IDO1 and HIV infectious disease: In the nonpathogenic model of SIV infection, the IDO activity in activated DC is downregulated though IFN-γ signaling and Toll-like receptor (TLR) stimulation by SIV and bacterial antigens, while in pathogenic SIV infection as well as in chronic HIV infection, IFN-γ responses remain high, leading to the persistence of elevated IDO activity [89, 90].