Clinical transplantation tolerance twelve years after prospective withdrawal of immunosuppressive drugs: studies of chimerism and anti-donor reactivity

Combined islet and hematopoietic stem cell allotransplantation: a clinical pilot trial to induce chimerism and graft tolerance

To prevent graft rejection and avoid immunosuppression-related side-effects, we attempted to induce recipient chimerism and graft tolerance in islet transplantation by donor CD34+hematopoietic stem cell (HSC) infusion. Six patients with brittle type 1 Diabetes Mellitus received a single-donor allogeneic islet transplant (8611 +/- 2113 IEQ/kg) followed by high doses of donor HSC (4.3 +/- 1.9 x 10(6) HSC/kg), at days 5 and 11 posttransplant, without ablative conditioning. An 'Edmonton-like' immunosuppression was administered, with a single dose of anti-TNFalpha antibody (Infliximab) added to induction. Immunosuppression was weaned per protocol starting 12 months posttransplant. After transplantation, glucose control significantly improved, with 3 recipients achieving insulin-independence for a short time (24 +/- 23 days). No severe hypoglycemia or protocol-related adverse events occurred. Graft function was maximal at 3 months then declined. Two recipients rejected within 6 months due to low immunosuppressive trough levels, whereas 4 completed 1-year follow-up with functioning grafts. Graft failure occurred within 4 months from weaning (478 +/- 25 days posttransplant). Peripheral chimerism, as donor leukocytes, was maximal at 1-month (5.92 +/- 0.48%), highly reduced at 1-year (0.20 +/- 0.08%), and was undetectable at graft failure. CD25+T-lymphocytes significantly decreased at 3 months, but partially recovered thereafter. Combined islet and HSC allotransplantation using an 'Edmonton-like' immunosuppression, without ablative conditioning, did not lead to stable chimerism and graft tolerance.

Effects of biomaterial-induced inflammation on fibrosis and rejection

Evidence is emerging that biomaterials cause inflammation by ligating innate immune receptors on antigen presenting cells. Although inflammation is usually viewed as detrimental, it has unexpected and potentially beneficial effects on fibrosis and transplant rejection. For example, the magnitude of inflammation due to a biomaterial is not predictive of the extent of fibrosis. Similarly, biomaterials do not always show adjuvancy. Some biomaterials suppressed T cell rejection responses in vivo and in vitro, while others non-specifically stimulated T cell proliferation. Understanding these complex inter-relationships is the key to designing a biomaterial that stimulates regeneration and induces tolerance in tissue engineering applications.

Protective conditioning against GVHD and graft rejection after combined organ and hematopoietic cell transplantation

We have performed combined organ and hematopoietic cell transplantation using a similar conditioning regimen in mice and humans. In the mouse model of MHC-mismatched combined heart and marrow transplantation, we compared conditioning of BALB/c hosts with total lymphoid irradiation (TLI: 10 doses of 240 cGy each) targeted to the spleen, lymph nodes and thymus to conditioning with a single dose of sublethal total body irradiation (TBI; 450 cGy). Conditioning also included three injections of anti-thymocyte serum (ATS), in both groups. C57BL/6 heart grafts, marrow cells and blood mononuclear cells were transplanted 24 h after the completion of irradiation. Blood mononuclear cells were added to the marrow cells to engender severe graft versus host disease (GVHD) that is present after combined organ and hematopoietic cell transplantation in humans given non-myeloablative conditioning. Both TLI and TBI conditioned groups accepted the organ grafts and became stable chimeras. However, the TBI group all died of GVHD during the 100-day observation period. The TLI group survived during the same period without clinical signs of GVHD. These hosts were tolerized to the donor organ grafts, since third party grafts were rejected rapidly when transplanted after 100 days. When NK T-cell-deficient CD1d(-/-) BALB/c hosts were used instead of wild-type hosts in the TLI/ATS conditioned group, then all hosts survived but all rejected the organ grafts and almost all failed to develop stable chimerism. None developed GVHD. Since host NK T cells were required for graft acceptance and NK T cells are activated after recognition of CD1d on antigen presenting cells, we compared heart and marrow graft survival from wild-type versus CD1d(-/-) donors after transplantation to TLI and ATS conditioned wild-type hosts. Whereas marrow and heart grafts from wild-type donors were accepted, almost all grafts from CD1d donors were rejected. Grafts from control Jalpha18(-/-) donors that were NK T cell deficient but expressed CD1d were all accepted. The results indicate that host NK T cells facilitate graft acceptance by recognizing CD1d on donor cells. We applied the TLI conditioning regimen using 10 doses of 80 cGy each and 5 doses of rabbit ATG to human recipients of HLA-matched G-CSF "mobilized" blood mononuclear cell transplants for the treatment of leukemia and lymphoma [R. Lowsky, T. Takahashi, Y.P. Liu, et al., Protective conditioning for acute graft-versus-host disease. N. Engl. J. Med. 353 (2005) 1321-1331.]. Currently more than 100 transplants have been performed, and the incidence of acute GVHD has been about 4% when both MRD and MUD transplants are combined. Almost all recipients became complete chimeras after receiving grafts that contained 2-3x10(8) CD3(+) T cells/kg. In further studies, we applied the same TLI and ATG conditioning regimen to combined kidney and G-CSF "mobilized" blood stem cell transplantation from HLA-matched sibling donors. The hematopoietic grafts in the latter protocol were selected CD34(+) cells with 1x10(6) CD3(+) T cells/kg added back to the hematopoietic cells. Preliminary results indicate that stable mixed chimerism can be achieved using this protocol allowing for complete immunosuppressive drug withdrawal without GVHD or subsequent rejection episodes. Thus, conditioning with TLI based regimens can simultaneously protect against organ graft rejection and GVHD. Levels of chimerism are dependent upon the content of donor T cells in the hematopoietic graft.

