Naturally occurring CD25(+)CD4(+) regulatory T cells (Tregs) actively engage in the maintenance of immunologic self-tolerance and immunoregulation. They specifically express the transcription factor Forkhead box P3 (Foxp3) as a master control molecule for their development and function. Although several cell-surface molecules have been reported as Treg-specific markers, such as CD25, glucocorticoid-induced TNFR family-related gene/protein and CTL-associated molecule-4, they are also expressed on activated T cells derived from CD25(-)CD4(+) naive T cells. To identify Treg-specific molecules controlled by Foxp3, we performed DNA microarray analysis by comparing the following pairs of cell populations: fresh CD25(+)CD4(+) T cells versus fresh CD25(-)CD4(+) T cells, activated CD25(+)CD4(+) T cells versus activated CD25(-)CD4(+) T cells and retrovirally Foxp3-transduced CD25(-)CD4(+) T cells versus mock-transduced CD25(-)CD4(+) T cells. We found that the Gpr83, Ecm1, Cmtm7, Nkg7, Socs2 and glutaredoxin genes are predominantly transcribed in fresh and activated natural Treg as well as in Foxp3-transduced cells, while insulin-like 7, galectin-1, granzyme B and helios genes are natural Treg specific but Foxp3 independent. G protein-coupled receptor 83 (Gpr83) expression on the cell surface of natural Treg was confirmed by staining with Gpr83-specific antibody. Retroviral transduction of either group of genes in CD25(-)CD4(+) T cells failed to confer in vitro suppressive activity. Thus, there are several genes that are expressed in a highly Treg-specific fashion. Some of these genes are controlled by Foxp3, and others are not. These genes, in particular, Gpr83, Ecm1 and Helios, could potentially be used as specific markers for natural Treg.

Murine CD4+CD25+ regulatory cells have been reported to express latency-associated peptide (LAP) and TGF-β on the surface after activation, and exert regulatory function by the membrane-bound TGF-β in vitro. We have now found that a small population of CD4+ T cells, both CD25+ and CD25−, can be stained with a goat anti-LAP polyclonal Ab without being stimulated. Virtually all these LAP+ cells are also positive for thrombospondin, which has the ability to convert latent TGF-β to the active form. In the CD4+CD45RBhigh-induced colitis model of SCID mice, regulatory activity was exhibited not only by CD25+LAP+ and CD25+LAP− cells, but also by CD25−LAP+ cells. CD4+CD25−LAP+ T cells were part of the CD45RBlow cell fraction. CD4+CD25−LAP−CD45RBlow cells had minimal, if any, regulatory activity in the colitis model. The regulatory function of CD25−LAP+ cells was abrogated in vivo by anti-TGF-β mAb. These results identify a new TGF-β-dependent regulatory CD4+ T cell phenotype that is CD25− and LAP+.

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CD4+CD25+ T cells play a pivotal role in immunological homeostasis by their capacity to exert immunosuppressive activity. However, the mechanism by which these cells function is still a subject for debate. We previously reported that surface (membrane) TGF-β produced by CD4+CD25+ T cells was an effector molecule mediating suppressor function. We now support this finding by imaging surface TGF-β on Foxp3+CD4+CD25+ T cells in confocal fluorescence microscopy. Then, using a TGF-β-sensitive mink lung epithelial cell (luciferase) reporter system, we show that surface TGF-β can be activated to signal upon cell-cell contact. Moreover, if such TGF-β signaling is blocked in an in vitro assay of CD4+CD25+ T cell suppression by a specific inhibitor of TGF-βRI, suppressor function is also blocked. Finally, we address the role of CTLA-4 in CD4+CD25+ T cell suppression, showing first that whereas anti-CTLA-4 does not block in vitro suppressor function, it does complement the blocking activity of anti-TGF-β. We then show with confocal fluorescence microscopy that incubation of CD4+CD25+ T cells with anti-CTLA-4- and rB7-1/Fc-coated beads results in accumulation of TGF-β at the cell-bead contact site. This suggests that CTLA-4 signaling facilitates TGF-β-mediated suppression by intensifying the TGF-β signal at the point of suppressor cell-target cell interaction.