IDO1 and neoplasm: For instance, upregulation of CD31 on DCs can tilt Th differentiation towards Tregs.82 Furthermore, DCs can stifle CD8+ T cells and natural killer (NK) cell proliferation and aid Treg cell differentiation by expressing IDO1, which degrades essential substrates for immune cell responses.83 The tumor milieu can also impede the anti-tumor capability of Plasmacytoid DCs (pDC).