How can we measure immunologic tolerance in humans?

High Throughput Human T Cell Receptor Sequencing: A New Window Into Repertoire Establishment and Alloreactivity

Recent advances in high throughput sequencing (HTS) of T cell receptors (TCRs) and in transcriptomic analysis, particularly at the single cell level, have opened the door to a new level of understanding of human immunology and immune-related diseases. In this article, we discuss the use of HTS of TCRs to discern the factors controlling human T cell repertoire development and how this approach can be used in combination with human immune system (HIS) mouse models to understand human repertoire selection in an unprecedented manner. An exceptionally high proportion of human T cells has alloreactive potential, which can best be understood as a consequence of the processes governing thymic selection. High throughput TCR sequencing has allowed assessment of the development, magnitude and nature of the human alloresponse at a new level and has provided a tool for tracking the fate of pre-transplant-defined donor- and host-reactive TCRs following transplantation. New insights into human allograft rejection and tolerance obtained with this method in combination with single cell transcriptional analyses are reviewed here.

Operational tolerance in kidney transplantation and associated biomarkers

In the 1960s, our predecessors won a historical battle against acute rejection and ensured that transplantation became a common life-saving treatment. In parallel with this success, or perhaps because of it, we lost the battle for long-lived transplants, being overwhelmed with chronic immune insults and the toxicities of immunosuppression. It is likely that current powerful treatments block acute rejection, but at the same time condemn the few circulating donor cells that would have been able to elicit immunoregulatory host responses towards the allograft. Under these conditions, spontaneously tolerant kidney recipients - i.e. patients who maintain allograft function in the absence of immunosuppression - are merely accidents; they are scarce, mysterious and precious. Several teams pursue the goal of finding a biomarker that would guide us towards the 'just right' level of immunosuppression that avoids rejection while leaving some space for donor immune cells. Some cellular assays are attractive because they are antigen-specific, and provide a comprehensive view of immune responses toward the graft. These seem to closely follow patient regulatory capacities. However, these tests are cumbersome, and require abundant cellular material from both donor and recipient. The latest newcomers, non-antigen-specific recipient blood transcriptomic biomarkers, offer the promise that a practicable and simple signature may be found that overcomes the complexity of a system in which an infinite number of individual cell combinations can lead possibly to graft acceptance. Biomarker studies are as much an objective - identifying tolerant patients, enabling tolerance trials - as a means to deciphering the underlying mechanisms of one of the most important current issues in transplantation.

A New Window into the Human Alloresponse

Alloreactive T lymphocytes are the primary mediators of allograft rejection. The size and diversity of the HLA-alloreactive T cell repertoire has thus far precluded the ability to follow these T cells and thereby to understand their fate in human transplant recipients. This review summarizes the history, challenges, and recent advances in the study of alloreactive T cells. We highlight the historical development of assays to measure alloreactivity and discuss how high-throughput T cell receptor (TCR) sequencing-based assays can provide a new window into the fate of alloreactive T cells in human transplant recipients. A specific approach combining a classical in vitro assay, the mixed lymphocyte reaction, with deep T cell receptor sequencing is described as a tool to track the donor-reactive T cell repertoire for any specific HLA-mismatched donor-recipient pair. This assay can provide mechanistic insights and has potential as a noninvasive, highly specific biomarker for rejection and tolerance.

Monitoring the Intracellular Tacrolimus Concentration in Kidney Transplant Recipients with Stable Graft Function

