IDO1 and neoplasm: All CD3+ T cell subtypes have been shown to be capable of promoting tumor progression, either through altered cytokine production such as IL-1, IL-4, TGF-β and IL-10 [34-36] or through cell-cell contact after being converted into FoxP3 regulatory T cells by the influence of tumor stroma derived immunosuppressive factors such as PGE2, TGF-β or IDO by-products [37,38].