Constitutive activation of signal transducer and activator of transcription 1 (STAT1) is a distinctive feature of Epstein-Barr virus (EBV)-immortalized B cells (lymphoblastoid cell lines [LCLs]). The expression of STAT1 in these cells is modulated by the latent membrane protein 1 (LMP1), but the mechanism of STAT1 activation has remained unclear. We demonstrate that the tyrosine phosphorylation of STAT1 in LCLs results from an indirect pathway encompassing an NF-kappaB-dependent secretion of interferons (IFNs). The cell culture supernatant of LCLs induced tyrosine phosphorylation of STAT1 in cells with no constitutively activated STAT1. Moreover, removal of supernatant from LCLs was sufficient to decrease the phosphorylation of STAT1. Inhibition of NF-kappaB activity by different pharmacological inhibitors (i.e., parthenolide, MG132 and BAY 11-7082) and by overexpressed mutated IkappaBalpha prevented the activation of STAT1. To identify the factors involved, we performed macroarray cDNA profiling with or without inhibition of NF-kappaB. The expression of several cytokines was NF-kappaB dependent among those alpha and gamma IFNs (IFN-alpha and IFN-gamma), known activators of STAT1. By real-time PCR and enzyme-linked immunosorbent assay we show that IFN-alpha and IFN-gamma are expressed and released by LCLs in an NF-kappaB-dependent manner. Finally, the blocking of the IFN-alpha and IFN-gamma by neutralizing antibodies led to the complete inhibition of tyrosine phosphorylation of STAT1. Taken together, our results clearly show that LMP1-induced tyrosine phosphorylation of STAT1 is almost exclusively due to the NF-kappaB-dependent secretion of IFNs. Whether this response, which is usually considered to be antiviral, is in fact required for the persistence of the virus remains to be elucidated.