Microchimerism and renal transplantation: doubt still persists

OBJECTIVE

We sought to study microchimerism in a group of kidney transplant recipients.

MATERIALS AND METHODS

In this study, the peripheral blood microchimerism (PBM) after renal transplantation was retrospectively evaluated in 32 male-to-female recipients of living unrelated or cadaveric donor renal transplants. Using a nested polymerase chain reaction (PCR) amplification specific for SRY region of the Y chromosome, microchimerism was detected with a sensitivity of 1:1,000,000. Recipients were compared according to the presence of PBM, acute and chronic rejection episodes, type of allotransplant, recipient and donor age at transplantation, previous male labor or blood transfusion, allograft function (serum creatinine level), and body mass index.

RESULTS

Among 32 recipients, 7 (21.9%) were positive for PBM upon multiple testing at various posttransplant times. All microchimeric recipients had received kidneys from living unrelated donors. No significant difference was observed with regard to other parameters. In addition the acute rejection rate in the microchimeric group was 3 (42%) versus 4 (16%) in the nonmicrochimeric recipients (not significant).

CONCLUSION

Our results suggested better establishment of microchimerism after living donor kidney transplantation. However, doubt persists concerning the true effect of microchimerism after renal transplantation. It seems that microchimerism alone has no major protective role upon renal allograft survival.

From current immunosuppressive strategies to clinical tolerance of allografts

In order to prevent allograft rejection, most current immunosuppressive drugs nonspecifically target T-cell activation, clonal expansion or differentiation into effector cells. Experimental models have shown that it is possible to exploit the central and peripheral mechanisms that normally maintain immune homeostasis and tolerance to self-antigens, in order to induce tolerance to alloantigens. Central tolerance results from intrathymic deletion of T cells with high avidity for thymically expressed antigens. Peripheral tolerance to nonself-molecules can be achieved by various mechanisms including deletion of activated/effector T cells, anergy induction and active regulation of effector T cells. In this article, we briefly discuss the pathways of allorecognition and their relevance to current immunosuppressive strategies and to the induction of transplantation tolerance (through haematopoietic mixed chimerism, depleting protocols, costimulatory blockade and regulatory T cells). We then review the prospect of clinical applicability of these protocols in solid organ transplantation.

Monitoring tolerance after human liver transplantation

The validation of reliable, non-invasive immunological assays evaluating anti-donor responsiveness in allograft recipients would provide a clinically relevant tool for the early detection of ongoing rejection process as well as for the identification of operational tolerance in the long term. A sequential approach towards immunological monitoring of allografts is proposed in this review: (i) investigations exploring the initial donor-recipient alloresponses, including the analysis of the cytokine network; (ii) investigations regarding graft acceptance and operational tolerance in long-term transplant patients, consisting in the analysis of regulatory T cells and of circulating precursors of dendritic cells, in the measurement of T cell alloreactivity as well as in the study of T cell receptor repertoires. Beside the conventional in vivo and in vitro immunological techniques, the potential applications of molecular imaging in transplantation also deserve further exploration, with particular respect to allograft immune monitoring. Enforced collaboration between transplant clinicians and immunologists will be required to develop the translational research protocols required for the development of immunological monitoring, within an international multicentric network.

Is clinical tolerance realistic in the next decade?

Tolerance to allografts would mean a better quality of life and prognosis for transplant patients. Despite the first descriptions of tolerance to alloantigens over 50 years ago, deliberately induced tolerance in the clinic on a wide scale remains a goal that is not quite in reach. However, much progress has been made in understanding tolerance in rodent models and in the few reports of induced or spontaneously occurring tolerance in humans. Here, we review this progress made in the quest to achieve clinical tolerance.

Tolerance Assays: Measuring the Unknown

Distinguishing transplant recipients who will benefit from a reduction in, or even the withdrawal of, immunosuppression from those who require intensive, lifelong immunosuppression will be dependent on developing strategies for immune monitoring. Currently, no assays have been shown to accurately predict the development or presence of donor-specific tolerance in humans after transplantation. In this overview we describe and discuss those assays that we believe may be useful for identifying tolerant transplant recipients. Validation of "tolerance" assays will be critical for the safe development of tolerance regimens in humans.

Contrasting CD25hiCD4+T cells/FOXP3 patterns in chronic rejection and operational drug-free tolerance

BACKGROUND

Although immunosuppression withdrawal in kidney recipients usually leads to rejection, in some patients it does not, leading to a state of clinical operational tolerance.

