OBJECTIVE: Immune cells contribute to angiotensin II (ATII)-induced vascular dysfunction and inflammation. Interferon-gamma (IFN-gamma), an inflammatory cytokine exclusively produced by immune cells, seems to be involved in ATII-driven cardiovascular injury, but the actions and cellular source of IFN-gamma remain incompletely understood. APPROACH AND RESULTS: IFN-gamma(-/-) and Tbx21(-/-) mice were partially protected from ATII-induced (1 mg/kg per day of ATII, infused subcutaneously by miniosmotic pumps) vascular endothelial and smooth muscle dysfunction, whereas mice overexpressing IFN-gamma showed constitutive vascular dysfunction. Absence of T-box expressed in T cells (T-bet), the IFN-gamma transcription factor encoded by Tbx21, reduced vascular superoxide and peroxynitrite formation and attenuated expression of nicotinamide adenosine dinucleotide phosphate oxidase subunits as well as inducible NO synthase, monocyte chemoattractant protein 1, and interleukin-12 in aortas of ATII-infused mice. Compared with controls, IFN-gamma(-/-) and Tbx21(-/-) mice were characterized by reduced ATII-mediated vascular recruitment of both natural killer (NK)1.1( ) NK-cells as the major producers of IFN-gamma and CD11b( )Gr-1(low) interleukin-12 secreting monocytes. Selective depletion and adoptive transfer experiments identified NK-cells as essential contributors to vascular dysfunction and showed that T-bet( )lysozyme M( ) myelomonocytic cells were required for NK-cell recruitment into vascular tissue and local IFN-gamma production. CONCLUSIONS: We provide first evidence that NK-cells play an essential role in ATII-induced vascular dysfunction. In addition, we disclose the T-bet-IFN-gamma pathway and mutual monocyte-NK-cell activation as potential therapeutic targets in cardiovascular disease.