IDO1 and viral infectious disease: Developed in periphery utilize CCR9/α4 integrin signals  In steady state, contribute to the maintenance of tolerance  In steady state express low levels of MHCII contributing to T cell disregard  In active state upregulate MHCII molecules inducing T cell proliferation  Produce IFN-1 and IL10contributing to T reg formation, and IDO and PDL1 increasing Treg density  Respond to viral infections secreting IFN1