on lymphocytes, monocytes, and granulocytes. In a multiple dose study, we observed time points where surface CXCR4 appeared fully occupied but MEDI3185 was not detectable in serum. These paradoxical results represented a type of assay interference, and by comparing pharmacokinetic, ADA and total CXCR4 results, the most likely reason for the free CXCR4 results was the emergence of neutralizing anti‐drug antibodies (ADA). The total CXCR4 assay was unaffected by ADA and provided a reliable marker of target modulation in both in vivo studies. © 2015 The Authors Cytometry Part B: Clinical Cytometry Published byWiley Periodicals, Inc.

Repeated Injections of IL-2 Break Renal Allograft Tolerance Induced via Mixed Hematopoietic Chimerism in Monkeys

Tolerance of allografts achieved in mice via stable mixed hematopoietic chimerism relies essentially on continuous elimination of developing alloreactive T cells in the thymus (central deletion). Conversely, while only transient mixed chimerism is observed in nonhuman primates and patients, it is sufficient to ensure tolerance of kidney allografts. In this setting, it is likely that tolerance depends on peripheral regulatory mechanisms rather than thymic deletion. This implies that, in primates, upsetting the balance between inflammatory and regulatory alloimmunity could abolish tolerance and trigger the rejection of previously accepted renal allografts. In this study, six monkeys that were treated with a mixed chimerism protocol and had accepted a kidney allograft for periods of 1-10 years after withdrawal of immunosuppression received subcutaneous injections of IL-2 cytokine (0.6-3 × 10(6) IU/m(2) ). This resulted in rapid rejection of previously tolerated renal transplants and was associated with an expansion and reactivation of alloreactive pro-inflammatory memory T cells in the host's lymphoid organs and in the graft. This phenomenon was prevented by anti-CD8 antibody treatment. Finally, this process was reversible in that cessation of IL-2 administration aborted the rejection process and restored normal kidney graft function.

Pre-transplant donor-specific Interferon-gamma-producing cells and acute rejection of the kidney allograft

BACKGROUND

Our retrospective study included a cohort of 47 patients who underwent living donor kidney transplantation.The pre-transplant frequencies of donor-specific Interferon-gamma (IFN-γ) producing cells were define dusing enzyme-linked immunosorbent spot (ELISpot) assay and correlated with incidence of acute cellular(ACR), antibody-mediated rejection (AMR) and kidney graft survival up to one year after transplantation.

RESULTS

We found a statistically significant correlation between the frequencies of IFN-γ-producing cells and the number of mismatches in HLA antigens between patients and their respective donors – for Class I – A and B (r = 0.399, p b 0.01) and for Class I and Class II antigens – A, B and DR (r = 0.409, p b 0.01). No significant relationship was observed between the numbers of IFN-γ-secreting cells and incidence of acute rejection (neither ACR, nor AMR). However, there was a trend of elevated frequencies of IFN-γ-producing cells in patients who developed ACR or AMR in comparison with kidney recipients free of rejection (91 ± 82 and 114 ± 75 vs. 72 ± 70/5 × 10(4) peripheral blood mononuclear cells respectively). Patients with concurrent acute cellular and antibody-mediated rejection had also higher numbers of IFN-γ-producing memory/effector cells compared to patients with cellular rejection only.

CONCLUSION

Pre-transplant determination of the numbers of IFN-γ-producing donor-specific memory cells using the ELISpot technique may provide clinically relevant results when evaluating the risk of development of acute cellular and antibody-mediated rejection. These frequencies are influenced by the degree of HLA mismatching between patients and their respective kidney donors.

Tolerance of Lung Allografts Achieved in Nonhuman Primates via Mixed Hematopoietic Chimerism

While the induction of transient mixed chimerism has tolerized MHC-mismatched renal grafts in nonhuman primates and patients, this approach has not been successful for more immunogenic organs. Here, we describe a modified delayed-tolerance-induction protocol resulting in three out of four monkeys achieving long-term lung allograft survival without ongoing immunosuppression. Two of the tolerant monkeys displayed stable mixed lymphoid chimerism, and the other showed transient chimerism. Serial biopsies and post-mortem specimens from the tolerant monkeys revealed no signs of chronic rejection. The tolerant recipients also exhibited T cell unresponsiveness and a lack of alloantibody. This is the first report of durable mixed chimerism and successful tolerance induction of MHC-mismatched lungs in primates.