METHODS

We compared these highly contrasted situations by analyzing blood cell phenotype and transcriptional patterns in drug-free spontaneously tolerant kidney recipients, recipients with chronic rejection, recipients with stable graft function under standard or minimal immunosuppression and healthy individuals

RESULTS

The blood cell phenotype of clinically tolerant patients did not differ from that of healthy individuals. In contrast, recipients with chronic rejection had significantly less CD25hiCD4+T cells and lower levels of FOXP3 transcripts compared with clinically tolerant recipients. Patients with chronic rejection also displayed CD25-CD4+T cells expressing NKG2D+CD94+ and CD57+CD27-CD28- cytotoxic-associated markers (P<0.05).

CONCLUSION

These data show that whereas clinically tolerant recipients displayed normal levels of CD25hiCD4+T cells and FOXP3 transcripts, chronic rejection is associated with a decrease in CD25hiCD4+T cells and FOXP3 transcripts, suggesting that clinically "operational tolerance" may be due to a maintained phenomenon of natural tolerance that is lacking in patients with chronic rejection.

Clinical operational tolerance after kidney transplantation

Induction of allograft-specific tolerance and the detection of a "tolerance" state in recipients under immunosuppression with long-term stable graft function are major challenges in transplantation. Clinical "operational tolerance," defined as stable and acceptable graft function without immunosuppression for years, is a rare event. There is no report on the clinical history of such patients. In this article, we report on the medical history of 10 kidney recipients who display an immunosuppressive drug-free "operational tolerance" for 9.4 +/- 5.2 years. Clinical factors that may favor such a tolerant state are underlined. Firstly, most of the patients interrupted immunosuppression over a long time period (until 4 years), which mimics the procedure of intentional immunosuppression weaning following liver transplantation. Secondly, donor age was younger (median 25 years) than the one of the general transplanted population, suggesting that graft quality is one of the conditions favoring "operational tolerance." Moreover, the "operationally tolerant" recipients may be 'low responders' to blood transfusions (PRA 6 +/- 5.4%, six blood transfusions). We also show that "operational tolerance" occurs in the presence of anti-donor class II antibodies, as assessed in two patients. Finally, two patients degraded their renal function 9 to 13 years after treatment withdrawal, however only one presented histological lesions of chronic rejection.

The immunological monitoring of alloreactive responses in liver transplant recipients: a review

The aim of this work is to review the current knowledge in the field of immunological monitoring of allogenic responsiveness in clinical liver transplantation. When compared to other solid-organ transplants, liver allografts are considered as immunologically privileged, and, accordingly, constitute a favorable setting to develop experimental as well as clinical strategies for minimization of immunosuppression and even induction of operational tolerance. The validation of simple, reliable, noninvasive assays exploring antidonor alloreactivity will constitute a crucial step toward implementing such approaches in the clinic. In contrast to research in rodents claiming the development of donor-specific tolerance in case of graft survivals of over 100 days without immunosuppression, it is impractical to confirm tolerance induction in this way in humans. Promising candidate assays include the detection of post-transplant immune deviation, of circulating precursors of dendritic cells subtypes, and of regulatory T cells. A conceptual framework for the development of tolerance assays in clinical liver transplantation is also proposed.

Development of tolerogenic strategies in the clinic

The study of tolerance in the clinic can be divided into three areas: (i) focused evaluation of existing tolerant transplant recipients as to their mechanism of tolerance; (ii) prospective tolerance trials, such as combined bone marrow and kidney transplantation as well as T cell depletion followed by subsequent weaning of immunosuppression; and (iii) immunologic assays to assess the likelihood of rejection or tolerance. Frankly, a very small number of patients have been transplanted with the intention of removing all immunosuppressive therapy, but several clinical trials with this aim are currently in progress, largely sponsored by the Immune Tolerance Network, a joint venture between the National Institutes of Health and the Juvenile Diabetes Research Foundation. Similarly, a reliable assay to assess tolerance has not yet been developed but a variety of approaches towards assessing rejection, and in some cases tolerance, are being developed. It would be accurate to state that many of the experimental and preclinical approaches to the induction of tolerance have resulted in better immunosuppression for human transplantation, but reliable tolerance strategies in humans have not yet been achieved. Combined bone marrow and kidney transplantation may be considered as one exception to this, but such a strategy is not generally applicable to the vast majority of solid organ transplant recipients. This review will summarize efforts to date, particularly focusing on kidney transplantation.

Use of highly sensitive mitochondrial probes to detect microchimerism in xenotransplantation models

Chimerism, defined as the co-existence of cells of different origin within the same organism, has received much attention in hematopoietic cell and organ transplantation because of the strict relationship between its establishment and the induction of specific tolerance. Traditional methods applied for chimerism detection, such as immunohistochemistry, cytogenetics, fluorescent-activated cell sorter analysis, and serological and biochemical testing, are limited by their sensitivity. We have established a highly sensitive molecular approach based on the amplification of the mitochondrial cytochrome B gene and tested its specificity and sensitivity level in six different mammalian species, including human, pig, mouse, rat, sheep and rabbit. Increased sensitivity of detection of specific amplification products was obtained by the non-radioactive Southern blot technique. This novel approach allows the detection of one cell against the background of 1 to 4 x 10(6) xenogenec cells and will be helpful for high-sensitivity analysis of donor cell engraftment after xenotransplantation procedures in these animal models.