Although monitoring the intracellular concentration of immunosuppressive agents may be a promising approach to individualizing the therapy after organ transplantation, additional studies on this issue are needed prior to its clinical approval. We investigated the relationship between intracellular and whole blood concentrations of tacrolimus (IC-TAC and WB-TAC, respectively), the factors affecting this relationship, and the risk of rejection based upon IC-TAC in stable kidney recipients. Both IC-TAC and WB-TAC were measured simultaneously in 213 kidney recipients with stable graft function using LC-MS/MS. The tacrolimus ratio was defined as IC-TAC per WB-TAC. The genetic polymorphism of ABCB1 gene and flow cytometric analyses were conducted to probe the correlation between tacrolimus concentrations and the immunoreactivity status as a potential risk of rejection, respectively. The correlation between IC-TAC and WB-TAC was relatively linear (r = 0.67; P<0.001). The factors affecting the tacrolimus ratio were sex, hematocrit, and the transplant duration, as follows: a high tacrolimus ratio was noted in female patients, patients with a low hematocrit, and patients with a short transplant period. However, the tacrolimus ratio did not reflect the prior clinical outcomes (e.g., rejection) or the genetic polymorphism of ABCB1. After stimulation with phorbol-12-myristate 13-acetate and ionomycin, the proportion of T cells producing interferon-gamma or interleukin-2 was higher in the low-IC-TAC group than in the high-IC-TAC group. Further studies are required to evaluate the value of the intracellular tacrolimus concentrations in several clinical settings, such as rejection, infection, and drug toxicity.

Tracking donor-reactive T cells: Evidence for clonal deletion in tolerant kidney transplant patients

T cell responses to allogeneic major histocompatibility complex antigens present a formidable barrier to organ transplantation, necessitating long-term immunosuppression to minimize rejection. Chronic rejection and drug-induced morbidities are major limitations that could be overcome by allograft tolerance induction. Tolerance was first intentionally induced in humans via combined kidney and bone marrow transplantation (CKBMT), but the mechanisms of tolerance in these patients are incompletely understood. We now establish an assay to identify donor-reactive T cells and test the role of deletion in tolerance after CKBMT. Using high-throughput sequencing of the T cell receptor B chain CDR3 region, we define a fingerprint of the donor-reactive T cell repertoire before transplantation and track those clones after transplant. We observed posttransplant reductions in donor-reactive T cell clones in three tolerant CKBMT patients; such reductions were not observed in a fourth, nontolerant, CKBMT patient or in two conventional kidney transplant recipients on standard immunosuppressive regimens. T cell repertoire turnover due to lymphocyte-depleting conditioning only partially accounted for the observed reductions in tolerant patients; in fact, conventional transplant recipients showed expansion of circulating donor-reactive clones, despite extensive repertoire turnover. Moreover, loss of donor-reactive T cell clones more closely associated with tolerance induction than in vitro functional assays. Our analysis supports clonal deletion as a mechanism of allograft tolerance in CKBMT patients. The results validate the contribution of donor-reactive T cell clones identified before transplant by our method, supporting further exploration as a potential biomarker of transplant outcomes.

Noninvasive methods to assess the risk of kidney transplant rejection

In current clinical practice, immune reactivity of kidney transplant recipients is estimated by monitoring the levels of immunosuppressive drugs, and by functional and/or histological evaluation of the allograft. The availability of assays that could directly quantify the extent of the recipient's immune response towards the allograft would help clinicians to customize the prescription of immunosuppressive drugs to individual patients. Importantly, these assays might provide a more in-depth understanding of the complex mechanisms of acute rejection, chronic injury, and tolerance in organ transplantation, allowing the design of new and potentially more effective strategies for the minimization of immunosuppression, or even for the induction of immunological tolerance. The purpose of this review is to summarize results from recent studies in this field.

Flow cytometry and solid organ transplantation: a perfect match

In the field of transplantation, flow cytometry serves a well-established role in pre-transplant crossmatching and monitoring immune reconstitution following hematopoietic stem cell transplantation. The capabilities of flow cytometers have continuously expanded and this combined with more detailed knowledge of the constituents of the immune system, their function and interaction and newly developed reagents to study these parameters have led to additional utility of flow cytometry-based analyses, particularly in the post-transplant setting. This review discusses the impact of flow cytometry on managing alloantigen reactions, monitoring opportunistic infections and graft rejection and gauging immunosuppression in the context of solid organ transplantation.