Cytokines: The Good, the Bad, and the Deadly

Over the past 30 years, the world of pharmaceutical toxicology has seen an explosion in the area of cytokines. An overview of the many aspects of cytokine safety evaluation currently in progress and evolving strategies for evaluating these important entities was presented at this symposium. Cytokines play a broad role to help the immune system respond to diseases, and drugs which modulate their effect have led to some amazing therapies. Cytokines may be "good" when stimulating the immune system to fight a foreign pathogen or attack tumors. Other "good" cytokine effects include reduction of an immune response, for example interferon β reduction of neuron inflammation in patients with multiple sclerosis. They may be "bad" when their expression causes inflammatory diseases, such as the role of tumor necrosis factor α in rheumatoid arthritis or asthma and Crohn's disease. Therapeutic modulation of cytokine expression can help the "good" cytokines to generate or quench the immune system and block the "bad" cytokines to prevent damaging inflammatory events. However, care must be exercised, as some antibody therapeutics can cause "ugly" cytokine release which can be deadly. Well-designed toxicology studies should incorporate careful assessment of cytokine modulation that will allow effective therapies to treat unmet needs. This symposium discussed lessons learned in cytokine toxicology using case studies and suggested future directions.

Long-term lung transplantation in nonhuman primates

Despite advances in surgical technique and clinical care, lung transplantation still remains a short-term solution for the treatment of end-stage lung disease. To date, there has been limited experience in experimental lung transplantation using nonhuman primate models. Therefore, we have endeavored to develop a long-term, nonhuman primate model of orthotopic lung transplantation for the ultimate purpose of designing protocols to induce tolerance of lung grafts. Here, we report our initial results in developing this model and our observation that the nonhuman primate lung is particularly prone to rejection. This propensity toward rejection may be a consequence of 1) upregulated nonspecific inflammation, and 2) a larger number of pre-existing alloreactive memory T cells, leading to augmented deleterious immune responses. Our data show that triple-drug immunosuppression mimicking clinical practice is not sufficient to prevent acute rejection in nonhuman primate lung transplantation. The addition of horse-derived anti-thymocyte globulin and a monoclonal antibody to the IL-6 receptor allowed six out of six lung recipients to be free of rejection for over 120 days.

Use of CTLA4Ig for induction of mixed chimerism and renal allograft tolerance in nonhuman primates

We have previously reported successful induction of renal allograft tolerance via a mixed chimerism approach in nonhuman primates. In those studies, we found that costimulatory blockade with anti-CD154 mAb was an effective adjunctive therapy for induction of renal allograft tolerance. However, since anti-CD154 mAb is not clinically available, we have evaluated CTLA4Ig as an alternative agent for effecting costimulation blockade in this treatment protocol. Two CTLA4Igs, abatacept and belatacept, were substituted for anti-CD154 mAb in the conditioning regimen (low dose total body irradiation, thymic irradiation, anti-thymocyte globulin and a 1-month posttransplant course of cyclosporine [CyA]). Three recipients treated with the abatacept regimen failed to develop comparable lymphoid chimerism to that achieved with anti-CD154 mAb treatment and these recipients rejected their kidney allografts early. With the belatacept regimen, four of five recipients developed chimerism and three of these achieved long-term renal allograft survival (>861, >796 and >378 days) without maintenance immunosuppression. Neither chimerism nor long-term allograft survival were achieved in two recipients treated with the belatacept regimen but with a lower, subtherapeutic dose of CyA. This study indicates that CD28/B7 blockade with belatacept can provide a clinically applicable alternative to anti-CD154 mAb for promoting chimerism and renal allograft tolerance.

The rejuvenating effects of leuprolide acetate on the aged baboon's thymus

BACKGROUND

We have previously demonstrated that the juvenile thymus plays an essential role in tolerance induced by both renal transplantation and a short course of calcineurin inhibitors. Aged thymi have a decreased ability to induce tolerance. Luteinizing hormone-releasing hormone (LHRH) is known to pharmacologically rejuvenate the thymus in rodents. In order to develop a clinically applicable regimen of transplantation tolerance in adults, we sought to determine if thymic rejuvenation would occur with LHRH agonism in non-human primates.

METHODS AND RESULTS

Thymic rejuvenation was evaluated by magnetic resonance imaging (MRI), histology, as well as in-vitro cellular and molecular tests. Four aged male hamadryas baboons underwent subcutaneous injection of a 3-month depot of Lupron (11.25mg; LI) and were followed for 3 months. Thymi increased volumetrically by MRI. After LI, thymic cellularity markedly increased within the cortical and medullary thymus. Additionally, a significant increase in the CD4(+)/CD45RA(hi+) population in the peripheral blood occurred for 50 days after LI, and flow cytometry of thymic tissue revealed a large increase in the percentage of CD4(+)/CD8(+) cells. TREC assay corroborated enhancement in thymic function.

