Immunosuppressive and metabolic agents that influence allo- and xenograft survival by in vivo expansion of T regulatory cells

The transplanted organs or cells survive if the recipient receives adequate long-term immunosuppressive therapy. Immunosuppressive therapy combined with cell-based strategies (eg, regulatory T cell [Treg]-based therapy) promotes graft survival. A combination of Treg-based therapy and minimal or no immunosuppressive drug therapy would have the potential to minimize the risks of the complications and side effects of these drugs. Fortunately, some immunosuppressive and other agents not only impede the effector T cell response, but also help generate new CD4⁺ Tregs from conventional effector T cells. These agents include IL-2, TGF-β, agents that block the CD40/CD40L costimulation pathway, mTOR inhibitors, and histone deacetylase inhibitors. Consequently, a state of relative unresponsiveness to the transplanted organ may be induced through the expansion of Tregs. We here review the effect of these various agents on expansion of CD4⁺ Tregs in allo- and xenotransplantation. The expansion of Tregs might allow a dose reduction of the standard immunosuppressive drugs.     

Costimulation blockade and its possible future use in clinical transplantation

The nonimmune effects of currently used immunosuppressive drugs result in a high incidence of late graft loss due to nephrotoxicity and death of patients. As an immune-specific alternative to conventional immunosuppressants, new biotechnology tools can be used to block the costimulation signals of T-cell activation. Many experimental studies--particularly preclinical studies in nonhuman primates--have focused on blocking the 'classical' B7/CD28 and CD40/CD40L pathways, which are critical in primary T-cell activation. Here, we review the limitations, the recent advances and the first large-scale clinical application of the CTLA4-Ig fusion protein to block the B7/CD28 costimulation pathway. We also focus on new B7/CD28 and tumor necrosis factor (TNF)/TNF-R family costimulatory molecules that can deliver positive or negative costimulation signals regulating the alloimmune response. Strategies that use single agents to block costimulation have often proved to be insufficient. Given the diversity of the different costimulation molecules, future strategies for human transplantation may involve the simultaneous blockade of several selected pathways or the simultaneous use of conventional immunosuppressants.     

New immunosuppressive agents in transplantation

Immunosuppressive agents have enabled the development of allogenic transplantation during the last 40 years, allowing considerable improvement in graft survival. However, several issues remain such as the nephrotoxicity of calcineurin inhibitors, the cornerstone of immunosuppressive regimens and/or the higher risk of opportunistic infections and cancers. Most immunosuppressive agents target T cell activation and may not be efficient enough to prevent allo-immunization in the long term. Finally, antibody mediated rejection due to donor specific antibodies strongly affects allograft survival.Many drugs have been tested in the last decades, but very few have come to clinical use. The most recent one is CTLA4-Ig (belatacept), a costimulation blockade molecule that targets the second signal of T cell activation and is associated with a better long term kidney function than calcineurin inhibitors, despite an increased risk of acute cellular rejection.The research of new maintenance long-term immunosuppressive agents focuses on costimulation blockade. Agents inhibiting CD40-CD40 ligand interaction may enable a good control of both T cells and B cells responses. Anti-CD28 antibodies may promote regulatory T cells. Agents targeting this costimulation pathways are currently evaluated in clinical trials.Immunosuppressive agents for ABMR treatment are scarce since anti-CD20 agent rituximab and proteasome inhibitor bortezomib have failed to demonstrate an interest in ABMR. New drugs focusing on antibodies removal (imlifidase), B cell and plasmablasts (anti-IL-6/IL-6R, anti-CD38…) and complement inhibition are in the pipeline, with the challenge of their evaluation in such a heterogeneous pathology.     

