Biomarkers of tolerance: searching for the hidden phenotype

Biomarkers of tolerance

PURPOSE OF REVIEW

As the induction and maintenance of donor-specific tolerance is a central aim in solid organ transplantation, it is essential that clinicians are able to identify and monitor tolerance accurately and reliably. This review highlights recent advances in defining sets of biomarkers in noninvasive samples that may guide minimization and withdrawal of immunosuppression in tolerant recipients.

RECENT FINDINGS

Recent studies in liver and kidney transplant recipients have identified distinct biomarker profiles that are associated with operational tolerance. Although there is some heterogeneity in the findings of these studies, these have suggested novel cellular mechanisms for the development of tolerance.

SUMMARY

Multiple platforms such as microarray gene expression analysis, flow cytometry, and immune cell functional assays have been used to discover and validate composite sets of biomarkers, which identify recipients with operational tolerance both in liver and kidney transplantation. These studies suggest that distinct cellular and molecular mechanisms lead to the development of tolerance in different transplanted organs. These putative biomarker profiles now need to be validated prospectively in trials of immunosuppression withdrawal and in novel approaches to induce transplant tolerance.

Non-invasive biomarkers to guide management following renal transplantation: the need for a multiplatform approach

PURPOSE OF REVIEW

Balancing the level of anti-rejection therapy is the key challenge facing clinicians managing recipients of a solid organ transplant. Identification of biomarkers in non-invasive samples will allow serial monitoring which can prompt changes in therapy at an earlier stage without the need for invasive tests, and before significant damage to the graft or side effects have occurred. In this review we provide an update on the present status of such biomarker discovery in renal transplantation.

RECENT FINDINGS

Previous studies focusing on candidate biomarkers have identified a number of genes that have the potential to identify patients undergoing rejection. Advances in technology now allow screening of samples for markers which have led to identification of further genes as well as adding microRNAs and proteins to the list. Similarly, by studying patients in whom anti-rejection therapy has been withdrawn, a gene signature for operational tolerance has been described.

SUMMARY

It has become clear that to obtain a test suitable for clinical purposes, combinations of markers are required to identify specific clinical phenotypes. Identification and validation of such marker sets in large cohorts is urgently required to allow progression to clinical trials with the ultimate goal of offering recipients personalized anti-rejection therapy regimes.

Relevance of regulatory T cell promotion of donor-specific tolerance in solid organ transplantation

Current clinical strategies to control the alloimmune response after transplantation do not fully prevent induction of the immunological processes which lead to acute and chronic immune-mediated graft rejection, and as such the survival of a solid organ allograft is limited. Experimental research on naturally occurring CD4⁺CD25^highFoxP3⁺ Regulatory T cells (Tregs) has indicated their potential to establish stable long-term graft acceptance, with the promise of providing a more effective therapy for transplant recipients. Current approaches for clinical use are based on the infusion of freshly isolated or ex vivo polyclonally expanded Tregs into graft recipients with an aim to redress the in vivo balance of T effector cells to Tregs. However mounting evidence suggests that regulation of donor-specific immunity may be central to achieving immunological tolerance. Therefore, the next stages in optimizing translation of Tregs to organ transplantation will be through the refinement and development of donor alloantigen-specific Treg therapy. The altering kinetics and intensity of alloantigen presentation pathways and alloimmune priming following transplantation may indeed influence the specificity of the Treg required and the timing or frequency at which it needs to be administered. Here we review and discuss the relevance of antigen-specific regulation of alloreactivity by Tregs in experimental and clinical studies of tolerance and explore the concept of delivering an optimal Treg for the induction and maintenance phases of achieving transplantation tolerance.