CONCLUSION

These data indicate that LI is associated with thymic rejuvenation in baboons, and further confirm that extrinsic factors play an important role in thymic rejuvenation in a non-human primate model.

Induction of tolerance through mixed chimerism

"Mixed chimerism" refers to a state in which the lymphohematopoietic system of the recipient of allogeneic hematopoietic stem cells comprises a mixture of host and donor cells. This state is usually attained through either bone marrow or mobilized peripheral blood stem cell transplantation. Although numerous treatment regimens have led to transplantation tolerance in mice, the induction of mixed chimerism is currently the only treatment modality that has been successfully extended to large animals and to the clinic. Here we describe and compare the use of mixed chimerism to establish transplantation tolerance in mice, pigs, monkeys, and in the clinic. We also attempt to correlate the mechanisms involved in achieving tolerance with the nature of the tolerance that has resulted in each case.

Alefacept promotes immunosuppression-free renal allograft survival in nonhuman primates via depletion of recipient memory T cells

Renal allograft tolerance has been achieved in MHC-mismatched primates via nonmyeloablative conditioning beginning 6 days prior to planned kidney and donor bone marrow transplantation (DBMT). To extend the applicability of this approach to deceased donor transplantation, we recently developed a novel-conditioning regimen, the "delayed protocol" in which donor bone marrow (DBM) is transplanted several months after kidney transplantation. However, activation/expansion of donor-reactive CD8(+) memory T cells (TMEM) occurring during the interval between kidney and DBM transplantation impaired tolerance induction using this strategy. In the current study, we tested whether, Alefacept, a fusion protein which targets LFA-3/CD2 interactions and selectively depletes CD2(high) CD8(+) effector memory T cells (TEM) could similarly induce long-term immunosuppression-free renal allograft survival but avoid the deleterious effects of anti-CD8 mAb treatment. We found that Alefacept significantly delayed the expansion of CD2(high) cells including CD8(+) TEM while sparing naïve CD8(+) T and NK cells and achieved mixed chimerism and long-term immunosuppression-free renal allograft survival. In conclusion, elimination of CD2(high) T cells represents a promising approach to prevent electively the expansion/activation of donor-reactive TEM and promotes tolerance induction via the delayed protocol mixed chimerism approach.

Tolerance induction: hematopoietic chimerism

PURPOSE OF REVIEW

Although numerous experimental models to induce allograft tolerance have been reported, it has been difficult to translate these basic studies to clinical transplantation. However, successful induction of tolerance in HLA-mismatched kidney transplantation has recently been reported. In this review, recent progress in tolerance induction in preclinical (nonhuman primates) and clinical transplantation is summarized.

RECENT FINDINGS

Among many clinical trials to induce renal allograft tolerance, success has so far been achieved only by combining donor bone marrow with organ transplantation. Induction of renal allograft tolerance by transient or durable mixed chimerism has been reported in HLA-matched or mismatched kidney transplant recipients. More recently, renal allograft tolerance by induction of full donor chimerism has also been reported using a more intensified preparative conditioning regimen.

SUMMARY

Durable allograft tolerance has been achieved by induction of hematopoietic chimerism in clinical kidney transplantation, with outstanding long-term results in successful cases. However, these approaches have been associated with higher early complications than are seen following transplantation with conventional immunosuppression. Improvements in the consistency and safety of tolerance induction and extension of successful protocols to other organs will be the next steps in bringing tolerance to a wider range of clinical applications.

Endogenous memory CD8 T cells are activated within cardiac allografts without mediating rejection

Endogenous memory CD8 T cells infiltrate MHC-mismatched cardiac allografts within 12-24 h posttransplant in mice and are activated to proliferate and produce IFN-γ. To more accurately assess the graft injury directly imposed by these endogenous memory CD8 T cells, we took advantage of the ability of anti-LFA-1 mAb given to allograft recipients on days 3 and 4 posttransplant to inhibit the generation of primary effector T cells. When compared to grafts from IgG-treated recipients on day 7 posttransplant, allografts from anti-LFA-1 mAb-treated recipients had increased numbers of CD8 T cells but these grafts had marked decreases in expression levels of mRNA encoding effector mediators associated with graft injury and decreases in donor-reactive CD8 T cells producing IFN-γ. Despite this decreased activity within the allograft, CD8 T cells in allografts from recipients treated with anti-LFA-1 mAb continued to proliferate up to day 7 posttransplant and did not upregulate expression of the exhaustion marker LAG-3 but did have decreased expression of ICOS. These results indicate that endogenous memory CD8 T cells infiltrate and proliferate in cardiac allografts in mice but do not express sufficient levels of functions to mediate overt graft injury and acute rejection.