Clinical transplantation tolerance

Transplantation is the treatment of choice for many if not most causes of end-stage organ failure. Over 20,000 organ transplant procedures were performed in the USA in 2009 to treat patients with failed or failing kidneys, livers, hearts, lungs, and intestines, and there remain 85,000 individuals waiting on the transplant list. Currently, in the USA, there are over 170,000 individuals living with a transplanted organ. Virtually, all of these individuals receive maintenance immunosuppression in an attempt to maximize the function and survival of the transplanted organ. However, it is clear that the long-term use of immunosuppressive agents is associated with an extensive list of undesirable side effects that have the potential to limit the survival of the patient and transplanted organ as well as to compromise quality of life. Although the ability to induce reproducibly a state of robust, stable tolerance would address this problem, tolerance remains an infrequent event in clinical transplantation that is largely a consequence of chance. Factors limiting the broader investigation of clinical transplantation tolerance include the lack of therapeutic regimens known to favor tolerance in humans, the lack of validated assays or biomarkers predictive of tolerance, and concerns about the safety and ethics of complete withdrawal of immunosuppression given the very good results achievable with current immunosuppression. Despite these barriers, a number of investigators have continued to conduct well-designed and carefully supervised studies with the long-term goal of making clinical transplantation tolerance more feasible. The aim of this review is to summarize the status of these studies.

Clinical operational tolerance after renal transplantation: current status and future challenges

In solid organ transplantation, the achievement of an immunosuppression (IS)-free state [also referred to as clinical operational tolerance (COT)] represents the ultimate goal. Although COT is feasible and safe in selected cases after liver transplantation, it is an exceptional finding after other types of solid organ transplantation. In the field of renal transplantation (RT), approximately 100 cases of COT have been reported to date, mainly in patients who were not compliant with their immunosuppressive regimens or in individuals who had previously received a bone marrow transplant for hematological disorders. On the basis of promising results obtained in animal models, several tolerogenic protocols have been attempted in humans, but most have failed to achieve robust and stable COT after RT. Molecule-based regimens have been largely ineffective, whereas cell-based regimens have provided some encouraging results. In these latter regimens, apart from standard IS, patients usually receive perioperative infusion of donor bone marrow-derived stem cells, which are able to interact with the immune cells of the host and mitigate their response to engraftment. Unfortunately, most renal transplant patients who developed acute rejection-occurring either during the weaning protocol or after complete withdrawal of IS-eventually lost their grafts. Currently, the immune monitoring necessary for predicting the presence and persistence of donor-specific unresponsiveness is not available. Overall, the present review will provide a conceptual framework for COT and conclude that stable and robust COT after RT remains an elusive goal and that the different strategies attempted to date are not yet reproducibly safe or effective.

Review: Pharmacodynamic monitoring of immunosuppression in kidney transplantation

Advances in immunosuppressive therapies have improved kidney transplant outcomes. However, immunosuppressant drug-induced toxicities continue to reduce tolerability and impact patient and graft survival. A major ongoing challenge in kidney transplantation is to establish ways of tailoring immunosuppressant therapy so as to maintain efficacy while minimizing toxicity. Pharmacodynamic monitoring by direct measurement of immune cell function has the potential to personalize immunosuppression. The purpose of this review is to provide the clinician with an overview of the methodology and use of immune function monitoring in the field of kidney transplantation.

The immunological monitoring of kidney and liver transplants in adult and pediatric recipients

Over the last half century, kidney and liver transplantation have been recognized as the treatment of choice for adult and children with end-stage renal or liver failure. Infants present a relative naïve immune system, but they are capable of mounting both cellular and humoral immune responses to the foreign antigens presented by the allograft. Immune monitoring is a way of measuring functional and molecular correlates of immune reactivity which may provide clinically useful information for identifying patients who have an increase risk of acute rejection prior to clinical symptoms or develop transplant tolerance. However, although numerous assays have been shown to predict rejection, to date no assays have been demonstrated to detect or predict transplantation tolerance. This is a summary of the published literature on promising antigen-specific and non-antigen-specific assays used for immunological monitoring in solid organ transplantation. This work also attempts to review their applicability to pediatric transplantation, specifically, pediatric kidney and liver recipients.

