Tregs: Where We Are and What Comes Next?

Abstract

Regulatory T cells are usually recognized as a specialized subset of CD4⁺ T cells functioning in establishment and maintenance of immune tolerance. Meanwhile, there is emerging evidence that regulatory T cells (Tregs) are also present in various non-lymphoid tissues, and that they have unique phenotypes credited with activities distinct from regulatory function. Their development and function have been described in plenty of manuscripts in the past two decades. However, with the deepening of research in recent years, emerging evidence revealed some novel mechanisms about how Tregs exert their activities. First, we discuss the expanding family of regulatory lymphocytes briefly and then, try to interpret how fork-head box P3 (Foxp3), a master regulator of the regulatory pathway in the development and function of regulatory T cells, functions. Subsequently, another part of our focus is varieties of tissue Tregs. Next, we primarily discuss recent research on how Tregs work and their faceted functions in terms of soluble mediators, functional proteins, and inhibitory receptors. In particular, unless otherwise noted, the term "Treg" is used here to refer specially to the "CD4⁺CD25⁺Foxp3^+" regulatory cells.

Keywords: Foxp3; amphiregulin; d-mannose; neuropilin-1; regulatory T cells; regulatory innate lymphoid cells.

Figure 1

Milestone discoveries in regulatory cells field and their expanding family members. There are two crucial aspects to the “regulatory lymphocytes”: (i). A growing number of other members of regulatory cells family are gradually emerging into our sight, such as γδ-Treg and ILCregs. (ii). Meanwhile, with regard to regulatory T cells, early work was focusing on identifying their markers; present work gradually shifts to distinct functions of Tregs and their metabolomics and genomics. Tregs, regulatory T cells; Bregs, regulatory B cells; iNKT, invariant natural killer T cells; GITR, glucocorticoid-induced tumor necrosis factor receptor family-related gene; Foxp3, fork-head box P3; ICOS, inducible T cell costimulator; exTreg, T-helper (Th) 17 cells derived from Foxp3⁺ T cells without Foxp3 expression; GARP, glycoprotein A repetitions predominant; Runx-CBFβ, runt-related transcription factor–core-binding factor subunit-β complex; NRP-1, neuropilin-1; LAG, lymphocyte-activation gene; STAT, signal transducer and activator of transcription; ILCreg, regulatory innate lymphoid cells.

Figure 2

Treg-mediated suppressive function via cyclic AMP (cAMP). ① Through expression of the ectoenzymes CD39 and CD73, Treg drives the accumulation of adenosine extracellularly, which disrupts Teff metabolism, leading to anergy. During this process, adenosine activities high-affinity A2a receptors, with a result of plenty of cAMP by means of adenylyl cyclase 9 (42). ② On the other hand, cAMP pool in Treg can be poured into Teff/APC by means of GJIC (43). Connexin43 plays an irreplaceable role in this delivery (44). ③ Finally, accumulative cAMP inhibits TCR-mediated signaling by preventing zeta-chain-associated protein kinase 70 (ZAP70) phosphorylation. This decreased TCR signaling leads to impaired Teff/APC activation and proliferation eventually (45).

Figure 3

Model for TGF-β production by Treg. ① The latent form of TGF-β can be released from both activated Tregs and T helper cells (53, 54). Inside latent TGF-β form, latency-associated peptide (LAP) is bound tightly to active TGF-β, acting as a shield which separate active TGF-β from its receptor. ② Glycoprotein A repetitions predominant protein (GARP), a kind of transmembrane anchor to keep latent TGF-β cling to the cell surface, is a cell surface receptor on activated Tregs, platelets, but negligible expressed on Th clones (55, 56). Combined with LAP and mature TGF-β, GARP represents the third part of a muti-protein complex in activated Tregs. Or, GARP can be regarded as a covalent receptor for latent TGF-β in active Tregs. Subsequently, association of TGF-β with GARP induces activation of the latent complex via integrin α_vβ₆ or integrin α_vβ₈ (57). ③ Active TGF-β interacts with its specific receptors to induce cellular responses. ④ Recent research disclosed that d-mannosem can upregulate levels of integrin a_vb₈ and reactive oxygen species generated by increased fatty acid oxidation, which facilitates activation of the latent TGF-β (49). ⑤ In synergy with IL-2, TGF-β promotes the conversion of naive CD4+ T cells toward Tregs by upregulating expression of Foxp3 (58, 59).

Figure 4

The IL-35 expression feedback loop and how it balances Th17 cells and iTr35. IL-35 induces naive T cells to differentiate toward regulatory iTr35 cells while the latter further secrete higher concentration of IL-35, forming a positive feedback cycle. Conversely, IL-35 generates a negative effect on the differentiation and function of Th17 cells.