Potential limitations of the NSG humanized mouse as a model system to optimize engineered human T cell therapy for cancer

The genetic modification of peripheral blood lymphocytes using retroviral vectors to redirect T cells against tumor cells has been recently used as a means to generate large numbers of antigen-specific T cells for adoptive cell therapy protocols. However, commonly used retroviral vector-based genetic modification requires T cells to be driven into cell division; this potent mitogenic stimulus is associated with the development of an effector phenotype that may adversely impact upon the long-term engraftment potential and subsequent antitumor effects of T cells. To investigate whether the cytokines used during culture impact upon the engraftment potential of gene-modified T cells, a humanized model employing T cells engrafted with a MART-1-specific T cell receptor adoptively transferred into NOD/Shi-scid IL-2rγ(-/-) (NSG) immune-deficient mice bearing established melanoma tumors was used to compare the effects of the common γ chain cytokines IL-2, IL-7, and IL-15 upon gene-modified T cell activity. MART-1-specific T cells cultured in IL-7 and IL-15 demonstrated greater relative in vitro proliferation and viability of T cells compared with the extensively used IL-2. Moreover, the IL-15 culture prolonged the survival of animals bearing melanoma tumors after adoptive transfer. However, the combination of IL-7 and IL-15 produced T cells with improved engraftment potential compared with IL-15 alone; however, a high rate of xenogeneic graft-versus-host disease prevented the identification of a clear improvement in antitumor effect of these T cells. These results clearly demonstrate modulation of gene-modified T cell engraftment in the NSG mouse, which supports the future testing of the combination of IL-7 and IL-15 in adoptive cell therapy protocols; however, this improved engraftment is also associated with the long-term maintenance of xenoreactive T cells, which limits the ultimate usefulness of the NSG mouse model in this situation.

Haplessly hoping: macaque major histocompatibility complex made easy

Major histocompatibility complex (MHC) gene products control the repertoire of T cell responses that an individual may create against pathogens and foreign tissues. This text will review the current understanding of MHC genetics in nonhuman primates, with a focus on Mauritian-origin cynomolgus macaques (Macaca fascicularis) and Indian-origin rhesus macaques (Macaca mulatta). These closely related macaque species provide important experimental models for studies of infectious disease pathogenesis, vaccine development, and transplantation research. Recent advances resulting from the application of several cost effective, high-throughput approaches, with deep sequencing technologies have revolutionized our ability to perform MHC genotyping of large macaque cohorts. Pyrosequencing of cDNA amplicons with a Roche/454 GS Junior instrument, provides excellent resolution of MHC class I allelic variants with semi-quantitative estimates of relative levels of transcript abundance. Introduction of the Illumina MiSeq platform significantly increased the sample throughput, since the sample loading workflow is considerably less labor intensive, and each instrument run yields approximately 100-fold more sequence data. Extension of these sequencing methods from cDNA to genomic DNA amplicons further streamlines the experimental workflow and opened opportunities for retrospective MHC genotyping of banked DNA samples. To facilitate the reporting of MHC genotypes, and comparisons between groups of macaques, this text also introduces an intuitive series of abbreviated rhesus MHC haplotype designations based on a major Mamu-A or Mamu-B transcript characteristic for ancestral allele combinations. The authors believe that the use of MHC-defined macaques promises to improve the reproducibility, and predictability of results from pre-clinical studies for translation to humans.

Prolonged survival of allogeneic islets in cynomolgus monkeys after short-term triple therapy

Preclinical studies in nonhuman primates (NHP) are particularly useful to evaluate the safety and efficacy of new therapeutic proteins developed for use in clinical transplantation. We hypothesized that a treatment that selectively destroys activated cytopathic donor reactive T cells while sparing resting and immunoregulatory T cells in a mouse model might also produce long-term drug-free engraftment and tolerance without the hazards of lymphopenia in the challenging nonhuman primate islet allograft model. Short-term treatment with a regimen consisting of rapamycin, and IL-2.Ig plus mutant antagonist-type IL-15.Ig cytolytic fusion proteins (triple therapy) posttransplantation results in prolonged, drug-free engraftment of cynomolgus islet allografts. Moreover slow progressive loss of islet function in some recipients was not associated with obvious pathologic evidence of rejection.

