CD4(+) T cells that express the forkhead box P3 (FOXP3) transcription factor function as regulatory T (Treg) cells and hinder effective immune responses against cancer cells. Abundant Treg cell infiltration into tumors is associated with poor clinical outcomes in various types of cancers. However, the role of Treg cells is controversial in colorectal cancers (CRCs), in which FOXP3(+) T cell infiltration indicated better prognosis in some studies. Here we show that CRCs, which are commonly infiltrated by suppression-competent FOXP3(hi) Treg cells, can be classified into two types by the degree of additional infiltration of FOXP3(lo) nonsuppressive T cells. The latter, which are distinguished from FOXP3(+) Treg cells by non-expression of the naive T cell marker CD45RA and instability of FOXP3, secreted inflammatory cytokines. Indeed, CRCs with abundant infiltration of FOXP3(lo) T cells showed significantly better prognosis than those with predominantly FOXP3(hi) Treg cell infiltration. Development of such inflammatory FOXP3(lo) non-Treg cells may depend on secretion of interleukin (IL)-12 and transforming growth factor (TGF)-β by tissues and their presence was correlated with tumor invasion by intestinal bacteria, especially Fusobacterium nucleatum. Thus, functionally distinct subpopulations of tumor-infiltrating FOXP3(+) T cells contribute in opposing ways to determining CRC prognosis. Depletion of FOXP3(hi) Treg cells from tumor tissues, which would augment antitumor immunity, could thus be used as an effective treatment strategy for CRCs and other cancers, whereas strategies that locally increase the population of FOXP3(lo) non-Treg cells could be used to suppress or prevent tumor formation.

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Regulatory T cells (Tregs) have an immunosuppressive role in the tumor microenvironment. Since effector Tregs (eTregs), which have highly suppressive functions, are located in a subpopulation of Foxp3 CD4 Tregs, the TCR-inducible costimulatory receptor (ICOS) was applied as a marker of eTregs that infiltrated gastric cancer tissue and the induction pathway of ICOS Foxp3 cells was analyzed by flow cytometry and immunohistochemistry. In tumor-infiltrating lymphocytes (TILs), ICOS Foxp3 CD4 T cells were abundantly observed in the late stages of gastric cancer. ICOS CD4 TILs exhibited the ability to produce IL-10, but not IFN-γ, TNF, or IL-17 and also to suppress the proliferation of CFSE-labeled responder CD8 T cells. With the agonistic ICOS-L protein (rICOS-L Ig), ICOS Foxp3 cells were efficiently induced from naive CD4 T cells under a stimulation with TGF-β and CD3/CD28 mAbs. Furthermore, when A*0201 PBMCs were cultured with the CMV or Melan-A antigenic peptide and rICOS-L Ig, the induction of CMV or Melan-A tetramer-binding CD8 T cells, respectively, was inhibited. The expression of ICOS in Foxp3 cells was closely related to plasmacytoid dendritic cells (pDCs) and their expression of ICOS-L and TLR9 as well as Helicobacter pylori infection. Collectively, our results demonstrate the potential of ICOS as a promising target for direct Treg-targeting therapeutic agents for gastric cancer, and that of eradicating therapy for H. pylori as an indirect immune therapy for gastric cancer.

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