Protective conditioning for acute graft-versus-host disease

BACKGROUND

Conditioning with total lymphoid irradiation plus antithymocyte serum protects mice against acute graft-versus-host disease (GVHD) after hematopoietic-cell transplantation. We tested this strategy in humans.

METHODS

Thirty-seven patients with lymphoid malignant diseases or acute leukemia underwent an experimental conditioning regimen with 10 doses of total lymphoid irradiation (80 cGy each) plus antithymocyte globulin, followed by an infusion of HLA-matched peripheral-blood mononuclear cells from related or unrelated donors who received granulocyte colony-stimulating factor.

RESULTS

Of the 37 transplant recipients, only 2 had acute GVHD after hematopoietic-cell transplantation. Potent antitumor effects in patients with lymphoid malignant diseases were shown by the change from partial to complete remission. In the transplant recipients who underwent conditioning with total lymphoid irradiation and antithymocyte globulin, the fraction of donor CD4+ T cells that produced interleukin-4 after in vitro stimulation increased by a factor of five, and the proliferative response to alloantigens in vitro was reduced, as compared with normal control subjects and control subjects who underwent conditioning with a single dose of total-body irradiation (200 cGy).

CONCLUSIONS

A regimen of total lymphoid irradiation plus antithymocyte globulin decreases the incidence of acute GVHD and allows graft antitumor activity in patients with lymphoid malignant diseases or acute leukemia treated with hematopoietic-cell transplantation.

Donor bone marrow transplantation: chimerism and tolerance

Infusion of donor bone marrow (DBM)-derived cells continue to be tested in clinical protocols intended to induce specific immunologic tolerance. Central clonal deletion of donor-specific alloreactive cells associated with mixed chimerism reliably produced long-term graft tolerance. In this setting, depletion of recipient T cells by antilymphocyte antibodies and subsequent repopulation by donor hematopoietic cells after donor bone marrow infusion (DBMI) are prerequisites for tolerance induction. Major advances have been made in animal models and in pilot clinical trials and the key questions with the future perspectives are presented in this article.

Transplantation tolerance in pediatric recipients: lessons and challenges

Clinical transplantation tolerance has remained an elusive goal in the 50 yr since it was first described in experimental animals. Greater understanding of the molecular mechanisms responsible for allorecognition have allowed for the development of promising immunosuppressive strategies that may bring us closer to reproducible induction of tolerance; consideration of past successes and failures from both clinical and basic science is required to define future challenges facing this field. This article reviews mechanisms of self and transplantation tolerance, translation of basic science research to clinical protocols in animals and human beings, the changing role of immunosuppression, complications following tolerance induction and controversies surrounding the choice of patients for tolerance trials with a focus on issues relevant to pediatric patients. The role of the Immune Tolerance Network is discussed along with realistic goals for tolerance induction in human beings over the next decade.

Operationally tolerant and minimally immunosuppressed kidney recipients display strongly altered blood T-cell clonal regulation

Most kidney transplant recipients who discontinue immunosuppression reject their graft. Nevertheless, a small number do not, suggesting that allogeneic tolerance state (referred to operational tolerance) is achievable in humans. So far, however, the rarity of such patients has limited their study. Because operational tolerance could be linked to anergy, ignorance or to an active regulatory mechanism, we analyzed the blood T-cell repertoire usage of these patients. We report on comparison of T-cell selection in drug-free operationally tolerant kidney recipients (or with minimal immunosuppression), recipients with stable graft function, chronic rejection and healthy individuals. The blood T cells of operationally tolerant patients display two major characteristics: an unexpected strongly altered T-cell receptor (TCR) Vbeta usage and high TCR transcript accumulation in selected T cells. The cytokine transcriptional patterns of sorted T cells with altered TCR usage show no accumulation of cytokine transcripts (IL10, IL2, IL13, IFN-gamma), suggesting a state of hyporesponsiveness in these patients. Identification of such a potential surrogate pattern of operational tolerance in transplant recipients under life-long immunosuppression may provide a new basis and rationale for exploration of tolerance state. However, these data obtained in a limited number of patients require further confirmation on larger series.

Short tandem repeat analysis to monitor chimerism in macaca fascicularis

Chimerism assessment following bone marrow transplantation (BMT) in cynomolgus monkeys (cynos) has been hampered by the lack of good engraftment markers. In human BMT, such markers have been provided by short tandem repeat (STR) loci. We tested the idea that techniques effective for detecting human STR could be readily adapted to cynos. Genomic DNA was extracted from cyno unseparated blood or peripheral cell subsets. With only slight modifications, reagents for detecting human STR alleles were used to amplify and detect cyno STRs and to quantitate allelic mixtures on an automated sequencer. Of the 15 STR loci tested, only CSF1PO, D18S51, and FGA successfully amplified, with seven, seven and two alleles, respectively. CSF1PO and D18S51 heterozygosity (80% and 55%, respectively) allowed use of these two loci for chimerism quantitation after BMT. The successful adaptation of human STR reagents to monitor chimerism in transplanted cynos will facilitate the use of this species in preclinical tolerance studies.

