Type I interferons (IFNs) play critical roles in the host defense by modulating the expression of various genes via the IFN-dependent activation of signal transducers and activators of transcription and NF-kappaB (nuclear factor kappa B) transcription factors. Previous studies established that IFNalpha/beta activates NF-kappaB to promote cell survival through a phosphatidylinositol 3-kinase (PI3K)/Akt pathway, which involves serine phosphorylation and degradation of IkappaB alpha. We now describe a second pathway by which IFNs activate NF-kappaB that is independent of IkappaB degradation. This pathway involves NF-kappaB-inducing kinase (NIK) and the tumor necrosis factor receptor-associated factor-2 (TRAF2) and results in IFNalpha/beta-induced processing of the p100/NF-kappaB2 precursor into p52. IFNalpha/beta stimulates NF-kappaB DNA binding and NF-kappaB-dependent transcription. Whereas expression of NIK and TRAF2 constructs causes NF-kappaB activation, expression of dominant negative NIK and TRAF2 constructs blocks IFN-promoted NF-kappaB activation and IFN-stimulated kappaB-dependent transcription and IFNalpha/beta-induced processing of the p100/NF-kappaB2 precursor into p52. In contrast, PI3K does not mediate IFNalpha/beta-induced p100 processing, although PI3K is involved in the pathway resulting in IkappaB alpha degradation. Moreover, whereas IFN promotes cell survival in lymphoblastoid cells, expression of dominant negative NIK and TRAF2 constructs enhances IFN-induced apoptosis. Our results for the first time place NIK and TRAF2, previously shown to function in TNF signaling, within the IFN signal transduction pathway. Thus, IFN induces NF-kappaB activation to mediate IFN-dependent cell survival signals through a "canonical" pathway of IkappaB alpha proteolysis mediated by PI3K/Akt and a "noncanonical" pathway of p100 processing mediated by NIK/TRAF.