IFN-gamma and IL-12 are crucial cytokines for cell-mediated immunity against intracellular pathogens. We have previously shown that human IL-12Rbeta1-deficiency leads to impaired IL-12 responsiveness and unusual susceptibility to infections due to mycobacteria and salmonellae. IL-23 is a cytokine with functions that partially overlap with those of IL-12. IL-23 consists of IL-12p40 and a novel p19 protein, and binds to a receptor complex comprising IL-12Rbeta1 and IL-23R. Thus, IL-12Rbeta1-deficiency may impair both IL-12- and IL-23 signaling, and both may contribute to the immunological phenotypes. To examine whether IL-12Rbeta1 is essential for IL-23 signaling in human T cells, we have studied IL-23 responsiveness of four IL-12Rbeta1 deficient individuals. Whereas IL-23 promoted IFN-gamma production by CD4(+) and CD8(+) T cells in controls, IL-12Rbeta1-deficient T cells lacked IL-23-induced IFN-gamma secretion, but responded normally to IL-2, IL-4, IL-15 and IL-18. We also show that induction of IFN-gamma production by IL-23 depends upon TCR-ligation and is enhanced by CD-28-costimulation. Furthermore, IL-23 cooperates with IL-12 and IL-18 in promoting IFN-gamma production in controls, but not in patients. We conclude that IL-12Rbeta1 -deficiency impairs IL-12- and IL-23-dependent signaling in human T cells. The syndrome caused by IL-12Rbeta1 -deficiency thus needs to be reinterpreted as resulting from defective IL-12- as well as IL-23-mediated immunity.