Revisiting liver transplant immunology: from the concept of immune engagement to the dualistic pathway paradigm

Ever since the demonstration that allografts are rejected through immune reactions of the host, clinical therapies for organ allografts have relied on immune suppression to prevent these destructive events. A growing body of clinical and experimental data suggests that allografts elicit multiple, interactive immune responses. The result is not inevitably graft rejection, and "spontaneous" acceptance of fully allogeneic liver grafts occurs in rodents without immunosuppression. A spectrum of results range from spontaneous acceptance without immunosuppression to rejection with immunosuppression. The "dualistic pathway paradigm" aims to reconcile apparently conflicting observations in liver transplantation and proposes that: (1) immune engagement between the host and the allograft is instrumental in both rejection and acceptance; (2) there exist in all mammalian species congruent interactive pathways of immune activation whereby the fate of the allograft is determined by the quantitative results of these interactions; (3) the dualistic effect of immunosuppressive drugs on pathways of immune activation, conferring the capacity for favorable or unfavorable graft outcome should be investigated in experimental models in which organ allografts are spontaneously accepted. In conclusion the design of clinical strategies based on this research may contribute to protocols resulting in allograft acceptance without chronic immunosuppression.

Tolerance induction in clinical transplantation

Introduction of modern immunosuppressive agents has led to great success of allotransplantation in humans, and survival rates for all solid organs have been dramatically improved. However, a constant proportion of organs is lost every year due to chronic allograft rejection and immunosuppressive drug toxicity. This has led to a situation where, despite the of donor organ shortage, about one third of the patients on the kidney transplant waiting list are listed for a retransplant. The induction of donor-specific tolerance has the potential of at least partially resolving this problem, since it might prevent chronic rejection and drug toxicity at the same time. For a variety of protocols, successful tolerance induction has been demonstrated in rodent models. However, translation of such protocols to large animal models and on clinical trials has turned out to be very difficult. This review briefly describes mechanisms and barriers to transplantation tolerance, and then focuses on pre-clinical and clinical studies in non-human primates and humans. We have divided the strategies into two groups, based on the principle mechanisms of tolerance induction: the first group are protocols not using hematopoietic stem cell transplantation (HCT) as part of there regimen. They rely mainly on intensive T cell depletion (either by total body irradiation, total lymphoid irradiation or treatment with T cell-depleting agents such as anti-thymocyte globulin, anti-CD52 antibody or CD3 immunotoxin), which have been combined with costimulatory blockade, signaling blockade or donor antigen infusion. The second group are HCT-based protocols combining HCT with T cell-depleting agents and cytoreductive treatment. So far, only two protocols (one with total lymphoid irradiation and anti-thymocyte globulin, but no HCT; one with HCT, cyclophosphamide, anti-thymocyte globulin and thymic irradiation) have been translated into successful human studies. We summarize and discuss the results of these trials and suggest goals for further studies for the development tolerance protocols applicable for a broad population of allograft recipients.

The road to tolerance: renal transplant tolerance induction in nonhuman primate studies and clinical trials

Organ transplantation has become a standard life-saving therapy for many causes of end stage organ failure. Although valuable, it remains hampered by the requirement for, and complications of, immunosuppression to prevent immune rejection of the transplanted organ. It is now clear that rejection can be avoided in some experimental systems without a requirement of immunosuppressive medication, and these experimental concepts are now making their way into the clinic in the form of early transplantation tolerance trials. This manuscript will discuss the most promising techniques for tolerance induction, namely, costimulation blockade, lymphocyte depletion, and mixed chimerism. Seminal preclinical studies will be cited and the results of initial clinical trials will be reviewed. The data to date indicate that while tolerance remains elusive, immunosuppression minimization is a feasible near-term alternative.

Clinical trials: where are we now?

Clinical trials in transplantation have focused on improving outcomes and minimizing side effects associated with renal transplantation. Although immunologic tolerance, which means complete freedom from immunosuppressive drugs and maintenance of excellent long-term graft function, has seldom been achieved, in rare cases, this has been accomplished. Most current clinical trials focus on minimization of steroid use and calcineurin inhibitor use as a step toward tolerance, sometimes termed prope tolerance. Alternatively, new immunosuppressive agents are studied to assess their efficacy in preventing graft rejection with the anticipation of lesser toxicity. This review is organized in a case presentation style with actual cases from the University of Wisconsin kidney transplant experience presented as illustrations of actual scenarios in clinical trials. Lessons learned from these particular patients are then summarized with reference to the literature associated with the case. Using this format, some of the important lessons learned from clinical trials are outlined and directions for future study are noted. Clinical trials have permitted a dramatic improvement in graft survival and lowering of infectious and malignant side effects over the past 30-40 years. Nevertheless, we remain far from achieving true tolerance in patients for a variety of reasons.