Novel strategies in immunosuppression: issues in perspective

Recent findings suggest that a chronic alloimmune response is playing the dominant role in late allograft loss, challenging the notion that most grafts are lost due to the inexorable progression of calcineurin inhibitor (CNI) nephrotoxicity. CNIs have failed to improve long-term outcomes and are associated with multiple metabolic derangements. Thus, improvement in long-term allograft outcomes may depend on new agents with novel mechanisms of action, devoid of the toxicities associated with CNIs. To meet this need, inhibitors of novel pathways in B cell and plasma cell activation have emerged to combat the humoral immune response including belimumab and atacicept, both promising targets of B-cell survival factors and bortezomib and eculizumab, agents currently in trials for desensitization protocols and treatment of antibody-mediated rejection. Promising agents for maintenance immunosuppression, used as monotherapy or synergistically, include monoclonal antibodies and fusion receptor proteins targeting the CD40-CD154 pathway (multiple anti-CD40 antibodies), the CD28-CD80/86 pathway (i.e., belatacept), the LFA3-CD2 pathway (i.e., alefacept), and small molecules such as tofacitinib, a janus kinase 1/3 inhibitor. The induction of allograft tolerance has been attempted with some success with simultaneous bone marrow/kidney transplantation from the same donor, albeit, limited by its associated toxicites. Finally, the exciting fields of tissue engineering and stem cell biology with the repopulation of decellularized organs is ushering in a new paradigm for transplantation. The era of simplified immunosuppression regimens devoid of toxicities is upon us with the promise of dramatic improvement in long term survival.     

Protein therapies and antiproliferatives: a new paradigm in immunosuppression

The development of immunosuppressive therapies has focused on inhibiting effects of the activated T cell. The introduction of powerful immunosuppressive agents that interrupt the effects of T-cell activation, such as the calcineurin inhibitors (CNIs), has revolutionized solid organ transplantation. However, the ubiquitous location of their targets causes a number of side effects, which can compromise recipient health and long-term allograft survival. Therefore, a common goal in the development of emerging immunosuppressive strategies is to maintain efficacy and minimize toxicities related to these immunosuppressant compounds. The rationale for CNI-free regimens that exploit combinations of antiproliferative and protein therapeutic agents is attractive. Recently, studies employing these agents in CNI-free regimens have begun to offer additional insight into both the potential benefits and limitations of currently available strategies. The currently available biologic agents provide either too potent immunosuppression (eg, T-cell depletion) or inhibit an aspect of T-cell activation too limited to provide adequate rejection prophylaxis (eg, interleukin 2 receptor [IL-2R] blockade). Growing evidence suggests that costimulation blockade, particularly those protein therapeutics targeting CD28 and CD40, provides the correct balance between immunosuppressive and low toxicity, with a more specific, nondepleting, and timely targeting of the immune response. Already, results from a phase 2 trial suggests that combination with a costimulation blockade using belatacept with mycophenolate mofetil as a maintenance therapy after induction with an IL-2R blocker is closest to fulfill this promise. Belatacept represents an emerging immunosuppression paradigm with maintenance protein therapy that fulfills the need of more selective immunosuppression with reduced toxicities, which offers the potential of improving long-term outcomes in renal transplant.     

Trends of rapamycin in survival benefits of liver transplantation for hepatocellular carcinoma

The proportion of liver transplantation (LT) for hepatocellular carcinoma (HCC) has kept on increasing over the past years and account for 20%-40% of all LT. Post-transplant HCC recurrence is considered the most important factor affecting the long-term survival of patients. The use of different types of immunosuppressive agents after LT is closely associated with an increased risk for HCC recurrence. The most commonly used conventional immunosuppressive drugs include the calcineurin inhibitors tacrolimus (FK506) and mammalian target of rapamycin inhibitor rapamycin (RAPA). Compared with tacrolimus, RAPA may carry an advantage in survival benefit because of its anti-tumor effects. However, no sufficient evidence to date has proven that RAPA could increase long-term recurrence-free survival and its anti-tumor mechanism of combined therapy remains incompletely clear. In this review, we will focus on recent advances in clinical application experience and basic research results of RAPA in patients undergoing LT for HCC to further guide the clinical practice.     

