CD4+ T helper (Th) cells play a central role in orchestrating host immune responses through their capacity to help other cells of the immune system. More recently, a novel CD4+ T cell subset termed Th17 cells has C646 cell line been identified, which expresses the transcription factor retinoid-related orphan receptor (ROR)-γt and produce the proinflammatory

cytokine interleukin (IL)-17 [1,2]. Although Th17 cells play a critical role in the pathogenesis of many inflammatory and autoimmune diseases [3,4], their prevalence among tumour-infiltrating lymphocytes (TILs) and function in human tumour immunity remain largely unknown. The results from two studies in prostate and ovarian cancer patients have suggested both beneficial and harmful implications of Th17 cells in tumour development [5,6]. Apart from its proinflammatory role, IL-17 up-regulates the production of a variety of proangiogenic factors, thus contributing to tumour angiogenesis and development. The basis for this discrepancy is not yet understood, and the presence or absence of the adaptive immune system has been suggested to account for it [7]. CD4+CD25+ regulatory T cells (Treg), constitutively expressing high levels of CD25 (the IL-2Rα chain) and the transcription

factor forkhead box P3 (FoxP3), are essential for maintaining peripheral tolerance, preventing autoimmune diseases and chronic inflammatory diseases [8–10]. Cyclopamine However, they also limit beneficial responses by suppressing sterilizing immunity and limiting anti-tumour immunity. The outcome

of this activity appears to promote the survival IMP dehydrogenase of cancer cells by affording protection from both the innate and adaptive immune systems. Several studies have shown that higher numbers of Treg were associated with progression in a variety of malignancies [11,12]. Antigen-specific Treg have also been demonstrated at the tumour site or in the draining lymph nodes, which suppress the proliferation of naive CD4+ T cells and inhibit IL-2 secretion by effector T cells upon activation by tumour-specific ligands [13,14]. In various animal models, depletion of Treg has been shown to induce immune responses and prevent the growth or trigger the regression of tumours when performed before or very early after tumour cell injection [15,16]. Depletion of immune cells before the adoptive transfer of tumour-reactive T cells has also been shown to be a promising result in human melanoma [17]. Apart from a functional antagonism between Treg and Th17 cells in autoimmunity [18], the differentiation of these two lineages is reciprocally regulated both in mice and human. It is now well established that although transforming growth factor (TGF)-β alone induces FoxP3+ regulatory T cells, TGF-β and IL-6 induce the differentiation of mouse naive T cells into Th17 cells by up-regulating the ROR-γt [19,20].