Operational Tolerance in Kidney Transplantation—Improved Terminology May Enable More Precise Investigation

Current status of tolerance in kidney transplantation

PURPOSE OF REVIEW

The attainment of tolerance remains a highly desirable goal in recipients of kidney transplants. Achievement of this goal would extend graft survival and eradicate toxicities related to long-term immunosuppression. Understanding mechanisms of tolerance and strategies to induce tolerance - their risk/benefit profiles - is essential for future success.

RECENT FINDINGS

Mechanistic studies of spontaneously tolerant kidney transplant recipients have uncovered potential roles for B or regulatory T cells, or both, in the maintenance of tolerance. Mixed hematopoietic chimerism has been the most commonly used approach to induce tolerance. Distinct protocols at three major transplant centers have led to successful withdrawal of immunosuppression in a subset of living donor kidney transplant recipients at the expense of complications such as infections and graft versus host disease. The addition of regulatory cell therapies to tolerance induction protocols could enhance success while minimizing complications.

SUMMARY

This review summarizes the features of spontaneous tolerance in kidney transplant recipients, the results of clinical trials of tolerance induction in the context of living donor kidney transplant, and potential measures to improve the safety and efficacy of tolerance induction strategies.

Immune monitoring as prerequisite for transplantation tolerance trials

Ever since its first application in clinical medicine, scientists have been urged to induce tolerance towards foreign allogeneic transplants and thus avoid rejection by the recipient's immune system. This would circumvent chronic use of immunosuppressive drugs (IS) and thus avoid development of IS-induced side effects, which are contributing to the still unsatisfactory long-term graft and patient survival after solid organ transplantation. Although manifold strategies of tolerance induction have been described in preclinical models, only three therapeutic approaches have been utilized successfully in a still small number of patients. These approaches are based on (i) IS withdrawal in spontaneous operational tolerant (SOT) patients, (ii) induction of a mixed chimerism and (iii) adoptive transfer of regulatory cells. Results of clinical trials utilizing these approaches show that tolerance induction does not work in all patients. Thus, there is a need for reliable biomarkers, which can be used for patient selection and post-therapeutic immune monitoring of safety, success and failure. In this review, we summarize recent achievements in the identification and validation of such immunological assays and biomarkers, focusing mainly on kidney and liver transplantation. From the published findings so far, it has become clear that indicative biomarkers may vary between different therapeutic approaches applied and organs transplanted. Also, patient numbers studied so far are very small. This is the main reason why nearly all described parameters lack validation and reproducibility testing in large clinical trials, and are therefore not yet suitable for clinical practice.

A composite score associated with spontaneous operational tolerance in kidney transplant recipients

New challenges in renal transplantation include using biological information to devise a useful clinical test for discerning high- and low-risk patients for individual therapy and ascertaining the best combination and appropriate dosages of drugs. Based on a 20-gene signature from a microarray meta-analysis performed on 46 operationally tolerant patients and 266 renal transplant recipients with stable function, we applied the sparse Bolasso methodology to identify a minimal and robust combination of six genes and two demographic parameters associated with operational tolerance. This composite score of operational tolerance discriminated operationally tolerant patients with an area under the curve of 0.97 (95% confidence interval 0.94-1.00). The score was not influenced by immunosuppressive treatment, center of origin, donor type, or post-transplant lymphoproliferative disorder history of the patients. This composite score of operational tolerance was significantly associated with both de novo anti-HLA antibodies and tolerance loss. It was validated by quantitative polymerase chain reaction using independent samples and demonstrated specificity toward a model of tolerance induction. Thus, our score would allow clinicians to improve follow-up of patients, paving the way for individual therapy.

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.