Chimeric Antigen Receptor (CAR) Treg: A Promising Approach to Inducing Immunological Tolerance

Abstract

Cellular therapies with polyclonal regulatory T-cells (Tregs) in transplantation and autoimmune diseases have been carried out in both animal models and clinical trials. However, The use of large numbers of polyclonal Tregs with unknown antigen specificities has led to unwanted effects, such as systemic immunosuppression, which can be avoided via utilization of antigen-specific Tregs. Antigen-specific Tregs are also more potent in suppression than polyclonal ones. Although antigen-specific Tregs can be induced in vitro, these iTregs are usually contaminated with effector T cells during in vitro expansion. Fortunately, Tregs can be efficiently engineered with a predetermined antigen-specificity via transfection of viral vectors encoding specific T cell receptors (TCRs) or chimeric antigen receptors (CARs). Compared to Tregs engineered with TCRs (TCR-Tregs), CAR-modified Tregs (CAR-Tregs) engineered in a non-MHC restricted manner have the advantage of widespread applications, especially in transplantation and autoimmunity. CAR-Tregs also are less dependent on IL-2 than are TCR-Tregs. CAR-Tregs are promising given that they maintain stable phenotypes and functions, preferentially migrate to target sites, and exert more potent and specific immunosuppression than do polyclonal Tregs. However, there are some major hurdles that must be overcome before CAR-Tregs can be used in clinic. It is known that treatments with anti-tumor CAR-T cells cause side effects due to cytokine “storm” and neuronal cytotoxicity. It is unclear whether CAR-Tregs would also induce these adverse reactions. Moreover, antibodies specific for self- or allo-antigens must be characterized to construct antigen-specific CAR-Tregs. Selection of antigens targeted by CARs and development of specific antibodies are difficult in some disease models. Finally, CAR-Treg exhaustion may limit their efficacy in immunosuppression. Recently, innovative CAR-Treg therapies in animal models of transplantation and autoimmune diseases have been reported. In this mini-review, we have summarized recent progress of CAR-Tregs and discussed their potential applications for induction of immunological tolerance.

Treg Heterogeneity, Function, and Homeostasis

“…Indirect evidence of this concept are the positive results obtained in vivo using Tregs with a chimeric antigen receptor (CAR) to treat autoimmune diseases as well as in transplantation. Such CAR-Tregs specifically migrate to target sites and exhibit more pronounced antigen-specific suppressive activity (124), thus filling a gap in a concrete set of Treg clones specific to antigenic determinants with respect to which tolerance is impaired.…”

Immunotherapy Deriving from CAR-T Cell Treatment in Autoimmune Diseases

“…Regulatory T cells (Tregs) also play a critical role in regulating the immune system by inhibiting the function of immune cells to keep immunologic self-tolerance and immune homeostasis, and an AID will occur when the specific transcription factor Forkhead box protein P3 (Foxp3) of Tregs is mutated or the CD4+CD25+ T cells are eliminated [50]. Therefore, applying Treg therapy in AIDs after being engineered to CAR-Tregs having antigen specificity may be a new choice [16,21,51,52]. CAR-Tregs can induce antigen-specific cytolysis of the targeted cell in a granzyme B-dependent way, suppressing antigen-specific effector T cells' (Teffs) response, and releasing immunosuppressive cytokines, like transforming growth factor β1 (TGF-β1) and IL-10 [53].…”

“…CAR-Tregs are transduced from T cells and expanded ex vivo with normal expressing levels of Foxp3 to keep the expanding ability of reaching the therapeutic number, but transformation from CAR-Tregs to effector CAR-T cells in an inflammatory milieu is the major safety issue [44]. Nevertheless, CAR-Tregs can suppress Teffs by the following mechanisms: releasing immunosuppressive cytokines, such as IL-10, IL-35, and TGF-β; competing binding molecules CD80/CD86 on APCs with cytotoxic T lymphocyte antigen 4 expressed by CAR-Tregs to CD28 expressed by Teffs; and inducing apoptosis of Teffs through Fas-ligand or granzyme B/A and perforin produced by CAT-Tregs [52].…”

Impact of infection on transplantation tolerance

“…CD4 + Treg adoptive transfer has shown promising efficacy in allogeneic mouse skin, cardiac, and islet transplantation models. 206 HLA-A2, a MHC class I molecule highly prevalent among human donors, has been used as a target antigen to generate CAR-Tregs for suppressing transplant immunity. 200 CARs in nTregs are capable of redirecting the specificity of nTregs toward the desired antigens without altering their regulatory phenotype or epigenetic stability.…”

“…Transfected CARs in nTregs are capable of redirecting the specificity of nTregs toward the desired antigens without altering their regulatory phenotype or epigenetic stability. 206 HLA-A2, a MHC class I molecule highly prevalent among human donors, has been used as a target antigen to generate CAR-Tregs for suppressing transplant immunity. 207 Infusion of HLA-A2-specific CAR Tregs (A2-CAR-Tregs) has been shown to completely prevent rejection of allogeneic skin graft in immune reconstituted humanized mice in the absence of any immunosuppression.…”