IDO1 and neoplasm: Jung et al. (2010) found that murine DC maturation depends on IDO expression via a HO-1-dependent pathway. They found that in IDO deficiency, there is a loss of maturation of DCs in vitro and in vivo. In addition, inhibition of HO by ZnPP abolished IDO expression and DC maturation, while the administration resulted in opposite effects. Therefore, studying the immunosuppressive function of leukocytes in the fetomaternal interface as well as in the tumor site may help reveal the role of HO-1 in immune regulation (Blancou and Anegon, 2010; Gabrilovich et al., 2012).