Innate immunity and resistance to tolerogenesis in allotransplantation

The development of immunosuppressive drugs to control adaptive immune responses has led to the success of transplantation as a therapy for end-stage organ failure. However, these agents are largely ineffective in suppressing components of the innate immune system. This distinction has gained in clinical significance as mounting evidence now indicates that innate immune responses play important roles in the acute and chronic rejection of whole organ allografts. For instance, whereas clinical interest in natural killer (NK) cells was once largely confined to the field of bone marrow transplantation, recent findings suggest that these cells can also participate in the acute rejection of cardiac allografts and prevent tolerance induction. Stimulation of Toll-like receptors (TLRs), another important component of innate immunity, by endogenous ligands released in response to ischemia/reperfusion is now known to cause an inflammatory milieu favorable to graft rejection and abrogation of tolerance. Emerging data suggest that activation of complement is linked to acute rejection and interferes with tolerance. In summary, the conventional wisdom that the innate immune system is of little importance in whole organ transplantation is no longer tenable. The addition of strategies that target TLRs, NK cells, complement, and other components of the innate immune system will be necessary to eventually achieve long-term tolerance to human allograft recipients.

Nonhuman primate transplant models finally evolve: detailed immunogenetic analysis creates new models and strengthens the old

Nonhuman primate (NHP) models play a critical role in the translation of novel therapies for transplantation to the clinic. However, although MHC disparity significantly affects the outcome of transplantation, until recently, experiments using NHP models were performed without the ability to rigorously control the degree of MHC disparity in transplant cohorts. In this review, we discuss several key technical breakthroughs in the field, which have finally enabled detailed immunogenetic data to be incorporated into NHP transplantation studies. These advances have created a new gold-standard for NHP transplantation research, which incorporates detailed information regarding the degree of relatedness and the degree of MHC haplotype disparity between transplant pairs and the precise MHC alleles that both donors and recipients express. The adoption of this new standard promises to increase the rigor of NHP transplantation studies and to ensure that these experiments are optimally translatable to patient care.

Heterogeneity within T Cell Memory: Implications for Transplant Tolerance

Adaptive immunity in both mouse and man results in the generation of immunological memory. Memory T cells are both friend and foe to transplant recipients, as they are intimately involved and in many cases absolutely required for the maintenance of protective immunity in the face immunosuppression, yet from the evidence presented herein they clearly constitute a formidable barrier for the successful implementation of tolerance induction strategies in transplantation. This review describes the experimental evidence demonstrating the increased resistance of memory T cells to many distinct tolerance induction strategies, and outlines recent advances in our knowledge of the ways in which alloreactive memory T cells arise in previously untransplanted individuals. Understanding the impact of alloreactive memory T cell specificity, frequency, and quality might allow for better donor selection in order to minimize the donor-reactive memory T cell barrier in an individual transplant recipient, thus allowing stratification of relative risk of alloreactive memory T cell mediated rejection, and conversely increase the likelihood of successful establishment of tolerance. However, further research into the molecular and cellular pathways involved in alloreactive memory T cell-mediated rejection is required in order to design new strategies to overcome the memory T cell barrier, without critically impairing protective immunity.

Effects of an agonist interleukin-2/Fc fusion protein, a mutant antagonist interleukin-15/Fc fusion protein, and sirolimus on cardiac allograft survival in non-human primates

BACKGROUND

To tilt the immunologic balance toward tolerance and away from rejection, non-human primate recipients of cardiac allografts were treated with interleukin (IL)-2/Fc, mutant (m) antagonist type mIL-15/Fc, and sirolimus.

METHODS

Heterotopic heart transplants were performed on 8 fully mismatched cynomolgus macaques. An untreated control recipient rejected its graft by post-operative Day 6. The remaining 7 animals received oral or intramuscular immunosuppression with sirolimus. A recipient treated with sirolimus alone rejected at the end of 28 days of immunosuppression. The remaining 6 monkeys also received IL-2/Fc and mIL-15/Fc intramuscularly until 28 days after transplant. One animal received a second 28-day course of fusion protein starting at Day 50. In these 6 animals, sirolimus was continued for 28 days (n = 4) or until protein levels were low (n = 2).

RESULTS

In the 4 monkeys treated with a 28-day course of sirolimus and fusion proteins, mean graft survival was 51.5 days (range, 28-76 days). The animal receiving a second course of fusion protein rejected its graft on Day 177, despite detectable levels of the fusion proteins and sirolimus. The central memory, effector memory, and naïve CD4(+) and CD8(+) T-cell populations in the peripheral blood did not change significantly during fusion protein administration. A 2.5-fold expansion in CD4(+)CD25(+) lymphocytes occurred in recipients treated with fusion proteins and sirolimus that was not observed in the recipient treated with sirolimus alone.

CONCLUSIONS

Although IL-2/Fc, mIL-15/Fc, and sirolimus administered in this manner permitted modest prolongation of graft survival and expansion of CD4(+)CD25(+) T cells, tolerance was not achieved.

