This is a competitive grant renewal application, whose overall goal is to understand the mechanisms of IFN-signaling. IFNs are pleiotropic cytokines that exhibit important antiviral and antineoplastic activities in vitro and in vivo. Although the mechanisms of IFN-dependent gene transcription via activation of the classic Jak/Stat-pathway are well understood, very little is known on the mechanisms of mRNA translation of IFN-sensitive genes and generation of protein products that mediate the biological effects of IFNs. We have identified a novel signaling pathway shared by both the Type I and II IFN receptors, involving activation of the PI 3'kinase and mTOR, and downstream engagement of the p70 S6 kinase. Our data indicate the function of this pathway is essential for mRNA translation of key IFN-sensitive genes and induction of the antiviral properties of IFNs. Interestingly, this pathway is also well-known to transduce mitogenic signals in response to growth factors and oncogenes. The surprising finding of engagement of this cascade by IFNs suggests that, under certain circumstances, mTOR-generated signals can mediate antiviral and, possibly, antitumor responses. The current proposal is a systematic approach to determine the role of this pathway in IFN-signaling. Specific aim A will examine the mechanisms of activation of mTOR by IFNs, and will define the role of Jak kinases in this process. Specific aim B will dissect the contributions of distinct IFN-regulated effectors downstream of mTOR in mRNA translation and induction of antiviral responses. Specific aim C will examine the functional relevance of 4E-BP1/2, Akt1/2 and p70S6k1/2 in the generation of the antiviral, immunoregulatory, and antitumor effects of IFNs in vivo, using different knockout mice. Finally, specific aim D will address the potential involvement of mTOR-pathways in the generation of the effects of IFNs on normal and leukemic hematopoiesis. Altogether, these studies should provide important information on the mechanisms by which signals are transduced by IFN receptors and advance our knowledge on the mechanisms of development of IFN-resistance. Identification of such mechanisms should prove valuable in designing better ways to use IFNs in clinical medicine and in the future development of novel, more potent and specific, pharmacological agents that target similar signaling pathways.