Strategies to prevent ischemic heart disease after kidney transplantation

Preventing ischemic heart disease (IHD) after kidney transplantation requires a comprehensive clinical strategy. Early referral and pretransplant screening for IHD may help prevent posttransplant IHD events. Perioperative β-blockade may also be effective. The management of traditional risk factors, both before and after transplantation, may include aspirin prophylaxis, cigarette abstinence, treatment of hypertension, treatment of dyslipidemias, and intensive blood glucose control. Although the risk for IHD can be reduced by minimizing the use of immunosuppressive agents that adversely affect cardiovascular risk factors, the management of risk factors must also include a strategy of optimal immunosuppression to prevent acute rejection and maximize long-term kidney function. A number of lifestyle modifications that may favorably affect cardiovascular disease risk factors should also be encouraged. In summary, a multidisciplinary approach that emphasizes evidence-based management of traditional risk factors is currently the best approach to reducing the risk for IHD after kidney transplantation.     

Antifungal attributes of immunosuppressive agents: new paradigms in management and elucidating the pathophysiologic basis of opportunistic mycoses in organ transplant recipients

The currently available immunosuppressive agents cyclosporine A, tacrolimus, and rapamycin have potent antifungal activity against a number of opportunistic fungi in organ transplant recipients, most notably, C. neoformans, Candida, and Aspergillus species. The targets of their antifungal activity are fungal homologs of the signaling molecules that mediate their immunosuppressive action in humans, which has implications for further unraveling the pathogenesis of these infections. Corroborative clinical data suggest that despite the apparent paradox between the antifungal activity of the immunosuppressive agents and the occurrence of fungal infections during their administration, the antifungal attributes of these drugs may influence the spectrum and clinical characteristics of these infections after organ transplantation. Finally, the potent synergistic interactions between the immunosuppressive agents and antifungal drugs against many pathogenic fungi, including those that are typically resistant to traditional antifungal agents, could potentially have a role in devising novel therapeutic strategies for opportunistic mycoses in transplant recipients.     

Individualizing early use of sirolimus in renal transplantation

One of the main goals in the current care of kidney transplant recipients is to extend long-term graft survival. Efficacious immunosuppressive agents devoid of nephrotoxicity are needed. In human clinical transplantation, sirolimus combined with other immunosuppressive drugs has proven to be a powerful immunosuppressant capable of preventing acute graft rejection, as well as of improving renal function, renal histology, and graft survival when compared with immunosuppressive regimens that include calcineurin inhibitors. The valuable experience gained through many clinical studies allows clinicians to plan sirolimus use. We present a review of the clinical experience and literature review on the use of sirolimus in the first 12 months posttransplantation.     

Review of select transplant subpopulations at high risk of failure from standard immunosuppressive therapy

Despite improvements in short-term graft and patient survival rates for solid organ transplants, certain subgroups of transplant recipients experience poorer clinical outcome compared to the general population. Groups including pediatrics, African-Americans, diabetics, cystic fibrosis patients, and pregnant women require special considerations when designing immunosuppressive regimens that optimize transplant outcomes. Problems specific to pediatric transplant recipients include altered pharmacokinetics of immunosuppressive drugs, such as cyclosporine (CsA) and tacrolimus (poor absorption, increased metabolism, rapid clearance), the need to restore growth post-transplantation, and a high incidence of drug-related adverse effects. African-Americans have decreased drug absorption and bioavailability, high immunologic responsiveness, and a high incidence of post-transplant diabetes mellitus. Diabetics and cystic fibrosis patients exhibit poor absorption of immunosuppressive agents, which may lead to underimmunosuppression and subsequent graft rejection. Pregnant women undergo physiologic changes that can alter the pharmacokinetics of immunosuppressives, thus requiring careful clinical management to minimize the risks of either under- or overimmunosuppression to mother and child. To achieve an optimal post-transplant outcome in these high-risk patients, the problems specific to each group must be addressed, and immunosuppressive therapy individualized accordingly. Drug formulation greatly impacts upon pharmacokinetics and the resultant level of immunosuppression. Thus, a formulation with improved absorption (e.g., CsA for microemulsion), higher bioavailability, and less pharmacokinetic variability may facilitate patient management and lead to more favorable outcomes, especially in groups demonstrating low and variable bioavailability. Other strategies aimed at improving transplant outcome include the use of higher immunosuppressive doses, different combinations of immunosuppressive agents, more frequent monitoring, and management of concurrent disease states.     

