IDO1 and neoplasm: Accordingly, lack of tryptophan and overproduction of kynurenine has been shown to not only diminish the proliferation and survival of T cells, but also shift the differentiation of T cells into Treg cells.830 Interestingly, evidence have shown that combined treatment using an IDO inhibitor and a tumor vaccine induce the upregulation of IL-6 in pDCs and in situ, resulting in the conversion of a majority of Tregs to Th17-like cells, with marked enhancement in the activation and anti-tumor efficacy of Teff cells.