Stimulation index for PCNA mRNA in peripheral blood as immune function monitoring after renal transplantation

Although more effective and potent immunosuppressive agents have recently reduced the incidence of acute rejection, drug-induced toxicity and infection caused by over-immunosuppression occasionally elicit a serious problem. However, no effective assay for evaluating overall patient's immune condition is in widespread use at present. We attempted to measure the stimulation index for mRNA of proliferating cell nuclear antigen (PCNA), which is synthesized in early G1 and S phases of the cell cycle and would be expected to reflect the proliferation capacity of T lymphocytes under the immunosuppressive condition. The stimulation index for PCNA mRNA seemed to be closely related to the immunosuppressive state of renal transplant recipients. Patients with stimulation index less than 2.0 tended to have viral reactivation after transplantation. It was suggested that PCNA mRNA monitoring in peripheral blood could provide a warning of possible over-immunosuppression as one simple assay for immune function monitoring.

Deficient TNF-alpha and IFN-gamma production correlates with nondetectable donor-specific cytotoxicity after clinical kidney transplantation

BACKGROUND

We previously reported that no cytotoxic T-lymphocyte precursor frequencies (CTLpf) were found in 60% of patients on azathioprine or mycophenolate mofetil+Pred long after kidney transplantation. We questioned whether the absence of donor-specific CTLpf was associated with low levels of stimulatory Th1 (tumor necrosis factor [TNF]-alpha, interferon [IFN]-gamma) or high levels of regulatory Th2 (interleukin [IL]-4, IL-6, IL-10) cytokines.

METHODS

In this study, peripheral blood monocyte cells (PBMC) were stimulated with irradiated donor cells. After 7 days, cytokine production was determined by cytokine bead array, and CTLpf by limiting dilution assay.

RESULTS

Patients with detectable CTLpf (> or =10/10(6) PBMC) had significantly higher levels of TNF-alpha (P=0.04) and IFN-gamma (P=0.02) than patients with nondetectable CTLpf (<10/10(6) PBMC). Donor-reactive IL-4, IL-6, and IL-10 production was comparable in both patient groups. Additionally, CTLpf was positively correlated with TNF-alpha (rs=0.54, P=0.0003) and IFN-gamma (rs=0.64, P<0.0001) production.

CONCLUSION

The absence of donor-specific CTLp after transplantation correlates with low levels of stimulatory cytokines, not with elevated levels of regulatory cytokines.

Microarrays: monitoring for transplant tolerance and mechanistic insights

With recent advances in immunology and a growing understanding of transplantation biology, the development of reliable assays that may be used for identification and prediction of the current state of an immune response (rejection and tolerance) are urgently needed to allow us to predict the development of immunologic graft injury, individualize immunosuppression, rationally minimize immunosuppressive drug toxicity, promote a better understanding of the mechanisms underlying stable graft acceptance, and aid in the design of tolerance-inducing clinical transplantation trials. Microarrays can provide nonbiased, simultaneous global expression patterns for more than 40,000 human genes across different experiments. High throughput microarray technology offers a means to study disease-specific transcriptional changes in tissue biopsy, peripheral blood, and biofluids.

Frontiers in nephrology: immune tolerance to allografts in humans

Vascularized allografts are rejected unless some indefinite modification to the recipient's immune system is made. This modification is typically achieved through the long-term administration of immunosuppressive drugs. Patients thus trade their end-stage organ failure for dependence on daily drug therapy and the accompanying chronic condition of immunodeficiency. However, it is clear from studies in experimental animals that rejection can be prevented through the use of several therapeutic approaches, including donor hematopoietic cell infusion, chimerism, T cell depletion, and/or co-stimulation blockade. Successfully treated animals avoid rejection beyond the period of therapy without a phenotype of chronic immunosuppression and are thus considered to be tolerant of their grafts. Although intriguing, this success in animals has yet to be reproducibly translated to the clinic, and human transplant recipients remain tethered to immunosuppressive drugs with rare exceptions. This article provides an overview of the existing, largely anecdotal, clinical experience with organ allograft tolerance. It reviews the various approaches that are being applied in pilot human trials and suggests avenues for future clinical investigation.

