Regulating the regulators: costimulatory signals control the homeostasis and function of regulatory T cells

Abstract

Costimulation is a concept that goes back to the early 1980s when Lafferty and others hypothesized that cell surface and soluble molecules must exist that are essential for initiating immune responses subsequent to antigen exposure. The explosion in this field of research ensued as over a dozen molecules have been identified to function as second signals following T-cell receptor engagement. By 1994, it seemed clear that the most prominent costimulatory pathway CD28 and functionally related costimulatory molecules, such as CD154, were the major drivers of a positive immune response. Then the immunology world turned upside down. CD28 knockout mice, which were, in most cases, immunodeficient, led to increased autoimmunity when bred into the non-obese diabetic background. Another CD28 family member, cytotoxic T-lymphocyte-associated protein 4, which was presumed to be a costimulatory molecule on activated T cells, turned out to be critical in downregulating immunity. These results, coupled with the vast suppressor cell literature which had been largely rebuked, suggested that the immune system was not poised for response but controlled in such a way that regulation was dominant. Over the last decade, we have learned that these costimulatory molecules play a key role in the now classical CD4(+)CD25(+)Foxp3(+) regulatory T cells (Tregs) that provide critical control of unwanted autoimmune responses. In this review, we discuss the connections between costimulation and Tregs that have changed the costimulation paradigm.

Fig. 1. Schematic model of the role of CD28 costimulation in Treg thymic development

In the absence of CD28 costimulation, high-affinity TCR signals could have resulted in an overlap in agonist peptides inducing the selection of autoreactive effector (blue line) versus regulatory (dotted red line) T cells. CD28 costimulation raises the total strength of signal to levels not permissive for effector T-cell development but allowing the upregulation of Foxp3 expression and the effective development of Tregs (red line).

Fig. 2. Model of biochemical signals downstream of CD28 costimulation that are important for Treg homeostasis

Upon stimulation by MHC-peptide complexes and B7 molecules, CD28 signals result in the translocation of PKCθ into lipid rafts and its recruitment into the immunological synapse (IS). These ‘signalsomes’ structures induce downstream biochemical cascades that lead to the transcription of the Foxp3 gene and the initiation of the Treg-developmental program, with the possible involvement of transcription factors NFAT and NF-κB. See text for details.

Fig. 3. Model of pathways involved in CTLA-4 control of Treg function

CTLA-4 could influence Treg suppressive through different pathways. In Treg-intrinsic pathways, CTLA-4 could alter TCR-mediated signaling and lead to increased suppression (1) and induce the production of immunosuppressive TGF-β, although TGF-β may function independently from CTLA-4 (2). In Treg-extrinsic pathways, CTLA-4 alters DCs by triggering the down-modulation of B7-1 and B7-2 (3) and the production of IDO (4) that interferes with activation of Teff by activating tryptophan catabolism and/or through downstream mediators HO-1 and CO. Finally, CTLA-4 is necessary for the generation of adaptive Tregs induced by TGF-β or IDO (5). See text for details.

Fig. 4. Influence of B7-1 and B7-2 molecules on Treg homeostasis versus generation of autoreactive T cells

Spleen and pancreatic lymph nodes were isolated from WT NOD mice or NOD mice heterozygous for B7-1 or B7-2 alone or for both B7-1 and B7-2. (A) The percentage of CD4⁺Foxp3⁺ Tregs was examined by flow cytometry in the spleen. (B) The percentage of autoreactive IGRP-specific CD8⁺ T cells was evaluated by flow cytometry using NRP-V7-H-2K^d tetramers (provided by the NIH MHC Tetramer Core Facility, Atlanta, GA) and anti-CD8 and anti-CD44 mAbs. Results shown were obtained in pancreatic lymph nodes and similar results were obtained in the spleen. (C) Ratio of the mean frequency of Treg (closed bar) and Teff (IGRP-specific T cells, hatched bar) in indicated NOD-B7-het mice divided by the frequency in NOD WT mice.