New concepts in organ transplantation

Long-term graft survival is the desired outcome of organ transplantation. The surrogate metric elimination of acute rejection episodes is not only inadequate but also deceptive, since this freedom does not promise long-term graft survival. Current clinical immunosuppressive agents have reduced acute rejection, but not prolonged graft survival. New paradigms in organ transplantation focus on adhesion-migration events using a selectin antagonist, an antisense oligonucleotide, and FTY 720; on peptide or allochimeric antigens on cytokine disruption, and on inhibition of costimulatory signals. Due to the array of adverse reactions to the available immunosuppressive drugs, these new approaches aim not only to augment long-term graft survival, but also minimize the associated toxicities.     

The Maastricht Transplant Center: Clinical setting and epitope searches in HLA class II molecules: Does the structural localization of a polymorphic site contribute to its immunogenicity?

Our understanding of the immunological processes influencing the clinical outcome after kidney transplantation has advanced majorly over the last few decades. However, many factors still restrict graft and patient survival. Within the Maastricht transplant center we have successfully implemented an alternative immunosuppressive regimen involving Tacrolimus monotherapy in order to minimize the adverse effects associated with long-term use of immunosuppressive drugs. This clinical development has an impact on pre-transplant risk stratification which requires that patients are closely monitored immunologically. In this review we will elaborate on our strategy regarding the analysis of epitopes in HLA-DQ and HLA-DP molecules. In this respect we have also looked at the immunodominance of certain epitopes by assessing their structural localization, conformation and physiochemical properties.     

New immunosuppressive therapies in renal transplantation: monoclonal antibodies

Remarkable advances in understanding the mechanisms of immune recognition and allograft rejection have been made in the past few years, leading to the development of innovative immunosuppressive strategies in the field of renal transplantation. Monoclonal antibodies (mAbs) have emerged as a new class of immunosuppressive agents, which appear to be effective (in both the treatment and the prevention of acute rejection) and well-tolerated in renal transplant recipients. The highly specific effects of these drugs make them less toxic than the oral long-term maintenance agents such as corticosteroids and the calcineurin inhibitors. Some of these mAbs have already confirmed their efficacy in preventing acute rejection in clinical phase III studies, and are now part of the well-established immunosuppressive regimens; these are the anti-CD25 mAbs (basiliximab and daclizumab). Other recently developed mAbs, like anti-CD52 (Campath-1H), anti-CD20 (rituximab), anti-LFA-1, anti-ICAM-1 and anti-tumour necrosis factor (TNF)-alpha (infliximab), are currently being tested, and show encouraging immunosuppressive potential. Blocking either the binding of cell-surface molecules or intracellular signal transduction, these mAbs could become an effective method to promote the holy grail of solid-organ transplantation, antigen-specific tolerance.     

