Optimizing the use of regulatory T cells in allotransplantation: recent advances and future perspectives

Abstract

Apart from clonal deletion of self-reactive T cells in the thymus, Tregs are the major regulators of immune responses in the periphery and maintain a state of self-tolerance free from autoimmune diseases. Due to their inherent suppressive function, Tregs are being explored for their therapeutic potential in preventing autoimmunity and improving survival of allografts. This review provides recent updates on Treg biology and their use in animal as well as clinical models of transplantation. We discuss potential problems that limit the widespread clinical application of Tregs and provide future perspectives on how to optimize their medical use. Special consideration is given to methods by which Tregs should be isolated and expanded in order to facilitate clinical therapies. We also focus on recent discussions of Treg stability and plasticity, with specific insights into preventing the loss of Treg suppression by allotransplant-mediated inflammation.

Keywords::

• allograft
• autoimmunity
• cell therapy
• Foxp3
• suppressor cells
• tolerance
• transplantation
• Tregs

Financial & competing interests disclosure

This work was funded by grants from the NIH (NIH-RO1-A1-090244) and the University of Toledo – Biomedical Science Innovation Award. The authors have no other relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript. This includes employment, consultancies, honoraria, stock ownership or options, expert testimony, grants or patents received or pending or royalties.

No writing assistance was utilized in the production of this manuscript.

Key issues

• • New recommendations for Treg nomenclature have recently been proposed. Tregs that develop in the thymus, traditionally natural Tregs (nTregs), should be referred to as thymus-derived Tregs (tTregs) to designate their thymic origin. Similarly, the traditionally defined adaptive Tregs (aTregs) and induced Tregs should be termed peripherally derived Tregs (pTregs) to reflect their peripheral origin.

• • Tregs must first be isolated from blood using cell surface markers, subsequently expanded in vitro, and finally re-isolated using novel and unique markers to eliminate both the expanded contaminating cells and those Tregs that have lost suppressive capacity.

• • nTregs are inherently more stable than aTregs; however, inhibition of methylation at the Treg-specific demethylation region can allow formation of stable aTregs in vitro.

• • In vitro generation of aTregs should follow a procedure designed to mimic the in vivo thymic generation of Tregs by T-cell receptor stimulation and subsequent Forkhead box protein 3 (Foxp3) expression to allow generation of more stable and potent Tregs.

• • Demethylation status of Foxp3 and associated genes should be used as a marker for stability and to define those Tregs that could be used in allotransplantation clinical trials.

• • Several new Phase I/II clinical trials using ex vivo expanded Tregs obtained from the patients themselves are ongoing to test the safety and efficacy of Treg infusion therapy in treating autoimmunity and allotransplantation.

• • Research in the next few years should focus on further clarifying the molecular basis of Foxp3 expression stability and the plasticity of suppressive function in Tregs. More recent reports have shown that Foxp3 expression stability and persistent suppressive function of Tregs may be regulated independently of each other, although both may be indispensable for bonafide suppressive Treg cells.

• • Transient loss of Foxp3 expression by bonafide Tregs, transient loss of suppressive function and acquisition of Foxp3 by non-bonafide Tregs may each hinder Treg-based therapies in the clinic. Since these events are triggered mostly by inflammatory environments in the local tissues, Treg therapy should be complemented with anti-inflammatory drugs to reduce Treg instability and plasticity.