Foxp3 Expressing CD4+CD25high Regulatory T Cells Are Overrepresented in Human Metastatic Melanoma Lymph Nodes and Inhibit the Function of Infiltrating T Cells

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Abstract
Dominant tolerance is mediated by regulatory T cells (Treg) that control harmful autoimmune T cells in the periphery. In this study, we investigate the implication of Treg in modulating infiltrating T lymphocytes in human metastatic melanoma. We found that CD4+CD25high T cells are overrepresented in metastatic lymph nodes (LNs) with a 2-fold increased frequency compared with both tumor-free LNs and autologous PBMCs. These cells express the Foxp3 transcription factor, display an activated phenotype, and display a polyclonal TCR Vβ chain repertoire. They inhibit in vitro the proliferation and cytokine production of infiltrating CD4+CD25− and CD8+ T cells (IL-2, IFN-γ) through a cell-contact-dependent mechanism, thus behaving as Treg. In some cases, the presence of Treg type 1/Th3-like lymphocytes could also be demonstrated. Thus, Treg are a major component of the immunosuppressive microenvironment of metastatic melanoma LNs. This could explain the poor clinical response of cancer patients under immunotherapeutic protocols, and provides a new basis for future immunotherapeutic strategies counteracting in vivo Treg to reinforce local antitumor immune responses.