Cyclosporine: a new and promising immunosuppressive agent

Cyclosporine is an exciting new agent that has been heralded as a major advance in chemical immunosuppression for organ transplantation. The drug is a potent, reversible suppressant of both humoral and cellular immunity and does not cause myelosuppression. Its mechanism of action appears to be selective for lymphocytes and may interrupt the necessary cellular signals required for proliferation of alloreactive T-cells. In early clinical trials cyclosporine has been shown to ameliorate renal allograft survival. A major concern about its widespread clinical use has been the observed nephrotoxicity and possible development of lymphomas in treated patients. Adverse side effects may be minimized by pharmacologic monitoring of drug levels. Future questions include the ideal dosage schedule, necessity for additional immunosuppressive agents, distinction between nephrotoxicity and rejection, and the precise mode of absorption, accumulation, and metabolism of the drug. Cyclosporine may be considered the prototype of a new generation of immunosuppressive agents that open up new perspectives in the field of immunoregulation. The ability to synthesize the compound may permit future biochemical manipulations to increase the immunobiologic specificity and decrease the toxicity of the drug.     

Current concepts and perspectives of immunosuppression in organ transplantation

Background While early surgical success made organ transplantation possible in the 1950s and 1960s, the breakthrough in clinical organ transplantation was achieved through the discovery and invention of modern immunosuppressive agents in the early/mid-1980s. Especially during the 1990s, a large array of immunosuppressants has expanded the armamentarium used to prevent and treat allograft rejection, resulting in an excellent short-term and an acceptable long-term outcome. However, these drugs have potent but still non-specific immunosuppressive properties and frequently show severe acute and chronic side effects, sometimes questioning the overall success. Concepts/Trends As the “Holy-Grail” of the transplant community, the induction of “true donor-specific tolerance” has not been achieved yet; current immunosuppressive strategies, in particular in Europe, include “individually tailored immunosuppressive” protocols, mostly based on specific immunologic and non-immunologic risk factors. These protocols allow for optimal immunosuppressive protocols for each patient group according to their needs by choosing the most suitable, well-tolerated combination of agents and the most effective doses to avoid acute rejection episodes (incidence and severity) and minimise drug-related toxicity to reduce long-term drug-related morbidity and mortality. Nevertheless, transplant recipient are still being forced to take a life-long course of chemical immunosuppressive agents to keep their graft, knowing about the possible life-threatening side effects. Summary We review current trends of immunosuppressive protocols in liver and kidney transplantation, focusing on calcineurin-inhibitor-sparing protocols, mammalian-target-of-rapamycin (mTOR) inhibitor based-protocols and corticosteroid-avoidance protocols, being aware of the fact, that most of these strategies could be applicable for other transplanted organs, too. Finally, we describe future trends and new developments that are rising on the horizon.     

Ageing and immunosuppression in kidney transplantation

Modern approaches to tailor-made, individualized immunosuppressive therapy for patients receiving organ transplantation require a rethinking of therapeutic strategies when it comes to older persons receiving kidney transplants, especially from deceased older donors. This review article makes the case for the use of calcineurin-inhibitor-free immunosuppressive induction/maintenance protocols in this "worst-case scenario" and discusses the theoretical and clinical data that support this recommendation. We will discuss modern theories of ageing, emphasizing the free-radical theory in relation to new insights into the mechanisms of innate immunity. In this context, a new, modified theory of ageing is presented. Increased generation of reactive oxygen species during ageing, via increased leakage of these oxidizing molecules from mitochondria, may contribute to senescence and age-related diseases by direct damage to intracellular DNA, proteins, and lipids. In addition, free-radical-mediated tissue injury, accompanied by induction of damage-associated molecular patterns, may result in activation of both inflammatory and vascular cells of the innate immune system, contributing (via inflammatory processes) to ageing and age-related diseases such as atherosclerosis. Calcineurin-inhibiting agents have been shown to induce oxidative stress and are thus defined as "proageing" drugs. Their use in older patients may aggravate the preexisting oxidized intracellular state and therefore should be avoided. In contrast, inosine-monophosphate dehydrogenase-inhibiting agents such as mycophenolate mofetil have been shown to even ameliorate oxidative stress and are thus defined as "antiageing" drugs. Therefore, their use for immunosuppression in older patients receiving kidney transplantation is suggested. This recommendation is supported by data from a prospective trial on the application of a calcineurin inhibitor-free, mycophenolate-mofetil--based induction/ maintenance immunosuppressive protocol in older recipients of kidneys from deceased older donors: the 5-year patient and 5-year allograft survival rates are currently 87% and 70%, respectively.     