Immune recognition and rejection of allogeneic skin grafts

The transplantation of allogeneic skin grafts is associated with a potent inflammatory immune response leading to the destruction of donor cells and the rejection of the graft. Shortly after transplantation, skin dendritic cells (DCs) migrate out of the graft through lymphatic vessels and infiltrate the recipient's draining lymph nodes where they present donor antigens via two mechanisms: the direct pathway, in which T cells recognize intact donor MHC antigens on donor DCs; and the indirect pathway, involving T-cell recognition of donor peptides bound to self-MHC molecules on recipient DCs. Some recent studies have suggested that T cells can become activated via recognition of donor MHC molecules transferred on recipient antigen-presenting cells (semidirect pathway). Activation of T cells via direct or indirect allorecognition is sufficient to trigger acute rejection of allogeneic skin grafts. In addition, allospecific antibodies contribute to the rejection process either by killing allogeneic targets in a complement-dependent fashion or by opsonizing donor cells and forming immune complexes. Finally, several studies demonstrate that NK cells, activated due to missing self-MHC class I molecules on allogeneic cells, are involved in allogeneic skin graft rejection via direct killing of donor cells and through the production of proinflammatory cytokines including IFN-γ and TNF-α.

Overcoming memory T-cell responses for induction of delayed tolerance in nonhuman primates

The presence of alloreactive memory T cells is a major barrier for induction of tolerance in primates. In theory, delaying conditioning for tolerance induction until after organ transplantation could further decrease the efficacy of the regimen, since preexisting alloreactive memory T cells might be stimulated by the transplanted organ. Here, we show that such "delayed tolerance" can be induced in nonhuman primates through the mixed chimerism approach, if specific modifications to overcome/avoid donor-specific memory T-cell responses are provided. These modifications include adequate depletion of CD8+ memory T cells and timing of donor bone marrow administration to minimize levels of proinflammatory cytokines. Using this modified approach, mixed chimerism was induced successfully in 11 of 13 recipients of previously placed renal allografts and long-term survival without immunosuppression could be achieved in at least 6 of these 11 animals.

T-lymphocyte homeostasis and function in infant baboons: implications for transplantation

Laboratory mice are born lymphopenic and demonstrate lymphopenia-induced proliferation that generates memory T cells, yet they are prone to immunologic tolerance. Here we tested whether these fundamental immunologic observations apply to higher animals by studying the immune system of infant baboons. Using flow cytometry of the peripheral blood cells, it was found that baboons are born relatively lymphopenic and subsequently expand their initially naïve T cell pool with increasing numbers of memory T cells. After transplantation of an artery patch allograft or xenograft, non-immunosuppressed recipients readily mounted an immune response against donor-type antigens, as evidenced by mixed lymphocyte reaction. Immunosuppression with anti-thymocyte globulin (ATG), anti-CD154 mAb, and mycophenolate mofetil prevented T cell-mediated rejection. After lymphocyte depletion with ATG, homeostatic T cell proliferation was observed. In conclusion, the baboon proved a suitable model to investigate the infant immune system. In this study, neonatal lymphopenia and expansion of the memory T cell population were observed but, unlike mice, there were no indications that infant baboons are prone to T cell tolerance. The expansion of memory T cells during the neonatal period or after induction therapy may actually form an obstacle to tapering immunosuppressive therapy, or ultimately achieving immunologic tolerance.

Preclinical and clinical studies on the induction of renal allograft tolerance through transient mixed chimerism

PURPOSE OF REVIEW

The present review updates the current status of research for induction of tolerance through a mixed chimerism approach in nonhuman primates and humans.

RECENT FINDINGS

Allograft tolerance has been successfully achieved with a nonmyeloablative conditioning regimen and donor bone marrow transplantation in human leukocyte antigen (HLA)-matched and mismatched kidney transplantation. In HLA-matched kidney transplantation, persistent mixed chimerism and renal allograft tolerance has been achieved in some patients. In HLA-mismatched combinations, induction of persistent mixed chimerism has not been achieved using a nonmyeloablative preparative regimen. Nevertheless, the transient mixed chimerism that has been achieved has resulted in long-term renal allograft tolerance in the majority of patients. Recent preclinical studies have demonstrated that the presence of heterologous memory T-cell responses observed in primates, but not in rodents, may be a major barrier for induction of durable chimerism and tolerance in primates. Strategies to overcome such memory T-cell responses may, therefore, be of great value in the development of reliable protocols for clinical tolerance induction.

SUMMARY

Induction of tolerance in clinical kidney transplantation has been achieved via mixed chimerism approaches. Improvements in the consistency and safety of tolerance induction and extension of successful protocols to other organs and to organs from deceased donors will all be among the next steps in bringing tolerance to a wider range of clinical applications.