Transplantation Tolerance

Tolerance following organ transplantation was first described in experimental models over 50 years ago. Reports of tolerance in clinical transplantation have appeared in the literature sporadically for decades. Despite this long-standing fascination with transplantation tolerance, the ability to reproducibly induce tolerance in humans undergoing organ transplantation has remained elusive. Recent advances in our knowledge of the mechanisms that contribute to the induction and maintenance of tolerance as well as those factors that oppose tolerance may allow the design of clinical trials aimed at introducing tolerance-inducing strategies into clinical transplantation.

Expansion and tissue infiltration of an allospecific CD4+CD25+CD45RO+IL-7Ralphahigh cell population in solid organ transplant recipients

It has been recently shown (Seddiki, N., B. Santner-Nanan, J. Martinson, J. Zaunders, S. Sasson, A. Landay, M. Solomon, W. Selby, S.I. Alexander, R. Nanan, et al. 2006. J. Exp. Med. 203:1693–1700.) that the expression of interleukin (IL) 7 receptor (R) α discriminates between two distinct CD4 T cell populations, both characterized by the expression of CD25, i.e. CD4 regulatory T (T reg) cells and activated CD4 T cells. T reg cells express low levels of IL-7Rα, whereas activated CD4 T cells are characterized by the expression of IL-7Rα^high. We have investigated the distribution of these two CD4 T cell populations in 36 subjects after liver and kidney transplantation and in 45 healthy subjects. According to a previous study (Demirkiran, A., A. Kok, J. Kwekkeboom, H.J. Metselaar, H.W. Tilanus, and L.J. van der Laan. 2005. Transplant. Proc. 37:1194–1196.), we observed that the T reg CD25⁺CD45RO⁺IL-7Rα^low cell population was reduced in transplant recipients (P < 0.00001). Interestingly, the CD4⁺CD25⁺CD45RO⁺IL-7Rα^high cell population was significantly increased in stable transplant recipients compared with healthy subjects (P < 0.00001), and the expansion of this cell population was even greater in patients with documented humoral chronic rejection compared with stable transplant recipients (P < 0.0001). The expanded CD4⁺CD25⁺CD45RO⁺IL-7Rα^high cell population contained allospecific CD4 T cells and secreted effector cytokines such as tumor necrosis factor α and interferon γ, thus potentially contributing to the mechanisms of chronic rejection. More importantly, CD4⁺IL-7Rα⁺and CD25⁺IL-7Rα⁺ cells were part of the T cell population infiltrating the allograft of patients with a documented diagnosis of chronic humoral rejection. These results indicate that the CD4⁺CD25⁺IL-7Rα⁺ cell population may represent a valuable, sensitive, and specific marker to monitor allospecific CD4 T cell responses both in blood and in tissues after organ transplantation.

Clinical transplantation tolerance: many rivers to cross

Modern immunosuppressive regimens for organ transplantation have resulted in excellent short-term results but less dramatic improvements in long-term outcomes. Moreover, they are associated with significant deleterious effects. One solution that should avoid the adverse drug effects and result in improved graft and patient longevity as well as positively impacting on the organ shortage is the establishment of transplantation tolerance. Ever since the original description of transplantation tolerance in rodent allografts, there have been significant efforts made to translate tolerance-promoting strategies to the clinical arena. However, >50 years later, we are still faced with significant barriers that are preventing such a goal from being widely attained. Nonetheless, pilot clinical tolerance protocols are underway in selected transplant recipients. In this review, we discuss the scientific and nonscientific issues that must be overcome for successful transplantation tolerance to become a clinical reality.