Crossing the bridge: large animal models in translational transplantation research

Many methods for reducing the immunosuppressive requirements of allotransplantation have been proposed based on a growing understanding of physiological and allospecific immunity. As these regimens are developed for clinical application, they require validation in models that are reasonably predictive of their performance in humans. This article provides an overview of the large animal models commonly used to test immunomodulatory organ transplant protocols. The rationale for the use of large animals and the effects of common immunosuppressants in the dog, pig, and non-human primate are reviewed. Promising methods for the induction of allospecific tolerance are surveyed with references to early human trials where appropriate.

Clinical transplantation tolerance: The promise and challenges

Organ transplantation is now well established as a preferred option for the treatment of end-stage organ failure. However, there is a severe shortage of donor organs and continued loss of a significant number of organ grafts due to chronic allograft dysfunction. Induction of tolerance of a transplant recipient toward their foreign organ graft, therefore, remains the "Holy Grail" of transplantation immunobiologists. Recently, clinical trials to explore pilot tolerance protocols in humans have been initiated. Defining the ideal strategy(ies) and the role of immunosuppressive drugs, developing tolerance assay(s), and enhancing cooperation between transplant professionals, industry, and the government are some of the challenges to achieving clinical transplantation tolerance. This article reviews the promise and the challenges of achieving clinical transplantation tolerance in human organ transplant recipients.

Approaches to transplantation tolerance in humans

Although transplantation tolerance to organ allografts has been achieved using a wide variety of immunologic interventions in laboratory animals, few tolerance induction protocols with complete immunosuppressive drug withdrawal have been tested in humans. Preclinical and clinical studies of the use of total lymphoid irradiation for the induction of chimeric and nonchimeric tolerance are summarized here.

Comprehensive examination of gene expression associated with long-term stable graft acceptance by renal transplant recipients

Expression levels of mRNA in peripheral blood mononuclear cells from five renal transplant recipients and five non-transplanted controls were analyzed with GeneChips (GeneChip Instrument system, Affymetrix, Santa Clara, CA, USA). All recipients had retained a well-functioning kidney graft for more than 15 yr on low-dose maintenance immunosuppression. Among a total of 12630 transcripts examined, significant differential expression was observed for 599 genes, whereby 470 genes were up-regulated and 129 down-regulated in the transplant recipients compared with controls. Of these, 192 up-regulated and 46 down-regulated genes showing a change greater than twofold were divided into eight functional categories as follows (numbers of genes, up/down): immune system (12/14), cell proliferation (17/3), oncology (15/3), transporter/receptor/binding protein (16/5), transcription factors (8/2), enzymes (17/4), expressed sequence tags (91/9), and others (16/6). Predictably, expression of immune-associated genes was decreased in the recipients. Significant reduction of expression levels of CD3, ICAM-1, and B7.2, which are critical molecules for interactions between antigen presenting cells and T cells, were observed. In T cell signal transduction, the Ras pathway was likely to be suppressed by activation of hVH-5. The present data help to elucidate the immunological status in long-term kidney graft recipients and may provide insights for future regimens to establish donor-specific hyporesponsiveness.

Clinical islet transplant: current and future directions towards tolerance

The ultimate goal of islet transplantation is to completely correct the diabetic state from an unlimited donor source, without the need for chronic immunosuppressive drug therapy. Although islet transplantation provides an opportunity to develop innovative strategies for tolerance in the clinic, both alloimmune and autoimmune barriers must be controlled, if stable graft function is to be maintained long-term. After islet extraction from the pancreas, the cellular graft may be stored in tissue culture or cryopreserved for banking, providing an opportunity not only to optimally condition the recipient but also to allow in vitro immunologic manipulation of the graft before transplantation, unlike solid organ grafts. As such, islets may be considered a "special case." Remarkable progress has occurred in the last three years, with dramatic improvements in outcomes after clinical islet transplantation. The introduction of a steroid-free, sirolimus-based, anti-rejection protocol and islets prepared from two (or rarely three) donors led to high rates of insulin independence. The "Edmonton Protocol" has been successfully replicated by other centers in an international multicenter trial. A number of key refinements in pancreas transportation, processing, purification on non-ficoll-based media, storage of islets in culture for two days and newer immunological conditioning and induction therapies have led to continued advancement through extensive collaboration between key centers. This review outlines the historical development of islet transplantation over the past 30 years, provides an update on current clinical outcomes, and summarizes a series of unique opportunities for development and early testing of tolerance protocols in patients.

Induction of kidney allograft tolerance after transient lymphohematopoietic chimerism in patients with multiple myeloma and end-stage renal disease

BACKGROUND

Two patients with end-stage renal disease secondary to multiple myelomas were treated with combined kidney and bone marrow transplantation in an effort to achieve donor-specific allotolerance through the induction of mixed lymphohematopoietic chimerism.

