ICOS⁺ Foxp3⁺ TILs in gastric cancer are prognostic markers and effector regulatory T cells associated with Helicobacter pylori

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.