IRF1 and viral load: Notably, while both type I and II IFNs induce the transcription factor IRF1, in vitro studies of cells obtained from unrelated children with inherited complete IRF1 deficiency and multiple, life-threatening diseases in early life caused by weakly virulent mycobacteria and related intra-macrophagic pathogens but no history of severe viral disease demonstrated that IRF1 is essential for IFN-γ-dependent macrophage immunity to mycobacteria, but largely redundant for type I IFN-dependent anti-viral immunity [182].