METHODS

Two female patients (55 and 50 years of age) with end-stage renal disease secondary to kappa light-chain multiple myelomas received a nonmyeloablative conditioning regimen that consisted of 60 mg/kg cyclophosphamide intravenously (IV) on days -5 and -4; 15 mg/kg equine anti-thymocyte globulin (ATGAM) IV on days -1, +1, and +3; and thymic irradiation (700 cGy) on day -1. On day 0, the recipients underwent kidney transplantation, followed by IV infusion of donor bone marrow (2.7x10(8) and 3.8x10(8) /kg nucleated cells, respectively) obtained from a human leukocyte antigen (HLA)-matched sibling. Cyclosporine A was administered IV at a dose of 5 mg/kg on day -1, then continued orally at 8 to 12 mg/kg per day until days +73 and +77, respectively, after which no further immunosuppression was given. Donor leukocyte infusions (1x10(7) /kg CD3+ T cells) were administered in an attempt to enhance the graft-versus-myeloma effect (days +66 and +112 in the first patient and day +78 in the second patient). Hematopoietic chimerism was monitored weekly by microsatellite assays.

RESULTS

Multilineage lymphohematopoietic chimerism (5%-80% donor CD3+ or CD3- cells, or both) was first detected during the second posttransplant week and was maintained for approximately 12 weeks, after which there was a gradual decline to undetectable levels (<1% donor cells) after day 105 in the first patient and after day 123 in the second patient. In both recipients, the blood urea nitrogen and creatinine levels returned to normal within 3 days. No rejection episodes have occurred. Quantification of urinary kappa light chains revealed a decline from 28 mg/dL to undetectable levels (<2.5 mg/dL) within 29 days in the first case and from 99.8 mg/dL to <10 mg/dL within 50 days in the second case. Both patients continue with normal kidney function and sustained anti-tumor responses, while receiving no immunosuppression for nearly 4 years and 2 years, respectively.

CONCLUSIONS

This nonmyeloablative regimen followed by combined HLA-matched donor bone marrow and renal allotransplantation is the first example of an intentional and clinically applicable approach to inducing renal allograft tolerance and achieving potent and sustained antitumor effects in patients with multiple myeloma.

H2-mismatched transplantation with repetitive cell infusions and CD40 ligand antibody infusions without myeloablation

Graft rejection and graft-versus-host disease are major problems in mismatched marrow transplants along with toxicity from standard myeloablative host treatments. We have established a tolerization model, using 1 Gy irradiation, which reduces stem cell capacity to < 10% of control while causing minimal myelosuppression, donor antigen pre-exposure (spleen cells), CD40-ligand antibody blockade and high levels of marrow (40 x 106 cells), which allows for stable long-term multilineage engraftment in H2-mismatched murine marrow transplants. We now show that the establishment of 'microchimaerism' (0.5-3.8%) sets the stage for macrochimaerism, with subsequent marrow infusions in H2-mismatched mice with CD40-ligand blockade only. Neither irradiation nor spleen cell exposure were necessary. When 40 x 106 bone marrow cells were infused on weeks 0, 12, 14 and 16, blood engraftment was about seven times the single 40 x 106 control. When marrow cells were given on weeks 0, 3, 4, 5 and 6, engraftment at 24 weeks post transplant was 17.9 +/- 1.2%, compared with 2.7 +/- 0.8% for the single 40 x 106 control (P = 0.009). We have shown stable, long-term multilineage chimaerism and established that the schedule of marrow administration, not the total cell dose, is critical for tolerization. This approach indicates that microchimaerism can tolerize for subsequent marrow infusions and produce macrochimaerism. This strategy could be applied in clinical human transplants.

Mixed chimerism and immunosuppressive drug withdrawal after HLA-mismatched kidney and hematopoietic progenitor transplantation

BACKGROUND

Rodents and dogs conditioned with total-lymphoid irradiation (TLI), with or without antithymocyte globulin (ATG), have been shown to develop mixed chimerism and immune tolerance without graft-versus-host disease (GVHD) after the infusion of major histocompatability complex (MHC)-mismatched donor bone marrow cells given alone or in combination with an organ allograft.

METHODS

Four human leukocyte antigen (HLA)-mismatched recipients of living donor kidney transplants were conditioned with TLI and ATG posttransplantation and infused with cyropreserved donor granulocyte colony-stimulating factor (G-CSF) "mobilized" hematopoietic progenitor (CD34+) cells (3-5x10(6) cells/kg) thereafter. Maintenance prednisone and cyclosporine dosages were tapered, and recipients were monitored for chimerism, GVHD, graft function, T-cell subsets in the blood, and antidonor reactivity in the mixed leukocyte reaction (MLR).

RESULTS

Three of the four patients achieved multilineage macrochimerism, with up to 16% of donor-type cells among blood mononuclear cells without evidence of GVHD. Prolonged depletion of CD4+ T cells was observed in all four patients. Rejection episodes were not observed in the three macrochimeric recipients, and immunosuppressive drugs were withdrawn in the first patient by 12 months. Prednisone was withdrawn from a second patient at 9 months, and cyclosporine was tapered thereafter.

