Deficiency in the transcription factor interferon regulatory factor (IRF)-2 leads to severely compromised development of natural killer and T helper type 1 cells

Interferon (IFN) regulatory factor (IRF)-2 was originally described as an a ntagonist of IRF-1-mediated transcriptional regulation of-IFN-inducible gen es. IRF-1(-/-) mice exhibit defective T helper type 1 (Th1) cell differenti ation. We have used experimental leishmaniasis to show that, like IRF-1-/- mice, IRF-2(-/-) mice are susceptible to Leishmania major infection due to a de feet in Th1 differentiation. Natural killer (NK) cell development is c ompromised in both IRF-1(-/-) and IRF-2-/- mice, but the underlying mechani sm differs. NK (but not NK+ T) cell numbers are decreased in IRF-2-/- mice, and the NK cells that are present are immature in phenotype. Therefore, li ke IRF-1, IRF-2 is required for normal generation of Th1 responses and for NK cell development in vivo. In this particular circumstance the absence of IRF-2 cannot be compensated for by the presence of IRF-1 alone. Mechanisti cally, IRF-2 may act as a functional agonist rather than antagonist of IRF- 1 for some, but not all, IFN-stimulated regulatory element (ISRE)-responsiv e genes.