Peripherally induced tregs - role in immune homeostasis and autoimmunity

Abstract

Thymically derived Foxp3(+) regulatory T cells (tTregs) constitute a unique T cell lineage that is essential for maintaining immune tolerance to self and immune homeostasis. However, Foxp3 can also be turned on in conventional T cells as a consequence of antigen exposure in the periphery, under both non-inflammatory and inflammatory conditions. These so-called peripheral Tregs (pTregs) participate in the control of immunity at sites of inflammation, especially at the mucosal surfaces. Although numerous studies have assessed in vitro generated Tregs (termed induced or iTregs), these cells most often do not recapitulate the functional or phenotypic characteristics of in vivo generated pTregs. Thus, there are still many unanswered questions regarding the T cell receptor (TCR) repertoire and function of pTregs as well as conditions under which they are generated in vivo, and the degree to which these characteristics identify specialized features of pTregs versus features that are shared with tTregs. In this review, we summarize the current state of our understanding of pTregs and their relationship to the tTreg subset. We describe the recent discovery of unique cell surface markers and transcription factors (including Neuropilin-1 and Helios) that can be used to distinguish tTreg and pTreg subsets in vivo. Additionally, we discuss how the improved ability to distinguish these subsets provided new insights into the biology of tTregs versus pTregs and suggested differences in their function and TCR repertoire, consistent with a unique role of pTregs in certain inflammatory settings. Finally, these recent advances will be used to speculate on the role of individual Treg subsets in both tolerance and autoimmunity.

Keywords: Helios; autoimmunity; immune tolerance; neuropilin-1; regulatory T cell.

Figure 1

Model depicting the generation and function of tTregs and pTregs. Nrp-1^hi tTregs are generated in the thymus and are important in maintaining immune homeostasis and controlling autoimmune responses. During the course of an immune response, Nrp-1^lo pTregs are generated in response to Ag presentation by specialized APCs and control effector T cells (Teff) at the site of inflammation. pTregs help in controlling inflammation locally and may be more effective than tTregs at suppressing Teff due to overlapping antigen specificity.

Figure 2

Stability of Treg subsets in MBP.TCR.Tg 1B3 mice using lineage reporter system. The MBP.TCR.Tg mouse when crossed onto RAG^−/− background lacks tTregs but generates pTregs in the periphery highlighted by red box (GFP⁺YFP⁻ subset). The FACS plots depicting expression of GFP and YFP by CD4⁺ T cells from LNs of 3- to 4-week-old MBP.TCR.Tg.RAG^−/−.Foxp3-Cre × R26-YFP or MBP.TCR.Tg.RAG^+/−. Foxp3-Cre × R26-YFP mice are shown. In the current gating strategy, GFP⁺YFP⁺ population represents the stable Treg subset whereas GFP⁻YFP⁺ gate represents unstable Tregs, which previously expressed Foxp3. Cells gated on CD4⁺ T cells are shown and numbers around the outlined areas indicate percent. Graph on bottom shows the frequency of GFP⁻YFP⁺ among YFP⁺ cells with each symbol representing an individual mouse and bars representing mean values for each group.

Figure 3

Distinct TCR repertoire of pTregs and tTregs. (A) Venn diagram showing distribution of unique and overlapping pTreg, tTreg and Tconv CDR3 sequences. Nrp-1^hi tTregs, Nrp-1^lo pTregs and CD4⁺Foxp3⁻ Tconv cells were sorted from MBP.TCR.Tg (1B3)-Foxp3.GFP mice. cDNA was amplified with Vα2-specific primers and amplicons were subcloned and sequenced to determine CDR3 sequences. (B) Frequency of unique CDR3 sequences (identified by peptide number along horizontal axis) in Nrp-1^hi tTregs (black bars; top graph), Foxp3⁻ Tconv cells (white bars; top graph) and Nrp-1^lo pTregs (gray bars; bottom graph) sorted from 1B3 mice. Data from one representative mouse is presented here.