IDO1 and neoplasm: A plethora of influences hampers the cytotoxic antitumor response, such as the suppression of CD8+ T cells through Tregs, tumor-induced hypoxia, lactic acid accumulation, the secretion and expression of immunomodulatory mediators (IL-10, Fas/FasL, CXCL17, VEGF and IDO) and the weakened stimulation by APCs and CD4+ T cells [123,124].