CD4 ؉ CD25 ؉ Foxp3 ؉ regulatory T cells (Tregs) are potent suppressors of the adaptive immune system, but their effects on innate immune cells are less well known. Here we demonstrate a previously uncharacterized function of Tregs, namely their ability to steer monocyte differentiation toward alternatively activated macrophages (AAM). AAM are cells with strong antiinflammatory potential involved in immune regulation, tissue remodeling, parasite killing, and tumor promotion. We show that, after coculture with Tregs, monocytes/ macrophages display typical features of AAM, including up-regulated expression of CD206 (macrophage mannose receptor) and CD163 (hemoglobin scavenger receptor), an increased production of CCL18, and an enhanced phagocytic capacity. In addition, the monocytes/ macrophages have reduced expression of HLA-DR and a strongly reduced capacity to respond to LPS in terms of proinflammatory mediator production (IL-1␤, IL-6, IL-8, MIP-1␣, TNF-␣), NFB activation, and tyrosine phosphorylation. Mechanistic studies reveal that CD4 ؉ CD25 ؉ CD127 low Foxp3 ؉ Tregs produce IL-10, IL-4, and IL-13 and that these cytokines are the critical factors involved in the suppression of the proinflammatory cytokine response. In contrast, the Tregmediated induction of CD206 is entirely cytokine-independent, whereas the up-regulation of CD163, CCL18, and phagocytosis are (partly) dependent on IL-10 but not on IL-4/IL-13. Together these data demonstrate a previously unrecognized function of CD4 ؉ CD25 ؉ Foxp3 ؉ Tregs, namely their ability to induce alternative activation of monocytes/macrophages. Moreover, the data suggest that the Tregmediated induction of AAM partly involves a novel, cytokineindependent pathway. alternatively activated macrophages ͉ mannose receptor ͉ phagocytosis ͉ proinflammatory response ͉ interleukin-10

Objective. In mice, CD4؉CD25؉ regulatory T cells play a pivotal role in preventing autoimmunity. Regulatory T cells are also present and functional in healthy humans. We investigated the presence, phenotype, and function of CD4؉CD25؉ regulatory T cells in peripheral blood (PB) and synovial fluid (SF) from patients with rheumatoid arthritis (RA).Methods. The presence and phenotype of CD4؉CD25؉ regulatory T cells were determined by flow cytometry. Anergy and suppressive activity were assessed by culturing CD4؉CD25؊ and CD4؉CD25؉ T cells with anti-CD3 monoclonal antibodies and antigen-presenting cells, followed by proliferation and cytokine detection.Results. The percentage of CD4؉CD25؉ T cells in RA SF was significantly increased compared with that in RA PB, and both of these percentages were higher than that in PB from controls. The cells in RA PB were similar in phenotype and function to CD4؉CD25؉ regulatory T cells from controls. In SF, however, ϳ40-50% of CD4؉CD25؉ T cells expressed an activated phenotype, i.e., CD69؉, class II MHC؉, OX-40؉, with high levels of CTLA-4 and glucocorticoid-induced tumor necrosis factor receptor. These synovial CD4؉CD25؉ T cells displayed an increased suppressive capacity compared with blood CD4؉CD25؉ T cells. However, this enhanced suppressive activity was counterbalanced, because activated responder T cells from SF were less susceptible to CD4؉CD25؉ T cell-mediated suppression than were responder cells from PB.Conclusion. We demonstrate that CD4؉CD25؉ regulatory T cells are present and functional in patients with RA, with higher numbers of regulatory T cells with increased suppressive activity found in SF compared with PB. These findings suggest a negative feedback system that is active at the site of inflammation. The balance between activated responder and regulatory T cells appears to influence the extent of immunoregulation in RA.

Anergic/suppressive CD4+CD25+ T cells exist in animal models but their presence has not yet been demonstrated in humans. We have identified and characterized a human CD4+CD25+ T cell subset, which constitutes 7–10 % of CD4+ T cells in peripheral blood and tonsil. These cells are a CD45RO+CD45RBlow highly differentiated primedT cell population that is anergic to stimulation. Depletion of this small subset from CD4+ T cells significantly enhances proliferation by threefold in the remaining CD4+CD25– T cells, while the addition of isolated CD4+CD25+ T cells to CD4+CD25– T cells significantly inhibits proliferative activity. Blocking experiments suggest that suppression is not mediated via IL‐4, IL‐10 or TGF‐β and is cell‐contact dependent. Isolated CD4+CD25+ T cells are susceptible to apoptosis that is associated with low Bcl‐2 expression, but this death can be prevented by IL‐2 or fibroblast‐secreted IFN‐β. However, the anergic/suppressive state of these cells is maintained after cytokine rescue. These human regulatory cells are therefore a naturally occurring, highly suppressive, apoptosis‐prone population which are at a late stage of differentiation. Further studies into their role in normal and pathological situations in humans are clearly essential.