CONCLUSIONS

Multilineage macrochimerism can be achieved without GVHD in HLA-mismatched recipients of combined kidney and hematopoietic progenitor transplants. Conditioning of the host with posttransplant TLI and ATG was nonmyeloablative and was not associated with severe infections. Recipients continue to be studied for the development of immune tolerance.

Recent advances in the immunology of chronic rejection

Chronic allograft rejection is a slowly progressive, insidious process in which the host immune system continues to mount an immunological attack on a transplanted organ, ultimately resulting in the failure of the graft. To varying degrees, all solid organ grafts are at risk for chronic rejection and undergo a stereotypic process of injury and inflammation, eventually leading to parenchymal fibrosis. The clinical consequences of chronic rejection are particularly apparent in thoracic transplantation, where both patient and graft survival decline steadily over time and the opportunities for re-transplantation or long-term extracorporeal support are limited. A variety of antigen-dependent and antigen-independent factors are known to modulate the propensity for an organ to undergo chronic rejection. Recent clinical and laboratory research has suggested that distinct immunologic mechanisms may underlie the process of chronic rejection. Ultimately, strategies to induce long-term tolerance to alloantigens will be necessary to prevent chronic rejection and to abrogate the deleterious sequelae of chronic immunosuppression.

Therapeutic approaches for transplantation

Developments in marrow and organ transplantation are mutually interactive. There have been several recent advances in stem cell transplantation: to ensure engraftment using larger doses of stem cells; to substantially reduce the incidence of graft-versus-host disease and marrow rejection using monoclonal antibodies; and to reduce toxicity of the preparative regimen through use of so-called nonmyeloablative regimens (mini-transplants). These advances may pave the way for generation of mixed hemopoietic chimerism as an aid to achieving tolerance to organ transplants. The use of short courses of T-cell-depleting antibodies, such as CD3 immunotoxin in primates and CAMPATH-1H in humans, has demonstrated that long-term graft survival may be possible without substantive long-term immunosuppressive treatment of the recipient. The demonstration in rodents that nondepleting antibodies to T cells can give rise to powerful regulatory mechanisms that maintain tolerance to grafts has initiated a major research effort in understanding how these regulatory T cells work, with the prospect of new therapeutic modalities to mimic or enhance their function.

Tolerance, mixed chimerism and protection against graft-versus-host disease after total lymphoid irradiation

Total lymphoid irradiation (TLI), originally developed as a non-myeloablative treatment for Hodgkin's disease, has been adapted for the induction of immune tolerance to organ allografts in rodents, dogs and non-human primates. Moreover, pretransplantation TLI has been used in prospective studies to demonstrate the feasibility of the induction of tolerance to cadaveric kidney allografts in humans. Two types of tolerance, chimeric and non-chimeric, develop after TLI treatment of hosts depending on whether donor bone marrow cells are transplanted along with the organ allograft. An advantageous feature of TLI for combined marrow and organ transplantation is the protection against graft-versus-host disease (GVHD) and facilitation of chimerism afforded by the predominance of CD4+ NK1.1(+) -like T cells in the irradiated host lymphoid tissues. Recently, a completely post-transplantation TLI regimen has been developed resulting in stable mixed chimerism and tolerance that is enhanced by a brief course of cyclosporine. The post-transplantation protocol is suitable for clinical cadaveric kidney transplantation. This review summarizes the evolution of TLI protocols for eventual application to human clinical transplantation and discusses the mechanisms involved in the induction of mixed chimerism and protection from GVHD.

Multicenter trial of one HLA-DR-matched or mismatched blood transfusion prior to cadaveric renal transplantation

BACKGROUND

The beneficial effect of blood transfusions before cadaveric renal transplantation on allograft survival, although previously well documented, has become controversial in light of their adverse effects. Recently, it has been suggested that their clinical benefits are due to HLA-DR sharing between the blood donor and recipient.

METHODS

In this prospective study, 144 naive patients were randomly assigned to receive one unit of blood matched for one-HLA-DR antigen (N = 49), or one unit of mismatched blood (N = 48), or to remain untransfused (N = 47). Graft survival and acute rejection rate were analyzed in 106 cadaveric renal allograft recipients receiving the same immunosuppressive protocol.

RESULTS

Graft survival was similar in the three groups at one and five years: 91.7 and 80% in untransfused patients, 90.3 and 79.3% in patients transfused with one DR-antigen-matched unit, and 92.3 and 83.7% in patients transfused with HLA-mismatched blood. The difference in the incidence of six-month post-transplant acute rejections was not statistically significant in the three groups: 12 out of 36, 33.3% in nontransfused patients; 6 out of 31, 19.4% in patients transfused with one DR-matched blood; and 13 out of 39, 33.3% in patients transfused with mismatched blood.

CONCLUSION

The results of our prospective randomized trial showed that in a population of naive patients, one transfusion mismatched or matched for one HLA-DR antigen given prior to renal transplantation had no significant effect on the incidence and severity of acute rejection, and did not influence overall long-term graft outcome. Considering the potentially deleterious adverse effects of blood transfusions, the costs, and the considerable logistical efforts required to select and type blood donors, such a procedure cannot be recommended in a routine practice for patients awaiting cadaveric kidney transplantation.