Foxp3+ regulatory T cell therapy for tolerance in autoimmunity and solid organ transplantation

Abstract

Regulatory T cells (Tregs) are critical for tolerance in humans. The exact mechanisms by which the loss of peripheral tolerance leads to the development of autoimmunity and the specific role Tregs play in allograft tolerance are not fully understood; however, this population of T cells presents a unique opportunity in the development of targeted therapeutics. In this review, we discuss the potential roles of Foxp3+ Tregs in the development of tolerance in transplantation and autoimmunity, and the available data regarding their use as a treatment modality.

Keywords: adoptive cell transfer; allograft tolerance; antigen-specific regulatory T cells; autoimmunity; regulatory T cells; solid organ transplantation.

Figure 1

Mechanisms by which Tregs function to suppress pro-inflammatory responses. 1) Release of perforin and granzymes leads to apoptotic pathways. 2) Treg derived CD25 scavenges IL-2, resulting in decreased CD4+/CD8+ effector T cell activation and leads to apoptosis. 3) CTLA-4 expressed on Tregs interacts with co-stimulatory molecules CD80/86 preventing APCs from effectively stimulating effector T cells. CTLA-4 can also directly interact with CD80/86 expressed on effector T cells. 4) Tregs expressing PD-1 prevent autoreactive and alloreactive B-cells from stimulating other T cells. 5) CD39 converts ATP to AMP, CD73 converts AMP to adenosine which then leads to suppression of effector T cells. 6) Tregs release IL-10, IL-35, and TGF-β to promote suppressive action (created with Biorender.com).

Figure 2

Allorecognition pathways. (1) Indirect allorecognition is activated following presentation of a donor peptide on a recipient APC with a recipient MHC class II molecule. These peptides are recognized by recipient CD4+ T cells. (2) Semi-direct allorecognition occurs after presentation of an intact donor-derived MHC class I peptide by a recipient APC to a recipient T cell. (3) Direct allorecognition occurs following presentation of a donor MHC class II peptide on a donor APC and/or presentation of a donor MHC class I peptide on another donor cell. These peptides are recognized by CD4+ T cells and CD8+ T cells respectively. (created with Biorender.com).

Figure 3

In-vitro methods of Treg expansions. Once Tregs are isolated from a source utilizing FACS or immunomagnetic sorting, they can be expanded in a (1) polyclonal fashion or (2-4) antigen specific manner. (1) Tregs are polyclonally expanded in the presence of anti-CD3/anti-CD28 antibodies resulting in mature Tregs with the expression of a diverse array of T-cell receptors (TCRs). (2) Genetically engineered Tregs undergo polyclonal expansion and also transfection with a lentiviral/retroviral vector coding for a specific TCR producing Tregs expressing only one specific TCR. (3) Chimeric antigen receptor (CAR) Tregs are expanded via polyclonal expansion and transfection with lentiviral/retroviral vectors expressing CARs fused to intracellular signaling domains. (4) In the context of transplantation, “Donor” specific Tregs can be expanded via co-culture with activated, allospecific antigen presenting cells (APCs). As depicted, these in-vitro methods of Treg expansion additionally require supplementation of culture media with IL-2 and other Treg inducing molecules, such as rapamycin or everolimus, in some culture protocols (created with Biorender.com).

Figure 4

CAR Treg constructions. CARs are created by fusing a specific antigen binding domain to a transmembrane domain and a single or multiple intracellular signaling domains responsible for T cell activation. In the context of Tregs, 1^st generation include an intracellular CD3 ζ signaling domain, 2^nd generation consist of intracellular CD3 ζ and typically CD28 and 3^rd generation CD3 ζ, CD28 and CD137 or other co-stimulatory molecule. 4^th generation CARs are created by modifying 2^nd or 3^rd generation CARs with the addition of a cytokine inducing domain, suicide gene, or safety switch (created with Biorender.com).