Treg cell-based therapies: challenges and perspectives

Abstract

Cellular therapies using regulatory T (T_reg) cells are currently undergoing clinical trials for the treatment of autoimmune diseases, transplant rejection and graft-versus-host disease. In this Review, we discuss the biology of T_reg cells and describe new efforts in T_reg cell engineering to enhance specificity, stability, functional activity and delivery. Finally, we envision that the success of T_reg cell therapy in autoimmunity and transplantation will encourage the clinical use of adoptive T_reg cell therapy for non-immune diseases, such as neurological disorders and tissue repair.

Fig. 1 |. Mechanisms of action of effector T cells versus T_reg cells.

a | The recognition of specific antigens presented or cross-presented by antigen-presenting cells (APCs) activates CD4⁺ and CD8⁺ T cells, respectively. Following activation, CD4⁺ T cells produce cytokines that help CD8⁺ T cells to differentiate into cytotoxic T lymphocytes (CTLs). CTLs then kill target cells by performing their cytotoxic activity in an antigen-specific and cell contact-dependent manner. b | Like effector T cells, regulatory T (T_reg) cell activation is antigen specific. However, once activated, T_reg cells exert bystander suppression, meaning that they have suppressive function regardless of their antigen specificity. T_reg cells suppress effector T cells using direct or indirect mechanisms of action. Finally, T_reg cells can spread their suppressive properties to neighbouring cells via a phenomenon named infectious tolerance. IDO, indoleamine-2,3-dioxygenase; IFNγ, interferon-γ; MHC, major histocompatibility complex; TCR, T cell receptor; TGFβ, transforming growth factor-β.

Fig. 2 |. Various T_reg cell products for T_reg cell-based therapy.

There are currently three main regulatory T (T_reg) cell products developed for adoptive cell therapy (ACT). a | The most widely used product in clinical trials is expanded polyclonal T_reg cells that were isolated from peripheral blood and expanded in vitro using high-dose IL-2 and anti-CD3/CD28 beads to generate a high number of T_reg cells for ACT. b | Another approach, mainly used in transplantation to prevent graft rejection, is the use of antigen-presenting cells (APCs) from the graft donor to specifically stimulate alloreactive T_reg cells from the recipient in vitro. These antigen-specific T_reg cells have been proven to be more potent than polyclonal T_reg cells. However, the cell yield is relatively low after expansion, and this approach cannot be adapted for the treatment of autoimmune diseases owing to the low precursor frequencies of specific autoantigen-reactive T_reg cells. c | A third approach is the expansion of polyclonal T_reg cells genetically engineered to express a synthetic receptor (chimeric antigen receptor (CAR) or artificial T cell receptor (TCR)) that recognizes a target antigen of interest. With this strategy, a high number of antigen-specific T_reg cells can be obtained after expansion.

Fig. 3 |. Characteristics of recombinant T cell receptors versus chimeric antigen receptors.

T cell receptors (TCRs) are commonly composed of a heterodimer of one α-chain and one β-chain that can bind to a specific peptide–major histocompatibility complex (MHC) and subsequently activate the CD3 complex that is made of four signalling subunits: one dimer of CD3ε and CD3γ chains, one dimer of CD3ε and CD3δ chains, and one homodimer of CD3ζ chains. Recombinant TCRs are generated through the integration of the genes that encode for the α- and β-chains of a TCR specific for an antigen of interest. Second-generation chimeric antigen receptors (CARs) are artificial immunoreceptors composed in their extracellular part of a single-chain variable fragment (scFv) capable of binding a target antigen that is linked to a transmembrane domain via a hinge and in their intracellular part of a co-stimulatory domain (CD28 or 4–1BB) and a CD3ζ domain that together effectively trigger the activation of the CAR-expressing T cell after binding of the antigen. ACT, adoptive cell therapy; CRS, cytokine release syndrome; ECM, extracellular matrix; T_reg cell, regulatory T cell.

Fig. 4 |. The future of T_reg cell-based therapy.

Universal donor regulatory T (T_reg) cells for adoptive cell therapy represent the ultimate goal to broaden the use of T_reg cells as therapeutics. Recent advances in genome editing and ongoing studies aimed at differentiating various T cell subsets from induced pluripotent stem (iPS) cells have the potential to enable the generation of T_reg cells compatible with any major histocompatibility complex (MHC) class and equipped with an adjustable immunoreceptor to confer the appropriate antigen specificity based on patient needs. T_reg cells will be isolated from a healthy donor (from peripheral blood (PB) or umbilical cord blood (UCB)) or differentiated from iPS cells, which can be derived from fibroblasts. T_reg cells will be expanded in vitro and subjected to genome editing using a non-viral approach, such as CRISPR–Cas9 technology, to equip the cells with a synthetic immunoreceptor and other payloads such as an orthogonal IL-2/IL-2 receptor (IL-2R) pair. a | By knocking out the donor HLA molecules and knocking in a non-classical HLA, T_reg cells can be made compatible with any HLA profile while being protected from natural killer cell-mediated cytotoxicity. Moreover, based on recent effector T cell studies, we envision that replacing the endogenous T cell receptor (TCR) with a synthetic immunoreceptor will confer the desired antigen specificity to T_reg cells to improve their potency while removing the endogenous TCR to prevent mispairing. In addition, the potency of the engineered T_reg cells will be optimized by equipping them with neomorphic properties, such as the expression of chemokine receptors to enhance their migration to the target tissue, the production of immunosuppressive cytokines to improve their suppressive activity as well as the expression of the orthogonal pair of IL-2/IL-2 receptor (IL-2R) to promote their specific survival. Importantly, technologies, such as the Synthetic Notch (SynNotch) system, may be used to control the expression and activity of these neomorphic properties. Finally, in case of an adverse event (for example, cell instability, off-target toxicity), engineered T_reg cells will express a suicide cassette to allow efficient and rapid elimination if necessary. b | Although most of the effort in the T_reg cell field to redirect the cell antigen specificity has been made using classic chimeric antigen receptors (CARs) or TCRs, the ideal engineered immunoreceptor should be universal with an adjustable antigen recognition domain (that is, the antigen recognition domain should be separated from the signalling domain of a conventional CAR, enabling one to pick and even switch the antigen specificity of the cells). To this aim, alternative approaches, such as by Xyphos Biosciences (convertible CAR), Unum Therapeutics (ACTR, antibody-coupled TCR) and CALIBR (switchable CAR), focus on the development of universal binding sites for antigen receptors to redirect tissue targeting. Although these approaches have been developed for effector T cells, they could also be translated to T_reg cell engineering to direct the cells to specific sites of inflammation using universal binding platforms. Similar to CARs, engineered TCRs are capable of redirecting the antigen specificity of T_reg cells, and novel TCR-based technologies are being evaluated. Notably, TCR2 Therapeutics has designed a T Cell Receptor Fusion Construct (TRuC) platform that consists of the fusion of a single-chain variable fragment (scFv) specific for a surface antigen of interest to the extracellular N termini of one of the TCR subunits, which enables the recognition of the antigen on target cells without the need for HLA presentation and engages the complete TCR machinery. MicAbody, modified MHC-class I-related chain A fused to an antibody; iNKG2D, inert natural killer group 2, member D receptor; PNE, peptide neo-epitope.