B-cell-dependent memory T cells impede nonmyeloablative mixed chimerism induction in presensitized mice

Presensitization to HLA antigens limits the success of organ transplantation. The achievement of donor-specific tolerance via mixed chimerism could improve outcomes of transplantation in presensitized patients. In presensitized B-cell-deficient μMT B6 mice, we developed nonmyeloablative bone marrow transplantation (BMT) regimens that successfully tolerized presensitized T cells, achieving long-term (LT) multilineage chimerism and tolerance to donor-type skin. To apply these regimens in wild-type (WT) animals while avoiding antibody-mediated destruction of donor bone marrow cells, presensitized WT B6 mice were rested >2 years to allow alloantibody clearance. However, chimerism and tolerance were not reliably achieved in LT presensitized WT B6 mice in which alloantibody had declined to minimal or undetectable levels before BMT. Strong antidonor memory T-cell responses were detected in LT presensitized WT B6 mice after rejection of donor bone marrow (BM) occurred, whereas levels of alloantibody remained consistently low. In contrast, presensitized μMT B6 mice had diminished memory T-cell responses compared to WT B6 mice. These data implicate T-cell memory, but not alloantibody, in rejection of donor BM in LT presensitized WT mice.

Contributions of direct and indirect alloresponses to chronic rejection of kidney allografts in nonhuman primates

The relative contribution of direct and indirect allorecognition pathways to chronic rejection of allogeneic organ transplants in primates remains unclear. In this study, we evaluated T and B cell alloresponses in cynomolgus monkeys that had received combined kidney/bone marrow allografts and myeloablative immunosuppressive treatments. We measured donor-specific direct and indirect T cell responses and alloantibody production in monkeys (n = 5) that did not reject their transplant acutely but developed chronic humoral rejection (CHR) and in tolerant recipients (n = 4) that never displayed signs of CHR. All CHR recipients exhibited high levels of anti-donor Abs and mounted potent direct T cell alloresponses in vitro. Such direct alloreactivity could be detected for more than 1 y after transplantation. In contrast, only two of five monkeys with CHR had a detectable indirect alloresponse. No indirect alloresponse by T cells and no alloantibody responses were found in any of the tolerant monkeys. Only one of four tolerant monkeys displayed a direct T cell alloresponse. These observations indicate that direct T cell alloresponses can be sustained for prolonged periods posttransplantation and result in alloantibody production and chronic rejection of kidney transplants, even in the absence of detectable indirect alloreactivity.

Transplantation tolerance: memories that haunt us

Transplant tolerance, which allows grafts--allogeneic cells, tissues, or organs--to be accepted without host immunosuppression, can be achieved in mice but not in primates. In this issue of Science Translational Medicine, Nadazdin et al. report that a high pretransplant frequency of graft-reactive memory T cells may inhibit the induction of transplant tolerance in nonhuman primates and lead to transplant rejection. Knowing the frequency of allograft-specific memory T cells in potential transplant recipients could aid clinical decision-making by guiding selection of the antigenic profile of the donor organ or by influencing the type of tolerance-induction protocol pursued.

Host alloreactive memory T cells influence tolerance to kidney allografts in nonhuman primates

Transplant tolerance, defined as indefinite allograft survival without immunosuppression, has been regularly achieved in laboratory mice but not in nonhuman primates or humans. In contrast to laboratory mice, primates regularly have high frequencies of alloreactive memory T cells (TMEMs) before transplantation. These TMEMs are poorly sensitive to conventional immunosuppression and costimulation blockade, and the presence of donor-reactive TMEMs in primates may account for their resistance to transplant tolerance protocols that have proven consistently effective in mice. We measured the frequencies of anti-donor TMEMs before and after transplantation in a series of rejecting and tolerant monkeys that underwent nonmyeloablative conditioning, short-term immunosuppression, and combined allogeneic kidney/cell transplantation. Transplants were acutely rejected in all the monkeys with high numbers of donor-specific TMEMs before transplantation. In contrast, long-term survival was observed in the recipients harboring lower frequencies of anti-donor TMEMs before transplantation. Similar amounts of TMEM homeostatic expansion were recorded in all transplanted monkeys upon hematopoietic reconstitution; however, only the tolerant monkeys had no expansion or activation of donor-reactive TMEMs after transplantation. These results indicate that the presence of high frequencies of host donor-reactive TMEMs before transplantation impairs tolerance induction to kidney allografts in this nonhuman primate model. Indeed, recipients harboring a low anamnestic reactivity to their donor before transplantation were successfully rendered tolerant via infusion of donor cells and short-term immunosuppression. This suggests that selection of allogeneic donors with low memory responses in recipients may be essential to successful transplant tolerance induction in patients.