Minimization of immunosuppression: transplant immunology

Induction of tolerance, which obviates the need for maintenance immunosuppression following organ transplantation remains elusive. In cardiac transplantation, ongoing immunosuppressive therapy is essential to ensure long-term graft survival. Although drug regimens have substantially improved in recent years, their adverse effects continue to cause significant morbidity and affect quality of life. Newer immunosuppressive therapies have been effective at reducing allograft rejection rates in the short term but long term outcomes have changed little in the last two decades. Minimization of immunosuppression requires appreciation of the potential consequence. High risk patients in particular need to be identified and excluded from low intensity immunosuppressive regimens. A variety options exist for lowering of immunosuppression and steroid weaning has now become common practice with about 40% of all cardiac transplant recipients remaining steroid free in the long term. Minimization of calcineurin inhibitor exposure may be achieved with concurrent use of the more potent anti-proliferative agents mycophenolate mofetil and sirolimus. Patients require close monitoring for rejection during weaning. In addition to the conventional clinical parameters which include therapeutic drug monitoring, endomyocardial biopsy and echocardiography, newer techniques for monitoring hold future promise. These include detection of circulating alloantibodies and quantitative measurement of the net state of immunosuppression (Cylex). However, the efficacy of these modalities requires further investigation.     

Effects of sotrastaurin, mycophenolic acid and everolimus on human B-lymphocyte function and activation

Humoral rejection processes may lead to allograft injury and subsequent dysfunction. Today, only one B-cell-specific agent is in clinical use and the effects of standard and new immunosuppressant substances on B-cell activation and function are not fully clarified. The impact of sotrastaurin, mycophenolic acid and everolimus on human B-lymphocyte function was assessed by analysing proliferation, apoptosis, CD80/CD86 expression and immunoglobulin and IL-10 production in primary stimulated B cells. In addition, B-cell co-cultures with pre-activated T cells were performed to evaluate the effect of the different immunosuppressive agents on T-cell-dependent immunoglobulin production. Sotrastaurin did not inhibit B-cell proliferation, CD80/CD86 expression, and IgG production and had only minor effects on IgM levels at the highest concentration administered. In contrast, mycophenolic acid and everolimus had strong effects on all B-cell functions in a dose-dependent manner. All immunosuppressive agents caused decreased immunoglobulin levels in T-cell-dependent B-cell cultures. The data provided here suggest that mycophenolic acid and everolimus, but not sotrastaurin, are potent inhibitors of human B-lymphocyte function and activation.     

What's Next in the Pipeline

The first decade of the new millennium has been disappointing for transplant therapeutics: no new immunosuppression agents have been approved. Several high profile drugs and biologics failed the rigors of clinical trials or had disappointing preclinical results (FTY720, FK778, anti‐CDI54, anti‐IL15, anti‐CD28, R3421). Several challenges face the industry and clinical investigators in bringing novel drugs to the clinic including the difficulty in targeting new endpoints for toxicities or chronic allograft disease since acute rejection has been reduced to below 15% as well as the Food and Drug Administration insistence of excluding the use of immunosuppression regimens embraced by the transplant community in control arms of clinical trials. Currently six new agents, 3 small molecules (ISA247, a semisynthetic analogue of cyclosporine; AEB071, a protein kinase C isoforms inhibitor; CP 690,550, a selective Janus kinase inhibitor) are in phase II trials and 3 biologics (belatacept, a second generation CTLA4Ig; efalizumab, a humanized antiCD11a [LFA1] monoclonal antibody; and alefacept, a LFA3‐IgG1 fusion receptor protein) are in phase II/III clinical trials. The preclinical pipeline is not only full but promises to address previously neglected targets and fulfill unmet medical needs in transplant therapeutics.