Perspectives on Regulatory T Cell Therapies

Abstract

Adoptive transfer in animal models clearly indicate an essential role of CD4+ CD25+ FOXP3+ regulatory T (T(reg)) cells in prevention and treatment of autoimmune and graft-versus-host disease. Thus, T(reg) cell therapies and development of drugs that specifically enhance T(reg) cell function and development represent promising tools to establish dominant tolerance. So far, lack of specific markers to differentiate human T(reg) cells from activated CD4+ CD25+ effector T cells, which also express FOXP3 at different levels, hampered such an approach. Recent identification of the orphan receptor glycoprotein-A repetitions predominant (GARP or LRRC32) as T(reg) cell-specific key molecule that dominantly controls FOXP3 via a positive feedback loop opens up new perspectives for molecular and cellular therapies. This brief review focuses on the role of GARP as a safeguard of a complex regulatory network of human T(reg) cells and its implications for regulatory T cell therapies in autoimmunity and graft-versus-host disease.

Fig. 1

CK1 inhibition affects FOXP3 expression in human T_reg cells, suggesting the potential use of small molecules to interfere with T_reg cell functions. Human alloantigen-specific T_reg cells were pretreated over night with lactose that impairs the lectin binding function of LGALS3 [11], CK1 kinase inhibitor (D4476, TOCRIS bioscience, Bristol, UK), and solvent control (DMSO) or irrelevant kinase inhibitors (PD98059 = MAP kinase inhibitor; SB202190 = p38 kinase inhibitor) and analyzed for intracellular FOXP3 as described [13].

Fig. 2

GARP as safeguard of the regulatory phenotype. Qualitative and quantitative differences of FOXP3 expression between T_reg (upper panel) and T_h cells (lower panel) with their mutually exclusive effector and regulatory functions can be explained in part by the existence of T_reg-specific control mechanisms. First of all, FOXP3 is expressed ab initio in T_reg cells, because of difference in the accessibility of the FOXP3 locus [15, 21], whereas it is induced with delay in T_h cells following TCR stimulation. Lineage-restricted induction of GARP due also to selective hypomethylation of the locus in T_reg cells [20] (indicated by lack of black dots) initiates a positive feedback loop that enhances and maintains high levels of FOXP3. This regulatory network is associated with redundant GARP/FOXP3-enhancing molecules, e.g. the ß-galactoside-binding protein LGALS3, which also reveals lineage-specific differences of DNA methylation [20], the endoprotease LGMN, receptor CD27 and CD83 suggesting a higher-order regulatory network [13].

Fig. 3

Adoptive T_reg cell therapy – perspectives. Based on the isolation of either CD4+ CD25-/int/+ T_h cells or CD4+ CD25^hi T_reg cells, two different strategies exist for adoptive regulatory ? cell applications, i) Isolation of antigen-specific CD4+ T_h cells from different disease settings (e.g. GvHD or autoimmunity) with the use of ‘classical’ antigen-specific stimulation/cloning procedures or by tretramer/ multimer separation [15], and re-programming/transdifferentiation of these disease-associated effector towards T_reg cells via genetic engineering with GARP as described for alloantigen-specific T_h cells [13]. ii) Isolation of T_reg cells and their preferable antigen-specific expansions with the need for re-isolation to exclude potential harmful antigen-specific T_h cells included in the preparation be means of T_reg cell-specific marker(s), including e.g. GARP or the recently described strategy of using CD121 (IL1R), recently identified as FOXP3-dependend molecule [11], and LAP as T_reg markers [19, 38, 48] (table 1).