ICOS and neoplasm: The ICOS-driven interaction between tumor associated CD4+ T cells and ICOSL-expressing TA-pDCs sustain the expansion of ICOS+ FoxP3+ Tregs in the tumor microenvironment and promote the secretion of IL-10 and TGF-β from Tregs, which support an immunosuppressive microenvironment and tumor progression [137,138,150,151,152,154,174,176,177].