Immuno-intervention for the induction of transplantation tolerance through mixed chimerism

The induction of transplantation tolerance could liberate organ transplant recipients from the complications of life-long chronic immunosuppression. The original description of tolerance induction through mixed hematopoietic chimerism in mice utilized lethal whole body irradiation as the preparative regimen for achieving mixed chimerism. While such a regimen might be acceptable for treatment of patients with malignancies, which might also respond to the therapeutic effects of radiation, its toxicity would be unacceptable for patients in need only of an organ transplant. Graft-vs.-host disease, which is frequently a complication of mismatched bone marrow transplantation, would likewise be unacceptable for ordinary clinical transplantation. Therefore, as we have extended the use of this modality for tolerance induction from mice to large animal models, we have attempted to design preparative regimens that avoid both of these complications. In this article, we review our studies of mixed chimerism in mice, miniature swine and monkeys, as well as the results of our recent clinical studies that have extended this treatment modality to a series of kidney transplant patients who have been successfully weaned from all immunosuppression while maintaining stable renal function for up to 8 years.

Human endothelial cells generate Th17 and regulatory T cells under inflammatory conditions

Organ transplantation represents a unique therapeutic option for irreparable organ dysfunction and rejection of transplants results from a breakdown in operational tolerance. Although endothelial cells (ECs) are the first target in graft rejection following kidney transplantation, their capacity to alloactivate and generate particular T lymphocyte subsets that could intervene in this process remains unknown. By using an experimental model of microvascular endothelium, we demonstrate that, under inflammatory conditions, human ECs induced proliferation of memory CD4(+)CD45RA(-) T cells and selectively amplified proinflammatory Th17 and suppressive CD45RA(-)HLA-DR(+)FoxP3(bright) regulatory CD4(+) T lymphocytes (Tregs). Although HLA-DR expression on resting microvascular ECs was sufficient to induce proliferation of memory CD4(+) T cells, Treg amplification was dependent on the interaction with CD54, highly expressed only under inflammatory conditions. Moreover, expansion of Th17 cells was dependent on IL-6 and STAT-3, and inhibition of either specifically impaired Th17, without altering Treg expansion. Collectively these data reveal that the HLA-DR(+) ECs regulate the local inflammatory allogeneic response, promoting either an IL-6/STAT-3-dependent Th17 response or a contact-CD54-dependent regulatory response according to the cytokine environment. Finally, these data open therapeutic perspectives in human organ transplantation based on targeting the IL-6/STAT-3 pathway and/or promoting CD54 dependent Treg proliferation.

Allospecific regulatory effects of sirolimus and tacrolimus in the human mixed lymphocyte reaction

BACKGROUND

Tacrolimus (TAC) and sirolimus (SRL), two commonly used immunosuppressive agents, have demonstrated contrasting immunoregulatory effects. We recently described factors affecting the generation of allospecific CD4CD25 forkhead/winged helix transcription factor P3 (FOXP3) T-regulatory (Treg) cells in mixed lymphocyte reaction (Treg MLR) and now report additional findings on the effects of TAC and SRL.

METHODS

TAC, SRL, or media without agents were added separately to MLRs using human leukocyte antigen two DR-matched and -mismatched healthy volunteers and prekidney transplant donor/recipient pairs. Concentrations correlated with subtherapeutic and therapeutic blood levels. Stimulation indices of H-TDR uptake, cell proliferation, and the generation of carboxy-fluorescein diacetate succinimidyl ester (CFSE) labeled CD4CD25FOXP3 cells by flow cytometry were initially compared. Each group of (non-CFSE labeled) MLR-generated cells were then added as third components to CFSE-labeled responding cells in freshly prepared primary MLRs, to determine allospecific and nonspecific inhibitory and Treg recruitment effects.

RESULTS

TAC inhibited stimulation indices and CD4CD25 FOXP3 cell generation in both human leukocyte antigen DR-matched and -mismatched pairs, particularly at therapeutic levels (≥5 ng/mL). SRL had an equivalent effect in matched pairs but was associated with a significantly higher %generation of CD4CD25FOXP3 cells than TAC. SRL-MLR-generated Tregs added as third components allospecifically inhibited MLR proliferation and recruited additional CFSE-labeled autologous Tregs compared with addition of TAC- or media-MLR-generated Tregs.

CONCLUSIONS

Calcineurin and mammalian target of rapamycin inhibitors have disparate effects on allospecific Treg generation using the Treg MLR. This assay can thereby be helpful in assessing allospecific regulatory effects of diverse immunosuppressive agents.