Regulatory T Cells

Different subsets, similar functions

Regulatory T cells (Tregs) play an important role in maintaining immune homeostasis1. Tregs suppress the function of other T cells to limit the immune response. Alterations in the number and function of Tregs has been implicated in several autoimmune diseases including multiple sclerosis, active rheumatoid arthritis, and type 1 diabetes. High levels of Tregs have been found in many malignant disorders including lung, pancreas, and breast cancers. Tregs may also prevent antitumor immune responses, leading to increased mortality.

CD4 and CD8 Tregs

Two major classes of Tregs have been identified to date: CD4 and CD8 Tregs. CD4 Tregs consist of two types, “natural” Tregs (nTregs) that constitutively express CD25 and FoxP3, and so-called adaptive or inducible Tregs (iTregs).

Natural Tregs originate from the thymus as CD4+ cells expressing high levels of CD25 together with the transcription factor (and lineage marker) FoxP3. nTregs represent approximately 5–10% of the total CD4+ T cell population, and can first be seen at the single-positive stage of T lymphocyte development.2 They are positively selected thymocytes with a relatively high avidity for self-antigens. The signal to develop into Treg cells is thought to come from interactions between the T cell receptor and the complex of MHC II with self peptide expressed on the thymic stroma.3 nTregs are essentially cytokine independent.

Adaptive or inducible Tregs originate from the thymus as single-positive CD4 cells. They differentiate into CD25 and FoxP3 expressing Tregs (iTregs) following adequate antigenic stimulation in the presence of cognate antigen and specialized immunoregulatory cytokines such as TGF-β, IL-10, and IL-4.4

FoxP3 is currently the most accepted marker for Tregs, although there have been reports of small populations of FoxP3- Tregs. The discovery of transcription factor FoxP3 as a marker for Tregs has allowed scientists to better define Treg populations leading to the discovery of additional Treg markers including CD127.

CD127 discovery

Research from the laboratories of Barbara Fazekas de St. Groth (Centenary Institute of Cancer Medicine and Cell Biology, Sydney, Australia) and Jeffrey Bluestone (UCSF Diabetes Center, San Francisco, CA, USA) and confirmed in laboratories at BD Biosciences demonstrated that CD127 expression is down-modulated on the Treg cells, inversely correlating with the expression of Treg marker FoxP3. It was also demonstrated that Tregs are also present (at varying levels) in populations of cells expressing high and low levels of CD25. These findings provided a new cell surface marker for Tregs,5,6 enabling isolation of viable cells by flow cytometric sorting for further downstream analysis.

CD127 is part of the heterodimeric IL-7 receptor that is composed of CD127 and the common g chain, which is shared by other cytokine receptors (IL-2R, IL-4R, IL-9R, IL-15R, and IL-21R). CD127 is expressed on thymocytes, T- and B-cell progenitors, mature T cells, monocytes, and some other lymphoid and myeloid cells. Studies have shown that IL-7R plays an important role in the proliferation and differentiation of mature T cells, and in vitro experiments show that the expression of CD127 is down-regulated following T cell activation.7-9 It is believed that FoxP3 interacts with the CD127 promoter and might contribute to reduced expression of CD127 in Tregs.6

Treg subsets

  Natural Treg (nTregs) Induced Tregs (iTregs)
  nTregs Tr1 Th3
Phenotype CD4+CD25int/high, CD127low CD4+CD25- CD4+CD25+ from CD25- precursors
Other associated markers CTLA4+, GITR+, FoxP3+, CD127low CD25low-variable, CD45RBlow, FoxP3- CD25low-variable, CD45RBlow, FoxP3+
Suppression Contact dependent, granzyme B-dependent, makes TGF-β Through cytokines produces IL-10 Through cytokines, produces TGF-β
Target cells APC and T effector cells T effector cells Not yet Identified
CD28 Involvement Thymic development and maintenance in periphery Not for development or function Not involved
In vivo Role Suppression of autoreactive T cells Mucosal immunity, inflammatory response Mucosal immunity, inflammatory response
In vitro Expansion Expandable using TCR/CD28 stimulation and IL-2 CD3, IL-10, rehnoic acid CD3, TGF-β